F PB96-91040I

NTSB/AAR-96/01

DCA95MA001

NATIONAL

TRANSPORTATION

SAFETY

BOARD

WASHINGTON, D.C. 20594

AIRCRAFT ACCIDENT REPORT

IN-FLIGHT ICING ENCOUNTER AND LOSS OF CONTROL

SIMMONS AIRLINES, d.b.a. AMERICAN EAGLE FLIGHT 4184

AVIONS de TRANSPORT REGIONAL (ATR)

MODEL 72-212, N401AM

ROSELAWN, INDIANA

OCTOBER 31,1994

VOLUME 1: SAFETY BOARD REPORT

6486C
The National Transportation Safety Board is an independent Federal agency dedicated to

promoting aviation, railroad, highway, marine, pipeline, and hazardous materials safety.

Established in 1967, the agency is mandated by Congress through the Independent Safety

Board Act of 1974 to investigate transportation accidents, determine the probable causes of

the accidents, issue safety recommendations, study transportation safety issues, and evaluate

the safety effectiveness of government agencies involved in transportation. The Safety

Board makes public its actions and decisions through accident reports, safety studies, special

investigation reports, safety recommendations, and statistical reviews.

Information about available publications may be obtained by contacting:

National Transportation Safety Board

Public Inquiries Section, RE-51

490 L’Enfant Plaza, S.W.

Washington, D.C. 20594

(202)382-6735

(800)877-6799

Safety Board publications may be purchased, by individual copy or by subscription, from:

National Technical Information Service

5285 Port Royal Road

Springfield, Virginia 22161

(703)487-4600
NTSB/AAR-96/01 PB96-910401

NATIONAL TRANSPORTATION

SAFETY BOARD

WASHINGTON, D.C. 20594

AIRCRAFT ACCIDENT REPORT

IN-FLIGHT ICING ENCOUNTER AND LOSS OF CONTROL

SIMMONS AIRLINES, d.b.a. AMERICAN EAGLE FLIGHT 4184

AVIONS de TRANSPORT REGIONAL (ATR)

MODEL 72-212, N401AM

ROSELAWN, INDIANA

OCTOBER 31, 1994

Adopted: July 9, 1996

Notation 6486C

Abstract: Volume I of this report explains the crash of American Eagle flight 4184, an

ATR 72 airplane during a rapid descent after an uncommanded roll excursion. The

safety issues discussed in the report focused on communicating hazardous weather

information to flightcrews, Federal regulations on aircraft icing and icing certification

requirements, the monitoring of aircraft airworthiness, and flightcrew training for unusual

events/attitudes. Safety recommendations concerning these issues were addressed to

the Federal Aviation Administration, the National Oceanic and Atmospheric

Administration, and AMR Eagle. Volume II contains the comments of the Bureau

Enquetes-Accidents on the Safety Board's draft of the accident report.

a-1 

Findings 21, 23, 24, 25, 26, 35, and 36 (and the corresponding text on pages 177, 178, 179, 179-

80,181, 193, and 194, respectively) are revised as follows: 

21. Prior to Before the Roselawn accident, ATR recognized the reason for the 

aileron behavior in the previous incidents and determined demonstrated that ice 

accumulation behind the deice boots, at an [angle of attack] sufficient to cause 

an airflow separation, would cause the ailerons to become unstable. Therefore, 

it would have been prudent for ATR to examine the combinations of icing 

conditions and airplane configurations that could produce the performance, 

stability, and control characteristics (including aileron hinge moment shifts) 

exhibited in the prior incidents, and the possible repercussions of such aileron 

hinge moment shifts had sufficient basis to modify the airplane and/or provide 

operators and pilots with adequate, detailed information regarding this 

phenomenon. 

23. ATRís proposed post-Mosinee AFM/FCOM changes, even if which were not

adopted by the DGAC and the FAA, would not have provided flightcrews with 

sufficient information to identify or recover from the type of event that occurred 

at Roselawn, and the actions taken by ATR following the Mosinee incident were 

insufficient. 

24. The 1992 ATR All Weather Operations brochure was misleading and minimized 

did not adequately communicate the known catastrophic potential of ATR 

operations in freezing rain. 

25. Information provided by ATR failed to disseminate adequate warnings and 

guidance to operators after the late 1980s and early 1990s about ice-related 

incidents did not give adequate warnings and guidance to operators about the 

adverse characteristics of, and techniques to recover from, ice-induced aileron 

hinge moment reversal events; and ATR failed to develop additional airplane 

modifications, which led directly to this accident. 

26. Prior to the Roselawn accident, the DGAC failed to require ATR to examine the 

combinations of icing conditions and airplane configurations that could produce 

the performance, stability, and control characteristics (including aileron hinge 

September 13, 2002 

 The National Transportation Safety Board adopted revisions 

to the findings and probable cause for this accident, as 

summarized below. 

For more information, see the full 

Response to Petition for Reconsideration. 
a-2 

moment shifts) exhibited in the prior incidents, and the possible repercussions of 

such aileron hinge moment shifts; take additional corrective actions, such as 

performing additional icing tests, issuing to issue more specific warnings 

regarding the aileron hinge moment reversal phenomenon,; developing 

additional airplane modifications, and providing to provide specific guidance on 

the recovery from a hinge moment reversal., which led directly to this accident. 

35. Because the DGAC did not require ATR, and ATR did not to provide to the 

operators of its airplanes, information that specifically alerted flightcrews to the 

fact that encounters with freezing rain could result in sudden autopilot 

disconnects, aileron hinge moment reversals, and rapid roll excursions, or 

guidance on how to cope with these events, the crew of flight 4184 had no 

reason to expect that the icing conditions they were encountering would cause 

the sudden onset of an aileron hinge moment reversal, autopilot disconnect, and 

loss of aileron control. 

36. Neither the flight attendantís presence in the cockpit nor the flightcrewís 

conversations with her contributed to the accident. However, aA sterile cockpit 

environment would probably have reduced flightcrew distractions and could 

have promoted a more appropriate level of flightcrew awareness for the 

conditions in which the airplane was being operated. 

The probable cause is amended as follows: 

The National Transportation Safety Board determines that the probable causes cause of 

this accident were was the loss of control, attributed to a sudden and unexpected aileron 

hinge moment reversal, that occurred after a ridge of ice accreted beyond the deice boots 

because: 1) ATR failed to completely disclose to operators, and incorporate in the ATR 

72 airplane flight manual, flightcrew operating manual and flightcrew training programs, 

adequate information concerning previously known effects of freezing precipitation on 

the stability and control characteristics, autopilot and related operational procedures 

when the ATR 72 was operated in such conditions; while the airplane was in a holding 

pattern during which it intermittently encountered supercooled cloud and drizzle/rain 

drops, the size and water content of which exceeded those described in the icing 

certification envelope. The airplane was susceptible to this loss of control, and the crew 

was unable to recover. 

Contributing to the accident were 1) 2) the French Directorate General for Civil 

Aviationís (DGACís) inadequate oversight of the ATR 42 and 72, and its failure to take 

the necessary corrective action to ensure continued airworthiness in icing conditions; 

2)3)the DGACís failure to provide the FAA with timely airworthiness information 

developed from previous ATR incidents and accidents in icing conditions, as specified 

under the Bilateral Airworthiness Agreement and Annex 8 of the International Civil 

Aviation Organization. Contributing to the accident were: 1) 3) the Federal Aviation 

Administrationís (FAAís) failure to ensure that aircraft icing certification requirements, 
a-3 

operational requirements for flight into icing conditions, and FAA published aircraft 

icing information adequately accounted for the hazards that can result from flight in 

freezing rain and other conditions not specified in 14 Code of Federal Regulations (CFR) 

Part 25, Appendics C, and 2) 4) the FAAís inadequate oversight of the ATR 42 and 72 to 

ensure continued airworthiness in icing conditions; and 5) ATRís inadequate response to 

the continued occurrence of ATR 42 icing/roll upsets which, in conjunction with 

information learned about aileron control difficulties during the certification and 

development of the ATR 42 and 72, should have prompted additional research, and the 

creation of updated airplane flight manuals, flightcrew operating manuals and training 

programs related to operation of the ATR 42 and 72 in such icing conditions. 

The following paragraph is removed from page 75: 

Hydraulically powered flight controls can overcome high control forces resulting from 

normal in-flight control surface hinge moments. If properly designed, they can also 

prevent control surface anomalies from being transmitted back through the control 

system and into the cockpit. According to ATR engineers, hydraulically powered 

ailerons were discussed during the preliminary design of the ATR 42. It was determined 

that adequate lateral control characteristics could be obtained without the additional 

weight and complexity of a hydraulic system. Hydraulic aileron control was again 

discussed informally among ATR engineers after an incident involving a Simmons 

Airlines ATR 42 in Mosinee, Wisconsin, in December 1988. ATR management has 

since stated that hydraulically powered ailerons have never been ìofficiallyî considered 

for either the ATR 42 or 72. 

The following sentence is removed from page 117: 

Also, with respect to Flight 4184, the chief test pilot for ATR testified that the type of roll 

anomaly the flight crew experienced would not have been recoverable by the average line 

pilot.

iii

CONTENTS

EXECUTIVE SUMMARY................................................................ vii

1. FACTUAL INFORMATION

1.1 History of Flight................................................................................... 1

1.2 Injuries to Persons................................................................................ 12

1.3 Damage to Airplane ............................................................................. 12

1.4 Other Damage ...................................................................................... 12

1.5 Personnel Information.......................................................................... 12

1.5.1 The Captain .......................................................................................... 12

1.5.2 The First Officer................................................................................... 13

1.5.3 The Flight Attendants .......................................................................... 14

1.5.4 Air Traffic Control Personnel.............................................................. 14

1.5.4.1 DANVILLE Sector Controller............................................................. 14

1.5.4.2 BOONE Sector Controller ................................................................... 15

1.5.4.3 BOONE Sector Developmental Controller ......................................... 15

1.6 Airplane Information ........................................................................... 15

1.6.1 Flight 4184 Dispatch Weight and Balance Information...................... 17

1.6.2 ATR 72 Wing Design History ............................................................. 17

1.6.3 ATR 72 Lateral Flight Control System Description ........................... 18

1.6.3.1 ATR 72 Directional Flight Control System......................................... 21

1.6.4 ATR 72 Stall Protection System.......................................................... 23

1.6.5 Autoflight System Description ............................................................ 24

1.6.6 ATR 72 Ice and Rain Protection Systems ........................................... 25

1.6.7 ATR 42/72 Type Certification History................................................ 30

1.6.7.1 General ................................................................................................. 30

1.6.7.2 ATR 72 Icing Certification Program ................................................... 30

1.6.7.3 Postaccident Certification Review....................................................... 35

1.7 Meteorological Information................................................................. 44

1.7.1 General ................................................................................................. 44

1.7.2 Flight 4184 Dispatch Weather Information......................................... 45

1.7.3 Weather Synopsis................................................................................. 48

1.7.4 Pilot Reports (PIREPs) and Other Weather Information .................... 53

1.7.4.1 Witness Descriptions of Weather Conditions ..................................... 54

1.7.5 Hazardous In-flight Weather Advisory Service (HIWAS) ................. 57

1.7.6 Information About Freezing Rain/Freezing Drizzle and General Icing

 Conditions .......................................................................................... 57

1.7.7 Classification of Icing Conditions ....................................................... 60
iv

1.7.8 Forecasting of In-flight Icing Conditions............................................ 61

1.8 Aids to Navigation ............................................................................... 63

1.9 Communications .................................................................................. 63

1.10 Aerodrome Information ....................................................................... 63

1.11 Flight Recorders................................................................................... 63

1.11.1 Cockpit Voice Recorder....................................................................... 63

1.11.2 Digital Flight Data Recorder................................................................ 64

1.12 Wreckage and Impact Information ...................................................... 64

1.12.1 General Wreckage Description............................................................ 64

1.12.2 The Wings ............................................................................................ 67

1.12.3 Empennage........................................................................................... 71

1.12.4 Engines and Propellers......................................................................... 72

1.13 Medical and Pathological Information ................................................ 73

1.14 Fire ....................................................................................................... 73

1.15 Survival Aspects .................................................................................. 73

1.16 Tests and Research............................................................................... 74

1.16.1 ATR 42/72 Lateral Control System Development History................. 74

1.16.2 Previous ATR 42/72 Incidents/Accidents ........................................... 75

1.16.3 Communication of Airworthiness Information Between FAA,

 DGAC and ATR................................................................................. 88

1.16.4 Investigation of Lateral Control System Behavior.............................. 91

1.16.5 Postaccident NASA Icing Research .................................................... 92

1.16.6 ATR 72 Icing Tanker Tests.................................................................. 94

1.16.7 Historical Aspects of Icing Research and Aircraft Icing Certification

 Requirements ..................................................................................... 97

1.17 Organizational and Management Information..................................... 100

1.17.1 Simmons Airlines................................................................................. 100

1.17.2 AMR Eagle Organization .................................................................... 101

1.17.3 FAA Oversight of Simmons Airlines/AMR Eagle.............................. 103

1.17.4 FAA Partnership in Safety Program .................................................... 103

1.17.5 Simmons Airlines/AMR Eagle Pilot Training .................................... 104

1.17.5.1 General Training Information.............................................................. 104

1.17.5.2 AMR Eagle Flight Training................................................................. 106

1.17.6 Flight and Airplane Operating Manual................................................ 107

1.17.7 Unusual Attitude and Advanced Maneuvers Training........................ 117

1.18 Additional Information ........................................................................ 118

1.18.1 Air Traffic Control............................................................................... 118

1.18.1.1 Chicago Area Airspace ........................................................................ 118

1.18.1.2 Air Traffic Control System Command Center .................................... 118
v

1.18.2 FAA Aircraft Certification................................................................... 122

1.18.3 Previous Safety Board Recommendations Regarding In-flight Icing. 126

1.18.4 Previous Safety Board Recommendations Regarding Unusual Attitude

 Training for Pilots .............................................................................. 136

1.18.5 Previous Safety Board Recommendations Regarding the Performance

 of ATR Airplanes and the Air Traffic Control System Command Center

140

1.18.6 Government Accounting Office (GAO) and Department of Transportation

 Inspector General (DOT/IG) Investigation of the Federal Aviation

 Administration ................................................................................... 146

1.18.7 Bilateral Airworthiness Agreement ..................................................... 150

1.18.8 Federal Regulations for Flight Operations in Icing Conditions.......... 155

1.18.9 New Technology .................................................................................. 156

1.18.9.1 Stall Protection System........................................................................ 156

2. ANALYSIS

2.1 General ................................................................................................. 158

2.2 Summary of Accident Sequence.......................................................... 159

2.3 Meteorological Factors ........................................................................ 161

2.3.1 General ................................................................................................. 161

2.3.2 Provisions of Weather Information to the Crew of Flight 4184 ......... 163

2.3.3 Icing Definitions .................................................................................. 165

2.3.4 Methods of Forecasting Icing Conditions ........................................... 166

2.4 ATR Flight Characteristics in Icing Conditions.................................. 167

2.5 ATR Certification for Flight Into Icing Conditions ............................ 170

2.5.1 Stall Protection Systems ...................................................................... 175

2.6 Continuing Airworthiness.................................................................... 176

2.6.1 Adequacy of Actions Taken by ATR After Previous ATR Incidents. 176

2.6.2 Continuing Airworthiness Oversight by DGAC ................................. 180

2.6.3 Continuing Airworthiness Oversight by FAA..................................... 181

2.7 ATR Certification and Continued Airworthiness Monitoring Under

 the Bilateral Airworthiness Agreement ............................................. 184

2.8 Air Traffic Control............................................................................... 187

2.9 Flightcrew Actions............................................................................... 190

2.9.1 Unusual Event Recovery...................................................................... 195

2.10 AMR Eagle/Simmons Airlines Management Structure

2.11 and FAA Oversight ............................................................................ 199

3. CONCLUSIONS

3.1 Findings................................................................................................ 203
vi

3.2 Probable Cause..................................................................................... 210

4. RECOMMENDATIONS................................................................... 211

5. APPENDIXES

Appendix A--Investigation and Hearing ............................................. 219

Appendix B--Cockpit Voice Recorder Transcript............................... 221

Appendix C--Excerpts from the FAA Special Certification Review

 of the ATR 72..................................................................................... 252

Appendix D--Photographs of Ice Accretions on the ATR 72

 During the Icing Tanker Tests ........................................................... 263

Appendix E--Doppler Weather Radar Wind and Windshear

 Calculations........................................................................................ 272

Appendix F--Doppler Weather Radar Images with Track of

 Flight 4184 Superimposed................................................................. 273

Appendix G--Discussion of Liquid Water Content and Liquid

 Water Drop Size................................................................................. 281

Appendix H--Listing of Previous Incident and Accident History for

 the ATR 42/72 Aircraft...................................................................... 287

Appendix I-- ATR All Weather Operations Brochure and ATR Icing

 Condition Procedures - Version 2.0 .................................................. 288
vii

EXECUTIVE SUMMARY

On October 31, 1994, at 1559 Central Standard Time, an Avions de

Transport Regional, model 72-212 (ATR 72), registration number N401AM,

leased to and operated by Simmons Airlines, Incorporated, and doing business as

American Eagle flight 4184, crashed during a rapid descent after an uncommanded

roll excursion. The airplane was in a holding pattern and was descending to a

newly assigned altitude of 8,000 feet when the initial roll excursion occurred. The

airplane was destroyed by impact forces; and the captain, first officer, 2 flight

attendants and 64 passengers received fatal injuries. Flight 4184 was a regularly

scheduled passenger flight being conducted under 14 Code of Federal

Regulations, Part 121; and an instrument flight rules flight plan had been filed.

The National Transportation Safety Board determines that the

probable causes of this accident were the loss of control, attributed to a sudden

and unexpected aileron hinge moment reversal that occurred after a ridge of ice

accreted beyond the deice boots because: 1) ATR failed to completely disclose to

operators, and incorporate in the ATR 72 airplane flight manual, flightcrew

operating manual and flightcrew training programs, adequate information

concerning previously known effects of freezing precipitation on the stability and

control characteristics, autopilot and related operational procedures when the ATR

72 was operated in such conditions; 2) the French Directorate General for Civil

Aviation’s inadequate oversight of the ATR 42 and 72, and its failure to take the

necessary corrective action to ensure continued airworthiness in icing conditions;

and 3) the French Directorate General for Civil Aviation's failure to provide the

Federal Aviation Administration with timely airworthiness information developed

from previous ATR incidents and accidents in icing conditions, as specified under

the Bilateral Airworthiness Agreement and Annex 8 of the International Civil

Aviation Organization.

Contributing to the accident were: 1) the Federal Aviation

Administration’s failure to ensure that aircraft icing certification requirements,

operational requirements for flight into icing conditions, and Federal Aviation

Administration published aircraft icing information adequately accounted for the

hazards that can result from flight in freezing rain and other icing conditions not

specified in 14 Code of Federal Regulations, Part 25, Appendix C; and 2) the

Federal Aviation Administration's inadequate oversight of the ATR 42 and 72 to

ensure continued airworthiness in icing conditions.
viii

The safety issues in this report focused on communicating hazardous

weather information to flightcrews, Federal regulations regarding aircraft icing

and icing certification requirements, the monitoring of aircraft airworthiness, and

flightcrew training for unusual events/attitudes.

Safety recommendations concerning these issues were addressed to

the Federal Aviation Administration, the National Oceanic and Atmospheric

Administration, and AMR Eagle. Also, as a result of this accident, on November

7, 1994, the Safety Board issued five safety recommendations to the Federal

Aviation Administration regarding the flight characteristics and performance of

ATR 42 and ATR 72 airplanes in icing conditions. In addition, on November 6,

1995, the Safety Board issued four safety recommendations to the Federal

Aviation Administration concerning the Air Traffic Control System Command

Center. In accordance with Annex 13 to the Convention on International Civil

Aviation, the Bureau Enquetes-Accidents provided comments on the Safety

Board's draft of the accident report that are contained in Volume II of this report.
ix

SELECTED ACRONYMS AND DEFINITIONS

AAS anti-icing advisory system

AC advisory circular; provides nonregulatory guidance to certificate

holders for a means (but not necessarily the only means) to comply

with Federal Aviation (FAA) Regulations

ACARS automatic communications and recording system

AD airworthiness directive; FAA regulatory requirement for immediate

mandatory inspection and/or modification

ADC air data computer

AEG FAA aircraft evaluation group

AIM Aeronautical Information Manual; a primary FAA publication whose

purpose is to instruct airmen about operating in the U. S. National

Airspace System

AIRMET Airman’s Meteorological Information; such advisories to flightcrews

include, but are not limited to, moderate icing and moderate

turbulence

AMM aircraft maintenance manual

AOA angle-of-attack (“vane” AOA is about 1.6 times the fuselage AOA for

the ATR 72)

AOM aircraft operating manual

ARTCC air route traffic control center

ASRS NASA’s Aviation Safety Reporting System

ATCSCC air traffic control system command center

BAA Bilateral Airworthiness Agreement

BEA French Bureau Enquetes-Accidents

CFR Code of Federal Regulations

CWA center weather advisories

CWSU center weather service unit

DGAC French Directorate General for Civil Aviation

EADI electronic attitude display indicator indicating pitch and roll attitudes

EDCT expect departure clearance time

EFC expect further clearance from ATC

EFIS electronic flight information system

EHSI electronic horizontal situation indicator

FAR/JAR Federal Aviation Regulations/Joint Airworthiness Requirements

FCOM flightcrew operating manual

G one G is equivalent to the acceleration due to Earth’s gravity

GPWS ground proximity warning system
x

HIWAS hazardous in-flight weather advisory service; continuous recorded

hazardous in-flight weather forecasts broadcast to airborne pilots over

selected VOR outlets defined as an HIWAS Broadcast Area

ICAO International Civil Aviation Organization

IFR instrument flight rules flight plan

IEP ice evidence probe

KIAS knots indicated airspeed

LWC liquid water content; the FAA defines LWC as the total mass of water

in all the liquid cloud droplets within a unit volume of cloud;

LWC/SLW refer to the amount of liquid water in a certain volume of

air

LWD left wing down

MFC multi-function computer

MVD median volumetric diameter; the FAA defines freezing drizzle as

supercooled water drops with MVDs between 50 and 300 microns

and freezing rain as supercooled water drops with MVDs greater than

300 microns (a micron is 1/1,000 of a millimeter) (“supercooled” is

the liquid state of a substance that is below the normal freezing

temperature for that substance)

NASA National Aeronautics and Space Administration (formerly the

National Advisory Committee for Aeronautics NACA))

NCAR National Center for Atmospheric Research

NOAA National Oceanic and Atmospheric Administration

NWS National Weather Service

NAWAU National Aviation Weather Advisory Unit; renamed Aviation

Weather Center subsequent to the accident

OIM operators information message

OAT outside air temperature

PIREP pilot report

POI FAA principal operations inspector

RWD right wing down

SAT static air temperature (synonymous with OAT)

SB service bulletin supplied by manufacturer

SIGMET significant meteorological information; such advisories to flightcrews

include, but are not limited to, severe and extreme turbulence and

severe icing

SPS stall protection system

STC supplemental type certificate

TAT total air temperature
xi

TCAS traffic alert and collision avoidance system

TRACON terminal radar approach control

TLU travel limiter unit, which limits rudder travel

VOR very high frequency omni-directional radio range navigation aid

Zulu Time coordinated universal time (UTC), time at the 0o

 longitude line that

passes through Greenwich, England, and is based on the 24-hour

clock

NATIONAL TRANSPORTATION SAFETY BOARD

WASHINGTON, D .C. 20594

AIRCRAFT ACCIDENT REPORT

IN-FLIGHT ICING ENCOUNTER AND LOSS OF CONTROL

SIMMONS AIRLINES, d.b.a. AMERICAN EAGLE FLIGHT 4184

AVIONS de TRANSPORT REGIONAL (ATR), MODEL 72-212, N401AM

ROSELAWN, INDIANA

OCTOBER 31, 1994

1. FACTUAL INFORMATION

1.1 History of Flight

On October 31, 1994, at 1559 Central Standard Time,1 an Avions de

Transport Regional, model 72-212 (ATR 72), registration number N401AM, leased

to and operated by Simmons Airlines, Incorporated, and doing business as (d.b.a.)

American Eagle flight 4184, crashed during a rapid descent after an uncommanded

roll excursion. The airplane was in a holding pattern and was descending to a newly

assigned altitude of 8,000 feet2 when the initial roll excursion occurred. The airplane

was destroyed by impact forces, and the captain, first officer, 2 flight attendants and

64 passengers received fatal injuries. Flight 4184 was a regularly scheduled

passenger flight being conducted under 14 Code of Federal Regulations (CFR) Part

121; and an instrument flight rules (IFR) flight plan had been filed.

The flightcrew reported for duty at 1039 in Chicago, Illinois, departed

Chicago's O'Hare International Airport (ORD) as flight 4101, on schedule at 1139,

and arrived in Indianapolis, Indiana (IND), at 1242. The trip sequence after IND

included a return leg to ORD, followed by a stopover at Dayton, Ohio (DAY), a

return trip to ORD, and a final stop in Champaign/Urbana, Illinois (CMI). The

captain was scheduled to complete only the first four segments of the first day's

schedule while the first officer was to fly all five segments. The accident occurred

 

1All times herein are Central Standard Time (CST) unless otherwise noted.

2All altitudes are expressed in relation to mean sea level (msl) unless otherwise noted.
2

on the second leg (from IND to ORD), while the first officer was performing the

duties of the flying pilot.

Prior to departure, the flightcrew received a company-prepared,

combined flight plan release and weather package. The meteorological information

provided to the crew did not contain airman's meteorological information (AIRMET)3

or any information regarding a forecast of light-to-moderate turbulence or in-flight

icing conditions along flight 4184's intended route of flight. According to testimony

of the Manager of Dispatch for Simmons Airlines, AIRMETs are available to

dispatchers for review and can be included in the flight release at the discretion of the

dispatcher. AIRMETs are also available for the pilots to review at the departure

station. There was no evidence to indicate whether the flightcrew of flight 4184 had

obtained this information.

Flight 4184 was scheduled to depart the gate in IND at 1410 and arrive

in ORD at 1515; however, due to a change in the traffic flow because of deteriorating

weather conditions (by the Traffic Management Coordinator) at ORD, the flight left

the gate at 1414 and was held on the ground for 42 minutes before receiving an IFR

clearance to ORD. At 1453:19, the ground controller at the IND air traffic control

(ATC) tower advised the crew of flight 4184 that, "...you can expect a little bit of

holding in the air and you can start 'em up [reference to engine start] contact the

tower when you're ready to go." The controller did not specify to the crew the reason

for either the ground or airborne hold.

At 1455:20, the IND local control (LC) controller cleared flight 4184 for

takeoff. The route for the planned 45-minute flight was to fly directly to IND VOR

(very high frequency omni-directional radio range) navigation aid via V-399 (Victor

Airway), then to BOILER VOR, directly to BEBEE intersection4 and thereafter to

ORD.

 

3According to the Aeronautical Information Manual (AIM), AIRMETs (Airman's Meteorological Information) are "inflight advisories concerning weather phenomena which are of operational interest to all aircraft and potentially hazardous

to aircraft having limited capability because of lack of equipment, instrumentation, or pilot qualifications. AIRMETs

concern weather of less severity than that covered by SIGMETs." AIRMETs cover large geographical areas similar to a

SIGMET [significant meteorological information], and include information regarding "moderate icing, moderate

turbulence, sustained winds of 30 knots or more at the surface...."

4An intersection is a point defined by any combination of intersecting courses, radials or bearings of two or more

navigational aids.
3

The data from the digital flight data recorder (FDR) indicated that the

flightcrew engaged the autopilot as the airplane climbed through 1,800 feet. At

1505:14, the captain made initial radio contact with the DANVILLE Sector (DNV)

Radar Controller and reported that they were at 10,700 feet and climbing to

14,000 feet. The DNV controller issued a clearance to the crew to proceed directly to

the Chicago Heights VOR (CGT). At 1508:33, the captain of flight 4184 requested

and received a clearance to continue the climb to the final en route altitude of

16,000 feet.

At 1509:22, the pilot of a Beech Baron, identified as N7983B, provided

a pilot report (PIREP) to the DNV controller that there was "light icing" at 12,000

feet over BOILER, and, at 1509:44, he reported the icing was "trace rime...."

According to the DNV controller, because the crew of flight 4184 was on the

frequency and had established radio contact, the PIREP was not repeated. The DNV

controller received additional PIREPs shortly after the accident.

At 1511:40, prior to flight 4184 establishing radio contact with the

BOONE sector controller,5 the ORD Terminal Radar Approach Control (TRACON)

West Arrival Handoff controller contacted the BOONE controller via telephone and

said, "...protect yourself for the hold." At 1511:45, the DNV controller contacted

flight 4184 and issued a clearance and a frequency change to the BOONE controller.

This transmission was acknowledged by the captain. About this same time, the FDR

indicated that the airplane was in a level attitude at an altitude of about 16,300 feet

and was maintaining an airspeed of approximately 190 knots indicated airspeed

(KIAS). About 2 minutes later, the captain of flight 4184 made initial radio contact

with the BOONE controller and stated, "...checking in at one six thousand we have a

discretion down to one zero thousand forty southeast of the Heights we're on our way

down now." The BOONE controller acknowledged the radio transmission and

provided the ORD altimeter setting. At 1513, flight 4184 began the descent to

10,000 feet. During the descent, the FDR recorded the activation of the Level III6

airframe deicing system and the propeller revolutions per minute (RPM) at 86

percent.

At 1517:24, the BOONE sector was advised by the ORD TRACON

arrival controller to issue holding instructions to those aircraft that were inbound to

 

5Refer to Section 1.5.4.3 for further information about the developmental controller and trainer handling aircraft in the

BOONE Sector.

6Refer to Section 1.6.6 for detailed information about the ATR-72 deicing system.
4

ORD.7 At 1518:07, shortly after flight 4184 leveled off at 10,000 feet, the crew

received a clearance from the BOONE controller that they were, "...cleared to the

LUCIT intersection8 via radar vectors turn ten degrees left intercept Victor 7 hold

southeast on Victor 7 expect further clearance (EFC) two one three zero [Zulu time]

[1530 CST]." The captain acknowledged the transmission. About 1 minute later, the

BOONE controller revised the EFC for flight 4184 to 1545.9 This was followed a

short time later by several radio transmissions between the captain of flight 4184 and

the BOONE controller in which he received approval for 10 nautical mile legs in the

holding pattern, a speed reduction,10 and confirmation of right turns while holding.

(See Figure 1 for holding location.)

At 1524:39, the captain of flight 4184 contacted the BOONE controller

and reported, "entering the hold." The crew then notified the company via the

automatic communications and recording system (ACARS) that they were delayed

and that the EFC was 1545. According to the FDR, the first holding pattern was

flown at approximately 175 KIAS with the wing flaps in the retracted position. The

airframe deice system was deactivated during this time, and the propeller speed was

reduced to 77 percent.

Recorded sounds on the cockpit voice recorder (CVR) began at 1527:59,

with the sounds of music playing in the first officer's headset and a flight attendant in

the cockpit discussing both flight and nonflight-related information with the pilots.

At 1533:13, the captain stated, "man this thing gets a high deck angle in these

turns...we're just wallowing in the air right now" [FDR data indicated that the vane

angle-of-attack (AOA)11 was 5 degrees]. The following exchange

 

7Between 1547:59 and 1558:28 there were seven aircraft holding in the BEARZ sector at HALIE intersection located 25

nautical miles northeast of the LUCIT intersection. The aircraft holding were a United Airlines B-757 at 11,000 feet; a

United Airlines B-767 holding at 12,000 feet; a USAir DC-9 holding at 13,000 feet; a United Airlines B-737 holding at

14,000 feet; a Northwest Airline Airbus A-320 holding at 15,000 feet; a Dassault Falcon 50 holding at 16,000 feet; and

an American Airlines Airbus A-300 holding at 17,000 feet.

8Located 18 nautical miles from the Chicago Heights VOR on the 157-degree radial.

9Arriving aircraft that preceded flight 4184 were slowed down because of deteriorating weather conditions and an

anticipated "rush" of arriving aircraft from the west; as a result, the BOONE sector controller issued two additional

EFC's to the flightcrew.

10The maximum airspeed for all propeller-driven airplanes (including turbopropeller) in holding is 175 KIAS.

According to the FDR data, flight 4184's indicated airspeed varied between 170 and 180 KIAS in the holding pattern.

11Vane AOA is herein referred to as "AOA" and is approximately 1.6 times the fuselage AOA, such that at 5 degrees

vane AOA, fuselage AOA is approximately 3 degrees.
5

Figure 1.--Holding pattern location; box identifies LUCIT intersection.
6

of conversation and sounds were recorded on the CVR:12

1533:19 First Officer you want flaps fifteen

1533:21 Captain I'll be ready for that stall procedure 

here pretty soon

1533:22 First Officer sound of chuckle

1533:24 Captain do you want kick 'em in (it'll) bring

the nose down

1533:25 First Officer sure

1533:26 Sound of several clicks similar to flap handle being

moved. [The FDR recorded the flaps moving to the

15-degree position and the aircraft AOA decreasing to

approximately 0 degrees].

1533:34 Sound of "whooler" similar to pitch trim movement

1533:39 Captain...the trim, automatic trim

1533:56 Sound of single tone similar to caution alert

At 1538:42, the BOONE controller issued a revised EFC of 1600 to

flight 4184. The captain acknowledged this transmission, and the CVR recorded the

flightcrew continuing their discussion with the flight attendant. At 1541:07, the CVR

recorded the sound of a single tone similar to the caution alert chime,13 and the FDR

recorded the activation of the "Level III" airframe deicing systems. About 3 seconds

later, both the CVR and FDR recorded an increase in the propeller speed from 77

percent to 86 percent.

At 1542:38, during the beginning of the third circuit of the holding

pattern, the CVR recorded the flight attendant leaving the cockpit followed by the

flightcrew discussing flight-related information. Also, during this time, the crew

 

12Refer to appendix B for a complete transcript of the CVR.

13This caution chime can be activated by any one of numerous aircraft systems, including the aircraft ice detection

system. See Section 1.6.6 for more information about the ice detection system.
7

received information from the company dispatch via the ACARS. The first officer

transmitted the updated EFC time and fuel data via the ACARS but was unsuccessful

in acknowledging a company-transmitted ACARS message. He succeeded in

sending another ACARS message; however, he was still unsuccessful in

acknowledging the company's messages.

At 1548:34, the first officer commented to the captain, "that's much

nicer, flaps fifteen." About 7 seconds later, the CVR recorded one of the two pilots

saying, "I'm showing some ice now." This comment was followed 2 seconds later by

an unintelligible word(s). The CVR group could not determine whether the word(s)

was spoken by the captain or the first officer. The captain remarked shortly

thereafter, "I'm sure that once they let us out of the hold and forget they're down

[flaps] we'll get the overspeed."14

At 1549:44, the captain departed the cockpit and went to the aft portion

of the airplane to use the restroom. During the captain's absence, both he and a flight

attendant spoke with the first officer via the inter-communication system (ICS) for

about 1 minute. The captain advised the first officer that the restroom was occupied

and that he would return shortly. The CVR indicated that the captain returned from

the restroom at 1554:13, and upon his return asked the first officer for a status update

regarding company and ATC communications. There was no verbal inquiry by the

captain about the status of the icing conditions or the aircraft deice/anti-icing

systems. At 1555:42, the first officer commented, "we still got ice." This comment

was not verbally acknowledged by the captain. The CVR indicated that the

flightcrew had no further discussions regarding the icing conditions.

At 1556:16, the BOONE controller contacted flight 4184 and instructed

the flightcrew to, "descend and maintain eight thousand [feet]." At 1556:24, the CVR

recorded a TCAS15 alert; however, there was no discussion between the flight

crewmembers regarding this alert. This was followed by a transmission from the

BOONE controller informing the crew that "...[it] should be about ten minutes till

you're cleared in." The first officer responded, "thank you." There were no further

radio communications with the crew of flight 4184.

 

14Reference is to the aural flap overspeed warning that activates if the aircraft speed exceeds 185 knots with the flaps in

the 15-degree position.

15The traffic alert and collision avoidance system is an airborne collision avoidance system based on radar beacon

signals that operate independent of ground-based equipment. TCAS II generates traffic advisories and resolution

(collision avoidance) advisories in the vertical plane.
8

At 1556:51, the FDR showed that the airplane began to descend from

10,000 feet, the engine power was reduced to the flight idle position, the propeller

speed was 86 percent, and the autopilot remained engaged in the vertical speed (VS)

and heading select (HDG SEL) modes. At 1557:21, as the airplane was descending

in a 15-degree right-wing-down (RWD) attitude at 186 KIAS, the sound of the flap

overspeed warning was recorded on the CVR. Five seconds later, the captain

commented, "I knew we'd do that," followed by the first officer responding, "I [was]

trying to keep it at one eighty." As the flaps began transitioning to the zero degree

position, the AOA and pitch attitude began to increase.

At 1557:33, as the airplane was descending through 9,130 feet, the AOA

increased through 5 degrees, and the ailerons began deflecting to a RWD position.

About 1/2 second later, the ailerons rapidly deflected to 13.43 degrees RWD,16 the

autopilot disconnected, and the CVR recorded the sounds of the autopilot disconnect

warning (a repetitive triple chirp that is manually silenced by the pilot). The airplane

rolled rapidly to the right, and the pitch attitude and AOA began to decrease (see

Figures 2 and 3 for graphical depictions of the airplane's flightpath and FDR/CVR

data). There were no recorded exchanges of conversation between the flightcrew

during the initial roll excursion, only brief expletive remarks followed by the sounds

of "intermittent heavy irregular breathing."

Within several seconds of the initial aileron and roll excursion, the AOA

decreased through 3.5 degrees, the ailerons moved to a nearly neutral position, and

the airplane stopped rolling at 77 degrees RWD. The airplane then began to roll to

the left toward a wings-level attitude, the elevator began moving in a nose-up

direction, the AOA began increasing, and the pitch attitude stopped at approximately

15 degrees nose down.

At 1557:38, as the airplane rolled back to the left through 59 degrees

RWD (towards wings level), the AOA increased again through 5 degrees and the

ailerons again deflected rapidly to a RWD position. The captain's nose-up control

column force exceeded 22 pounds,17 and the airplane rolled rapidly to the right, at a

rate in excess of 50 degrees per second.

 

16Maximum designed aileron deflection is 14 degrees in either direction from neutral.

17The DFDR records data that indicate when more than 22 pounds of force are applied to the captain's and first officer's

control columns in both nose-up and nose-down directions.
150

130

I

90

70

I

30

13

Flight Path 3-D View with X-Y Axis Plane Projection

(DCA95MA001, 10/31/94, Roselawn IN, ATR-72-212, N401AM)

. .

.

* . . * I

.8

.

.

.

.

150

130

Figure 2.—Flightpath of airplane in holding pattern.

11

According to the FDR data, the captain's nose-up control column force

decreased below 22 pounds as the airplane rolled through 120 degrees, and the first

officer's nose-up control column force exceeded 22 pounds just after the airplane

rolled through the inverted position (180 degrees). Nose-up elevator inputs were

indicated on the FDR throughout the roll, and the AOA increased when nose-up

elevator increased. At 1557:45, as the airplane rolled through the wings-level

attitude (completion of first full roll), the captain said "alright man" and the first

officer's nose-up control column force decreased below 22 pounds. The nose-up

elevator and AOA then decreased rapidly, the ailerons immediately deflected to

6 degrees LWD and then stabilized at about 1 degree RWD,18 and the airplane

stopped rolling at 144 degrees right wing down.

At 1557:48, as the airplane began rolling left, back towards wings level,

the airspeed increased through 260 knots, the pitch attitude decreased through

60 degrees nose down, normal acceleration fluctuated between 2.0 and 2.5 G,19 and

the altitude decreased through 6,000 feet. At 1557:51, as the roll attitude passed

through 90 degrees, continuing towards wings level, the captain applied more than 22

pounds of nose-up control column force, the elevator position increased to about 3

degrees nose up, pitch attitude stopped decreasing at 73 degrees nose down, the

airspeed increased through 300 KIAS, normal acceleration remained above 2 G, and

the altitude decreased through 4,900 feet.

At 1557:53, as the captain's nose-up control column force decreased

below 22 pounds, the first officer's nose-up control column force again exceeded

22 pounds and the captain made the statement "nice and easy." At 1557:55, the

normal acceleration increased to over 3.0 G, the sound of the ground proximity

warning system (GPWS) "Terrain Whoop Whoop" alert was recorded on the CVR,

and the captain's nose-up control column force again exceeded 22 pounds.

Approximately 1.7 seconds later, as the altitude decreased through 1,700 feet, the

first officer made an expletive comment, the elevator position and vertical

acceleration began to increase rapidly, and the CVR recorded a loud "crunching"

sound. The last recorded data on the FDR occurred at an altitude of 1,682 feet

(vertical speed of approximately 500 feet per second), and indicated that the airplane

was at an airspeed of 375 KIAS, a pitch attitude of 38 degrees nose down with 5

 

18Prior to this point, vane AOA remained over 5 degrees, and aileron position had been oscillatory. Aileron position

stabilized after vane AOA decreased below 5 degrees.

19Normal acceleration, as stated in this report, refers to the acceleration of the center of gravity of the airplane along its

vertical axis, which is 90 degrees to the airplane's longitudinal and lateral axes. The values are shown in units of "G"

force; and one (1) G is equivalent to the acceleration due to Earth's gravity.
12

degrees of nose-up elevator, and was experiencing a vertical acceleration of 3.6 G.

The CVR continued to record the loud crunching sound for an additional 0.4 seconds.

The airplane impacted a wet soybean field partially inverted, in a nose down, leftwing-low attitude.

There were no witnesses to the accident, which occurred during the

hours of daylight at 41o 5' 40" north latitude and 87o 19' 20" west longitude. The

elevation of the accident site was about 675 feet.

1.2 Injuries to Persons

Injuries Flightcrew Cabincrew Passengers Other Total

Fatal 2 2 64 0 68

Serious 0 0 0 0 0

Minor 0 0 0 0 0

None 0 0 0 -- 0

Total 2 2 64 0 68

1.3 Damage to Airplane

The airplane was destroyed by impact forces. The estimated value of the

airplane was $12,000,000.

1.4 Other Damage

The airplane struck the ground in a 20-acre soybean field. The field was

determined to be an environmental hazard; and the expense of reconditioning the

land for agricultural use was estimated at $880,000.

1.5 Personnel Information

1.5.1 The Captain

The captain, age 29, held an Federal Aviation Administration (FAA)

Airline Transport Pilot (ATP) certificate, No. 572598812, with a multiengine land

airplane rating, and type ratings in the Shorts SD3 and the ATR 42/72. Additionally,

he held commercial pilot and flight instructor certificates with single-engine land,

multiengine land and instrument airplane ratings. He was issued
13

an FAA First Class Airman Medical Certificate on October 31, 1994, with no

limitations.

The captain had gained his initial flying experience (prior to

employment with Simmons Airlines) in general aviation aircraft. He was hired by

Simmons Airlines on July 1, 1987, as a first officer for the Shorts 360 and progressed

to a captain on the Shorts. The captain transitioned to the ATR and attained his type

rating in the ATR 72 on March 17, 1993. According to company records, the captain

had accumulated 7,867 hours of total flight time as of the date of the accident, with

1,548 hours total time in the ATR. All of the flight time he had accrued in the ATR

was as the pilot-in-command. His most recent 14 CFR 121 proficiency check was

successfully accomplished on April 25, 1994. He attended recurrent training on

October 9, 1994, and satisfactorily completed a line check on June 8, 1994, which

was administered by a check airman from the American Eagle Training Center.

A review of the captain's airman certification records and FAA

accident/incident and violation histories revealed no adverse information. He held a

valid Illinois driver's license with no history of automobile accidents or violations in

the preceding 3 years. Interviews with other crewmembers, check airmen and

instructors subsequent to the accident described the captain's performance in positive

terms. Several pilots stated that he solicited input from first officers, considered their

opinions and promoted a relaxed atmosphere in the cockpit.

A review of the captain's activities in the 3 days before the accident

showed that he had flown a 3-day trip schedule that ended at 1930 on the day before

the accident. He successfully completed an FAA Airman Medical examination on the

morning of October 31. According to witnesses, the captain appeared rested. It

could not be confirmed through the company's records if this was the first trip pairing

for this captain and this first officer.

1.5.2 The First Officer

The first officer, age 30, held a commercial pilot certificate,

No. 2316882, with single and multiengine land airplane and instrument ratings.

Additionally, he held both ground and flight instructor certificates. He was issued an

FAA First Class Airman Medical Certificate on August 8, 1994, with no limitations.
14

The first officer had gained his flying experience (prior to employment

with Simmons Airlines) in general aviation aircraft. He was hired by Simmons

Airlines on August 14, 1989, for his current position and, according to company

records, had accumulated a total flight time of 5,176 hours as of the date of the

accident, with 3,657 hours in the ATR. His most recent 14 CFR Part 121 proficiency

check was successfully accomplished on September 7, 1994, and he attended

recurrent training on September 9, 1994.

The first officer's airman certification, and FAA accident/incident and

violation histories were reviewed and no adverse information was revealed. He held

a valid Wisconsin driver's license with no history of automobile accidents or

violations in the preceding 3 years. Crewmembers, check airmen and instructors,

who were interviewed subsequent to the accident, described the first officer's

performance in positive terms.

A review of the first officer's activities in the 3 days preceding the

accident showed that he had been off duty and spent the majority of the time at his

family's ranch. According to witnesses, the first officer appeared alert during the

break period prior to the accident flight.

1.5.3 The Flight Attendants

There were two flight attendants aboard flight 4184 at the time of the

accident. The senior flight attendant was employed by Simmons Airlines on

January 17, 1988, and received training on the Shorts 360 and the ATR 42/72

airplanes. She successfully accomplished her ATR recurrent training on

April 12, 1994.

The junior flight attendant was hired by Simmons Airlines on October 6,

1994, and successfully completed her initial training in October on the Saab 340,

Shorts 360 and the ATR 42/72 airplanes. Flight 4184 was the first line trip for the

junior flight attendant.

1.5.4 Air Traffic Control Personnel

1.5.4.1 DANVILLE Sector Controller

The controller was employed by the FAA on July 30, 1982. He began

his duty at the Chicago air route traffic control center (ARTCC) on October 27,
15

1982, and became a full performance level (FPL) controller on February 27, 1986.

He was issued an FAA Second Class Airman Medical Certificate with no waivers or

limitations in February 1994.

1.5.4.2 BOONE Sector Controller

The controller was employed by the FAA on October 30, 1987, and

graduated from the FAA Academy in January 1988. He began his duty at the

Chicago ARTCC on January 21, 1988, and became an FPL for the South Area on

April 8, 1993. He was issued an FAA Second Class Airman Medical Certificate with

a limitation to wear corrective lenses for nearsightedness in July 1994. At the time of

the accident, he was conducting on-the-job training and instructing the

developmental controller.

1.5.4.3 BOONE Sector Developmental Controller

The controller was employed by the FAA on September 26, 1989, and

had graduated from the FAA Academy in December of 1989. She began her duty at

the Chicago ARTCC on December 20, 1989. She was issued an FAA Second Class

Airman Medical Certificate in July 1994, with no waivers or limitations.

At the time of the accident, the developmental controller was receiving

on-the-job training at the BOONE Sector radar position and had accumulated

87.16 hours of the allotted 120 hours in this position. She was previously certified at

two radar positions (Pontiac and Danville) and seven manual assist positions,

including the DANVILLE and BOONE sectors.

1.6 Airplane Information

N401AM, ATR serial number 401, was a pressurized, high wing, two

engine, turbopropeller airplane. It was manufactured in Toulouse, France, on

February 2, 1994, and at the time of the accident was owned by and registered to

AMR Leasing Corporation, a subsidiary of AMR Corporation. N401AM was issued

a French Export Certificate and a U.S. Certificate of Airworthiness on March 24,

1994. The airplane was placed into service with Simmons Airlines on March 29,

1994, and was maintained in accordance with the its Continuous Airworthiness and

Maintenance Program (CAMP). According to ATR, a total of 154 ATR airplanes are

currently in operation in the United States. The total includes 103 ATR 42 airplanes

and 51 ATR 72 airplanes.
16

At the time of the accident, the airplane had accumulated 1,352.5 hours

of flight time in 1,671 flights. The maintenance records revealed that the airplane

had been in compliance with all applicable airworthiness directives (ADs). On the

day of the accident, the airplane had been dispatched with two deferred maintenance

items: an inoperative No. 2 bleed valve, and an inoperative cargo door warning

system.

The airplane was equipped with a Honeywell Electronic Flight

Information System (EFIS) that displays the aircraft attitude, heading, and other

flight-related information. The airplane's attitude is displayed on a cathode ray tube

(CRT) for each pilot and on a mechanical "standby" attitude display indicator (ADI)

located on the center instrument panel. The CRT attitude display is referred to as an

Electronic Attitude Display Indicator (EADI), and its operation is such that the

horizon sphere symbol moves relative to the airplane symbol to indicate pitch and roll

attitudes. The EADI, unlike the mechanically operated ADI, will not tumble or lose

its reference during extreme attitude changes. The portion of the sphere that

represents the sky is colored blue, and that representing the ground is shaded brown.

The pitch scale is marked in 5-degree increments to plus and minus 80 degrees. The

roll scale displays 0, 10, 20, 30, 45 and 60 degree orientation marks.

The EADI also displays chevrons that point toward the horizon and are

fully visible above a 45-degree nose-up and below a 30-degree nose-down pitch

attitude. The chevrons are used to orient the pilot to the horizon and to aid in the

recovery from an unusual attitude. The tip of the chevron [below the horizon line]

becomes visible at a pitch attitude of approximately 10 degrees nose down. The

investigation revealed that these chevrons are not typically visible to the pilot through

the "normal" range of pitch attitudes; however, the pilots do see the chevrons when

performing emergency descent procedures during training. In addition to the

chevrons, the EADI displays an "eyelid," which is shaded either blue or brown,

depending on the aircraft's pitch attitude. The system logic was designed so that the

eyelid would remain visible when the EADI pitch attitude indication was at or

beyond the maximum normal display limits of the horizon reference line. The eyelid

horizon symbol and the chevrons are meant to facilitate pilot orientation to the

horizon during extreme pitch attitudes.

The Electronic Horizontal Situation Indicator (EHSI) combines several

displays on one screen to provide a moving-map depiction of the airplane position.

The display shows the airplane's position relative to VOR radials, localizer and

glideslope beams, as well as providing real-time information for heading, course
17

selection, distance, groundspeed, desired track, bearings, glideslope or glidepath

deviations, and other navigational features. The EHSI also incorporates a four-color

weather radar and displays 3 levels of detectable moisture with four separate colors.

According to the ATR 72 Flight Crew Operating Manual (FCOM), the following

colors are used to depict the various cloud densities:

Level Weather Mode Map Mode

Level 0 No Detectable Clouds Black

Level 1 Normal Clouds Green

Level 2 Dense Clouds Yellow

Level 3 Severe Storm Red

Because this information is not recorded on the FDR, and the flightcrew

did not make any comments referencing the weather radar, it could not be determined

during the investigation if the weather radar was being used during the accident

flight.

1.6.1 Flight 4184 Dispatch Weight and Balance Information

The dispatch information for flight 4184 indicated that it was released

from IND at a gross takeoff weight of 45,338 pounds [maximum gross takeoff weight

is 47,400 pounds], with a calculated zero fuel weight of 40,586 pounds. The

computed weight of flight 4184 included 11,934 pounds for 64 passengers and

baggage/cargo, and 5,060 pounds for fuel. The center of gravity was calculated to be

22 percent mean aerodynamic chord (MAC).20 The calculated gross weight of the

airplane at the time of the accident was approximately 43,850 pounds.

1.6.2 ATR 72 Wing Design History

According to the manufacturer, the ATR 72 wing was developed by

Aerospatiale, based on a modified National Advisory Committee for Aeronautics

(NACA) 43XXX "5 digit series" non-laminar21 airfoil design. [NACA was renamed

the National Aeronautics and Space Administration (NASA) in 1958.] The NACA

airfoil designs have been used for airplanes manufactured worldwide.

 

20The ATR 72 maximum allowable MAC range for flight is 10 to 39 percent.

21Laminar flow is the smooth movement of air in parallel layers with very little mixing between layers. Each layer has a

constant velocity but is in motion relative to its neighboring layers.
18

The ATR 72 wing is a non-laminar flow design; thus, the boundary layer

airflow was not intended to remain laminar.22

1.6.3 ATR 72 Lateral Flight Control System Description

The ATR 72 lateral flight control systems consist of moveable,

unpowered23 ailerons plus hydraulically actuated wing spoilers that supplement the

ailerons. The ailerons are aerodynamically balanced through the use of an offset

hinge line, geared trailing edge balance tabs, and exposed horns (see figures 4 and 5).

The exposed horns, which are also weighted for mass balance of the ailerons, are

mounted on the outboard tips of the ailerons and extend spanwise beyond the tips of

the wings.

The ailerons are connected to the cockpit control wheels by a series of

cables, bellcranks, and carbon-fiber push-pull rods. A tension regulator maintains 20

to 25 deca-newtons (daN)24 of cable tension. An electric trim actuator motor is

connected to the left aileron balance tab. The maximum deflections for the ailerons,

control wheels, and the balance tabs are approximately +/-14 degrees, +/-65 degrees,

and +/-4 degrees, respectively. The lateral control system is augmented with one

hydraulically actuated spoiler on the upper surface of each wing, just inboard of the

ailerons. The hydraulic control for each spoiler is controlled mechanically by the

aileron control linkage. The hydraulic actuator for each side is designed to activate at

an aileron deflection of 2.5 degrees trailing edge up, and spoiler deflection is linear

up to 57 degrees for 14 degrees of aileron deflection.

According to ATR engineers, the design of the lateral control system

produces roll rates and maximum control wheel forces of less than 60 pounds, as

required in 14 CFR Parts 25.143 and 25.147. The control wheel forces required to

move unpowered ailerons are a function of aileron hinge moments and mechanical

 

22Fifty percent of the ATR wing is located in the propeller slipstream, resulting in turbulent airflow along the entire

airfoil chord for that portion of the span. The remainder of the wing (outside the propeller slipstream) has a slight airfoil

surface discontinuity at the junction of the removable leading edges and center wing section (located at 16 percent

chord). This chordwise discontinuity results in boundary layer transition from the laminar regime to the turbulent

regime, if it has not already occurred.

23Refers to flight controls that are not hydraulically assisted.

24According to the ATR 72 Aircraft Maintenance Manual, 10 daN is equivalent to 22.48 [foot] pounds. Thus, 20 to 25

daN would be equivalent to 44.96 to 56.2 [foot] pounds.
19

ROLL CONTROL

CONTROL WHEEL

Y

—

F.D.A.U, 7

\

CABLE TENSION REGULATOR

+

I

POSITION I

%. -

TRANSMlllER= I -AD

, TRIM ACTUATOR

m“

Figure 4.—Roll control diagram.
20

AILERON ASSEMBLY

. . -->.. }.... . . . . . . . . . . . .. . . . . . . . .. . . . .. . . . . . . . . .

a

Figure 5.--Aileron assembly.
21

gearing between the aileron hinges and the cockpit control wheels. Aileron hinge

moments are a function of the air pressure distribution on the surface of the aileron

and associated balance devices, as well as the chordwise location of the aileron hinge

line.

The aileron systems on the ATR 42 and 72 utilize the horns and balance

tabs to provide an "aerodynamic power assist" in the direction of the deflection,

which results in aileron controllability without hydraulic actuators. Under normal

airflow conditions, deflection of the ailerons requires a control wheel force that

progressively increases as aileron deflection increases. Without the horns and tabs,

the control forces in flight would be excessively high. The forward aileron hinge line

provides aileron deflection stability, while the balance horns and tabs provide aileron

deflection controllability.

Compared to a hydraulically powered aileron system, the ATR's

unpowered, aerodynamically balanced aileron control system is light weight, requires

a minimal number of components, and is inexpensive to manufacture. However,

during some airflow separation conditions, unpowered aileron control systems can be

susceptible to undesirable aileron hinge moment changes (including asymmetric

hinge moment reversals) and subsequent uncommanded aileron deflections.

1.6.3.1 ATR 72 Directional Flight Control System

ATR 72 directional control is accomplished with the rudder and its

associated systems: the Travel Limiter Unit (TLU); the Releasable Centering Unit

(RCU); the yaw damper; and the rudder trim system (see Figure 6). The rudder is

mechanically connected to the cockpit rudder pedals through a series of cables,

springs, bellcranks, and push-pull rods, and has a maximum low speed deflection of

27 degrees each side of neutral.

The TLU limits the rudder travel to about 3 degrees each side of neutral

(6 degrees total) at speeds above 185 KIAS, using a "U-shaped" mechanical stop that

extends around the lower portion of the rudder. The TLU is normally controlled

automatically via the multi-function computer and airspeed data obtained from the air

data computer (ADC). The TLU high speed mode occurs when both ADCs sense an

indicated airspeed greater than 185 knots. Reversion to the low speed mode (full

rudder deflection) occurs when at least one ADC senses an

23

indicated airspeed that is less than 180 knots. The TLU function can be overridden by

the pilot through the activation of the TLU override switch located in the cockpit, and

full rudder authority will be available 15 seconds after override switch activation.

The Safety Board reviewed operational procedures in both the American

Eagle ATR 72 Aircraft Operating Manual (AOM) and the ATR Flight Crew Operating

Manual (FCOM). Both publications are identical in their respective description of the

"Aileron Jam" procedures and state in the "Comments" section that, "there is no

indication of an aileron jam other than an inability to operate the control wheel

laterally." The procedures also state that the bank angle is limited to a maximum of 25

degrees "due to reduced roll control efficiency." The procedures do not indicate that

the rudder is the primary source of lateral control in the event of jammed ailerons or

that the rudder travel is limited to 6 degrees at airspeeds above 185 KIAS. Also, the

procedures for both aileron jams and spoiler jams do not indicate the need to use the

TLU override switch to restore full rudder authority over 185 KIAS.

1.6.4 ATR 72 Stall Protection System

The ATR 72 stall protection system (SPS) offers the pilot three different

devices that provide warnings prior to the airplane reaching AOAs consistent with

"clean" and ice-contaminated flow separation characteristics. These devices are: an

aural warning and a stick shaker, both of which activate simultaneously when the

AOA reaches a predetermined value that affords an adequate margin prior to the onset

of adverse aerodynamic characteristic(s); and a stick pusher that activates when the

AOA reaches a subsequently higher value that has been determined to be nearer to the

onset of stall or aileron hinge moment reversal. The activation of the stick pusher

results in an immediate and strong nose-down movement of the control column.

The SPS on the ATR 72 is controlled by two multi-function computers

(MFC), each of which uses information from the following sources for activation: the

AOA probes; the flap position; engine torque; airplane on-ground/in-flight indication;

horn anti-ice status; airplane altitude above or below 500 feet; and the presence or

absence of optional deicers on the inner leading edges. The stick pusher, which is

mechanically linked to the left control column cable, moves the column to the 8-

degree nose-down position when the MFC stick pusher activation criteria are met.
24

The SPS logic also uses AOA probe information to reduce the triggering

threshold when the AOA is rapidly moving toward positive values. According to the

aircraft maintenance manual (AMM) for the ATR 72, the phase lead of the triggering

threshold has a maximum value of plus 3 degrees AOA and does not intervene when

the anti-icing system is engaged.25 The SPS is designed so that a single failure of any

component in the system cannot cause the loss of the stick pusher function, improper

activation of the stick pusher, the loss of the aural warning alert, or the loss of both

stick shakers.

The SPS on the ATR 72, as well as the ATR 42, has icing and nonicing

AOA triggering thresholds for each flap configuration. The SPS activates at lower

AOAs when the anti-icing system is activated to account for aerodynamic changes

when flying in 14 CFR Part 25, Appendix C, icing conditions. These SPS "icing"

AOA thresholds do not account for more adverse aerodynamic changes that may result

from flight in freezing drizzle/freezing rain or other icing conditions outside those

defined by 14 CFR Part 25, Appendix C.

1.6.5 Autoflight System Description

The ATR 72 is equipped with a Honeywell SPZ-6000 Digital Automatic

Flight Control System (DAFCS). The following subsystems are included: the Attitude

and Heading Reference System (AHRS), the Air Data System, the Electronic Flight

Instrument System (EFIS), the Flight Guidance System (FGS), and the PRIMUS 800

Color Weather Radar System.

The DAFCS is a completely automatic flight control system that provides

fail-passive flight director guidance, autopilot, yaw damper and pitch trim functions.

The autopilot computers monitor the system continuously and alert the flightcrew to

faults that have been detected in the system. The autopilot system design incorporates

the use of two in-flight bank angle selections: "HIGH" bank angle (27 degrees) and

"LOW" bank angle (15 degrees). These bank angle limits are manually selected by

the pilot and are used to control the maximum amount of bank angle executed by the

autopilot during turns.

The "limitations" section of the ATR 72 Aircraft Flight Manual (AFM)

provides a brief description of the flight regimes during which the autopilot may be

 

25Refer to Section 1.6.6 for further information regarding the SPS.
25

operated. Both the ATR and AMR Eagle/Simmons Airlines operating manuals

permitted, as of the time of the accident, the use of the autopilot for holding and flight

operations in icing conditions. The American Eagle ATR 42/72 Operation Manual,

Volume 1, Conditionals Section, stated, in part:

...effective management of all flightdeck resources is an absolute

necessity for safe and efficient operation of a two crew aircraft. The

design features of the ATR, including AFCS Flight Director/Autopilot

system, provide the crew an opportunity to effectively manage the

flight deck environment during all phases of flight, including

abnormal and emergency procedures. However, periodic "hand

flying" of the aircraft will ensure basic pilot skills are retained....

The autopilot will disengage automatically if the computer senses any one

of a variety of system faults or malfunctions, including the exceeding of a

predetermined rate of travel for the ailerons (3.6 degrees per second). If the aileron

rate monitor is tripped, power will be removed from the autopilot aileron servo motor

and servo clutch, and the flightcrew will receive an aural and visual warning alert in

the cockpit.

The MFC also monitors the trailing edge flaps and sounds an alarm if the

airplane exceeds an airspeed of 185 knots with the flaps extended at the 15-degree

position. If the flaps are in the retracted position, the MFC will inhibit flap extension

above an indicated airspeed of 180 knots (KIAS). After this accident, ATR Service

Bulletin (SB) ATR 72-27-1039, dated January 12, 1995, provided a means to remove

the flap extension inhibit logic so that flightcrews could select flap extensions in

emergencies above 180 KIAS.

1.6.6 ATR 72 Ice and Rain Protection Systems

The ATR 42 and 72 ice protection system is a combination of deicing and

anti-icing systems. This system consists of the following:

1. a pneumatic system (leading edge inflatable boots) that permits

deicing of critical airframe surfaces, i.e., outboard and inboard

wing sections, the horizontal stabilizer leading edges, and the

vertical stabilizer (optional);
26

2. a pneumatic system for deicing the engine air intakes;

3. electrical heating for anti-icing of the propeller blades, the

windshield and forward portion of the side windows, the pitot

tubes, static ports, TAT [total air temperature] probe, and the

AOA vanes;

4. electrical heating for anti-icing of the aileron, elevator and

rudder balance horns;

5. and a windshield wiping system for the forward windows.

The ice protection systems are controlled and monitored from control

panels located in the cockpit. In addition, there is an illuminated Ice Evidence Probe

(IEP) located outside and below the captain's left side window. The IEP is visible to

both pilots and provides visual information regarding ice accretion. The IEP is

molded in the shape of an airfoil with spanwise ridges to increase its ice accretion

efficiency and is not equipped with an anti-ice or deice system. The probe is designed

to retain ice until sublimation or melting has occurred and is intended to provide the

flightcrew with a visual means of determining that other portions of the airframe are

either accreting ice or are free of ice.

Additionally, an Anti-Icing Advisory System (AAS), which employs a

Rosemont ice detector probe, is mounted on the underside of the left wing leading

edge between the pneumatic boots. The AAS provides the flightcrew with a visual26

and aural alert when ice is accreting on the detector probe. The aural alert chime is

inhibited when the deice boots are activated. The visual alert will remain illuminated

as long as ice is detected, regardless of whether deice boots are activated. (See Figure

7 for diagram of ATR 72 ice protection system.)

The AAS was designed to enhance ice detection by using the Rosemont

ultrasonic (harmonic/vibrating) ice detector probe which senses ice accretions. The

AAS warning alarm signal is generated by the probe on the underside of the left wing.

It is approximately 1/4 inch in diameter and 1 inch long and vibrates along its axis on

a 40 kHz [kilohertz] frequency. The system detects changes in vibration frequency

resulting from the increased mass of accumulated

 

26The visual alert consists of an amber light that illuminates on the instrument panel, below the central crew alerting

system (CCAS).
27

ATR 72- ICE PROTECTION *m

Figure 7.--Ice protection system.
28

ice, which, in turn, activates the visual and aural ice accretion alerts in the cockpit. If

ice is detected, the Rosemont probe will initiate a heat cycle to remove the accretion

and start the ice detection process again. According to ATR and the manufacturer of

the Rosemont probe, the detection system may not reliably detect large supercooled

drops that are near freezing (such as freezing drizzle/freezing rain) because there may

not be enough heat transfer to freeze the large water drops that contact the probe. The

ATR 72 ice protection system was designed with three levels of operation, and

provides the flightcrew with the ability to choose the level(s) of protection based on

environmental conditions.

* Level I - activates all probe and windshield heating systems,

and, according to the ATR 72 Flight Crew Operating Manual

(FCOM), must be in operation at all times after engine start and

during flight operations.

* Level II - activates the pneumatic engine intake boots, electric

propeller heaters, elevator, rudder and aileron horn heat, and

electric side window heaters. According to the American Eagle

AOM, the Level II protection must be in operation when

atmospheric icing conditions exist.27 Propeller RPM

[revolutions per minute] must be at or above 86 percent during

Level II operation to ensure adequate propeller deicing.28

* Level III - activates the wing, horizontal and vertical stabilizer

leading edge boots (if installed), and must be used at the first

visual identification of ice accretion or when alerted to ice

accretion by the AAS. Level III ice protection must remain

activated for as long as ice is accreting on the

 

27Page 8 of the LIMITATIONS Section of the American Eagle (Simmons) Aircraft Operating Manual, Part 1, states that

atmospheric icing conditions exist when the "Outside Air Temperature (OAT) on the ground and for takeoff is at or below

5 degrees C or when the Total Air Temperature (TAT) in flight is at or below 7 degrees C and visible moisture in any form

is present (such as clouds, fog with visibility of less than one mile, rain, snow, sleet, and ice crystals)."

28According to ATR, the propeller RPM must be increased to 86 percent in icing conditions because the increased

rotational speed will prevent the formation of and/or improve the shedding of ice and will subsequently prevent the

formation of ice aft of the deice boots in the area of the propeller slipstream. Tests conducted by ATR indicate that

operation with propeller RPMs below 86 percent does not affect the formation of ice behind the wing deice boots in front

of the aileron, or the airflow over the ailerons.
29

airframe. [ATR recommends that flightcrews use the IEP as a

means of determining when the airframe is free of ice.]

Activation of the Level II ice protection system causes the SPS to use the

lower "icing" AOA threshold and the "Icing AOA" annunciator is illuminated. The

ATR 72 aural stall warning and stick shaker AOA threshold decreases from 18.1

degrees to 11.2 degrees in cruise flight, and to 12.5 degrees when either the flaps are

extended to 15 degrees or for 10 minutes after takeoff. The stall warning threshold

returns to 18.1 degrees when the "Icing AOA" is deselected by deliberate pilot action

(does not automatically return to 18.1 degrees when level II is deactivated).

The purpose of the pneumatic deice boot system installed on the ATR 42

and 72 is to remove ice that has accumulated on the leading edges of the wings,

horizontal and vertical stabilizers, and engine inlets. This is accomplished

mechanically by changing the shape of the leading edge with alternately

inflating/deflating tubes within each of the boots. This method of ice protection is

designed to remove ice after it has accumulated on the airfoil surface rather than to

prevent the accretion on the airfoil surface, such as with an anti-ice system. Most

pneumatic deice boot designs have the inflation tubes oriented spanwise. However,

the boots used on the ATR 72 are oriented chordwise and cover about 7 percent of the

chord of the upper wing surface. The boots consist of two sets of chambers, "A" and

"B," that inflate on an alternating schedule to shed ice at selected time intervals.

When the boots are not inflated, they are held in a streamlined position conforming to

their respective structure by a vacuum. The vacuum is provided by a venturi29 which

uses engine bleed air to create a negative pressure within the boots. Two separate

switches mounted in the cockpit control the automatic inflation and cycle modes

(FAST and SLOW) of the boots and provide an override capability in the event of a

failure of the normal system. The system is designed so that the boots will

automatically inflate either on a 1 minute (FAST) or 3 minute cycle (SLOW). There is

no provision for manual control by the pilot of the duration of the boot inflation.

 

29A tube or port of smaller diameter in the middle than at the ends. When air flows through such a tube or port, the

pressure decreases as the diameter becomes smaller, the amount of the decrease being proportional to the speed of the

flow and the amount of the restriction.
30

1.6.7 ATR 42/72 Type Certification History

1.6.7.1 General

The ATR 42-200 and -300 airplanes were certified under JAR (Joint

Airworthiness Requirements) 25 by the DGAC on October 25, 1985. Under the

Bilateral Airworthiness Agreement30 (BAA) with the United States, the ATR 42 was

type certificated by the FAA in accordance with 14 CFR Part 25, and began

commercial operations with Command Airways on January 24, 1986. Since that time,

several derivatives of the ATR 42 (-200,-300,-320) have received certification under

the ATR 42 FAA-Type certificate. Additionally, seven models of the ATR 72 (-101, -

102, -201, -202, -210, -211, -212), have been certified, some of which initially began

operations in the United States with Executive Airlines, on January 10, 1990.

1.6.7.2 ATR 72 Icing Certification Program

The ATR 72 was certificated for flight into known icing conditions in

accordance with FAR/JAR Part 25.1419 and Appendix C, and the DGAC Special

Condition B6 (SC B6) and its interpretive material. FAR/JAR Part 25.1419, Ice

Protection, requires that a manufacturer demonstrate safe operation of the aircraft in

the maximum continuous and maximum intermittent icing envelopes specified in

Part 25, Appendix C. (See Figure 8 for graph from Appendix C.) Appendix C icing

envelopes specify the water drop mean effective diameter (MED),31 liquid water

content (LWC),32 and the temperatures at which the aircraft must be able to safely

operate. The envelopes specify a maximum MED of 50 microns,33 which, by

definition, do not include freezing drizzle or freezing rain.34 (See figure 9.)

 

30Refer to Section 1.18.7 for further information regarding the ATR 42 and 72 certification process under the Bilateral

Airworthiness Agreement.

31According to the FAA, the mean effective diameter is the apparent median volumetric diameter (MVD) that results

from having to use an assumed drop size distribution when analyzing data from rotating multi-cylinder cloud sampling

devices (old-style technology). Modern cloud sampling devices measure the drop size distributions directly and can

determine the actual MVD.

32According to the FAA, LWC is the total mass of water contained in all the liquid cloud droplets within a unit volume of

cloud. Units of LWC are usually grams of water per cubic meter of air (g/m3

). The terms LWC and SLW refer to the

amount of liquid water in a certain volume of air.

33A micron is 1/1000 of a millimeter (mm). A 0.5 mm mechanical pencil is 500 microns in diameter, or 10 times greater

than the largest MED defined in Appendix C.

34FAA icing experts have defined freezing drizzle as supercooled water drops with MVD's between 50 and 300 microns

and freezing rain as supercooled water drops with MVD's greater than 500 microns.
31

Figure 8.—14 CFR Part 25 Appendix C Icing Envelope.
32

DROPLET SIZE

Figure 9.—Drop size diagram.
33

Compliance with FAR/JAR 25.1419 must be demonstrated through analysis,

experimentation, and flight testing.

According to both ATR and the DGAC, SC B6 and its interpretive

material were developed by the DGAC to improve airplane icing certification within

the FAR/JAR 25 Appendix C envelopes and were first applied to the ATR 72

certification. SC B6 established specific additional icing certification requirements

because FAR and JAR 25.1419 do not explicitly require the determination of the

effects of ice on aircraft handling characteristics and performance. In addition,

FAR/JAR 25.1419 left uncertainties about how to determine performance decrements,

how they should be applied to the flight manual, which flight characteristics should be

reviewed, and how safe flight in various flight phases should be demonstrated.

SC B6 requires that the effects of ice accretion on protected and

unprotected surfaces be evaluated when establishing performance data for takeoff,

climb, cruise, descent and landing phases of flight. Performance data for each flight

phase should be based on speed, thrust/power, and drag changes related to expected

ice accumulations. Any potential performance penalties associated with flight in icing

conditions must be added to the airplane flight manuals. Handling characteristics

addressed by SC B6 include 1 G stalls, zero G pushovers,35 stall warnings, changes in

takeoff speeds, vibrations, and stability/maneuverability. SC B6 also addresses other

issues, such as: ice shapes found in typical flight phases; possible failures or

malfunctions of the ice protection system; effects of ice shapes on the performance

and handling characteristics; and testing of artificial ice shapes.

FAA aircraft icing certification advisory material includes AC 20-73 and

the Aircraft Icing Handbook (AIH). These materials, which provide guidance and are

not mandatory, include information on aircraft design, testing, and operational

concerns for flight in icing conditions as specified in FAR 25 Appendix C. These

publications do not provide design guidance concerning flight in freezing

drizzle/freezing rain.

AC 20-73 does state that "Service experience indicates that holding in

icing conditions for as much as 45 minutes is an operational condition that may be

 

35A flight maneuver where nose-down elevator input is made to achieve a zero vertical G load. The intent is to evaluate

tailplane AOA margins and hinge moment characteristics.
34

encountered...it is recommended that the tests include a continuous exposure for at

least 45 minutes." The AC further states, "The 45 minute holding criterion should be

used in developing critical ice shapes for unprotected surfaces of the aircraft for which

operational characteristics of the overall airplane are to be analyzed." Both the AIH

and AC 20-73 state, in part, "If the analysis shows that the airplane is not capable of

withstanding the 45 minute holding condition, a reasonable period may be established

for the airplane, but a limitation must be placed in the Airplane Flight Manual." The

AIH states that for the 45-minute holding pattern evaluation, an MED of 22 microns

and a LWC of 0.5 grams per cubic meter should be used. These values are at the

center of the Appendix C envelopes and do not represent worst-case conditions within

the Appendix C envelopes. ATR tests with ice shapes resulting from flight in 22

micron/0.5 gram per cubic meter conditions showed that the ATR 42 and 72 could

safely fly for at least 45 minutes in those conditions.

According to the FAA team leader for the ATR special certification, prior

to beginning the process of certifying an airplane under 14 CFR, Part 25, an aircraft

manufacturer and the FAA agree to the icing certification basis/requirements that will

be applied to their specific aircraft when the icing certification submissions are

reviewed by the FAA. The FAA applies the requirements of 14 CFR, Part 25.1419, as

well as additional requirements based on FAA staff experience, advisory circular

guidelines, and AIH guidelines. According to FAA icing certification experts, a

combination of natural icing condition tests, icing tanker/icing tunnel tests, dry wind

tunnel tests, flight tests with artificial ice shapes, and computer analyses are typically

performed.

As a part of the ATR 72 icing certification, ATR performed computer

analyses of ice accretion characteristics on its airfoils (wings, horizontal and vertical

stabilizers) using ice accretion simulation software developed by ONERA.36 ATR

also conducted flight tests using artificial ice shapes, as well as flights in natural icing

conditions. The tests conducted by ATR in natural icing conditions did not capture

data points near the zero-degree temperature boundary of the Appendix C icing

envelopes. The number of data points attained during natural icing tests were limited

to available icing conditions.

 

36ONERA is the French counterpart to the National Aeronautics and Space Administration.
35

1.6.7.3 Postaccident Certification Review

Subsequent to the accident involving flight 4184, the Safety Board

recommended that the FAA conduct a Special Certification Review (SCR) of Model

ATR 42 and 72 series airplanes. The Safety Board also recommended that flight tests

and/or wind tunnel tests be conducted as part of that review to determine the aileron

hinge moment characteristics of the airplanes while operating at different airspeeds

and in different configurations during ice accumulation, and with varying AOA

following ice accretion.

On December 12, 1994, a 10-person team, including six certification

specialists from the FAA and four specialists from the DGAC, began the certification

review process.

The SCR team participated in the creation of two telegraphic

airworthiness directives (ADs). AD T94-25-51, issued on December 9, 1994,

prohibited flight into known or forecast icing conditions for the ATR fleet. AD T95-

02-51, issued on January 11, 1995, restored flight in icing conditions upon

incorporation of certain flight and dispatch restrictions, and procedures.

On September 29, 1995, the FAA published the SCR report.37 The team

focused on the following major issue areas during its investigation:

Certification Basis - The basic Model ATR 42 was approved by the

FAA on October 25, 1985 [Type Certificate (TC) A53EU]. The

certification basis for the airplane is 14 CFR Part 25, as amended by

Amendment 25-1 through Amendment 25-54, with certain special

conditions not related to icing. The basic Model ATR 72 was

approved by the FAA on November 15, 1989, as an amendment to TC

A53EU. The ATR 72 211/212 model (the accident airplane) was

approved by the FAA on December 15, 1992.

Review of Certification Practices and Results - The icing

certification program conducted for the ATR 42 and 72 demonstrated

the adequacy of the anti-ice and deicing systems to protect the airplane

against adverse effects of ice accretion in

 

37See Appendix C for Executive Summary, Conclusions and Recommendations of the FAA SCR Report.
36

compliance with the requirements of FAR/JAR 25.1419. The wing

deicing system has demonstrated acceptable performance in the

meteorological conditions defined in the FAR/JAR 25 Appendix C

envelope. Additionally, during the icing tanker testing conducted at

Edwards Air Force Base (AFB), California, the proper functioning of

the wing deicing boots was observed to correlate with Aerospatiale

(ATR) test data within the Appendix C envelope. The certification

program for the ATR 72-201/202 and ATR 72-211/212 icing systems

was documented thoroughly using sound procedures and was

processed and conducted in a manner consistent with other FAA icing

certification programs. All data reviewed showed compliance with

FAR 25/JAR 25.1419. The SCR team concluded that results show a

good correlation with Special Condition B6 stall requirements and

also with FAR/JAR 25.203 (handling qualities). The ATR 42 and

ATR 72 series airplanes were certificated properly in accordance with

DGAC and FAA regulations, practices, and procedures.

Autopilot Certification Procedures and Characteristics - The

Honeywell Automatic Flight Control System (AFCS) was approved by

the DGAC in accordance with the FAA certification basis that existed

for each successive ATR series airplane. System design parameters

for performance and servo authority meet those specified by FAR

25.1329 and AC 25.1329-1A. The system installation and monitor

design is supported by the Aerospatiale Safety Assessment Automatic

Pilot System and Honeywell DFZ-6000 Safety Analysis for critical

and adverse failure cases. The equipment qualification and

subsequent performance and malfunction flight tests that were

performed are consistent with acceptable industry practices and

procedures and are similarly consistent with practices and procedures

accepted by the FAA in the past for other aircraft. The SCR team

concluded that the Honeywell AFCS installed in the successive ATR

series airplanes was certificated properly to the requirements of the

FARs.

Review of Pertinent Service Difficulty Information - While all

icing-related accident and incident information was not examined to

the full extent of the Roselawn accident due to time and resource

limitations, certain important aspects of the event history were
37

studied and some conclusions were possible. Events of unacceptable

control anomalies were associated with severe icing conditions such

as freezing rain/freezing drizzle, and, in a few cases, the icing was

accompanied by turbulence. These other roll anomaly events provided

no evidence that the ATR 72 had any problems with any icing

conditions for which it was certificated. Appendix 8 contains a

tabulation of events that were known to the SCR team.

Environmental Conditions Outside the Appendix C Envelope -

Weather observed in the area of the accident appears to have included

supercooled water droplets in the size range of about 40 to 400

microns. This weather phenomenon is defined by the SCR team as

Supercooled Drizzle Drops (SCDD).

While the physics of formation are not the same, freezing drizzle and

SCDD can be considered to present the same icing threat in terms of

adverse effects. The difference between them is that freezing drizzle

is found at the surface, while SCDD is found aloft with air at

temperatures above freezing underneath. Freezing rain contains

droplets in the range of 1,000 to 6,000 microns. Collectively, all these

large drops are referred to as supercooled large droplets (SLD). When

used herein, the aerodynamic effects of SCDD and freezing drizzle are

synonymous. While the effects of ice accreted in SLD may be severe,

the clouds that produce them tend to be localized in horizontal and/or

vertical extent.

The scientific investigation of SCDD and the body of knowledge on

this subject are relatively new. SCDD is not universally understood in

the aviation community. SCDD may be considered to icing as the

microburst is to wind shear. Both have been unrecognized until recent

times. Since they may be very severe, but are localized in extent and

are difficult to detect until the airplane has encountered the condition,

pilot awareness and prompt action to exit the condition are relied upon

for now. Some researchers have observed that the effects of ice

accreted in SCDD are far more severe than those of freezing rain.
38

Considering all available data, the SCR team has determined that the

icing conditions of the accident environment were well outside the

Appendix C icing envelope. This report contains a detailed

description of this phenomenon; several short and long term

recommendations are made.

Analysis of Aileron Hinge Moment Characteristics - The flight test

data and qualitative assessments made by the DGAC during basic

certification of the ATR 42 and 72, and the ATR 72-211/212, did not

indicate that any unsafe or atypical lateral control wheel force

characteristics exist. This conclusion also was based on the

comprehensive assessment of the airplane in icing conditions

conducted in accordance with Special Condition B6. The original

certification test program did lack an evaluation of airplane

characteristics with asymmetrical ice shapes; however, such an

evaluation is not considered standard practice. Ice asymmetry was

considered unlikely due to system design and Airplane Flight Manual

(AFM) procedures.

Wind tunnel data and analysis have shown that a sharp-edge ridge on

the wing upper surface in front of one aileron only can cause

uncommanded aileron deflection. By using a very conservative

analysis, these data show that keeping the wings level at 175 knots

indicated airspeed (KIAS) takes approximately 56 pounds of control

wheel force. These force levels were not seen during any of the icing

tanker tests. However, during the first series of tests in the icing cloud

behind the tanker, a ridge of ice did build up behind the deicing boots

in a similar location to the wind tunnel model, but it was not sharpedged and only extended spanwise approximately 40 percent in front

of the ailerons due to the dimension of the icing cloud. However,

these tests indicated that a mechanism existed that could actually

produce such a ridge in actual icing conditions. Even though high

lateral wheel forces were not seen during the tanker tests, icing

specialists indicated that under slightly different conditions of the

icing environment, other shapes could develop. Since the ice ridge

sheds in a random manner, and in light of the airflow difference over

the wings during maneuvering and turbulence or due to aerodynamic

effects, an assumption was made that there could be a significant

difference in ice accretion between the left and right wings.
39

Additional flight tests were conducted by Aerospatiale with artificial

ice shapes duplicating the ice that accreted during the tanker tests in

freezing drizzle conditions. Initially, these shapes were applied in

front of the aileron in a random pattern to duplicate the shedding that

was observed during the tanker tests. Additionally, a series of flight

tests were conducted with ice shapes covering full and partial spans of

the wing. The results of these tests coincided with the results obtained

from the tanker tests. Further testing by Aerospatiale with more

asymmetry and with sharper edge shapes indicated higher lateral

control forces, although not as high as those derived from the initial

wind tunnel studies.

FAA/Air Force Icing Tanker Testing - Two series of icing tanker

tests were performed at Edwards AFB, California, in support of the

investigation of the October 31, 1994, accident. A United States Air

Force jet airplane (similar to a Boeing Model 707) specially modified

to produce an icing cloud was used to simulate the conditions believed

to have existed at the time of the accident. Direct results of the icing

tanker tests were used to determine possible (1) immediate and long

term changes to the aircraft, (2) changes to flight crew operations

procedures, (3) changes to the Master Minimum Equipment List

(MMEL), and (4) changes to flight crew training.

The first tanker test took place December 13 - 22, 1994; the second

test program took place March 4 - 7, 1995. Both test programs were

conducted as similarly as possible so that the results of the two tests

could be compared directly.

Approval of Modified Deicing Boots - Aerospatiale developed a

modification that consists of an increase in coverage of the active

portion of the upper surface of the outer wing deicing boots from

5 percent chord on the ATR 42 and 7 percent chord on the ATR 72 to

12.5 percent chord for both airplane models. These enlarged wing

deicing boots were certificated by extensive dry air and icing wind

tunnel tests, and by dry air and natural icing flight tests conducted by

Aerospatiale and FAA flight test pilots. In addition, an ATR 72 fitted

with the modified boots was flown behind the
40

icing tanker at Edwards AFB. The results of all these tests revealed

that the modified boots perform their intended function within the

icing requirements contained in Appendix C of Part 25 of the Federal

Aviation Regulations. All U. S.-registered Model ATR 42 and ATR

72 series airplanes were modified with the new boots prior to June 1,

1995.

Aerospatiale developed the deicing boot modification to provide an

increased margin of safety in the event of an inadvertent encounter

with freezing rain or freezing drizzle (SLD). With the ability to

recognize that an inadvertent encounter had occurred, flight crews

would be afforded an increased opportunity to safely exit those

conditions. However, even with improved boots installed, Model

ATR 42 and 72 airplanes, along with all other airplanes, are not

certificated for flight into known freezing drizzle or freezing rain

conditions.

Operational Considerations that May Require Changes - Several

recommendations regarding operational considerations for the

turboprop transport fleet were made. These recommendations include

changes to flight crew and dispatcher training, expanded pilot reports,

Air Traffic Control and pilot cooperation regarding reporting of

adverse weather conditions, flight crew training in unusual attitude

recovery techniques, aircraft systems design and human factors, and

MMEL relief.

Changes to the Certification Requirements (Appendix C) - The

FAA recognizes that the icing conditions experienced by the accident

airplane, as well as other airplanes involved in earlier accidents and

incidents (see Appendix 8), may not be addressed adequately in the

certification requirements. Therefore, the FAA has initiated the

process to create a rulemaking project under the auspices of the

Aviation Rulemaking Advisory Committee (ARAC). The ARAC will

form a working group, made up of interested persons from the U.S.

aviation industry, industry advocacy groups, and foreign

manufacturers and authorities. The ARAC working group will

formulate policy and suggested wording for any proposed rulemaking

in the area of icing certification.
41

According to the SCR report, the team concluded, based on their review

and evaluation of the data, that:

1. The ATR 42 and ATR 72 series airplanes were certificated

properly in accordance with the FAA and DGAC certification

basis, as defined in 14 CFR parts 21 and 25 and JAR 25,

including the icing requirements contained in Appendix C of

FAR/JAR 25, under the provisions of the BAA between the

United States and France.

2. The Roselawn accident conditions included SCDD outside the

requirements of 14 CFR Part 25 and JAR 25. Investigations

prompted by this accident suggest that these conditions may not

be as infrequent as commonly believed and that accurate

forecasts of SCDD conditions do not have as high a level of

certitude as other precipitation. Further, there are limited means

for the pilot to determine when the airplane has entered

conditions more severe than those specified in the present

certification requirements.

The SCR team also made the following recommendations:

• The current fleet of transport airplanes with unboosted

flight control surfaces should be examined to ascertain that

inadvertent encounters with SLD will not result in a

catastrophic loss of control due to uncommanded control

surface movement. The following two options should be

considered:

1. The airplane must be shown to be free from any hazard

due to an encounter of any duration with the SLD

environment, or

2. The following must be verified for each airplane, and

procedures or restrictions must be contained in the AFM:

a. The airplane must be shown to operate safely in the

SLD environment long enough to identify and

safety exit the condition.
42

b. The flight crew must have a positive means to

identify when the airplane has entered the SLD

environment.

c. Safe exit procedures, including any operational

restrictions or limitations, must be provided to the

flight crew.

d. Means must be provided to the flight crew to

indicate when all icing due to the SLD

environment has been shed/melted/sublimated from

critical areas of the airplane.

• FAR 25.1419, Appendix C, should be reviewed to

determine if weather phenomena which are known to exist

where commuter aircraft operate most often should be

included...;

• Rulemaking and associated advisory material should be

developed for airplanes with unpowered flight control systems

to address uncommanded control surface movement

characteristics that are potentially catastrophic during

inadvertent encounters with the SLD environment. Discussions

about these new criteria should consider the criteria already

contained in the certification requirements...;

• Existing criteria used for evaluation of autopilot failures

[should] be used to evaluate the acceptability of the dynamic

response of the airplane to an uncommanded aileron deflection.

Moreover, since both of these events (failure/hardover aileron

deflection) can occur without pilots being directly in the loop,

the three-second recognition criteria used for cruise conditions

also should be adopted;
43

• Policy should be developed to assure that on-board computers

do not inhibit a flightcrew from using any and all systems

deemed necessary to remove an airplane from danger;

• Airplane Flight Manuals (AFM) should be revised to clearly

describe applicable icing limitations;

• The FAA/JAA harmonization process for consideration of

handling qualities and performance of airplanes while flying in

icing conditions should be accelerated...;

• Evaluate state-of-the-art ice detector technology to determine

whether the certification regulations should be changed to

require these devices on newly developed airplanes;

• Flightcrew and dispatcher training related to operations in

adverse weather should be reevaluated for content and

adequacy;

• Flightcrews should be exposed to training related to extreme

unusual attitude recognition and recovery;

• Pilots should be encouraged to provide timely, precise, and

realistic reports of adverse flight conditions to ATC. The

tendency to minimize or understate hazardous conditions should

be discouraged;

• An informational article should be placed in the Winter

Operations Guidance for Air Carriers, or airline equivalent,

which explains the phenomenon of uncommanded control

surface movement and the hazard associated with flight into

SLD conditions;

• MMEL relief for all aircraft, particularly items in Chapter 30

(Ice and Rain Protection), should be reviewed for excessive

repair intervals; and
44

• Methods to accurately forecast SLD conditions and mechanisms

to disseminate that information to flightcrews in a timely

manner should be improved.

1.7 Meteorological Information

1.7.1 General

At the time of the accident, there was no significant meteorological

information (SIGMET)38 indicating the existence of icing conditions, and stations

along flight 4184's route of flight were not reporting any freezing precipitation. The

only relevant in-flight icing weather advisory (AIRMET "Zulu") indicated, "light to

occasional moderate rime icing in clouds and in precipitation freezing level to 19,000

feet." There were no additional reports of any significant weather phenomenon in the

vicinity of the LUCIT intersection.

The Safety Board performed an in-depth study of the environmental

conditions to define the weather phenomenon in which flight 4184 was operating until

the time of the accident. Because of the complexity of the environmental conditions,

it was necessary to collect and document data from numerous sources, and to

determine the pertinent weather products, services, and actions of agencies and

individuals involved. In addition to information received from the FAA, National

Weather Service (NWS), and Simmons/AMR Eagle, numerous individuals were

interviewed, including pilots operating in the area of the LUCIT intersection at the

time of the accident. Also, data were collected from the WSR-88D Doppler weather

radar sites located in Romeoville, Illinois, and Indianapolis, Indiana.

The Safety Board also sought additional input to define the

meteorological environment encountered by flight 4184 from scientists of several

organizations, including the National Oceanic and Atmospheric Administration

(NOAA), the National Center for Atmospheric Research (NCAR), and the University

of Wyoming. The Safety Board also received multispectral digital data from the

Geostationary Operational Environmental Satellite (GOES 8), and reviewed the data

on the Safety Board's Man computer Interactive Data Access System (McIDAS)

Computer Workstation.39

 

38SIGMET is defined as significant meteorological information. It is an in-flight advisory for the en route environment,

indicating weather phenomenon severe enough to represent a concern to all categories of aircraft. Among other weather

phenomena, the SIGMET includes information about severe icing which affects an area of at least 3,000 square miles.

39McIDAS is an interactive meteorological analysis and data management computer system. McIDAS is administered by

personnel at the Space Science and Engineering Center at the University of Wisconsin, Madison, Wisconsin.
45

1.7.2 Flight 4184 Dispatch Weather Information

FAA Order 8400.10, Chapter 7, Section 2, paragraph 1423, "Operational

Requirements - Flightcrews," provides guidance to POI's regarding the weather

information that a carrier should provide to its pilots. The Order states, in part:

Flightcrews need accurate weather information to determine the

present and forecast weather conditions on any planned operation.

For example, for adequate flight planning, flightcrews should know

existing and expected weather conditions at the departure airport,

along the planned route of flight, and at destination, alternate, and

diversionary airports.

A. Preflight Planning: Operational flight planning decisions

require consideration of the following weather information:

• Area Forecasts

• AIRMETs, SIGMETs, and Convective SIGMETs

• Icing (location, type and severity)

• Center Weather Advisories (CWA) are not specified in this Order40

At Simmons Airlines, the flightcrew's primary source of weather

information is provided by the dispatch office and is typically presented in the flight

release paperwork. The Manager of Dispatchers at Simmons testified at the Safety

Board's public hearing that at the time of the accident, it was standard policy at

Simmons to provide the flightcrew with surface weather observations, terminal

 

40A CWA is issued by the meteorologist located in the ARTCC for significant meteorological hazards i.e., icing or

turbulence.
46

forecasts, and SIGMETs. AIRMETs and CWAs are not normally included in the

flight release but may be included at the discretion of the dispatcher. AIRMETs are

continually available at the dispatcher's station while CWAs must be requested.

According to the Manager of Dispatchers, although a flightcrew is only

provided selected weather information, it is the duty and responsibility of the

dispatcher to review all of the available weather information, including area forecasts,

station forecasts, airport weather observations, AIRMETs, SIGMETs, and a variety of

computer-generated prognostic charts, to ensure that the flight release is "current,

accurate and pertinent to the planned route of flight." Upon completion of the flight

release, it is provided to the flightcrew approximately 60 to 75 minutes prior to

departure. The flightcrew receives significant weather information (i.e.,

thunderstorms, turbulence, icing, etc.) updates, as necessary, and, at the discretion of

the dispatcher, via the ARINC ACARS [Aeronautical Radio Inc., Automatic

Communications and Recording System].

The following is a partial summary of weather information that was

included in the Flight Release prepared for the crew of Flight 4184 at 1255:

Surface weather observations for ORD and IND...

IND...1251...Measured ceiling 1,300 feet overcast; visibility

5 miles; fog; temperature 61 degrees F; dew point 57 degrees F;

winds 160 degrees at 7 knots; altimeter setting 29.73 inches of Hg.,

rain ended 1204.

ORD...1250...1,100 feet scattered, measured ceiling 2,500 feet

overcast; visibility 4 miles; moderate rain, fog; temperature

47 degrees F; dew point 45 degrees F; winds 050 degrees at

16 knots; altimeter setting 29.89 inches of Hg.

Terminal Forecast for IND...Prepared by the National Weather

Service indicated that from 1200 to 1600...clouds at 800 feet

scattered, a ceiling at 1,500 feet overcast, with a visibility of

greater than 6 miles; the winds would be from 130 degrees at

8 knots; and it could be expected that occasionally the ceiling

would be 800 feet broken; and visibility would be 4 miles, with

light rain, fog.
47

The Terminal Forecast for ORD prepared by American Airlines

Weather Services staff indicated that from 1300 to 1700 the clouds

would be 1,500 feet scattered, with a ceiling at 3,500 feet overcast;

visibility 6 miles; light rain; winds 050 degrees at 12 knots gusts to

22 knots; occasional ceiling 500 feet broken, 1,200 feet overcast;

visibility 2 miles; moderate rain showers; fog.

The Flight Release also contained the following description of weather

conditions, valid from 0900 on October 31 to 0300 on November 1. The following

summarized information was prepared by American Airlines Weather Services:

A surface low located Southern Missouri will move into Ohio by 0300

on November 1. A quasi-stationary front located central New Jersey,

Southern Ohio, Southern Missouri will become a warm front and

move to Ohio, Southern New York by 0300 November 1. A cold front

out of the surface low Missouri, Northwest Arkansas will move to

Ohio Central Kentucky, Southern Louisiana by 0300 November 1.

Scattered to occasional moderate broken showers will fall over

portions of Missouri, Arkansas, Iowa, Wisconsin, Illinois today

spreading and intensifying into Indiana, Michigan, Ohio. Some

flurries light snow showers will develop tonight over the Western

Great Lakes portions of Wisconsin, Michigan, possibly Illinois.

Thunderstorm Outlook...Isolated becoming widely scattered to

scattered along the cold front Southern Missouri moving eastward

with the front to Southern Illinois, Indiana, Ohio....

The following wind and temperature deviation41 information was also

included in the Flight Release:

• Boiler VOR at 10,000 feet wind 220 degrees at 19 knots

temperature deviation 0 degrees C;

 

41The difference between the actual temperature and the temperature in the Standard Atmosphere for a given altitude.
48

• LUCIT Intersection at 10,000 feet wind 230 degrees at 11 knots

temperature deviation minus 1 degrees C;

• Chicago Heights VOR at 10,000 feet 230 degrees at 8 knots

temperature deviation minus 1 degrees C.

There were no Terminal SIGMECs42 issued by American Airlines

Weather Service staff for ORD in effect for the time of the accident. The Terminal

SIGMECs are issued when conditions exist for moderate or greater icing, low level

turbulence, low level windshear, and/or thunderstorm activity in the vicinity of the

terminal area. These reports are valid for the terminal area, which is defined as about

a 25 nautical mile radius of the airport. The Terminal SIGMECs are produced by

American Airlines meteorological specialists and forwarded to the Simmons flight

dispatch center for release among their crews.

In addition, the Safety Board found that there are neither FAA

regulations, ATC procedures, nor Simmons' policies that would prohibit aircraft from

holding in known or forecast icing conditions.

1.7.3 Weather Synopsis

The surface weather and upper air conditions for the area of Roselawn,

Indiana, were summarized from the National Weather Service (NWS) Weather

Depiction Chart recorded at 1600. The charts revealed a low pressure center in the

area of west central Indiana and "...cloud ceilings of less than 1,000 feet and/or

visibilities of less than 3 miles, in rain," occurring in northern Indiana. Further, a

"moderate" cold front extended from the low pressure center and extended in a

southwesterly direction. A moderate stationary front was also present and extended

eastward from the center of the low pressure area. In addition, precipitation in the

form of rain and rainshowers associated with this system were occurring to the north

(ahead) of the stationary front and west (behind) of the cold front. The accident site

was north of the stationary front, where surface temperatures of plus 7o

 C were being

reported.

The NWS's 1800 analysis of the 850 millibar data (recorded about 5,000

feet msl) indicated an area of low pressure with the center located in west

 

42The SIGMEC is a weather product issued exclusively by the American Airlines weather service staff.
49

central Indiana; and a northerly flow over northern Illinois and southwesterly flow

over eastern Indiana. The temperatures were near 3 degrees C with moisture evident

in the area where flight 4184 was holding. The 1800 analysis of the 700 millibar data

(recorded about 10,000 feet msl) indicated an area of low pressure, with the center

located in northern Illinois, and a southwesterly flow over the accident area.

Temperatures were near minus 4 degrees C with moisture evident in northern Indiana.

At 500 millibars (about 18,000 feet), the center of the low pressure area was located in

northeastern Iowa and had a southwesterly flow over the area of the accident.

Temperatures were near minus 18 degrees C with moisture evident in the area.

Surface weather observations surrounding the accident site were as

follows:

Gary, Indiana (GYY) [located 32 miles north of the accident site]:

1545...Record...800 feet scattered, estimated ceiling 1,700 feet

overcast; visibility 7 miles; light rain showers; temperature 44 degrees

F; winds 020 degrees at 13 knots gusts 30 knots; altimeter setting

29.68 inches of Hg.; [remarks] pressure falling rapidly; ceiling ragged.

1645...Record...800 feet scattered, estimated ceiling 1,700 feet

overcast; visibility 5 miles; light rain showers, fog; temperature

43 degrees F; winds 020 degrees at 18 knots gusts to 43 knots;

altimeter setting 29.65 inches of Hg.; [remarks] ceiling ragged.

South Bend, Indiana (SBN) [located about 58 nautical miles

northeast of the accident site]: 1552...Record...Measured ceiling 1,400

feet overcast; visibility 3 miles; moderate rain; temperature

44 degrees F; dew point 43 degrees F; winds 050 degrees at 11 knots;

altimeter setting 29.71 inches of Hg.

1652...Record...Measured ceiling 1,400 feet overcast; visibility

4 miles; light rain, fog; temperature 44 degrees F; dew point

42 degrees F; winds 050 degrees at 11 knots; altimeter setting

29.65 inches of Hg.; [remarks] precipitation very light.

Chicago, O'Hare Airport (ORD) [located about 60 nautical miles

north-northwest of the accident site]: 1550...Record...sky partly
50

obscured, measured ceiling 1,100 feet broken, 2,500 feet overcast;

visibility 2 1/2 miles; moderate rain; fog; temperature 44 degrees F;

dew point 43 degrees F; winds 020 degrees at 20 knots gusts to

30 knots; altimeter setting 29.81 inches of Hg.

1650...Record special...sky partly obscured; measured ceiling

1,100 feet broken, 2,500 feet overcast; visibility 3 miles; moderate

rain, fog; temperature 42 degrees F; dew point 41 degrees F; winds

010 degrees at 22 knots gusts 30 knots; altimeter setting 29.80 inches

of Hg.

Lafayette, Indiana (LAF) [located about 44 nautical miles southsoutheast of the accident site]: 1545...Record...measured ceiling 600

feet broken, 1,000 feet overcast; visibility 2 1/2 miles; light rain, fog;

temperature 47 degrees F; dew point 46 degrees F; winds 060 degrees

at 16 knots; altimeter setting 29.57 inches of Hg.; [remarks] pressure

falling rapidly.

1645...Record...measured ceiling 400 feet overcast; visibility 1 mile;

light rain showers, fog; temperature 48 degrees F; dew point 47

degrees F; winds 070 degrees at 12 knots gusts to 18 knots; altimeter

setting 29.53 inches of Hg.

Indianapolis, Indiana (IND) [located about 94 nautical miles southsoutheast of the accident site]: 1455...Record...1,600 feet scattered,

estimated ceiling 9,500 feet broken, 18,000 feet overcast; visibility 6

miles; fog; temperature 65 degrees F; dew point 57 degrees F; winds

130 degrees at 9 knots; altimeter setting 29.64 inches of Hg.; pressure

falling rapidly.

1551...Record...measured ceiling 9,500 feet broken, 20,000 feet

overcast; visibility 6 miles; fog; temperature 64 degrees F; dew point

58 degrees F; winds 130 degrees at 10 knots; altimeter setting 29.59

inches of Hg.; few stratocumulus.

The weather conditions reported by the airport operator at the Lowell

Airport, located about 12 nautical miles northwest of the accident site, were: clouds

approximately 1,400 feet broken, 3,000 feet overcast; the winds were estimated to be

from the southwest at 20 knots and "gusty;" and there was light drizzle falling. The
51

time of the report was unknown; however, it was estimated that the observation was

made about 30 minutes after the accident.

At Demotte, Indiana, which is about 9 miles north-northeast of the

accident site, no precipitation was recorded between 1500 and 1545, and 0.1 inch of

precipitation was recorded between 1545 and 1600.

Upper air information recorded from onboard sensors from six aircraft

operating within about an 80 nautical miles radius of Chicago between the hours of

1430 and 1800 were reviewed. Three of the aircraft were approaching or departing to

the southwest through the southeast of Chicago, and three of the aircraft were

approaching or departing through the east of Chicago. The following is a summary of

the information prepared by investigators:

For all aircraft, at approximately 3,000 feet, the temperature varied

between 1.5 to .5 degrees C, and at approximately 6,000 feet, the

temperature varied between about minus 2 and minus 3 degrees C. At

approximately 10,000 feet, the temperature varied between about

minus 3 to minus 4 degrees C for the aircraft flying to the southwest

through the southeast of Chicago, and between minus 5 to minus

6 degrees C for aircraft flying to the west and east of Chicago. From

an aircraft flying to the northwest of Chicago the temperature was

about minus 7.5 degrees C.

At 1742, an aircraft recorded the wind and temperature at 10,000 feet

as 170 degrees at 25 knots and about minus 4 degrees C, respectively.

This aircraft was located about 45 nautical miles north-northwest of

the accident site.

At 1437, a second aircraft recorded the wind and temperature at

10,000 feet to be about 180 degrees at 20 knots and about minus

4 degrees C, respectively. This aircraft was located about 50 nautical

miles north-northwest of the accident site.

Static Air Temperatures (SAT) in degrees C calculated from Total Air

Temperatures recorded during the final moments of flight 4184 are as follows:

Height (Feet) SAT (Degrees C)
52

15,100 minus 9.0

14,100 minus 7.0

13,800 minus 9.0

12,800 minus 8.0

11,800 minus 6.0

11,500 minus 5.0

11,200 minus 4.0

10,800 minus 3.0

10,500 minus 2.5

10,200 minus 2.5

 9,800 minus 2.0

Upper level wind data were obtained from the WSR-88D Doppler

Weather Radar (located at Romeoville, Illinois (KLOT), about 46 nmi and

312 degrees from the accident site) velocity azimuth display (VAD) vertical wind

profile (VWP) product.43 The product is based on data obtained within a 22 nmi

radius of the KLOT radar site. Weather radar images from KLOT for 1530 to 1600, at

the elevation angles of 1.5, 2.4, 3.4, and 4.3 degrees were also reviewed. At an

elevation angle of 1.5 degrees, the radar beam center is located about 9,500 feet msl,

in the area where flight 4184 was holding. The images showed a changing pattern of

weather radar echoes. The weather radar echo intensities varied from weak to

moderate at the 1.5 degree elevation angle, and the radar echoes recorded at this angle

revealed movement to the northeast. About 5,000 feet, the movement of the echoes

was determined to be from 190 degrees at 25 knots; at about 10,000 feet, the echo

movement was from 200 degrees at 40 knots; and at about 14,000 feet, the echo

movement was from 195 degrees at 50 knots. A "bright band"44 was not evident in the

data recorded east of the radar, although a bright band could be seen in the data to the

north through west of KLOT. The radar images (1.5 degrees elevation) with the

ground track of flight 4184 superimposed are contained in Appendix F of this report.

The GOES 8 data were displayed and reviewed on the Board's McIDAS

Workstation. The Longwave Infrared (LWIR) Imager data showed radiative45

temperatures in the area of the LUCIT intersection of about minus 13 degrees C at

1432 and 1445. At 1515, colder radiative temperatures (higher cloud tops) were noted

to the south and east of LUCIT. Radiative temperatures of about minus 35 degrees C

 

43Refer to Appendix E for VAD VWP information.

44Bright band refers to the enhanced radar echo of snow as it melts to rain.

45Radiative temperatures pertain to atmospheric temperatures sensed by the GOES 8 satellite.
53

were noted in the area of LUCIT at 1532, with an area of colder radiative temperatures

to the north of LUCIT at 1545. At 1602, an area of relatively warm radiative

temperatures were recorded in the area of the LUCIT intersection. The upper air data

from Peoria (PIA) for 1800, which was in the colder air mass, showed that a

temperature of minus 35 degrees C corresponded to an approximate cloud height of

about 27,000 feet. Radiative temperatures (GOES 8 LWIR data) and estimated cloud

heights for the location of the accident are as follows:

Time Temperature (degrees C) Height (feet)46

1432 minus 13 17,100

1445 minus 12 16,600

1515 minus 20 20,600

1532 minus 38 29,200

1545 minus 17 19,100

1602 minus 21 21,100

Looping47 of the GOES 8 LWIR data indicated cloud movement towards

the northeast. The GOES 8 visible image showed brighter clouds about 5 nautical

miles west of the accident site at 1532. The clouds moved to about 9 nautical miles

north of the accident site at 1545, and, at 1602, the brighter clouds were located about

25 nautical miles to the north of the accident site. The visible data also showed

multiple cloud layers in the area of the accident and the presence of rolling wave cloud

features called Kelvin-Helmholtz Waves. These wave cloud tops were estimated from

GOES 8 LWIR data to be about 17,000 feet. These waves are present in the

atmosphere and are generated by windshear.

1.7.4 Pilot Reports (PIREPs) and Other Weather Information

A review of the numerous PIREPs revealed that there were about

13 reports that referenced icing conditions within an approximately 100 nautical mile

radius of Roselawn from 1500 to 1700. These reports were provided by the pilots of

various types of aircraft, at altitudes of between 4,000 and 21,000 feet msl. Of these

reports, six indicated "light rime or mixed" icing, four specified "light to

 

46The cloud heights were estimated from a combination of SAT data from flight 4184 and PIA upper air data for 1800.

47Displaying the satellite images one frame at a time on a video monitor to produce a continuous motion picture.
54

moderate icing," one specified "moderate mixed icing," two indicated "light mixed

icing," and two indicated no icing conditions. The following reports were from pilots

operating near the Boiler VOR, which is located approximately 35 nmi southsoutheast of the accident site: at 1510, a Beech Baron reported light rime icing at

12,000 feet; at 1617, a Saab 340 reported light to moderate rime icing at 15,000 feet;

and at 1657, a Saab 340 reported light to moderate rime icing at 13,000 to 16,000 feet.

One pilot, whose airplane was located about 100 nautical miles west of the accident

site, reported "freezing rain" and "negative icing" at an altitude of 4,000 feet msl.

1.7.4.1 Witness Descriptions of Weather Conditions

Pilots in flight.--The captain of a Northwest Airlines Airbus A-320 stated

in an interview after the accident that he had been holding in the area of HALIE

intersection (located about 26 nautical miles north-northeast of the LUCIT

intersection) between 1610 and 1640 and encountered icing conditions. The captain

stated that the icing began in the descent from about 14,000 feet at HALIE, and

continued to approximately 2,000 feet msl while on approach to ORD.

The captain said that he noticed light precipitation and light visible

moisture; however, the size of any drops were unknown. He said that there were no

drops "splattering" on the windshield, only frozen particles characterized as light

snow, and light sleet. He stated further that he estimated the intensity of the rime

icing to be light to moderate, and that the icing did not present a problem for the

airplane anti-icing systems. He also said that only light precipitation was showing on

the airborne weather radar.

The captain also estimated that between 1/2 to 3/4 inch of ice

accumulated rapidly on the icing probe and that it remained until they were on "short

final" into ORD (about 2,000 feet msl). He said that they had been in the icing

conditions about 30 minutes, and that the shape of the ice was "jagged to bumpy."

These conditions were reported by the captain to ATC as "light rime."

The captain of a Boeing 727 (KIWI flight 17) that was also in close

proximity to the Roselawn area at the time of the accident stated that his aircraft had

been in clouds that contained rain and "light to moderate icing and light turbulence."

He estimated that the icing levels existed between 5,000 and 15,000 feet. The captain

did not provide this information as a PIREP to ATC.

About 1611, the BOONE sector controller solicited a PIREP from the

crew of a second Boeing 727 (KIWI flight 24), which was located about 10 nautical
55

miles east of the accident site and heading northbound at an altitude of about

9,000 feet. The following pilot report was recorded:

...well we're in and out of some pretty heavy rain with some sleet in

it...started about fourteen thousand feet and it's continuing still.

During the interview with the crew of KIWI flight 24 after the accident,

they described the precipitation as being "more like rain and snow mixed" and not "ice

pellets" or "frozen rain."

On October 31, 1994, there were no SIGMETs or Convective SIGMETs

in effect at the time and for the area of the accident. Also, there were no Chicago

Center Weather Service Unit (CWSU) Center Weather Advisories in effect at the time

and for the area of the accident. Convective SIGMET 14C48 (issued at 1355 and valid

until 1555), for an area over 100 nautical miles west/southwest of the Indianapolis

Airport, was valid while flight 4184 was holding on the ground. FAA Order 7110.65,

"Air Traffic Control," Chapter 2, "General Control," Section 6, "Weather

Information," states, in part, that "...the controller shall advise pilots of hazardous

weather that may impact operations within 150 nautical miles of their sector of

jurisdiction....Tower cab and approach control facilities may opt to broadcast...alerts

only when any area described is within 50 nautical miles of the airspace under their

jurisdiction." The BOONE and the DANVILLE sector controllers issued weather

information at 1454 and 1456, respectively, approximately 20 minutes prior to flight

4184 arriving on their radio frequencies.

The following summarized AIRMETs (updates 2 and 3), issued at 0845

and 1445, respectively, and valid until 1500 and 2100, respectively, included the route

from Indianapolis, Indiana (IND) to ORD:

AIRMET Sierra Update 2 for IFR indicated occasional cloud ceilings

below 1,000 feet and visibility below 3 miles in precipitation and fog.

 

48Developing line of embedded thunderstorms 15 miles wide, moving from 250 degrees at 30 knots, cloud tops to 30,000

feet.
56

AIRMET Tango Update 2 for turbulence indicated occasional

moderate turbulence below 12,000 feet.

AIRMET Zulu Update 2 for Icing and Freezing Level indicated that

light to occasional moderate rime icing in cloud and in precipitation -

freezing level to 19,000 feet. Also, the freezing level was estimated to

be from 2,000 to 5,000 feet, sloping to the north, and up to 8,000 feet,

on along a line that was defined as Oswego, Kansas, to Burlington,

Iowa, to Detroit, Michigan.

AIRMET Sierra Update 3 for IFR indicated occasional cloud ceilings

below 1,000 feet and visibility below 3 miles in precipitation and fog.

AIRMET Tango Update 3 for turbulence indicated occasional

moderate turbulence below 12,000 feet.

AIRMET Zulu Update 3 for Icing and Freezing Level indicated light

to occasional moderate rime icing in cloud and in precipitation,

freezing level to 19,000 feet. The freezing level was estimated to be

4,000 to 5,000 feet in the northern portion of area, sloping to 8,000 to

11,000 feet in the southern portion of area.

The AIRMETs issued at 0845 and valid until 1500 were not included in

the flight release provided to the crew of flight 4184. Also, the updates to the

AIRMETs were not provided to the crew prior to flight 4184's departure from

Indianapolis.

The AIRMETs, which were issued by the National Weather Service's

National Aviation Weather Advisory Unit (NAWAU)49 in Kansas City, Missouri,

covered a large geographical area that encompassed several states. The Manager of

the weather advisory unit stated during the Safety Board's public hearing that the

notation, "icing in precipitation," contained in the icing AIRMETs, indicated the

possibility of freezing rain and/or freezing drizzle from the freezing level to

19,000 feet. He stated that "...it is our intent as forecasters that 'in precip'

includes...what's been characterized in earlier statements here as the freezing drizzle

 

49Subsequent to this accident, the NAWAU was renamed the Aviation Weather Center (AWC).
57

regime, and/or freezing rain for that matter." The notation, "Icing in Precipitation" is

not defined in the Aeronautical Information Manual (AIM), in AC-00-45C, or in any

other documentation readily available to pilots, and is routinely cited in AIRMETs.

1.7.5 Hazardous In-flight Weather Advisory Service (HIWAS)

The HIWAS provides continuous recorded hazardous in-flight weather

forecasts to in-flight pilots over selected VOR frequencies. The HIWAS broadcast

consists of a summary of weather products, including AIRMETs, SIGMETs, and

Center Weather Advisories.

The HIWAS broadcast recorded by the Kankakee, Illinois, Automated

Flight Service Station (AFSS) at 1500 included information that light to occasional

moderate rime icing was forecast in the clouds and in precipitation. The affected

altitude of this forecasted icing condition extended upward from the freezing level to

FL190 (19,000 feet). In addition, occasional moderate turbulence was forecast below

12,000 feet. The HIWAS broadcast was relevant for an area that was within a 150

nautical mile radius of the ORD, Pontiac, Illinois (PNT), Polo, Illinois (PLL), and

Burlington, Iowa (BRL) VORs. The area identified as affected by forecast turbulence

and forecast icing conditions encompassed the accident site location.

In addition, the HIWAS broadcast recorded at 1507 by the Terre Haute,

Indiana, AFSS for the area extending 150 nautical miles radially from the Nabb,

Indiana (ABB), Pocket City, Indiana (PXV), Shelbyville, Indiana (SHB), and Terre

Haute, Indiana (HUF) VORs reported AIRMETs for "occasional moderate turbulence"

below 12,000 feet throughout the area, as well as icing conditions above the freezing

level. This same broadcast was recorded at 1506 for the area extending radially from

the Lafayette, Indiana (BVT) VOR.

1.7.6 Information About Freezing Rain/Freezing Drizzle and General Icing

Conditions

Estimates of Liquid Water Content (LWC) for the flightpath of flight

4184 were made using the available meteorological data, including the data from the

KLOT Doppler weather radar. Based on the KLOT WSR-88D radar data, LWCs

ranging from less than .01 to 0.7 gram per cubic meter were estimated for the time that

flight 4184 was in the hold at LUCIT. Using the Safety Board Program ICE4A,50 the

 

50A computer program developed by the Safety Board to estimate liquid water content in the atmosphere.
58

LWC was estimated to be about 0.74 gram per cubic meter for an altitude of about

10,000.

A second method of estimating the LWC, found in the Forecasters' Guide

on Aircraft Icing, published by the Air Force, indicated a LWC of about 0.59 gram per

cubic meter at an altitude of approximately 10,000 feet. A scientist from NCAR

indicated that given the conditions that existed in the area at the time of the accident,

Supercooled51 Liquid Water (SLW) contents of between 0.3 to 1 gram per cubic meter

were reasonable assumptions. In addition, estimates of LWC for this time period were

also made by the French Bureau Enquetes-Accidents (BEA) that showed they ranged

from 0.3 to 0.7 gram per cubic meter. (See Appendix G.)

The water droplet sizes were estimated using the data from the KLOT

Doppler weather radar. These drop sizes were determined using an assumed LWC of

0.1 to 1 gram per cubic meter and the measured reflectivity in the area of the LUCIT

intersection. These calculations indicated that the drop sizes ranged from about 100 to

2,000 microns in diameter.

The icing conditions associated with SLW droplets with diameters of 50

microns to 500 microns have been defined by both the FAA and the icing research

community as freezing drizzle, while those with drop diameters greater than 500

microns are referred to as freezing rain. The NCAR scientist testified that the

presence of freezing drizzle/freezing rain in the atmosphere is "difficult to forecast"

and "usually not detected." Additionally, because of its insidious nature, the ice that

results from freezing drizzle/freezing rain is not always apparent to pilots; thus,

avoidance is not always possible. The ice that forms from freezing drizzle/freezing

rain accretes not only on the protected surfaces of the aircraft but also aft of the

protected surfaces. Though ice accretions on the protected surfaces can be removed

using conventional aircraft deice/anti-ice systems, the ice that has accreted behind the

protected surfaces remains on aircraft until it is removed naturally through

sublimation, melting, aerodynamic force, or a combination of these factors.

A research professor at the University of Wyoming provided testimony at

the Safety Board's public hearing regarding the environmental conditions necessary

for freezing drizzle/freezing rain to exist. He stated that cloud drops, drizzle drops,

and rain drops are defined according to the size of the water droplets. These

 

51Supercooled is the liquid state of a substance that is below the normal freezing temperature for that substance.

Regarding airframe ice accretion, supercooled rain drops, freezing rain, supercooled drizzle drops and freezing drizzle are

considered synonymous terms.
59

definitions are the basis for determining the type of freezing moisture conditions and

the severity of the resulting icing phenomenon. The research specialist defined the

following drop sizes: Cloud drops are typically less than 50 microns in diameter and

fall at speeds of less than 5 centimeters per second (cm/s); drizzle drops are typically

50 to 500 microns in diameter and fall at speeds of between 5 and 60 cm/s; rain drops

are typically greater than 500 microns in diameter and fall at speeds greater than 160

cm/s.

According to the scientist from NCAR, the formation process for freezing

rain or freezing drizzle can be divided into two basic categories. In the first category,

the atmospheric temperature must be below 0 degrees C throughout the majority of the

altitudes, with an embedded layer of air in which temperatures are greater than 0

degrees C. The process begins with snow formed in the clouds above the layer of

warm air, and, as it falls through the layer of warm air, it melts and forms drizzle or

rain. The resulting droplets continue to fall and reenter the layer(s) of cold air

(temperatures less than 0 degrees C) but remain in their liquid state. The drizzle or

rain drops, depending upon their size, freeze on contact with various surfaces.

The second category does not involve an ice phase in the formation of

freezing rain or freezing drizzle. The process begins when the water droplets grow to

either drizzle or rain drop size without having evolved from a snow flake and melting.

The droplets are formed at cloud-drop size (less than 50 microns) and continue to

grow at a slow rate through a process known as "condensational growth." However,

the droplet growth is often accelerated considerably through a second process known

as "collision coalescence," which results when cloud size water drops that are larger

than their neighbors begin to fall. These drops fall at different speeds, collide with

other cloud drops and coalesce with them, thereby increasing their mass at a faster rate

than condensation alone. As the drops increase in weight, they continue to fall at an

accelerated rate, colliding with more water droplets, thereby creating drizzle or rain.

The freezing drizzle or freezing rain can occur either near the earth's

surface or further aloft in the atmosphere. This process is not temperature dependent

and can occur in clouds that are colder than 0 degree C, as long as the
60

clouds do not contain a significant amount of ice (since the presence of ice tends to

deplete the SLW in the cloud). According to the professor from the University of

Wyoming, about 25 percent of the time freezing rain or freezing drizzle is produced by

the collision-coalescence process. This is based on data for freezing precipitation that

falls to the ground.

1.7.7 Classification of Icing Conditions

The FAA addresses the reporting of icing conditions in the AIM.

According to AIM Section 7-1-25, Meteorology, Paragraph 7-20, "PIREPs Relating to

Airframe Icing," it states, in part:

a. The effects of icing on aircraft are cumulative - thrust is

reduced, drag increases, lift lessens and weight increases. The

results are an increase in stall speed and a deterioration of

aircraft performance...it takes but 1/2 inch of ice to reduce the

lifting power of some aircraft by 50 percent and increases in the

frictional drag by an equal percentage;

b. A pilot can expect icing when flying in visible precipitation,

such as rain or cloud droplets, and the temperature is 0 degrees

Celsius or colder. When icing is detected, a pilot should do one

of two things (particularly if the aircraft is not equipped with

deicing equipment), he should get out of the area of

precipitation or go to an altitude where the temperature is above

freezing....Report icing to ATC/FSS, and if operating IFR,

request new routing or altitude if icing will be a hazard. The

following describes how to report icing conditions:

1. Trace - Ice becomes perceptible. Rate of accumulation is

slightly greater than the rate of sublimation. It is not

hazardous even though deicing/anti-icing equipment is

not utilized unless encountered for an extended period of

time (over 1 hour).
61

2. Light - The rate of accumulation may create a problem if

flight is prolonged in this environment (over 1 hour).

Occasional use of deice/anti-icing equipment

removes/prevents accumulation. It does not present a

problem if the deicing/anti-icing equipment is used.

3. Moderate - The rate of accumulation is such that even

short encounters become potentially hazardous and use of

deicing/anti-icing equipment or flight diversion is

necessary.

4. Severe - The rate of accumulation is such that

deicing/anti-icing equipment fails to reduce or control the

hazard. Immediate flight diversion is necessary.

In addition, the following are the AIM definitions of the two different

types of ice:

Rime Ice - rough, milky, opaque ice formed by the instantaneous

freezing of small supercooled water droplets;

Clear Ice - a glossy, clear or translucent ice formed by the relatively

slowly freezing of large supercooled water droplets. The large

droplets spread out over the airfoil prior the complete freezing,

forming a sheet of clear ice.

The AIM does not define "Mixed Ice;" however, a definition is found in

AC-00-45C, Aviation Weather Services, as a combination of clear and rime ice. The

FAA Aircraft Icing Handbook defines mixed icing conditions as, "a subfreezing cloud

composed of snow and/or ice particles as well as liquid droplets."

1.7.8 Forecasting of In-flight Icing Conditions

According to information received from NCAR and the NWS, icing

forecast techniques are currently predicated on relative humidity and temperature

fields. This method enables a forecaster to determine the potential for icing conditions

and typically covers large areas. However, these forecasts do not include SLW

(Supercooled Liquid Water) content or provide explicit water droplet sizes.

According to the NCAR scientist, it is not possible, using temperature and humidity
62

data, to accurately determine the severity of the icing conditions that may exist. The

scientist stated, "severity depends in the liquid water content of the clouds, how much

water mass you are actually intercepting with your airplane, how large the droplets are

and the temperature."

One icing forecast produced after the accident, using a state-of-thescience atmospheric model developed by NOAA and NCAR, provided no indications

of freezing rain or freezing drizzle in the Roselawn area for the time of the accident.

The NCAR scientist stated, "...models aren't perfect. Forecasts aren't perfect...even

though it's current state-of-the-art atmospheric modeling." She also said that, with

continuing deployment of Radar Wind Profilers, the use of WSR-88D and terminal

Doppler weather radar (TDWR), multispectral satellite data, aircraft-transmitted

atmospheric reports, and the sophisticated mesoscale atmospheric models, it is

possible to refine the current icing forecasts.

The FAA-sponsored research and development program for forecasting

icing conditions has focused primarily on the creation of new mathematical algorithms

that enhance the weather information received from weather forecasting models in

operation at the National Meteorological Center. The FAA's Advanced Weather

Product Generator (AWPG) program was intended to be a comprehensive automated

aviation weather warning display for the CWSU, Flight Service Station (FSS) and

other related system users, capable of generating and displaying icing forecasts and

icing data. The AWPG program was canceled in 1994, "due to prioritization, based

upon severe budget constraints...." According to the NCAR scientist, although the

majority of the development work has been accomplished and in-flight icing research

is continuing, the program funding will cease in 1996. The scientist also stated that

the "freezing rain" and "freezing drizzle" algorithms were informally reviewed and

tested during the winters of 1993, 1994, and 1995 but that they have not been

validated.

On May 3, 1995, the Safety Board received a letter from the FAA

regarding its ongoing activities involving the forecasting of in-flight icing. The FAA

stated, "In-flight icing forecast research is currently being performed...this research is

intended to develop methodologies for determining the location of supercooled liquid

in clouds which produces icing conditions...Additional research is planned and ongoing to enable the determination of icing severity and to diagnose icing in real time."
63

1.8 Aids to Navigation

Not Applicable

1.9 Communications

Not Applicable

1.10 Aerodrome Information

Not Applicable

1.11 Flight Recorders

1.11.1 Cockpit Voice Recorder

N401AM was equipped with a Fairchild model A-100A CVR, Serial

Number 60753. The recorder was transported to the Safety Board's audio laboratory

on November 1, 1994. The CVR group convened on November 2, 1994, and again on

December 7, 1994.

Examination of the CVR revealed that the exterior received minor

structural damage that consisted of several small dents in the outer casing. The

interior of the recorder, including the magnetic tape, was intact and did not sustain

heat or impact damage. The recording consisted of three channels of good quality

audio information. One channel contained the cockpit area microphone audio signal.

The other two channels contained the captain and first officer audio panel signals.

The timing on the tape was established using the known time of a specific air traffic

control transmission recorded on a cassette tape provided by the FAA.

The audio portion of the recording started at 1527:59 and continued

uninterrupted until 1557:57,52 when electrical power was removed from the unit. The

CVR group, consisting of representatives from the parties to the investigation,

collectively transcribed the tape in its entirety and had the opportunity to review the

transcript. About 8 minutes of nonaviation-related conversation between the

flightcrew and a flight attendant were not included in the transcript made available

 

52The CVR and FDR times were correlated.
64

to the public. However, the transcript does specify the time when these discussions

began and ended (identified as "non-pertinent pilot and flight attendant conversation")

and it includes all other conversations and sounds recorded on the CVR.

The final 2 minutes of the recording were reviewed using a sound

spectrum analyzer. The data obtained from the spectrum analysis were used to

complete the verification of certain cockpit sounds and to determine the elapsed time

between key events.

1.11.2 Digital Flight Data Recorder

The airplane was also equipped with a Loral/Fairchild flight data

recorder, model F800, Serial Number 4838. The recorder was capable of recording 25

hours of operational data and was configured to record approximately 115 parameters.

The recorder was transported to the Safety Board's FDR laboratory on November 1,

1994, for readout and evaluation.

The FDR sustained extreme impact damage to both external and internal

components. However, the crash-survivable memory module unit was found intact

with no evidence of internal damage to the recording medium. All of the recorded

information, with the exception of the last second of operational data, was recovered

and analyzed.

1.12 Wreckage and Impact Information

1.12.1 General Wreckage Description

The airplane impacted the ground in a nose-down, partially inverted

position at a high rate of speed. Fragmented airplane wreckage was found in and

around three impact craters. A complete survey of the accident scene and aircraft

structure was accomplished; however, the severity of the damage precluded a

complete accounting of all the airplane structure. (See Figure 10a.)

Two smaller impact craters, consistent with the size of the left and right

engines, were found on both sides of the larger, main impact crater (the size and

orientation of the three craters, identified as crater 1, crater 2A, and crater 2B, are

shown in Figure 10b). Most of the human remains, as well as portions of the


67

airplane structure and components, were located to the east-northeast of the impact

craters. Portions of the wings and empennage were found to the south and southwest

of the craters. The first major airplane structure found in the debris field, located

farthest southwest from the impact craters, was a portion of the left elevator.

Numerous small lightweight pieces of composite material were found about 1,000 feet

southwest of the elevator. The last pieces in the debris field, farthest northeast from

the impact craters, consisted of a main landing gear assembly (main landing gear

wheel and tire) a portion of the flightcrew/passenger oxygen bottle, and two hydraulic

pumps located in the lower section of the airplane.

An outboard portion of the left wing was found south of crater 1. The

horizontal stabilizer was found west-southeast of the impact craters, and portions of

the left elevator and left aileron were found southwest of the creek bordering the field.

The majority of the fuselage disintegrated into small unidentifiable pieces

with only the aft end of the fuselage, the horizontal and vertical stabilizers, and both

wing tips located as intact assemblies at the site. The forward fuselage and cockpit

had been completely destroyed. Portions of all flight controls were found in the debris

field, some of which were located nearly 1,000 feet from the impact craters.

A section of the left wing rear spar approximately 7 1/2 feet long (inboard

and outboard of wing rib No. 13), several engine accessories, and portions of the

engine mount frame were found in crater 1. Numerous other pieces of the left wing

and left engine were found northeast of the main impact crater. Engine accessory

components were found in crater 2A, and parts from the right engine were found

northeast of craters 2A and 2B.

The balance horns for the elevators, ailerons, and the rudder and portions

of all flight controls and doors were found in the debris field.

1.12.2 The Wings

The outboard portions of both wings were found to have separated

approximately 10 feet inboard from their respective tips. On both outboard wing

sections, the upper skin extended almost 2 feet farther inboard than the lower skin.

The wing structure is constructed of a composite material outboard of wing rib
68

 No. 13, and no definitive failure modes were determined from the fracture surfaces of

the outboard wing pieces.

The outboard portion of the left wing was found 390 feet from the

southwest edge of crater No. 1. The outboard attach fitting for the left aileron was

found in an approximate 3-foot-long depression adjacent (northeast) to the wing

section. Another large portion of the left wing was found approximately 75 feet

northeast of crater 2B. The majority of the leading edge was found with the wing

section but was partially separated. The outboard third of the rear spar and a portion

of the outboard lower skin had separated.

The right wing sustained substantially more damage than the left wing,

especially in the area of the flap. The outboard section of the right wing was found

approximately 80 feet east of crater 2B. Other than this piece, only a few small

portions of the right wing were recovered.

Examination of the leading edges of the left and right outboard sections

of the wings revealed minor damage, and the deicing boots were intact and properly

bonded. The filler in the spanwise seam between the leading edge and the upper skin

surface was found to be intact and flush on both outboard wing sections. All of the

vortex generators on the left wing were found mounted in their normal positions. The

upper surface of the outboard portion of the right wing sustained impact damage, and

only six vortex generators were found mounted in their normal positions. Impactrelated damage to the right wing in the area of the vortex generators precluded

complete documentation.

The leading edges of both wings on the ATR 72, inboard of the engines,

contain a piano hinge along the lower surface, with the upper surface attached to the

wing structure by screws. The forward half of the hinge is normally attached to the

leading edge and the aft portion of the hinge is attached to a flange on the wing lower

skin. The hinge pin is held in place by a hinge pin stop on each end. The stop on the

right end of the hinge (as viewed from above) consists of a plug inserted into the aft

half hinge. The stop at the left end of the hinge (as viewed from above) consists of a

plate riveted onto the forward half hinge and includes a solid hinge tooth that blocks

movement of the pin after the plug and the pin are first installed.

Examination of the airplane wing structure revealed that a 44-inch

portion of the forward half hinge was found attached to a piece of the left wing
69

leading edge that measured about 51 inches in length, including the outboard edge.

The outboard 31 inches of the 44-inch hinge section was straight, and the hinge pin

stop was attached. A 38-inch portion of the corresponding aft half-hinge remained

attached to the lower flange and left wing skin. Dirt was found embedded in the pin

"through-holes" in the hinge teeth. Further examination of the inside of the throughholes revealed no evidence of smearing damage; however, two through-holes at the

inboard end of the forward half hinge were elongated. The hinge pin was absent from

the left wing hinge pieces. Fretting damage53 was observed on inboard and outboard

faces of several hinge teeth.

The left wing piano hinge section, mated to a portion of the left wing

structure, was transported to the Safety Board's laboratory for examination. The

examination revealed that the through-holes bore evidence that was indicative of the

pin having been in its normal mounted position at the time of impact.

Except for the four outboard teeth on the forward (leading edge) half

hinge, the full span of the right wing hinge was found in three pieces, with the largest

measuring 75 inches long. A portion of the lower flange and leading edge

corresponding to the longest hinge piece was also found. The hinge pin was found in

the two longest hinge pieces, and the teeth on the forward half hinge of these pieces

were broken at the base. The smallest of the three hinge pieces was attached to the

wing flange and consisted of the outboardmost two teeth. The "plug-type" hinge pin

stop was not found, but there was a circular area void of white paint where the pin

stop had been installed. The "plate-type" hinge pin stop was not located, nor was the

lower flange inboard of the two hinge teeth.

The ailerons from both wings were recovered. Two pieces of the left

aileron with its balance horn, and the mating inboard portion, measuring

approximately 57 inches, were found embedded leading-edge-down on the south side

of the creek, approximately 700 feet southwest of the impact craters.

The left outboard aileron hinge fitting was found in a small ground

depression beside the left wing tip. The web of the fitting between the aileron and the

wing rear spar was broken near the location of the rear spar, and the forward end of

the broken web was bent inboard. The fractures where the outboard aileron

 

53According to the American Society for Metals definition of metallurgical terms, fretting is the "action that results in

surface damage...when there is relative motion between solid surfaces in contact under pressure."
70

hinge fitting had separated from the wing and the aileron bore evidence indicative of

tensile overload.

Marks consistent with impact from the upper and lower aileron stops

were observed on the middle aileron hinge fitting. The white paint and green primer

were missing from the center of the lower stop, although both were present near the

edges. The paint and primer were found on the upper stop, although the center of the

stop was slightly darker in color than the surrounding surface.

The right aileron and the balance horn were found in several pieces near

the impact craters. The largest portion of the aileron was the inboard portion, which

measured approximately 54 inches long. The outboard portion measured

approximately 50 inches but only consisted of the leading edge and front spar. Both

of these pieces were crushed aft. The forward outboard edge of the counterweight

horn was crushed downward and aft. The forward inboard edge was crushed

outboard, and the right aileron trim tab had broken into two spanwise pieces but was

complete.

The outboard aileron hinge fitting was found attached to the wing. Marks

consistent with impact from the upper and lower aileron stops were observed on the

middle aileron hinge fitting, and no white paint or green primer was observed at the

center of either aileron stop or on the surrounding surface.

The majority of both wing flaps was recovered, and evidence found on

the flap tracks and other parts connected to the wing indicated that the flaps were

attached to their respective wing structure at the time of impact. Further examination

revealed that the interconnect rod and the mushroom-shaped pin between the inboard

and outboard flaps of both the left and right wings were intact. The flap

interconnection shaft between the left and right wings was found in numerous pieces.

The trailing edge fairings from both wings between the flaps and the

ailerons were recovered. Because of a previously identified problem (addressed by an

airworthiness directive) of aileron interference with the wing flap, the right fairing and

the outboard piece of the right outboard flap were examined at the Safety Board's

laboratory to determine if the right flap might have contacted the right aileron during

flap retraction. The examination revealed no evidence of fiberglass carbon fibers (the

flap is constructed of composite material) embedded in the fiberglass composite

fairing. The electronic flap control switch, located under the
71

 center pedestal in the cockpit, was found in an intermediate position, between the

second and third selections of the flap control lever, with a twisted shaft.

1.12.3 Empennage

Large portions of the tailcone and the vertical stabilizer, with the rudder

attached, were found connected to a portion of the aft fuselage, located approximately

200 feet east of crater 2B.

The horizontal stabilizer was found intact, approximately 165 feet west of

crater No. 1. The stabilizer leading edge and deicing boot received minor damage,

including a puncture of the lower surface of the left horizontal stabilizer near midspan.

Both sides of the horizontal stabilizer bore evidence of wrinkling in the upper skin.

Depressed areas were also observed in the upper skin between the ribs, mostly in the

outboard portion of the horizontal stabilizer. All vortex generators were intact and

attached. The deicing boot material was relatively intact and exhibited cuts and

scratch marks that were consistent with ground impact.

The horizontal stabilizer fittings that attach the horizontal stabilizer to the

vertical stabilizer consist of six attachment lugs, three on each side of centerline.

Examination of this area revealed that the left side lugs had pulled through the bottom,

and the fractures on the left lugs were indicative of tensile overload. The right lugs

were intact, and no deformation was observed on the right forward lug. The middle

and aft lugs were bent outboard, with greater deformation on the aft flange.

The left elevator was found in three pieces, and the left elevator trim tab

was found in two pieces. Both elevator sections were broken in the same approximate

location as the elevator trim tab center hinge. The entire right elevator was found in

four pieces. The right elevator trim tab was found in two pieces and had broken near

the center hinge.

Both elevators exhibited damage to the stops consistent with over-travel

impact. The left elevator contained damage to the upper and lower surfaces of the

attach fitting cutout at the locations of the middle and outboard fittings, consistent

with over-travel impact with the fitting.

The right side of the vertical stabilizer had a vertical break in the skin,

approximately 4 feet long and located aft of frame 44 (aft pressure bulkhead
72

location). The upper left side of the stabilizer had an L-shaped break (approximately

10 inches by 12 inches) in the skin in the same general area. The vortex generators on

each side of the vertical stabilizer were intact and attached.

The vertical stabilizer fittings (attaching the vertical and the horizontal

stabilizer) consist of six double-flange lugs, three on each side of centerline. The left

lugs were intact, and the bolts and spherical bearings were attached. The outboard

flange of the right lugs had broken off at the base, and the three bearings were

missing. The bolts on the forward and aft fittings remained; the bolt at the middle

fitting was missing.

The rudder was found attached to the vertical stabilizer. Pieces of

separated rudder skin were found in the beginning of the debris field near the left

elevator piece. Both sides of the attach fittings for the rudder contained gouges, and

rudder skin on both sides aft of each fitting was damaged consistent with impact with

the fittings. The upper fitting at the lower rudder hinge point was broken.

1.12.4 Engines and Propellers

The two Pratt & Whitney PW-127 engines and their respective Hamilton

Standard propellers were found separated from their airframe engine mounts and

located in the vicinity of craters 2A and 2B. The engines and propellers were removed

from the accident site for further examination and disassembly.

The Engines.--Both engines sustained impact damage that fragmented

portions of the engines forward of the high pressure diffuser case. Examination of the

first stage power turbine blades revealed evidence of circumferential rubbing with

corresponding smears due to radial contact with the shroud. The second stage power

turbine blades were deformed and fractured in a direction opposite to normal rotation.

The remaining internal components of both engines also revealed

evidence of rotational smearing, rubbing and blade fractures in a direction opposite to

their normal rotation. The damage sustained by these components indicated that at the

time of impact, the engines were producing power.

The Propellers.--The eight composite propeller blades sustained various

degrees of impact damage. The majority of one blade was found mounted in the

propeller hub that was located on the north side of the impact crater.
73

Numerous pieces of blade were scattered near the impact crater, and all eight blades

were identified and recovered. The damage sustained by the propellers was consistent

with rotation under power at the time of impact.

1.13 Medical and Pathological Information

Due to the catastrophic destruction of the airplane, identification of the

flight crewmembers was conducted using deoxyribonucleic acid (DNA) protocols at

the Armed Forces Institute of Pathology (AFIP) in Washington, D.C. Following the

identification, muscle tissue samples from both pilots were forwarded to the FAA's

Civil Aeromedical Institute (CAMI) for toxicological analysis. Both pilots tested

negative for alcohol and other drugs.

1.14 Fire

Not Applicable

1.15 Survival Aspects

The accident was not survivable because the impact forces exceeded

human tolerances, and no occupiable space remained intact. The Newton County

Coroner's Office investigative report stated that the occupants sustained fatal injuries

due to, "multiple anatomical separations secondary to velocity impact of aircraft

accident."

The emergency response by the Newton County Sheriff's Department, the

Lincoln Township Volunteer Fire Department and the Indiana State Police was

initiated by several telephone calls to the emergency dispatch service about 1600. The

aircraft wreckage site, which covered approximately 20 acres, was declared a

"biohazard" area, and access to the site was restricted to essential personnel. The

monitoring of the site and access control were conducted by the Sheriff's department.

The procedures imposed for working in this type of environment required the Safety

Board's investigative team, including the party members, to wear personal protective

gear while working on the site.
74

1.16 Tests and Research

1.16.1 ATR 42/72 Lateral Control System Development History

The Safety Board reviewed historical information regarding the

development of the ATR 42 and 72, including a presentation by ATR engineering

personnel on the development of the ATR 42 and 72 lateral control systems.

ATR engineers stated that the initial ATR 42 aileron system development

included multiple balance/hinge moment-related configuration changes to achieve the

desired roll efficiency, hinge moment characteristics, and roll trim characteristics.

Several ATR 42 developmental aileron configurations produced aileron hinge moment

reversals at low AOAs. According to ATR engineers, the final ATR 42 aileron design

was a "compromise of acceptable roll rates and hinge moments," and resulted in the

aileron hinge moment reversals being delayed to about 25 degrees AOA. ATR

indicated that the aileron hinge moment reversals were linked to aerodynamic stall.

The susceptibility to hinge moment reversal from aerodynamic stall is a characteristic

of aerodynamically balanced control surfaces at high AOAs, and the characteristics

can vary among configurations.

According to ATR officials, the ATR 42 SPS was designed to provide a

margin between "normal" aircraft operations and the higher AOAs found to be

associated with undesirable handling characteristics, including, but not limited to,

aileron hinge moment reversals. SPS AOA thresholds were established for both a

"clean" and "iced" airplane. The SPS threshold values for the airplane with ice

contamination were established based upon the AOAs at which undesirable handling

characteristics, including aileron hinge moment reversals, occurred during the icing

certification handling tests. This SPS design was carried forward during the

development of the ATR 72.

During the ATR 72 development stages, efforts were made to achieve the

needed roll and AOA performance by various means. Initial aileron configurations

resulted in hinge moment reversals at AOAs deemed to be too low by ATR. Vortex

generators were then added to the upper wing surface of the ATR 72, in front of the

ailerons, which delayed the aileron hinge moment reversal to 25 degrees vane AOA.

The installation of the vortex generators, which proved effective in postponing the

flow separation in the area of the ailerons and the resulting aileron hinge moment

reversal, prompted ATR to develop similar aileron vortex generators for the ATR 42

as a product improvement.
75

Further performance enhancements desired for the ATR 72, series 210,

required an increase in maximum AOA capability. ATR subsequently added more

vortex generators of a different design (co-rotative) in front of the ailerons. This

change increased the aileron hinge moment reversal AOA to 27 degrees.

Hydraulically powered flight controls can overcome high control forces

resulting from normal in-flight control surface hinge moments. If properly designed,

they can also prevent control surface hinge moment anomalies from being transmitted

back through the control system and into the cockpit. According to ATR engineers,

hydraulically powered ailerons were discussed during the preliminary design of the

ATR 42. It was determined that adequate lateral control characteristics could be

obtained without the additional weight and complexity of a hydraulic system.

Hydraulic aileron control was again discussed informally among ATR engineers after

an incident involving a Simmons Airlines ATR 42 in Mosinee, Wisconsin, in

December 1988. ATR management has since stated that hydraulically powered

ailerons have never been "officially" considered for either the ATR 42 or 72.

The Safety Board reviewed graphical data from developmental test flights

in which aileron hinge moment reversals were encountered during flight test stall

demonstrations. The graphs indicated that aileron hinge moment reversal occurred at

or above the current "clean"54 airplane stick pusher activation AOA. The stall speeds

noted on the graphs where the hinge moment reversals occurred were about 100 knots

indicated airspeed (KIAS), and the flight test pilot indicated that the control forces

required to counteract the uncommanded aileron deflections were "not excessive."

ATR engineers agreed in principle that airfoil contamination, such as icing, could tend

to lower the AOA at which the aileron hinge moment reversal occurs, and that icing

conditions beyond those specified for certification could lower the AOA at which the

aileron hinge moment reversals occur to below the certified icing stall protection

system (SPS) AOA thresholds.

1.16.2 Previous ATR 42 and 72 Incidents/Accidents

The service histories of the ATR 42 and 72 airplanes were examined by

the Safety Board, with an emphasis placed on previous roll control incidents. Twentyfour roll control incidents were found to have been reported since 1986, all

 

54"Clean" refers to a wing surface that is free of any contamination, such as ice.
76

of which involved the ATR 42.55 The Safety Board determined that 13 of the 24 roll

control incidents were related to icing conditions. Of these 13 icing-related incidents,

the following 5 occurred in weather conditions consistent with freezing

drizzle/freezing rain, and involved varying degrees of uncommanded aileron

deflections with subsequent roll excursions:

• AMR Eagle/Simmons Airlines at Mosinee, Wisconsin,

December 22, 1988;

• Air Mauritius over the Indian Ocean, April 17, 1991;

• Ryan Air over Ireland, August 11, 1991;

• Continental Express at Newark, New Jersey, March 4, 1993;

• Continental Express at Burlington, Massachusetts, January 28,

1994.

All five of these incidents were investigated by either the Safety Board,

the French Bureau Enquetes - Accidents (BEA), or ATR/Aerospatiale. The Safety

Board conducted investigations of the incidents that occurred at Mosinee, Wisconsin,

and Newark, New Jersey. The BEA participated in the investigation of the Mosinee

incident and received information from ATR regarding the incidents in Ireland and

over the Indian Ocean. The FAA participated in the investigation of the Mosinee and

Newark incidents, and ATR participated in the investigation of all five incidents.

ATR used available data from the incidents and its six degrees-offreedom (6 DOF) numerical simulation to study the airplane performance and identify

any abnormal aerodynamic characteristics. In each incident, ATR identified

significant drag increases, and, in some cases, found significant decreases in lift

coefficient. ATR attributed the drag increases primarily to propeller ice accretions

that resulted from the propellers being operated at speeds of 77 percent, rather than the

required 86 percent.

 

55See Appendix H for a listing of ATR 42/72 incidents/accidents in icing conditions or roll control problems.
77

In the case of the accident involving flight 4184, the Aerospatiale 6 DOF

simulations have indicated intermittent periods of moderate drag increase well prior to

the event, imperceptible (less than 3 percent) drag increase just prior to the event, a

slight right roll and yaw increment just prior to the event, and normal aileron

effectiveness throughout the departure, climb, and initial descent.

Based on the available information, the Safety Board determined on June

6, 1990, that the probable cause of the December 22, 1988, ATR 42 incident at

Mosinee was "a stall induced by the accretion of moderate to severe clear icing."

According to ATR, the DGAC and the FAA were provided copies of the

ATR analysis of the Mosinee incident. The Safety Board was not provided a copy of

this analysis until after the Roselawn accident. The ATR analysis of the Mosinee

incident contained the following conclusions:

• The autopilot disengagement occurred owing to its internal

safety devices. The ailerons tended to adopt the zero hinge

moment position in the absence of pilot reaction. The

maximum deflection reached was minus 12.5 degrees. This

deflection introduced a high roll rate which added to the wing

drop to take the aircraft to an 80 degree bank attitude;

• Two other roll excursions corresponding to increasing AOA

were checked by the control surfaces. The increased engine

power and descending flightpath made the aircraft fly at an

AOA such that the roll excursions disappeared and/or could be

easily controlled by the pilot;

• It should be noted that throughout the incident, i.e., 30 seconds

in all, the control surfaces remained effective and, owing to

their action alone, enabled the aircraft to recover a normal

attitude, although control stability was affected, owing to the

changes in hinge moment according to angle of attack, which

were probably due to the presence of ice on the airfoil beyond

the deicers, as is the case on all aircraft in freezing rain

conditions.
78

The Safety Board compared the December 22, 1988, incident at Mosinee,

Wisconsin, to the Roselawn accident, and the following similarities have been

noted:

• both events occurred with the autopilot initially engaged, while

operating in icing conditions consistent with freezing

drizzle/freezing rain;

• both airplanes were turning with the AOA increasing, when the

ailerons began to deflect in the direction of the turn;

• in both events, the autopilot disconnected automatically prior to

SPS activation and the ailerons immediately deflected rapidly to

nearly their full travel limit at rates in excess of pilot input or

autopilot capabilities;

• both airplanes rolled in the direction of the aileron deflection;

• in both events, the aileron deflection was rapid and oscillatory

at elevated AOAs, and was stable at lower AOAs.

The Safety Board also noted the following differences between the

Mosinee incident and the Roselawn accident:

• the ATR involved at Mosinee had a significant loss of speed

due to ice accretion prior to the incident; conversely, the

Roselawn accident only showed small, intermittent speed

losses;

• at Mosinee, the initial uncommanded aileron deflection

occurred at 11.5 degrees vane AOA and 154 KIAS, while at

Roselawn, the aileron deflection occurred at approximately 5.2

degrees vane AOA and 184 KIAS;

• at Mosinee, the airspeed and vertical acceleration did not

exceed 190 KIAS and 1.7 G after the event, while at Roselawn,

the airspeed and vertical acceleration exceeded 370 KIAS and

3.0 G after the event;
79

• at Mosinee, the ice accretion and uncommanded aileron

deflection occurred at flaps 0, while at Roselawn, ice accreted

with the flaps set at 15 degrees, and the aileron deflection

occurred when the flaps were retracted to 0 degrees;

• the Mosinee flightcrew did not use the Level III deicing system

before the event, while at Roselawn, the FDR data indicate that

Level III ice protection was activated 17.5 minutes before the

event.

The Mosinee flightcrew filed a report with NASA's Aviation Safety

Reporting System (ASRS) regarding the incident and indicated that they had

encountered clear ice that they were unable to see on the airframe. The crew's report

did not mention side window icing, but it did state, "...to keep this airplane safe we

need some indicator to let us know we have ice on the airframe we cannot see."

In addition, ATR sent an “Operators Information Message” (OIM) in

January 1989, regarding the Mosinee incident. The message characterized the event

as follows:

The A/C was submitted to freezing rain. This freezing rain affected

control forces on the ailerons in such a manner that the autopilot was

no longer able to maintain the bank angle in the procedure turn. As a

consequence, the A.P. [autopilot] was normally disconnected by its

monitoring system. The A/C rolled to a large bank angle until the

pilot took over the control manually. From that point the response of

the A/C to pilot aileron inputs was correct except that the wing

heaviness was present for about 20 seconds as long as incident [AOA]

was not significantly reduced. The rest of the flight was uneventful

including the landing on an ice covered runway. Taking into account

the information presently available the A/C manufacturer considers

that nothing needs to be changed on the A/C or in the operating

procedures. This position has the agreement of the French

airworthiness authority....

It is emphasized that aircraft ice protection systems are designed

basically to cope with the supercooled cloud environment (not
80

freezing rain). Supercooled cloud water droplets have median

volumetric diameter (MVD) of 5 to 50 microns. Freezing rain MVD

is as great as 1300 microns. Large droplets of freezing rain impact

much larger areas of aircraft components and will in time exceed the

capability of most ice protection equipment. Flight in freezing rain

should be avoided where practical.

On February 8, 1989, ATR provided Flight Safety International (FSI)

with an ATR 42 icing model for implementation in the FSI ATR 42 simulators. This

was followed on June 26, 1990 by a similar icing model for the FSI ATR 72

simulators. These icing simulation packages provided demonstration of low, medium

and high ice accretion rates, resulting in loss of airspeed (the rate of these losses were

dependent upon the ice accretion rate selected) that flightcrews were intended to

recognize and activate the deice boots. The airspeed losses cease upon activation of

the deice boots. The simulation induces a roll to the right or left (random) after the

AOA has increased beyond the stick shaker activation AOA (11.2 degrees vane AOA

if the Level II anti-icing is selected). These icing simulations do not include any

change that would demonstrate rapid and uncommanded aileron and control wheel

deflections to near their full travel limits with high, unstable control wheel forces.56

In 1990, ATR added vortex generators forward of the ailerons on all ATR

42 airplanes. According to ATR statements provided to the Safety Board after the

Roselawn accident, the vortex generators increased the AOA at which the airflow

separation occurred and would provide an additional AOA margin of several degrees

between the normal operating AOA and the aileron hinge moment reversal AOA. In

1990, the DGAC provided the FAA with the certification documentation necessary for

the installation of the vortex generators on the ATR 42. Subsequently, on September

18, 1992, the FAA issued AD 92-19-01 requiring the installation of the vortex

generators as terminating action for AD 89-09-05 (AFM limitations prohibiting the

use of the autopilot in icing conditions). In the discussion section of the Notice of

Proposed Rulemaking (NPRM) for AD 92-19-01, the FAA stated that:

...flight testing and analysis have demonstrated that installation of

vortex generators on the upper surface of the Model ATR 42 wing

significantly improves the effectiveness of the ailerons, which

 

56ATR’s post-Roselawn “freezing drizzle” simulation package, provided to FSI on January 30, 1996, demonstrates these

characteristics.
81

reduces the severity of the roll upset that can occur with asymmetric

ice accumulations resulting from icing conditions such as freezing

rain.... The FAA has determined that long term continued operational

safety will be better assured by design changes to remove the source

of the problem rather than by repetitive inspections or special

operating procedures. Long term special operating procedures may

not be providing the degree of safety assurance necessary for the

transport airplane fleet....

ATR had also developed the Anti-Icing Advisory System (AAS) for the

ATR 42 and 72. The DGAC issued CN 89-120-023B, which required the installation

of the AAS and SPS by October 1, 1989. The FAA subsequently issued AD 89-24-07,

which required the installation of the AAS on U.S.-registered ATR airplanes.

In an August 28, 1989, response letter to the FAA regarding the Notice of

Proposed Rule Making (NPRM), the Air Line Pilots Association (ALPA) expressed

its concerns to the FAA about the installation of the AAS on ATR airplanes. ALPA

stated that "...we question whether or not the modifications proposed will solve the

problem...." Additionally, ALPA stated in this letter that:

...We are also concerned with the premise that this aircraft was not

certified for flight into freezing rain. The FAA has not gone far

enough in outlining the procedures pilots should take when confronted

with the possibility of flight into freezing rain....Since freezing rain

cannot be predicted with any reasonable certainty, should pilots

refrain from flight into any icing conditions? How can pilots

determine if their aircraft will be subjected to freezing rain? And if

their aircraft are subjected to unexpected freezing rain, will the

modifications proposed in the AD be effective in ensuring the

continued safe flight of this aircraft? All other aircraft types were not

certificated for flight into freezing rain as well, yet these same aircraft

have not experienced the serious loss of control incidents as the ATR

42 has. Perhaps anti-ice/deice systems of other aircraft types have

been more thoroughly designed to compensate for operations in all

icing conditions thus recognizing the inability of predicting freezing

rain.
82

On July 9, 1991, ATR made the following conclusions after its

investigation of the April 17, 1991, Air Mauritius incident:

During Air Mauritius flight MK121 on Wednesday 17th April 1991,

performed in potential icing conditions (external temperature minus 3

degrees C, presence of clouds), ATR 42 s/n 208 registered 3BNAP,

started a moderate roll excursion at flight level 160 on AP

disconnection;

The crew had previously observed an appreciable speed decrease.

After manual take over, the flight was continued without any anomaly;

Analysis of the DFDR and the simulations made afterwards lead

[ATR] to believe that this aircraft was subjected to ice accretion which

downgraded drag and lift performance, and was not reproducible by

the certificated ice simulation models and not detected by the crew

and the ice detector (transparent ice, location...;)

The ice accretion caused dissymetry ("heavy wing") which was

difficult to control by the autopilot. The unusual control forces then

encountered by the crew on disconnection led to a 40 degree roll

excursion. Use of roll control then allowed the normal situation to be

quickly restored;

The propeller speed directive for potential icing conditions - Np

greater than 86 percent was not respected; this contributed

significantly to the thrust/drag deficit;

The modifications decided upon as a result of the incident on aircraft

23 [ATR 42 operated by Executive Airline, San Juan Puerto Rico], in

particular as [it] regards indication of roll out of trim and which will

be retrofitted in the medium term of the fleet, would certainly have

made it possible to limit the roll excursion on autopilot

disconnection....As these control forces may be unusual, it would be

desirable for the crews to be trained to face these roll out-of-trim

situations.
83

On November 13, 1991, ATR made the following conclusions from its

investigation of the August 11, 1991, Ryan Air incident:

On Sunday, August 11, 1991, at 1440, in cruise, during flight

RYR 123, ATR 42 SN 161 RYAN AIR (EI-BYO), stalled in icing

conditions at FL [flight level] 180 [18,000] after prolonged

deceleration at cruise power. After manual control recovery, the flight

continued at FL 140 without any further incident;

An analysis of the weather conditions in the area showed that the

aircraft probably flew in the cold frontal zone where the air

temperature was minus 10 degrees C at FL 180....The extra moisture

may have triggered off the severe ice conditions;

FDR information shows that the stall warning threshold (angle of

incidence 11.5 degrees) is reached at the same time of the AP

disconnection; this leads us to think that the AP was disconnected by

the stall warning;

FDR information shows that the airframe de-icing system was

switched on only 2 minutes and 30 seconds before the incident; the

anti-icing systems (propellers, horns, side windows) were selected

without setting Np at 86 percent;

This incident is the consequence of non-observance by the crew of

procedure and limitation as described in the ATR 42 AFM, probably

in severe icing conditions, namely: late activation of ice protection

systems, propellers left at Np=77 percent (minimum allowed 86

percent), no immediate speed recovery initiated at stall warning

activation (the elevator remains in pitch up position during 12 seconds

leading the aircraft to the stall);

ATR has decided to launch a new campaign of information for the

crews related to icing conditions and to introduce in the ATR 42

checklist an "icing conditions" checklist.

ATR's analysis also stated, "Crew noticed ice on side window...." The

Ryan Air flightcrew had reported that a "large sudden accretion of ice was observed

on windscreen...."
84

In December 1992, ATR sent all ATR operators a brochure entitled All

Weather Operations, (See Appendix I) which addressed the operation of ATR

airplanes in various weather conditions, including icing. This brochure also contained

a section dedicated to discussing freezing rain and stated, in part, "Although freezing

rain is not part of certification cases, it must be taken into account for operations in

icing conditions." The brochure also provides a discussion of the following items:

• the physics of freezing rain;

• meteorological conditions conducive to freezing rain (temperature

inversions);

• the potential for ice accretion aft of the leading edges of airframe

components;

• the potential for asymmetric wing lift and "associated increased

aileron forces necessary to maintain coordinated flight before

aerodynamic stall;"

• the difficulty in visually detecting the presence of associated clear

ice (transparent, shiny);

• the need to avoid freezing rain where practical;

• ways of avoiding freezing rain (reviewing weather charts, PIREPs,

AIRMETs, SIGMETs, monitoring outside air temperature data for

temperature inversions);

• operating procedures for freezing rain encounters (monitor the

autopilot for roll retrim messages, increase speed as much as

possible, extend flaps as close as possible to VFE [design flap

limiting speed], avoid excessive maneuvering);

• alternative actions for exiting freezing rain conditions (climb or

alter course);
85

• procedures to follow in the event of a roll axis "anomaly:"

"disconnect AP holding control stick firmly. Possible abnormal

rolls will be better felt when piloting manually."

An investigation of an incident involving an ATR 42 operated by

Continental Express in Newark, New Jersey, on March 4, 1993, was commenced by

the Safety Board on March 5, 1993. The pilots of the Continental Express flight

provided the following ASRS report regarding the events:

Apparently our problem was caused by ice formation on top of the

wing in an unprotected area...Ice was noted accumulating on the side

windows. The outside temp was fluctuating between 0 and minus 3

degrees C throughout the descent...Passing approximately 7 NM and

approaching the final fix the FO [first officer] began a power

reduction in order to reduce speed so that the aircraft could be

configured in the normal landing profile. It was at this time during the

speed reduction the autopilot disconnected and the aircraft

immediately rolled to the right...Both pilots immediately grabbed the

controls to bring the wings level and nose back up. It took full aileron

travel to do so. The aircraft returned to normal flight and was now

being hand flown by the FO. Shortly after, the same flight

characteristic was observed and the aircraft once again was recovered.

At this time, the trims were checked and were found to be normally

positioned. The same flight characteristics were then observed for a

third time. The captain took control of the aircraft. The trims were

checked a second time along with the spoiler lights on the overhead

panel, again found to be normally positioned. On the fourth roll, it

was observed that prior to the roll, the flight controls became spongy

and rough air disturbance could be felt over the ailerons. The aircraft

was recovered again, and the captain observed that there was

approximately 3 inches of ice aft of the leading edge boots spanning

the entire length of the wing. The ice extended back as far as could be

observed....

ATR participated in the investigation of the Newark incident and

concluded in its March 25, 1993, "Preliminary Report"57 that:

 

57This was the only report provided by ATR regarding the Newark incident.
86

ATR 42 MSN 259 operated by CONTINENTAL AIRWAYS

encountered a sudden lateral jerk reaching a peak of 52 degrees bank

angle. Aircraft was flying at a speed of 170 knots in heavy turbulent

atmosphere conditions; recorded TAT was close to 0 degrees C;

selected configuration was flap 0. After the anomaly, pilot quickly

recovered normal aircraft attitude and flightpath; he then performed

safe landing after normal selection of flap 15 and flap 30.

ATR further described the "anomaly" that had occurred, and stated, in

part:

...banking tendency to the right; right hand bank angle increases (delta

= 10 degrees). AP disconnects. At the time of the disconnection,

local AOA of 7 degrees and VC = 170 knots; immediately after the

disconnection, rapid left aileron deflection is observed (7 degree

increase - right bank order). Simultaneously the right bank angle goes

further to the right; a strong input to the left (to the aileron stop - equal

to 14 degrees) stops the roll excursion at 52 degrees. Converging

oscillation in bank angle is then observed.

The analytical descriptions made by ATR are consistent with the FDR

data. However, the Safety Board has delayed the issuance of a probable cause

pending the results of the investigation involving flight 4184.

Continental Express did not, nor were they required to, notify the Safety

Board of the ATR 42 incident in Burlington, Massachusetts, on January 28, 1994.

However, Continental Express did notify ATR of the incident, and also sent the

airplane's FDR to ATR for readout and analysis. ATR’s March 17, 1994, analysis

concluded the following:

...roll excursion on autopilot disconnection was observed on ATR 42

N15818 (MSN 153) operated by Continental Express. The aircraft

was then in cruise, at flight level 160, at 144 knots, in icing conditions

with propeller/horn anti-icing and wing/engine de-icing selected, Np

at 86 percent. There was a quick takeover by the pilot and the flight

continued without any other problems at flight level 120;
87

AP disconnection by the STALL warning as the local angle of attack

was greater than 11.2 degrees (threshold in icing conditions). The

local angle of attack went on increasing and reached a maximum of

12.4 degrees. On AP disconnection negative deflection (probably not

commanded) of the LH aileron (minus 10.5 degrees), with the control

column not held, which accentuated the roll movement to the left (30

degrees per second) which was quickly countered by the pilot who

deflected the aileron positively on to its stop (right turn). The

maximum bank angle reached was 54 degrees to the left;

This incident revealed an evolution in drag (and lift) which was

incompatible with the most severe assumptions envisioned by the

certification regulations (conventional icing, leading edge shapes).

This type of evolution was similar to the one observed in the incidents

concerning aircraft 161 and 208 and for which the assumption of a

low pollution [contamination], but covering the major part of the

chord, had been made. This assumption was associated with the

turbulence generated by operation at Np = 77 percent in icing

conditions, which was not the case for this flight. This type of

evolution which was characterized by a continuous considerable

reduction in the cruise speed - with constant power lever position -

was tolerated by the crew without reaction and resulted in activation

of the stall warning and automatic AP disconnection just after the first

sign of natural stalling. Takeover of the aircraft by the crew was

quick and easy.

In each of the five prior incidents, the airplanes accreted ice while in a

flaps 0 configuration, pitched nose up as airspeed decreased (resulting from drag

increase), and experienced roll excursions immediately following the disengagement

of the autopilot and an uncommanded deflection of the ailerons. In each case, the

flightcrews were able to regain full control of the aircraft and complete the flight

successfully by either increasing power, reducing the pitch attitude or extending the

flaps to 15 degrees, which reduced the AOA.

In addition to the installation of vortex generators, the installation of an

anti-icing advisory system (AAS), the development of icing simulator training

packages, and the distribution of the All Weather Operations brochure, ATR also took

the following actions in response to the prior icing incidents:
88

• issued "All Operators Messages" to inform ATR operators of its

conclusions with regard to some of the investigated icing incidents,

related airframe modifications, and changes to operating

procedures; and

• conducted operational visits to ATR operators to respond to

specific concerns expressed by the pilots and operators.

ATR's knowledge of the aileron hinge moment behavior and the

associated autopilot behavior of the ATR 42 in freezing rain conditions, as discussed

in its incident analyses and 1992 All Weather Operations brochure, was not explicitly

incorporated into the ATR airplane flight manuals, aircraft operations manuals or pilot

training programs. Also, the DGAC and FAA did not recommend or require ATR or

its operators to include this information in the specific aircraft manuals or pilot

training programs.

1.16.3 Communications of Airworthiness Information Between FAA,

DGAC and ATR

According to ATR, the DGAC and BEA were provided copies of the

ATR analyses for each of the five prior icing incidents. Testimony provided by two

FAA staff members indicated that the FAA had not been provided ATR's analyses of

these icing-related incidents. ATR stated that the FAA was provided a copy of the

analysis for the Mosinee incident shortly after it was completed, but could not verify

that the FAA had been provided copies of its analyses of the other four icing incidents.

FAA staff members also testified that based on their understanding of the Bilateral

Airworthiness Agreement (BAA), it was both ATR's and the DGAC's responsibility to

provide the FAA with such information.

The Special Assistant to the Director, FAA Aircraft Certification,

testified that the BAA is the "foundation of the FAA's aircraft certification

system...[and] is a technical agreement between governments." He said that the BAA

was intended to be, among other things, a means for establishing a direct link between

the FAA and a foreign airworthiness authority. The Special Assistant also stated that

under the BAA, both contracting parties [the U.S. and France] are required to "keep

the other party informed" of "information concerning continued airworthiness." The

BAA does not specifically require the DGAC or any other airworthiness authority to

provide the FAA with the manufacturer's incident/accident analyses.
89

The standards set forth in the International Civil Aviation Organization's

(ICAO) Annex 8, Part II, paragraph 4, "Continuing airworthiness of aircraft," state, in

part;

4.2.2 The State of Manufacture of an aircraft shall transmit any

generally applicable information which it has found necessary for the

continuing airworthiness of the aircraft and for the safe operation of

the aircraft (hereinafter called mandatory continuing airworthiness

information)....

Note 1. - In 4.2, the term "mandatory continuing airworthiness

information" is intended to include mandatory requirements for

modification, replacement parts or inspection of aircraft and

amendment of operating limitations and procedures. Among such

information is that issued by Contracting States in the form of

airworthiness directives.

On April 27, 1995, the Safety Board investigated a Northwest Airlines

Airbus A320 that had experienced severe pilot-induced roll oscillations of 30 degrees

while on final approach to runway 18 at Washington National Airport, Washington,

D.C. The Safety Board learned that a temporary revision to a procedure in the Airbus

flightcrew operating manual had been reviewed by the DGAC and that the DGAC had

determined that regulatory action was not required. The investigation also revealed

that the FAA did not perform a review of this information to determine if regulatory

action was required. The Safety Board concluded that information regarding

undesirable flight characteristics in the A320 had not been "effectively disseminated

from the manufacturer to the different airworthiness authorities, operators and

flightcrews." Furthermore, the Safety Board expressed its concern that, "...other

useful and perhaps critical information of a similar nature is not being effectively

communicated," and on November 14, 1995, recommended to the FAA that it and the

DGAC "establish policy and procedures to assure effective dissemination of all

essential information regarding airworthiness problems and corrective actions in

accordance with ICAO Annex 8, Part II, paragraph 4. ([Safety Recommendation] A95-109)."

In its response of January 29, 1996, to the Safety Board, the FAA stated,

in part, that it was:
90

...working closely with the French [DGAC] to determine the adequacy

of Airbus Industrie's reporting process for information concerning

continued airworthiness and safe operation of its aircraft...[and that]

...appropriate certificate management offices, principal maintenance

inspectors, and the Seattle aircraft evaluation group have also been

asked to review the procedures that Airbus Industrie uses to

disseminate continued airworthiness information to its operators....

In a letter dated March 20, 1996, the Safety Board noted that:

[t]he FAA’s actions are only partially responsive to A-95-109, because

they are limited to the problems noted with Airbus Industrie. Safety

Recommendation A-95-109 was directed at the broad policy and

procedures issues for effective dissemination of essential

airworthiness information between the FAA and the DGAC, not

merely Airbus Industrie’s reporting processes. The intent of A-95-109

was to gain improvements in the overall reporting system. Further,

similar concerns about adequate dissemination of critical

airworthiness information have arisen during the investigation of the

American Eagle ATR-72 accident at Roselawn, Indiana....

Based on the understanding that the FAA would submit a more complete

reply to the recommendation, it was classified “Open--Await Response” pending

further evaluation of the issues and clarification of the FAA’s planned actions.

In a letter dated May 7, 1996, the FAA further responded to the

recommendation by stating:

Under the United States/French agreement regarding Certificates of

Airworthiness for Imported Aircraft, all essential information related

to airworthiness problems and corrective actions on all imported

French aeronautical products come to the [FAA] through the French

[DGAC]. Currently, the United States and France are completing a

new agreement regarding the Promotion of Aviation Safety. The new

agreement, which is scheduled for completion by June 1996, will

replace the current agreement regarding Certificates
91

of Airworthiness for Imported Aircraft. Under the new agreement, the

FAA and the DGAC will codevelop procedures to define the roles and

responsibilities of the FAA and the DGAC. The FAA intends to

define the information to be made available on continuing

airworthiness and corrective actions. It is anticipated that the

procedures will be fully implemented by mid-1997.

In addition to this effort, the FAA will meet with the DGAC to discuss

the importance of transmitting any generally applicable information

found necessary for the continuing airworthiness of and for the safe

operation of imported French aircraft. The FAA will also discuss with

the DGAC the feasibility of having access to the DGAC electronic

data base containing reports of failures, malfunctions, defects, and

incidents of French-designed aircraft models which are on the U.S.

registry.

By letter dated May 15, 1996, the Safety Board classified Safety

Recommendation A-95-109 “Open--Acceptable Response,” pending implementation

and review of the agreement regarding the Promotion of Aviation Safety.

1.16.4 Investigation of Lateral Control System Behavior

During the investigation, the Safety Board examined the possible reasons

for flight 4184's rapid right-wing-down aileron deflection at the point of autopilot

disengagement. The aileron deflection rate, which was in excess of 50 degrees per

second, exceeded the deflection rate capability of a pilot (determined to be about 30

degrees per second), the autopilot servo motor (determined to be about 9 degrees per

second), and a runaway aileron trim (determined to be 37 degrees per second). The

FDR data indicated that the autopilot servo motor disconnected at the time of the rapid

aileron deflection, and the aileron trim was in the neutral position and had not moved

since the initial climb phase of the flight.

The Safety Board also examined other possible mechanisms (both

mechanical and nonmechanical) that would have resulted in this type of aileron

deflection behavior. They included aerodynamic forces that would have resulted in

unbalanced aileron hinge moments between the left and right ailerons, a spoiler

system force input that would have "back driven" the aileron control rods, and ice

contamination of the flap leading edges that could have impinged on the aileron
92

control rods. These possibilities were examined thoroughly. Based on the consensus

of the party participants, all of the possibilities were discounted by analysis, except for

the aerodynamic force/unbalanced aileron hinge moment scenario. This aerodynamic

force/unbalanced hinge moment phenomenon was found to have been cited by ATR in

its written analysis of the 1988 Mosinee, Wisconsin, ATR 42 incident. ATR

attributed this aileron behavior to the accretion of ice, aft of the wing de-ice boots and

in front of the ailerons, as a result of flight in freezing rain.

In consideration of all available information, the Safety Board requested

that ATR perform wind tunnel tests to determine the type and location of arbitrary

artificial ice shape(s) that would result in similar aileron and airplane behavior

exhibited by flight 4184. The wind tunnel tests revealed that one ice shape, similar to

a 3/4-inch-high wooden "quarter-round" molding, induced an aileron hinge moment

reversal at very low AOAs. This shape also resulted in low drag when mounted on the

upper wing surface, in a limited span, forward of the aileron.

ATR conducted high-speed taxi tests with these simulated ice shapes

mounted on the upper surface of the wing of an ATR 72. The tests were performed at

airspeeds up to 100 KIAS. The tests revealed that asymmetric placement of the shapes

induced an asymmetric aileron hinge moment reversal with control wheel forces

remaining within the certification limits (40 pounds continuous, and 60 pounds

maximum) at this airspeed.

ATR also conducted similar high speed taxi tests with these simulated ice

shapes mounted on a Fokker F-27, a Saab 340, and an Embraer 120. They reported

that these airplanes also exhibited aileron hinge moment responses similar to the ATR

72, but with varying wheel force magnitudes that were specific to each airplane. The

results were qualitatively evaluated by ATR; and no numerical data were recorded.

1.16.5 Postaccident NASA Icing Research

During the course of this investigation, the Safety Board requested and

received the assistance of aircraft icing specialists assigned to NASA's Lewis

Research Center in Cleveland, Ohio. The NASA Lewis icing specialists provided

technical guidance during the initial review of the Roselawn FDR data, meteorological

data, and ATR icing certification data. They subsequently supported
93

the Safety Board's investigation by performing icing tunnel tests on an airfoil section

very similar to that of the ATR 72, by performing computer simulations of the ice

accretion characteristics of the ATR 72 airfoil, and by performing computer

simulations of the airflow about the ATR 72 airfoil with various ice accretions found

in the icing tunnel tests.

In the icing tunnel tests, the specialists varied the icing conditions and

airfoil AOAs parametrically to document general ice accretion trends in various icing

conditions, including those consisting of large water droplets at near freezing

temperatures.58 The results of the tests are summarized in the Safety Board's Icing

Tunnel Test, Icing Computer Simulation, and Airflow Simulation Factual Report. The

tests indicate that by increasing either the mean volumetric diameter (MVD), the

Liquid Water Content (LWC), or the Total Air Temperature (TAT), the aft chordwise

accretion limit increased on the upper and lower surfaces of the airfoil, until such time

that the amount of water or heat was too great to permit sufficient heat transfer to form

ice (liquid water runs off the trailing edge of the airfoil).

The tests also found that there was an increase in the aft chordwise

accretion limit that occurred between 34 and 35°F TAT, regardless of the MVD/LWC

combination tested, with significant random, chordwise sliding and shedding of the

ice accretions at different points along the span of the airfoil section. This sliding and

shedding could result in spanwise asymmetry between left and right wings on a

complete airplane (these tests were performed on a limited-span wing section).

Additionally, it was found that decreasing the AOA increased the aft chordwise

accretion limit on the upper surface of the airfoil and decreased the aft chordwise

accretion limit on the lower surface. Conversely, increasing the AOA increased the

aft chordwise accretion limit on the lower surface of the airfoil and decreased the aft

chordwise accretion limit on the upper surface.

The tests also showed that ice accretions on the negative pressure side

(upper surface for a typical wing in flight -- lower surface for a typical horizontal tail

in flight) of the airfoil would result in airflow separation on the negative pressure side

starting at the trailing edge and moving forward as the AOA increased. If a hinged

control surface is located at the trailing edge of an airfoil section that is

 

58This was NASA Lewis' first research effort specifically involving water drop size distributions that are considerably

larger than those specified in 14 CFR Part 25, Appendix C, and in temperatures that are near freezing. Consequently, the

results of this research should be used with caution pending further research and validation.
94

experiencing airflow separation on the negative pressure side, the moment about the

hinge of the control surface could tend to deflect the trailing edge towards the

negative pressure side. The magnitude of this hinge moment is a function of the

pressure gradient about the control surface and the chordwise location of the hinge

line.

The NASA Lewis research further revealed that ice accretions of large

supercooled water drops could extend beyond the active portion of the deice boot on

the ATR 72 wing, and trailing edge airflow separations could occur at lower than

normal AOAs. The tests also found that such ice accretions at near freezing

temperatures could shed randomly, resulting in spanwise ice shape asymmetry.

1.16.6 ATR 72 Icing Tanker Tests

A series of flight tests were conducted by ATR at Edwards Air Force

Base, California, in December 1994. The flight tests utilized an Air Force NKC-135A

tanker that was flown ahead of the ATR 72, and a Learjet that was fitted with

instrumentation that measured drop size, LWC and temperature. The tanker was

equipped with a boom diffuser and interchangeable nozzles to produce a range of

icing conditions. These tests were conducted to confirm that the ATR 72 met the

certification standards specified in 14 CFR Part 25, Appendix C, and to evaluate the

ATR 72 ice accretion characteristics when exposed to large, supercooled droplets at

near freezing temperatures (conditions outside of the Appendix C envelope).

During the icing tanker testing, static air temperatures (SAT) at altitude

were varied from minus 9.2 degrees Celsius to minus 0.4 degrees; water drop MVDs

were varied between 24 and 140 microns, and LWCs were varied between 0.20 and

0.89 grams per cubic meter. The test procedure involved establishing the desired air

temperature and airspeed, sampling the tanker cloud with the instrumented Learjet,

and exposing the ATR 72 to the tanker cloud for the planned period of time, followed

by maneuverability checks and 1 G decelerating stalls by the ATR 72. The

decelerating stalls were performed to observe the control wheel force/aileron hinge

moment behavior at AOAs up to stick pusher with each type of accretion.
95

The icing tanker test results indicate that in icing conditions

representative of those specified in 14 CFR Part 25, Appendix C, the ATR 72

accretes ice within the active area of the deice boots, in both the flaps 0 and 15

configurations. These tests showed that the deice boots shed the ice effectively

during normal boot cycling, with no resulting aileron hinge moment reversals

occurring prior to stick pusher.

The tests in conditions that exceeded 14 CFR Part 25, Appendix C,

icing conditions (freezing drizzle) showed that the ATR 72 accreted ice both within

and aft of the active area of the deice boots, in both the flaps 0 and 15

configurations. The deice boots shed the ice in the active area effectively during

normal boot cycling, but developed a jagged, spanwise ridge of ice near the aft edge

of the boot, on the upper wing surface (8 percent chord at flaps 0, 9 percent chord at

flaps 15). The aft limit of the upper surface accretion extended back to

approximately 14 percent chord with decreasing ice thickness. Intentional 1 G

decelerating stall maneuvers resulted in aileron hinge moment reversals that

occurred at AOAs of 12 degrees for flaps 15 accretion followed by flaps 15 stall

maneuver and at 7 to 11 degrees AOA for flaps 15 accretion followed by flaps 0

stall maneuver.

The tests also revealed that there were distinct, recognizable ice

accretion patterns on the aft portion of the side windshield, which exceeded the 14

CFR Part 25, Appendix C, icing conditions. Also, there was very little change in

ice accretion characteristics with "ice-phobic" chemicals applied. 59

The postaccretion 1 G decelerating stall maneuvers were performed

starting at approximately 175 KIAS. The icing stall protection system (SPS) AOA

schedule for stick shaker and pusher at flaps 0 are 11.2 degrees and 15.3 degrees

AOA, respectively. At flaps 15, the icing SPS AOA shaker/pusher schedule is

12.5 degrees and 16.4 degrees AOA, respectively. In all of the 104 through 140

micron MVD tests (outside FAR 25 Appendix C envelope) during which ice

accretion occurred and the subsequent stall maneuvers were performed at flaps 0,

the hinge moment reversals occurred between shaker and pusher AOA. In all of the

104 to 140 micron MVD tests (outside of the 14 CFR Part 25 Appendix C

envelope) during which ice accretion occurred at flaps 15, the subsequent stall

maneuver resulted in hinge moment reversals prior to or at shaker AOA, regardless

 

59Ice phobic chemicals are used to prevent the accretion of ice on the surface of a wing. The chemicals are typically

dispensed in liquid form from outlets on the wing surface.
96

of whether the maneuver was performed at flaps 15 or 0. Control wheel forces

subsequent to the aileron hinge moment reversals averaged 30 to 40 pounds, with a

maximum momentary peak of 77 pounds occurring during one test.

The ice accretions documented in the NASA icing tunnel tests

performed for the Safety Board were similar in some aspects to those observed in

the tanker tests. The FAA, NASA and ATR concluded that the differences in these

accretions were significant enough to warrant further development of the icing

tunnel and icing computer simulation capability with respect to icing conditions

outside of the 14 CFR Part 25 Appendix C envelope.

ATR subsequently developed artificial ice shapes based on the findings

of the Edwards AFB tanker tests. Flight tests at flaps 0 with the "flaps 15" artificial

ice shapes resulted in airplane behavior consistent with the autopilot disconnect,

uncommanded aileron deflection, and initial roll excursion identified in the data

from flight 4184. ATR's flight tests with these artificial shapes resulted in the

following ATR conclusions:

• control wheel forces subsequent to an aileron hinge moment

reversal did not vary significantly with airspeed;

• an ice shape height of one-half inch or more was required to

induce a premature aileron hinge moment reversal. Increasing the

ice shape to a height more than one-half inch only slightly

increased the severity of the aileron hinge moment reversal;

• sharp edges on the ice shapes reduced the AOA at which the

aileron hinge moment reversal occurred, and increased the

resulting control wheel forces;

• aileron hinge moment reversals induced by full span ice shapes

were not significantly more severe than those resulting from

partial span ice shapes;

• the most severe aileron hinge moment reversals were encountered

with "flaps 15" ice shapes flown in a flaps 0 configuration;
97

• moving the ice shape location from 5 percent to 13 percent

chordwise reduced the magnitude of the control wheel forces

resulting from the aileron hinge moment reversals.

During both series of icing tanker tests at Edwards AFB, it was

determined that two generally accepted methods of calculating MVD and LWC

provided significantly different results. One method was developed by Particle

Measuring Systems, the manufacturer of the instruments used to measure the icing

conditions, and the other method was developed by NCAR. It was found that when

processing any given set of raw icing data collected behind the icing tanker, the

two methods provided MVD and LWC results that differed by as much as a factor

of 2. These results are attributed to the different mathematical equations used by

the two methods.

Following these flight tests, ATR designed extended chord deice boots

for the area of the wing outboard of the engines, which included the area in front of

the ailerons. ATR conducted a second series of icing tanker flight tests at Edwards

AFB with the new deice boots. In simulated icing conditions, consistent with those

estimated to have existed in the Roselawn area at the time of the accident, no ice

accreted aft of the new extended chord deice boots. (See Appendix D for

photographs from both Edwards AFB tanker tests.)

In early 1995, ATR also published the ATR Icing Conditions

Procedures brochure. The brochure described the icing tanker tests conducted at

Edwards AFB and summarized its findings. In addition, the brochure provided

recommended procedures for flight in freezing rain or drizzle. These procedures

provided for the identification of visual cues, established recommended airplane

configurations, and defined actions related to lateral trim and autopilot functions.

1.16.7 Historical Aspects of Aircraft Icing Research and Aircraft Icing

Certification Requirements

The existing FAA/JAA aircraft icing certification requirements are

based on envelopes defined by the minimum and maximum values of mean effective

water drop size, liquid water content, and air temperature. The boundaries of the

14 CFR Part 25, Appendix C icing envelopes are based upon recommended values

cited in the National Advisory Committee for Aeronautics (NACA) Technical Note

(TN) 1855 (March 1949), and are statistical boundaries derived from hundreds of
98

hours of in-flight icing data collected by NACA in the United States from the late

1940s to the early 1950s. NACA TN 2738 (July 1952) shows that these data were

collected and categorized by geographical location within the United States, namely,

the eastern U.S. region, the plateau region, and the pacific coast region. The data

were further categorized by cloud type during the icing encounter (layer or cumulus)

and probability of encounter.

NACA TN 2738 shows that the drop sizes and liquid water contents of

the pacific coast region were considerably greater than those of the plateau or

eastern U.S regions. For example, the maximum mean effective drop size shown

for cumulus clouds at a probability of 0.00160 was determined to be over 80

microns for the pacific coast region, and about 57 microns for the plateau region,

whereas there was no cumulus recorded data for the eastern U.S. region. The

respective regional values for layer clouds are: 78 microns, 53 microns, and 46

microns.

The Safety Board compared the existing 14 CFR Part 25 1419,

Appendix C icing envelopes with the NACA TN 2738 data and found that the

Appendix C Maximum Continuous icing envelope coincides approximately with

the eastern U.S. layer cloud icing data having a probability of 0.001, and the

Appendix C Maximum Intermittent icing envelope coincides approximately with

the pacific coast cumulus cloud icing data having a probability of 0.001. The

NACA data indicate that the pacific coast layer cloud maximum drop size (78

microns) was 70 percent larger than that of the eastern U.S. layer cloud (46

microns), and the associated liquid water contents in the pacific coast cloud data

were 3 times higher than that of the eastern U.S. cloud data of 40 microns. These

larger pacific coast layer cloud drops at higher liquid water contents are not

represented in the NACA TN 1855 or Appendix C envelopes.

NACA TN 1855 provides a recommended envelope for freezing rain

icing conditions, citing a temperature range of 25 to 32 degrees F, a liquid water

content of 0.15 grams per cubic meter, a drop size of 1,000 microns, and a

horizontal extent of 100 miles. The authors of the NACA TN 1855 wrote, in

regards to freezing rain, "observational data are not available for this class, since,

in the only case in which data have been taken, the water content of the rain was

 

60The maximum drop size in the NACA TN 2738 statistical data occurs at an LWC of 0 and at a temperature of 32

degrees Fahrenheit.
99

too low to measure in the presence of the clouds through which it was falling. For

this reason, the values for the proposed conditions were calculated....based on an

assumed rate of rainfall of 0.10 inch per hour, with drops 1 millimeter in diameter."

The NACA TN 1855 concludes:

It is not intended that each icing condition tabulation should be

specified as a design requirement for all components of the airplane,

but rather that each condition be considered as a possible

meteorological situation to be encountered and, therefore, worthy of

some attention. For example, the designer, having a certain

component of the airplane in mind, should review the listing to

determine which icing condition would probably affect that

component and, therefore, should be included in the design

calculation. For his part, the operator should consider the listing as

indicative of the wide variations of conditions through which his

aircraft might be called upon to operate.

The existing FAA icing advisory material, including AC 20-72, and

the recently revised FAA Aircraft Icing Handbook, do not contain any design or

certification guidance concerning freezing drizzle or freezing rain. The

predecessor to the FAA's Aircraft Icing Handbook, the Engineering Summary of

Airframe Icing Technical Data (ADS-4, issued December 1963), discusses

designing for exposure to freezing rain in several instances, and concludes, "flight

through freezing rain can have adverse effects on aircraft performance....In any

aircraft design, the effect of freezing rain should be considered in addition to the

current design procedures for normal (small droplet) icing conditions."

In 1981, the Safety Board published the finding of its safety study

entitled Aircraft Icing Avoidance and Protection.61 Based on the findings of the

study, the Safety Board recommended to the FAA, among other things, that it

revise the 14 CFR Part 25, Appendix C, icing certification envelopes to include

freezing rain. Further, in 1983, Dr. Richard Jeck (then of the Naval Research Lab;

with the FAA since 1990) published a report62 for the FAA in which he noted that

although icing research and commercial aircraft continue to encounter icing

conditions outside of the Appendix C envelope (such as freezing drizzle and

 

61National Transportation Safety Board, Safety Report, NTSB-SR-81-1, September 9, 1981.

62A New Database of Supercooled Cloud Variables for Altitudes Up to 10,000 Feet AGL and the Implications for Low

Altitude Aircraft Icing, Dr. Richard Jeck, August 1983, DOT/FAA/CT-83/21 (NRL Report 8738).
100

freezing rain), "...Data on freezing rain or freezing drizzle are essentially absent

from the Icing Data Base at this writing...." Dr. Jeck's 1983 report also contained

the following findings that are of significance to the accident flight 4184:

In addition to the engineering concerns, there have been calls for

improved icing forecasts and for redefining the icing severity

classifications in terms of quantitative LWC values instead of the

relative and ambiguous, "trace," "moderate," etc, that is now in use....

In 1952, after the NACA researchers became aware of the

seriousness of the runoff errors for measurements at temperatures

just below 0 degrees C, they must have reexamined their data and

concluded that not more than about 5 percent of the reported

measurements would be affected....

[NACA researchers recalled] that severe icing was observed on the

windshield of their C-46 research aircraft with only 0.15 grams per

cubic meter of LWC when the MVD was an unusually large 50

microns, which apparently led the author [NACA] to stress the

potential importance of the larger MVD's because of the greater

collection efficiencies associated with them....

The accreted rime [from rain] usually breaks away in 1 to 3-inch

wide pieces at random positions along the wing. The instances noted

by the author [Politovich and Sands] all occurred at ambient

temperatures of not more than 2 or 3 degrees Celsius below freezing

so that softening of the ice may have been expected anyway. In

addition, the efficiency of this impact-assisted deicing is probably a

function of speed....

1.17 Organizational and Management Information

1.17.1 Simmons Airlines

Simmons Airlines originated as a small commuter airline based in

Marquette, Michigan, in 1979. The airline provided scheduled commuter service

with 5-passenger Piper Aerostars and eventually expanded into larger aircraft:

Piper Navajos, Embraer Bandeirantes, and the Shorts. Simmons Airlines joined the
101

American Eagle system as an independent airline on April 16, 1986, and provided

principal air transportation from smaller communities to the hubs of American

Airlines.

On August 8, 1988, Simmons Airlines was purchased by AMR Eagle,

a subsidiary of AMR Corporation, the parent company of American Airlines. In

December 1992, AMR acquired Metro Airlines and merged it with Simmons.

Simmons Airlines serves 30 cities from its Chicago hub and 31 cities from its

Dallas/Ft. Worth hub. At the time of the accident, Simmons employed

approximately 3,300 employees, operated a fleet of 79 aircraft, including 32 Saab

340s, 25 ATR 42s and 22 ATR 72s, and dispatched approximately 565 flights per

day.

1.17.2 AMR Eagle Organization

The senior management of Simmons Airlines is comprised of the

President (who reports to the President of AMR Eagle); a Vice President of Flight

Operations; a Vice President of Maintenance and Engineering; a Vice President of

Finance/Administration; a Vice President of Airline Services; and a Director of

Personnel. The flight operations management structure consists of the Vice

President of Flight Operations who oversees the following:

Director of Flight Operations

Manager of Dispatch

MQT Technical Publications

Chicago (ORD) Chief Pilot

Dallas/Ft. Worth (DFW) Chief Pilot

ATR Fleet Manager

Saab Fleet Manager

Manager of Crew Scheduling

The management structure of AMR Eagle consists of the Chairman,

who reports to the President of AMR Corporation; a President, to whom the four

individual carriers report; a Vice President; Director of Flight Operations; Director

of Maintenance and Engineering; a Manager of Crew Planning; a Senior Systems

Analyst and the Director of the American Eagle System Operations Control Center

(AESOCC).
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AMR Eagle owns three other airlines: Executive Airlines,

headquartered in San Juan, Puerto Rico; Wings West Airlines, headquartered in

San Luis Obispo, California; and Flagship Airlines, headquartered in Nashville,

Tennessee. These airlines conduct operations under the auspices of AMR Eagle

but maintain their individual FAA operating certificate identities. They operate in

accordance with their respective FAA operating specifications, and the compliance

of each is overseen by an FAA Principal Operations Inspector (POI).

While AMR Eagle does not hold an FAA air carrier operating

certificate, its corporate organization and responsibilities are similar to those of an

operating air carrier. It also performs the following functions for each of the four

carriers:

Pilot Recruitment and Hiring

Pilot Training and Checking

Crew Planning and Aircraft Acquisition

Airline Planning and Marketing

In addition, AMR Eagle has centralized the crew scheduling, flight

dispatch, and pilot training of each of the carriers by collocating them at the AMR

facility in Ft. Worth, Texas. However, the dispatch and crew scheduling remain a

function of individual carriers, and pilot training is conducted by employees from

each carrier. AMR Eagle also coordinates the route planning, development of

aircraft operating procedures and the related manuals, and allocation of aircraft

among the individual carriers. The flight operations, in-flight services and

recordkeeping are the responsibility of the individual carrier.

At the Safety Board's public hearing, the Vice President stated that the

AMR Eagle organization serves as a coordinator between the four Eagle carriers

and that the AMR Eagle staff interacts with the staff of the carriers to facilitate a

joint decision to "standardize those decisions as much we can." He also stated that

AMR Eagle does not exercise operational control over the individual carriers and

that the "objective of AMR Eagle is to ensure the consistency of operations and

encourage the airlines to operate at the highest level of safety possible."

Additionally, the Vice President stated that American Eagle is a "generic

name...[with] no organizational entity...[and] it [AMR Eagle] exists for several

purposes. Number one, it exists to provide technical support to those airlines that

operate as American Eagle. It also exists to provide some oversight to ensure that

it complies with the Federal Aviation Regulations and with the company policies

and procedures." AMR Eagle, as part of its technical support function, gathers
103

both the aircraft and crewmember data from the airline, the manufacturer and the

FAA, and consolidates and publishes the pertinent operating manuals and

documents.

1.17.3 FAA Oversight of Simmons Airlines/AMR Eagle

The FAA certificate holding office for Simmons Airlines was

transferred from the FAA's Grand Rapids, Michigan, Flight Standards District

Office (FSDO) to the DFW Certificate Management Office (CMO) in October of

1992. The transfer of the certificate occurred 2 days after the Grand Rapids FSDO

rejected Simmons' ground deicing program.63

The American Eagle Training Center (AETC) in Dallas is overseen by

the FAA Program Manager at the DFW CMO. The FAA "Focal Point" coordinator

in Dallas is the repository for information flowing between AMR Eagle, the four

AMR Eagle carriers, and the FAA. The FAA coordinator's role is to assist in the

facilitation of "standardization" between the four AMR Eagle carriers and their

respective FAA POIs. The POI for Simmons Airlines characterized the relationship

between AMR Eagle and the individual carriers as one in which AMR Eagle tried to

implement changes without the carriers' knowledge or understanding. He also said

that the "Focal Point" coordinator routinely disseminated information to the individual

carriers to determine whether they had a complete understanding of the proposed

changes.

1.17.4 FAA Partnership in Safety Program

The Partnership in Safety Program was introduced to a portion of the

aviation industry by the President of AMR Eagle in June 1994, during the Safety

Board's Public Forum on Commuter Airline Safety. The following is an excerpt

from the AMR presentation at the forum:

AMR Eagle makes extensive use of comprehensive internal audit

programs using company evaluators to conduct ongoing inspections

to ensure the standards of American Eagle are maintained. This

 

63The FAA's certificate holding offices for the other AMR Eagle carriers (with individual oversight responsibility) are

located in San Juan, Puerto Rico (Executive Airlines); San Jose, California (SJC) (Wings West Airlines); and Nashville,

Tennessee (BNA) (Flagship Airlines).
104

commitment to internal evaluation programs is made in concert with

the FAA Partnership in Safety Program -- a program that is designed

to achieve the highest possible level of carrier and FAA

communication and coordination on issues relating to daily

operations, aircraft manufacturer information, and internal FAA

guidance. This program serves both the FAA and the carrier by

insuring a high degree of regulatory compliance, and at the same

time insuring the carrier's ability to use its assets effectively in its

operation.

On February 8, 1995, at the request of the Safety Board, the FAA

provided documentation describing of the Partnership in Safety Program. The

written material outlined general program structure that could be used to implement

an internal safety program; however, there were no specific goals or expectations

cited to assess the success or failure of the program. The Safety Board requested

more specific information from the FAA regarding the Partnership in Safety

Program, and, on May 12, 1995, the Safety Board received a response that "...no

written documentation on this program currently exists."

FAA AC-120-59 provides guidance to 14 CFR Part 121 air carriers for

the establishment and conduct of an internal audit program. The POI for Simmons

Airlines testified that Simmons Airlines did not have a formal internal evaluation

program at the time of the accident, but that AMR Eagle had contracted with the

American Airlines Safety department to conduct annual safety audits. He said that

the audits that he was familiar with did not reveal any "irregularities." The FAA

Program Manager for the AMR Eagle Training Center testified that he was familiar

with the safety audits that were conducted and while the "training center does not

have a dedicated internal evaluation program that you could identify with an

advisory circular," an internal evaluation is performed as part of the carriers'

internal evaluation program.

1.17.5 Simmons Airlines/AMR Eagle Pilot Training

1.17.5.1 General Training Information

Simmons Airlines and the other AMR Eagle carriers conduct ATR

pilot ground and simulator training at the AMR Eagle Training Center in Ft. Worth,

Texas, and ATR 42/72 simulator training in Houston, Texas, and Wilmington,
105

Delaware. The Ft. Worth training center is staffed by a program manager, and

instructors from Simmons Airlines and the other three Eagle carriers.

All training at the Ft. Worth Training Center is conducted in

accordance with the FAA "Approved Training Manual" (ATM). Any changes to

the ATM or the training curriculum must be accomplished through a process that

includes approval from the management of each of the four AMR Eagle carriers

operating that equipment, their respective POIs and AMR Eagle management.

The instructor who provided the captain and first officer of flight 4184

with ground instruction during their training session prior to the accident discussed

the dissemination of information. He stated that operating bulletins from the

manufacturer [ATR] were provided to AMR Eagle but not directly to the training

center instructors, and that "typically" the information from the bulletins was

passed down by "word of mouth." The manufacturer bulletins that are received by

AMR Eagle are evaluated and approved by the individual carriers and the FAA.

Once a bulletin change has been approved, it is incorporated into the airplane

operations manual, disseminated to all the AMR Eagle airlines and incorporated

into the training curriculum.

The instructor also stated that ground school instructors were not

included on the company's computer "E-mail" system and that information from the

company in Dallas was disseminated through their supervisors. Also, the instructor

stated that one of the other ground instructors, who is also a line pilot, often

provided the remaining instructors with aircraft operations messages that had been

distributed to the line pilots by the company. The Safety Board found that the

special holding procedure developed after the accident involving flight 4184 was

initially disseminated to flightcrews with the flight releases for the AMR Eagle

ATR flights. This procedure was also conveyed to all AMR Eagle pilots and those

training center instructors responsible for teaching flight-related procedures via

the company's "E-mail" system. All AMR Eagle pilots and training center

instructors are required to read the E-mail promulgated by the company.

According to Simmons Airlines training personnel, both the initial and

recurrent pilot training programs include a review of prior incidents and accidents

involving the ATR 42 and 72. Simmons had not provided guidance to the

instructors about the previous ATR icing incidents, and ATR did not provide

specific findings about all of the icing incidents to AMR Eagle or its airlines.
106

However, ATR had provided some information64 via ATR-generated "Operator

Information Messages" (OIM). In addition, the Safety Board interviewed several

flightcrew members, some of whom stated that they had not received any

information about the previous ATR icing incidents during their respective ground

school sessions.

1.17.5.2 AMR Eagle Flight Training

The ATR simulators utilized by American Eagle are classified

"Level C".65 Currently, there are no simulators capable of projecting specific

exterior visual cues for ice accretions; thus, the pilot's simulator training relies on

the Anti-ice Advisory System (AAS) for icing identification. According to the

American Eagle ATR 42 and 72 Operating Manual, Volume II, the Anti-ice

Advisory System (AAS) is considered a secondary means for ice detection, while

crew "vigilance" and visual detection is primary. Flightcrew members are taught

that there are several primary visual cues that can be used to confirm ice accretions

on the airplane. They include the formation of ice on the propeller spinners and/or

the ice evidence probe located near the left side window.

The training center check airman, who had performed the accident

captain's line check, stated that he had observed other pilots operate the ATR in

icing conditions. He stated that the pilots he observed typically activated the level

three ice protection when icing was detected by the AAS, but that he had also seen

pilots activate the system when ice was visually observed on the aircraft but not yet

detected by the AAS.

Several pilots were interviewed subsequent to the accident regarding

their understanding of airframe ice detection. One pilot stated that the captains

with whom he was familiar "usually" waited until they received the AAS alert

before they activated the level three ice protection. Another pilot said that the AAS

"rarely came on" before the crew visually detected the icing conditions.

A review of the AMR ATR pilot training curriculum, as well as other

related information received from AMR Eagle, revealed that simulator sessions on

operations in icing conditions included information about the identification of icing

 

64 See section 1.16.2, Previous ATR 42 and 72 Incidents/Accidents, for further information.

65Level C simulators incorporate full motion with full visual graphics.
107

conditions, both visually and with the automated systems on the airplane, and the

operation of the anti-ice/deicing systems. AMR Eagle stated, in part, the following

regarding the simulator training sessions:

...at the time of the accident, every other training flight in the

simulator [was] conducted in an icing environment condition....A

demonstration of stalls to stick pusher activation is made when these

maneuvers are first introduced to ensure the crewmember has good

operational knowledge of pusher operation and appropriate recovery

procedures....Crewmembers are taught to initiate recovery at the first

indication of any of the following: stick shaker, stall "cricket" (aural

warning), airframe buffet or stick pusher....If the simulation is set for

icing conditions, a crewmember is not permitted to perform stall

maneuvers without the appropriate [ice protection] equipment being

turned on. Permitting training in an incorrect configuration would be

classified as negative training...we [AMR Eagle] were never

informed by ATR of any simulator icing package which would

provide special or unique handling characteristics during icing

simulations, or which might be cause for modifying any of the

industry standard training procedures....In our extensive experience

in using these simulators, there have never been indications or

reports of roll off characteristics when the anti-ice/deice equipment is

being operated in accordance with prescribed procedures.

1.17.6 Flight and Airplane Operating Manual

The manuals that were issued to Simmons Airlines ATR pilots, and

that were in effect at the time of the accident, include the American Eagle/Simmons

Airlines, Inc., ATR 42/72 Airplane Operating Manual Volumes I and II (AOM), the

Flight Manual - Part 1, (FM), and Jeppesen Airway Manuals. The American

Eagle/Simmons Airlines, Inc., ATR-42/72 Operating Manual (AOM) Volume I and

the ATR FAA-approved Airplane Flight Manual (AFM) are required to be onboard

the airplane.

Section 4 of the American Eagle Flight Manual - Part 1, presents,

among other things, the company's policy on flight crewmembers leaving their

stations during a flight. This section also quotes a portion of 14 CFR §121.542,
108

which describes the nonessential duties of a flight crewmember during critical

phases of flight:

(a) No certificate holder shall require, nor may any flightcrew

member perform, any duties during a critical phase of flight

except those duties required for the safe operation of the

aircraft. Duties such as company required calls made for such

non-safety related purposes as ordering galley supplies and

confirming passenger connections...are not required for the

safe operation of the aircraft;

(b) No flight crewmember may engage in, nor may any pilot in

command permit, any activity during a critical phase of flight

which could distract any flight crewmember from the

performance of his or her duties or which could interfere in

any way with the proper conduct of those duties. Activities

such as eating meals, engaging in nonessential conversations

within the cockpit and nonessential communications between

the cabin and cockpit crew, and reading publications not

related to the proper conduct of the flight are not required for

the safe operation of the aircraft;

(c) For the purpose of this section, critical phase of flight includes

all ground operations involving taxi, takeoff and landing, and

all other flight operations conducted below 10,000 feet, except

cruise flight. A critical phase of flight may also include any

other phase of a particular flight as deemed necessary by the

captain.

According to testimony provided at the public hearing by both AMR

Eagle and FAA personnel, since flight 4184 was holding at 10,000 feet, this phase

of flight is not considered "critical," and the sterile cockpit rule was not in effect.

The guidance provided by AMR Eagle/Simmons Airlines to their

pilots regarding flight operations in icing conditions is described in the American

Eagle Flight Manual - Part 1, Section 9, Weather and Section 6, En-route. The

company requires their pilots to provide PIREPs "...when encountering inflight

icing conditions," and use specific terminology (extracted from the FAA's AIM)

when providing icing conditions PIREPs to either ATC or an FSS. The PIREPs
109

provided by company pilots are required to be made "as soon as practicable" and

expressed in terms of "trace, light, moderate, and severe, rime and clear" with the

type of aircraft in which these conditions were encountered also identified.

A review of the air traffic control conversations with the flightcrew of

flight 4184 revealed that neither crewmember provided a PIREP about the icing

conditions they were encountering during the holding pattern circuits.

Aviation Weather AC 00.6A provides information on conditions

favorable to the formation of structural icing. It states, "The condition most

favorable for very hazardous icing is the presence of many large, supercooled water

drops. Conversely, an equal or lesser number of smaller droplets favors a slower

rate of icing."

The American Eagle ATR 42/72 Airplane Operating Manual, Volume

1, Limitations Section, in effect at the time of the accident, provided pilots with the

following information regarding atmospheric icing conditions:

Atmospheric Icing Conditions Exist When: OAT [Outside Air

Temperature] on the ground and for takeoff is at or below 5 degrees C or when the

TAT [Total Air Temperature] in flight is at or below 7o

 C and visible moisture in

any form is present (such as clouds, fog, with visibility of less than one mile, rain,

snow, sleet and ice crystals).

Operations In Icing Conditions: For Operations in atmospheric icing

conditions:

Np [Propeller speed] below 86 % is prohibited

• Horns, propellers, side windows and engine anti-icing must be

selected ON.

• Eng[ine] start rotary selector must be placed to CONT[inuous]

RELIGHT.
110

• Airframe deicing must be selected ON at first indication of ice

accretion.

The American Eagle ATR 42/72 Airplane Operating Manual Vol. I,

"Conditionals" section, pages 41 and 42, outlined the use of the anti-ice/deice

systems. The manual states that Level I ice protection must be selected for all

flight operations. Also, for all takeoffs and flight operations in atmospheric icing

conditions, Level II protection must be selected in addition to Level I, and

whenever ice is building on the airframe, Level III protection must also be selected.

The American Eagle ATR 42/72 AOM, Volume II, cautions pilots that "some types

of ice accumulation might not be detected by the Anti-ice Advisory System

(AAS)."

Guidance provided to pilots in the American Eagle ATR 42/72

Airplane Operating Manual states that it is not necessary to have ice buildup on the

leading edges of the wing and stabilizer surfaces prior to activation of the Level III

ice protection system. It states that the Level III system, "must be selected "ON" as

soon as, and as long as, ice accretion develops on the airframe."

The American Eagle Flight Manual, Part 1, Section 3, Dispatch, Icing

Dispatch Policy and Procedures, pages 25 and 26, discussed the company policy

regarding dispatch of aircraft into icing conditions. The policy stated, in part:

B. No aircraft shall be dispatched, continue to operate en route or

land when in the opinion of the captain or dispatcher icing

conditions are expected that might adversely affect safety.

1. When freezing precipitation is reported at the time of departure

at the departure airport, and the surface temperature is at or

below freezing, no aircraft shall be dispatched except in strict

compliance with the approved ground deicing program,

including compliance with the appropriate hold-over

restriction.

2. When freezing rain is reported or anticipated at the estimated

time of arrival at the destination, or alternate airport(s), the

aircraft shall be dispatched and operated so as to avoid flight

into freezing rain conditions.
111

C. In making the decision to operate in freezing precipitation,

special consideration should be given to:

• surface temperature

• temperature aloft and depth of any temperature inversion

• intensity of precipitation

• types of de-ice/anti-ice fluids available

• anticipated turn around and taxi times

• SIGMET information regarding in-flight icing

• PIREPs indicating the presence of in-flight icing

D. If current weather reports and briefing information indicate

that forecast icing conditions that would otherwise prohibit the

flight will not be encountered, the flight may be dispatched.

On January 10, 1994, AMR Eagle issued an information bulletin that

addressed the company policy regarding release or departure of aircraft during

icing conditions. The bulletin stated, in part, the following:

The AFM [FAA-Approved ATR Flight Manual] will not specify

'...light or moderate icing only...', and furthermore, there are

generally no AFM restrictions prohibiting flight in a certain type of

ice (i.e. rime ice, clear ice, freezing rain, etc.). The only existing

exception is the ATR 42/72 AFM's which state that flight in freezing

rain...should be avoided.... (emphasis added)

The January 10 bulletin highlighted information contained in Part 1 of

the AMR Eagle Flight Manual, which stated, in part, "...strict compliance with the

policies, procedures, and regulations as covered in this manual is required." The

aforementioned bulletin was not required by AMR Eagle to be incorporated into

the flight manual and "...could be retained or discarded at the pilot's option." This

information was not added to the "limitations sections" of either the ATR 42 or

ATR 72 AFMs. There was a statement, in the Normal Procedures/Flight

Conditions section of the ATR 42 AFM, section 3-02, page 1, dated March 1992,

"Operation in freezing rain must be avoided."
112

A review of the Normal Procedures/Flight Conditions section of the

ATR 72 AFM, and the ATR Flightcrew Operating Manuals (FCOM) for both the

ATR 42 and ATR 72 aircraft revealed that neither publication contained the

statement, "Operation in freezing rain must be avoided." Additionally, these

manuals did not contain any information prohibiting flight in freezing rain, or any

limitation when operating in such conditions. At the Safety Board's public hearing,

the ATR Vice President, Flight Operations for North America, testified that the

omission of this information from the manuals was "not intentional."

As mentioned earlier, ATR published a brochure in 1992 entitled, All

Weather Operations, which contained information regarding the operation of the

ATR airplanes in various weather conditions that included icing. In this brochure,

ATR stated on page 24, "...flight in freezing rain should be avoided where

practical." The brochure also provided information to pilots on how to recognize

freezing drizzle and freezing rain conditions and stated, "...as soon as possible,

leave freezing rain conditions. This can usually be accomplished by climbing to a

higher altitude into the positive temperature region or by altering course." The

brochure was provided by ATR as general information and was not a required

addition, substitution, or revision to any of the FAA-approved ATR flight or

operating manuals. ATR distributed the All Weather Operations brochure to all

ATR operators, including Simmons Airlines, and also attempted to send a copy to

all ATR pilots directly. Simmons Airlines/AMR Eagle did not distribute the

brochure to its pilots because some of the information was contrary to Federal

Aviation Regulations and some of the operational information was more permissive

than the approved Aircraft Operating Manual (AOM). Also, Simmons

Airlines/AMR Eagle indicated that while it did use some of the information from

the brochure to enhance the AOM, the ATR All Weather Operations brochure

consolidated information that already existed in the various ATR and Simmons

Airlines/AMR Eagle flight manuals, specifically in the "Conditional" section of the

Aircraft Operating Manual (AOM).

The American Eagle Flight Manual, Part 1, stated that the dispatch of

airplanes "shall be" conducted so as to avoid flight into freezing rain conditions.

Neither the Flight Manual, Part 1, nor the AOM state that flight in freezing rain

"should" or "must" be avoided, as stated in the ATR 42 AFM.

Also, the American Eagle ATR 42/72 AOM, "Conditionals" section,

contains guidance for pilots regarding winter operations. The AOM states, in part:
113

Cruise - Crew vigilance in observing formation of ice is the primary

means of determining the aircraft has entered ice accretion

conditions. Visual indication can usually be detected on such

surfaces as windshield wipers, prop spinners [model 42], ice

evidence probe [model 72] and wing leading edges and engine inlets.

In conditions of potential clear icing, periodic cycling of the airframe

boots will cause any clear ice to crack making its visual detection

much easier.

In extended or severe icing conditions, a noticeable decrease in the

level of performance or significant vibrations may occur due to

propeller residual icing....

Further review of the AMR Eagle and Simmons Airlines guidance

material available to flightcrews revealed that there are no definitions or

explanations for the terms "extended" or "indeterminate," as it equates to time. The

FAA AC-00-45C, entitled Aviation Weather Services does not include the terms

"extended" or "indeterminate," but it does state that a "prolonged" period of time in

icing conditions is considered to be "over one hour."

The Manager of Flight Standards for the American Eagle Training

Center testified at the Safety Board's public hearing that the company does not have

a policy that states a specific period of time in which an airplane can remain in

icing conditions before an alternate course of action is to be taken by the

flightcrew. In addition, he stated that it is "...a crew decision based on the

environment that he may be in at the time. He may be in between layers but still in

the icing environment. He may be just accumulating light rime ice. The most

important thing to me as a pilot after I've been in a hold for some time would be my

fuel supply."

On January 23, 1989, Simmons Airlines distributed a memorandum to

its pilots entitled, “Loss of Aircraft Stability,” which summarized the December 22,

1988, Simmons ATR 42 incident at Mosinee, Wisconsin. The memorandum

provided a summation of the Aerospatiale/ATR OIM concerning the Mosinee

incident, as well as a copy of the January 6, 1989, Simmons Airlines memorandum

entitled, “Flight Into Icing Conditions.” The January 6 memorandum stated, in

part:
114

...if icing or adverse weather is experienced, make a PIREP so your

fellow pilot may benefit from your experience...This is important if

the weather is better or worse than forecast...The temperature range

favorable for ice formation is generally 0 to -15 degrees Celsius.

However, supercooled water droplets in liquid form in temperatures

above freezing, can freeze on impact with the aircraft. Exercise

caution when operating your aircraft near the freezing level in visible

moisture. Freezing rain may also form ice on an aircraft that is

operating near the freezing level (+\- a few degrees above and below

the OAT 0 degrees Celsius). This phenomena is usually associated

with a temperature inversion. If freezing rain is encountered, you

should exit the condition immediately. This diversion should consist

of a turn toward better conditions and/or a climb to warmer altitude.

Freezing rain and clear ice can be very difficult to recognize on an

aircraft, therefore it is strongly recommended when operating in

conditions favorable to this type of icing that an extra vigilance be

maintained. This should include periodic cycling of the wing boots

to aide in the detection of ice...The weather radar may also be useful

when operating in visible moisture, near the freezing level. The use

of weather radar may help identify areas of greater precipitation. An

aircraft may be dispatched into forecast freezing rain. However, our

aircraft are not to be operated in known freezing rain or severe ice....

On November 15, 1991, the Director of Operations (DO) for Simmons

Airlines distributed a memorandum to the company pilots entitled, Flight

Operations in Freezing Rain. The memorandum provided guidance to the pilots

regarding the operation of the ATR aircraft in freezing rain and freezing drizzle

conditions. The memo stated in part:

A) No aircraft shall dispatch through known or probable icing

conditions unless the requirements of the Minimum Equipment

Manual are met....

B) Intentionally Left Blank.

C) It is emphasized that aircraft ice protection systems are

designed to cope with the supercooled cloud environment
115

(not freezing rain). Large droplets (1,300 microns, large rain

droplets) of freezing rain impact much larger areas of the

aircraft components and will in time exceed the capability of

most ice protection equipment. Therefore, flight in freezing

rain should be avoided where practical. Simmons aircraft are

certified for flight into freezing drizzle and light freezing rain

as long as the aircraft meets the requirements of paragraph A

above.

The Simmons DO testified that the memorandum had been rescinded

because it was in conflict with the approved AFM. According to Simmons

officials, the rescinding documentation was electronically distributed (via computer

E-mail) and that a copy of the document (requested by the Safety Board at the

public hearing) cannot be located.

The Simmons DO also testified about several information bulletins

highlighting practices or procedures contained in Part 1 of the American Eagle

Flight Manual. One specific bulletin stated in part, "...If planned routing to the

destination or alternate will allow the aircraft to avoid areas of freezing rain during

the approach and landing, note that light freezing rain shall be given the same

consideration. However, freezing drizzle does not require the same restrictions."

The DO stated that it was his understanding, based on the guidance "...per this

bulletin here, it indicates that you would be allowed to fly in freezing drizzle...."

The American Eagle Flight Manual - Part 1, Chapter 6, En-route,

page 8, describes the use of the anti-icing/deicing system. The manual states, in

part:

Flight crews and dispatchers shall recognize anti-ice/deicing

equipment as an aid in descending or ascending through and during

emergency flight in severe icing conditions. Operations requiring

anti-ice/deicing use shall be based on the consideration that such

equipment will permit extended operations only in light ice.

(emphasis added)

The Safety Board also reviewed the AMR Eagle guidance and

procedures for pilots when holding. According to the Flight Manual - Part 1,

Section 8, Communications:
116

Pilots shall, except in an emergency, and then when possible, comply

with ATC clearances and instructions. This does not preclude a pilot

from questioning any clearance or instruction-on the contrary, the

company expects its pilots, in the fulfillment of their responsibilities,

to question any clearance or instruction received that in their opinion

is unreasonable or not in compliance with good operating

procedure...

The Flight Manual - Part 1, Chapter 6, En-route, states that the

maximum holding speed for a turboprop is 175 KIAS. The manual encourages the

pilot to request a deviation from ATC if a higher speed is required. The AMR

Eagle ATR 42/72 Operating Manual, Volume 3, states that the airspeeds depicted

in the holding charts are predicated on the aircraft in a "clean"66 configuration and

a holding speed of VMHBO (minimum control speed, high bank mode, zero flap

configuration) in icing conditions.

A newly trained Simmons first officer interviewed by the Safety Board

said that if the speed of the airplane was above VMHBO upon entering the holding,

he was trained to extend the flaps to slow the airplane. Another first officer

estimated that before the accident, 65 percent of the captains with whom he was

familiar typically extended flaps while holding in clear air and 100 percent of the

captains extended the flaps while holding in icing conditions. A line/proficiency

check airman stated that the use of flaps in holding is not prohibited and that some

pilots use flaps because it "makes the aircraft more stable and drops the nose."

In the December 1993, issue of the Simmons Flight Operations News

Letter, the section entitled “Aircraft Ice” states, in part:

...Anytime ice accumulates on the aircraft during flight it must be

treated seriously. Not only does the performance deteriorate, but any

encounter with severe ice - including freezing rain - for a prolonged

period of time may cause control problems beyond that of the

intended design. When it is possible stay out of icing conditions.

Delaying a descent into a cloud layer or requesting an alternate

altitude or route to stay clear of known ice will decrease the amount

 

66"Clean" refers to the aircraft being in a minimum drag configuration, e.g., landing gear in the up position and the flaps

fully retracted.
117

of total ice build-up and any potential problem related to ice

accumulation....

1.17.7 Unusual Attitude and Advanced Maneuvers Training

The FAA defines an "unusual attitude" as "...any airplane attitude not

normally required for instrument flight." According to the Instrument Flying

Handbook published by the Department of Transportation and the FAA, an unusual

attitude may result from:

...a number of conditions, such as turbulence, disorientation,

instrument failure, confusion, or preoccupation with cockpit

duties....Since unusual attitudes are not intentional maneuvers during

instrument flight...they are often unexpected, and the reaction of an

inexperienced or inadequately trained pilot to expect abnormal flight

attitudes is usually instinctive rather than intelligent and deliberate....

A review of the AMR Eagle training syllabus that was in effect prior to

the accident for both the ground and simulator training programs revealed that

there were no formal "advanced maneuvers" or "unusual attitude" training sessions

being conducted. Also, there were no company documents available to indicate

whether any AMR Eagle ATR pilots had been shown an unusual aircraft attitude on

the EADI [electronic attitude display indicator]. At the time of the accident, there

were no FAA requirements for air carriers operating under 14 CFR Part 121 to

conduct training involving the recovery from an unusual attitude or the

performance of advanced maneuvers. Moreover, there were no data or algorithms

to support roll anomalies in the ATR 42/72 simulators. Also, with respect to flight

4184, the chief test pilot for ATR testified that the type of roll anomaly the

flightcrew experienced would not have been recoverable by the average line pilot.

The FDR data from flight 4184 revealed that primarily nose-up

elevator inputs (never exceeding 8 degrees) were made throughout the roll

excursions, including those periods when the airplane was in an inverted or nearly

inverted attitude. The FDR data also revealed that left rudder inputs were made

throughout the upset; however, because the airspeed was in excess of 185 KIAS,

the travel limiter unit (TLU) limited the rudder deflection, and the rudder travel did

not exceed 2.3 degrees.
118

1.18 Additional Information

1.18.1 Air Traffic Control

1.18.1.1 Chicago Area Airspace

In 1994, Chicago's O'Hare International Airport was classified as the

busiest airport in the United States, with 882,000 flights. The airspace extending

beyond a 40-mile radius of O'Hare is controlled by the Chicago Air Route Traffic

Control Center (ARTCC). The airspace within that 40-mile radius is controlled by

the Chicago Terminal Radar Approach Control (TRACON).

The airspace controlled by the Chicago ARTCC consists of

approximately 109,000 square mile in five states. The airspace is further divided

into seven areas: north; northeast; east; southeast; south; southwest and northwest.

The south area is again divided into seven sectors, five low altitude sectors, 0 to

10,000 feet; and two high altitude sectors, above 10,000 feet. The five low altitude

sectors include the BOONE sector, which is approximately 1,400 square miles and

is supported by three air route surveillance and two airport surveillance radars

(ASR).

1.18.1.2 Air Traffic Control System Command Center

The predecessor to the FAA's Air Traffic Control System Command

Center (ATCSCC) was the Central Flow Control Facility (CFCF), originally

located at the FAA Headquarters in Washington, D.C. The CFCF was established

with the objective of balancing aircraft flow to minimize delays to the user

(primarily airlines) without exceeding controller capacity. The CFCF was renamed

the Air Traffic Control System Command Center (ATCSCC) and was relocated to

Herndon, Virginia, on March 26, 1994.

The basic mission of the ATCSCC is to manage the flow of air traffic

throughout the National Airspace System (NAS), and to achieve the optimum use

of the navigable airspace while minimizing the effect of air traffic delays on the

user without exceeding operationally acceptable levels of traffic. The ATCSCC

consists of the following five operational units:
119

1) the Traffic Management Function (TMF), which is

responsible for coordination and approval of all major intercenter flow control restrictions on a system basis in order to

obtain maximum utilization of the airspace;

2) the Central Altitude Reservation Function, which is

responsible for coordinating, planning, and approving special

user requirements;

3) the Airport Reservation Office, which is responsible for

approving IFR [instrument flight rules] flights at designated

high density airports (John F. Kennedy, LaGuardia, O'Hare,

and Washington National) during specified hours;

4) the ATC Contingency Command Post, which is a facility that

enables the FAA to manage the ATC system when significant

portions of the system's capabilities have been lost or are

threatened; and

5) the Central Flow Weather Service Unit (CFWSU) which is

staffed by National Weather Service personnel and provides

24-hour service to the ATCSCC and users as needed.

The ATCSCC is operational 24 hours a day. Generally, two

controllers and one supervisor are assigned to midnight shifts, and seven crews

(with eight controllers per crew) rotate to work the day shifts. Personnel at the

facility are typically full performance level (FPL) air traffic controllers and are

normally assigned to the facility for 2 or 3 years. The controllers are provided

training about standard operating procedures (SOPs) during 80 hours of classroom

training.

All operating positions at the ATCSCC are linked through the Apollo

computer system which enables communications between all ATC en route

facilities and specific terminal facilities. The flow control workload is typically

distributed to specialists at the ATCSCC by dividing the country into two

geographical areas, east and west. The east area includes the boundaries of Boston,

New York, Cleveland, Washington, Atlanta, Jacksonville, Memphis, Indianapolis,

and Miami Air Route Traffic Control Centers (ARTCC). The west area includes:
120

Seattle, Salt Lake City,Denver, Oakland, Los Angeles, Albuquerque, Minneapolis,

Chicago, Fort Worth, Houston, and Kansas City ARTCCs. The sectors can be

reassigned as necessary to utilize the system to the fullest extent. In conjunction

with the ATCSCC, the CFWSU and the Airport Reservation Office also provide

services for each sector area. The primary function of the CFWSU is to provide

meteorological expertise and advice to senior level air traffic flow

managers/controllers. Meteorological support is also provided to the 20 ARTCCs

and high traffic volume facilities.

The ATCSCC specialists have several tools available for monitoring

traffic, one of which is the aircraft situation display (ASD). The ASD is a

computer system located in Cambridge, Massachusetts, that receives radar track

data from all 20 contiguous ARTCCs via satellite link, organizes the data into a

mosaic display, and presents digital information on a computer screen. The ASD is

not a radar display and only updates approximately every 3 minutes. The visual

display provides the traffic management coordinator with multiple methods of

selecting and highlighting either individual aircraft or groups of aircraft for

analysis. The user also has the option of superimposing selected aircraft positions

over any number of background displays, which include ARTCC boundaries, any

stratum of en route sector boundaries, navigational fixes, airways, military and

other special use airspace, airports, and geopolitical boundaries. All ARTCCs, the

26 terminal facilities, and some users, such as American Airlines, are equipped

with ASD stations. By using the ASD, the traffic coordinator can monitor any

number of individual aircraft flow situations or view the entire system-wide traffic

flows. Each ATCSCC specialist maintains direct contact with the facilities in his

or her area so that special traffic flow programs can be implemented if necessary.

According to FAA Order 7110.65, Air Traffic Control Handbook,

Pilot/Controller Glossary, the Control Departure Time (CDT) program is the "flow

control process whereby aircraft are held on the ground at the departure airport

when delays are projected to occur in either the en route system or the terminal of

intended landing. The purpose of these programs is to reduce congestion in the air

traffic system or to limit the duration of airborne holding in the arrival center or

terminal area. A CDT is a specific departure slot shown on the flight progress strip

as an expected departure clearance time (EDCT)."

Controllers maintain an awareness of the expected hourly demand in a

given area or airport, based on information published in the Official Airline Guide

(OAG). When a situation requires the implementation of a traffic flow program,
121

the computer will arbitrarily assign EDCTs to the affected flights. If a

nonscheduled flight, such as a military or general aviation aircraft, requests a

clearance to a destination with a program in effect and has not been assigned an

EDCT, the controller is required to request a time from the Command Center.

One of the duties of the ATCSCC specialist is to retrieve a

"Verification and Analysis Report" from the computer every 2 hours, as well as at

the end of the program to determine its effectiveness. The verification and analysis

report lists all of the aircraft that departed a given airport and the actual departure

time. This information is used by the controller to verify that the specific flight did

depart during the EDCT time. Tracking of a specific aircraft is not required unless

that aircraft has been holding for longer than 15 minutes. Subsequent holds of

14 minutes or less by various sectors are not recorded. Thus, an aircraft can move

from one ARTCC to the next, or from controller to controller, holding each time

for up to 14 minutes with no recorded delays.

The crew of flight 4184 was directed to hold on the ground by the

Indianapolis ground controller because a ground delay program was in effect for

the flights into O'Hare due to deteriorated weather conditions at O'Hare. As a

result, flight 4184 held on the ground approximately 42 minutes prior to receiving a

takeoff clearance, and then because the weather at O'Hare had deteriorated further,

held again in flight for approximately 35 minutes because of multiple expect

further clearances (EFCs). In testimony provided by the South Area Supervisor for

the Chicago ARTCC, he stated that proper notification to the Traffic Management

Coordinator (TMC) of the excessive holding time experienced by flight 4184

(greater than 15 minutes) had not occurred, as required. Additionally, the TMC

stated in an interview after the accident that when flight 4184 was released from

IND, there were no flights holding for landing at O'Hare. However, in anticipation

of a "rush" of arriving aircraft from the west, she informed the controllers to

"expect holding on the east side [of the sector]." In addition, the TMC stated that

she had not been informed that the BOONE sector was in a holding status at the

time of the accident.

According to the National Traffic Management Officer (NTMO) on

duty at the ATCSCC during the periods before and after the accident:

The purpose of the EDCT is to permit aircraft to sit on the ground

then arrive at the destination with no delay except what is needed en
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route (spacing requirements) to reduce airborne holding and save

fuel. Center controllers use "call for release" option to keep the flow.

It is common to use both EDCT's and call for release simultaneously.

During the course of the on-scene portion of the investigation, Safety

Board investigators made a request to the FAA to hold all ATCSCC documentation

regarding flow control that was related to the accident. However, this was

interpreted as a request to hold the data from the Chicago Traffic Management

Unit. The policy regarding requested information that was in effect at the

ATCSCC only required the retention of certain facility paperwork for 15 days. The

Safety Board reiterated its request in writing on November 15, 1994; however, this

request was not forwarded to the ATCSCC until November 17, 1994. As a result,

the data pertaining to flight 4184 was not held by the facility and could not be

recovered.

1.18.2 FAA Aircraft Certification

The Safety Board reviewed the FAA's organizational units responsible

for aircraft certification and oversight. The Aircraft Certification Directorates are

described in FAA Order 8000.51, dated February 1, 1982, which contains the

duties, responsibilities and authority of each Directorate. The order specifies the

need for "timeliness" in "monitoring continuing airworthiness" issues and states

that the Aircraft Evaluation Group (AEG) will be responsible for providing all

applicable technical services to the Flight Standards Division and Aircraft

Certification Offices.

The FAA Air Transportation Inspector's Handbook, Order 8400.10,

directs Flight Standards personnel responsible for the investigation of aircraft

incidents and accidents to contact the AEG office for assistance and background

information. The Order generally describes the AEG office as a unit of the Flight

Standards (FS) office, collocated with the Aircraft Certification Office (ACO). The

ACO is responsible for providing initial operational evaluation of each aircraft type

for FS approval in the aircraft certification process. The AEG, which consisted of

12 specialists, is responsible for monitoring the fleet service history of an aircraft

to fulfill the responsibilities of maintaining continued airworthiness. According to

the Order, AEG responsibilities also include performing operational evaluations of

the aircraft, providing guidance relating to its airworthiness, the receipt and

maintenance of service difficulty reports (SDRs), and the evaluation of

supplemental type
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certificates (STCs). The AEG specialists are fully qualified FS aviation safety

inspectors in the areas of operations, airworthiness, and avionics.

The Aircraft Certification Directorate procedures are outlined in FAA

Order 8100.5, Paragraph 305(b), which states that each Aircraft Certification

Office is responsible for keeping the Directorate informed of significant accidents

and incidents as referenced in chapter 7 of the Order. Chapter 7 of the Order,

entitled Service Difficulties, describes the issuance of airworthiness directives, and

section 702 of the chapter, "Accident Investigations," has been "reserved," and

provides no information or guidance regarding the proper procedures for reporting

the findings of the accidents and incidents to the Directorate. The Washington

Headquarters Directives Checklist, Order WA 0000.4T, dated February 2, 1995,

lists FAA Order 8100.5, issued October 1, 1982, as being a current order:

The Operations Unit Supervisor for the FAA AEG testified that they:

...cover approximately 60 airplanes in the U.S. inventory [and]

perform several functions. We have a sister organization that's an

airworthiness organization that performs MRB (maintenance review

board) activities, which is analyzing the initial maintenance program

on newly type [certificated] airplanes. And we do, in the operations

unit, we do two Boards; the Flight Standardization Board and the

Flight Operations Evaluation Board....We do continuing

airworthiness activities in concert with the certification offices...and

we participate in in-service history - following an aircraft from...its

type certificate until it's taken out of revenue service.

The unit supervisor also testified that the AEG office does not

maintain a data base for incident/accident history for specific aircraft. He said

"...we're not that sophisticated. We do obviously keep records, especially within

the Flight Standardization Board...but we don't particularly have a database."

There was no formal tracking system available from which to obtain

background information regarding the incident/accident history of the ATR

airplanes in icing conditions. The Safety Board was provided a briefing paper

entitled, ATR-42 Icing History, written to the Manager, Seattle Aircraft Evaluation
124

Office, from the Manager, Seattle Aircraft Evaluation Group,67 on March 25, 1989.

According to the briefing paper, it was believed that the ATR 42 had an

"...apparent inability to carry ice or at least perform reliably in icing conditions."

The briefing paper also stated, in part:

...As of this date there are 10 icing-related incidents, reasonably well

documented, in which abnormal flight characteristics were

demonstrated by the airplane. A continuing airworthiness

statement68 ...the subject of which was "ATR 42 Icing Problems,"

prepared by Robert McCracken, ANM-113, annotates 5 of those 10

incidents and summarizes, briefly, the evolution of concern with

those incidents. As a result of the incidents prior to the December

22, 1988, incident with Simmons Airlines, the manufacturer in

concert with the [FAA's] Brussels Office has published revision No.

6 to the ATR 42 AFM....

We feel that revision 6, as far as it goes, is a definite step in the right

direction, however, it is our understanding that the manufacturer has

not expressed an interest in mandating the aircraft changes....

All along there has been a perceived reluctance on the part of the

manufacturer to accept the fact there is an icing problem with the

ATR 42. They have continually questioned the competence of the

aircrews and the training programs in dealing with flight in icing

conditions....

It is thought that control forces are building up due to lift distortions

on the wing caused by ice build-up, and when the build-up of control

forces exceed those which the autopilot can handle, the autopilot

disconnects and aileron displacement causes the aircraft to pitch

left....

 

67The Aircraft Evaluation Group evolved from the consolidation of the Flight Standardization Board (FSB) and the

Flight Operations Evaluation Board.

68The "continuing airworthiness statement" referenced in the briefing paper was requested by Safety Board

investigators. The FAA responded that there "is no official document called a continuing airworthiness statement in

FAA terminology." The author of the requested document indicated "I do not remember preparing the specific

document...I could well have done so, and suspect that it was a briefing paper prepared to alert management to possible

problem areas regarding the ATR 72 airplane."
125

Pragmatically we feel that the design of the wing has been the

singular problem. It has been our observation on line operations that

this wing is very efficient, and it follows that any distortion of

airflow would be extremely disruptive. Operators and the industry as

a whole are used to operating aircraft of the size and general type as

the ATR 42 with heavy thick airfoils that will carry a "ton of ice."

This wing will not....

Another problem seems to have been that the aircraft was certificated

under the Bilateral Agreements, which in this case made it difficult

to collate, coordinate, and disseminate information between the

manufacturer, regulatory entities, and operators....

In the context of problem solving we would like to see flight tests on

the ATR series aircraft with irregular ice shapes emulating "runback" i.e., small distortions that have not been test flown to date.

Intuitively, it seems that a high performance wing and boots do not

go together.

The unit supervisor who generated the 1989 briefing paper testified at

the Safety Board's public hearing that he made the statement regarding the

"...perceived reluctance on the part of the manufacturer to accept the fact that there

is an icing problem with the ATR 42" because he was "not familiar with the ATR

manufacturer." He stated, "I had noticed, however, with some of the past [ATR]

incidents, that there was...often a mention of a crew following improper

procedures...and coming from a training background, I took note of that." The unit

supervisor also testified that as he became more familiar with people from ATR, "I

found that they were in fact not reluctant. That they were doing a lot to deal with

these issues." He also stated that "it appeared, however, that when I wrote the

letter [briefing paper] that that was [not] the case."

The unit supervisor also testified that he perceived "another problem"

existed with the ATR's certification under the bilateral [airworthiness] agreement.

He said:

...[under the bilateral airworthiness agreement] it's more difficult for

us. For instance, we do not have an operational bilateral...and what

this does when you're working in a bilateral situation, it does
126

impede in a certain sense information flow. And in the AEG, we

being a small part of the flight standards, deal in the information.

We collect information in the field. We supply it to the certification

office, and vice versa. That process was difficult for me as I got

involved....I think we could do more to smooth out the lines of

communication within the bilateral...."

1.18.3 Previous Safety Board Recommendations Regarding

In-flight Icing

On September 9, 1981, the Safety Board published a report entitled

Aircraft Icing Avoidance and Protection. The primary issues discussed in the

report included icing standards for aircraft certification, weather

forecasting/dissemination, and aircraft performance in icing conditions. The report

targeted general aviation, air taxi and commuter size aircraft as those most

vulnerable to "aircraft structural icing" because they are regularly flown at altitudes

that are conducive to atmospheric icing conditions. The Safety Board's report

indicated that during the period 1976 through 1979, there were no commercial

aviation accidents in the United States attributed to aircraft icing. This successful

period was due in part to the fact that the majority of the flights were being

conducted by large aircraft that were capable of "...operating above the prevalent

icing regimes," with "relatively sophisticated deicing and anti-icing equipment on

those aircraft."

The report reflected the Safety Board's concerns about aircraft

operations in icing conditions and the varying consequences that ice accretions had

on different aircraft types. Based on its findings, the Board stated that "...a

forecasting system is needed which will allow the pilot to determine the icing

effects on his or her particular aircraft at any of the various stages of his or her

flight and to prepare from this a safe flight plan." Thus, the Safety Board issued to

the Federal Coordinator for Meteorological Services and Supporting Research,

Safety Recommendations A-81-113 and -114, which stated, in part, respectively:

A-81-113

...develop instruments to measure temperature, liquid water content,

drop size distribution, and altitude in the atmosphere, on a real-time

basis, that are economical to use on a synoptic time and grid scale

and;
127

A-81-114

Use the developed instrumentation to collect icing data on a real-time

basis on a synoptic grid and, in turn, develop techniques to forecast

icing conditions in terms of liquid water content, drop size

distribution, and temperature.

On May 12, 1994, the Safety Board classified both recommendations,

"Closed--Acceptable Alternate Action" because the issues discussed in the

recommendations were addressed in a report published in 1982, entitled, "A Report

on Improving Forecasts of Icing Conditions for Aviation." Further, the Aircraft

Icing Program Counsel was established in 1984 to continue the study of icing

forecast methods. In 1986, a second report was published entitled, "National

Aircraft Icing Technology Plan," which also addressed the improved aircraft icing

detection technologies on current generation aircraft. This plan also promoted the

development of aircraft ice detection technology that would be needed by 1995 to

meet the goals set for the new generation of aircraft that were in development.

Also, based on the findings of the study, the Safety Board issued

Recommendations A-81-115 through -118 to the FAA. The first recommendation

stated:

A-81-115

Evaluate individual aircraft performance in icing conditions in terms

of liquid water, drop size distribution, and temperature, and establish

operational limits and publish this information for pilot use.

The FAA initially responded to the Board's recommendation on

December 21, 1981, and cited in its correspondence that:

Full implementation of this recommendation would be dependent

upon prior implementation of Safety Recommendations A-81-113

and -114...For a pilot to utilize operational limits in terms of liquid

water content, drop size distribution, and temperature, information

on icing forecasts and actual conditions must be available to him in

terms of these parameters. We can envision that implementation of

this concept would entail considerable expense, both in measuring

the atmospheric parameters and in providing information for pilot
128

use in aircraft flight manuals. During certification in icing, the

aircraft is evaluated in terms of liquid water content, drop size

distribution and temperature to establish adequacy of the ice

protection system and to demonstrate the capability of the aircraft to

operate safely in the defined atmospheric conditions. Limited

certification in terms of liquid water content, drop size distribution,

and temperature is not permitted. As there are no limitations in terms

of these parameters for an aircraft certificated in icing, there would

be little or no need to provide such information to pilots. (The

exception to this is freezing rain, freezing drizzle, and mixed

conditions...). We believe the present icing certification philosophy

and criteria are basically sound and this is reflected in the accident

statistics....In view of this, the cost of implementing

Recommendation A-81-115, and the fact that icing certification does

not allow limitations in terms of atmospheric icing parameters, the

FAA cannot concur....

The Safety Board emphasized in its April 16, 1982, response to the

FAA that, "...the basic concept of enabling an operator to determine the effects of

icing conditions, stated in parametric terms, upon a specific aircraft is valid.

Forecasts issued in terms of intensity levels ('light,' 'moderate,' 'severe') do not

apply equally to all aircraft, for example moderate icing to a large transport aircraft

might be severe to a small general aviation aircraft...."

The FAA's June 7, 1982, response to the Safety Board stated, in part:

The present FAA icing standards require an ice protection system

which permits flight in maximum icing conditions. The rules do not

allow certification for less extreme conditions...because variables

such as liquid water content, droplet size and outside air temperature

are not controllable by the pilot. These conditions may change so

rapidly that diversion to areas where less severe icing conditions

exist may not be possible....Providing icing forecasts and airplane

operating limits in parametric terms...could therefore prove

hazardous for an aircraft with only a limited capability to operate

safely in icing conditions....To allow certification with operating

limitations in terms of the above parameters would therefore degrade

the level of safety....Forecasts issued or icing conditions
129

described in terms of intensity levels...should not affect the capability

of icing certified aircraft to operate safely in icing conditions

regardless of the size or category of the aircraft. This is because

icing-certified aircraft are evaluated to the full icing envelope

expected in nature and defined in 14 CFR 25, Appendix C.

The FAA closed the correspondence with, "...We believe

implementation of A-81-115 would involve considerable expense with little or no

tangible benefit being realized...."

The Safety Board reiterated its position in its October 24, 1983,

written response to the FAA, which stated, in part:

We maintain the position that pilots, particularly those involved in

general aviation, air taxi, and commuter aircraft need more

information concerning the potential severity of icing and its effect

upon aircraft that they are flying.

The Safety Board's stated in its October 2, 1987, follow-up response to

the FAA:

...in both Advisory Circulars, 29-2 and 23.1419-1, it is recommended

that a statement be included in the flight manuals that the prescribed

flight test environment does not include freezing rain and/or mixed

conditions and that these conditions may exceed the capabilities of

an ice protection system.

The Board believes that a pilot flying into known or forecast icing

conditions needs more information than is presently provided.

Based on the FAA's unfavorable response of June 7, 1982, the Safety

Board classified Recommendation A-81-115, "Open--Unacceptable Response."

In its December 11, 1989, final response to the Safety Board, the FAA

cited Advisory Circular 29-2 and Advisory Circular 23.1419-1 (subsequently

superseded by AC-23.1419-2 on January 3, 1992), which provide a description of

the effects of icing on aircraft performance and flight characteristics. The
130

information and actions contained in the ACs do not include flight testing in

conditions that extend beyond those specified in Appendix C, such as freezing

drizzle and freezing rain.

In the Safety Board response to the FAA, dated April 11, 1990, it

stated:

...Considerable important research has been conducted, and the

results have been published in research and academic papers, as well

as discussed with pilots at FAA safety seminars. However, because

the FAA has not related this information to individual aircraft, pilots

have not benefited completely from this information. Because this

information has not been effectively used, Safety Recommendation

A-81-115 has been classified as "Closed--Unacceptable Action."

The Safety Board's 1981 icing report also identified the need for the

FAA to review and revise the icing certification criteria in 14 CFR Part 25,

Appendix C, based on the fact that this criteria was determined by, and established

for, aircraft in use some 40 years ago. The Safety Board believed that because of

advancements in technology, i.e., "deicing and anti-icing equipment, and

improvements in the instruments used to measure atmospheric icing parameters," it

was necessary for the FAA to also advance the criteria to keep pace with

technology. Thus, it issued Safety Recommendation A-81-116, to the FAA, which

stated:

Review the icing criteria published in 14 CFR 25 in light of both

recent research into aircraft ice accretion under varying conditions of

liquid water content, drop size distribution, and temperature, and

recent developments in both the design and use of aircraft; and

expand the certification envelope to include freezing rain and mixed

water droplet/ice crystal conditions, as necessary. (Class III, Longer

Term Action) (A-81-116)

The FAA initially responded to the recommendation with a discussion

about the "low probability of occurrence" in such conditions as freezing drizzle,

freezing rain and mixed water droplet/ice crystals. They also stated, in part,

"...indications are that it would be excessively penalizing and economically
131

prohibitive to require compliance with such criteria as part of a normal icing

certification."

The Safety Board responded to the FAA on April 16, 1982, and "took

exception" to the FAA's position that certification requirements for these

conditions (freezing rain, freezing drizzle and/or mixed) should be elective. The

Safety Board believed that "operation in freezing rain, freezing drizzle and mixed

conditions occurs often enough to warrant inclusion of such conditions in the

certification criteria, especially considering their hazardous nature."

The Safety Board sent a follow-up response to the FAA's June 7, 1982,

letter on October 24, 1983, and stated, in part:

...In a recent analysis of an annual compilation of icing accidents, 28

percent were found to involve freezing rain. Consequently, such an

occurrence cannot be considered a rare event. Freezing rain also is

the most likely condition to be encountered during VFR flight in that

it is often encountered below the clouds in relatively good visibility

at altitudes most frequently utilized by smaller aircraft.

Based on the FAA's unfavorable responses, the Safety Board

continued to classify A-81-116 as "Open--Unacceptable Response."

In 1986, the FAA sent a follow-up letter to the Safety Board stating

that:

The FAA has reconsidered the issue of considering freezing rain and

drizzle as a criterion of aircraft for flight in icing conditions. The

FAA has concluded that current research and development

efforts...will provide the data needed to form a basis for determining

the feasibility of any rulemaking action....

The Safety Board responded to the FAA in March of 1987, and stated

that, "while the Safety Board is concerned about the lack of action since this

recommendation was issued, it is encouraging that the FAA has reconsidered....

Pending the Board's review of the final action taken, Safety Recommendation A81-116 has been classified as "Open--Acceptable Response."
132

Additional correspondence between the FAA and the Safety Board

resulted in Safety Recommendation A-81-116 being reclassified as "Open--

Unacceptable Response" in April 1990. The FAA's most recent response before

the accident was received on September 16, 1994, and stated, in part:

...The FAA has reviewed the research and development projects that

have been conducted on various icing issues and especially with

respect to the adequacy of the icing criteria published in 14 CFR Part

25....The FAA has concluded that the icing criteria published in 14

CFR Part 25 is adequate with respect to the issues outlined in Safety

Recommendation A-81-116 and A-81-118. Thus, the FAA has met

the intent of the safety recommendation.

The Safety Board responded to the FAA on July 12, 1995, and

indicated that although the Board noted that the FAA had reviewed the icing

criteria published in 14 CFR Parts 25, 91 and 135, and concluded that they were

adequate with respect to the issues outlined in Safety Recommendations A-81-116

and -118, the Board did not agree with the FAA's conclusions.

Further, information gleaned from the icing study prompted the Safety

Board to issue recommendation A-81-118 to the FAA because it was believed that

the definition of "severe icing" as found in the Aeronautical Information Manual

(AIM) was not consistent with its use in the Federal Aviation Regulations. The

recommendation asked the FAA to:

Reevaluate and clarify 14 CFR 91.209(c) and 135.227(c) to insure

that the regulations are compatible with the definition of severe icing

established by the Federal Coordinator for Meteorological Services

and Supporting Research as published in the Airman's Information

Manual. (Class II, Priority Action) (A-81-118)69

The FAA's initial response in December 1981 was favorable and

acknowledged that:

 

6914 CFR Part 91.209(c) was changed to 14 CFR Part 91.527(c); and 14 CFR Part 135.227 (c) was changed to

paragraph "(d)."
133

...the content of the rules in Parts 91 and 135 are not consistent with

the definition of severe icing contained in the Airman's Information

Manual and used by the National Weather Service. Accordingly, we

agree that clarification of the current regulation is necessary. This

incompatibility will be corrected in both Sections 91.209(c) and

135.227(c) in the next major review of these rules.

On June 7, 1982, the FAA responded to the Safety Board with a

proposed amendment to the definition of "severe" icing found in the AIM. This

amendment was believed to be "more compatible" with the language of 14 CFR 91

and Part 135. The Safety Board took exception in its October 23, 1983, response

and stated, in part, "...This is in fact changing the established definition of severe

icing and stating in effect that there are no conditions so severe that a properly

certificated aircraft cannot safely fly in them."

In April 1990, the Safety Board sent a follow-up response to the FAA

and expressed "disappointment" with its failure to "implement this Safety

Recommendation [A-81-118] after 8 years." However, in consideration of the ongoing research by the FAA, the Safety Board stated that it would monitor the

progress of this issue and reclassified the recommendation "Open--Acceptable

Response," pending further response.

The most recent FAA response to the Safety Board before the accident

was received on September 16, 1994, and said, in part:

...the FAA has reviewed the research and development projects that

have been conducted on various icing issues and especially with

respect to the adequacy of the icing criteria published in 14 CFR Part

25...The FAA has reviewed the study of aviation requirements

described in the "National Plan to Improve Aircraft Icing Forecasts."

The FAA has also analyzed extensive in-flight icing data that were

obtained from various European agencies as well as from research

projects in the United States. As a result...the FAA has concluded

that 14 CFR 91 and 14 CFR 135 are adequate to ensure that the

intent of this safety recommendation is addressed, and I plan no

further action.

The FAA concluded its response letter as follows:
134

The FAA has put in place major programs in recent years which have

addressed various anti-ice and deicing issues. At the same time the

FAA has sponsored or collaborated on numerous icing

programs...However, none of this work has established the

foundation or justification to revise 14 CFR Parts 25, 91, or 135 as

requested by these safety recommendations...I [the FAA

Administrator] consider the FAA's actions to be complete on the

safety recommendations.

The Safety Board's July 12, 1995, response letter to the FAA stated:

The Safety Board notes that the FAA has reviewed the icing criteria

published in 14 CFR Parts 25, 91, and 135 and has concluded that

they are adequate with respect to issues outlined in Safety

Recommendations A-81-116 and -118. The Safety Board does not

agree. The content of 14 CFR 91.527(c) and 14 CFR 135.227(e) still

is not consistent with the provisions defined in section 34, Appendix

A, of 14 CFR Part 135. Under certain ice protection provisions

defined in section 34 Appendix A of 14 CFR Part 135, flight into

known severe icing conditions is permitted. However, severe icing,

as currently defined, includes hazardous environmental conditions

that existing deicing/anti-icing equipment is unable to reduce or

control, and immediate diversion is necessary.

In light of the accident on October 31, 1994, near Roselawn, Indiana,

involving a Simmons Airlines ATR-72-210 airplane in which

structural icing may have been involved, the Safety Board believes

the issue of icing criteria, as related to the design and use of

transport-category aircraft, warrants reexamination by the FAA and

the aviation industry. Investigation, testing, and analysis following

the ATR-72 accident, and testimony at the Safety Board's associated

public hearing for that accident, have underscored the need to amend

the icing criteria as they pertain to 14 CFR Parts 25, 91, and 135.

Accordingly, the Safety Board classifies Safety Recommendations

A-81-116 and -118 "Open--Unacceptable Response," pending further

action by the FAA on this matter.
135

The FAA responded to the Safety Board on August 28, 1995, in regard

to Safety Recommendations A-81-116 and -118, and stated:

The Federal Aviation Administration (FAA) has taken actions to

address the ATR-72 aircraft design and operation in icing conditions.

The FAA is currently evaluating similar aircraft designs to ensure

there are no adverse characteristics when operating in icing

conditions. The final phase of this evaluation is to review current

certification requirements, applicable operating regulations, and

forecast methodologies associated with ice under varying

environmental conditions. The FAA plans to conduct an

international meeting in the spring of 1996 with representatives from

airworthiness authorities, the aviation industry, the NTSB, and other

interested parties. This meeting will include a comprehensive review

of all aspects of airworthiness when operating in icing conditions and

determine where changes or modifications can be made to provide an

increased level of safety.

The Safety Board responded to the FAA on November 20, 1995, and

indicated that the Board notes and supports the FAA's intention to convene an

international meeting of representatives from foreign airworthiness authorities, the

aviation industry, and other interested parties in 1996. However, the Safety Board

maintains its position that in light of the accident involving flight 4184 and the

subsequent flight testing and analysis, the issues raised in Safety Recommendations

A-81-116 and -118 underscore the need to amend the icing certification

regulations. Thus, the Safety Board classified recommendations A-81-116 and -

118, "Open--Unacceptable Response," pending further actions by the FAA. Based

on a new recommendation issued with this report, the Safety Board classifies

recommendations A-81-116 and -118 as "Closed—Unacceptable

Action/Superseded."

The Safety Board's 1981 icing report also cited information about the

causes of various icing conditions and the detrimental effects that such conditions

have on aircraft performance. The report provided a description of the formation

and effects of "clear ice," and cited, in part:

Clear ice is a glossy, clear-to-translucent accumulation formed by

large water droplets or raindrops which spread and freeze on
136

contact, forming a sheet of smooth ice. It is a hazardous icing

condition because it accumulates rapidly and is dense and heavy. It

often spreads beyond the effective area of deicing or anti-icing

surfaces and adheres strongly to the aircraft's surfaces.

Based on this information, the Board issued Safety Recommendation

A-81-117, to the FAA, encouraging it to:

Establish standardized procedures for the certification of aircraft

which will approximate as closely as possible the magnitudes of

liquid water content, drop size distribution, and temperature found in

actual conditions, and be feasible for manufacturers to conduct

within a reasonable length of time and at a reasonable cost.

(Class III, Longer Term Action) (A-81-117)

After several follow-up letters between the two agencies, the FAA

again responded in regards to Safety Recommendation A-81-117 on October 24,

1983, and stated that it was reviewing the icing criteria for normal icing

certification. This review was to include the consideration of freezing rain and

freezing drizzle; however, the FAA believed that the latter would be considered

"elective" rather than a requirement of the normal icing certification.

The FAA provided a final response to the Safety Board on December

1, 1986, and stated that it had reconsidered the issue of including freezing rain and

freezing drizzle as a criterion in the certification of aircraft for flight in icing

conditions. The response letter also stated that research and development data was

needed to determine the basis for rulemaking action, and that once the data was

received, the FAA would determine the appropriate course of action. Based on this

response, on March 12, 1987, the Safety Board classified Safety Recommendation

A-81-117 "Closed--Acceptable Action."

1.18.4 Previous Safety Board Recommendations Regarding Unusual

Attitude Training for Pilots

The Safety Board has addressed the issue of "unusual attitude"

recognition and recovery training for transport-category pilots four times in the

past 27 years. One recommendation resulted from the investigation of an accident

in a United Airlines Boeing 727 that occurred on November 16, 1968, near the

Detroit
137

Metropolitan Airport. The Safety Board issued Safety Recommendation A-70-021 to

the FAA, which encouraged the FAA to require commercial airlines to provide

additional training to flightcrews regarding unusual attitudes, and require the pilot to

demonstrate periodically, proficiency in the area of recovery from unusual attitudes. It

was also recommended that aircraft simulators be utilized to provide flightcrew

familiarization in the following areas: 1) The various instrument displays associated

with and resulting from encounters with unusual meteorological conditions; 2) The

proper flightcrew response to the various displays; and 3) Demonstration of and

recovery from possible ensuing unusual attitudes.

The FAA did not respond favorably, and, on August 17, 1972, the

Safety Board classified recommendation A-70-021, "Closed--Unacceptable Action."

On September 15, 1972, the Safety Board issued Safety

Recommendation A-72-152, following an accident on March 31, 1971, at Ontario,

California, involving a Boeing 707/720B which crashed after the flightcrew lost

control while attempting a 3-engine missed approach on a proficiency check

flight.70 Although the Safety Board attributed the probable cause of the accident to

the failure of the aircraft's rudder actuator, the Board expressed concern regarding

the flightcrew's ability to rapidly assess the situation and effect a recovery.

Safety Recommendation A-72-152 asked that the FAA require pilots to

demonstrate their ability to recover from abnormal regimes of flight and unusual

attitudes solely by reference to flight instruments. The use of simulators was

recommended for this purpose. The Safety Board noted that if current simulators

were not capable of being used for this purpose, the simulators should be modified.

The FAA's response to the safety recommendation stated:

The simulator is not capable of simulating certain regimes of flight

which go beyond the normal flight envelope of the aircraft. Further,

since an aircraft simulator is not required as part of an air carrier

training program, the FAA cannot require that it be replaced or

modified to simulate regimes of flight outside the flight envelope of

the aircraft.

 

70Aircraft Accident Report—"Western Air Lines, Inc., Boeing 720-0478, N3166, Ontario International Airport,

Ontario, California, March 31, 1971" (NTSB/AAR-72-18)
138

The Safety Board was disappointed that the FAA declined to

implement A-72-152, and, on January 16, 1973, classified this safety

recommendation, "Closed--Unacceptable Action."

On July 10, 1991, the Safety Board investigated an accident involving

a L'Express Airlines, Beech 9971 that crashed while conducting an instrument

landing system approach to runway 5 at the Birmingham Airport (BHM),

Birmingham, Alabama. The Safety Board found that the current Federal

regulations do not require instrument-rated pilots to maintain proficiency in the

ability to recognize and recover from unusual aircraft attitudes. It also found that

the difficulty the L'Express flightcrew had controlling the airplane may have been

exacerbated because they had not received unusual attitude recognition and

recovery training from the company. Based on this accident, the Safety Board

issued Safety Recommended A-92-20 to the FAA which stated:

Require recurrent training and proficiency programs for instrument

rated pilots to include techniques for recognizing and recovering

from unusual attitudes.

The FAA's July 9, 1992, response to the Safety Board stated, in part:

...the FAA believes that pilot flight crewmembers must be proficient

in the recovery from unusual flight attitudes and has designed the

flight training requirements to address this skill. Recovery from

unusual flight attitudes is required in order for individuals to receive

a private pilot certificate. Additionally, the instrument rating

practical test standards require pilots who obtain an instrument rating

to be proficient in the recovery from unusual flight attitudes.

Likewise, the practical test standards for an airline transport pilot

require pilots to recover from specific flight characteristics for a

particular type aircraft.

The Safety Board was disappointed with the FAA's response and

responded with a second letter reiterating the importance of such training. The

Safety Board believed that instrument-rated pilots should receive recurrent training

 

71Aircraft Accident Report--"L'Express Airlines, Inc., Flight 508, Beech 99, N7217L, Weather Encounter and Crash

Near Birmingham, Alabama, July 10, 1991" (NTSB/AAR-92/01)
139

in techniques for recognition and recovery from unusual attitudes because this

training would greatly enhance a pilot's ability to safely recover from an unusual

attitude. Therefore, the Safety Board classified recommendation A-92-20 on

January 26, 1993, "Closed--Unacceptable Action."

On June 25, 1991, the Safety Board issued Safety Recommendation

A-93-72, following the accident involving a Beech 1900 that crashed near Block

Island, Rhode Island, on December 28, 1991.72 The recommendation asked the

FAA to:

Consider an amendment to 14 CFR Part 135 to require that commuter

air carriers perform certain hazardous training, testing, and checking

maneuvers, such as engine-out operations, and recovery from

unusual flight attitudes, in approved flight simulators to the

maximum extent feasible.

The FAA stated in its response to A-93-72, that it was considering new

air carrier training requirements, in particular, requiring certain 14 CFR Part 135

air carriers to conduct their pilot training in accordance with the standards set forth

in 14 CFR Part 121.

On August 29, 1995, the Safety Board classified Safety

Recommendation A-93-72, "Closed--Acceptable Action," based on the FAA's

February 3, 1995, response in which it stated that rulemaking (NPRM) actions were

in progress to require pilots of scheduled 14 CFR Part 135 air carriers operating

aircraft that required two or more pilots, or seated 10 or more passengers, to

receive training under the provisions of 14 CFR Part 121. The proposed rule

would permit the use of sophisticated aircraft simulators to conduct the training.

The FAA's final rule was adopted in December 1995.

On August 16, 1995, the FAA disseminated a new Flight Standards

Handbook Bulletin (HBB) for Air Transportation (HBAT), HBAT 95-10, entitled

Selected Event Training, to its POIs. The bulletin contains "...guidance and

information on the approval and implementation of 'Selected Events Training' for

operators training under 14 CFR Part 121, who use flight simulation devices as part

of their flight training programs."

 

72National Transportation Safety Board, Safety Report (NYC-92-F-A053)
140

The bulletin states that the selected events training is "voluntary flight

training in hazardous inflight situations which are not specifically identified in

FAA regulations or directives." Some of the examples of these selected events

include: false stall warning at rotation; excessive roll attitude (in excess of 90

degrees); and high pitch attitude (in excess of 35 degrees). The bulletin further

states that this training program was developed jointly by the FAA and the aviation

industry in response to previously issued Safety Board recommendations

addressing the need for unusual events and unusual attitude training for Parts 135

and 121 air carrier pilots.

1.18.5 Previous Safety Board Recommendations Regarding the

Performance of ATR Airplanes and the Air Traffic Control System

Command Center

As a result of this accident, on November 7, 1994, the Safety Board

issued the following safety recommendations to the FAA:

A-94-181

Conduct a special certification review of the ATR 42 and ATR 72

airplanes, including flight tests and/or wind tunnel tests, to determine

the aileron hinge moment characteristics of the airplanes operating

with different airspeeds and configurations during ice accumulation

and with varying angles of attack following ice accretion. As a result

of the review, require modifications as necessary to assure

satisfactory flying qualities and control system stability in icing

conditions. (Class II, Priority Action)

A-94-182

Prohibit the intentional operation of ATR 42 and ATR 72 airplanes

in known or reported icing conditions until the effect of upper wing

surface ice on the flying qualities and aileron hinge moment

characteristics are examined further as recommended in A-94-181

and it is determined that the airplane exhibits satisfactory flight

characteristics. (Class I, Urgent Action)

A-94-183

Issue a general notice to ATC personnel to provide expedited service

to ATR 42 and ATR 72 pilots who request route, altitude,
141

or airspeed deviations to avoid icing conditions. Waive the 175 knot

holding speed restriction for ATR 42 and ATR 72 airplanes pending

acceptable outcome of the special certification effort. (Class I,

Urgent Action)

A-94-184

Provide guidance and direction to pilots of ATR 42 and ATR 72

airplanes in the event of inadvertent encounter with icing conditions

by the following actions: (1) define optimum airplane configuration

and speed information; (2) prohibit the use of autopilot; (3) require

the monitoring of lateral control forces; (4) and define a positive

procedure for reducing angle of attack. (Class I, Urgent Action)

A-94-185

Caution pilots of ATR 42 and ATR 72 airplanes that rapid descents

at low altitude or during landing approaches or other deviations from

prescribed operating procedures are not an acceptable means of

minimizing exposure to icing conditions. (Class I, Urgent Action)

In a letter dated December 2, 1994, the FAA responded positively to

all of the recommendations. The Safety Board evaluated the FAA’s reply and

classified the FAA’s responses to each of the recommendations in a letter dated

January 9, 1995.

With regard to Safety Recommendation A-94-181, the FAA stated that

it agreed with the recommendation and that it had established a special certification

review (SCR) team, comprised of representatives from the FAA and the French

DGAC, to:

conduct a special certification review of the ATR-42 and ATR-72

series airplanes. The team will also require flight tests and/or wind

tunnel tests as necessary to determine control system performance,

particularity in roll of airplanes operating with different airspeeds

and configurations during ice accretion. Included in the review will

be an evaluation of aileron hinge moment characteristics. As a result

of the review, the FAA will require modifications, as necessary, to

ensure satisfactory flying qualities and control system
142

stability in icing conditions. The team is expected to prepare a

formal report by February 1, 1995. On November 16, 1994, the FAA

issued telegraphic airworthiness directive (AD) T94-24-51

applicable to all model ATR-42 and ATR-72 series airplanes. The

AD requires a revision to the FAA-approved airplane flight manual

to prohibit operation of the autopilot in icing conditions when the

airplane is operated in moderate or greater turbulence, or if any

unusual lateral trim situation is observed.

The FAA advised the Safety Board that the certification review team

expected to complete its formal report by February 1, 1995. Based on these

actions, on January 9, 1995, the Board classified A-94-181 "Open--Acceptable

Response," stating that the Board was waiting for completion of the work of the

special certification team and that it looked forward to receiving the results

contained in its formal report.

In a letter dated April 19, 1996, the FAA advised the Safety Board that

it had conducted an SCR of the ATR 42 and ATR 72 airplanes. On September 29,

1995, the team issued its final report, a copy of which was provided to the Safety

Board. Based on its review of the SCR report and the verification of the viability

of the flight operations restrictions imposed on ATR 42 and ATR 72 airplanes, the

Safety Board classifies Safety Recommendation A-94-181 "Closed—Acceptable

Action."

With regard to Safety Recommendations A-94-182 and A-94-184, in a

letter dated December 2, 1994, the FAA outlined several actions that had been

taken since the accident. Those actions included a meeting of ATR operators, FAA

representatives, pilot and industry organizations, and the airframe manufacturer and

the issuance of telegraphic AD T94-24-51 on November 16, 1994, which

prohibited the use of the autopilot on the ATR 42/72 in icing conditions or

moderate or greater turbulence, and specified certain procedures in the event of

unusual trim situations. The actions also included the issuance of Flight Standards

Information Bulletin (FSIB) 94-16, ATR 42 and ATR 72 Operating Procedures in

Icing Conditions, on November 18, 1994, that directed the POIs for ATR operators

to ensure that several actions were accomplished immediately. Those actions

included verification that the procedures in AD T94-24-51 were accomplished, that

an attached list of pilot procedures were immediately distributed to all operators

and flightcrews of ATR airplanes, and that special dispatch procedures for icing

operations were in place.
143

Further, the FAA conducted followup teleconferences to verify that the provisions

of FSIB 94-16 had been implemented, and special surveillance procedures,

including a substantial increase in en route inspections, were implemented to verify

that the revised procedures were in place and being used.

Before the Safety Board had formally responded to the FAA’s actions

relevant to A-94-182 and A-94-184, on December 9, 1994, the FAA issued AD

T94-25-51 applicable to the ATR fleet to prohibit flight into icing conditions. On

January 9, 1995, the Safety Board classified A-94-182 and A-94-184 "Open—

Acceptable Response," pending any corrective actions based on the SCR, as

recommended in A-94-181.

In a letter dated January 18, 1995, the FAA responded further to Safety

Recommendations A-94-182 and A-94-184 stating that on January 11, 1995, it had

issued AD T95-02-051 and FSIB 95-01, ATR 42 and ATR 72 Airworthiness

Directive T95-02—51 Compliance Procedures.

On February 24, 1995, the Safety Board classified A-94-182 and A-94-

184 "Open—Acceptable Action," pending notification from the FAA that

terminating actions (to correct the characteristics that led to the special flight

restrictions on the airplanes) had been taken and that the results of the SCR team

had been published.

Based on the results of the SCR, which was enclosed with a letter from

the FAA dated April 19, 1996, and the verification of the viability of the flight

operations restrictions imposed on the ATR airplanes, the Safety Board classifies

A-94-182 "Closed—Acceptable Action."

With reference to A-94-184, in the April 19, 1996, letter, the FAA

advised the Safety Board that it had issued a supplemental notice of proposed

rulemaking (NPRM) on January 19, 1996, to require revised flightcrew procedures

with respect to flight in large droplet freezing precipitation (freezing drizzle)

conditions, and that these revised procedures for the ATR were identical for all

other affected airplanes. In addition, the FAA stated that it will issue one final

regulatory document incorporating the NPRM and supplemental NPRM. The

Safety Board looks forward to receiving this information. Consequently, A-94-

184 remains classified “Open—Acceptable Action.”
144

With regard to A-94-183, the FAA responded on December 2, 1994,

that it had issued GENOT (general notice) RWA 4/85, dated November 11, 1994,

that directed air traffic personnel to provide priority handling to pilots of ATR 42

and ATR 72 airplanes when they requested route, altitude or airspeed deviations

to avoid icing conditions. The GENOT also advised that air traffic personnel

should be aware that the normal holding airspeeds for the ATR 42 and ATR 72

airplanes have been waived and that, when speeds in excess of 175 knots (as

published in the Aeronautical Information Manual for turbopropeller airplanes)

are used, the airplanes may not remain within the confines of the holding pattern

airspace. In a letter to the FAA, dated January 9, 1995, the Safety Board

classified A-94-183 "Closed—Acceptable Action."

With regard to A-94-185, on December 2, 1994, the FAA advised the

Safety Board that its FSIB 94-16:

cautions pilots that rapid descents at low altitude or during landing

approaches or any deviations from these approved procedures as a

means of minimizing exposure to icing condition should be avoided.

Strict adherence to AD limitations and approved procedures is

required.

In a reply to the FAA dated January 9, 1995, the Safety Board noted:

that the FAA has included in FSIB 94-16 specific precautions to

pilots not to use rapid descents at low altitudes or during instrument

approaches as a means to minimize exposure to icing conditions. It

also urged strict adherence to AD limitations regarding the use of

autopilot and other approved procedures. The Safety Board is aware

that the FAA has taken actions to verify pilot understanding and

compliance by conducting en route inspections and visiting airline

operations. Therefore, the Safety Board classifies A-94-185

"Closed--Acceptable Action."

In addition, as a result of this accident, the Safety Board issued the

following safety recommendations to the FAA on November 6, 1995:
145

A-95-103

Require the Air Traffic Control System Command Center to retain all

flow control-related facility documents for 15 days, regardless of

title, name or form number, for reconstruction purposes. (Class II,

Priority Action)

A-95-104

Develop a list of documents to be completed by the Air Traffic

Control System Command Center personnel in the event of an

incident or accident. (Class II, Priority Action)

A-95-105

Revise Order 8020.11, “Aircraft Accident and Incident Notification,

Investigation and Reporting,” to include the Air Traffic Control

System Command Center (DCC) facility. Ensure that the SCC

facility is assigned specific requirements to be included in an

accident/incident package. (Class II, Priority Action)

A-95-106

Revise FAA Order 7210.3, “Facility Operation and Administration,”

Chapter 3, “Facility Equipment,” Section 4, “Recorders,” paragraph

3-41, “Assignment of Recorder Channels,” to include the Air Traffic

Control System Command Center facility, listing the recorded

positions and their priority. (Class II, Priority Action)

On February 2, 1996, and on May 1, 1996, the FAA responded to the

Safety Board concerning Safety Recommendations A-95-103 through A-95-106.

In its reply to the FAA on June 13, 1996, the Safety Board noted the following:

The Safety Board notes that the FAA developed a list of documents

that will be retained by the DCC facility for 15 days and will be

provided to investigators in the event of an incident or accident.

Therefore, the Safety Board classifies Safety Recommendations A95-103 and -104 "Closed—Acceptable Action."

The Safety Board notes that the FAA reviewed the requirements of

the DCC and issued a general notice that revised Order 8020.11 to
146

include the facility. Therefore, the Safety Board classifies Safety

Recommendation A-95-105 "Closed—Acceptable Action."

The Safety Board notes that the FAA revised Order 7210.3 to

include the DCC positions and their priority. Therefore, the Safety

Board classifies Safety Recommendation A-95-106 "Closed—

Acceptable Action."

1.18.6 Government Accounting Office (GAO) and Department of

Transportation Inspector General (DOT/IG) Investigation of the

Federal Aviation Administration

In September 1993, the General Accounting Office (GAO) published

a report, at the request of Congressman James Oberstar, former Chairman of the

House Subcommittee on Aviation, regarding the adequacy of the FAA's aircraft

certification process and design criteria for transport category aircraft to ensure

that the FAA met all applicable safety standards. The following is an excerpt

from the findings discussed in the GAO's report:73

The FAA has not ensured that its staff is effectively involved in a

certification process that delegates the vast majority of

responsibilities to aircraft manufacturers. Despite the National

Academy of Sciences' recommendation in 1980 that the FAA

develop a more structured role in the certification process, the

agency has increasingly delegated duties to manufacturers without

defining such a role. The report stated that the FAA now delegates

up to 95 percent of the certification activities to manufacturers

without defining (1) critical activities in which FAA staff should be

involved, (2) guidance on the necessary level and quality of the

oversight of designees, and (3) standards to evaluate staff members'

performance. As a result, FAA staff no longer conduct all of such

critical activities as the approval of test plans and analyses of

hypothetical failures of systems. Because FAA has increased

delegation over the last 13 years, its ability to effectively oversee

 

73United States General Accounting Office. Report to the Chairman, Subcommittee on Aviation, Committee on Public

Works and Transportation, House of Representatives, Aircraft Certification, New FAA Approach Needed to Meet

Challenges of Advanced Technology. September 1993. Report GAO/RCED-93-155.
147

and add value to the certification process as well as understand new

technologies has been questioned by internal reviews and FAA and

industry officials.

The GAO found, for example, that between fiscal years 1990 and

1992, only 1 of the 12 FAA engineers responsible for approving aircraft computer

software attended a software-related training course. The GAO said that FAA

officials acknowledged that inadequate training over the last decade had limited

the certification staff's ability to understand areas of dramatic technological

advancement. As a result, the FAA developed a new training program intended to

improve the competence of the staff; however, the program was found to lack the

necessary structure to establish specific training requirements for staff in their

areas of responsibility.

The GAO's report issued recommendations to the Secretary of

Transportation, suggesting that the FAA, "define a minimum effective role for the

agency in the certification process by identifying critical activities requiring the

FAA's involvement or oversight; establish guidance and the necessary level and

quality of the oversight of the designees; and develop measures through which a

staff member's effectiveness can be evaluated." The GAO also recommended that

the FAA formally examine the need to hire experts in areas of technological

advancement, require an expert's involvement early in the certification process

and at other key junctures, establish specific training requirements, and identify

training in new technologies that is available at universities, industry, and other

government agencies.

In addition, the GAO report stated:

...After maintenance and design problems with a McDonnell

Douglas DC-10 aircraft were found to have contributed to an

accident resulting in 273 fatalities, in 1979, the Secretary of

Transportation established a "blue-ribbon" committee to assess the

adequacy of the FAA's certification program. Under the direction

of the National Academy of Sciences, the committee reported in

1980 that the FAA's system of delegation to Designated

Engineering Representatives (DERs) was sound, in part because the
148

FAA reserved most of the critical activities, such as approving all

test proposals, for its own staff.74 The report warned however, that

the FAA's technical competence was falling far behind the DERs to

the point that the agency's oversight was becoming superficial. The

Academy called on the FAA to establish a "higher esprit de corp"

by hiring, retaining, and training highly competent engineers....

...Acknowledging that its staff was falling behind industry in

technical competence, the FAA established a program in 1979 to

increase staff members' knowledge of state-of-the-art technologies.

Under the National Resource Specialist (NRS) Program, the FAA

identified a need for expertise in 23 areas, including crash

dynamics, fuel and landing gear systems, advanced materials,

advanced avionics, and the effects of such environmental factors as

ice. [emphasis added] Experts in the program were to be

responsible for maintaining the highest level of expertise in their

particular specialty and acting as advisers to staff during the

certification process. However, the FAA never fully implemented

the program. Of the 23 positions the FAA identified as critical,

only 11 were authorized. According to the manager of the NRS

program, the FAA intended to authorize all of the positions but did

not do so because it could not attract qualified individuals to fill

them....

For example, according to the certification staff, the FAA has no

one who is maintaining state-of-the-art expertise in the effects of ice

on new airplane designs, as the relevant position in the program has

been vacant since 1987. The effects ice has on different aircraft

designs vary greatly, making it imperative that the FAA have an

expert in this area, [emphasis added] according to the acting

manager of the Propulsion Branch at the Los Angeles Aircraft

Certification Office (AC). Because the position has not been filled

and engineers with some expertise in this area are retiring, the new

staff are falling farther behind in understanding the principles and

effects of ice, he stated....

 

74Improving Aircraft Safety: FAA Certification of Commercial Passenger Aircraft, National Academy of Sciences,

National Research Council, Committee on FAA Airworthiness Certification Procedures (June 1980).
149

Comments from the FAA's Aircraft Certification Service Director to

the GAO indicated that it was the FAA's belief that the staff was not "falling

behind in understanding the principles and effects of ice." The Director also

stated that the FAA had recently issued regulations governing an airline's ground

operations during icing conditions. As a result, the GAO confirmed with the

acting manager of the Propulsion Branch at the Los Angeles AC office the

accuracy of the point made in the draft report. Although acknowledging that new

regulations governing airline operations had been issued by FAA headquarters,

the acting manager stated that new certification staff were falling behind in

understanding the principles and effects of ice on aircraft designs because the

FAA had not hired an NRS on icing to assist staff in understanding those

principles and effects.

The GAO recommended that the Secretary of Transportation direct

the Administrator of the FAA to formally examine the need to hire NRSs in areas

of technological advancement over the last 14 years and to require NRS

involvement early in the certification process and at other key certification

junctures.

The DOT responded that the FAA does not need to formally examine

the need to hire experts in areas of technological advancement because the FAA

periodically assesses the NRS Program. However, the GAO report details

examples provided by NRS and FAA staff in which the FAA staff has fallen

farther behind in some areas because the FAA has not fully staffed the program.

In addition, three members of the National Academy of Sciences' committee

stated in 1980 that the NRS program has been an inadequate response to the

Academy's call for greater competence by the FAA in the certification process, in

part because it has been understaffed.

On April 15, 1994, the Office of the Inspector General (IG) of the

Department of Transportation published a report entitled the Federal Aviation

Administration, Responsiveness to Suspected Aircraft Maintenance and Design

Problems. The report stated, in part:

...The Office of the Inspector General (OIG), Department of

Transportation, conducted an inspection of the Federal Aviation

Administration's ability to identify and respond to suspected aircraft

maintenance and design problems. This inspection was initiated in
150

response to growing concerns about the FAA's ability to correct

suspected aircraft problems--particularly after the October 1992

El Al Airlines crash in the Netherlands. During the inspection, we

contacted 89 representatives from FAA, the National Transportation

Safety Board, aircraft manufacturers, and aircraft operators....

The report also stated that:

Our review concludes that the FAA's ability to identify, evaluate,

and correct suspected aircraft maintenance and design problems is

hampered by inadequate oversight of the FAA's engineers' activities

and decisions, and insufficient analysis capability. This conclusion

applies primarily to the FAA's Transport Airplane Directorate

(TAD)...Specifically, TAD's ability to identify and respond to

suspected aircraft maintenance and design problems is hampered by

inadequate oversight because no normal system exists to ensure

aircraft problems do not fall into a "black hole," and no adequate

documentation, tracking and reporting archival and research

mechanism exists to enable the FAA to recall incidents, other than

engineer's memories...TAD makes limited use of, and has no

specific requirement for trend analysis.75

The Inspector General recommended that the TAD develop and

implement a formal tracking system to ensure adequate accountability and timely

resolution of reported aircraft maintenance and design problems. The FAA did

not concur. Additionally, it was recommended that the TAD develop and

implement standard procedures for documenting research of suspected aircraft

problems. Again the FAA did not concur and stated that the current systems and

procedures meet the intent of a "formal" tracking system.

1.18.7 Bilateral Airworthiness Agreement

The ATR 42/72 was type certificated in the United States under an

agreement between the United States and France, enacted in 1973. The Bilateral

Airworthiness Agreement (BAA) is an "enabling" document that is less formal

 

75Office of Inspections and Evaluations, Office of the Inspector General, U.S. Department of Transportation. Report

on Federal Aviation Administration, Responsiveness to Suspected Aircraft Maintenance and Design Problems, April

15, 1994. Report E5-FA-4-009.
151

than an international treaty, and is executed between Chiefs of State without

senatorialapproval. Typically, the BAA with the United States develops when a

foreign country has manufactured "civil aeronautics products" it intends to export

to the United States and has a competent civil airworthiness authority. Since the

agreements are technically oriented and are not trade agreements, they are

intended to prevent unnecessary repetitive certification activities by facilitating

cooperation and acceptance of findings between the exporting country's

airworthiness authority and the FAA.

In addition to certification-related responsibilities, the agreement

states:

...The aeronautical authorities of each Contracting Party shall keep

the aeronautical authorities of the Other Contracting Party fully

informed of all mandatory airworthiness modifications and special

inspections which they determine are necessary in respect of

imported or exported products to which this agreement applies.

...The aeronautical authorities of the exporting State shall, in respect

of the products produced in that State,...assist the aeronautical

authorities of the importing State in determining whether major

design changes and major repairs made under the jurisdiction of the

importing State comply with the laws, regulations and requirements

under which the product was originally certificated and approved.

They shall also assist the aeronautical authorities of the importing

State in analyzing those major incidents occurring on products to

which this Agreement applies and which are such as would raise

technical questions regarding the airworthiness of such products....

The FAA, on behalf of the U.S. State Department, must evaluate the

technical competence, capabilities, regulatory authority and efficacy of the foreign

country's airworthiness authority. Further, the FAA assesses the foreign country's

laws and regulations, and the state-of-the-art design and manufacturing capability.

The FAA Team Leader for the ATR Special Certification Review

testified at the Safety Board's public hearing about the certification process for the

ATR 42/72. The following is a brief description of testimony provided by the

team leader regarding the ATR certification process by both the FAA and DGAC:
152

...Under that bilateral, each of the participants have some rather

well-defined roles...the DGAC has the certification authority and

the FAA has the validating authority.

In addition to determining the certification basis, another major role

at this stage in the process is the development of policy and

guidance for the benefit of both ATR and DGAC...this was done

largely by means of issue papers...There were a large number of

issue papers on the ATR-42...actually 98 issue papers. Issue papers

are used as a tool to transfer previous experience that we may have

had on other programs...things that other manufacturers may have

had some difficulty with....

The DGAC applies our regulations...our policy...and any guidance

given to them along the way...The 'flight manual,' the official

document that is part of the type design of the airplane...we do not

approve that document the DGAC does on our behalf. However,

we review it thoroughly and make changes as necessary...and only

when we're satisfied with the contents of the AFM do we then

authorize the DGAC to sign it on our behalf.

...the validation of data...in general, we rely on the guidance that we

have given the DGAC in specific cases...if there is an area of

misunderstanding or disagreement, that's where this issue paper

process comes in...flight testing is a definite part of each bilateral

approval. However, the flight testing is not really an

evaluation...the idea behind the FAA pilot flying the airplane

is...first, it's familiarity with the airplane so he can fulfill his duties

later throughout the life of the airplane. Also, it's to determine the

suitability for use in airline service. The AEG also participates in

this evaluation and typically it's a fairly short involvement for the

flight test. Typically it's roughly ten hours of [total] flying...four

flights...usually one at night to check the lighting....

[Regarding an evaluation of the aircraft in icing conditions] we do

not specifically go out and seek icing conditions during the flight

evaluation....
153

Before the U.S. airworthiness certificate can be issued, the FAA must

determine that the aircraft conforms to the applicable U.S. airworthiness

requirements, which, in the case of the ATR 42 and 72, is 14 CFR Part 25. Under

the BAA and by Federal regulation, a foreign-built aircraft is entitled to a U.S.

type certificate if the exporting State certifies, and the FAA finds, that the aircraft

does conform to the type design and appropriate certification requirements. The

FAA can make a determination based in whole, or in part, on the exporting State's

certification, provided a BAA exists. Also, under the bilateral agreement, the

FAA does not have to conduct any flight testing of the airplane prior to the

issuance of the U.S. airworthiness certificate.

On March 4, 1987, and May 8, 1987, the Safety Board conducted

investigations of two Construcciones Aeronauticas, S.A. (CASA) 212 airplanes,

involved in accidents at the Detroit Metropolitan Wayne County Airport,

Romulus Michigan, and the Mayaguez Airport, Mayaguez, Puerto Rico,

respectively.76 The investigation of both accidents revealed that the FAA's

certification of the CASA 212 under the Bilateral Airworthiness Agreement was

deficient. One of the Safety Board's conclusions in both accidents stated that,

"The bilateral type certification project of the CASA C-212 was not managed

effectively by the FAA. The reorganizational changes, personnel changes, and the

limited availability of resources in the engineering and operations departments of

the FAA are contributing factors." Based on these investigations, the Safety

Board issued the following recommendation to the FAA:

A-88-100

Complete as soon as possible and make findings available to the

Safety Board the report on the in-house review of the bilateral

aircraft type certification program and corrective actions taken or

contemplated as a result of the review.

On January 1, 1990, the Safety Board classified recommendation A88-100 as "Closed--Acceptable Action" after the FAA conducted a review of both

the CASA 212 certification process and the BAA. The FAA subsequently

produced a report entitled, Review of the Construcciones Aeronauticas, S.A.

 

76NTSB Aircraft Accident Report--"Fischer Bros. Aviation, Inc., dba Northwest Airlink, Flight 2268, (CASA) C-212-

CC, N160FB, Detroit Metropolitan Wayne County Airport, Romulus, Michigan, March 4, 1987" (NTSB/AAR-88/08)

and NTSB Aircraft Accident Report "Executive Air Charter, Inc., d.b.a. American Eagle Flight 5452, CASA C-212,

N432CA, Mayaguez, Puerto Rico, May 8, 1987." (NTSB/AAR-88/07)
154

CASA 212 Certification Program and the U.S. Import Type Certification Process.

The intent of this report was to evaluate the working relationships and the

implementation of the BAA procedures, and to identify areas where improvements

could be made to accomplish the objectives of the aircraft certification program

regarding imported products. The FAA published the report in March 1988, and,

in addition to the review of the type certification of the CASA 212, the FAA's

performance regarding the BAA procedures for type certification and how

airworthiness issues were resolved after certification were also examined. The

FAA review team believed that the findings "...can be applied across the

directorate system and should be incorporated as such." These

findings/conclusions resulted in 17 recommendations to FAA management as a

"start toward achieving that quality improvement." The recommendations

included the subject of training of FAA personnel about product certification

under BAA procedures, and the development of documentation to standardize the

directorate organization, procedures, responsibilities, and functions of those

organizations.

One of the issues discussed in the report pertained to the "Follow-On

Type Certification" which was described in terms of the FAA questioning the

foreign airworthiness authorities and manufacturer's compliance with the U.S.

interpretation of FAR 25 (certification basis). The review team concluded:

The follow-on certification issues were not performed efficiently

and effectively. This can be attributed to several factors. First,

there was a lack of continuity of staffing....The other major factor,

which is a result of the first, was a lack of accountability. No

apparent tracking or management control mechanisms were in place

to assure that the issues were being handled in a manner and time

period appropriate to their safety implications.

A second issue discussed in the report was the "Present Import Type

Certification System - Seattle." The review team found that two mistakes can be

made in a certification project: 1) certification of a product that does not meet

14 CFR Part 25 standards, and 2) disparate treatment of applicants. The team

concluded:

The organization, defined roles, and established procedures within

the Transport Airplane Certification Directorate have gone a long

way toward ensuring standardization of import certification

programs. However, the high level of management control appears
155

to be preventing timely decisions and publication of policy material.

Issue papers are being used as a means of documentation and

standardization. This goes beyond the original scope intended for

issue papers. Certification engineers seem unfamiliar with the

bilateral concept and unsure of the depth of involvement required

for certification. To further complicate this, there seems to be a

lack of resources to do the task at hand....The Standardization

Branch seems to have reached a critical stage in coping with the

increasing European workload and complexity of foreignmanufactured airplanes.

The report cited, in the discussion about the "Present Import Type

Certification System - Washington," that "some concern was expressed about the

many BAA's having different language and scope and, in many cases, being

obsolete in dealing with today's environment of increased unilateral certification

programs." The team concluded that "there is a need to review all BAA's for

consistency in language and scope and for currency."

1.18.8 Federal Regulations for Flight Operations in Icing Conditions

There are several Federal Aviation Regulations that either impose

limitations on the operation of aircraft in icing conditions, or provide guidance to

pilots when operating conditions are conducive to icing. Both 14 CFR Part

91.527(b) and 135.227(b), state:

Except for an airplane that has ice protection provisions that meet

section 34 of Appendix A, or those for transport category airplane

type certification, no pilot may fly--

(1) Under IFR into known or forecast light or moderate icing

conditions; or

(2) Under VFR into known light or moderate icing conditions;

unless the aircraft has functioning deicing or anti-icing

equipment protecting each propeller, windshield, wing

stabilizing or control surface....
156

In addition, 14 CFR Part 91.527(c) and 135.227(d) state:

Except for an airplane that has ice protection provisions that meet

section 34 of Appendix A, or those for transport category airplane

type certification, no pilot may fly an aircraft into known or forecast

severe icing conditions.

As for those aircraft being operated under 14 CFR Part 121,

paragraph 121.629, states:

No person may dispatch or release an aircraft, continue to operate

an aircraft en route, or land an aircraft when in the opinion of the

pilot in command or aircraft dispatcher (domestic and flag carriers

only) icing conditions are expected or met that might adversely

affect the safety of flight.

1.18.9 New Technology

1.18.9.1 Stall Protection System

Stall protection systems on transport-category airplanes typically use

a fuselage mounted angle-of-attack (AOA) sensor. Such sensors cannot detect

airflow separations on an airfoil, therefore their usefulness for stall protection

systems is limited to airfoil configurations where aerodynamic characteristics are

known. Since small amounts of contamination, such as ice on the surface of an

airfoil, can significantly alter the aerodynamic characteristics of an airfoil, typical

stall warning devices do not account for the presence and effect of contaminants.

The Safety Board has investigated several aircraft accidents in which airframe ice

and/or snow contamination was found to have been a contributing factor.

There is new technology available that can detect airflow separation

on aerodynamic surfaces. One new system measures the pressure in the airflow

above the upper wing surface with a probe located at about 70 percent chord

(varies by airplane), inboard of the ailerons. The system has been shown to

effectively detect upper wing surface turbulence associated with airflow

separation, both in flight and during the takeoff roll, once the airplane has
157

accelerated to at least 50 knots. According to a Society of Automotive Engineers

(SAE) technical paper,77 developmental testing found that:

Conventional stall warning systems, which use a fuselage mounted

AOA sensor, do not measure the actual stalling condition at the

wing. The key to determining an early stall due to the presence of

contamination is to measure the flow directly at the lifting surface.

Local velocity changes in a region above the upper surface of the

wing provide a consistent indication of an approaching

aerodynamic stall even when contamination is present. This

method of stall warning also offers new levels of safety during low

level windshear recovery and takeoff performance monitoring.

ATR had established a test program that evaluated the effectiveness

of this device before this accident. The system was tested on an ATR 72 in

January 1994; and it was tested on ATR-42-500 in December 1994. In addition to

ATR airplanes, tests have been conducted with the detection device mounted on a

Cessna 421 (cabin-class piston twin), a NASA Sabreliner (business jet), and a

Fokker 100 (passenger jet). Additionally, wind tunnel tests were conducted with

various amounts of surface roughness and ice shapes on various airfoil designs.

A second new type of airflow separation detection system measures

the change in sound (amplitude and frequency) of the airflow over the surface of

an airfoil. This system had not been flight tested before this accident, but

subsequent wind tunnel tests at the NASA Lewis Research Center showed

consistent reliability in the detection of airflow separation.

 

77SAE Technical Paper 922010, Stall Warning Using Contamination Detection Aerodynamics, by Paul Catlin, B.F.

Goodrich Aerospace Avionics Systems, Presented at Aerotech '92, October 1992.
158

2. ANALYSIS

2.1 General

The flightcrew was properly certificated, and each crewmember had

received the training and off-duty time prescribed by the Federal regulations. There

was no evidence of any preexisting medical condition that might have affected the

flightcrew's performance.

The air traffic controllers involved with flight 4184 were properly

certificated and provided the required services to the flightcrew. The performance of

the FAA's air traffic management and weather dissemination systems is discussed

later in this report.

The airplane was certificated, equipped and maintained in accordance

with Federal regulations and approved procedures. There was no evidence of

preexisting mechanical malfunctions or other failures of the airplane structure, flight

control systems, powerplants or propellers that would have contributed to the

accident.

The evidence revealed that the crew of flight 4184 experienced a sudden

autopilot disconnect, uncommanded aileron deflection, and rapid roll of the airplane

consistent with airflow separation near the ailerons caused by a ridge of ice that

formed aft of the deice boots, on the upper surface of the wing.

The accident was unsurvivable, and the catastrophic impact and

destruction of the airplane precluded a complete inventory of components. However,

all major structural pieces were recovered and examined. Based on the ground scars,

distribution of the wreckage, damage to the horizontal stabilizer, elevators, outboard

wing sections and the ailerons, FDR data and sounds recorded on the CVR, the

Safety Board concludes that the outboard portion of both wings and the horizontal

tail separated in flight, in close proximity to the ground. The structural separation

was due to excessive aerodynamic loads.

After summarizing the accident sequence, this analysis addresses the

meteorological conditions that existed in the area of the LUCIT intersection at the

time of the accident, the provision of weather data to the flightcrew, icing definitions,

methods of forecasting icing, ATR 72 flight characteristics with ice accretions, the

FAA's certification of the ATR 42 and 72, DGAC and FAA
159

oversight of the continuing airworthiness of ATR airplanes, the Bilateral

Airworthiness Agreement between France and the United States, Air Traffic Control

policy and practices, the flightcrew's actions during the flight, unusual event/attitude

recovery, and the management structure of Simmons Airlines/AMR Eagle and its

oversight by the FAA.

2.2 Summary of Accident Sequence

FDR data revealed that at 1517, while the airplane was descending to

10,000 feet, the flightcrew activated the anti-icing/deicing system to Level III, an

action that is required whenever the airplane is accreting ice. At that time the

propeller speed was set at 86 percent of maximum RPM, which is also a requirement

for flight in actual or potential icing conditions (total air temperature less than +7

degrees C in the presence of visible moisture). At 1523, just prior to the airplane

entering the holding pattern at LUCIT, the Level III anti-icing/deicing system was

deactivated. At 1525, as the airplane was entering the holding pattern, the propeller

speed was reduced to 77 percent. According to AMR Eagle procedures, this action is

consistent with the reduction of anti-icing/deicing systems to Level I, which is

appropriate only for flight outside of actual or potential icing conditions.

The FDR indicated that at 1540, the Level III ice protection system was

activated and the propeller speed was increased to 86 percent. However, FDR data

also revealed that subsequently on two occasions during the holding pattern

preceding the initial upset, there was evidence of small drag increases that were

probably the result of ice accretions on the airplane. The first drag increase occurred

at approximately 1533 (about 24 minutes before the upset78) just before the flaps

were extended to 15 degrees. The second increase was evident at about 1551 (6

minutes before the upset). It is likely that the airplane intermittently encountered

areas of large supercooled drizzle/rain drops while it was holding which contributed

to the formation of a ridge of ice on the upper surface of the wing, aft of the wing

deice boots, in front of the ailerons.

The crew received a clearance to descend to 8,000 feet. At 1557:23, as

they were descending at 185 KIAS, the CVR recorded the activation of the aural flap

overspeed warning. The flightcrew retracted the flaps in response to the

 

78The total time from the start of the hold to the upset was about 39 minutes.
160

warning. The FDR data indicate that as the flaps retracted, the autopilot increased

the pitch attitude to maintain a preset vertical speed for the descent.

As the airplane pitched nose up and the AOA increased through 5

degrees, the airflow in the area of the right aileron began to separate from the wing

upper surface because of the ice ridge. As the AOA continued to increase, the

airflow separation in the area of the right aileron also increased, causing a reversal of

the right aileron hinge moment characteristics. Although the right aileron hinge

moment reversal caused the ailerons to deflect rapidly to a right-wing-down (RWD)

position, the AOA was not sufficient to activate the stall warning system prior to the

aileron deflection. The autopilot could not control the aileron deflection rate, which

exceeded that allowed by the autopilot so the autopilot disconnected.

Within 0.25 seconds of the autopilot disconnection, the ailerons fully

deflected to the RWD position79 and the airplane rolled rapidly to the right until

reaching 77 degrees RWD. An immediate nose-down elevator deflection reduced the

AOA; and the ailerons were deflected LWD by the flightcrew to counter the right

roll. The airplane began to roll back towards a wings-level-attitude. The crew then

applied 2 to 3 degrees of left rudder and nose-up elevator. The flightcrew's aileron

and rudder control inputs reduced the bank angle to 55 degrees RWD. However, as

the AOA increased to more than 5 degrees, the airflow over the right aileron

separated again, resulting in a second aileron hinge moment reversal and rapid RWD

aileron deflection.

The airplane rolled again to the right for 9 seconds, rolling

approximately 1 and 1/4 times. During this roll, elevator position increased to 8

degrees nose-up, the pitch trim remained constant, the airspeed increased to more

than 250 KIAS, the vertical acceleration increased to more than 2 G, the AOA

remained greater than 5 degrees, and the aileron position remained RWD and

oscillatory. The nose-up elevator and constant pitch trim resulted in the AOA

remaining above the airflow separation AOA of 5 degrees during this 9 second

period. The reduction of nose-up elevator deflection at the end of this 9 second

period resulted in the AOA decreasing. As the AOA decreased through 5 degrees,

the airflow over the right aileron reattached, allowing the flightcrew to regain aileron

control. The ailerons momentarily deflected to 6 degrees LWD and then stabilized

close to the neutral position. The airplane immediately began rolling in the LWD

 

79Both the Safety Board and ATR analyses indicate that neither the autopilot, the roll spoiler system, nor any other

airplane system were capable of generating this rapid aileron deflection.
161

direction, back towards a wings level attitude, with nearly neutral aileron position

and 2 degrees nose-left rudder. At this point, the airplane was descending through

approximately 6,000 feet, at a rate of about 400 feet per second (24,000 feet per

minute).

As the airplane rolled toward wings level, the normal acceleration

increased to 2 G, and the pitch attitude stopped decreasing at 73 degrees nose down.

The flightcrew accelerated the pullout with a 2 to 3 degree nose-up elevator

deflection. As the airplane descended through 3,700 feet, the airspeed increased

through 327 KIAS, the pitch attitude increased through 60 degrees nose down, the

roll angle decreased through 50 degrees RWD, the vertical acceleration increased

through 2.3 G, and the captain made the statement "nice and easy." About 3 seconds

later, as the airplane rolled through wings level, and the normal acceleration

increased to more than 3 G, the GPWS began sounding its "TERRAIN TERRAIN"

warnings. Approximately 1.7 seconds later, as the altitude decreased through 1,700

feet, the first officer made an expletive comment, the elevator position and vertical

acceleration began to increase rapidly (to more than 3.7 G), and the CVR recorded a

loud crunching sound. The CVR and FDR data end 0.5 second later. Analysis of the

airplane wreckage indicates that the outboard 10 feet of the left and right wings, as

well as the horizontal stabilizer, separated from the airframe at a very low altitude.

The first officer's expletive comment occurred when the airplane was

descending through 1,700 feet, which was most likely just after the airplane

descended through the base of the clouds (the clouds were broken at about

2,100 feet). The Safety Board concludes that both pilots saw the ground, realized

their close proximity, nose-down attitude, and high descent rate, and made an

additional nose-up elevator input. This elevator input combined with the high

airspeed (about 115 KIAS over the certified maximum operating airspeed) resulted in

excessive wing loading and the structural failure of the outboard sections of the

wings.

2.3 Meteorological Factors

2.3.1 General

Based on the analysis of all available data, reports from pilots and

evaluations by several atmospheric scientists and researchers, the Safety Board

concludes that flight 4184 encountered a mixture of rime and clear airframe icing in
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supercooled cloud and drizzle/rain drops, while in the holding pattern at the LUCIT

intersection. The supercooled drops in the area were estimated to be greater than 100

microns in diameter, with some as large as 2,000 microns. The liquid water content

(LWC) was estimated to have varied from less than 0.1 to nearly 1.0 gram per cubic

meter. The ambient air temperature in the area of the holding pattern (10,000 feet)

was about minus 3 degrees C, with the freezing level between 7,000 and 8,000 feet,

and the cloud tops between 19,000 and 30,000 feet. In addition, there were ice

crystals present in the atmosphere along the flightpath traversed by flight 4184.

The LWC estimates were predicated on the maximum and minimum

reflectivity values from the WSR-88D Doppler weather radar located at Romeoville,

Illinois (KLOT), the upper air data for the area of the LUCIT intersection, and

calculations performed using the Safety Board's ICE4A computer program and a

mathematical equation developed by the U.S. Air Force.

The drop sizes in the area of the accident were estimated using the

WSR-88D radar data. PIREPs support the existence of large drops in the area east of

the accident site. The captain of one airplane stated that there was a mixture of rain

with snow at 1610:52, when they reported to ATC, "well we're in and out of some

pretty heavy rain with some sleet in it, started about fourteen thousand feet and it's

continuing still." At the time of this report, as estimated from data recorded by the

WSR-88D, there was a weak weather echo that included an area of precipitation with

an estimated LWC of 0.1 gram per cubic meter. Based on this LWC and radar

reflectivity, the drop sizes were likely as great as 2,000 microns. This is consistent

with the report of "rain" by the captain.

The existence of large supercooled drops in the accident area is also

supported by the distribution of radiative temperatures measured by the GOES 8

satellite, which included temperatures that were greater than minus 18 degrees C.

Such temperatures are conducive to the presence of large supercooled drops. Also,

the GOES 8 satellite imagery indicated the presence of rolling wave cloud features,

which are indicative of windshear. The presence of windshear conditions can, and

often does, result in the broadening of the drop size distributions to include large

drops.
163

2.3.2 Provision of Weather Information to the Crew of Flight 4184

AIRMET "Zulu," Update 3, issued at 1445, by the National Aviation

Weather Advisory Unit (NAWAU), Kansas City, forecast "light to moderate icing in

clouds and in precipitation" for the large area covered by the AIRMET. The forecast

was consistent with PIREPs for the area. However, the icing and turbulence

information strip issued about 1530 by the Center Weather Service Unit (CWSU)

meteorologist at the Chicago ARTCC, which was pertinent to a smaller, more

defined area around northwest Indiana (including the accident site), only indicated

the possibility of light rime/mixed icing in clouds at or below 18,000 feet. The

CWSU meteorologist stated that he did not forecast moderate icing for this area

because there were no applicable PIREPs to indicate the existence of such conditions

in that particular area.

The Safety Board concludes that the forecasts produced by the National

Weather Service (NWS) were substantially correct based on the available

information, and the actions of the forecasters at the NAWAU and the CWSU

meteorologists at the Chicago ARTCC were in accordance with NWS guidelines and

procedures. Further, based on information provided by the controllers after the

accident, it appears that the Chicago ARTCC controllers were only aware of the

CWSU forecast of light icing, and not the NAWAU forecast of light to moderate

icing, as noted in the updated AIRMET.

However, the Safety Board does have some concerns about the lack of

weather information disseminated to the crew of flight 4184. Specifically, the

information contained in AIRMETs "Zulu," "Sierra" and "Tango," and Update 2, was

available well in advance of flight 4184's departure, and was pertinent to flight

4184's route of flight. This information was not, and typically would not be,

included in the weather portion of the flight release provided by Simmons

Airlines/AMR Eagle. Further, it could not be determined if the flightcrew had

obtained the updated weather information via the HIWAS while en route or prior to

the recorded conversations on the CVR.

14 CFR Part 121.601 (b) and (c) state, in part, respectively, "...before

beginning a flight the aircraft dispatcher shall provide the pilot in command with all

available weather reports and forecasts of weather phenomena that may affect the

safety of flight...," and that during a flight the dispatcher shall provide "any

additional available information of meteorological conditions including adverse

weather phenomena." FAA Order 8400.10, paragraph 1423, requires that AIRMET
164

information be considered in the preflight planning process; however, Center

Weather Advisories (CWAs) are not required to be included or considered.

Simmons Airlines dispatchers review the AIRMETs, but they do not typically

include them in the flight release package. CWAs are not included in the release

packages because they are not required. The Safety Board is concerned that because

Simmons Airlines dispatchers do not include AIRMETs (which include information

regarding moderate icing) and CWA information, flightcrews may not be provided

"...all available weather reports and forecasts of weather phenomena...." necessary to

make informed decisions.

Although the Safety Board concludes that the actions of the crew of

flight 4184 (see section 2.9 of this report) would not have been significantly different

even if they had received the AIRMETs, the Safety Board nonetheless believes that

Simmons Airlines/AMR Eagle should require its dispatchers to include in the flight

release AIRMETs and CWAs that are pertinent to the route of flight so that

flightcrews can consider this information in their preflight and in-flight decisions.

Further, the Safety Board believes that the FAA should direct its POIs to ensure that

all air carriers require their dispatchers to provide pertinent information, including

AIRMETs and CWAs, to flightcrews for preflight and in-flight planning purposes.

With regard to the availability of in-flight weather information, the

Safety Board notes that the HIWAS broadcast generated by the Kankakee AFSS

included all of the icing information contained in AIRMET "Zulu," Update 3.

Although the HIWAS broadcast generated by the Terre Haute AFSS indicated that

icing was forecast above the freezing level, it did not indicate the icing levels, the

intensity and type of icing, or the existence of icing conditions in clouds and

precipitation included in AIRMET "Zulu," Update 3. The Safety Board understands

that the HIWAS broadcasts are intended to provide hazardous weather information in

a short format that will facilitate the pilot's understanding of the potentially

hazardous conditions. However, the Safety Board concludes that safety would be

enhanced if the information were presented more consistently among HIWAS

stations and if those broadcasts included all of the information pertinent to the safety

of flight, such as the altitudes of the icing conditions, the intensity and type of icing,

and the location of the actual or expected icing conditions (i.e. in clouds and

precipitation). The Safety Board believes that the FAA should require that HIWAS

broadcasts consistently include all pertinent information contained in weather reports

and forecasts, including In-Flight Weather Advisories (AIRMETs, SIGMETs, and

Center Weather Advisories). The Safety Board also believes that
165

the FAA and air carriers should reemphasize to pilots that HIWAS is a source of

timely weather information and should be used whenever aircraft are operating in or

near areas of potentially hazardous weather conditions.

2.3.3 Icing Definitions

The AIM sets forth the following icing definition:

1) Trace - Ice becomes perceptible. Rate of accumulation is

slightly greater than the rate of sublimation. It is not

hazardous even though deicing/anti-icing equipment is not

utilized unless encountered for an extended period of time

(over 1 hour);

2) Light - The rate of accumulation may create a problem if flight

is prolonged in this environment (over 1 hour). Occasional

use of deice/anti-icing equipment removes/prevents

accumulation. It does not present a problem if the

deicing/anti-icing equipment is used;

3) Moderate - The rate of accumulation is such that even short

encounters become potentially hazardous and use of

deicing/anti-icing equipment or flight diversion is necessary;

4) Severe - The rate of accumulation is such that deicing/antiicing equipment fails to reduce or control the hazard.

Immediate flight diversion is necessary.

While these icing severity definitions provide some basis for assessing

ice accumulation in PIREPs, they are subjective and are of limited use to pilots of

different aircraft types. For example, using these definitions, "light" icing for a

Boeing 727 could be "severe" icing for an ATR 72 or a Piper Malibu. The icing

report provided by the captain of the A-320 Airbus that was holding at the HALIE

intersection, near Roselawn, indicated that he observed about 1 inch of ice

accumulate rapidly on his aircraft's icing probe. The captain provided a PIREP to

ATC and reported the icing as "light rime." He stated in an interview after the

accident that the anti-ice equipment on the airplane "handled the icing adequately,"

and he believed the icing intensity to have been "light to moderate."
166

The Safety Board concludes that icing reports based on the current icing

severity definitions may often be misleading to pilots, especially to pilots of aircraft

that may be more vulnerable to the effects of icing conditions than other aircraft.

The Safety Board believes that the FAA should develop new aircraft icing intensity

reporting criteria that are not subjective and are related to specific types of aircraft.

In addition, the investigation revealed a problem with the aviation

community's general understanding of the phrase "icing in precipitation," which is

used by the NWS but is not defined in any aeronautical publications, including

advisory circulars (ACs), Part 1 of the Federal Aviation Regulations or the

Aeronautical Information Manual (AIM). This phrase is often contained in in-flight

weather advisories; however, it does not typically specify types of precipitation.

According to the NWS, this phrase is intended to include freezing drizzle and

freezing rain. The Safety Board concludes that defining “icing in precipitation” in

such publications would make pilots and dispatchers more aware of the types of

precipitation and icing conditions that are implied by this phrase. Therefore, the

Safety Board believes that the FAA should provide a definition of the phrase "icing

in precipitation" in the appropriate aeronautical publications.

Further, the Safety Board believes that the FAA should require all

principal operations inspectors (POIs) of 14 CFR Part 121 and 135 operators to

ensure that training programs include information about all icing conditions,

including flight into freezing drizzle/freezing rain conditions.

2.3.4 Methods of Forecasting Icing Conditions

The current methods of forecasting icing conditions are of limited value

because they typically cover very large geographic areas and do not provide specific

information about LWC or water drop sizes. Present forecast techniques use only

relative humidity and temperature. According to the scientist from NCAR who

testified at the Safety Board’s public hearing, it is not possible to infer the severity of

icing using only temperature and humidity. The severity of the icing also depends on

the LWC and the size of the water droplets, information which is not currently

identified and forecasted.

A current state-of-the-art atmospheric model was employed by NCAR in

an attempt to determine if the icing conditions that are presumed to have been

present in the accident area could have been forecast accurately. The atmospheric
167

modeling did not generate a forecast of freezing rain or freezing drizzle for the area

of the LUCIT intersection. The scientist from NCAR testified that "...models aren't

perfect, forecasts aren't perfect... even though it's the current state-of-the-art of

atmospheric modeling."

There were no (and still are not any) reliable methods for flightcrews to

differentiate, in flight, between water drop sizes that are outside the 14 CFR Part 25,

Appendix C, icing envelope and those within the envelope. Further, although side

window icing was recognized as an indicator of ice accretions from freezing drizzle

during flight tests of an ATR 72 after the accident, the crew of flight 4184 could not

have been expected to know this visual cue because its significance was unknown to

the ATR pilot community at the time. Moreover, in-service ATR incidents and pilot

reports have shown that side window icing does not always accompany ice

accretions aft of the deice boots, which ATR has stated only occurs in freezing

drizzle and/or freezing rain.

The Safety Board acknowledges the efforts of atmospheric research in

the meteorological community and hopes that its important findings will eventually

provide the aviation industry with a better understanding of the freezing drizzle/rain

phenomenon. The Safety Board concludes that the continued development of

equipment to measure and monitor the atmosphere (i.e., atmospheric profilers, use of

the WSR-88D and Terminal Doppler weather radars, multispectral satellite data,

aircraft-transmitted atmospheric reports, and sophisticated mesoscale models), and

the development of computer algorithms, such as those contained in the FAA's

Advanced Weather Products Generator (AWPG) program to provide comprehensive

aviation weather warnings, could permit forecasters to refine the data sufficiently to

produce more accurate icing forecasts and real-time warnings. Therefore, the Safety

Board believes that the FAA should continue to sponsor the development of methods

to produce weather forecasts that define very specific locations of potentially

hazardous atmospheric icing conditions (including freezing drizzle and freezing rain)

and to produce short-range forecasts ("nowcasts") that identify icing conditions for a

specific geographic area with a valid time of 2 hours or less.

2.4 ATR Flight Characteristics in Icing Conditions

As discussed previously, the evaluation of meteorological data indicates

that the range of water droplet sizes at Roselawn probably varied from cloud drops of

less than 50 microns to drops as large as 2,000 microns. The 100 to 140 micron

MVD drop sizes in the December 1994 ATR icing tanker tests resulted
168

in ice ridges just aft of the active portion of the deice boots and subsequent autopilot

and aileron behavior comparable to that noted in the FDR data for the accident.

Control wheel force data from the icing tanker tests and the subsequent flight tests

with artificial ice shapes indicated that the freezing drizzle ice shapes caused trailing

edge flow separation and subsequent aileron hinge moment reversals. Therefore, the

Safety Board concludes that the ATR 42 and 72 can experience ice-induced aileron

hinge moment reversals, autopilot disconnects, and rapid, uncommanded rolls if they

are operated in near-freezing temperatures and water droplet MVDs typical of

freezing drizzle.

The freezing drizzle encounters in the December 1994 ATR icing tanker

tests resulted in ice ridge accretions aft of the deice boots in both the flaps 0 and

flaps 15 configurations. However, the tanker test results showed that at flaps 15,

there was no pronounced ice ridge on the lower wing surface as there was when the

ice accreted at flaps 0. Further, there was a much smaller drag increase when the ice

accreted at flaps 15 than there was when the ice accreted at flaps 0. Based on the

small drag increases apparent in the data from flight 4184, it is apparent that the ice

ridge that formed during the accident flight developed and grew primarily after the

flaps were extended to 15 degrees.

Also, the ridge of ice that formed in the tanker tests and in the

NASA-Lewis icing tunnel tests tended to shed pieces randomly along the span of the

wing, resulting in broken, jagged ridges. Although these tests only involved

exposing a small portion of the outboard section (including the aileron) of one wing

to freezing drizzle, it is likely that the random nature of the partial ice shedding

would result in airflow asymmetry over the left and right ailerons in a natural

encounter of the airplane with freezing drizzle. Such asymmetry could cause an

aileron hinge moment reversal.

Because the ailerons on the ATR 72 are not hydraulically actuated, a

pilot would have to overcome manually the rapid increase in force produced by a

hinge moment reversal. For the accident conditions at the initiation of the aileron

hinge moment reversal (185 KIAS, 1.0 G), ATR indicates that approximately 60

pounds of force on the control wheel would have been required to maintain a wingslevel-attitude. This amount of force is in compliance with Federal Aviation

Regulations for temporary control wheel forces. However, the Safety Board

concludes that rapid, uncommanded rolls and sudden multiple onsets of even

60 pounds of control wheel force without any form of warning or pilot training for
169

such unusual events would, and most likely did in this case, preclude the flightcrew

from effecting a timely recovery.

The Safety Board recognizes that the risk of another ATR 42 or 72

accident resulting from an uncommanded aileron excursion in freezing

drizzle/freezing rain has been reduced by the addition of extended deice boots,

improved operational procedures, extensive crew training, and heightened awareness

by pilots. Because wind tunnel and in-flight tanker tests have been performed for

only a limited range of icing and flight conditions, the Safety Board remains

concerned whether, even with the improvements, the airplane can be controlled

under all naturally occurring combinations of conditions of liquid drop size and

content, temperature, airplane configuration, load factors, speeds, and time of

exposure. Moreover, the Safety Board found that ATR’s post-Roselawn brochure

entitled, “ATR Icing Conditions Procedures,” still does not adequately address or

clearly represent the exact nature of the ATR ice-induced aileron hinge moment

reversal.

Additionally, as part of the investigation, the Safety Board reviewed

historical accident and incident data of other similar turbopropeller aircraft. The data

did not show other airplane models to have a similar incident/accident history

involving uncommanded aileron excursions in the presence of freezing

drizzle/freezing rain. One possible reason for this is that other model aircraft use

hydraulically powered ailerons, smaller mechanical ailerons with larger hydraulically

powered spoilers, or different balance/hinge moment control devices to provide

adequate roll control with less propensity for aileron hinge moment reversals. The

Safety Board understands that ATR is considering design changes to the lateral

control system for current and future ATR airplanes that are expected to reduce the

susceptibility to flow separation-induced aileron hinge moment reversals and/or

uncommanded aileron deflections. The Safety Board concludes that such design

changes, if effective, would reduce the need to rely on the changes to flight

operations and pilot training that have already been mandated to ensure the safety of

flight. Thus, the Safety Board believes that the FAA should encourage ATR to test

lateral control system design changes and, if they correct the aileron hinge moment

reversal/uncommanded aileron deflection problem, require these design changes on

all existing and new ATR airplanes.
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2.5 ATR Certification For Flight into Icing Conditions

The Safety Board has found no evidence that the ATR 42 and 72 were

not properly certificated for flight into icing conditions under FAR/JAR

Part 25.1419, FAR/JAR Part 25, Appendix C, and DGAC Special Condition B6. The

results of a thorough review of the original airplane certification and the subsequent

"Special Certification Review," including icing tanker tests, indicate that the airplane

met the existing regulations. However, the investigation has raised a number of

concerns relating to the process for certifying an airplane for flight into icing

conditions.

Among these concerns are the regulatory authorities' acceptance of a

limited number of icing test data points, most of which are not near the boundaries of

the envelope; the limited range of conditions (LWC and MVD size) provided by the

Appendix C icing certification envelope; the lack of standardized methods for

processing LWC and MVD data; the implied authorization of flight into conditions

beyond the envelope; and the certification of stall protection systems that are

intended to prevent exposure to undesirable (even dangerous) characteristics of the

airplane without a requirement for the manufacturer to advise the FAA, operators and

pilots of such characteristics.

This investigation has revealed that the ATR 42 and 72 were not

required to be tested throughout a significant portion of the icing conditions that are

specified in the Appendix C icing envelope. The limited number of test points

accepted by the FAA as sufficiently comprehensive were well within the boundaries

of the envelope and did not include the warmer, near freezing conditions at the upper

boundary of the Appendix C envelope in which run-back icing and asymmetric

sliding/shedding are likely to occur. Thus, by allowing limited data well within the

envelope to suffice for certification purposes, the FAA effectively precluded any

chance of identifying the phenomena that led to flight 4184's ice ridge buildup,

uncommanded aileron deflection and loss of control.

The Safety Board's concern about the adequacy of Appendix C criteria

was heightened by the results of one December 1994 ATR icing tanker test in which

ice accumulated behind the active portion of the ATR 72's deice boots during

exposure to water droplet sizes of only 57 microns MVD, which is only slightly

outside the Appendix C envelope. Further, data developed by NACA, the NASA

predecessor, indicated in the 1950s that MVDs of 70 microns or more could be

encountered in layer clouds. Flight in layer clouds is not an unusual event in this
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country, but flight into layer clouds can result in encounters with icing conditions

beyond those set forth in 14 CFR Part 25, Appendix C. Several ATR 42 icing

incidents with ice aft of the boots (Air Mauritius, Ryan Air, and Continental Express

at Burlington) occurred in layer clouds, which supports the conclusion that icing

encounters in high altitude layer clouds can exceed the capabilities of aircraft

certified to the Appendix C envelope.

Thus, because the Appendix C envelope is limited and does not include

larger water drop conditions, such as freezing drizzle or freezing rain (conditions that

can be routinely encountered in winter operations throughout much of the northern

United States, and were most likely encountered by flight 4184), the Safety Board

concludes that the current process by which aircraft are certified using the Appendix

C icing envelope is inadequate and does not require manufacturers to sufficiently

demonstrate the airplane's capabilities under a sufficiently realistic range of icing

conditions.

In addition, the lack of standardized methods for processing icing data

to determine MVDs raises concern that certification icing tests may be conducted at

actual MVDs below the calculated values. For example, during the series of icing

tanker tests at Edwards AFB, it was determined that two generally accepted methods

of calculating MVD and LWC provided significantly different results. One method

was developed by Particle Measuring Systems, the manufacturer of the instruments

used to measure the icing conditions, and the other method was developed by NCAR.

It was found that when processing any given set of raw icing data collected behind

the icing tanker, the two methods provided MVD and LWC results that differed by as

much as a factor of 2. These differences are attributed to the different mathematical

equations used by the two methods and raise concerns about the accuracy of the

results. Therefore, it is possible that airplanes certificated in accordance with

Appendix C criteria may not actually have been tested in the icing conditions

described in the certification documentation. Thus, the Safety Board believes that

the FAA should revise the icing certification requirements and advisory material to

specify the numerical methods to be used in determining median volume diameter

(MVD) and liquid water content (LWC) during certification tests.

Further, although no aircraft are certified for flight into freezing drizzle

or freezing rain, the ATR 72 flight manual did not specify the operational limits and

capabilities of the airplane in conditions such as freezing drizzle and freezing rain.

Although the "Normal Procedures/Flight Conditions" section of the FAA-approved
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ATR 42 flight manual (AFM), section 3-02, page 1, dated March 1992, contained the

statement, "Operation in freezing rain must be avoided," the "Normal

Procedures/Flight Conditions" section of the ATR 72 AFM did not contain the same

statement, or any other limitation or prohibition of operation on the ATR 72 in such

conditions. At the Safety Board's public hearing, the ATR Vice President, Flight

Operations for North America, testified that the omission of this information from

the ATR 72 manuals was "not intentional."

Currently, FAA ground deice and anti-ice programs permit operators to

dispatch aircraft into freezing drizzle and light freezing rain80 provided they use

Type II anti-ice fluid and respect the specified holdover timetables. Specifically,

Flight Standards Information Bulletin (FSIB) for Air Transport (FSAT), 95-29, dated

October 25, 1995, states that Type II deicing fluid will be used when "operating

during light freezing rain and freezing drizzle weather conditions" and that the "use

of special procedures (i.e. visual inspections, remote deice capability) is required."

The Safety Board recognizes that the FAA's intent of this FSAT is to provide

operators with the means to dispatch airplanes that will quickly depart and climb

through the freezing drizzle or light freezing rain conditions and that the FAA's

permission of limited operations in freezing drizzle and light freezing rain is

apparently based on the assumption that the airplane will depart within the prescribed

"holdover" time of the anti-ice fluid, and transit through the freezing drizzle/light

freezing rain conditions with minimal exposure. However, FSAT 95-29 does not

specifically state that continued flight in such conditions is prohibited. The Safety

Board is concerned that in some situations it may be necessary to operate in such

conditions for an extended period of time. One such situation is the failure of an

engine shortly after takeoff, which could require maneuvering for an indeterminate

period of time while returning to the departure airport where freezing drizzle or light

freezing rain conditions are known to exist.

Further, although it is known by many in the aviation community that

flight into freezing drizzle or freezing rain is not safe, the Safety Board is unaware of

an explicit provision in the Federal Aviation Regulations that prohibits flight into

freezing drizzle and freezing rain. Additionally, as was noted in the Safety Board's

1981 study on aircraft icing, airplanes certificated for flight into known icing are

 

80The National Center for Atmospheric Research (NCAR) definition for light freezing rain is: "measured intensity up

to 0.10 in/hr (2.5 mm or 25 gr/dm2

/hr); Maximum 0.01 inch in 6 minutes from scattered drops that, regardless of

duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen."
173

authorized to fly into weather conditions that produce "severe" icing under 14 CFR

Parts 91, 135 and 121. However, by definition, severe icing conditions result in a

rate of ice accumulation that exceeds the capabilities of the airplane deice/anti-icing

system or that require immediate diversion from the planned route of flight.

The Safety Board is concerned that these unclear and inconsistent

messages to pilots about the operation of aircraft that are certified for flight in icing

conditions may create the misconception that flight in freezing drizzle and/or

freezing rain is acceptable when it is not. Such confusing and apparently

contradictory information could have contributed to the belief by Simmons

Airlines/AMR Eagle management that it was permissible for ATR 42 and 72

airplanes to be dispatched and flown into conditions of freezing drizzle and light

freezing rain when it disseminated a memorandum to its pilots in 1991 setting forth

the conditions for such flights.

The Safety Board concludes that no airplane should be authorized or

certified for flight into icing conditions more severe than those to which the airplane

was subjected in certification testing unless the manufacturer can otherwise

demonstrate the safety of flight in such conditions. Thus, the Safety Board believes

that the FAA should revise its certification regulations to ensure that airplanes are

properly tested for all conditions in which they are authorized to operate, or are

otherwise shown to be capable of safe flight into such conditions. If safe operation

cannot be demonstrated by the manufacturer, operational limitations should be

imposed to prohibit flight in such conditions, and flightcrews should be provided

with the means to positively determine when they are in icing conditions that exceed

the limits for aircraft certification.

Many of the concerns raised about icing certification criteria in this

investigation were previously identified by the Safety Board and were the basis for

safety recommendations issued in 1981 to the FAA and NOAA as a result of the

Safety Board's study on icing avoidance and protection. The study raised concerns

about the adequacy of the Appendix C envelope and icing certification, the inability

to properly measure icing and atmospheric parameters forecast, the inadequacy of the

icing severity definitions, and the inconsistency of being legally permitted to operate

in conditions that are more severe than those to which the airplane is subjected

during the certification process.

As a result of the study, the Safety Board recommended that the FAA

evaluate individual aircraft performance in icing conditions and establish operational
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limits in terms of LWC and MVD for pilot use; review icing criteria in 14 CFR Part

25 and expand the certification envelope to include freezing rain as necessary;

establish standardized procedures for icing certification; and resolve the

incompatibility between the regulations and the definition of severe icing provided in

the AIM.

However, the FAA has not acted positively on these recommendations.

For example, in its June 7, 1982, response to Safety Recommendation A-81-115,

which requested the evaluation of individual aircraft performance in icing conditions

and establishing operational limits, the FAA stated, in part:

...Forecasts issued or icing conditions described in terms of

intensity levels...should not affect the capabilities of icing

certified aircraft to operate safely in icing conditions regardless of

the size of aircraft. This is because icing-certified aircraft are

evaluated to the full icing envelope expected in nature as defined

in 14 CFR 25, Appendix C.

Safety Recommendation A-81-115 was classified "Closed—

Unacceptable Action" on April 11, 1990.

In a subsequent response, dated August 28, 1995, to other Safety Board

1981 recommendations, which were still being held as "Open," the FAA again stated

that it did not intend to initiate any action because:

the FAA has put in place major programs in recent years which

have addressed various anti-ice and deice issues. At the same

time the FAA has sponsored or collaborated on numerous icing

programs....However, none of this work has established the

foundation or justification to revise 14 CFR Parts 25, 91 or 135 as

requested by these safety recommendations. The FAA considers

its actions to be complete.

The FAA has continually indicated in its responses to the 1981 safety

recommendations during the past 14 years that sufficient research and data collection

had been accomplished and that icing was not a significant problem for airplanes

certified under 14 CFR Part 25, Appendix C. Despite the funding of research and

occasionally providing positive written responses to some of the safety

recommendations, the Safety Board found that the FAA's actions were not adequate
175

to satisfy the intent of these recommendations; and in a November 20, 1995, letter it

classified A-81-116 and -118 as "Open--Unacceptable Response." The Safety Board

concludes that if the FAA had acted more positively upon the safety

recommendations issued in 1981, this accident may not have occurred.

Additionally, the Safety Board understands that the FAA, as a result of

this accident, is currently planning a review of its icing certification and operational

regulations, including the icing severity definitions issue. The Board supports and

encourages this action. However, the Safety Board believes that the FAA should

revise 14 CFR Parts 91.527 and 135.227 in a timely manner to ensure that the

regulations are compatible with the published definition of severe icing, and to

eliminate the implied authorization of flight into severe icing conditions for aircraft

certified for flight in such conditions.

Finally, the Safety Board notes that Special Condition B6, developed by

the French DGAC in the 1980's and initially applied during the ATR 72 certification,

includes a "zero G" flight test maneuver (pushover) designed to identify ice-induced

elevator hinge moment reversals. The Safety Board understands that at least some

manufacturers in the world aviation community (including the United States) are

concerned that Special Condition B6 is too demanding, particularly the tailplane

icing pushover test. However, the Safety Board concludes that the addition of a test

procedure to determine the susceptibility to aileron hinge moment reversals in both

the clean and iced-wing conditions could help to prevent accidents such as that

involving flight 4184. Thus, the Safety Board believes that the FAA should develop

a test procedure similar to the tailplane icing pushover test to determine the

susceptibility of airplanes to aileron hinge moment reversals in the clean and icedwing conditions.

2.5.1 Stall Protection Systems

The Safety Board is concerned that the FAA and other airworthiness

authorities still permit airplane manufacturers to use stall protection systems (SPS) to

prevent flightcrews from experiencing known undesirable flight characteristics

unique to their particular aircraft design without requiring the manufacturers to

reveal these characteristics to the airworthiness authorities, operators, and pilots.

According to ATR, its use of an SPS to prevent, among other things, aileron hinge

moment reversals in the clean and iced configurations was not explained to the

airworthiness authorities or the operators because ATR was not required to do so.

The Safety Board concludes that the failure of the DGAC and the FAA to require
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that they be provided with documentation of known undesirable post-SPS flight

characteristics contributed to their failure to identify and correct, or otherwise

properly address, the abnormal aileron behavior early in the history of the ATR icing

incidents. Therefore, the Safety Board believes that the FAA should require aircraft

manufacturers to provide, as part of the certification criteria, information to the FAA

and operators about any known undesirable flight characteristics beyond the SPS and

related shaker/pusher flight regime.

2.6 Continuing Airworthiness

2.6.1 Adequacy of Actions Taken by ATR After Previous ATR Incidents

ATR's analysis of the 1988 ATR 42 incident at Mosinee, Wisconsin,

concluded that ice had formed on the upper surface of the wings, aft of the deice

boots, because the airplane had been operated in conditions that were outside the

certification envelope. ATR's analysis also concluded that the ice accretion changed

the aileron hinge moments, resulting in an autopilot disconnect, uncommanded

aileron deflection, and subsequent roll excursions. ATR stated that “...the ailerons

tended to adopt the zero hinge moment position in absence of pilot reaction....” The

zero hinge moment position referred to by ATR was 12.5 degrees. ATR further

stated that, “...the control surfaces remained effective and, owing to their reaction

alone, enabled the aircraft to recover a normal attitude, although control stability was

affected, owing to the changes in hinge moment according to angle of attack, which

was probably due to the presence of ice on the airfoil beyond the deicers, as is the

case on all aircraft in freezing rain conditions.”

Further, ATR had experienced aileron hinge moment reversals during

the development of both the ATR 42 and 72. For example, the aileron hinge moment

reversals that resulted during early flight tests of the ATR 72 occurred at low AOAs

on a “clean” wing. The occurrence of hinge moment reversals prompted ATR in the

late 1980s to develop and install vortex generators that were intended to raise the

AOA at which the ailerons would become unstable and a hinge moment reversal

would occur. In addition, during development of the ATR 72-210 series airplanes,

ATR added additional vortex generators to further increase the AOA at which the

aileron hinge moment reversals occurred. ATR also specifically designed the stall

protection system to activate at AOAs lower than the AOA at which the ailerons

would become unstable. The knowledge gained from flight testing about hinge

moment reversals, the findings from the previous incidents, and ATR’s active
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participation in the study of tailplane icing hinge moment reversals, leads the Safety

Board to conclude that ATR recognized the reason for the aileron behavior in the

previous incidents and determined that ice accumulation behind the deice boots, at an

AOA sufficient to cause an airflow separation, would cause the ailerons to become

unstable. ATR had sufficient basis to modify the airplane and/or provide

airworthiness authorities, operators, and pilots with adequate, detailed information

regarding this phenomenon.

Following the Mosinee incident, ATR issued an “Operators Information

Message” (OIM), added the Anti-Icing Advisory System (AAS) and vortex

generators in front of the ailerons to all ATR airplanes, proposed airplane flight

manual (AFM) and flightcrew operating manual (FCOM) changes and developed an

icing package for the ATR simulators. The 1989 OIM characterized the Mosinee

incident in a manner that could have been interpreted by operators and pilots to

indicate only that the ailerons became “stiff” or hard to move because of an accretion

of ice, and that the autopilot was unable to move the ailerons and correct the

increasing roll attitude. The OIM did not indicate that an ice accretion behind the

deice boots in front of the ailerons, could cause them to overpower the autopilot,

move uncommanded in an abrupt manner to their full travel limits, cause rapid rolls

and create unusual lateral control behavior.

The 1989 icing simulation package provided to simulator manufacturers

and aircraft operators by ATR for use in their ATR 42 training programs did not

adequately present the effects of the icing event experienced by the Mosinee

flightcrew or the crew of flight 4184. The modification did not model abrupt, full

aileron and control wheel deflections with high control wheel forces that would

typically be necessary to recover from an aileron hinge moment reversal. Further,

AMR Eagle stated that in its simulator program, the pilots were taught to initiate

recovery from a stall at the first indication of stick shaker, stall aural warning,

airframe buffet or stick pusher, and that any training in an incorrect configuration,

such as further increasing the AOA beyond stall warning to cause the airplane to roll

off, would be classified as "negative" training.

In 1996, after the Roselawn accident, ATR provided simulator operators

with another, more accurate icing simulation package. This simulator package would

have provided the crew of flight 4184 with a higher awareness of the potential effects

of icing conditions on ATR airplanes and the ability to recognize and probably

recover from an uncommanded, unstable aileron movement. The Safety Board

concludes that the 1989 icing simulation package developed by ATR
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for the training simulators did not provide training for pilots to recognize the onset of

an aileron hinge moment reversal or to execute the appropriate recovery techniques.

The Safety Board notes that when ATR developed the AAS and the

vortex generators following Mosinee, it also proposed changes to the ATR 42 AFM

and FCOM. These changes were adopted in part by the German and Canadian

airworthiness authorities; however, the DGAC and the FAA did not require these

changes. In its 1992 Airworthiness Directive (AD) requiring the installation of the

vortex generators, the FAA indicated that the vortex generator modification would

“remove” the source of the abnormal aileron behavior; thus, it did not require the

inclusion of ATR’s proposed AFM or FCOM changes. The DGAC did not require

the AFM or FCOM changes because they addressed a condition outside the

certification envelope of the airplanes.

Although the AFM/FCOM changes proposed by ATR did enhance the

existing icing information in the manuals, these changes did not warn pilots of

unexpected autopilot disconnects or rapid and uncommanded aileron deflections to

near their full travel limits with high, unstable control wheel forces. Thus, the Safety

Board concludes that ATR’s proposed AFM/FCOM changes, even if adopted by the

DGAC and the FAA, would not have provided flightcrews with sufficient

information to identify or recover from the type of event that occurred at Roselawn,

and the actions taken by ATR following the incident at Mosinee were insufficient.

In 1990, ATR conducted flight tests with run-back ice shapes developed

from the 1989 British blower tunnel tests and found that the ice shapes, although

located aft of the boot on the under surface of the wing, did not adversely affect the

stability and control of the airplanes. However, the height of the shapes was 1/4

inch, which ATR indicates was not sufficient to initiate an aileron hinge moment

reversal prior to SPS activation.

Following the two ATR 42 incidents in 1991 (involving Ryan Air and

Air Mauritius), which ATR also attributed to operation in icing conditions outside of

the certification envelope, ATR published its 1992 All Weather Operations

brochure.81 The brochure, which was sent to all ATR operators, provided

 

81The All Weather Operations brochure, in which ATR consolidated general aircraft operating information for flight in

all types of weather conditions (including freezing drizzle and freezing rain), was not provided by Simmons

Airlines/AMR Eagle to its pilots. Simmons Airlines/AMR Eagle management stated that the brochure was not

disseminated because some information was contrary to Federal regulations and because most of the information already

existed in the various approved flight and operating manuals.
179

information about freezing rain, including temperature ranges that could produce

such conditions. It stated “Aileron forces are somewhat increased when ice accretion

develops, but remain otherwise in the conventional sense,” which is inconsistent with

the actual rapid and uncommanded aileron and control wheel deflections to near their

full travel limits with unusually high, unstable control wheel forces. The brochure

also stated “Freezing rain is capable of rapidly covering an aircraft with a sizable

layer of clear ice, well beyond the usual accretion areas around the stagnation point.”

However, the statement does not specifically indicate that ice may accumulate a

significant distance beyond the deice boots, although the wing leading edges and

windscreen may be free of ice. Finally, the brochure stated that “Should the aircraft

enter a freezing rain zone, the following procedures should be applied: Autopilot

engaged, monitor retrim roll left/right wing down messages. In case of roll axis

anomaly, disconnect autopilot holding the control stick firmly.” However, this does

not indicate that a roll trim message may not occur, or could occur coincident with

the autopilot disconnecting (as it did with flight 4184), thus precluding sufficient

time for the flightcrew to perform the recommended procedures, nor does it advise

flightcrews to expect sudden autopilot disconnects, rapid and uncommanded aileron

and control wheel deflections to near their full travel limits with unusually high,

unstable control wheel forces. Therefore, the Safety Board concludes that the ATR

All Weather Operations brochure was misleading and minimized the known

catastrophic potential of ATR operations in freezing rain.

Following the 1993 incident at Newark, New Jersey, which ATR

attributed to turbulence and freezing rain, and the incident about 1 year later at

Burlington, Massachusetts (both of which involved aileron-hinge moment reversals

in icing conditions), ATR had sufficient knowledge to conclude that the ATR 42 had

a significant, recurring airworthiness problem in icing conditions outside the

Appendix C icing certification envelope. Although ATR knew that the icing

conditions encountered in these incidents were outside the icing certification

envelope, ATR also knew that the airplanes were being flown more than occasionally

into such conditions and that neither the vortex generators nor the operational

information it had disseminated had corrected the problem or prevented recurrence.

Therefore, the Safety Board concludes that ATR's failure to disseminate adequate

warnings and guidance to operators about the adverse characteristics of,
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and techniques to recover from, ice-induced aileron hinge moment reversal events,

and ATR's failure to develop additional airplane modifications, led directly to this

accident.

2.6.2 Continuing Airworthiness Oversight by DGAC

ATR provided the DGAC (but not the FAA) with copies of all its

incident analyses, including the incident at Mosinee, Wisconsin. Thus, the DGAC

should have been fully aware that ATR had concluded that the Mosinee and other

incident flightcrews had flown their airplanes into icing conditions that were beyond

the Appendix C icing certification envelope. The DGAC should have recognized

from the ATR analyses that such incidents resulted in unexpected autopilot

disconnects, and rapid, uncommanded aileron and control wheel deflections.

As discussed in Section 2.6.1, following the 1991 Ryan Air and Air

Mauritius incidents, ATR developed its 1992 All Weather Operations brochure in

which the aileron behavior was vaguely discussed without directly alerting operators

or pilots to the specifics of the prior incidents or providing explicit guidance on how

to cope with aileron hinge moment reversals. The DGAC did not require ATR to

provide more specific information to operators and pilots, nor did it require ATR to

do further research and testing in icing conditions. Nonetheless, because ATR had

indicated that the airplanes in these incidents were inappropriately flown in icing

conditions beyond the certification envelope, and that in most cases the pilots had

not increased the propeller speed to 86 percent (as required by the aircraft flight

manual procedure for flight in icing conditions), it was reasonable for the DGAC to

accept ATR's commitment to educate flightcrews with the All Weather Operations

brochure as an adequate response at that time. However, the Safety Board concludes

that the DGAC did not require ATR to include adequate information about sudden

autopilot disconnects, and rapid, uncommanded aileron and control wheel deflections

in its All Weather Operation brochure, nor did the DGAC require that ATR

flightcrews receive mandatory training on this subject.

During its investigation of the 1993 Continental Express incident at

Newark, New Jersey, ATR concluded that turbulence was a primary factor in the

upset of the airplane. Excessive ice accumulation on the wings was also identified

but attributed to freezing rain and the flightcrew's failure to increase the propeller

speed from 77 percent to 86 percent as required by the flight manual. However, the

DGAC should have known that the amount of turbulence in that incident (+/-0.3 G)

was too low a turbulence level to cause aileron hinge moment reversals in a
181

transport category aircraft, and therefore should have recognized that the freezing

rain encounter was the reason for the unstable aileron behavior. Further, the 1994

investigation of the Continental Express incident at Burlington, Massachusetts,

provided data that led to the conclusion that an ice-induced aileron hinge moment

reversal occurred after "severe" ice had caused the airplane to decelerate and pitch up

despite proper use of all ice protection procedures.

Based on the long history of ATR incidents in icing conditions,

especially those that occurred after 1992, the DGAC should have recognized that the

vortex generators, the AAS, and the All Weather Operations brochure were not

sufficient to correct or prevent the recurrence of the ice-induced aileron hinge

moment reversal problem. Further, it should have been clear that the ATR airplanes

were still being flown into icing conditions that were beyond the Appendix C

envelope or were otherwise conducive to aileron hinge moment reversals.

The Safety Board concludes that following the 1994 Burlington

incident, the DGAC should have required ATR to take further action to correct the

ice-induced aileron instability and to ensure that all operators and regulators were

aware of ATR's analyses of the incidents and the characteristics of the phenomenon.

Further, the Safety Board concludes that the DGAC's failure to require ATR to take

additional corrective actions, such as performing additional icing tests, issuing more

specific warnings regarding the aileron hinge moment reversal phenomenon,

developing additional airplane modifications, and providing specific guidance on the

recovery from a hinge moment reversal, led directly to this accident.

2.6.3 Continuing Airworthiness Oversight by FAA

As early as 1981, the Safety Board had recommended that freezing rain

be included in the Appendix C envelope because aircraft operate in such

conditions.82 In 1983, Dr. Richard Jeck, now one of the FAA's experts in aircraft

icing, raised similar concerns within the FAA. Following the 1988 Mosinee

incident, the FAA became aware that the ATR 42 was susceptible to aileron hinge

moment reversals in freezing drizzle/light freezing rain conditions. In a 1989 letter

to the FAA, the Air Line Pilots Association (ALPA) stated that the AAS and vortex

generator modifications to the ATR airplanes were a positive step forward in taking

corrective action. However, ALPA questioned whether they were adequate to solve

 

82See discussion in section 1.18.3.
182

the problem, and also stated its concern that pilots still had no definitive way of

identifying when they encounter icing conditions that are outside the certification

envelope. The FAA, which had indicated that the vortex generators would correct

the aileron anomaly, did not respond to ALPA's concerns except to state that freezing

rain is a "...rare, low altitude phenomena, that is generally easy to forecast and

therefore avoid[able]."

The Safety Board is concerned that the FAA apparently misunderstood

the function of the vortex generators when it approved their installation on the ATR

42 and rescinded the previously imposed flight restrictions. The language used in

the airworthiness directive (AD) indicated that the FAA believed that the installation

of vortex generators would eliminate the aileron hinge moment reversal problem,

rather than only delaying the onset of the hinge moment reversal to a greater AOA.

The Board is also concerned that apparently neither ATR nor DGAC corrected the

FAA's misunderstanding.

In 1989, the manager of the FAA's airworthiness evaluation group

(AEG) stated in a briefing paper that there had been 10 ATR icing incidents that

warranted further study and that "...in the context of problem solving we would like

to see flight tests on the ATR series aircraft with irregular shapes emulating "runback" [ice]...." Despite the Safety Board’s request for information, the FAA was

unable to provide the Safety Board with a copy of any FAA response to the concerns

raised in this paper.

Further, in 1991, the FAA led an industry/government team in

developing a more detailed understanding of the icing accidents attributed to

tailplane icing. Freezing drizzle and freezing rain were a primary topic of discussion.

Following the 1991 Ryan Air and Air Mauritius incidents, neither ATR

nor DGAC provided the FAA with copies of ATR's analyses of these incidents.

Although some FAA staff may have been aware of these incidents and the 1992 ATR

All Weather Operations brochure, the FAA may still not have had sufficient

information to recognize that the ATR 42's susceptibility to aileron hinge moment

reversal required further action by ATR.

Following the 1993 Newark, New Jersey, and the 1994 Burlington,

Massachusetts, incidents, ATR and DGAC again did not provide the FAA with

copies of ATR's analyses. Important information regarding these incidents was not
183

provided to the FAA following the 1994 incident at Burlington. However, the FAA

should have had sufficient information regarding specific events and general

concerns to recognize the significance of the ATR problems in icing conditions and

to recognize that the actions taken by ATR were insufficient to correct the aileron

hinge moment reversal problems. The Safety Board concludes that the FAA's

failure, following the 1994 Continental Express incident at Burlington, to require

that additional actions be taken to alert operators and pilots to the specific icingrelated problems affecting the ATRs, and to require action by the manufacturer to

remedy the airplane's propensity for aileron hinge moment reversals in certain icing

conditions, contributed to this accident. The determination by the Safety Board that

the FAA's role in the causation of this accident was contributory and not directly

causal stems from the failures of ATR and DGAC to provide important information

to the FAA, and from the FAA's more secondary role than the State of manufacture

in the chain of assuring continued airworthiness of the ATR airplanes.

The Safety Board evaluated the role of the FAA’s AEG to determine

why it did not act to correct the problem with the ATR. FAA Order 8430.6C states

that an AEG inspector will, "Provide expert information on aircraft in support of

accident/incident investigations and assist in the development of corrective actions."

In testimony at the Safety Board's public hearing, the AEG operations unit supervisor

stated that:

...all three boards, the Maintenance Review Board, Flight

Standardization Board and the Flight Operations Evaluation

Board ha[d] only just [consolidated to form the AEG]. So we had

to bridge the gap...and use some of that pre-existing material [for

guidance] for a period of time because the new material has been

in draft until just a short time ago and it was somewhat

incomplete....

The supervisor testified further that "...the AEG does not maintain a data

base of incident/accident information...." Also, it was found that the AEG did not

regularly use other data bases within the FAA and outside the FAA from which

incident/accident data may be derived to formally monitor trends that could

compromise the continued airworthiness of aircraft that it had been assigned to

oversee. However, the supervisor stated that the AEG office "does keep records" and

obtains incident and accident information through a line of communication with the

FAA's other Flight Standards organizations or through information gathered from

ADs. The information that had been gathered by the AEG regarding the previous
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ATR incidents and the foreign accidents was general and had been difficult to obtain,

particularly with regard to the BAA. The supervisor also testified that under the

BAA, the lines of communication need to be "defined."

This deficiency in communication resulted in the AEG's failure to

receive pertinent documentation regarding the ATR icing incidents (such as the ATR

analyses) that could have been used to monitor the continued airworthiness of the

airplane. Further, this is not the first time that the Safety Board has identified

problems with the timeliness and effectiveness of the FAA's continuing airworthiness

oversight of foreign-built aircraft. The Safety Board noted in its 1987 report on the

crash of a CASA C-212-CC that the FAA's monitoring of airworthiness issues

relating to that aircraft was inadequate. Specifically, that investigation revealed that

the FAA delayed for more than 3 years taking actions to correct known issues of

noncompliance with 14 CFR Part 25, and that there "was an apparent lack of

standardization and coordination" among various offices within the FAA.83

The Safety Board concludes that the lack of defined lines of

communication and adequate means to retrieve pertinent airworthiness information

prevented the AEG from effectively monitoring the continuing airworthiness of

aircraft. Therefore, the Safety Board believes that the FAA should develop an

organizational structure and communications system that will enable the AEG to

obtain and record all domestic and foreign aircraft and parts/systems manufacturers'

reports and analyses concerning incidents and accidents involving aircraft types

operated in the United States, and ensure that the information is collected in a timely

manner for the effective AEG monitoring of the continued airworthiness of aircraft.

2.7 ATR Certification and Continuing Airworthiness Monitoring Under

the Bilateral Airworthiness Agreement

The Bilateral Airworthiness Agreement between the U.S. and France84

eliminates a significant amount of duplication in the overall certification of an

aircraft. This method of certification relies significantly upon the airworthiness

authority of the exporting country to review manufacturer data and ensure adherence

 

83NTSB Accident Report AAR-88-08, Fischer Bros. Aviation, Inc., dba Northwest Airlink, Flight 2268, Construcciones

Aeronauticas, S. A. (CASA) C-212-CC, Detroit Metropolitan Wayne County Airport, Romulus, Michigan, March 4,

1987, p.44.

84Described in section 1.18.7.
185

to the U.S. type certification procedures and requirements. It is generally an

appropriate process if the FAA is adequately involved in monitoring the certification

by a foreign airworthiness authority. However, it appears that the FAA has

implemented the process with extremely limited "hands-on" involvement in the

development, construction and flight testing of the aircraft.

The FAA team leader for the ATR certification testified about the

original ATR certification process:

...[the original certification process] takes generally five

years....The major way that we try to stay in the loop during this is

with meetings at the facility....In a perfect world, we would

schedule four meetings during this period to go over there with a

team...and review what has been done to date....But the budget

realities being what they are, the last several projects that we have

been working on, the best we've been able to do is generally two

meetings.

The team leader also stated:

...Generally half way through the process, ...we review all that's

been done to date, we see if there are any problem areas....If we

can do it, we will bring the team back over again just before the

airplane begins to go into flight testing. That's usually a good

time to catch any little things before they start a lot of expensive

flight testing....And if possible, we like to get the team back over

for the final type board. Although on the last several programs

that I'm familiar with, we've sent the project manager alone just to

make sure that everything has been done satisfactorily....

The Safety Board is concerned about the FAA's limited involvement

during the initial certification of the ATR 42 and 72. For example, there were

several meetings in which only one person from the FAA reviewed vast amounts of

certification documentation and had to resolve problems from many technical

disciplines. Further, because FAA personnel were either unavailable, or budget

constraints restricted travel, issues involving noncompliance or other concerns were

resolved only through "issue papers." An issue paper, of which there were more than

90 for the ATR 42 and 17 for the ATR 72, describes the FAA position regarding a

certification issue and the method(s) necessary to achieve compliance.
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For the ATR, the FAA delegated the compliance oversight for the issue papers to the

DGAC.

Included in the certification process is the FAA review of test data,

including data acquired from flight tests. According to testimony provided by the

FAA ATR certification team leader, the FAA does not flight test the aircraft; rather,

it conducts "evaluation" flights for the purpose of "familiarity with airplane...and [to]

determine suitability for use in airline service...." The FAA conducted about 10

hours of evaluation flights on the ATR; however, none of these flights duplicated

any tests required for certification, and none were conducted in icing conditions.

The Safety Board concludes that the FAA's limited involvement in the

ATR 42 certification does not appear to have resulted in an improperly certificated

airplane (ATR 42/72). However, such excessive reliance on a foreign airworthiness

authority could result in improper certification of an aircraft. Therefore, the Safety

Board believes that the FAA should review and revise, as necessary, the manner in

which it monitors a foreign airworthiness authority's compliance with U.S. type

certification requirements under the Bilateral Airworthiness Agreement (BAA).

The Safety Board is also concerned about the process by which the FAA

ensures the continuing airworthiness of airplanes certificated under the BAA. The

FAA did not receive pertinent information about the airworthiness of the ATR 42

and 72 series airplanes, including ATR’s analyses of the icing-induced aileron hinge

moment reversal incidents in 1991, and those in 1993 and 1994. The FAA could

have been more aggressive in requesting data from the DGAC following these

incidents. However, the DGAC should have, on its own accord, taken actions to

make sure that the FAA was provided with all information about the ATR incidents

to ensure FAA involvement in the continuing airworthiness of the airplane.

Unfortunately, this is not the only foreign manufactured airplane for

which the United States has failed to receive information that was critical to the

monitoring of the continued airworthiness of the airplane. In 1995, the Safety Board

investigation of an incident in which a Northwest Airlines Airbus A320 had

experienced severe roll oscillations while on final approach to runway 18 at

Washington National Airport, Washington, D.C., revealed a lack of communication

between the airworthiness authorities and Airbus. Prior to the Northwest A320

incident, the DGAC had determined that a temporary Airbus revision to a procedure

in its flightcrew operating manual, addressing flight conditions conducive to such

oscillations, did not require regulatory action. However, the FAA was unable to
187

determine if regulatory action was required because the pertinent information had not

been provided by either the DGAC or Airbus. In a letter to the FAA dated November

14, 1995, the Safety Board concluded that information regarding undesirable flight

characteristics in the A320 had not been "effectively disseminated from the

manufacturer to the different airworthiness authorities, operators and flightcrews."

Further, the Safety Board expressed its concern that, "...other useful and perhaps

critical information of a similar nature is not being effectively communicated," and in

Safety Recommendation A-95-109, asked that the FAA "in conjunction with the

French [DGAC] establish policy and procedures to assure effective dissemination of

all essential information regarding airworthiness problems and corrective actions in

accordance with ICAO Annex 8, Part II, paragraph 4."

The FAA’s first response (dated January 29, 1996) to this

recommendation addressed only communication of airworthiness information related

to Airbus Industries, and its subsequent response (dated May 7, 1996) went further.

(See section 1.16.3, Communication of Airworthiness Information Between FAA,

DGAC and ATR.)

As stated previously, on May 15, 1996, the Safety Board classified

Safety Recommendation A-95-109 “Open--Acceptable Response,” pending

implementation and review of the agreement regarding the Promotion of Aviation

Safety.

The Safety Board concludes that the FAA's ability to monitor the

continued airworthiness of the ATR airplanes has been hampered by an insufficient

flow of critical airworthiness information. The DGAC’s apparent belief that such

information was not required to be provided under the terms of the BAA raises

concerns about the scope and effectiveness of the BAA. Thus, the Safety Board

believes that the FAA should establish policies and procedures to ensure that all

pertinent information is received, including the manufacturer's analysis of incidents,

accidents or other airworthiness issues, from the exporting country's airworthiness

authority so that it can monitor and ensure the continued airworthiness of airplanes

certified under the BAA.

2.8 Air Traffic Control

The primary air traffic control issues examined by the Safety Board

were the ground delay and airborne holding of flight 4184, the traffic flow into
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Chicago's O'Hare International Airport, and the dissemination of significant weather

information to the flightcrew.

At 0800, on the day of the accident, the Chicago Air Route Traffic

Control Center (ARTCC) Traffic Management Coordinator (TMC) requested that a

ground delay program be implemented for aircraft scheduled to land at O'Hare

International Airport between the hours of 1200 and 1800 because of the forecast of

unfavorable weather conditions. Flight 4184 was released from IND into an area of

forecast icing conditions after a 42-minute ground hold with the anticipation that the

flight would probably hold en route. The area supervisor at the Chicago ARTCC

testified that it is considered an acceptable practice to issue a holding clearance for

turbopropeller aircraft operating in "light" or "moderate" icing conditions. Icing

conditions often do not exist even though such conditions are forecast. The

supervisor stated that the controllers would be "very responsive" if a pilot indicated

that they were holding in icing conditions and "wanted to get out," or rejected a

holding pattern because of icing conditions. The supervisor also stated that on the

day of the accident, he was not aware of any flightcrews rejecting holding

instructions because of icing conditions. Because forecasts of hazardous weather

may not be precise, and because airplanes can encounter a variety of icing conditions

including those considered to be "severe," and exit the conditions safely, efficient

use of airports is typically achieved by dispatching aircraft at rates that may require

holding if the weather deteriorates. Therefore, the Safety Board concludes that

although the controlling facilities were aware that light icing conditions were

forecast for the area of the LUCIT intersection, flight 4184 was properly released

from Indianapolis because there were viable options for pilots who chose to avoid

holding in icing conditions.

Periodically, throughout the day of the accident, all sector controllers

responsible for aircraft inbound to ORD were advised that the holding of aircraft was

possible, and that if the majority of traffic was inbound from the east, the west

sectors would hold aircraft, and vice versa. At 1452, when the Chicago Center Flow

Control (CFC) advised the Indianapolis Clearance Delivery (CD) that flight 4184

was released, the CD was also told "...that fix is in the hold so he might do some

holding when he gets up here...." At 1517, approximately 22 minutes after flight

4184 departed, the sectors responsible for inbound flights from the east [which

included flight 4184] were instructed to implement holding because a "rush" of

inbound aircraft were arriving from the west sector. FAA Order 7110.65 states that

air traffic services will be provided to aircraft on a "...first come first served basis as

circumstances permit." However, this is not always feasible during periods of high
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traffic volume or adverse weather conditions. During these periods, the primary

responsibility of traffic management is to ensure the safe and orderly flow of air

traffic, which may require the holding of aircraft in some sectors while allowing

other aircraft to continue inbound to their destination. The Safety Board concludes

that under the circumstances on the day of the accident, the controllers acted

appropriately in the management of traffic flow into ORD, which necessitated the

holding of flight 4184 in the BOONE sector.

At 1524:40, flight 4184 entered the holding pattern, and, according to

ATC procedures, the flight became an "arrival delay" when it was still holding after

15 minutes. FAA Order 7210.3, Facility Operation and Administration, Chapter 4,

paragraph 4-73, requires that air traffic personnel report delays of 15 minutes or

more that occur in facilities or airspace under the control of the Air Traffic Control

System Command Center (ATCSCC). ATCSCC personnel use these reports as a

guide to determine the success of a particular traffic management program.

Typically, controllers advise their supervisor(s) or the TMC of the specific delay(s).

The supervisor or TMC in turn advises the ATCSCC specialist so that a course of

action can be planned and implemented to alleviate the en route holding. However,

the Safety Board found that on the afternoon of the accident, because the supervisor

did not advise the ARTCC TMC that flight 4184 was an arrival delay, as required by

the FAA Order, the TMC was not aware that the BOONE sector was holding aircraft.

However, the Safety Board concludes that the supervisor's failure to report flight

4184 as an arrival delay did not affect the operation of the flight because, according

to the ATCSCC, it would not have amended the flow control program based on one

delayed aircraft. Nevertheless, testimony and statements from the controllers

indicate that any flight would have been rerouted to accommodate a pilot who

expressed concern about holding in icing conditions. Therefore, the Safety Board

concludes that the supervisor's failure to alert the ATCSCC that flight 4184 had been

holding for more than 15 minutes did not contribute to the accident.

Controllers are required by FAA Order 7110.65 to solicit a pilot report

when certain weather conditions that are specified in the order are either forecast or

reported for the area of jurisdiction. "Lighter or greater..." icing conditions, which

include freezing rain, are one of the five conditions specified by the Order for which

a controller will solicit a PIREP. When the BOONE sector controller assumed

control of the position and received a briefing by the departing controller, he was

told, "...no one was complaining about the weather." This included flight 4184

which had been on the radio frequency for approximately 3 minutes when the

BOONE controller assumed control. Because there were no PIREPs provided to
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the previous controller and the crew of flight 4184 did not provide a PIREP of icing

conditions at the LUCIT intersection, it was reasonable for the controller to assume

that there were no significant weather events in that area, and that the crew of flight

4184 was not experiencing any problems that would have required the controller to

take alternative actions. Nonetheless, the Safety Board believes the FAA should

revise FAA Order 7110.65, “Air Traffic Control,” Chapter 2, “General Control,”

Section 6, “Weather Information,” paragraph 2-6-3, “PIREP” Information, to include

freezing drizzle and freezing rain. These conditions should also be clearly defined in

the Pilot/Controller Glossary.

2.9 Flightcrew Actions

As noted in section 2.2, flight 4184 entered the holding pattern at 77

percent propeller RPM, which is consistent with the use of Level I ice protection and

would have been appropriate only if the flight were operating outside of all clouds

and precipitation. However, the drag increase noted at approximately 1533 was

evidence of flight in at least intermittent icing conditions in clouds or precipitation.

Further, on two occasions while flight 4184 was holding, the CVR recorded a single

tone chime identified as a caution alert. The caution alert can be activated by one of

several different aircraft systems, including the ice detection system. The flightcrew

did not increase the propeller RPM to 86 percent and activate the ice protection

system when the first caution alert chime sounded at 1533:56, but following the

second caution alert at 1541:07, the FDR indicated that the flightcrew did activate

the Level III ice protection system and increased the propeller RPM to 86 percent.

Because there was no discussion between the crewmembers regarding the 1533:56

caution alert, it is possible that it was activated by one of the other aircraft systems.

Assuming the chime at 1533:56 was activated by the ice detection system, consistent

with the drag increase noted at that time on the FDR, the Safety Board concludes that

the flightcrew’s failure to increase the propeller RPM to 86 percent and activate the

Level III ice protection system was not a factor in the accident because: 1) according

to ATR, the increased propeller RPM is necessary to increase the ice shedding

capabilities of the propeller blades; 2) the increased propeller RPM will prevent the

formation of ice aft of the deice boots in the area of the propeller slipstream, but will

not prevent the formation of ice on the wing in the areas behind the deice boots, in

front of the ailerons, or the airflow over the ailerons.

There was no discussion recorded on the CVR to suggest that the flight

crewmembers had a safety concern about the icing conditions in which they were
191

holding. Two comments by the crewmembers recorded on the CVR indicated they

were aware that ice was accreting on the airframe. The first comment, “I’m showing

some ice,” occurred about 9 minutes before the initial upset of the airplane, and the

second comment, “we still got ice” occurred about 2 minutes before the upset.

Neither comment indicated the type or amount of ice, nor did the comments suggest

that the crew was aware the ice accretions were related to an encounter with freezing

drizzle or freezing rain. The comments only indicate that the flightcrew was aware

that they were operating in an icing environment. Further, the flightcrew responded

appropriately to the caution alert at 1541:07 by increasing the propeller RPM to 86

percent and activating the deice boots 5 seconds later, or about 16 minutes before the

upset.

Although AMR Eagle cautioned pilots in its 1989 memorandum about

flight in freezing rain, the information and training provided by Simmons

Airlines/AMR Eagle to its flightcrews did not prohibit holding in icing conditions

that were perceived to be within the capabilities of the airplane. In addition, the crew

did not have specific training or information necessary to determine that the airplane

was operating in conditions (freezing rain) beyond those for which it had been

certificated.

It is generally understood that flight in icing conditions in any aircraft at

low airspeeds with the flaps and/or the landing gear extended for a long period of

time is not a good operating practice because such exposure increases the likelihood

of a significant accumulation of ice on the flaps and/or landing gear, which could

result in increases in weight and drag, and a decrease in aircraft performance.

However, at the time of the accident, the AFM issued by ATR and approved by the

both the DGAC and FAA did not prohibit (either implicitly or explicitly), holding

with flaps 15 in icing conditions nor did it address the use of flaps in icing

conditions. According to AMR Eagle, the flaps 0 holding data provided by ATR for

use in the AOM is advisory in nature and does not prohibit holding with flaps 15.

Additionally, the basis upon which ATR recommends holding with the flaps up is,

according to AMR Eagle, predicated on “...the most economical holding

configuration with respect to fuel consumption...this configuration provides for the

lowest dispatch fuel requirement and consequently the highest available payload.”

Based on the information provided by ATR at the time of the accident,

holding with flaps 15 extended at 175 KIAS provides a more desirable operating

margin for stall protection than the flaps 0 configuration. Further, ATR’s 1992 All

Weather Operations brochure advised flightcrews that if they recognized that they
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were in freezing rain, they should, “extend flaps as close to Vfe as possible.” This

position was reiterated at the Safety Board’s public hearing by ATR’s chief test pilot,

who stated, in part:

...as I told you not only nobody knew the pattern associated with

the large droplets but even more, nobody knew that it would have

[been] aggravated in the flaps 15 [configuration]. Flaps 15 on its

own right, selection of flaps 15, is [not] wrong and never was

made illegal.... You know, it [is a] means to reduce [the] angle of

attack....

Because there was no prohibition against flap extension in icing

conditions, and no published information explaining the potential consequences of

extending the flaps in icing conditions, the crew of flight 4184 would not have had

reason to believe that the extension of the flaps would result in an adverse ice

accumulation in front of the ailerons. In addition, the flightcrew’s training was such

that the only performance degradation they would expect from ice accumulation

would have been a continuous loss of airspeed and subsequent stall condition with

stick shaker activation, rather than an aileron hinge moment reversal at an airspeed

well above stall speed, that would suddenly overpower and disconnect the autopilot

and cause the ailerons and control wheel to move uncommanded to near their full

travel limits with no stick shaker activation.

The flightcrew's apparent lack of concern regarding the prolonged

operations in icing conditions may have been influenced by their extensive

experience of safely flying commuter aircraft in winter weather conditions, especially

in icing conditions that are prevalent in the Great Lakes region. In addiction, they

were probably confident in the ability of the airplane deicing system to adequately

shed the ice that had been accumulating on the wings and in their ability to perform

safely under the existing circumstances. The flightcrew was operating in icing

conditions that exceeded the limits set forth in 14 CFR Part 25, Appendix C,

resulting in a complete loss of aircraft control. However, the insidious nature of

these icing conditions was such that the ice accumulation on the observable portions

of the wings, windshield and other airframe parts was most likely perceived by the

flightcrew as nonthreatening throughout the holding period. Moreover, the

flightcrew was undoubtedly unaware that the icing conditions exceeded the

Appendix C limits and most likely had operated in similar conditions many times

prior to the accident, since such conditions occur frequently in the winter throughout

the Great Lakes and northeastern parts of the United States.
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Further, the flightcrew entered the holding pattern with the belief that

the holding would be of a short duration, unaware that it would be continually

extended in short increments for a total of 39 minutes. Therefore, the Safety Board

concludes that if a significant amount of ice had accumulated on the wing leading

edges so as to burden the ice protection system, or if the crew had been able to

observe the ridge of ice building behind the deice boots or otherwise been provided a

means of determining that an unsafe condition could result from holding in those

icing conditions, it is probable that they would have exited the conditions.

The Safety Board is aware that ATR provided information to the

operators of its airplanes that indicated that encounters with certain freezing

precipitation conditions could result in “roll axis anomalies.” However, this

information was vague and did not indicate to flightcrews how to determine that they

were in freezing rain, nor did it specifically alert them that encounters with freezing

rain could result in sudden autopilot disconnects, uncommanded aileron movements

and rapid roll excursions. Therefore, the crew of flight 4184 had no reason to expect

that the icing conditions in which they were operating could cause the autopilot to

disconnect unexpectedly because of the onset of an aileron hinge moment reversal

and cause a loss of normal, stable aileron control. In addition, ATR did not provide,

nor did the regulatory authorities require, training or guidance to pilots about the roll

axis “anomalies” or the recovery techniques if such an event should occur. Thus, the

Safety Board concludes that the crew of flight 4184 was not provided with adequate

information by the manufacturer or the regulatory authorities to recognize and cope

with the problems they experienced during an encounter with freezing rain.

Although the flightcrew did not indicate that it was concerned about

holding in icing conditions, the Safety Board notes that there were some potentially

distracting events that occurred during the hold. About 15 minutes of personal

conversation took place between a flight attendant and the captain that was recorded

on the CVR from 1528:00 to 1542:38. The CVR also recorded a music station

playing on the ADF frequency for about 18 minutes, as well as the sounds of the

captain's departure from the cockpit for about 5 minutes to use the rest room.

According to 14 CFR Part 121.542 (the “sterile cockpit” rule) and FAA

staff testimony at the public hearing, holding at 10,000 feet or above is not considered

to be a “critical” phase of flight. Thus, the presence of the flight attendant in the

cockpit and the ensuing conversation were not in violation of AMR Eagle policy or

Federal regulations.
194

Although the presence of the flight attendant and the music could have

been a distraction to the flightcrew, both pilots appeared to be attentive to flightrelated duties both immediately before, as well as during the roll upset. Thus, the

Safety Board also concludes that neither the flight attendant's presence in the cockpit

nor the flightcrew's conversations with her contributed to the accident.

The Safety Board did note, however, that the AMR Eagle ATR 72 flight

manual provides the captain with the authority to declare "...any other phase of a

particular flight...." a critical phase depending on the circumstances and thus to

invoke the sterile cockpit rule at the captain's discretion. The Safety Board

concludes that a sterile cockpit environment would have reduced flightcrew

distractions and could have heightened the flightcrew’s awareness to the potentially

hazardous environmental conditions in which the airplane was being operated.

However, the sterile cockpit environment would not have increased the flightcrew’s

understanding of the events that eventually transpired when the autopilot

disconnected and the ailerons and control wheels suddenly and rapidly moved

uncommanded to their full travel limits.

Had ATR provided the flightcrew with the detailed information about

these characteristics that were previously known, and have been made available since

this accident, there would have been a basis to question the flightcrew’s situational

awareness and action. However, without the appropriate information about the

aileron hardover induced by an aerodynamically unstable aileron system (as a result

of flight in freezing precipitation/large droplets), the Safety Board concludes that the

flightcrew’s actions were consistent with their training and knowledge.

Nonetheless, the Safety Board does believe that Simmons Airlines/AMR

Eagle should encourage its captains to observe a sterile cockpit environment when an

airplane is holding, regardless of altitude, in meteorological conditions, such as

convective areas and icing conditions that have the potential to demand significant

attention by a flightcrew. The Safety Board also believes that the FAA should evaluate

the need to require a sterile cockpit environment for airplanes holding in such weather

conditions as icing and convective activity, regardless of altitude.

Finally, regarding the captain's decision to take a restroom break while

in the extended holding period, the Safety Board notes that the break occurred during

a period of relatively low workload, with the first officer performing the
195

"flying pilot" duties and the autopilot engaged. Because the workload would have

increased substantially once the flight was cleared out of the hold and the approach

was commenced, it was appropriate for the captain to choose that time to take such a

break. Therefore, the Safety Board concludes that the captain's departure from the

cockpit to use the rest room during this period of time was neither prohibited by

Federal regulations nor inconsistent with Simmons Airlines/AMR Eagle policies and

procedures, and did not contribute to the accident.

14 CFR Part 121.561 states that a pilot should provide a PIREP

"whenever he encounters a meteorological condition...inflight, the knowledge of which

he considers essential to the safety of other flights." Simmons Airlines/AMR Eagle

policies are more specific in that they require the flightcrew to provide a PIREP to

ATC "...when encountering, among other conditions, inflight icing conditions...."

Further, the AIM "urges" pilots to "...cooperate and promptly volunteer reports...." of

hazardous weather conditions, including "...icing of light degree or greater...." Flight

4184 had been operating in icing conditions for more than 24 minutes; however, no

PIREP was provided. The evidence suggests that the flightcrew was not concerned

about the icing conditions and did not consider this environment to be a threat to either

their safety or that of other flights. Therefore, although the Simmons Airlines/AMR

Eagle policy did require the reporting of such conditions, and it would have been

prudent for the flightcrew to report the icing conditions as suggested in the AIM, the

Safety Board concludes that this did not contribute to the accident.

In addition, the crew received an aural TCAS alert of "traffic traffic"

shortly before the roll excursion. This particular type of alert is advisory in nature

and did not require a verbal acknowledgment or response by either crewmember, nor

did it require a pilot to make radio contact with the ATC controller unless a conflict

was perceived. Radar and FDR data indicate that the traffic which triggered the alert

was several miles away from flight 4184 and was not a factor in the accident. The

Safety Board concludes that although flight 4184 was close enough to a second

aircraft to activate the TCAS alert, the proximity of the two airplanes to one another

did not contribute to the accident.

2.9.1 Unusual Event Recovery

The Safety Board attempted to determine why the crew of flight 4184

was unable to successfully recover the airplane and prevent the accident when the

flightcrews of the airplanes involved in the prior incidents were able to do so. At
196

the time of the roll upset, flight 4184 was most likely operating in instrument

meteorological conditions (IMC), which precluded the flightcrew's use of visual cues

outside the airplane for attitude reference. According to the FDR data, the airplane

initially rolled to the right, reversed direction momentarily and subsequently rolled

again to the right.

The Safety Board notes that the ATR aileron hinge moment incidents

prior to this accident occurred with the flaps retracted and involved large, long-term

speed losses from ice-induced drag that are normally recognizable by pilots. Flight

4184 did not experience large or long-term drag increases while holding.

Analysis of the data collected during the icing tanker test and the data

from the previous ATR ice-induced aileron hinge moment reversal incidents suggest

that the successful recoveries may have been attributable, in part, to rapid pilot

corrective action. Also, because in the prior incidents the flaps had not been

extended and therefore were not retracted after ice was accreted, the airplanes were

not trimmed for flight at AOAs that were significantly higher than the aileron hinge

moment reversal AOA, as was the case with flight 4184 when the flaps were

retracted. Thus, in the previous incidents, a small speed increase would permit the

airplanes to maintain level flight at an AOA below the aileron hinge moment reversal

AOA. However, once the flaps were retracted by the crew of flight 4184, there was a

need to significantly increase the airplane's speed and trim nose down to keep the

AOA below that at which the aileron hinge moment would reverse. Because the

crew had not been alerted to or trained to recognize this situation, they were

confronted with a more difficult task than that which confronted the flightcrews of

the airplanes involved in the prior incidents. Additionally, the crew of flight 4184

could not redeploy the flaps to reduce the AOA because of the flaps 15 Vfe lockout.

The second roll event was not terminated before the airplane rolled

1 and 1/4 times, and pitched down to a nearly vertical attitude. Further, throughout

the second roll event, the elevators were deflected in a primarily nose-up position by

both crewmembers, and the rudder was deflected most of the time from 2 to

3 degrees nose left. Although the crew was applying corrective rudder during the

roll excursion, the aileron inputs by the flightcrew were not sufficient to effect

recovery. Aileron control during this time was most likely very difficult and

confusing as a result of the multiple encounters with high control wheel forces and

unusual oscillatory aileron behavior associated with the aileron hinge moment

reversal.
197

When the crew relaxed the back pressure on the control column, thereby

reducing the nose-up elevator, the AOA decreased below the hinge moment reversal

threshold, and the crew regained control of the ailerons and initiated recovery at

6,000 feet. At this point, the airplane was in a very steep, high speed descent, in a

near-inverted attitude that would most likely have been unfamiliar to the crew,

considering their lack of unusual attitude recovery training in this airplane. The

FDR, CVR, and wreckage distribution data show that in the next 9 seconds, the crew

had leveled the wings and was bringing the nose up towards a level attitude.

However, the airplane was moving at 375 KIAS with a load factor rapidly increasing

through 3.7 G when the outboard sections of the wings and the horizontal tail

separated from the airplane.

At the time of the accident, the AMR Eagle pilot training program did

not include an "unusual attitude" or "advanced maneuvers" segment (nor was such

training required). During simulator training, AMR Eagle pilots were not exposed to

aircraft attitudes that were typically beyond those used for normal operations or

considered unusual, and they only experienced an abnormal pitch attitude when they

practiced emergency descents. Although both crewmembers of flight 4184 were

certified flight instructors,85 it is likely that this was the first time they had

experienced such unexpected and excessive roll and pitch attitudes in the ATR 72.

Shortly after the upset, when the airplane rolled to an inverted position and

progressed into a steep nose-low attitude, the captain told the first officer to "mellow

it out." However, there was no evidence on the CVR to indicate that the captain was

conveying information to the first officer about the airplane's attitude, airspeed or

altitude.

The lack of unusual attitude training may have significantly hampered

the immediate recovery of the airplane once the upset occurred. However, because

the flightcrew was not aware that icing conditions could cause a sudden autopilot

disconnect, rapid and uncommanded aileron and control wheel deflections to near

their full travel limits with unusually high, unstable control wheel forces, the crew

was confronted with a situation that could have been perceived as having been

induced by the autopilot, a structural failure, or a mechanical malfunction. Because

the upset occurred suddenly and without forewarning, the crew did not have time to

assess the situation and determine the appropriate corrective actions before the roll

attitude exceeded 90 degrees. The Safety Board concludes that unusual attitude

 

85A requisite for a flight instructor certificate, set forth by the FAA, is the demonstration of an entry and recovery from

a spin.
198

training would have assisted the flightcrew in its recovery efforts and might have

prompted the captain to provide useful information to the first officer to facilitate a

timely recovery of the airplane. However, the Safety Board also concludes that

without the knowledge of the ice-induced aileron hinge moment reversal problem,

the flightcrew’s execution of conventional unusual attitude recovery techniques may

have been ineffective.

In four separate safety recommendations over the past 27 years, the

Safety Board has addressed the issue of unusual attitude training. The FAA's

unfavorable responses and failure to require such training have resulted in the Safety

Board classifying the FAA's past actions as "Unacceptable" in three of the four cases.

In the fourth case, Safety Recommendation A-93-72, the FAA's actions to

promulgate rules to bring most 14 CFR Part 135 scheduled passenger operators

under 14 CFR 121 training requirements (which include the use of simulators) was

classified "Closed—Acceptable Action" on August 29, 1995. However, the Safety

Board remains concerned that this does not necessitate a requirement to provide

unusual event/attitude training.

Based on the circumstances of this accident, the historical data of

similar accidents, and safety recommendations previously issued by the Safety

Board, the FAA, in August 1995, in joint cooperation with the aviation industry,

issued an FAA Inspector Handbook Bulletin detailing a program that encourages air

carriers to implement advanced maneuver/unusual attitude training in their pilot

training programs. AMR Eagle implemented an unusual attitude training curriculum

into its pilot training syllabus, action that the Safety Board supports. Additionally,

the Safety Board is encouraged by the FAA's latest position regarding unusual

attitude/events training; however, there remains a concern that the lack of a required

program might result in some carriers not providing unusual attitude training, and

that their respective training programs might be insufficient to demonstrate the cause

for and the recovery from aircraft attitudes that are not considered to be "normal."

Therefore, the Safety Board believes that the FAA should amend the Federal

Aviation Regulations to require air carriers to provide standardized training that

adequately addresses the recovery from unusual events and attitudes, including

extreme flight attitudes, in large, transport category airplanes.
199

2.10 AMR Eagle/Simmons Airlines Management Structure and FAA

Oversight

As noted in section 1.17, Simmons Airlines is one of four regional air

carriers owned by AMR Eagle. AMR Eagle does not hold an FAA-issued Air

Carrier certificate, but it serves as a coordinator among the four individual air

carriers, providing centralized crew scheduling, flight dispatch, and pilot training

facilities, which are staffed by employees of the individual carriers. In addition,

AMR Eagle centrally coordinates pilot recruitment and hiring, pilot training and

checking, aircraft acquisition, and airline planning and marketing. AMR Eagle also

develops and publishes standardized company manuals, such as aircraft operating

manuals and flight manuals, which are applicable to each of the four carriers.

Although major management decisions affecting Simmons Airlines

operations are often made by AMR Eagle management personnel who are not

directly involved with Simmons Airlines or its operations, the Safety Board found no

evidence that the Simmons Airlines/AMR Eagle management structure adversely

affected safety, or that it was a factor in this accident.

The FAA oversight of Simmons Airlines and the other air carriers

operating under AMR Eagle is accomplished by FAA principal operations inspectors

(POIs) assigned to each of the carriers. In addition, another FAA employee, known

as the Focal Point Coordinator (FPC), serves as a liaison between AMR Eagle

management and each of the individual POIs. When addressing matters of

compliance within their assigned airline, each POI interacts directly with the

appropriate management individual(s) from that airline. However, when addressing

matters that require coordination with AMR Eagle management, such as a

modification to the flight manuals, the POIs can only interact indirectly with AMR

Eagle through the FPC. Changes to published procedures or operating specifications

are proposed by AMR Eagle management and reviewed independently by each of the

four POIs. Once agreed upon by the POIs, the changes are then issued by AMR

Eagle.

Because the FAA's method of exercising oversight of Simmons Airlines

and the other AMR Eagle carriers relies on the FPC to coordinate the flow of

information communicated between the POIs and AMR Eagle, this structure

effectively insulates the POIs from direct contact with key decision-making

personnel at AMR Eagle. In a recent accident investigation involving Flagship

Airlines, another one of the four AMR Eagle carriers, the Safety Board examined
200

these same organizational and surveillance issues and concluded that the structure of

the FAA and its oversight of AMR Eagle did not provide for adequate interaction

between the POIs and AMR Eagle management personnel.86 While the Safety Board

found no evidence that this method of oversight contributed in any way to the

Flagship accident or this accident, the Safety Board remains concerned about the lack

of direct communication between the POIs of the individual air carriers and AMR

Eagle management. In its previous recommendation in conjunction with the Flagship

accident, the Safety Board urged the FAA to:

A-95-99

Review the organizational structure of the FAA surveillance of

AMR Eagle and its carriers with particular emphasis on the

positions and responsibilities of the Focal Point Coordinator and

principal inspectors, as they relate to the respective carriers.

In a letter to the Safety Board dated February 13, 1996, the FAA

responded to recommendation A-95-99 as follows:

There are four American Eagle air carriers located in the FAA’s

Southern, Western-Pacific, and Southwest regional offices. Each

air carrier is owned by American Eagle and has an individual air

carrier certificate issued by the FAA. In July 1990, the flight

standards division managers of the affected regions, in

coordination with FAA headquarters, designated a focal point to

coordinate the FAA approval/acceptance process among the

principal inspectors of each carrier. A Memorandum of

Understanding was signed by the respective regional division

managers formalizing the process. Recently, an action plan was

developed to review the organizational structure and effectiveness

of the American Eagle oversight process. The review should be

completed by February 29, 1996, and a final report issued by

March 15, 1996. I will apprise the Board of the findings of the

review as soon as it is completed.

In its subsequent letter, dated May 31, 1996, concerning Safety

Recommendation A-95-99, the FAA stated, in part:

 

86NTSB/AAR-95/07, "Uncontrolled Collision with Terrain, Flagship Airlines, Inc., d.b.a. American Eagle Flight 3379,

BAe Jetstream 3201, N918AE, Morrisville, North Carolina, December 13, 1994."
201

In February 1996, the FAA conducted an evaluation of the

organizational structure and effectiveness of the American Eagle

oversight process, including the focal point process. There are

four American Eagle air carriers located in the FAA's Southern,

Western-Pacific, and Southwest regional offices. Each FAA

region having FAA air carrier certificate oversight for American

Eagle assigned a management representative to the evaluation

team. The team reviewed the effectiveness of the 1990

Memorandum of Understanding (MOU) and interviewed FAA

principal inspectors from the respective certificate holders, flight

standards district office management, and American Eagle

management. The FAA and American Eagle focal points were

also interviewed during the evaluation. The evaluation included a

visit to American Eagle headquarters, the crewmember training

center, and the dispatch facility located in Fort Worth, Texas. On

March 1, 1996, the FAA issued its final report resulting from its

evaluation. The major findings of the evaluation are as follows:

• Overall surveillance of the four air carriers meets and/or

exceeds the requirements of 14 CFR Part 121 and FAA orders.

• The focal point process provides a quality review yet is slow at

times.

• The 1990 MOU needed to be revised to reduce coordination

time between the FAA and American Eagle for items requiring

FAA approval or acceptance.

As a result of this evaluation, the FAA revised the MOU on

March 1, 1996, to reflect the concerns revealed during the

evaluation. I have enclosed a copy of the final report and a copy

of the revised MOU for the Board's information.

I believe that the FAA has met the full intent of this safety

recommendation, and I consider the FAA's action to be

completed.
202

The FAA's action adequately addresses the issues raised by the Safety

Board. Therefore, the Safety Board classifies Safety Recommendation A-95-99

"Closed—Acceptable Action."
203

3. CONCLUSIONS

3.1 Findings

1. The flightcrew was properly certified and qualified in accordance with

applicable regulations to conduct the flight.

2. The Chicago air route traffic control center (ARTCC) sector controllers

were properly certified and trained to perform their duties.

3. The ATR 72 was certificated, equipped, and maintained in accordance

with Federal regulations and approved procedures.

4. There was no evidence of an aircraft structural or system failure that

would have either been causal or contributing to the accident.

5. Flight 4184 encountered a mixture of rime and clear airframe icing in

supercooled cloud and drizzle/rain drops. Some drops were estimated

to be greater than 100 microns in diameter, and some were as large as

2,000 microns.

6. The forecasts produced by the National Weather Service (NWS) were

substantially correct, and the actions of the forecasters at the National

Aviation Weather Advisory Unit (NAWAU) and the meteorologists at

the Chicago ARTCC's Center Weather Service Unit (CWSU) were in

accordance with NWS guidelines and procedures.

7. Safety would be enhanced if the hazardous in-flight weather advisory

service (HIWAS) information were presented more consistently and

had included all of the information pertinent to the safety of flight, such

as the altitudes of the icing conditions, the intensity and type of icing,

and the location of the actual or expected icing conditions (e.g. in

clouds and precipitation).

8. The flightcrew's actions would not have been significantly different

even if they had received the available AIRMETs.

9. The flightcrew’s actions were consistent with their training and

knowledge.
204

10. PIREPs [pilot reports] of icing conditions, based on the current icing

severity definitions, may often be misleading to pilots, especially to

pilots in aircraft that may be more vulnerable to the effects of icing

than other aircraft.

11. The aviation community's general understanding of the phrase "icing in

precipitation," which is used by the NWS and is often contained in inflight weather advisories, does not typically specify types of

precipitation. The provision of a definition in aviation publications,

such as the Aeronautical Information Manual (AIM) or Part 1 of the

Federal Aviation Regulations, would make pilots and dispatchers more

aware of the types of precipitation and icing conditions that are implied

by this phrase.

12. Continued development of equipment and computer programs to

measure and monitor the atmosphere could permit forecasters to

produce real-time warnings that define specific locations of potentially

hazardous atmospheric icing conditions (including freezing drizzle and

freezing rain) and short range forecasts ("nowcasts") that identify icing

conditions for a specific geographic area with a valid time of 2 hours or

less.

13. The 14 Code of Federal Regulations (CFR) Part 25, Appendix C,

envelope is limited and does not include conditions of freezing drizzle

or freezing rain; thus, the current process by which aircraft are certified

using the Appendix C icing envelope is inadequate and does not

require manufacturers to sufficiently demonstrate the airplane's

capabilities in all the possible icing conditions that can, and do, occur

in nature.

14. No airplane should be authorized or certified for flight into icing

conditions more severe than those to which the airplane was subjected

in certification testing, unless the manufacturer can otherwise

demonstrate the safety of flight in such conditions.

15. If the FAA had acted more positively upon the Safety Board's aircraft

icing recommendations issued in 1981, this accident may not have

occurred.
205

16. ATR 42 and 72 ice-induced aileron hinge moment reversals, autopilot

disconnects, and rapid, uncommanded rolls could occur if the airplanes

are operated in near freezing temperatures and water droplet median

volume diameter (MVDs) typical of freezing drizzle.

17. At the initiation of the aileron hinge moment reversal affecting flight

4184, the 60 pounds of force on the control wheel required to maintain

a wings-level-attitude were within the standards set forth by the Federal

Aviation Regulations. However, rapid, uncommanded rolls and the

sudden onset of 60 pounds of control wheel force without any warning

to the pilot, or training for such unusual events, would most likely

preclude a flightcrew from making a timely recovery.

18. ATR is considering design changes to the lateral control system for

current and future ATR airplanes that will reduce the susceptibility to

flow separation-induced aileron hinge moment reversals. Such design

changes could minimize the reliance on the changes to flight operations

and pilot training that have already been mandated.

19. The French Directorate General for Civil Aviation (DGAC) and the

Federal Aviation Administration (FAA) failed to require the

manufacturer to provide documentation of known undesirable postSPS [stall protection system] flight characteristics, which contributed

to their failure to identify and correct, or otherwise properly address,

the abnormal aileron behavior early in the history of the ATR icing

incidents.

20. The addition of a test procedure, similar to the "zero G" flight test

maneuver (pushover) designed to identify ice-induced elevator hinge

moment reversals, could determine the susceptibility of an aircraft to

aileron hinge moment reversals in both the clean and iced-wing

conditions and could help prevent accidents such as the one involving

flight 4184.

21. Prior to the Roselawn accident, ATR recognized the reason for the

aileron behavior in the previous incidents and determined that ice

accumulation behind the deice boots, at an AOA sufficient to cause an

airflow separation, would cause the ailerons to become unstable.

Therefore, ATR had sufficient basis to modify the airplane and/or
206

provide operators and pilots with adequate, detailed information

regarding this phenomenon.

22. The 1989 icing simulation package developed by ATR for the training

simulators did not provide training for pilots to recognize the onset of

an aileron hinge moment reversal or to execute the appropriate

recovery techniques.

23. ATR’s proposed post-Mosinee AFM/FCOM changes, even if adopted

by the DGAC and the FAA, would not have provided flightcrews with

sufficient information to identify or recover from the type of event that

occurred at Roselawn, and the actions taken by ATR following the

Mosinee incident were insufficient.

24. The 1992 ATR All Weather Operations brochure was misleading and

minimized the known catastrophic potential of ATR operations in

freezing rain.

25. ATR failed to disseminate adequate warnings and guidance to

operators about the adverse characteristics of, and techniques to

recover from, ice-induced aileron hinge moment reversal events; and

ATR failed to develop additional airplane modifications, which led

directly to this accident.

26. The DGAC failed to require ATR to take additional corrective actions,

such as performing additional icing tests, issuing more specific

warnings regarding the aileron hinge moment reversal phenomenon,

developing additional airplane modifications, and providing specific

guidance on the recovery from a hinge moment reversal, which led

directly to this accident.

27. The FAA's failure, following the 1994 Continental Express incident at

Burlington, Massachusetts, to require that additional actions be taken

to alert operators and pilots to the specific icing-related problems

affecting the ATRs, and to require action by the manufacturer to

remedy the airplane's propensity for aileron hinge moment reversals in

certain icing conditions, contributed to this accident.
207

28. The FAA Aircraft Evaluation Group (AEG) did not receive in a timely

manner, from all sources, pertinent documentation (such as the ATR

analyses) regarding the previous ATR icing incidents/accidents that

could have been used to monitor the continued airworthiness of the

airplane.

29. The ability of the FAA's AEG to monitor, on a real-time basis, the

continued airworthiness of the ATR airplanes was hampered by the

inadequately defined lines of communication, the inadequate means for

the AEG to retrieve pertinent airworthiness information, and the

DGAC's failure to provide the FAA with critical airworthiness

information, because of the DGAC's apparent belief that the

information was not required to be provided under the terms of the

Bilateral Airworthiness Agreement (BAA). These deficiencies also

raise concerns about the scope and effectiveness of the BAA.

30. The FAA's limited involvement in the ATR 42 certification does not

appear to have resulted in an improperly certificated airplane (ATR

42/72). However, the FAA's excessive reliance on a foreign

airworthiness authority may result in tacit approval of the certification

of a foreign-manufactured airplane without sufficient oversight and is

not in the best interest of safety.

31. The nearby air traffic control facilities were aware that light icing

conditions were forecast for the area of the LUCIT intersection.

Nonetheless, the release of flight 4184 from Indianapolis was proper

because there were viable options for pilots who chose to avoid

holding in icing conditions.

32. Under the circumstances on the day of accident, the controllers acted

appropriately in the management of traffic flow into O’Hare

International Airport (ORD), which necessitated the holding of flight

4184 in the BOONE sector.

33. The air traffic control (ATC) traffic management coordinator failed to

report flight 4184 to the air traffic control system command center

(ATCSCC) as an arrival delay, and he failed to alert the ATCSCC that

flight 4184 had been holding for more than 15 minutes. However, this
208

lack of information did not affect the operation of the flight and did not

contribute to the accident.

34. Because there were no PIREPs [pilot reports] provided to the Boone

sector controller by other pilots, and because the crew of flight 4184

did not provide a PIREP of icing conditions at the LUCIT intersection,

it was reasonable for the controller to conclude that there were no

significant weather events in that area and that the crew of flight 4184

was not experiencing any problems that would have warranted

precautionary action by the controller.

35. Because the DGAC did not require ATR, and ATR did not provide to

the operators of its airplanes, information that specifically alerted

flightcrews to the fact that encounters with freezing rain could result in

sudden autopilot disconnects, aileron hinge moment reversals, and

rapid roll excursions, or guidance on how to cope with these events, the

crew of flight 4184 had no reason to expect that the icing conditions

they were encountering would cause the sudden onset of an aileron

hinge moment reversal, autopilot disconnect, and loss of aileron

control.

36. Neither the flight attendant's presence in the cockpit nor the

flightcrew's conversations with her contributed to the accident.

However, a sterile cockpit environment would probably have reduced

flightcrew distractions and could have promoted an appropriate level of

flightcrew awareness for the conditions in which the airplane was

being operated.

37. The flightcrew’s failure to increase the propeller RPM to 86 percent

and activate the Level III ice protection system in response to the

1533:56 caution alert chime was not a factor in the accident.

38. Had ice accumulated on the wing leading edges so as to burden the ice

protection system, or if the crew had been able to observe the ridge of

ice building behind the deice boots or otherwise been provided a means

of determining that an unsafe condition was developing from holding

in those icing conditions, it is probable that the crew would have exited

the conditions.
209

39. The captain's departure from the cockpit to use the rest room while the

airplane was in the holding pattern was neither prohibited by Federal

regulations nor inconsistent with Simmons Airlines/AMR Eagle

policies and procedures and did not contribute to the accident.

40. Although the Simmons Airlines/AMR Eagle policy does require

flightcrews to provide a PIREP of icing conditions, and it would have

been prudent for the crew of flight 4184 to provide such a report, their

failure to do so did not contribute to the accident.

41. Although the crew of flight 4184 received an aural traffic alert and

collision avoidance system (TCAS) alert shortly before the roll

excursion, this alert was not perceived by the crew as a conflict, and the

proximity of the two airplanes to one another did not contribute to the

accident.

42. Both pilots saw the ground, realized their close proximity and high

descent rate, and made a nose-up elevator input that, combined with the

high airspeed (about 115 KIAS over the certified maximum operating

airspeed) resulted in excessive wing loading and structural failure of

the outboard sections of the wings.

43. Although both crew members of flight 4184 were certified flight

instructors, this was probably the first time they had experienced such

unexpected and excessive roll and pitch attitudes in the ATR 72. If the

operators had been required to conduct unusual attitude training, the

knowledge from this training might have assisted the flightcrew in its

recovery efforts and might have prompted the captain to provide useful

information to the first officer to facilitate a timely recovery of the

airplane.
210

3.2 Probable Cause

The National Transportation Safety Board determines that the

probable causes of this accident were the loss of control, attributed to a sudden

and unexpected aileron hinge moment reversal that occurred after a ridge of ice

accreted beyond the deice boots because: 1) ATR failed to completely disclose to

operators, and incorporate in the ATR 72 airplane flight manual, flightcrew

operating manual and flightcrew training programs, adequate information

concerning previously known effects of freezing precipitation on the stability and

control characteristics, autopilot and related operational procedures when the ATR

72 was operated in such conditions; 2) the French Directorate General for Civil

Aviation’s (DGAC’s) inadequate oversight of the ATR 42 and 72, and its failure

to take the necessary corrective action to ensure continued airworthiness in icing

conditions; and 3) the DGAC’s failure to provide the FAA with timely

airworthiness information developed from previous ATR incidents and accidents

in icing conditions, as specified under the Bilateral Airworthiness Agreement and

Annex 8 of the International Civil Aviation Organization.

Contributing to the accident were: 1) the Federal Aviation

Administration’s (FAA’s) failure to ensure that aircraft icing certification

requirements, operational requirements for flight into icing conditions, and FAA

published aircraft icing information adequately accounted for the hazards that can

result from flight in freezing rain and other icing conditions not specified in 14

Code of Federal Regulations (CFR) Part 25, Appendix C; and 2) the FAA’s

inadequate oversight of the ATR 42 and 72 to ensure continued airworthiness in

icing conditions.
211

4. RECOMMENDATIONS

As a result of the investigation of this accident, the National

Transportation Safety Board makes the following recommendations:

--to the Federal Aviation Administration:

Direct principal operations inspectors (POIs) to ensure that all 14

Code of Federal Regulations (CFR) Part 121 air carriers require

their dispatchers to provide all pertinent information, including

airman’s meteorological information (AIRMETs) and Center

Weather Advisories (CWAs), to flightcrews for preflight and inflight planning purposes. (Class II, Priority Action) (A-96-48)

Require that Hazardous In-flight Weather Advisory Service

(HIWAS) broadcasts consistently include all pertinent information

contained in weather reports and forecasts, including in-flight

weather advisories, airman’ s meteorological information

(AIRMETs), significant meteorological information (SIGMETs),

and Center Weather Advisories (CWA’s). (Class II, Priority

Action) (A-96-49)

Encourage principal operations inspectors (POIs) and operators to

reemphasize to pilots that Hazardous In-flight Weather Advisory

Service (HIWAS) is a source of timely weather information and

should be used whenever they are operating in or near areas of

potentially hazardous weather conditions. (Class II, Priority

Action) (A-96-50)

Revise the existing aircraft icing intensity reporting criteria (as

defined in the Aeronautical Information Manual (AIM) and other

Federal Aviation Administration (FAA) literature) by including

nomenclature that is related to specific types of aircraft, and that is

in logical agreement with existing Federal Aviation Regulations

(FARs). (Class II, Priority Action) (A-96-51)

Publish the definition of the phrase “icing in precipitation” in the

appropriate aeronautical publications, emphasizing that the
212

condition may exist both near the ground and at altitude. (Class II,

Priority Action) (A-96-52)

Continue to sponsor the development of methods to produce

weather forecasts that both define specific locations of atmospheric

icing conditions (including freezing drizzle and freezing rain) and

produce short-range forecasts (“nowcasts”) that identify icing

conditions for a specific geographic area with a valid time of

2 hours or less. (Class II, Priority Action) (A-96-53)

Revise the icing criteria published in 14 Code of Federal

Regulations (CFR), Parts 23 and 25, in light of both recent research

into aircraft ice accretion under varying conditions of liquid water

content, drop size distribution, and temperature, and recent

developments in both the design and use of aircraft. Also, expand

the Appendix C icing certification envelope to include freezing

drizzle/freezing rain and mixed water/ice crystal conditions, as

necessary. (Class II, Priority Action) ) (A-96-54) (Supersedes A81-116 and-118)

Revise the Federal Aviation Regulations (FAILs) icing certification

requirements and advisory material to specify the numerical

methods to be used in determiningg median volumetric diameter

(MVD) and liquid water content (LWC) during certification tests.

(Class II, Priority Action) (A-96-55)

Revise the icing certification testing regulation to ensure that

airplanes are properly tested for all conditions in which they are

authorized to operate, or are otherwise shown to be capable of safe

flight into such conditions. If safe operations cannot be

demonstrated by the manufacturer, operational limitations should be

imposed to prohibit flight in such conditions and flightcrews should

be provided with the means to positively determine when they are in

icing conditions that exceed the limits for aircraft certification.

(Class II, Priority Action) (A-96-56)

Require all aircraft manufacturers to provide, as part of the

certification criteria, information to the FAA and operators about

any known undesirable characteristics of flight beyond the protected
213

(stall system and related shaker/pusher) flight regime. (Class II,

Priority Action) (A-96-57)

Develop an icing certification test procedure similar to the tailplane

icing pushover test to determine the susceptibility of airplanes to

aileron hinge moment reversals in the clean and iced-wing

conditions. Revise 14 CFR Parts 23 and 25 icing certification

requirements to include such a test. (Class II, Priority Action)

(A-96-58)

Encourage ATR to test the newly developed lateral control system

design changes and upon verification of the improved or corrected

hinge moment reversal/uncommanded aileron deflection problem,

require these design changes on all new and existing ATR airplanes.

(Class-II, Priority Action) (A-96-59)

Revise 14 CFR Parts 91.527 and 135.227 to ensure that the

regulations are compatible with the published definition of severe

icing, and to eliminate the implied authorization of flight into severe

icing conditions for aircraft certified for flight in such conditions.

(Class II, Priority Action) (A-96-60)

Require all principal operations inspectors (POIs) of 14 CFR Part

121 and 135 operators to ensure that training programs include

information about all icing conditions, including flight into freezing

drizzle/freezing rain conditions. (Class II, Priority Action) (A-96-

61)

Develop an organizational structure and a communications system

that will enable the Aircraft Evaluation Group (AEG) to obtain and

record all domestic and foreign aircraft and parts/systems

manufacturers’ reports and analyses concerning incidents and

accidents involving aircraft types operated in the United States, and

ensure that the information is collected in a timely manner for

effective AEG monitoring of the continued airworthiness of aircraft.

(Class II, Priority Action) (A-96-62)

Review and revise, as necessary, the manner in which the FAA

monitors a foreign airworthiness authority’s compliance with U.S.
214

type certification requirements under the Bilateral Airworthiness

Agreement (BAA). (Class II, Priority Action) (A-96-63)

Establish policies and procedures to ensure that all pertinent

information is received, including the manufacturer’s analysis of

incidents, accidents or other airworthiness issues, from the

exporting country’s airworthiness authority so that the FAA can

monitor and ensure the continued airworthiness of airplanes

certified under the Bilateral Airworthiness Agreement (BAA).

(Class II, Priority Action) (A-96-64)

Evaluate the need to require a sterile cockpit environment for

airplanes holding in such weather conditions as icing and convective

activity, regardless of altitude. (Class II, Priority Action) (A-96-65)

Amend the Federal Aviation Regulations to require operators to

provide standardized training that adequately addresses the

recovery from unusual events, including extreme flight attitudes in

large, transport category airplanes. (Class II, Priority Action) (A96-66)

Revise FAA Order 8400.10, Chapter 7, Section 2, paragraph 1423

(Operational Requirements - Flightcrews) to specify that Center

Weather Advisories (CWAs) be included and considered in the

flightcrew’s preflight planning process. (Class II, Priority Action)

(A-96-67)

Revise FAA Order 7110.65, “Air Traffic Control,” Chapter 2,

“General Control,” Section 6, “Weather Information,” paragraph

2-6-3, “PIREP” Information, to include freezing drizzle and

freezing rain. Additionally, these conditions should be clearly

defined in the Pilot/Controller Glossary. (Class II, Priority Action)

(A-96-68)

Conduct or sponsor research and development of on-board aircraft

ice protection and detection systems that will detect and alert

flightcrews when the airplane is encountering freezing drizzle and

freezing rain and accreting resultant ice. (Class II, Priority Action)

(A-96-69)
215

--to the National Oceanic and Atmospheric Administration:

Develop methods to produce weather forecasts that both define

specific locations of atmospheric icing conditions (including

freezing drizzle and freezing rain), and that produce short range

forecasts (“nowcasts”) that identify icing conditions for a specific

geographic area with a valid time of 2 hours or less. Ensure the

timely dissemination of all significant findings to the aviation

community in an appropriate manner. (Class II, Priority Action)

(A-96-70)

--to AMR Eagle:

Require dispatchers to include in the flight release airman’s

meteorological information (AIRMETs) and center weather

advisories (CWAs) that are pertinent to the route of flight so that

flightcrews can consider this information in their preflight and inflight decisions. (Class II, Priority Action) (A-96-71)

Encourage captains to observe a “sterile cockpit” environment

when an airplane is holding, regardless of altitude, in meteorological

conditions such as convective areas or icing conditions, that have

the potential to demand significant attention of a flightcrew.

(Class II, Priority Action) (A-96-72)

Conduct a procedural audit to eliminate existing conflicts in

guidance and procedures between the Aircraft Flight

Flight Operations Manuals, and other published material.

Priority Action) (A-96-73)

Also as a result of this accident, the Safety Board issued

safety recommendations to the FAA on November 7, 1994:

Manuals,

(Class II,

the following

Conduct a special certification review of the ATR 42 and ATR 72

airplanes, including flight tests and/or wind tunnel tests, to

determine the aileron hinge moment characteristics of the airplanes

operating with different airspeeds and configurations during ice

accumulation and with varying angles of attack following ice

accretion. As a result of the review, require modifications as
216

necessary to assure satisfactory flying qualities and control system

stability in icing conditions. (Class II, Priority Action) (A-94-181)

Prohibit the intentional operation of ATR 42 and ATR 72 airplane

in known or reported icing conditions until the effect of upper wing

surface ice on the flying qualities and aileron hinge moment

characteristics are examined further as recommended in A-94-181

and it is determined that the airplane exhibits satisfactory flight

characteristics. (Class I, Urgent Action) (A-94-182)

Issue a general notice to ATC personnel to provide expedited

service to ATR 42 and ATR 72 pilots who request route, altitude,

or airspeed deviations to avoid icing conditions. Waive the

175 knot holding speed restriction for ATR 42 and ATR 72

airplanes pending acceptable outcome of the special certification

effort. (Class I, Urgent Action) (A-94-183)

Provide guidance and direction to pilots of ATR 42 and ATR 72

airplanes in the event of inadvertent encounter with icing conditions

by the following actions: (1) define optimum airplane configuration

and speed information; (2) prohibit the use of autopilot; (3) require

the monitoring of lateral control forces; (4) and define a positive

procedure for reducing angle of attack. (Class I, Urgent Action)

(A-94-184)

Caution pilots of ATR 42 and ATR 72 airplanes that rapid descents

at low altitude or during landing approaches or other deviations

from prescribed operating procedures are not an acceptable means

of minimizing exposure to icing conditions. (Class I, Urgent

Action) (A-94-185)

In addition, the Safety Board issued the following safety

recommendations to the FAA on November 6, 1995:

Require the Air Traffic Control System Command Center to retain

all flow control-related facility documents for 15 days, regardless of

title, name. or form number, for reconstruction purposes. (Class II,

Priority Action) (A-95-103)
217

Develop a list of documents to be completed by the Air Traffic

Control System Command Center personnel in the event of an

incident or accident. (Class II, Priority Action) (A-95-104)

Revise Order 8020.11, “Aircraft Accident and Incident Notification,

Investigation and Reporting,” to include the Air Traffic Control

System Command Center (DCC) facility. Ensure that the DCC

facility is assigned specific requirements to be included in an

accident/incident package. (Class II, Priority Action) (A-95-105)

Revise FAA Order 7210.3, “Facility Operation and

Administration,” Chapter 3, “Facility Equipment,” Section 4,

“Recorders,” paragraph 3-41, “Assignment of Recorder Channels,”

to include the Air Traffic Control System Command Center facility,

listing the recorded positions and their priority. (Class II, Priority

Action) (A-95-106)

BY THE NATIONAL TRANSPORTATION SAFETY BOARD

James E. Hall

Chairman

John Hammerschmidt

Member

John J. Goglia

Member

George W. Black

Member

Vice Chairman Robert T. Francis did not participate.

July 9, 1996

219

APPENDIXES

APPENDIX A

INVESTIGATION AND HEARING

1. Investigation

The Safety Board was notified of the accident by the FAA

Communications Center, Washington, D. C., at approximately 1700 eastern

standard time on October 31, 1994. A full Go-Team was dispatched to Roselawn,

Indiana, at approximately 2100 that evening via the FAA's Gulfstream IV aircraft.

The Investigator-in-Charge (IIC) was Mr. Gregory A. Feith, and Chairman James

Hall was the Board Member who accompanied the team to the site. The on-scene

investigation was conducted over a period of 9 days. The follow-up investigative

activities were conducted at various locations, including Toulouse, France, and

Edwards Air Force Base, California, and involved extensive operational,

engineering, airworthiness, air traffic control, and aircraft performance issues.

Investigative groups were convened at the Safety Board's

Headquarters in Washington, D. C., to read out the cockpit voice recorder (CVR)

and flight data recorder (FDR) after they were recovered from the accident

airplane and transported to the Safety Board.

The following were designated as parties to the investigation:

1. The Federal Aviation Administration (FAA)

2. Simmons Airlines, Inc./AMR Eagle

3. Air Line Pilots Association (ALPA)

4. Avions de Transport Regional ( ATR)

5. National Air Traffic Controllers Association (NATCA)

6. National Weather Service (NWS)

7. National Aeronautics and Space Administration (NASA)

8. Honeywell, Inc.

An accredited representative from the Bureau Enquetes-Accidents

(BEA) and the Direction General a l'Aviation Civile (DGAC) participated in all

investigative activities.
220

2. Public Hearing

A public hearing, chaired by Member John Hammerschmidt, was held

in Indianapolis, Indiana, from February 27 through March 3, 1995.
221

APPENDIX B

COCKPIT VOICE RECORDER TRANSCRIPT

Transcript of a Fairchild A-100A cockpit voice recorder (CVR), s/n 60753, installed

on an American Eagle ATR 72, N401AM, which was involved in a collision with terrain

near Roselawn, Indiana, on October 31, 1994.

LEGEND

HOT Crewmember hot microphone voice or sound source

RDO Radio transmission from accident aircraft

CAM Cockpit area microphone voice or sound source

INT Transmissions over aircraft interphone system

CTR Radio transmission from Chicago Center

ADF Transmission received over aircraft’s ADF radio

PA Transmission made over aircraft Public Address system

AEC Radio transmission from American Eagle Chicago Operations Control

KW17 Radio transmission from KIWI flight seventeen.

-B Sounds heard only through both pilots’ hot microphone systems

-1 Voice identified as Pilot-in-Command (PIC)

-2 Voice identified as Co-Pilot

-3 Voice identified as 1st female Flight Attendant

-4 Voice identified as 2nd female Flight Attendant

-? Voice unidentified

* Unintelligible word

@ Non pertinent word

# Expletive

% Break in continuity

( ) Questionable insertion

[ ] Editorial insertion

.... Pause

Note 1: Times are expressed in central standard time (CST).

Note 2: Non pertinent conversation where noted refers to conversation that does not directly concern the

operation, control, or condition of the aircraft, the effect of which will be considered along with

other facts during the analysis of flight crew performance.
222

NATIONAL TRANSPORTATION SAFETY BOARD

Engineering and Computer Services Division

Washington, D.C. 20594

SPECIALIST’S FACTUAL REPORT OF INVESTIGATION

Cockpit Voice Recorder

DCA 95 MA 001

October 16, 1995

The following corrections on page #8 of the original transcript have been approved by the CVR

Group:

1. Modify editorial comment at time 1542:40;

CAM [sound of several clicks similar to cockpit door being opened and closed]

Modify editorial comment at time 1542:46;

CAM [low frequency sound decreases slightly in volume]

Albert G. Reitan

Transportation Safety Specialist
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENTSTART of RECORDINGSTART of TRANSCRIPT

1527:59ADF-2 [sound of music]

1528:00CAM-1 did that transmit?1528:02CAM-2 looks like it did.1528:06CAM-3 ***.1528:07CAM-2 I didn’t see the transmit thing go off because I was distracted.1528:11CAM-3 wow, ***.1528:18CAM-? ** see what’s going on up here.1528:21CAM [sound of loud music]1528:26CAM-3 is that like stereo, radio. ... you don’t have a hard job at all.

... we’re back there slugging with these people. *****.

1528:38HOT-1 yeah you are.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1528:40CAM-3 **.1528:44HOT-1 we do have it pretty easy. I was telling Jeff I don’t think I’d

ever want to do anything else but this .

1528:51CAM-3 *****.1528:53CAM-2 no,  .1528:54CAM-3 ** *.1528:55CAM-2 just wanted to see your reaction. I, I like dealing people

in a way it’s kinda’ neat to be able to talk to them.

1529:03CAM [miscellaneous non-pertinent conversation between

captain and flight attendant continues]

1530:00CAM-3 I know.1530:00CAM-3 and how late are we going to be?1530:01CAM-1 well.1530:02CAM-3 we already got two people that have already missed their

flight.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1530:05CAM-1 oh really.1530:06CAM-3 three fifteen is one of them.1530:08CAM-1 three fifteen, three fifteen?1530:10CAM-3 it’s all your fault.1530:11CAM-3 uh huh. we weren’t due into Chicago until three fifteen.

***.

1530:15CAM-3 ***.1530:20CAM-1 she’s lying then.1530:23CAM-3 you know what we deal with out here?1530:26CAM-2 ** four fifteen.1530:28CAM-1 ya, you should hit her.1530:29CAM-3 yeah.1530:30CAM-1 three fifteen eastern time.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1530:34 - 1531:11CAM [miscellaneous non-pertinent conversation between pilot

and flight attendant continues]

1531:11CAM-3 what do you all do up here when *** when auto-piloting?

just hang out?

1531:17CAM-2 you still gotta tell it what to do.1531:20CAM-1 if the auto-pilot didn’t work, he’d be one busy little bee right

now.

1531:23CAM-2 [sound of laughter}1531:25CAM-3 so does the FO’s do a lot more work than you do?1531:28CAM-1 yep.1531:29CAM-3 [sound of laughter]1531:30CAM-2 not really.1531:31 - 1533:10CAM [non-pertinent pilot and flight attendant conversation continues]1533:13HOT-1 man this thing gets a high deck angle in these turns.1533:15HOT-2 yeah.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1533:17HOT-1 we’re just wallowing in the air right now.1533:19HOT-2 you want flaps fifteen?1533:21HOT-1 I’ll be ready for that stall procedure here pretty soon.1533:22HOT-2 [sound of chuckle]1533:24HOT-1 do you want kick ‘em in (it’ll) bring the nose down.1533:25HOT-2 sure.1533:26CAM [sound of several clicks similar to flap handle being

moved]

1533:29HOT-1 guess Sandy’s going “ooo”.1533:34CAM [wailing sound of “whooler” similar to pitch movement]1533:36HOT-1 so anyway ..1533:37CAM-3 aah.1533:39HOT-1 .. the trim, automatic trim.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1533:41CAM-3 what were they telling me about, what if, ***** something

about rain. they always trick the hiring people. * aboutrain, ** some little person that talks.

1533:56CAM [single tone similar to caution alert chime]1533:57HOT-1 rain?1533:58HOT-2 no, this one maybe?1533:59HOT-3 sounds like it said something about the rain, or.1534:01CAM-5 glide slope, whoop whoop, pull-up, whoop whoop pull-up.1534:05HOT-1 that one?1534:07CAM-3 ya, but there’s something else.1534:09HOT-2 no, like I said. it’s a rain cloud they say, well how you know?

because this thing tells us. it’ll tell you, terrain, terrain.

1534:18CAM-3 that’s what it says, terrain ***.1534:19HOT-1 I think it’s this thing here.1534:20CAM-3 ya.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1534:21HOT-1 I don’t know ..1534:23CAM-5 too-low, terrain, too-low terrain.1534:25 - 1538:47CAM [non-pertinent pilot and flight attendant conversation continues]1537:40CAM [wailing sound for 1.0 seconds similar to “whooler” pitch

trim movement]

1538:43CTR Eagle flight one eight four, expec

two two zero zero.

1538:47RDO-1 OK, we’ll expect further two two z

uh, one eight four.

1538:55 - 1542:34CAM [non-pertinent pilot and flight attendant conversation continues]

1538:55ADF-2 [sound of music similar to standa

tion continues]

1541:07CAM [single tone similar to caution alert chime]
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1541:12CAM [low frequency sound starts and increases slightly in frequency similar to increase in propeller RPM]1542:15CAM [wailing sound for 0.5 seconds similar to “whooler” pitch

trim movement]

1542:20CAM [sound of eight clicks]1542:38CAM-3 see you all.1542:39CAM-1 alright.1542:40CAM [sound of several click similar to cockpit door being

opened and closed]

1542:41

[Hereafter, all cockpit conversation and radio transmissions relating to flight 4184 have been transcribed in

their entirety.]

1542:46CAM [low frequency sound decreases slightly in similar to decrease is propeller RPM]1543:16HOT-2 let’s see, we got about uh, thirty six hundred pounds of

fuel?

1543:19HOT-1 uh huh.1543:27
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENTHOT-2 they sent us a message see that dispatch?1543:30HOT-1 does it work?1543:32HOT-2 so they must have got that message that we were in a hold

there.

1543:35HOT-1 why, what happened?1543:37HOT-2 um,1543:40HOT-1 oh, you got this?1543:42HOT-2 yeah. it just came up on its own.1543:51HOT-1 so did you send ‘em uh, the new updated uh, EFC?1543:56HOT-2 yeah. I just threatened to send it. it says acknowledge it.

how do you acknowledge it? this is the only way I knowhow.

1544:03HOT-1 yeah, you just uh, send ‘em something.1544:06HOT-2 should I tell ‘em how much fuel we got?1544:07HOT-1 sure.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1544:09HOT-2 **.1544:14HOT-? *.

1544:19ADF-2 [sound of music similar to standa

tion continues]

1544:25HOT-2 space, f-u-e-l is that?1544:36HOT-1 **** thirty six hundred pounds **.1545:10HOT-2 crews receive dummy messages but I don’t what that

***.

1545:14HOT-1 acknowledge message one two one one? they sending

you another message?

1545:18HOT-2 see that was in there before.1545:20HOT-1 oh OK, that’s an old one?1545:21HOT-2 yeah, I think ...1545:27HOT-1 did you send ‘em something?1545:29HOT-2 I think that’s if you send them that uh, acknowledged it’s

called **.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1545:34HOT-1 essential data *** .1545:36HOT-2 yeah.

1545:48ADF-2 [sound of music similar to standa

tion stops]

1545:48PA-1 well folks once again, this is the captain. you’re uh, do regret to

inform that, air traffic control has put us into a holding patternup here, we’re holding for approximately twenty minutes out ofChicago at this time but uh, I guess the congestion an’ traffic’scontinued on uh, they need us to hold out here for somespacing. they’re saying at this point uh, on the hour beforewe depart the hold though that may not hold uh, we may notbe here the full thirteen minutes. we’ll be sure to keep youupdated. once we leave the hold we’ll let you know more ifthey tell us the hold is going to be a little bit longer. I doapologize for all these delays. chances are that all theflights in and out of Chicago here this afternoon are going tobe delayed as well. this is not just aircraft in the air rightnow but this is also uh, for aircraft that were in the air earlier,aircraft on the ground and aircraft that are going to be departing. so uh, once again chances are that your flight would bedelayed also and you’ll still have a real good chance of makingyour connection. if not, they’ll uh, automatically re-book youon the next flight in Chicago.

1546:51HOT-1 did you get another note?1546:55HOT-2 no. I’m just trying to figure this out. this uh, ... messages.

there isn’t any company messages. but I think that’s thenumber you put in there you just hit this number. and, after
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT

you write a message, you know with the free text, you dothis and see to make sure if they acknowledge **.

1547:21HOT-1 why did they do that?1547:23HOT-2 this a free text one to send your own stuff. that’s what I

say.

1547:28HOT-1 did you tell them the new, the new delay times er the EFC

is zero zero?

1547:31HOT-2 yeah. but I didn’t do it on that line, I did it on uh, uh, uh,

the delays.

1547:39HOT-1 OK they know so they know OK. what if you went like

this? messages, message received, acknowledgedthirty nine twenty, so ...

1547:52HOT-2 I, I just typed that one in myself and I, I hit enter.1547:56HOT-1 oh, OK.1547:57HOT-2 and uh, I don’t know if that means they sent me that I’m

supposed to acknowledge that and hit it or if I’m sup.. or ifthat’s us sending them a message for them to acknowledge us.

1548:05HOT-1 oh, I don’t know.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1548:05HOT-? **.1548:06HOT-? .. confused on that.1548:09HOT-1 yeah, I’ll get my little uh, ACARS book and read it.1548:13HOT-2 one guy told me that system you, send another message

and you type that number in to see if they got it. if they uh,if write in there “acknowledged”, message thirty ninetwenty from you.

1548:24HOT-1 huh.1548:26HOT-2 I guess.1548:33CAM [sound of click]1548:34HOT-2 that’s much nicer, flaps fifteen.1548:36HOT-1 yeah.1548:43HOT-? I’m showing some ice now.1548:45HOT-? **.1548:46
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENTHOT-1 I’m sure that once they let us out of the hold and forget

they’re down we’ll get the overspeed.

1548:48HOT-2 [sound of chuckle]1548:57HOT-1 good, I can’t hold any more man, that big (cup) needs out

right now.

1548:59HOT-2 [sound of chuckle] they’re gonna be giving you dirty looks,

man.

1549:02HOT-1 oh man, oh yeah, I know they are. people do. it’s either

that or pee on ‘em.

1549:05CAM [sound of ding dong similar to flight attendant call bell]1549:05HOT-1 [sound of clink similar to seat belt being unfastened]1549:06HOT-2 yeah, I’ll talk to her.1549:06CAM [sound of ding dong similar to flight attendant call bell]1549:07CAM [sound of clunk]1549:08INT-1 what’s up?1549:08INT-4 it’s just me.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1549:09INT-1 huh?1549:10INT-4 I’m uh, it was just me.1549:11INT-1 oh.1549:12INT-4 I’m just wondering how much gas do we got.1549:14INT-1 how much gas we have?1549:15INT-4 yeah.1549:16INT-1 we got more than plenty of gas. we can be out here for

a long time.

1549:19INT-4 cool, OK. just, was worried. maybe you’d have to divert

somewhere, and really make these people ...

1549:25INT-2 sixty miles from Chicago.1549:26INT-1 oh, yeah.1549:26INT-4 six, sixty miles?1549:27INT-2 yeah.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1549:28INT-4 yeah, but they’re still gonna hold us, huh?1549:30INT-1 ‘til, about another ten minutes.1549:32INT-4 and that’s not a for sure thing, is it?1549:34INT-1 eehh ya, pretty for sure as of right now unless they decide

to make it different. how’s that, for an answer?

1549:40INT-4 [chuckle] same like the other one.1549:42INT-1 yeah, I know.1549:43CAM [sound of clunk]1549:44CAM-1 talk to her bro.1549:45INT-2 OK.1549:49INT-4 bye.1549:50INT-2 hey. you there?1550:41CAM [sound of ding dong similar to flight attendant call bell]
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1550:43INT-2 hello.1550:43INT-4 *, are you sure you can handle it up there?1550:46INT-2 I’ll try.1550:47INT-4 ‘K uh,1550:48INT-2 why do you ...1550:49INT-4 turn it down. it needs to be cooler back here. it’s hot.1550:51INT-2 I’m uh, it’s all the way down now.1550:53INT-4 OK thanks.1550:53INT-2 it’s been,1550:54INT-4 it’s been down?1550:55INT-2 yeah, well, I’ll, I’ll chill it up up with * too.1550:59INT-4 really, well we’re sweatin’ [sound of panting]1551:01INT-2 you know why?
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1551:02INT-4 you wanta hear us breathe heavy? [sound of chuckle]1551:03INT-2 it’s it’s, one of the bleeds are off.1551:06INT-4 OK.1551:07INT-2 one of the for the air conditioning.1551:08INT-4 yeah.1551:09INT-2 and it’s, your side.1551:10INT-4 oh *.1551:11INT-2 it’s the one that gives you most of the air back there.1551:13INT-4 figures.1551:14INT-2 so now you got y-y-you got less than uh, half, not only that

it’s your your half. [sound of chuckle]

1551:20INT-4 OK.1551:22INT-2 I’ll try.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1551:22INT-4 OK, well here. Orlando wants to talk to you.1551:24INT-2 Orlando does?1551:35INT-2 hello.1551:39INT-1 hey bro.1551:39INT-2 yeah.1551:40INT-1 gettin’ busy with the ladies back here.1551:41INT-2 oh.1551:43INT-4 [sound of snicker]1551:45INT-1 yeah, so if so if I don’t make it up there within the next say,

fifteen or twenty minutes you know why.

1551:49INT-2 OK.1551:50INT-1 OK.1551:51INT-2 I’ll uh, when we get close to touchdown I’ll give you a ring.1551:53
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENTINT-1 there you go.1551:54INT-2 *.1551:55INT-1 no, I’ll I’ll be up right now. there’s somebody in the bathroom so ****.1551:55CAM [wailing sound similar to “whooler” pitch trim movement for

two seconds]

1551:59INT-1 talk to you later.1552:00INT-2 OK.

1553:36KW17 good afternoon Chicago, Kiwi Air 

twenty for eleven.

1553:42CTR Kiwi Air seventeen Chicago cente

timeter two niner seven niner.

1553:48CAM [sound of two clicks]1554:13CAM [sound of several clicks similar to cockpit smoke door being operated]1554:16CAM-1 we have a brand new hombre.1554:20
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENTCAM [sound of two clicks similar to captain’s seat moving laterally and forward]1554:25HOT-2 oh yeah.1554:30CAM [sound of click similar to lap belt being fastened]1554:24HOT-1 [sound similar to captain’s hot microphone bumping

against object]

1553:47KW17 two niner seven niner, roger.

1554:38HOT-2 hello.

1553:39CTR Kiwi Air seventeen, expedite you

down to eleven, please.

1553:42KW17 expedite to eleven, Kiwi Air seven

1554:47HOT-1 did you get any more messages from the cabbage patch?1554:49HOT-2 no. I sent them another message saying did you get our

twenty two hundred uh, out of the hold thing through.

1554:52CAM [sound of click similar to shoulder harness being fastened]1554:55HOT-1 *.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1554:56HOT-2 you know the other mode about delays and just asked

them if they got that.

1555:04HOT-1 [sound of sigh] **.1555:04HOT-2 enough playing with that.1555:05HOT-1 where’s the uh, where’s the connecting gates? did we

throw those away?

1555:09HOT-2 uh, I didn’t throw ‘em away.1555:12HOT-1 how do you how do you get connecting gates?1555:14HOT-2 i- in-range one.1555:23HOT-1 and you haven’t heard any more from this chick in, this

controller chick huh?

1555:26HOT-2 no, not a word. where’d it go anyway?.1555:30HOT-1 I don’t know. I must have thrown it away.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT

1555:32CTR Kiwi Air seventeen, fly a heading 

radar vectors for your descent.

1555:37KW17 * Air seventeen * zero five zero.

1555:42HOT-2 we still got ice.1555:46CAM [sound similar to paper being torn from ACARS printer]1555:47HOT-1 here.1555:58HOT-2 get a message?1555:59HOT-1 you did.1556:01HOT-2 understand a definite maybe on twenty two, release time.

[sound of “ha, ha”]

1556:08HOT-B [sound of beep similar to frequency change on VHF comm]1556:11HOT-1 I’ll be right back. ‘K, I’m a talk to the company.

1556:14.7CTR Eagle flight one eighty four, desc

tain eight thousand?

1556:15.8RDO-1 Chicago, do you copy forty, one e
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT

1556:20.1AEC forty one eighty four, go ahead.1556:21.7 - 1556:47.0RDO-1 yeah, we’ve already been talking 

ACARS but so they are aware of know if you guys got the word on out here uh, we got three, thirty tthirty three hundred pounds of fuzero zero, for uh, EFC so in abouminutes we’ll find what the new wyou tell me about um, there’s this his Frankfurt connection uh, do yabout that?

1556:24CAM-5 traffic, traffic.

1556:27.8CTR Eagle flight one eighty four, desc

eight thousand.

1556:31.6RDO-2 down to eight thousand. Eagle fl

1556:38.3CAM [wailing sound similar to “whooler” pitch trim movement]

1556:44.9CTR Eagle flight one eighty four uh, sh

utes uh, till you’re cleared in.

1556:48.3AEC uh, I can double check on that uh

message to dispatch to see if you thirty on the fuel and estimated o
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT

and, did you have that uh, Frankfany chance?

1556:50.1RDO-2 thank you.

1556:53.1HOT-2 they say ten more minutes.

1557:01.5CTR Kiwi Air seventeen, fly a heading 1557:02.0RDO-1 um, no I sure don’t but I pulled up 

the ACARS and is says it’s going helps you any at all.

1557:05.0KW17 Kiwi Air seventeen, heading three 

1557:07

[sound of light tapping heard on first officer channel]

1557:08.8AEC let me check.

1557:16.3HOT-1 are we out of the hold?

1557:17.3HOT-2 uh, no, we’re just goin’ to eight thousand.1557:19.4HOT-1 OK.1557:20.0HOT-2 and uh, ten more minutes she said ....
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1557:22.1CAM [sound of repeating beeps similar to overspeed warning

starts and continues for 4.6 seconds]

1557:23.3HOT-2 ... oop.

1557:24.7CTR Kiwi Air seventeen, descend and 

sand.

1557:26.2HOT-1 we, I knew we’d do that.1557:27.4HOT-2 I’s trying to keep it at one eighty.

1557:28.2KW17 Kiwi Air seventeen, eleven point f

1557:29.2HOT-2 [ramping repetitive thud sound]1557:28.9HOT-B [wailing sound for 1.2 seconds similar to “whooler” pitch

trim movement]

1557:29.9HOT-1 oh.1557:31.2HOT-B [wailing sound for 1.7 seconds similar to “whooler” pitch

trim movement]

1557:32.8HOT-2 oops, #.
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1557:33.0CAM [sound of three thumps followed by rattling]1557:33.5CAM [sound of three sets of repetitive rapid triple chirps similar

to auto-pilot disconnect warning lasting 1.09 seconds]

1557:33.8HOT-2 #.1557:35.2CAM [single horn similar to altitude alert signal]

1557:35.6CTR Kiwi Air seventeen, direct Chicag

Midway.

1557:36.9HOT-? OK1557:37.0HOT-B [intermittent heavy irregular breathing starts and continues to end of recording]

1557:39.0KW17 direct the Heights direct Midway, 

1557:38.8

[repetitive thumping sound heard on first officers channel]

1557:39.9HOT-? oh #.1557:42.4HOT-1 OK.1557:43.7CAM [single horn similar to altitude alert signal]
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENT1557:44.0CAM [sound of “growl” starts and continues to impact]1557:44.2HOT-1 alright man, ...1557:45.8HOT-1 OK, mellow it out.1557:45.8CAM [sound of repeating beeps similar to overspeed warning

starts and continues to impact]

1557:46.7HOT-2 OK.1557:47.1HOT-1 mellow it out.1557:47.7HOT-2 OK.1557:48.1HOT-1 auto-pilot’s disengaged.1557:49.4HOT-2 OK.1557:52.8HOT-1 nice and easy.1557: 54.9CAM-5 terrain, whoop whoop.1557:56.6HOT-2 aw **.1557:56.7
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME & TIME &SOURCE CONTENT SOURCE CONTENTCAM [loud crunching sound]1557:57.1END of RECORDINGEND of TRANSCRIPT
252

APPENDIX C

EXCERPTS FROM THE FAA SPECIAL CERTIFICATION

REVIEW OF THE ATR

Executive Summary

On October 31, 1994, an accident involving an Aerospatiale Model ATR-72 series airplane

occurred when the airplane was enroute from Indianapolis to Chicago. Although the official

cause of the accident has not been determined, preliminary information from the accident

investigation indicates that, following exposure to a complex and severe icing environment

including droplets much larger than those specified in certification criteria for the airplane,

and during a descending turn immediately after the flaps were raised, the ailerons abruptly

deflected in the right-wing-down direction, the autopilot disconnected, and the airplane

entered an abrupt roll to the right, which was not fully corrected before the airplane impacted

the ground.

As a result of this accident, the National Transportation Safety Board (NTSB) recommended

that the Federal Aviation Administration (FAA) conduct a Special Certification Review

(SCR) of Model ATR-42 and -72 series airplanes. The NTSB also recommended that flight

test and/or wind tunnel tests be conducted as part of that review. These tests would be

performed to determine the aileron hinge moment characteristics of the airplanes while

operating at different airspeeds and in different configurations during ice accumulation, and

with varying angles of attack following ice accretion.

A ten-person team was formed, including six certification specialists from the FAA, and four

specialists from the Direction Générale de l’Aviation Civile (DGAC), which is the

airworthiness authority for France. Hundreds of hours were spent investigating the

certification and performance of ATR-42 and ATR-72 series airplanes over a six-month

period, at eight venues both in the United States and in France. 

During its investigation, the SCR team participated in the creation of two telegraphic

airworthiness directives (AD). Telegraphic AD T94-25-51, which was issued on December

9, 1994, while the special review team was in France, prohibited flight into known or

forecast icing conditions for the ATR fleet. The second telegraphic AD, T95-02-51,

restored flight in icing conditions upon incorporation of certain flight and dispatch

restrictions and procedures. That telegraphic AD was signed on January 11, 1995--only 72

days after the accident, including three major year-end holidays.

In accordance with its charter, the SCR team focused its attention on the following major

categories during its investigation:

CERTIFICATION BASIS

The basic Model ATR-42 was approved by the FAA on October 25, 1985 [Type Certificate

(TC) A53EU]. The certification basis for the airplane is 14 CFR Part 25, as amended by

Amendment 25-1 through Amendment 25-54, with certain special conditions not related to

icing. The basic Model ATR-72 was approved by the FAA on November 15, 1989, as an

1
253

amendment to TC A53EU. The ATR-72-2 11/2 12 model (the accident airplane) was

approved by the FAA on December 15, 1992.

REVIEW OF CERTIFICATION PRACTICES AND RESULTS

The icing certification program conducted for the ATR-42 and -72 demonstrated the

adequacy of the anti-ice and de-icing systems to protect the airplane against adverse effects

of ice accretion in compliance with the requirements of FAR/JAR 25.1419. The wing

deicing system has demonstrated acceptable performance in the meteorological conditions

defined in the FAR/JAR 25 Appendix C envelope. Additionally, during the icing tanker

testing conducted at Edwards Air Force Base (AFB), California, the proper functioning of

the wing deicing boots was observed to correlate with Aerospatiale (ATR) test data within

the Appendix C envelope. The certification program for the ATR-72-201/202 and

ATR-72-21 1/212 icing systems was documented thoroughly using sound procedures and

was processed and conducted in a manner consistent with other FAA icing certification

programs. All data reviewed shows compliance with FAR 25/JAR 25.1419. The SCR team

concluded that results show a good correlation with Special Condition B6 stall requirements

and also with FAR/JAR 25.203 (handling qualities). Model ATR-42 and ATR-72 series

airplanes were certificated properly in accordance with DGAC and FAA regulations

practices, and procedures.

AUTOPILOT CERTIFICATION PROCEDURES AND CHARACTERISTICS

The Honeywell Automatic Flight Control System (AFCS) was approved by the DGAC in

accordance with the FAA certification basis that existed for each successive ATR series

airplane. System design parameters for performance and servo authority meet those

specified by FAR 25.1329 and AC 25. 1329-1A. The system installation and monitor design

is supported by the Aerospatiale Safety Assessment Automatic Pilot System and Honeywell

DFZ-6000 Safety Analysis for critical and adverse failure cases. The equipment

qualification and subsequent performance and malfunction flight tests that were performed

are consistent with acceptable industry practices and procedures and are similarly consistent

with practices and procedures accepted by the FAA in the past for other aircraft. The SCR

team concluded that the Honeywell AFCS installed in the successive ATR series airplanes

was certificated properly to the requirements of the FAR’s.

REVIEW OF PERTINENT SERVICE DIFFICULTY INFORMATION

While all icing-related accident and incident information was not examined to the full extent

of the Roselawn accident due to time and resource limitations, certain important aspects of

the event history were studied and some conclusions were possible. Events of unacceptable

control anomalies were associated with severe icing conditions such as freezing rain/freezing

drizzle and, in a few cases, the icing was accompanied by turbulence. These other roll

anomaly events provided no evidence that the ATR-72 had any problems with any icing

conditions for which it was certificated. Appendix 8 contains a tabulation of events that

were known to the SCR team.

2
254

ENVIRONMENTAL CONDITIONS OUTSIDE THE APPENDIX C ENVELOPE

Weather observed in the area of the accident appears to have included supercooled water

droplets in the size range of about 40 to 400 microns. This weather phenomenon is defined

by the SCR team as Supercooled Drizzle Drops (SCDD).

Freezing drizzle and SCDD can be considered to present the same icing threat in terms of

adverse effects. While the physics of formation are not the same, the difference between

them is that freezing drizzle is found at the surface, while SCDD is found aloft with air at

temperatures above freezing underneath. Freezing rain contains droplets in the range of

1,000 to 6,000 microns. Collectively, all these large drops are referred to as supercooled

large droplets (SLD). When used herein, the aerodynamic effects of SCDD and freezing

drizzle are synonymous. While the effects of ice accreted in SLD may be severe, the clouds

that produce them tend to be localized in horizontal and/or vertical extent.

The scientific investigation of SCDD and the body of knowledge on this subject is

relatively new. SCDD is not universally understood in the aviation community. SCDD may

be considered to icing as the microburst is to wind shear. Both have been unrecognized until

recent times. Since they may be very severe, but are localized in extent and difficult to

detect until the airplane has encountered the condition, for now, pilot awareness and prompt

action to exit the condition are relied upon. Some researchers have observed that the effects

of ice accreted in SCDD are far more severe than those of freezing rain.

Considering all available data, the SCR team has determined that the icing conditions of the

accident environment were well outside the Appendix C icing envelope. This report

contains a detailed description of this phenomenon, several short and long term

recommendations are made.

ANALYSIS OF AILERON HINGE MOMENT CHARACTERISTICS

The flight test data and qualitative assessments made by the DGAC during basic certification

of the ATR-42 and -72, and the ATR-72-2 11/212, did not indicate that any unsafe or

atypical lateral control wheel force characteristics exist. This conclusion also was based on

the comprehensive assessment of the airplane in icing conditions conducted in accordance

with Special Condition B6. The original certification test program did lack an evaluation of

airplane characteristics with asymmetrical ice shapes; however, such an evaluation is not

considered standard practice. Ice asymmetry was considered unlikely due to system design

and Airplane Flight Manual (AFM) procedures.

Wind tunnel data and analysis have shown that a sharp-edge ridge on the wing upper surface

in front of one aileron only can cause uncommanded aileron deflection. By using a very

conservative analysis, these data show that keeping the wings level at 175 knots indicated

airspeed (KIAS) takes approximately 56 pounds of control wheel force. These force levels

were not seen during any of the icing tanker tests. However, during the first series of tests in

3
255

the icing cloud behind the tanker (see below), a ridge of ice did buildup behind the deicing

boots in a similar location to the wind tunnel model, but it was not sharp-edged and only

extended spanwise approximately 40 percent in front of the ailerons due to the dimension of

the icing cloud. However, these tests indicated that a mechanism existed that could actuaIly

produce such a ridge in actual icing conditions. Even though high lateral wheel forces were

not seen during the tanker tests, icing specialists indicated that under sIightly different

conditions of the icing environment, other shapes could develop. Since the ice ridge sheds

in a random manner, and in light of the airflow difference over the wings during

maneuvering and turbulence or due to aerodynamic effects, an assumption was made that

there could be a significant difference in ice accretion between the left and right wings.

Additional flight tests were conducted by Aerospatiale with artificial ice shapes, duplicating

the ice that accreted during the tanker tests in freezing drizzle conditions. Initially, these

shapes were applied in front of the aileron in a random pattern to duplicate the shedding that

was observed during the tanker tests. Additionally, a series of flight tests were conducted

with ice shapes covering full and partial spans of the wing. The results of these tests

coincided with the results obtained from the tanker tests. Further testing by Aerospatiale

with more asymmetry and with sharper edge shapes indicated higher lateral control forces,

however, not as high as those derived from the initial wind tunnel studies.

FAA/AIR FORCE ICING TANKER TESTING

Two series of icing tanker tests were performed at Edwards AFB, California in support of

the investigation of the October 31, 1994, accident. A United States Air Force jet airplane

(similar to a Boeing Model 707) specially modified to produce an icing cloud was used to

simulate the conditions believed to have existed at the time of the accident. Direct. results of

the icing tanker tests were used to determine possible (1) immediate and long term changes

to the aircraft, (2) changes to flight crew operations procedures, (3) changes to the Master

Minimum Equipment List (MMEL), and (4) changes to flight crew training.

The first tanker test took place December 13-22, 1994; the second test program took place

March 4-7, 1995. Both test programs were conducted as similarly as possible so that the

results of the two tests could be compared directly.

APPROVAL OF MODIFIED DEICING BOOTS

Aerospatiale developed a modification that consists of an increase in coverage of the active

portion of the upper surface of the outer wing deicing boots from 5 percent chord on the

ATR 42 and 7 percent chord on the ATR-72 to 12.5 percent chord for both airplane models.

These enlarged wing deicing boots were certificated by extensive dry air and icing wind

tunnel tests, and by dry air and natural icing flight tests conducted by Aerospatiale and FAA

flight test pilots. In addition, an ATR-72 fitted with the modified boots was flown behind

the icing tanker at Edwards AFB. The results of all these tests revealed that the modified

boots perform their intended fiction within the icing requirements contained in Appendix

C of Part 25 of the Federal Aviation Regulations. All U. S.-registered Model ATR-42 and

ATR-72 series airplanes were modified with the new boots prior to June 1, 1995.

4
256

Aerospatiale developed the deicing boot modification to provide an increased margin of

safety in the event of an inadvertent encounter with freezing rain or freezing drizzle (SLD).

With the ability to recognize that an inadvertent encounter had occurred, flight crews would

be afforded an increased opportunity to safely exit those conditions. However, even with

improved boots installed, Model ATR-42 and -72 airplanes, along with all other airplanes,

are not certificated for flight into known freezing drizzle or freezing rain conditions.

OPERATIONAL CONSIDERATIONS THAT MAY REQUIRE CHANGES

Several recommendations regarding operational considerations for the turboprop transport

fleet were made. These recommendations include changes to flight crew and dispatcher

training, expanded pilot reports, Air Traffic Control and pilot cooperation regarding

reporting of adverse weather conditions, flight crew training in unusual attitude recovery

techniques, aircraft systems design and human factors, and MMEL relief.

CHANGES TO THE CERTIFICATION REQUIREMENTS (APPENDIX C)

The FAA recognizes that the icing conditions experienced by the accident airplane, as well

as other airplanes involved in earlier accidents and incidents (see Appendix 8), may not be

addressed adequately in the certification requirements. Therefore, the FAA has initiated the

process to create a rulemaking project under the auspices of the Aviation Rulemaking

Advisory Committee (ARAC). The ARAC will form a working group, made up of

interested persons from the U.S. aviation industry, industry advocacy groups, and foreign

manufacturers and authorities. The ARAC working group will formulate policy and

suggested wording for any proposed rulemaking in the area of icing certification.

REPORT RECOMMENDATIONS

This report contains 14 specific recommendations in the areas of airplane certification

testing and operational considerations.

5
257

SCR Team Conclusions

€ ATR-42 and ATR-72 series airplanes were certificated properly in accordance with

the FAA and DGAC certification bases, as defined in 14 CFR parts 21 and 25 and

JAR 25, including the icing requirements contained in Appendix C of FAR/JAR 25,

under the provisions of the BAA between the United States and France.

€ The Roselawn accident conditions included SCDD outside the requirements of 14

CFR part 25 and JAR 25. Investigations prompted by this accident suggest that these

conditions may not be as infrequently as commonly believed and that accurate

forecasts of SCDD conditions does not have as high a level of certitude as other

precipitation. Further, there are limited means for the pilot to determine when the

airplane has entered conditions more severe than those specified in the present

certification requirements.

66
258

SCR Team Recommendations

The 14 recommendations made by the ATR-42 and ATR-72 Airplane Special Certification

Review Team are listed below:

RECOMMENDATION 1 The current fleet of transport airplanes with unboosted

flight control surfaces should be examined to ascertain

that an inadvertent encounter with SLD will not result in a

catastrophic loss of control due to uncommanded control

surface movement. The following two options should be

considered:

1. The airplane must be shown to be free from any hazard due to an encounter of

any duration with the SLD environment or

2. The following must be verified for each airplane, and procedures or restrictions

must be contained in the AFM:

a The airplane must be shown to operate safely in the SLD environment

long enough to identify and safely exit the condition.

b. The flight crew must have a positive means to identify when the airplane

has entered the SLD environment.

c. Safe exit procedures, including any operational restrictions or limitations,

must be provided to the flight crew.

d. Means must be provided to the flight crew to indicate when all icing due

to the SLD environment has been shed/melted/sublimated from critical areas

of the airplane.

RECOMMENDATION 2 I FAR 25.1419, Appendix C, should be reviewed to

determine if weather phenomena which are known to exist

where commuter aircraft are operated most often should

be included. The following steps should be taken:

1. Scientifically define the SLD environment using the appropriate parameters

(LWC, droplet diameter, temperature, horizontal extent).

2. Foster development and validation of analytical tools, computer codes, and test

methods to reliably predict and test impingement limits, shape, texture, location, and

aerodynamic effects of ice accretions in SLD conditions.

67
259

3. Evaluate current certification policy and procedures to determine whether new

information regarding the SLD environment should be included.

4. Develop Advisory Circulars or other guidance materials to aid in future aircraft

certification programs.

RECOMMENDATION 3 Rulemaking and associated advisory material should be

developed for airplanes with unpowered flight control

systems to address uncommanded control surface

movement characteristics that are potentially catastrophic

during an inadvertent encounter with the SLD

environment. Discussions about these new criteria should

consider the criteria already contained in the certification

requirements, as summarized below:

When the aircraft is flying manually:

. . . the “strength of pilots” limits may not exceed:

-60 lbs. for temporary application for roll control

-5 lbs. for prolonged application for roll control

[These forces have been changed to harmonize with JAR 25.143(c)

per Amendment 25.84, effective July 10, 1995, as shown below.]

€

. . . the “strength of pilots limits” for conventional wheel type controls may not

exceed:

-50 lbs. for temporary application for roll control - two hands

available for control

-25 lbs. for temporary application for roll control - one hand available

for control

-5 lbs. for prolonged application for roll control

68
260

When the autopilot is engaged:

~

€ACJ 25.1379 . .~ of c~

 pertinent excerpts of ACJ 25.1329 include the

following:

- A load on any part of the structure greater than its limit load.

- Bank angles of more than 60° enroute or more than 30° below a

height of 1,000 ft.

- Hazardous degradation of the flying qualities of the airplane.

- Hazardous height 10SS in relation to minimum permitted height for

autopilot use.

- Engagement or disengagement of a mode leading to hazardous

consequences.

● FAR 25.1329 and AC 25.1329-1A

- Roll force to overpower greater than 30 pounds.

- A load in excess of structural limits or beyond 2g.

. Climb, Cruise, Descent, and Holding: Recovery action should not be initiated

until three seconds after the recognition point.

RECOMMENDATION 4 The SCR team recommends that existing criteria used for

evaluation of autopilot failures be used to evaluate the

acceptability of the dynamic response of the airplane to an

uncommanded aileron deflection. Moreover, since both of

these events (failure/hardover, aileron deflection) can

occur without the pilots being directly in the loop, the

three-second recognition criteria used for the cruise

conditions aIso should be adopted.

RECOMMENDATION 5 I Policy should be developed to assure that on-board

computers do not inhibit a flight crew from using any and

all systems deemed necessary to remove an airplane from

danger.

RECOMMENDATION 6 Airplane Flight Manuals should be revised to clearly

describe applicable icing limitations.

69
261

RECOMMENDATION 7 The FAA/JAA harmonization process for consideration of

handling qualities and performance of airplanes while

flying in icing conditions should be accelerated. The

following specific points should be included:

1. SC B6 does not specify that ice be accreted in one configuration e.g., flaps up, and then

demonstrated in subsequent configurations that maybe more adverse. This condition should

be considered as a possible revision for future regulatory change.

2. SC B6, Section 4.3.1, states in part :

“Flight tests in measured natural icing conditions should include observations of

actual ice shapes to allow correlation to be made with the predicted shapes in

identical conditions, in terms of location, general shape and, where possible,

thickness.”

The regulation is unclear as to how artificial ice shapes used in flight testing must be

correlated with natural ice accretions. This point should be considered for future regulatory

change.

RECOMMENDATION 8 Evaluate state-of-the-art ice detector technology to

determine whether the certification regulations should be

changed to require these devices on newly developed

airplanes.

RECOMMENDATION 9 Flight crew and dispatcher training related to operations in

adverse weather should be re-eva.lusted for content and

adequacy.

RECOMMENDATION 10 Flight crews should be exposed to training related to

extreme unusual attitude recognition and recovery.

RECOMMENDATION 11 1 Pilots should be encouraged to provide timely, precise,

and realistic reports of adverse flight conditions to ATC.

The tendency to minimize or understate hazardous

conditions should be discouraged.

RECOMMENDATION 12 I An informational article should be placed in the Winter

Operations Guidance for Air Carriers, or airline

equivalent which explains the phenomenon of

uncommanded control surface movements and the hazards

associated with flight into SLD conditions.

70
262

RECOMMENDATION 13 MMEL relief for all aircraft, particularly items in Chapter

30 (Ice and Rain Protection), should be reviewed for

excessive repair intervals.

RECOMMENDATION 14 J Methods to accurately forecast SLD conditions, and

mechanisms to disseminate that information to flight

crews in a timely manner should be improved.

71









272

APPENDIX E

DOPPLER WEATHER RADAR

WIND AND WINDSHEAR CALCULATIONS

Winds from the WSR-88D Doppler Weather Radar at KLOT and Wind

Shear Calculations

Upper winds were obtained from the WSR-88D doppler weather radar VAD Vertical

Wind Profile (VWP) product for 1611. The following winds were estimated from the

1548 data. The doppler weather radar is located at Romeoville, Illinois (KLOT) about

46 nautical miles northwest of the accident site. The VAD VWP product samples the

volume of atmosphere at about a 22 nautical mile radius of KLOT. Wind speed is in

knots and wind direction is in degrees true.

Height feet AGL

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

11,000

Wind Direction

360

027

030

030

050

052

063

090

122

163

180

Wind Speed

15

40

40

40

40

40

35

25

15

20

25

Wind Shear values based on the above wind profile are as follows:

Altitude Interval (Feet) Wind Shear (sec-1)

7,000 to 8,000 .029

8,000 to 9,000 .025

9,000 to 10,000 .022

10,000 to 11,000 .014
273

APPENDIX F

DOPPLER WEATHER RADAR IMAGES

WITH TRACK OF FLIGHT 4184 SUPERIMPOSED

KLOT WSR-88D Doppler Weather Radar Images with the Track of

Flight 4184 Superimposed

This Appendix contains WSR-88D Doppler Weather Radar Images from KLOT. The

radar ground track of the last circuit of Flight 4184 in the hold at the LUCIT

intersection is superimposed on the images. In the images colors correspond to weather

radar echo intensities [see the vertical color baron the right side of the images]. The

intensities are measured in dBZ [see Table A Below]. The times of the images are

21242, 21302, 2136Z, 2142Z, 2148Z, 2154Z, and 2200Z. The elevation angle is set

to 1.5 degrees. The accident site is located about 132 degrees at 46 nautical miles from

KLOT. At an elevation angle of 1.5 degrees the radar beam center in the area of the

accident was at about 9,500 feet. The width of the beam was about 4,600 feet.

dBZ

0t029

30 to 39

40 to 44

45 to 49

50 to 54

55 or greater

Table A

Intensity

Weak

Moderate

Strong

Very Strong

Intense

Extreme







281

APPENDIX G

DISCUSSION OF LIQUID WATER CONTENT

AND LIQUID WATER DROP SIZE

I

Drop Diameters

Precipitation Intensity millimeters per hour.

Drop Diameter millimeters.

1 millimeter = 1,000 microns.

Popular Name Precipitation Intensit y Drop Diamete r

Fog Trace .01

Mist .05 .1

Drizzle .25 .2

Light Rain 1.00 .45

Moderate Rain 4.00 1.0

Heavy Rain 15.0 1.5

From Physics of the Air, Humphreys, Third Edition, 1940 .

The following cloud droplet size scale is from: Forecasters’

Guide on Aircraft Icing, March 1980, Air Weather Service .

Category

Small

Droplet Diameter

< 10 microns

Medium 10 to 30 microns

Large 30 to 100 microns

Freezing rain or drizzle 100 to 1,000 microns

According to a research professor from the University of Wyoming

the diameter of drizzle drops are from 50 to 300 to 400 microns.

The diameter of cloud drops are less than these values and the

diameter of rain drops are greater than these values. According

to a scientist from NCAR drizzle drops have a diameter of 50 to

500 microns and rain drops have a diameter greater than 500

microns.

French meteorologists define rain as having a drop diameter of

greater than 500 microns. Drizzle is defined as having a drop

diameter of less than 500 microns.

Liquid Water Content Calculation

Liquid Water Content (LWC) in grams per cubic meter, Icing
282

II

Intensity based on the definition of *icing intensitie s

established by the National Committee for Aviation Meteorology o n

February 25, 1964, and Rate of Ice Accumulation in inches pe r

minute were obtained from the NTSB Computer Program ICE4A.

* Icing Intensities

Heavy (Severe) . ..Accumulation of 1/2 inch of ice on a small prob e

per 10 miles. Moderate. . .Accumulation of 1/2 inch per 20 miles.

Light. . .Accumulation of 1/2 inch per 40 miles.

The following is output from ICE4A. . .

Assumptions. . .

Cloud Base 959 millibars (about 1,500 feet), temperature 4

degrees C, moist adiabatic ascent in cloud, LWC = .25 times the

adiabatic LWC.

TAS = Aircraft True Airspeed meters per second.

LWC = .72

Severe.

LWC = .76

Severe.

LWC = .72

Severe.

LWC = .76

Severe.

LWC = .72

Severe.

LWC = .76

Severe.

Altitude 9,700 feet // TAS = 75

Rate of Ice Accumulation = . 120 // Icing Intensity

Altitude 10,600 feet // TAS = 75

Rate of Ice Accumulation = . 134 // Icing Intensity

Altitude 9,700 feet // TAS 100

Rate of Ice Accumulation = .170 // Icing Intensity

Altitude 10,600 feet // TAS 100

Rate of Ice Accumulation = .179 // Icing Intensity

Altitude 9,700 feet // TAS 125

Rate of Ice Accumulation = .212 // Icing Intensity

Altitude 10,600 feet // TAS 125

Rate of Ice Accumulation = .223 // Icing Intensity
283

I I I

Note: The above values should only be viewed as possible

estimates.

The following was obtained from the Forecasters’ Guide On

Aircraft Icing; Air Weather Service; Scott AFB, Illinois; March

1980:

LWC = .348 (W0 -W1) P/T

As noted in the report, this represents a practical upper limit

of the LWC in cumuliform clouds at flight level. The LWC in

stratiform clouds averages about 1/2 the value computed for

cumuliform clouds.

W0 = saturation mixing ratio at cloud base (grams per kilogram) .

WI = saturation mixing ratio at flight level (grams per

kilogram) .

P = pressure at flight level (millibars) .

T = cloud temperature at flight level (degrees Kelvin) .

Given: cloud base = 2,000 feet; temperature = 4 degrees C at

cloud base; P at cloud base = 937 millibars // W0 = 5.46 grams

per kilogram.

At 10,000 feet; P = 692 millibars; T = -4 degrees C (269.16

degrees Kelvin) // W1 = 4.14 grams per kilogram.

Therefore, LWC = .59 grams per cubic meter for stratiform

conditions and 1.18 grams per cubic meter for cumuliform

conditions for an altitude of 10,000 feet.

The following relationship from a paper by *Greene and Clark

relates LWC to weather radar reflectivity:

M = 3.44X10-3Z4/ 7 where M = LWC in grams per cubic meter and

Z = weather radar reflectivity in millimeters to the sixth power

per cubic meter (mm6/m3) . The following Table relates M to

weather radar reflectivity in dBZ:

dBZ = 10 X Log(Z)

Z = 10dbZ/10

Reflectivity (dBZ) LWC (grams per cubic meter)

<5 <.0 1

1 0 . 0 1
284

IV

15 .02

20 . 05

25 . 09

30 . 18

35 .34

40 . 66

45 1.28

An exponential drop-size distribution proposed by Marshall and

Palmer (1948) is assumed.

According to a Research Professor from the University of Wyoming

at a weather radar reflectivity of 20 to 25 dBZ “ you’re getting

well up into the millimeter sizes and the Marshall Palmer is

probably much more appropriate [than a monodisperse drop size

distribution] then.” [ Public Hearing February 27, 1995].

* Vertically Integrated Liquid Water - A New Analysis Tool;

Monthly Weather Review; Vol. 100 No. 7; July 1972.

Weather Radar Echo Intensities (maximum and minimum)

approximate area enclosed by the track [last circuit

at the LUCIT intersection] of Flight 4184; elevation

degrees.

for the

in the hold

angle = 1.5

From KLOT WSR-88D Data:

Time Weather Echo Intensity

1524 . . . Maximum dBZ 20 to <25 Minimum <5 dBZ

1530 . . . Maximum dBZ 20 to <25 Minimum <5 dBZ

1536 . . . Maximum dBZ 25 to <30 Minimum <5 dBZ

1542 . . . Maximum dBZ 20 to <25 Minimum <5 dBZ

1548 . . . Maximum dBZ 15 to <20 Minimum <5 dBZ

1554 . . . Maximum dBZ 20 to <25 Minimum <5 dBZ

1600 . . . Maximum dBZ 35 to <40 Minimum <5 dBZ

Figure A [from Penn State University] relates the number of drops

per cubic centimeter and the radar reflectivity factor for

various droplet radii. Given the number of drops per cubic

centimeter and the diameter of the drops a Liquid Water Content

(LWC) can be calculated: LWC = (.52) *N*D3/106 LWC in grams per

cubic meter; N number of drops per cubic centimeter; and D drop

diameter in microns. A monodisperse droplet distribution is

assumed (better representation for a drizzle situation as opposed

to a convective situation) . Althoughr review of the chart shows

that the same value for reflectivity factor can result from a few

large drops per cubic centimeter or many smaller drops per cubic

centimeter LWC values need to be checked for reasonableness. For
285

v

example, one 200 micron diameter drop per cubic centimeter would

result in a reflectivity factor of 20 dBZ as would about ten

thousand 50 micron drops per cubic centimeter. However, the LWC

in the first case is estimated as 4.2 and in the second case 650;

both values not realistic given the conditions. A concentration

of .04 per cubic centimeter of droplets with a 200 micron

diameter results in a reflectivity factor of about 5 dBZ and a

LWC of about .17. A concentration of .1 per cubic centimeter of

droplets with a 200 micron diameter results in a reflectivity

factor of about 10 dBZ and a LWC of about .42. A concentration of

.00003 per cubic centimeter of droplets of a 1000 micron diameter

results in a reflectivity factor of about 15 dBZ and a LWC of

about .02.

If a monodisperse distribution is assumed (all drops the same

size) approximate drop sizes can be calculated from the WSR-88D

data. A monodisperse distribution is a better representation of a

drizzle situation than a Marshall - Palmer size distribution. If

a maximum SLW content of 1 gram per cubic meter is assumed and

given the maximum and minimum reflectivity values for the area

outlined by the track of Flight 4184 (see dBZ values above) drop

diameters ranging from 100 to some as large as 2,000 microns are

possible.

Estimates of drop diameters using Figure A:

40 dBz // .0003 drops per cubic centimeter // drop diameter 2,000

microns // LWC = 1.3 grams per cubic meter.

25 dBZ // .00001 drops per cubic centimeter // drop diameter

2,000 microns // LWC = .042 gram per cubic meter.

25 dBZ // .01 drops per cubic centimeter // drop diameter 600

microns // LWC = 1.1 grams per cubic meter.

5 dBZ // .00001 drops per cubic centimeter // drop diameter 800

microns // LWC = .003 gram per cubic meter.

5 dBZ // 2 drops per cubic centimeter // diameter 100 microns //

LWC = 1.0 gram per cubic meter.


288

APPENDIX I

ATR ALL WEATHER OPERATIONS BROCHURE

AND ATR ICING CONDITIONS PROCEDURES - VERSION 2.0
289

Icing conditions

There are a great number

of occasions, referred to as

“icing conditions",

where an aircraft is subject

to accretion of frozen water under

various forms.

I n order to illustrate its variability, an essential

feature of aircraft icing, the main occurrences

of this phenomenon will be recalled hereafter.

EXPOSURE TO PRECIPITATION in cold weather

of a still or taxying aircraft can lead to various

types of accretion ranging from dry snow to

clear ice, with all possible intermediates such as

coarse frost or mixtures of these elements,

depending upon the conditions.

A characteristic of these accretions is the wide

coverage of aircraft surfaces, changing airfoil

shapes and consequently their aerodynamic properties, and possible local accumulations that

may affect the normal functioning of systems or

the clearance of moving parts.

GROUND ICING CONDITIONS exist when the

temperature is at or below 5° C (410 F), when

operating on ramps, taxiways and runways

where surface snow, standing water or slush is

present. In such conditions, limited aircraft surfaces can be contaminated during aircraft operations on ground (tests have shown that such

ground contaminants cannot impact airfoils).
290

ATMOSPHERIC ICING can occur when the

Outside Air Temperature (OAT) on the ground

and for take-off is at or below 5° C (41” F) or

when TAT in flight is at or below 7° C (45° F),

and visible moisture in any form is present (such

as clouds, fog with visibility of less than one

mile, rain, snow, sleet and ice crystals).

Whether ice accretion will really occur or not

depends upon many factors that are not available

to the pilots, therefore only visual evidence will

tell the crew that accretion does exist.

In flight, the accretion will normally be limited to

areas around the aerodynamic stagnation point,

that is the leading edges of airfoils (including

propellers), aircraft nose, spinners...

“~

*,.,,*

The accretion however can have a large variety of

shapes and textures, ranging from clear, thin ice

difficult to detect to coarse rime with single or

double horn form (fig. 1 and 2).

FREEZING RAIN is a fairly rare phenomenon

where supercooled raindrops freeze when

impacting the aircraft or arty obstacle. Generally

associated with abnormal atmospheric parameters such as inverted temperature gradients with

altitude, freezing rain is capable of rapidly covering an aircraft with a sizeable layer of clear ice,

well beyond the usual accretion areas around

stagnation points. H
291

Theory and tests

As the aircraft external shapes are carefully optimized from an aerodynamic point

of view, it is no wonder that any deviation from the original lines due to ice accretion

leads to an overall degradation of performance

and handling, whatever the type. The real surprise comes from the amount of degradation

actually involved and its lack of “logical” relationship with the type of accretion. Systematical

wind tunnel tests have been carried out by

various institutes and manufacturers during the

last decades, providing a wealth of results that

have been largely confirmed by flight tests on

different types of jets and turboprops.

The main effects of ice accretion can be summarized as follows.

Fig. 3- Effect of certified ice shapes on lift curve - Flaps 30°, gear down standard

de-icers

LIFT

The lift curves are substantially modified compared to clean aircraft (fig. 3):

€ reduction of lift at a given angle of attack,

€reduction of maximum lift,

€ reduction of maximum lift angle of attack.

When the maximum lift capability of the wing

decreases by 25%, the actual stall speed is 12%

higher then the basic stall speed (aircraft clean).

So an iced aircraft (fig. 3) flying at a given

speed (and thus at a given CL) will have stall

margin reduced either looking at angle of attack

(6°5 less margin) or looking at stall speed (12%

less margin).

More surprising is the fact evidenced by fig. 4:

the bulk of the maximum lift degradation is

already there for accretions as small as a few

millimeters.

A Clmax decrease of 0.5 typically means a stall

Fig. 4- Effect of ice shape on CLmax - Wind tunnel tests - Flaps 15° speed increase of 10 kt for an ATR 42 with
292

flaps 15°. The ATR 42 wind tunnel test results

with single or double horn shapes are consistent

with the curves derived from extensive tests carried on conventional airfoils by the Swedish -

Soviet working group on flight safety.

DRAG

The drag polar is also heavily affected (fig. 5)

€ superior drag at a given angle of attack,

€ superior drag at a given lift,

€ best lift/ drag ratio at a lower lift coefficient.

PERFORMANCE

The drag and lift penalties described in the paragraphs above give a good idea of the performance impacts that could be expected from ice

accretion.

Beyond those main phenomenon, other effects

should not be underestimated: as an example ice

accretion on prop blades will reduce the efficiency and the available thrust of propeller driven

aircraft.

On the other hand, ice weight effect will remain

marginal when compared to other penalties.

Fig. 5- ATR 72- Effect of certified ice shapes on drag polar. Flaps 0° . Standard

de-icers

son to be symmetrical or regular along the entire

span of the wing.

Other potentially hazardous effects are linked to

tail surface icing : reduced maximum lift and

stall angle of attack may result in tail surface

stall under conditions where, if clean, it would

properly do its job.

These conditions are those of high negative

angle of attack and downloads on the tail surHANDLING

I

faces, found for extreme maneuvers at flap settings higher then 35°.

In order to ensure a satisfactory behaviour, air- Separated airflow on the tail surface can also

craft are carefully designed so that stall will seriously affect elevator behaviour when

occur initially in the inner part of the wings and

spread out towards the tips as angle of attack

increases. Roll moments and abruptness of lift

drop are then minimized.

This stall behaviour can be completely jeopardized by ice accretions that have no particular reamanually actuated, as aerodynamic compensation of control surfaces is a fine tuned and delicate technique.

Similar anomalies can affect other unpowered

controls (such as ailerons) when ice accretion

exists. 9

294

Nevertheless, all aircraft are equal when ground

icing is concerned, and from this point of view,

the “clean aircraft concept” at take-off is strictly

enforced by Airworthiness Authorities through

FAR regulations (sections 121.629, 91.209 and

135.227), and recalled through FAA Advisory

Circular AC 20-117 issued in 1982, and updated

in April 83 and in March 1988. Recent accidents

of improperly de-iced jets at take-off unfortunately confirm the necessity to comply with these

requirements.

Furthermore, wind tunnel tests have demonstrated the disastrous effects of a complete wing

upper surface coverage with frost.

Degradations up to 40% in CLmax and 5° in stall

angle of attack were evidenced in a take-off

configuration, likely to prevent any take-off at all.

As far as flight is concerned, current regulations

(FAR and JAR) are very explicit on icing conditions definition and on related system tests but

remain rather vague on aircraft handling and

performance requirements applicable to aircraft

with ice accretion.

Generally, it is only stated that the aircraft must

be able to operate safely in the maximum defined icing conditions and that acceptable handling characteristics with real ice accretion and

with simulated ice shapes (fig. 6) must be

demonstrated.

It therefore appears very desirable to provide specific interpretative material, and in this respect

the two Airworthiness Manual Advisories

525/2-X (Jan 88) and 525/5-X (Feb 88) from

DOT Canada constitute the first achievement of

this kind, addressing specifically handling and

performance requirements for flight in icing I

conditions.
295

Years before, during ATR 42 certification process and flight tests, requirements beyond existing regulations were agreed with French

DGAC, and these requirements have been formalized later in the form of a Special Condition

B6 for ATR 72 certification. Performance and

handling requirements are comprehensively

addressed by this document, with special

emphasis on polluted aircraft stall characteristics

and tail surface behaviour (pushover demonstration at high flaps settings).

I

The main purpose of this special condition is to

ensure that the safety level and margins in icing

conditions as defined by regulations remain

equivalent to what exist without ice. JAR

Authorities and FAA have been approached in

order to promote this Special Condition as a

basis for future requirements applicable to all

new propeller driven aircraft. _

Highlights of special condition B6 (basis for ATR 72 certification in icing)

● AII phases of flight considered, including take off.

€ Ice shapes determined by proven theoretical methods, leading to horn forms.

€ Minimum roughness is specified.

€ Depending on deicers used, residual ice on protected ports must be simulated.

€ Failure cases of de-icers must be demonstrated to determine handling and performance penalties; and procedures to

continue safely the flight when needed. 

€ Handling and performance tests must be conducted in all flight phases with appropriate simulated ice shapes.

Adequate behaviour must be demonstrated in terms of controllability, manoeuverability, stability, ability to trim, vibrations, buffeting. Particular attention must be paid to stall characteristics, stall speeds, and stall warning (artificial or natural) must ensure the same margin as for a clean aircraft. 

Push over manoeuvers down to 0 g with full flaps must be demonstrated with no abnormality in the most critical cases.

€ Minimum operational speeds must be defined so as to provide the same manoeuverability margin as those required

 on the clean aircraft. 

 €Aircraft Flight Manual must provide all necessary data applicable to the aircraft with ice accretion.

 ● Real icing tests must be conducted in variousconditions to confirm the validity of theoretical shapes, and ensure that

performance and handling degradations have been established on a conservative basis, with special attention to stall

warning. .
296

System description

On a turboprop the ancillary power

available (bleed air and electrical

power) being less than on a jet, a

permanent thermal protection is

unpracticable in particular for the

airframe.

A solution consists in installing a

pneumatic de-icing system on the

critical exposed parts (i.e. air-frame)

complemented by an electrical

anti-icing protection for the parts

on which a pneumatic de icing

device is not applicable (i.e rotating

components like the propellers,

windshields, probes).

This philosophy is applied

on all new generation

turboprop airplanes.

On all ATR aircraft the ice protection

system can be summarized as follows

(fig. 7):

€ A PNEUMATIC SYSTEM supplying the de

icing for the critical areas of the airframe:

€

€

€

€

wing and horizontal leading edges,

engine air intakes and ducts to engines

1.

The engines supply bleed air through the

HP ports.

ELECTRICAL HEATING (anti icing) of:

probes and windshields (always selected

ON),

€ inner leading edge of propeller blades (outer

part is de-iced by centrifugation),

€ flight control horns.

The power is supplied primarily by AC wild frequency current.

To operate efficiently, a sequential operation of

de-icing system must be performed in fast or

low mode depending on external temperatures.

1 : some countries, these protections must be used

as soon as anti-icing system is selected ON.
297

Fig. 8- Pneumatic de icing PNEUMATIC SYSTEM (AIRFRAME)

system

The switching temperatures have been determined

during flight tests. Thanks to the two cycles system efficiency the boots life can be optimized.

Fig. 9- Electrical ice ELECTRICAL SYSTEM (PROPELLER )

protection lime sequence

MONITORING ICE ACCRETION

The ice accretion is primarily detected by observing the natural stagnation points : windshield,

airframe leading edge, wipers, side windows

and propeller spinners.

At night wing lights are used to assist in ice

accretion detection.

Since the ATR 72 fuselage is longer than the

ATR 42’s one, the propeller spinners are not

visible from the cockpit. For this reason, an Ice

Evidence Probe (IEP), visible by both pilots, has

been installed on the ATR 72.

The FEP indicates ice accretion and is shaped to

retain ice until all other parts of airframe are free

of ice. On the ATR 42, this function is performed by the propeller spinner.

In addition to the previous primary ice accretion

recognition means, an Anti icing Advisory

System (AAS) is installed on the ATR (fig. 10).

It includes :

€ an electronic ice detector

€ three lights in the cockpit on the central

panel between the two pilots : ICING (amber),

ICING AOA (green), DE ICING (blue).

This system is not a primary system but has

been designed to alert the crew on the correct

procedures when flying in icing conditions as

detailed later in the document.

The electronic ice detector is located under the

left wing and alerts the crew as soon as and as

diagrum

The temperature reference prevents the possibili1 long as ice accretion develops on the probe.

ty of “run back” on the propeller, a phenomenon Aural and visual alerts are generated (amber

where melted ice freezes again aft of the de ICING light on the central panel and single

icers, with heavy performance losses. chime).

299

Advantages and limitations

The associated

performance

penalties are the

price of safety

The technology adopted by all the new tech

nology turboprop manufacturers is the pneumatic system for the following reasons:

● The pneumatic system induces a very small

penalty on the bleed air supplied by the engines

Consequently the performance of tbe engine!

and of the aircraft are only slightly reduced. The

average air flow picked-up is about 100 times

lower than what would be necessary with hot air

anti-icing.

● The weight of such a system complies with

the turboprop design objectives in terms of

weight savings, It is not the case with an electrical ice protection system which requires large

First the life duration of such a system is directly

related to the number of inflation/deflation

cycles. Consequently, use it when needed and

only when needed.

Secondly it does not completely prevent icing of

leading edges. The system limits the amount of

ice adhering to airfoil but cannot eliminate all

the ice accretion because of continuous accretion

between two consecutive boots cycles. Remains

of ice may also exist after a de-icing cycle

(fig. 11).

These limitations are known by all the manufacturers but are differently accounted for by each

of them. ■

ATR philosophy

In line with the Special Condition previously

presented, ATR philosophy is to propose a global solution maintaining equivalent safety levels

and margins in icing conditions as defined by

certification and in normal cases, taking into

account system performance and limitations.

After extensive testing with real and simulated

ce shapes, this philosophy translates into :

SPECIFIC PROCEDURES

These procedures (recalled hereafter) essentially

maintain safety margin through increases in

minimum speed for each phase of the flight.

INFORMATION

All effects on handling are clearly described,

and performance penalties quantified in the

manuals. ti
300

Before take-off

Flight dispatch must take

into account severe icing

conditions for both routing

and cruise flight level selection.

Preparation and operation

of the ATR following cold soak

in very low temperature require

particular precautions.

Following recommendations

which complement normal

operating instructions should be

observed when applicable.

EXTERIOR SAFETY INSPECTION

As stated in the operational requirements AC

20-117, no person may take-off an aircraft when

frost snow or ice is adhering to the wing, control

surfaces or propeller of the aircraft.

Perform normal exterior inspection.

Check that the following items are free of frost,

ice or snow.

Deice as necessary:

● engine inlets, cowling and drains, propellers,

● pack inlets,

● landing gear assemblies, landing gear doors,

● drains, pitot and static vents, angle of attack

sensors,

● fuel tank vents,

● all external surfaces (fuselage, wings, tail

surfaces, vertical and horizontal stabilizers,

control surfaces) including gaps between

fixed and movable control surfaces.

What are the ground de-icing and anti-icing

procedures ?

● Ground de-icing is the cold weather procedure by which snow, ice, rime and/or slush are

removed from the surfaces and all openings and

hinge points of the aircraft.

● Ground anti-icing is a precautionary measure which uses anti-icing fluids to prevent rime,
301

ice or snow forming or accumulating on the surfaces of a clean aircraft.

Two types of fluids can respectively be used:

Type I fluids (low viscosity), used for the

de-icing, consist of a minimum of 80% of inhibited glycol and phosphates. They are designed

according to AEA, AMS or MIL specifications.

Type 11 fluids (high viscosity), used for the

de and anti-icing, are composed of a minimum

of 50% of glycol and polymer. These fluids are

said to possess non-Newtonian characteristic

(change in state as a result of surface tension).

NOTE

Only KILFROST ABC 3, HOECHST 1704 LTV 88 and

SPCA AD 104 fluids meet the AEA type II fluid

specification, including holdover requirements.

The holdover time of the type 11 advanced fluid

are considerably increased in comparison wit]

the type I fluids.

It should be carefully noted than strict adherence

to adequate procedures by both qualified ground

servicing crews (application of fluids) and pilots

(holdover times) is essential.

De-icing / anti-icing maybe performed in Hotel

mode provided bleed 2 is selected OFF.

ONLY EXCEPTION TO CLEAN AIRCRAFT

CONCEPT AT TAKE-OFF

Limited frost accretion on lower wing surfaces

due to cold fuel remaining and high ambient

humidity.

Frost is a light, powdery, crystalline ice which

forms on the exposed surfaces of a parked aircraft when the temperature of the exposed surfaces is below freezing (while the free air temperature may be above freezing).

Frost degrades the airfoil aerodynamic characteristics. However, performance decrement at

take-off due to 2 mm of frost located on lower

surface of the wing only is covered by performance decrement taken into account preventively for take-off in atmospheric icing conditions.

Take-off may be performed with frost on the

wings provided:

The frost is located on the lower surface of

the wing only.

Frost thickness is limited to 2 mm.

A visual check of the leading edge, upper

surface of the wing, tailplane, control surfaces and propellers is performed to make

certain that those surfaces are totally cleared

of ice.

Performance decrement and procedures defined for take-off in atmospheric icing conditions are applied.
3 0 2

ENGINE STARTING TAXIING

Perform normal cockpit preparation with the following procedures modifications:

OVDB VALVE override control sw

FULL CLOSE

Provided ENG 2 air intake and both pack inlets

are free of snow, frost, ice:

ENG 2 IN HOTEL MODE START

The standard single engine taxi procedure may

still be used provided the friction coefficient

remains at or above 0.3 (braking action medium,

snowtam code 3) and nose wheel steering is not

used with too large deflections. If the OAT is

very low, it may be necessary anyway to start up

engine 1 early enough to get the necessary oil

warm up time (refer to notes 2 and 3 under).

On icy taxiways or in the presence of slush (low

friction coefficients), it is recommended to use

both engines and differential power for taxi. H

1. Starting on aircraft batteries is possible without special precautions down to -15° C (5° F).

2. When starting the engine in extremely cold conditions:

● start up time is slightly increased;

● oil pressure raising time is considerably increased: OIL LO PRESS red warning, may be activated for 6 0 seconds;

● after the initial increased raising time, OIL PRESS will be higher than usual (up to 70 PSI) for several minutes.

3. PL motion above FI is only allowed when oil temperature is at or above 0° C (32° F): this warm up time may take up to

4 minutes when OAT is -35° C (- 30° F).

4 . During cockpit preparation, both packs should be used to warmup cabin and cockpit while running engine 2 in Hotel

mode. Using gust lock stop power with HI FLOW selected (together with all doors, particularly cargo, closed) is recommended for warm up with OAT below -15° C (5° F).

5. Below -15° C (5° F’), several equipment items (e.g. fuel flow, pressurization ind., ADU, AFCS control box) may be not working initially but will automatically recover as cabin and cockpit warm up takes place and compartment temperature rises.
3 0 3

Operations in atmospheric icing conditions

As explained in the previous

chapters, and to take into account

the system limitations,

ATR brings a global solution

to maintain equivalent

safety levels and margins

in icing conditions

as provided in normal operations.

Anti-icing procedures and speed limitations have been developed and must be

complied with as soon as and as long as

icing conditions are met which may occur even

before ice accretion actually takes places. The

procedures and speed limitations do apply until

the aircraft is clear of ice.

Fig. 12

Why should we apply relevant icing procedures,

and above all observe relevant speed limitations

as soon as we fly into icing conditions and even

before ice accretion actually takes place ?

Answer should be obvious to all pilots and

based on two main factors:

● Even small ice accretion is enough to destroy lift significantly (refer to fig, 4 page 4).

● Small ice accretions are most often difficult

to observe and may even be missed by the

ice detector,

The conclusion is straight forward : as soon as

there is a possibility of icing, play the game as if

icing were really there !

Stall occurs at higher speeds (fig. 12) when ice

accretion spoils the airfoil, therefore the stall

warning threshold must be reset to a lower value

of angle of attack.

Thanks to the computing power of MFC on

ATR 72, stick pusher threshold activation is also

lowered accordingly.

The minimum manoeuver/operating speeds defined for normal (no icing) conditions must be

increased. These new minimum operating

speeds are called MINIMUM ICING SPEEDS. They

arc given in the approved AFM (chap. 6) in the

FCOM (2.02) and in the check list booklet.

Ice accretion may also affect forces required to

manoeuver flight controls.

On the ATR:

● rudder forces are not affected;

● aileron forces are somewhat increased when

ice accretion develops, but remain otherwise

in the conventional sense;

● pitch forces arc not affected in flaps 0°, 15°

and 30° (see further: take-off after type II

fluid use).
3 0 4

PERFORMANCE WITH ICE ACCRETION

When flying in icing conditions, remain "performance minded”. Make sure your planned

cruise level is coherent with the ceiling computed in icing conditions.

Example:

ATR 72- TWIN engine

Weight 20 T, ISA +20°C

Normal cond. Icing cond.

Service ceiling* FL 180 FL 200

* The service ceiling is computed with a 300

ft/mn residual rate of climb in normal conditions

and 100 ft/mn in icing conditions.

NOTE

Do not attempt, in icing conditions, to fly above

the service ceiling computed in normal conditions (refer FCOM 3.04), as your residual rate of

climb is reduced.

As far as the single engine ceiling is concerned,

it is clear that loss of performance are minimized by selecting flaps 15°.

Example:

ATR 72- One engine out

Weight 20 T, ISA -10°C

Flaps Normal ICING

cond. cond.

Gross ceiling* 0 FL 160 FL 130

15 -- FL 140

* The gross ceiling is computed in drift down

This is the reason why, if obstacle limitations

exist whenever minimum icing speeds are imposed (icing AOA light illuminated), single engine

critical phases (final take-off climb, en route

drift down procedure) must be performed with

flaps 15° configuration.

If no obstacle limitation exists, flaps 0 may be

used for single engine cruise in order to benefit

from a higher cruise speed but a lower cruising

altitude.

Never climb below minimum icing speed

The minimum icing speed is always close to the

best climb gradient speed. Any attempt to climb

at a speed lower than the minimum icing speed

IS hazardous and can only lead to reduced climbing performance.

When flying close t o top of icing clouds

(even a few hundred feet below) never try to

exchange speed for height when already

flying minimum icing speeds!

As mentioned here above no benefit can be

taken by reducing the prescribed minimum icing

speed even for the last hundred feet!

Never try to fly

above your

practical ceiling:

BE PERFORMANCE

MINDED.

Minimum icing

speed must NEVER

be deliberately

transgressed.

conditions.
3 0 5

C RUISE IN ICING CONDITION ~ USE OF AFCS IN ICING CONDITIONS

I

When flying in icing conditions, do not forget to: When climbing with A/P selected ON (V/S

● set NP at/or above 86% :

NP = 86% corresponds to the minimum rotating

speed required to provide effective propeller

de-icing (centrifugal effect is predominant to

physically eliminate ice on the blades).

Sticking to NP 77% may lead to blade contamination resulting in drastic thrust reduction and

drag increase, which could, in extreme cases,

push the aircraft down to stall in level flight.

● Compare predicted data (FCOM 3.05) to

observed performance.

For example: ATR 72, 20 T, ISA, FL 220, NP

86%. Speed is reduced from 197 kt in clear

conditions to 188 kt IAS in icing conditions.

● Set the internal bug to the speed given in the

manuals and never accept any SLOW indication.

€ If necessary, push the throttle to Max

climb or even Max cont., and change your flight

level and/or your route.

mode), be sure the required vertical speed is

compatible with the minimum icing speed.

Otherwise the speed may regress down to the

stall speed. Flying a 5° pitch basic mode is

always safe but it is more consistent to use the

IAS mode set at a speed equal to or greater than

the minimum speed (VmLB or VmHB in accordance with the required selection of LB or HB

on auto pilot). ■

Particular attention should

be paid to aileron mistrim message

(flashing on ADU and EADI):

if this message appears, apply

Aileron mistrim procedure.
3 0 6

Procedures

Before presenting the procedures,

it is necessary to recall the properties

of the ground anti-icing fluids.

T he type II fluids are used for their antiicing qualities. Under the effect of the

speed they spread out on the control surfaces, especially the lower surface of the elevator through the elevator gap during rotation

while taking-off.

Tests have been performed on ATR development

aircraft.

Results and relevant information are gathered

together in service information letters referenced

SIL ATR 42.30.5007 and SIL ATR 72.30.6001.

This phenomenon temporarily changes the trim

characteristics of the elevator and can lead to an

increase in control forces necessary to rotate,

these forces become more noticeable when the

.4 . . . . . . . .:

center of gravity is forward with temperatures

around 0° C (32° F).

Depending upon the fluid type, this effect can

double temporarily the pilot force necessary to

move the elevator and achieve the required rotation rate.

This problem is legitimate and not associated to

any other control or performance problem. This

phenomenon can be perfectly controlled by the

pilots and the take-off path remains unaffected.

I

1.

2.

3.

4.

5.

6.

These procedures are applicable to all flight phases including take-off.

Ice accretion may he primarily detected by observing the Icing Evidence Probe (IEP). At night, this IEP is automatically illuminated when NAV lights are turned ON. Ice accretion may be detected on propeller spinner, windshield, airframe (leading

edges), wipers and side windows on the ATR 42.

Clear ice accretion may be difficult to detect. If clear ice is suspected, temporary selection of airframe boots is recommended as the action of the boots will shatter the ice and make its observation much more obvious.

With very cold OAT, delay start of take-off roll until oil temperature is at least 45° C (113° F); this is necessary to guarantee inlet splitter de icing capability.

When ice accretion is visually observed de-icers must he selected and maintained ON as long as ice continues to accumulate.

Ice detector may also help the crew to determine continuous periods of ice accretion. Nevertheless it may not detect certain

ice accretion forms. 1
3 0 7

The following procedures must be applied for

take-off and for flight in icing conditions.

NOTE

Permanent heating (probes/windshield) is

always selected ON.

NOTES

1.

Take-off may be scheduled using normal

minimum V2.

2.

Horns anti icing must not be selected ON to

avoid lowering the stall warning threshold.

T A K E- OFF IN GROUND ICING CONDITIONS ~

WITHOUT ATMOSPHERIC ICING CONDITIONS ~ TAKE -OFF IN ATMOSPHERIC ICING

! CONDITIONS

When taking off from a contained runway ~

(slush, snow, supercooled water, . ..). without ~| Operational speeds must be increased whenever

atmospheric icing condition (no air contami- ~ possible wing leading edge pollution during

nants such as fog), wing leading edge pollution ] take-off due to air contaminants is anticipated.

is not anticipated during the take-off run and I

consequently operational speed increase needs1 Standard take-off procedure must be used with

not to be considered. ~ the following addition : for take-off with atmosHorns anti-icing should therefore not be selected pheric icing conditions, refer to appropriate

ON in order to avoid lowering the stall warning speeds and performance penalties to take into

threshold. Icing AOA light should not be illumi- account possible ice accretion during take-off

nated. It is better to maintain low VI (and V2) run.

on this type of runway, in case an aborted takeoff would have to be performed.

Note that propellers and brakes however may be ▲ RUNWAY IS CONTAMINATED (water, ice,

affected by these contaminants. Propeller anti- snow, slush) use the relevant performance penalicing should therefore be selected and it is ties defined in the performance section (FCOM

recommended to cycle after take-off the landing 3.03). At very low speeds using reverse on

gear in order to avoid wheel brake freezing. contaminated runways should be limited to

avoid contaminant projections at the level of

Before take-off cockpit windshield which may reduce visibility

ENG START ROTARY SEL CONT RELIGHT to zero (snow, slush).

PROPELLERS ANTI ICING ONL Y ON

After take-off

LANDING GEAR (if possible) CYCLE

PROP ANTI ICING AS RQD

ENG START ROTARY sel AS RQD
ENTERING ICING CONDITIONS (IEP) : when there is no more ice visible on the

IEP, the whole aircraft is cleared of residual ice.

ANTI ICING On the ATR 42, end of ice accretion can be

(PROP - HORNS - SIDEWINDOWS) ON checked on the propeller spinner.

ICING AOA light check ILLUMINATED

As long as ICING AOA green caption is illumICING SPEED Bugged and OBSERVED nated,

PROP mode sel according to SAT

CL set FOR NP > 86% MINIMUM ICING confirm

ICE ACCRETION and/or speed deceleration SPEED bugged and observed

MONITOR

Maintening de icing equipment in operation

unnecessarily is very detrimental to boots life. [n

AT FIRST VISUAL INDICATION OF ICE order to remind the crew to check i f ice accreACCRETION, AND AS LONG AS ICE tion has ceased and, when ascertained, to switch

ACCRETION DEVELOPS ON AIRFRAME the de-icing boats OFF, the de-icing blue light on

memo panel will blink if de-icers are still ON

ENTERING ICING CONDITIONS more than 5 minutes after the ice detector has

procedure CONFIRM COMPLIED WITH stopped to signal ice accretion (icing amber

MINIMUM lCING CONFIRM light OFF).

SPEED B UGGED AND OBSERVED

ENG START ROTARY sel CONT RELIGHT When no more residual ice,

DEICING (ENG then airframe) ON ICING AOA PUSH TO CANCEL

ENG and AIRFRAME

mode sel ACCORDING TO SAT

SPEED DECELERATION Monitored against relevant DESCENT

FCOM predicted values

Normal or icing approach

conditions CONFIRM

LEAVING ICING CONDITIONS Relevant approach speeds BUGGED

Relevant performance restrictions up to landing

De-icing, continuous relight and anti-icing may APPLY

be switched OFF, but ICING AOA caption must

not be cancelled until it is visually confirmed The procedure to follow in case of landing with

that the aircraft is cleared of any residual ice. a defective airframe de-icing system is given in

Experience has shown that, when the aircraft is 1| the FCOM 2.05 and in the check-list.

flown in warmer temperature, the last part to /

clear on the ATR 72 is the Icing Evidence Probe i
LANDING ON SLIPPERY RUNWAYS

Under these circumstances, the recommended

procedure is :

.

.

●

●

●

✎

Use the longest runway compatible with

crosswind limits. Avoid tailwind landings.

Avoid a long landing and put the aircraft

down in the touch-down zone.

After touch-down, lower the nose. Select

ground idle then reverse (use of reverse a

high power down to very low speeds may

reduce visibility as contaminant are blown

up by reversed air flow) and apply the brakes

symmetrically.

If no deceleration is felt, do not use alternate

brakes, do not pump the brakes as the antiskid system will always stop the aircraft in a

shorter distance than the pilot can by modulating the brakes.

In an emergency, reverse may he used until

standstill.

Reduce to taxi speed prior to turning off the

runway.

SLIPPERY RUNWAYS AND CROSSWINDS

(LANDING)

The wind component at right angles to the landing direction tends to push the aircraft to the

down-wind side of the runway. The aircraft

additionally tends to behave like a weather vane

and yaws into the wind. This creates a side component of reverse thrust which also pushes the

aircraft downwind.

The counter-acting side force required to keep

the aircraft on the centerline is provided by tire

traction. However on wet and/or slippery surfaces, tire traction is considerably reduced. So

when directional control becomes doubtful,

release the brakes and reduce reverse to ground

idle. Use rudder to re-align the aircraft with the

runway, reapply reverse and use the brakes as

required to stop the aircraft.

PARKING

When OAT is below -5° C (23° F), particularly

in wet conditions, avoid leaving the aircraft with

parking brake engaged and use chocks instead

whenever possible,

When severe cold soak is expected (temperature

below -20° C (- 4° F) for a prolonged time)

avoid immobilisation of the aircraft with propeller brake engaged. It is recommended to remove

the batteries and keep them in heated storage. ■
Freezing rain

Freezing rain is a precipitation of I FREEZING RAIN LOCALIZATION

large supercooled water drops.

These drops (negative temperature)

may be transformed into clear ice

when impacting the aircraft’s skin

Freezing rain seldom occurs and is seldom

encountered at high altitudes unless associated

with large storm systems such as thunderstorms.

It is normally a low altitude weather phenomena

in slightly negative temperature and is mainly linked to the presence of a front

condition. (temperature ranging from -5° to 0° C — 23° to

32° F).

AIthough freezing rain is not part of certification cases, it must be taken into account

for operations in icing conditions.

Freezing rain normally occurs as a result of weather conditions where temperature increases

with altitude (temperature inversion, ref.

fig. 13), Warm rain falls from or passes through

this warm layer into a region of subfreezing temperature and typically becomes supercooled.

These supercooled large rain drops will then

freeze upon impact with an object.

Impact of these large drops on the leading edge

of an aircraft wing or other aerodynamic surfaces, under certain conditions, can cause the

entire surface to become incrusted with ice.

To protect an aircraft from freezing rain of this

type would require that the entire aircraft rather

than only the leading edges, be equipped with

de-icing and anti-icing systems. This is obviously impracticable.

Ice accretion due to freezing rain may result in

asymmetrical wing lift and associated increased

aileron forces necessary to maintain coordinated

flight before aerodynamic stall.

AVOIDANCE

I

Freezing rain conditions are usually predictable,

recognizable and avoidable.

These conditions are predictable:

● on ground, by

- consulting weather chart

- reading AIREP and AIRMET message

● in flight, by

- listening to SIGMET message

- monitoring outside air temperature for the presence of temperature inversion condition.

A .

3

g

<

1:

Critical

zone

-10 0 10 °C

10 30 50 °F

Temperature

Fig. 13- Temperature i n version is a :one where

temperature increases with altitude.
311

Freezing rain and certification

Advisory circular 20.117 states

“ It is emphasized that aircraft ice protection systems are designed basically to cope with supercooled cloud water environment (not freezing

rain). Supercooled cloud water droplets have a median volumetric diameter(MVD) of 5 to 50 microns. Freezing rain MVD is as great as 1300

microns. Large drops of freezing rain impact much larger areas of aircraft

componements and will, in time, exceed the capability of most ice protection equipments. Flight in freezing rain should be avoided where

practical. ”

These conditions are recognizable :

If heavy rain occurs whenever the flight crew

have identified conditions propitious to freezing

rain formation, it is highly probable that freezing

rain is involved. Heavy rain is visually detectable (at night by use of the landing lights) and

can be heard striking the fuselage,

If all above conditions are met, this heavy rain

will lead to clear ice building on aircraft.

This accretion :

€ is transparent and consequently more difficult to detect but gives an unusual shiny

aspect to the covered surfaces ;

€ adheres to most of the surfaces of the aircraft, whereas the de-ice system is only designed for de-icing the leading edges, and

limits the efficiency of the de-icing boots,

Should the aircraft enter in a freezing rain zone,

he following procedure should be applied,

AP engaged,

RETRIM ROLL L/R WING DOWN " messages

MONITOR

In case of roll axis anomaly, disconnect AP holding the control stick firmly. Possible abnormal

roll will be felt better when piloting manually.

SPEED INCREASE

Increase the speed as much as performance and

weather conditions (turbulences) will allow.

Extend flaps as close as possible to respective

VFE. ■
312

Maintenance recommendations

Beyond the previous procedures,

a particular care must be taken

for the maintenance when

operations in icing conditions

are performed.

Some maintenance recommendations are presented hereafter.

LANDING GEAR CLEANING

whatever external conditions exist, the landing

gears should not be cleaned with high pressure

water which can cause grease to be washed

ding gears with low pressure water mixed with

glycol.

Hot air may also be used to remove snow, slush

or ice accumulations on the landing gears. This

method should be considered for brake units as

the spraying of de-icing fluid between stator and

rotor discs can affect the brakes performance.

TIRES

Tires can become frozen to the ground under

ground icing condition. In such a case hot air

may be used to warm and free the tires. Do not

use hot air temperatures above 80° C ( 176° F).

away and electrical plugs possibly damaged or )

contaminated. I LANDING GEAR SERVICING

Plain water should not be used in cold weather \

conditions since it could re-freeze on the landing I When charging the landing gears shock asborgears components and cause latches, locks, sli- ~ bers in a hangar, the difference in inside / outside

ding parts to jam or electrical continuities to he ~ temperatures should be taken into consideration

lost. It is therefore preferable to clean the kin- as it affects the struts height.

\
313

FLIGHT CONTROLS ! FUEL SYSTEM

Flight surfaces are controlled by cables and rods The fuel tanks and surge tanks should be drained

through pulleys and bellcranks. De-icing fluids ~at each line check, whatever conditions exist.

have a detrimental effect on bearing lubrica- When the airplane is parked or operated for a

tions. The direct spraying of fluid on these long time in negative temperatures, the water in ~ 

mechanisms, particularly in the wing rear spar the fuel can freeze and could cause engine suparea should be avoided when possible, ply difficulties or plugging of the fuel tank venInspections of the roll control mechanism are ting duct.

planned in the aircraft maintenance program for If the airplane was parked for a long time at subgenerai condition corrosion or excessive play at ~ zero temperatures and the drainage is made in

a C interval. Additional checks may be advi - these conditions, water may not be evacuated as

sable for airplanes subject to frequent de-icing it has become frozen. The drainage should thereoperations. fore be performed when possible after the airplane has remained again some time in a positive

ambient atmosphere (hangar, airfield). This can

PNEUMATIC D E-ICING SYSTEM also be achieved after a refueling operation with

warm incoming fuel.

To prevent water accumulation in the pneumatic ~

de-icing system, it is recommended to periodi- ~

cally blow the air distribution circuit and to -

“dry” the system. This can be achieved by blowing compressed air into the circuit through the

regulator/shut-off valve port.

DOORS

During on ground aircraft handling operations,

precipitations can cause water to enter into the

doors mechanisms, If the aircraft is then cold

soaked at very low temperatures, the door operaDE-ICERS (BOOT S ) ting mechanism may become jammed or hard to

open. In that case hot air only should be used to

De-icing boots should be seasonally checked for dry and free the system. Again de-icing fluids

debonding, presence of pin holes and blisters should not be used to avoid wash out of the

and repaired or replaced whenever necessary, to grease. ■

ensure their efficient operation (ref. CMM ~

AERAZUR 30-11 ; 30-21). I
314

,,*

,.

I

..

Despite continuous emphasis.

on icing hazards, accidents

and incidents linked to icing

continue to occur in air

transport.

ATR certification process and

philosophy is on the safe side,

and covers all predictable cases

of icing occurrences, provided

some basic rules are respected: ●●

● ✌

If you have any doubt about

the proper ground de-icing of

your aircraft, DO NOT TAKE OFF.

As soon as you enter icing

conditions, do not wait for

actual ice accretion to play the

game: turn Icing AOA green

light ON and observe minimum

icing speed right away. Cancel

Icing AOA only when you are

sure there is no more ice on

the aircraft.

AAS is designed to help you

but remember You are in charge. No system can replace

PILOT JUDGMENT and GOOD

AIRMANSHIP.
+?m

Version 2.0

317

The Edwards Air Force Base Testing

The temperatures

and droplet sizes

selected for

these tests were

based upon the

meteorologic

analysis of

conditions at

the time of the

October 31

accident of

Flight 4184.

SPECIFIC PROCEDURES

The tests conducted at Edwards Air Force Base had

two objectives:

1) To confirm that the ATR meets certification

standards for icing conditions; and

2) To observe the ice accretion characteristics at

marginal freezing temperatures of large water

droplet sizes that are outside certification

standards.

Tire temperatures and droplet sizes selected for

these tests were based upon the meteorologic

analysis of conditions at the time of the October

31 accident of Flight 4184.

The target exposure time of 17.5 minutes was

selected to match the time interval in the

accident between ice detector activation

and the roll upset.

IMPORTANT NOTICE:

This choice of 17.5 minutes does not imply

that such is the minimum time to reach a

critical level of ice buildup in real flight

conditions.

EQUIPMENT USED

The testing involved the following aircraft:

● USAF KC-135 – This aircraft has a special

water spray array on the end of the refueling

boom. The diameter of the “cloud” produced

by the array was approximately 8 feet, The size

of the water droplets and the liquid water

content of the cloud was adjusted by varying

air pressure in the spray system and changing

nozzles on the spray array.

.

●

LearJet 36A - This aircraft was equipped

with sensors to measure the characteristics

of the cloud. This aircraft also functioned as

a chase plane.

ATR 72-212- A stock production aircraft was

used. The normal crew escape hatch was

replaced with a special hatch equipped with two

video cameras, Tire cameras were pointed to the

outboard sections of the wings and were

equipped with zoom lenses. They were

controlled by one of the two crew members.

DEFINITIONS OF TERMS USED

MVD - Mean Volumetric Diameter

In simple terms, this is the water droplet size. In

nature, the droplets that make up clouds and

precipitation are a variety of sizes. Some air

masses may contain a greater proportion of large

diameter droplets, while others are composed

primarily of small droplets. The data collected

by the LearJet calibration aircraft was prncessed

through a variety of mathematical analysis

programs to produce an MVD value that might

be thought of as an average drop size.

WC- Liquid Water Content

This is the number of grams of water that are

contained in one cubic meter of the cloud (g/m3).

It might be thought of as describing the density 1

of the cloud.
318

Purpose of the Tests

T

he tests were designed to examine both the upper

limits of the certification envelope at marginal

freezing temperatures and the icing characteristics

of very large droplets not covered by certification.

Marginal freezing temperature tests were conducted

at a SAT of approximately -2C (TAT of approximately

+3C). Altitude was varied as necessary to achieve

and maintain the desired temperature, and most testing

was conducted between 11,000 and 13,000 feet. Air

speed used during accretion was between 175 KIAS

and 180 KIAS and was selected to replicate the

Flight 4184 accident scenario.

Certification standards call for a droplet size for the

test temperature range of 40 microns in diameter, but

most of the tanker test program flights occurred at an

MVD in excess of 70 microns — or almost double

the regulatory requirement. The LWC by regulation

is. 15 g/m3

. The tests were actually conducted with a

LWC of approximately .45 g/m3 — or approximately

3 times the regulatory requirement. For the purposes

of this document, these tests will be referred to as the

“70 micron tests”,

The tests to explore very large water droplet sizes

were conducted at an MVD of approximately 180

microns and the LWC was approximately .35 g/m3

. The U.S. Air Force

These tests will be referred to as “the 180 micron had never before tests.” The U.S. Air Force had never before applied

droplets of this size to any aircraft, either military applied droplets

or civilian. of this size to

any aircraft,

either military

or civilian.
319

Summary of findings

The following is a brief summary of the findings. The video, now in production, will cover the tests in

more detail and provide specific values when all data reduction is complete. In all ice accretion conditions

tested, the aircraft was flown down to stick pusher. The target application time of 17.5 minutes was

achieved with no difficulty for all tests.

70 Micron Tests

FLAPS 0° AND 15°

Ice Accretion Pattern

● The deice boots provided full protection.

● The windshield heating unit provided a clear area both on the forward window and on the side window.

The only ice accretion noted on the side windows was some small random droplets or “globs.” Coverage

on the forward side window was less than 10 percent with no discernible pattern. There was no accretion

on the aft side window.

● The ice detector actuated within 1.5 minutes of entering the icing “cloud.”

● The ice evidence probe accreted ice very quickly and, during the course of the test. created a “double

horn” shape of approximately 4 to 5 inches in height and approximately 1-inch thick.

Handling Characteristics

● At no time were there any changes in handling characteristics other than a small lateral imbalance that

would be expected with ice accretion occurring on only one wing tip.

70 Micron exposure
320

180 Micron Tests

(Outside Certification Envelope) . . .. . . . . . . . . “?,----- —— ..—

180 Micron Test:

Accretion

Flaps 15°

Accretion

Flaps 0°

FLAPS 0°

Ice Accretion Pattern

● Ice accretion occurred at the top edge of the

boot in the form of a “saw tooth" ridge.

This ridge would build to a height of

between 3/4 inch and 1 inch and then break

away in the airstream. It is estimated that at

any point in time this buildup may have

covered approximately 60 percent of the

leading edge area. Some accretion appeared

on the lower surface of the wing back as far

as approximately 50 percent chord.

● Minor ice accretion recurred on the lower

aft surface of the aileron control horn. The

aileron gap remained clear at all times.

● Vortex generators remained clear with only

occasional minor accretion at the tips,

which cleared itself periodically.

Handling Characteristics

● Handling characteristics remained essentially

normal, with only a “wing heavy” tendency

that would be expected under asymmetric

accretion,

FLAPS 15°

Ice Accretion Pattern

● While the boots kept the leading edge area

clear, ice accretion did recur behind the

boots to about 16 percent chord. This

accretion consisted of a major ridge, at

about 10 percent chord, and a second

minor ridge at about 14 percent chord. This

second ridge reached a height of only

about 1/4 inch. However, the forward ridge

was able to reach a height of approximately

3/4 inch during the 17.5-minute exposure

time. The lower surface of the wing

remained clean.

● The vortex generators remained clear with

only occasional minor accretion at the tips,

which cleared itself periodically.
321

FLAPS 15°

Handing Characteristic

● In checking the handling characteristics in

this test, the autopilot was used to simulate the

accident scenario. The autopilot was engaged at

175 KIAS and was capable of holding the lateral

forces. The aircraft was then accelerated to

185 KIAS and the flaps retracted to copy the

accident scenario. No abnormalities were noted.

AFTER FLAPS RETRACTION

Handling Characteristics (Flaps 0°)

● The aircraft then began a deceleration with

autopilot engaged.

● Aileron mistrim messages appeared on the ADU

prior to autopilot disconnect at approximately

125 KIAS. The aircraft was then hand flown

down to stick pusher. As stick pusher was

approached, the aircraft exhibited a tendency for

the ailerons to deflect in the direction of the

contaminated wing. The maximum lateral forces

noted, even with the asymmetric ice accretion,

were approximately 35-to-40 pounds.

In addition to the tests described above, a number of

other tests were conducted, including failure modes

of outboard boots and aileron horn heat as well as

operation at 77 percent Np. Details of these tests will

be covered in the video.

Ice Evidence Probe

180 Micron Exposure

The maximum

lateral forces

noted, even

with the

asymmetric ice

accretion, were

approximately

35-to-40 pounds.
322

After 30 seconds of exposure. After one minute of exposure.

Visual cues identified with freezing min or freezing drizzle are characterized as a

dispersed granular ice pattern, spanning the entire height of either side window,

covering all or part of the window from front to back.

After five minutes of exposure. (Right window) After ten minutes of exposure.