PB96-917003

NTSB/SIR-96/03

NATIONAL

TRANSPORTATION

SAFETY

BOARD

WASHINGTON, D.C. 20594

SPECIAL INVESTIGATION REPORT

ROBINSON HELICOPTER COMPANY R22 LOSS OF MAIN

ROTOR CONTROL ACCIDENTS

6405B
The National Transportation Safety Board is an independent Federal agency dedicated to

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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

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(703)487-4600
NTSB/SIR-96/03 PB96-917003

NATIONAL TRANSPORTATION

SAFETY BOARD

WASHINGTON, D.C. 20594

SPECIAL INVESTIGATION REPORT

ROBINSON HELICOPTER COMPANY R22 LOSS OF MAIN ROTOR

CONTROL ACCIDENTS

Adopted: April 2, 1996

Notation 6405B

Abstract: This report examines the loss of main rotor control accidents involving Robinson

Helicopter Company R22 helicopters, When similar accidents occurred involving the Robinson R44

helicopters, the scope of the report expanded to include those accidents. The safety issues

discussed in the report include the need for appropriate measures to reduce the probability of loss of

main rotor control accidents; the need for continued research to study flight control systems and

main rotor blade dynamics in lightweight, low rotor inertia helicopters; the need for operational

requirements to be addressed during future certification of lightweight, low rotor inertia helicopters;

and the need for the Federal Aviation Administration (FAA) to review and revise, as necessary, its

procedures to ensure that internal recommendations, particularly those addressed in special

certification reviews, are appropriately resolved and brought to closure. Safety recommendations

concerning these issues were made to the FAA and the National Aeronautics and Space

Administration.

iii

iii

CONTENTS

Executive Summary.........................................................................................................................................iv

Chapter 1 - Introduction..................................................................................................................................1

Chapter 2 - R22 Design ....................................................................................................................................5

General Description .............................................................................................................................5

Flight Control System ..........................................................................................................................5

Main Rotor System ..............................................................................................................................7

Flight Control Responsiveness............................................................................................................7

Original Certification...........................................................................................................................8

Recent Robinson Helicopter Company Design Changes..................................................................8

Chapter 3 - Accident Data and Accident Investigation Review.................................................................10

Characteristics of Accidents Involving Loss of Main Rotor Control.............................................13

Accident Investigation Review..........................................................................................................14

Chapter 4 - FAA Oversight............................................................................................................................18

Certification Reviews.........................................................................................................................18

Recent FAA Actions...........................................................................................................................21

Flight Testing of the R22 and R44 ....................................................................................................22

Georgia Tech Study............................................................................................................................23

Chapter 5 - Summary Discussion..................................................................................................................25

Findings...............................................................................................................................................29

Recommendations..............................................................................................................................31

APPENDIX A-Summary Reports of R22 Loss of Main Rotor Control Accidents...................................33

APPENDIX B-Summary Reports of R44 Loss of Main Rotor Control Accidents...................................66

APPENDIX C-Main Rotor Hub Teeter Angle and Rpm Decay Survey....................................................71

APPENDIX D-Applicable Safety Recommendations and Related Correspondence................................77
iv

Executive Summary

The National Transportation Safety Board's special investigation of accidents involving loss

of main rotor control by the Robinson Helicopter Company (RHC) R22 was prompted, in part, by

an accident that occurred during an instructional flight near Richmond, California, on June 29,

1992. The flight instructor had 2,000 hours in the R22, and the student had 4 hours. The findings

in that accident—that the helicopter was being operated at normal main rotor revolutions per

minute (rpm) within the approved flight envelope and with no indication of weather being a

factor—coupled with the Safety Board's difficulty in determining the causes of many similar loss of

main rotor control accidents in the past, led the Safety Board to investigate these accidents as a

group in an attempt to find common factors and to develop appropriate recommendations to prevent

occurrence of similar accidents in the future.

For this special investigation, the Safety Board reviewed fatal accidents involving

certificated helicopters; reexamined the available wreckage of the R22 from the Richmond accident

and other accidents; reviewed the Federal Aviation Administration's (FAA) original certification

process, certification requirements, and subsequent reviews of the R22 certification; and reviewed

the Safety Board's recommendation history for the R22. In addition, the Safety Board analyzed

various potential scenarios that could lead to loss of main rotor control.

During the Board's special investigation, the FAA implemented several operational changes,

primarily to ensure that pilots of the R22 and flight instructors were better trained and more

proficient and that flights in R22s in certain adverse weather conditions were restricted. There have

been no loss of main rotor control R22 accidents in the United States since the changes were

implemented more than a year ago.

The following issues are addressed in this special investigation report:

• The implementation of appropriate measures to reduce the

probability of loss of main rotor control accidents.

• The need for continued research to study flight control systems and

main rotor blade dynamics in lightweight, low-rotor inertia

helicopters.

• The establishment of operational requirements to be addressed

during future certification of lightweight, low rotor inertia

helicopters.

• The need for the FAA to review and revise, as necessary, its

procedures to ensure that internal recommendations, particularly

those addressed in special certification reviews, are appropriately
v

resolved and brought to closure.

As a result of this special investigation, recommendations concerning these issues were

made to the FAA and the National Aeronautics and Space Administration (NASA).

Chapter 1 - Introduction

On June 29, 1992, at 1242 Pacific daylight time, a Robinson Helicopter Company R22

helicopter, N83858, operated by the Sierra Academy of Aeronautics, Inc., lost main rotor control

and broke up in flight during an instructional flight near Richmond, California.1

 Witnesses

observed the tailboom and main rotor separate from the helicopter in flight. A certificated flight

instructor (CFI) was providing a primary flight lesson to his student, who was recording her lesson

(cockpit interphone and radio communications) with a microcassette tape recorder. The recording

revealed no operational difficulties during the engine start, ground checks, takeoff, or the 17-minute

flight en route to a practice area. The low rotor rpm2

 warning horn was checked and operated

normally on the ground.

While en route and following a climb, the CFI instructed the student to perform a left turn. 

According to the recording, the student completed the turn using a shallow bank. While cruising

southbound at 2,000 feet, with no prior indication of an anomaly, an undetermined event interrupted

the CFI's speech and culminated in the breakup of the helicopter. A wind-like background noise

then became evident on the tape and muffled the student's exclamation, "Help!" The helicopter

rapidly descended and crashed into San Pablo Bay, 3 miles northwest of Richmond, California. 

The CFI, who had accumulated about 2,000 hours of R22 flight time, and the student pilot were

killed. The student pilot had 4 hours of total flight time, all in the R22 as a pre-solo student.

The record of the flight provided by the audiotape showed that neither pilot had voiced any

concern about the operation of the helicopter before the breakup. The low rotor rpm warning horn

did not activate before or during the breakup sequence. The Safety Board's sound spectral analysis

of the audiotape indicated that the helicopter was being operated at normal main rotor rpm. No

unusual rotor system noises were heard before the event. The analysis of the audiotape indicated

that the main rotor rpm did not decay before the breakup. Analysis of the recorded primary and

secondary air traffic control radar data found that the initial breakup had occurred at 2,000 feet

mean sea level. The helicopter's indicated airspeed, calculated from available radar data, was

normal for cruise flight.

The wreckage was recovered from San Pablo Bay; examination of the wreckage produced

no evidence of preimpact control system or airframe failures that might have initiated the

breakup. Evidence of control interference was not found. The swashplate, spindle bearings, and

engine exhibited no signs of preimpact damage. The main rotor mast assembly, with the main

rotor blades attached, was recovered about 970 feet north of the main wreckage. The assembly

had separated from the upper portion of the helicopter's transmission housing. One main rotor

blade was found curled 39o

 upward, and both main rotor blades exhibited multiple red paint

smears that appeared to match the tailboom paint. The aft portion of the tailboom (aft of the first

 

 1For more detailed information, read Brief of Accident File 1003, accident number LAX92FA267.

 2

Revolution of the blade per minute.
2

bay area) was not recovered. However, a main rotor blade had left its impression in the crushed left

side of the tailboom's first bay area. Both pitch change links3 exhibited bending overload failures,

and the spindle tusks4 were fractured from each spindle, consistent with damage resulting from the

divergence of the main rotor blades from their normal plane of rotation. (Pitch change links and

spindle tusks are addressed later in the report in Chapter 2.)

The Safety Board could find no evidence of the specific event that caused or allowed the

main rotor blades to diverge from their normal flightpath plane and strike the airframe. The

circumstances of the accident did not suggest failure or inability to maintain adequate rotor rpm, nor

did they support that low-G maneuvering (that part of the flight envelope generally accepted to be

less than 0.5 positive G) precipitated the event.5

Since 1981, the Safety Board has investigated or researched 31 R22 and three R446

accidents (domestic and foreign) involving an in-flight loss of main rotor control and contact of the

main rotor blades with the tailboom or fuselage of the helicopter.7

 Because of the circumstances of

the accident near Richmond, California, and the Safety Board's difficulty in determining the specific

reasons for the loss of control of the main rotor blade and the precise mechanism by which the

blade severs the fuselage, the Safety Board conducted this special investigation to identify common

factors in these accidents and to recommend appropriate measures to prevent future accidents.

 

 3Pitch change links are metal links that connect the swashplate to the pitch horn of the blades to control

the blade angle-of-attack. See Figure 1.

 4

A tusk is the inner portion of the blade spindle that contacts the droop stops during start-up or shut

down. It is designed to prevent the blades from drooping too low when centrifugal and aerodynamic forces

are too low to support the blade in the plane of rotation.

 5

In the 1980s, low main rotor rpm and low-G maneuvering were associated with several R22 accidents as

determined by the FAA, RHC, and the Safety Board.

 6

The R44 is a four-place version of the R22.

 7

The Safety Board's special investigation initially focused on R22 accidents in which the main rotor

blade diverged from its normal path and struck the helicopter. When similar R44 accidents occurred, the

special investigation was expanded to include those accidents. (See Appendix A for a summary of each R22

accident and Appendix B for a summary of the R44 accidents.) The Safety Board’s review of R22 accident

reports disclosed that 13 of the reports did not contain sufficient information to adequately support the

previously issued probable causes. The Safety Board has revised the probable causes of these accidents and

changed the corresponding Brief of Accidents accordingly (see Chapter 5 for more discussion).
3

Main

Rotor

Hub

Teeter

Bolt

\ /Droop -.

Horn

I

Pitch

Link

LI

r

l\ . ----

1

Spindle

Tusk Teeter Spindle

stop

~

I

I Main Rotor !

Shaft

Swashplate

I

.

I

●

Figure 1 --R22 Main Rotor Hub and Assembly
4

Chapter 2 of the special investigation contains a description of the R22 design and

manufacture. Chapter 3 provides an overview of fatal accident data involving certificated

helicopters; the chapter also provides data related to pilot experience and some general

characteristics of loss of main rotor control accidents investigated by the Board. Chapter 4 tracks

the involvement of the Federal Aviation Administration (FAA) with the R22, related technical

studies, and actions taken to reduce the potential for loss of main rotor control accidents. Chapter 5

contains a summary discussion of the issues and further actions needed. The last section of Chapter

5 presents the Safety Board's findings and safety recommendations made as a result of this special

investigation.
5

Chapter 2 - R22 Design

General Description

The Robinson R22 helicopter is a two-place, light utility aircraft powered by a four-cylinder

Textron Lycoming 0-320 reciprocating engine (see Figure 2). The helicopter has design features

common to some helicopters but has several innovative design features unique to the R22 (and

R44).

Flight Control System

The R22's flight control system is similar to those in other conventional helicopters in

directional, lift, and maneuvering controls. The R22 uses a standard tail rotor system and tail rotor

pedals for directional control. The collective and cyclic control mechanisms8

 are also standard for

controlling lift, steady flight, and maneuvers.

However, the cyclic control is shaped differently from those in other helicopters. The R22's

cyclic control is shaped like a "T," with a vertical component between the pilot seats. The top part

of the "T" is angled slightly upwards from the center to the outboard ends to provide leg-to-handle

clearance for the nonflying pilot. The handles are attached vertically to the outboard ends of the "T"

for each pilot. The top part is hinged to the vertical component to allow the vertical position of the

handles to vary. It was noted that if the flying pilot holds the handle in a comfortable position, the

handle for the nonflying pilot may be in an awkwardly high position. However, the FAA has

reviewed the R22 cyclic control system effectiveness from a human performance perspective and

found it satisfactory. The FAA has also evaluated and approved the supplemental type certificate

for an alternative R22 cyclic control that has a more conventional design.

 

 8Collective is the flight control, located on the pilot's left side, that controls total lift of the rotor system. 

The collective changes the angle-of-attack of both main rotor blades equally. The cyclic is the flight control

that the pilot grips with the right hand to control the tilt of the main rotor system and thus the direction of

flight. The cyclic consists of push-pull tubes to a non-rotating swashplate, converted to rotating swashplate

via bearings, to the pitch change links to control the pitch of the blades.
n

~’-“ ““N--42 lN—

I

!--- Q I

1

1

1

!

I .

6aM

i

—

Figure 2--Three-Dimensional View of Robinson Helicopter Company R22
7

Main Rotor System

The main rotor system utilizes a two-blade, rigid-in-plane design.9 The rotor blades are

connected to the main rotor hub (see Figure 1) through individual flapping hinges. 10 The flapping

hinges are part of a teetering main rotor hub that is hinged to the main rotor mast. In most twobladed semi-rigid systems, the advancing blade flaps up, causing the retreating blade to flap down;

however, the R22 main rotor blades are individually hinge-pinned and therefore can flap

independently relative to each other. The total diameter of the R22 main rotor disc (two opposite

blades connected by the hub) measures 25 feet, 2 inches, and each blade weighs approximately 26

pounds.

The R22 uses main rotor blades designed according to National Advisory Committee for

Aeronautics (NACA) 63-015 airfoil specifications. The R22 main rotor blades are constructed at

the RHC's manufacturing facility with a 7.2-inch chord (width of blade) and are each 12.58 feet

long.

The R22 is operated at close to its maximum gross weight (1,370 pounds) with two people

on board and a full tank of fuel, resulting in operations routinely conducted near the upper limit of

the helicopter's operating envelope. This condition requires that the helicopter be operated near the

maximum design lift capability of the main rotor system. To gain the needed lift, the R22's main

rotor blade angle-of-attack will on occasions be near the stall angle-of-attack during normal

operations. According to RHC and a simulation study conducted by the Georgia Institute of

Technology (Georgia Tech),11 large, abrupt control movements may produce main rotor blade stall

and rapid decay of the rotor rpm.

Flight Control Responsiveness

The RHC, many R22 pilots, and some test pilots have indicated that the flight controls on

the R22 are more sensitive than on other light helicopters. That is, the R22 is highly responsive

in pitch and roll to small flight control inputs. In fact, in a February 13, 1984, memo from an

FAA helicopter test pilot to the FAA Supervisor, Flight Test Section, ANM-176W, the writer

stated that, "The aircraft in general is very quick. The aircraft reaction per inch of control input is

high, making pilot-induced oscillations (PIO) and overcontrolling tendencies much more

 

 9The rigid-in-plane term is used to describe the difference between the semi-rigid and the RHC design. 

The rigid-in-plane design allows the blades to move independent of each other in the vertical but not the

horizontal plane of rotation. Hence, if one blade accelerates, the other blade will mirror the acceleration, but

if one blade flaps (up or down), the other blade is not influenced directly.

 10Flapping is the vertical movement of the blade as a result of aerodynamic forces. Coning is the upward

bending of the blades caused by the resultant forces of lift and centrifugal force.

 11See Chapter 4 for more details about the simulation study.
8

noticeable than in other helicopters." Further, a 1994 report of a special certification review

(SCR)12 by the FAA stated that, "Informal interviews were conducted with individuals from the

operational community who were familiar with the R22. These individuals, who consisted of FAA

flight standards pilots, FAA certification test pilots and operators of the R22, frequently expressed

the opinion that this helicopter is very sensitive, requiring the pilots to be attentive at all times. 

However, those individuals interviewed stated that the aircraft did not have any unusual handling

characteristics." Some pilots have stated that this greater responsiveness to control inputs causes

R22 pilots to be highly alert, sharpening their piloting skills. However, the sensitivity of the R22

flight controls suggests that the greater responsiveness combined with limited pilot skills,

proficiency, or alertness could be a factor in some of the 31 accidents that the Board reviewed.

The Safety Board was unable to compare the response rates of the R22 to cyclic control

input with the response rates of other helicopters because the data in the flight regimes of

importance13 to this investigation for other helicopters were not readily available. In fact, such data

are difficult to obtain. Flight tests and computer simulations, which are quite extensive, are the best

source of such data, and because of the lack of an FAA requirement for such data, they are not

always available.

The Safety Board compared flight control response rate data of the R22 to Department of

Defense helicopter military specification MIL-H-8501A requirements. The data showed that

although the R22 is very responsive, it could meet the military's standards for flight control

response during instrument approaches.

Original Certification

The RHC began its concept design phase for the R22 helicopter in 1974. Following the

concept design phase, an application for certification (for research and development, with

limitations) was issued by the FAA on January 6, 1975. After the R22 helicopter was flight-tested,

the FAA issued Type Certificate No. H10WE to the RHC on March 16, 1979, approving the R22

design. Following the R22, the RHC requested certification of the R22 Alpha and was granted

certification on October 12, 1983. The R22 Beta model was approved by the FAA on August 12,

1985, with the R22 Mariner (equipped with floats) shortly after. The R22 helicopter is built at the

RHC factory in Torrance, California, under Production Certificate No. 424WE, which was granted

to the RHC by the FAA on March 6, 1981. There have been more than 2,550 R22s built, and about

850 are registered in the United States.

According to a 1989 FAA study of rotorcraft operations, about 80 percent of the R22 flight

hours logged that year were for instructional flights, 5 percent were for personal flying, and the

 

 12See Chapter 4 for more information about the SCR.

 13The flight regimes of importance to this investigation are those involving flight into turbulence, low-G

maneuvers, low main rotor rpm maneuvering, and large, abrupt pilot inputs.
9

remainder were for business or corporate purposes, including aerial application and observation.

Recent Robinson Helicopter Company Design Changes

According to the RHC, all R22 helicopters produced after serial number 2510 have an

electronic fuel control governor14 as standard equipment, and a kit became available to install the

new governor in other models of the R22 about August 1995. The electronic governor reduces

pilot workload, especially in critical times. It also prevents underspeeds, thereby preventing rotor

stall under certain conditions, and it prevents overspeeds that can overstress the rotor system.

In addition to the new governor, other design changes, according to the RHC, have been

developed or are under development that could lessen pilot workload. RHC announced in February

1996 the availability of the newly certificated R22 Beta 2, which incorporates the Lycoming O-360

engine. The O-360 will deliver about 13 percent more horsepower in some instances. The extra

horsepower will aid in maintaining or recovering rotor rpm more rapidly if the rpm becomes too

low. The larger engine will incorporate an automatic carburetor heat control. The carburetor heat

control could be set at takeoff, and when changes are required in flight, the movement of the

collective would cause an increase or decrease in the carburetor heat control. This also reduces

pilot workload at critical times, and also ensures that the carburetor heat will be automatically

reduced providing maximum power upon landing.

A new main rotor blade for the R22 is also being developed. According to the RHC, the

new blade would have the same dimensions as the present blade but would have a stainless steel

outer covering similar to that in use on the R44. The use of stainless steel provides increased inertia

in both the rotational and flapping directions leading to better rotor system performance. After

completion of testing on the new blade and FAA approval of the R22 main rotor blades, the RHC

plans to install the new blades on each production helicopter and modify each R22 when it is

returned to the RHC for its 2,000 hour overhaul.15 A certification date has not been set for the new

R22 model utilizing the stainless steel main rotor blades.

Obviously, design changes that reduce pilot workload (especially during critical phases of

flight) or enable the pilot to keep his or her attention focused outside of the cockpit will improve the

safety of the operation of the R22, or any other aircraft.

 

 14Device sustaining main rotor rpm by varying the throttle with inputs from the ignition system.

 15The RHC encourages operators to return their R22s to the RHC factory for a complete overhaul at

2,000-hour intervals. The Safety Board is not aware of any other manufacturer with a similar program.
10

Chapter 3 - Accident Data and Accident Investigation Review

Since the first R22 in-flight loss of main rotor control accident, which occurred near

Livermore, California, on November 11, 1981, the Safety Board has investigated or reviewed

investigative data from 31 R22 accidents in which the main rotor blade diverged from its normal

path and struck the helicopter. When similar R44 accidents occurred, the special investigation was

expanded to include those accidents. Consequently, three R44 accidents involving an in-flight loss

of main rotor control and contact of the main rotor blades with the tailboom or fuselage of the

helicopter have also been reviewed. Twenty-one of the accidents were domestic accidents; ten

occurred in other countries. The Safety Board participated in R22 and R44 investigations in

England, Germany, and Switzerland, and reviewed data files and reports received from Germany,

Switzerland, England, Australia, and New Zealand.

The following characteristics were common to the 31 R22 and three R44 accidents:

• an in-flight breakup or main rotor blade contact to the airframe occurred in flight

before collision with any object or terrain;

• there was no evidence of an initiating airframe or engine component malfunction;

• flight into adverse weather such as low visibility or ceilings was not involved; and

• pilot impairment from drugs or alcohol was not involved.

The Safety Board wanted to compare how often these types of accidents occurred per

flight hour in R22s with the rate at which these accidents occurred in other helicopters. The

Board did not attempt to calculate accident rates using the foreign R22 accidents because

comparable activity data (flight hours) were not available for foreign operations. Instead, the

Board concentrated on the U.S. R22 helicopter accidents for which information was available in

the Safety Board’s aviation accident data base.

The Board did not compare only accidents in which the main rotor blade contacted the

airframe by helicopter model because blade contact with the airframe following a loss of control

for non-mechanical reasons may be unique to the R22, while other helicopters might react

differently when subjected to similar loss of control conditions. That is, a large, abrupt pilot

input might lead to the loss of control of an R22, subsequent airframe contact by the main rotor

and a fatal crash, while a similar pilot input might lead to loss of control without main

rotor/airframe contact in another helicopter model but still involve a fatal crash. To permit a

more appropriate comparison, the Board examined fatal helicopter in-flight loss of control

accidents regardless of main rotor/airframe contact, provided that the loss of control was not
11

attributable to a preexisting mechanical condition or an encounter with weather.16 For the purpose

of making comparisons, certain loss of control accidents were selected by the Board for study. 

These accidents, which will be referred to as loss of control (LOC) accidents, involved an in-flight:

• loss of main rotor control;

 • structural failure of the main rotor blade that did not involve preexisting fatigue of

rotor blade materials; or

• loss of aircraft control or collision with terrain for unknown reasons, in the absence

of structural failure, encounter with instrument meteorological conditions, or pilot

impairment from drugs or alcohol.

From 1981 through 1994, the Safety Board investigated 500 fatal accidents involving U.S.-

registered helicopters. Ten helicopter models were involved in 43 accidents that met the above

criteria as LOC accidents:17 the Bell 47, Bell 204, Bell 206, Bell 212, Enstrom F28, Hiller UH12,

MBB BO 105, Hughes 269, Hughes 369, and the Robinson R22. These ten helicopter models were

involved in 357 of the 500 fatal accidents during the time period. 

For each helicopter model, the fatal LOC accident rate per 100,000 flight hours was

calculated using FAA estimated activity data for each model.18 The accident rates for fatal nonLOC and all fatal accidents were also calculated per 100,000 flight hours. This information is

summarized in Table 1.

 

 16Encounters with instrument conditions were not included because R22s are not approved for flight in

instrument conditions, and because accidents involving such encounters are usually related to the instrument

flight skills of the pilot.

 17Safety Board records include one additional LOC accident involving a Brantly B-2, and two involving a

Fairchild-Hiller FH1100 helicopter. However, relatively few of these helicopters are in service;

consequently, sufficiently reliable estimates of aircraft utilization were not available to calculate accidents

rates. Further, three additional comparison accidents involving amateur-built helicopters were also

excluded.

 18General Aviation Activity and Avionics Survey, Federal Aviation Administration: Washington, DC,

1981-1992. General Aviation and Air Taxi Activity and Avionics Survey, Federal Aviation Administration: 

Washington, DC, 1993. General Aviation and Air Taxi Activity Survey, Federal Aviation Administration: 

Washington, DC, 1994. Data for 5 flight hour values were missing. These values were estimated by linear

interpolation.
Table 1-U.S. Loss of control1 (LOC), non-loss of control (non-LOC), and all fatal

helicopter accidents, flight hours, and corresponding accident rates for the years

1981-1994. by helicopter model.

,1 Fatal Accidents

Fatal Accidents per 100,000 flight hours I

Helicopter Non- Flight Nonmodela LOC LOC All hoursb LOC LOC All

Bell 206 2 119 121 13,369,702 0.015 0.890 0.905

Hughes 369 2 38 40 3,00,236 0.067 1.267 1.333

Hiller UH12 1 13 14 987,796 0.101 1.316 1.417

Enstrom F28 1 16 17 845,032 0.118 1.893 2.012

MBB BO 105 1 12 13 806,750 0.124 1.487 1.611

Bell 212 1 3 4 497,129 0.201 0.603 0.805

Hughes 269 5 28 33 1,992,301 0.251 1.405 1.656

Bell 47 6 44 50 2,343,215 0.256 1.878 2.134

Bell 204 1 2 3 227,683 0.439 0.878 1.318

Robinson R22 23 39 62 1,524,483 1.509 2.558 4.067

Totals 43 314 357

1LOSS of control (LOC) accidents, involved an in flight: loss of main rotor control; structural failure of the main rotor blade that did not involve preexisting fatigue of rotor blade materials; or, loss of aircraft control or collision with terrain for unknown reasons, in the absence of structural failure,

encounter with instrument meteorological conditions, or pilot impairment due to drugs or alcohol.

a Accidents involving Fairchild Hiller FH1100 (two fatal LOC accidents) and Brantly B2 (one fatal LOC accident) helicopters were excluded because reliable utilization

data were not available.

b General Aviation Activity and Avionics Survey, Federal Aviation Administration: Washington, D.C. 1980-1992. Nine missing data values were imputed by linear

interpolation. General Aviation and Air Taxi Activity and Avionics Survey, Federal Aviation Administration: Washington, D.C., 1993. Preliminary 1994 data from

Federal Aviation Administration, Washington, D.C.
13

As shown in Table 1, seven of the 10 helicopter models had only one or two LOC accidents during

the period examined. The Hughes 269 had five LOC accidents, and six such accidents involved the Bell

47. However, the Robinson R22 was involved in 23 LOC accidents during the period, 21 of which involved

loss of main rotor control. In fact, per flight hour, the R22 was involved in the most LOC, non-LOC, and

total fatal accidents.

The Safety Board recognizes that the aircraft activity data gathered and reported by the FAA

are based on a survey. The data are subject to reporting and measurement error. Each year, the

FAA reports the standard error statistic associated with each activity estimate for each model. It did

not appear that any particular model was subject to consistently poor reporting during the period

studied.

Finally, the Safety Board decided to compare the accident history of the R22 with the Bell

47. Although it is an older design, the Bell 47 was selected for this last comparison because, like

the R22, it is a relatively lightweight, two-place, low inertia, helicopter that is also used extensively

for training. Both the R22 and Bell 47 have similar high utilization rates, and the standard errors

associated with these utilization estimates are low and comparable. The Safety Board used these

standard error statistics to estimate the upper and lower bounds of the accident rates for each

helicopter.19

Using these estimates of flight hours, the Bell 47 accident rates ranged from 0.155 to 0.729

fatal LOC accidents per 100,000 flight hours, compared to 0.964 to 3.300 for the R22. Accident

rates for fatal non-LOC accidents ranged from 1.342 to 5.346 for the Bell 47, and from 1.635 to

5.589 for the R22. It is important to note that these intervals overlap for non-LOC accidents, but

not for LOC accidents. That is, the lower bound rate of LOC accidents for the R22 is notably

greater than the upper bound rate of LOC accidents for the Bell 47. Statistically, the R22 and Bell

47 were about equally likely to be involved in non-LOC accidents, but the R22 was more likely

than the Bell 47 to be involved in LOC accidents.

 From its statistical review of fatal helicopter accidents that occurred between 1981 and

1994, the Safety Board concludes that, compared to other helicopter models that have had fatal

LOC accidents, Robinson R22s were involved in more fatal LOC accidents per flight hour.

Characteristics of Accidents Involving Loss of Main Rotor Control

The Safety Board compiled pertinent data from each of the 31 worldwide R22 in-flight

loss of control accidents involving main rotor airframe contact. Only eight of the accidents

involved winds or gusts known to be greater than 15 knots. In two accidents, the wind

conditions at the accident site were not known. Wind data for most of the other accidents were

obtained from the location of the closest-known official weather observatory. Of the 31 flights,

 

 19The estimated 99 percent confidence interval for total flight hours during the period ranges from

822,983 to 3,879,368 for the Bell 47, and from 697,759 to 2,385,420 for the R22.
14

18 were conducted for personal business, nine involved dual flight instruction,20 and three involved

unaccompanied students. Only one of the accident flights involved commercial operation other

than flight instruction. Further details of the accidents are contained in the summaries of the cases

in Appendix A.

The Board was specifically interested in the flight experience of the pilots involved in these

31 accidents. The Safety Board used available records to compute the median time in R22s, total

time in all helicopters, and total time in all aircraft for the pilots involved in each of the 31

accidents. In Table 2, these median flight hours are summarized by type of pilot assumed to be

flying. Median flight hours are given for the pilots-in-command for all of the 31 accidents, for both

the instructors and the students in the dual instructional flights, for the pilots-in-command of the

non-dual-instructional flights, and for the least experienced pilot. For the dual instructional flights,

least experienced pilot was the student; for the non-dual-instructional flights, it was the pilot-incommand.

Accident Investigation Review

Safety Board staff reviewed in detail six of the most recent R22 accident investigations21 in

which the helicopter lost main rotor control and broke up in flight to review the type and severity of

the physical damage. These six accidents were chosen because their wreckage remained available

for detailed examination. (See Chart 1.) In each case, the flight control system was extensively

damaged above the swashplate; no prior mechanical failures were evident; and the main rotor

blades had struck the structure of the helicopter. In each case, engine failures were conclusively

ruled out, and no evidence of a precipitating flight control failure was found.

The Safety Board's materials laboratory examined components from these R22 accidents. 

The lab examined pitch change links, masts, mast supports, drive assemblies, main and tail rotor

blades, hubs, droop stops, spindles, flight control tubes, bearings, and other pertinent items. In all

cases, the laboratory personnel found no evidence of fatigue failure, inadequate materials, or

improper maintenance. All of the R22 control and rotor system components examined in the

Board's materials lab revealed evidence of overload failures. No evidence suggested a failure of the

parts as causal to the in-flight rotor/fuselage contacts. 

 

 20This groups a "demonstration" flight for a non-pilot and a flight involving a flight instructor and

another rated pilot (in which the exact purpose was unknown) with other accidents that were dualinstructional flights.

 21Malabar, Florida, 1/30/92 (MIA92FA072); Maricopa, Arizona, 3/4/92 (LAX92FA137); Mt. Pleasant,

Tennessee, 5/6/92 (ATL92FA096); Richmond, California, 6/29/92 (LAX92FA267); Martinez, California,

9/30/92 (LAX92FA410); and Knightdale, North Carolina, 9/28/94 (ATL94FA179).
15

Table 2-Median flight hours in R22s, all helicopters, and all aircraft of

the pilots assumed to be handling the helicopter at the time of the

accident, for the 31 worldwide R22 accidents in which the main rotor

contacted the airframe.

Median flight hours in

Type of operation All All

Pilot Numbera R22s helos aircraft

All flights

Pilot-in-command 30 127.5 180 790

Least experienced pilotb 30 52.5 76 290

Dual instructional flights

Instructor 9 451 451 772

Student 9 4 4 190

Non-dual-instructional flights

Pilot-in-command 21 85 123.5 792

aFlight hours data were available for 30 of the 31 accidents.

bThe least experienced pilot was the pilot who had accumulated the fewest

flight hours in the R22. For the dual instructional flights, it was the student;

for the non-dual-instructional flights, it was the pilot-in-command.
16

Chart #1 Robinson R22 Accident Wreckage Component Comparisons

MALABAR MARICOPA MT PLEASANT RICHMOND MARTINEZ KN IGHTDALE

01/30/92 03104/92 05/06/92 06/29/92 09/30/92 09/28/94

Indents in hub from Strong indents in Slight indents in hub Indents in hub Indents in hub from Indents in hub from

HUB spindle tusks hub from spindle from spindle tusks from spindle tusks spindle tusks spindle tusks

contact

SPINDLES Both tusks sheared One tusk sheared Both tusks sheared Both tusks sheared One tusk twisted at One tusk sheared

the tip

DROOP STOPS Slightly compressed Crushed and Slightly compressed Bolt hole One crushed with Crushed and

and distorted deformed and distorted deformed; stops bolt hole deformed deformed

crushed

PITCH CHANGE Both fractured at Both fractured at Both fractured at upper Both fractured at One fractured at Both fractured at

LINKS upper adjustment upper adjustment adjustment threads upper adjustment blade horn, other at upper adjustment

threads threads threads upper adj. threads threads

SWASHPLATE Chord arm fractured, Intact Scoring on chord arm Fractured at chord Chord arm had deep Scoring on chord

ASSEMBLY upper swashplate from blade horn; upper arm and upper lateral indents arm

intact mast tube fitting swashplate

fractured

UPPER MAIN Slight indentations Torsional twisting Contact from hub; 25° Contact from hub Contact from hub Separation above

ROTOR SHAFT from hub contact and bending, and bend in upper main with slight bending with slight bending swashplate; No

contact from hub rotor shaft below hub of shaft of shaft torsion

TX & M R MAST Fractured at upper Fractured at upper Intact upper trans. cap Fractured at upper Fractured at upper Intact. Case

trans. cap trans. cap and shaft. trans. cap trans. cap fractured

LORD MOUNTS Intact Impressions in trans. Intact Trans. deck Trans. deck distorted Trans. deck

deck distorted and bent and bent distorted and bent

Both blade chords One fractured 48” One slightly coned One blade fractured One fractured 16” One fractured 39”

MAIN ROTO R fractured through to from blade tip upwards, other severely 24” from hub, rivet from tip from blade tip

BLADES main spar curled down and impressions along

fractured 22” from blade to 49” from

blade horn. tip

TAILBOOM No indications of Severe torsional No indications of blade Tailcone severed Severe torsional Tailboom severed:

blade strike twisting and strike and missing aft of twisting and blade strikes in two

separation at 1st hay. First bay separation at places

fuselage exhibits slap to left fuselage; blade strike

side 53” from forward end

COCKPIT Left passenger door Strike from main Left forward door frame No indications of Right skid and right Plexiglas strike

struck by main rotor rotor blade at left and bulkhead struck by blade strike upper windscreen

blade door, airframe, and main rotor blade struck by main rotor

forward skid blade
17

The Safety Board has determined that most of the damage occurred after the main rotor

blades began to diverge from their normal plane of rotation. The angles of the blade strikes to the

fuselage could not have been achieved unless the hub had teetered22 past its normal limits to the

point of contacting or breaking the main rotor shaft or mast. In addition, the blades would have had

to have been significantly out of plane for the spindle tusks to have contacted the droop stops or for

the pitch change links to have failed. Divergent main rotor behavior could have overloaded the

pitch change links.

A hub can become overteetered in a number of ways. Mechanically, the blade pitch control

system could fracture or could separate. Typically, these failures should produce identifiable

signatures in the wreckage. None were found.

In addition to the mechanically induced mechanism for overteetering, large, abrupt flight

control inputs could directly induce overteetering or high blade angles, which in turn could induce

mast bumping.23

The Safety Board is aware of only two cases in which an R22 exhibited signs of significant

mast bumping in which the helicopter was able to land. Therefore, once mast bumping occurs, the

margin for maintaining structural integrity is very small. Once overteetering and mast bumping

occurs, structural failure of the main rotor mast or shaft is highly likely and would be quickly

followed by overload of the pitch control system of the blade. The available wreckage from all six

accidents is consistent with this scenario.

 

 22The angle formed when the plane of rotation of the main rotor system is not aligned perpendicular to

the mast. The angle is measured from the horizontal portion of the hub and the mast.

 23Mast bumping occurs when a portion of the rotor system (two blades connected by the hub) exceeds the

teetering limit and strikes the mast of the helicopter, usually with sufficient force to cause mast deformation

or mast failure.
18

Chapter 4 - FAA Oversight

Certification Reviews

The R22 has been the subject of three FAA special certification reviews (SCRs).24

Following several R22 fatal accidents that occurred in 1981 and 1982, an SCR of the R22 by the

FAA, with the participation of RHC personnel, was completed on October 24, 1982. The FAA's

report of that review included four recommendations to the RHC for future actions: (1) conduct

rotor hub teetering and rpm decay studies; (2) issue an operations bulletin to make operators aware

of light helicopter problems specific to helicopters similar to the R22 by focusing on rpm decay and

recovery problems, throttle coordination problems, attention to proper maneuvers during student

instruction, and careful student monitoring during student solo flights; (3) issue a service bulletin

and provide a kit to enable the FAA to issue a priority airworthiness directive (AD) to make

installation of a low rpm warning light mandatory; and (4) raise the rpm limit for activation of the

low rotor rpm light from 91 percent to 97 percent. No discrepancies in the original certification

were found.

As a result of this SCR, the RHC conducted flight tests and published a report on the

results.25 In addition, the RHC issued a safety notice to advise operators of the R22's sensitivities to

low rotor rpm. The FAA raised the rpm threshold at which the warning horn (and newly installed

light) would be required to annunciate to 95 percent (from 91 percent) and issued an AD mandating

the installation of a low rotor rpm warning light.

The Safety Board noted that the SCR included a statement that, "The certification rules

relating to [flight] characteristics, policy, guidance and advisory circulars will be reviewed for

applicability to small FAR 27 rotorcraft. Especially, the 1-[second] delay time in correcting for

power loss, control response and [aircraft] dynamic stability during maneuvers." The SCR

recommended the following:

The airman certification rules, FAR 61, guidance [material], [flight] test guides

and the basic helicopter handbook, AC61-13B, should be revised to reflect the

safety needs of small rotorcraft as learned from the R22 accident records. The

current rules and practice [are] inadequate for students and [flight] instructors

training in small helicopters. Pilot certification and [flight instructor] ratings in

 

 24The SCRs provided to the Safety Board followed the format specified in FAA Orders 8110.4 (p. 21 and

p. 167, dated 1/31/77) and Rotorcraft Directorate Standard Procedures (dated 10/1/92).

 25Main Rotor Hub Teeter Angle and Rpm Decay Survey. October 26, 1982. (RHC RTR-073. See

Appendix C for relevant excerpts.)
19

rotorcraft are currently insufficient and unsafe. An FAA [flight standards] and

engineering review team will be formed26 to recommend specific changes.

On November 23, 1982, the Manager of the Western Aircraft Certification Field Office

(WACO) wrote to the Acting Manager of the FAA Flight Standards Division that RHC flight tests

showed that, "the R22 had no unusual flight characteristics when flown within the operating

limitations....Attempts to achieve a rapid rotor decay could only be accomplished with throttle

chops....The aircraft does have a relatively high roll rate sensitivity....The aircraft responds rapidly

to any control input, and the student must be made aware of this.[emphasis in original]" The letter

ends, "In conclusion, the R22 helicopter is a safe, airworthy aircraft when it is flown within its

operating limitations. It is highly responsive to small control inputs and as a result must be treated

gently by its operator (as any helicopter must be). The aircraft has a low inertia rotor system which

will decay rapidly during throttle chop maneuvers,27 but control is maintained throughout the

transient rotor droop, and rpm builds quickly back to power-off levels.28 It is an aircraft which can

foster good helicopter flying techniques in the student pilot."

The Safety Board reviewed the records of the Los Angeles Aircraft Certification Office

(LAACO) concerning action taken as a result of the SCR and found an internal FAA memorandum,

dated March 29, 1983, directed to the Manager, Helicopter Policy and Procedures Staff, ASW-110

from the manager of the FAA WACO, ANM-170. In that memorandum, the manager of the

WACO addressed the low-G maneuvering problem of the R22 and stated that, "This office is of the

opinion that the R-22's low g maneuvering characteristics are more sensitive to control inputs than

other helicopters but are still acceptable and within the criteria established in the regulations. 

Critical situations such as mast bumping can only be created through abnormal or aggressive

control inputs." He further stated that, "The FAA, however, has not addressed its problem as

perhaps it should have. We have found nothing published by the FAA which describes the problem

of helicopter control during low g maneuvering and the appropriate recovery techniques." 

The memo also stated that quantitative dynamic stability testing data were obtained on the

recent FAA-monitored refly (repeat of flight testing previously conducted) and that consideration

should be given to evaluating the dynamic stability characteristics of all future helicopters. The

memo concluded that the low-G control characteristics of the R22 are acceptable when the R22 is

flown in a normal and reasonable manner. The memo recommended that the FAA initiate action

to warn pilots of the dangers of low-G flight and expand dynamic stability testing to better

establish helicopter handling qualities. The Safety Board's subsequent review of the FAA records

 

 26To the Board's knowledge, no FAA team was ever formed.

 27A throttle chop maneuver is one in which a pilot intentionally reduces the throttle to simulate engine

failure.

 28Power-off levels range from 459 to 561 rpm. This range is considered acceptable for autorotative rpm.
20

found no documentation of any FAA action taken as a result of the FAA memorandum. However,

the Safety Board notes that such dynamic testing was performed on the R44.

On January 25, 1988, the manager of the FAA's Southwest Region, Aircraft Certification

Division, requested another SCR of the R22. (In 1987, there were three accidents involving the

R22 involving in-flight loss of control.) The SCR was completed on March 23, 1988, and found

that the R22 met all requirements of 14 CFR Part 27.29 The review found no new areas of concern.

 After the review, the SCR team recommended that a research program be initiated through the

FAA technical center to study potential rulemaking changes in the following areas: (1) specific

aircraft response rates to control inputs; (2) change in control force with cyclic and collective

displacement; (3) rotor speed decay rates after throttle chops; and (4) speed decay rates during

autorotation touchdown. The Safety Board could not find any records indicating that the FAA had

ever addressed the merits of or conducted research in response to any of the 1988 SCR team

recommendations.

On January 18, 1994, following additional accidents and Safety Board recommendations,

the FAA LAACO issued another SCR of the R22. The review found, as in the two previous

certification reviews, that the R22 met all 14 CFR Part 27 certification requirements. The FAA

concluded that the R22 is used extensively by training facilities and that the flight characteristics,

sensitivity to flight control inputs and high rotor rpm decay rate, are inherent to this aircraft because

of its low gross weight. The FAA also concluded that structural and mechanical integrity of the

rotorcraft did not appear to be an issue and that the rotor rpm decay rate is similar to other

helicopters. The SCR report recommended that a research and development (R&D) program be

established to provide data to support rulemaking and recommended that the program include rotor

decay rates related to a minimum standard, and allowable rotorcraft response rates to abrupt control

inputs. Additionally, it was suggested that a method of tracking accident rates be established. 

During the Board's special investigation, no evidence was found that the FAA had established such

an R&D program or determined that such a program was unnecessary. 

On May 3, 1996, the FAA provided to Safety Board staff a copy of a Research Project

Initiative (RPI) that had not been signed, had no RPI number, and had no approval attached to it. 

The Safety Board could find no evidence that the RPI had been funded. The FAA also provided

documentation of other actions taken by the FAA from 1982 through 1995 concerning the R22. 

However, the documentation did not link these actions to the recommendations made in the SCRs

nor provide evidence of any procedures that would have brought the recommendations to closure. 

In fact, the FAA's May 3, 1996, letter to the Board states that, "While present documents do not

specifically detail follow-up procedures, they are being revised to do so."

 

 29In a May 3, 1996, letter to the Safety Board's Office of Aviation Safety from the FAA"s Associate

Administrator for Regulation and Certification, the 1988 SCR was classified as an internal review and not

an SCR.
21

Recent FAA Actions

On July 22, 1994, the FAA published a special airworthiness alert (SAA) addressing the

operation of R22 helicopters. Among other things, the SAA cautioned R22 pilots to avoid abrupt

cyclic inputs and to reduce maneuvering speeds to the extent possible. (The alert was updated and

expanded to address the R44 and reissued on January 17, 1995, and was distributed to pilots and

operators of the R22 and R44.) 

Also, in response to the Board's July 1994 safety recommendations, the FAA convened an

Aircraft Certification Panel (technical panel) in July 1994 to study the R22 loss of main rotor

control accidents and to recommend appropriate action to prevent these types of accidents. The

panel was directed to consider the appropriateness of reducing the R22 helicopter "never exceed

airspeed" (Vne), the subject of Safety Board Safety Recommendation A-94-143.30 In addition, the

panel was to conduct research to determine possible deficiencies and their remedies with respect to

aircraft design, operating procedures, operating limitations, and 14 CFR Part 27 airworthiness

certification criteria, the subject of another Safety Board recommendation, A-94-144. In November

1994, the technical panel selected the Georgia Institute of Technology (Georgia Tech) to perform

simulation studies of the R22 main rotor system. (The Georgia Tech simulation studies are

discussed in more detail later in this section of the report.)

While the FAA's technical panel and Georgia Tech were conducting the research outlined

above, the FAA initiated other actions to reduce the potential for pilot actions resulting in loss of

main rotor control accidents in R22 and R44 helicopters. On January 10, 1995, the FAA issued

Priority Letter ADs 95-02-03 and 95-02-04 prohibiting flight of the R22 and R44 in surface winds

greater than 25 knots, gusting winds greater than 15 knots, and in moderate, severe, or extreme

turbulence. These ADs were amended and re-released as ADs 95-04-14 and 95-04-13 on March 2,

1995.31

On January 10, 1995, the FAA also issued a special airworthiness information (SAI)

bulletin to all pilots and operators of the R22 and R44. The bulletin warned airmen to operate

the R22 and R44 within the helicopters' normal flight envelopes and to avoid excess winds,

altitudes, and other conditions unfavorable to the helicopters. The FAA issued a flight standards

information bulletin on January 18, 1995, to all FAA Flight Standards inspectors to advise them of

 

 30The Safety Board recommendations referenced in this special investigation report are contained in

Appendix D. All safety recommendations that were issued before this special investigation report and that

were in an "open" status have been either closed superseded, closed acceptable alternate action, or closed

acceptable action. (Appendix D also contains the Safety Board's letters to the FAA in which the

recommendations were closed.)

 31ADs 95-04-14 and 95-04-13 were identical to the previously issued ADs except that the requirement for

turbulence to be based on area forecasts was deleted because area forecasts cover widely varying areas that

usually contain mountainous as well as flat terrain. The ADs were further amended in January 1996 to

reduce the unintended effect on experienced R22 and R44 pilots.
22

the information contained in the Safety Board's recommendation letter of January 6, 1995, and the

SAI. On February 15, 1995, the FAA issued a flight standardization board (FSB) report for the R22

and R44. The FSB recommended stringent requirements for all future training in the two helicopter

models. The FSB also recommended future research into several areas of helicopter hardware and

operations, including revision of 14 CFR Part 27 to consider main rotor system inertia in singleengine helicopters.

On February 23, 1995, the FAA issued Special Federal Aviation Regulation (SFAR) 73,

promulgating special rules for pilots operating under 14 CFR Part 61.32 The SFAR alters the

normal biennial flight review requirements by requiring R22 and R44 pilots to perform proficiency

flight reviews in the R22 and R44, as appropriate; increasing the amount of dual training required

before a pilot may receive his or her private pilot-helicopter certificate; and mandating special

awareness training specific to the R22 and R44 helicopters. The SFAR expires on December 31,

1997, or sooner if superseded or rescinded.

In June 1995, the technical panel released its report, dated March 17, 1995, to the Safety

Board. The report summarizes the panel's actions and outlines recommendations for further design

changes, operating limitations, and future actions. Specifically, the panel recommended the

following: (1) that the R22 be reconfigured with an electronic engine rpm governor similar to that

previously installed in the R44;33 (2) that the low rpm warning threshold be increased to activate at

a higher rpm and the audio warning be added through the R22's intercom system; (3) that the

operating limitations be changed to increase the minimum power-on rpm limit to 97 percent; (4)

that the cyclic control be removed for all passengers in the left seat; and (5) that normal flight

operations with the governor switched off be prohibited. The technical panel further recommended

that the simulation and modeling program initiated by Georgia Tech be continued until Safety

Board concerns and any deficiencies discovered by simulation were satisfied.

Flight Testing of the R22 and R44

As a result of the several R22 accidents involving excessive teetering and blade contact with

the tailboom, RHC conducted in 1982 a series of flight tests to ensure that adequate teetering

clearances existed during all normal flight regimes and that the rotor decay rates were not

excessive. The flight tests consisted of throttle chops and flight control inputs. Parameters of pitch,

roll, and teeter angle were recorded as were other parameters such as main rotor rpm, airspeed,

altitudes, and acceleration. Pitch and roll rate responses to cyclic input were calculated.

 

 32SFAR No. 73; Part 61—Certification: Pilots and Flight Instructors, issued February 23, 1995.

 33As previously noted, according to the RHC, all new R22 helicopters produced after serial number 2510

have the new governor as standard equipment, and a kit became available to install the electronic governor

in other models of the R22 about August 1995.
23

The tests determined that the margin of tailboom clearance was satisfactory during all

maneuvers and that adverse flight characteristics were not obtained during any of the maneuvers,

even those outside the normal flight envelope. Following power changes, a reduction of the

collective stick resulted in a positive response of the rpm, and the helicopter remained controllable

with normal inputs. The RHC concluded that the R22 rotor system would not stall, exceed its teeter

clearance, or contact the tailboom when the aircraft is flown within its approved limitations.

The tests showed that, during cruise, the cyclic is forward and to the right of neutral while

the rotor plane is tilted aft and to the left, about 5o

. Changes to the cyclic generally produce like

changes in the tilt of the rotor plane. For example, if the cyclic is moved aft and left from the cruise

position, the tilt of the rotor plane can be expected to increase in the aft and left direction.

During July 1995, the FAA participated in flight tests of the R44 at the RHC facilities in

Torrance, California. The R44 was fully instrumented to record information from the main rotor

system, helicopter performance information, and flight control positions. The flight tests were

accomplished to confirm that the R44 can be operated safely. According to the FAA, the results

indicated that the R44 could safely perform any nominal flight activity without main rotor

divergence tendencies. The flight tests comprised engine power reductions, push-overs, and normal

flight training maneuvers. 

A Safety Board staff review of the flight test results showed that some of the flight test

maneuvers were performed with large control inputs and at substantial input rates. A standard used

by one helicopter manufacturer is to input 10 percent of the total control input available in 0.1

second. The R44 was subjected to a 10 percent cyclic push in 0.17 second continuing to 25 percent

in 0.4 second.

The FAA reported that all flights were flown consistent with flight test procedures and that

at no time was the safety of the flight questionable. Unfortunately, because tests were not (and

could not safely be) conducted to determine the helicopters' response to large, abrupt cyclic inputs

while in steady state flight with the cyclic already forward and to the right (normal high speed

forward flight), the results of the flight tests did not provide the data needed to determine the

mechanism for the blade diverging into the body.

Georgia Tech Study

The FAA awarded a grant to the Georgia Tech School of Aerospace Engineering to develop

a computer-based mathematical simulation of the R22 to allow the study of the aircraft and rotor

system dynamics. The development of a simulation model had been recommended by the Safety

Board and the FAA's R22 technical panel. The objective of this effort was to use the simulation to

study the effects of flight control inputs, rotor rpm stall, low-G maneuvers, and turbulence on the

operation of the helicopter and the three-hinge rotor system. A final report of the study was

provided to the Safety Board in the first week of March 1996.
24

Georgia Tech researchers stated to the Safety Board that the research was concluded in

December 1995 when the available funds were expended and before the mathematical model was

thoroughly validated by comparison to flight test data. Modeling of such a complex system required

more resources than had been allotted for the project. According to the researchers, more research,

including future testing that produces excessive flapping or rotor divergences, is needed to fully

validate the mathematical modeling of the R22 in the areas of flapping and divergent tendencies.

 The FAA then opted to simulate induced tail rotor strikes through the use of large flight

control inputs and large gusts. FAA and Georgia Tech experts described the simulated flight control

inputs as extreme and the gusts like an encounter with a sharp thermal-induced gust. Although

some of the more severe flight control inputs or gusts did result in tailboom contacts, Georgia Tech

engineers pointed out that the model appeared to be numerically unstable in that induced roll rates

were not properly damped. Therefore, calculated roll excursions were artificially large, and the

tailboom contacts were not considered valid. However, each main rotor strike was preceded by

mast bumping and/or extremely high blade angle of attacks.

In effect, the simulations, based on the nature of the modeling, became invalid when roll

rates became large, the blade angle of attacks became high, or once the rotor hub contacted the

mast. In addition, rotor decay histories could not be evaluated because the model was based on a

constant rotor speed rather than an engine/power-based model that would react to rotor blade

aerodynamic performance factors. Also, the FAA decided to forgo a methodical buildup of control

inputs and evaluated large control movements; it is unknown if smaller control inputs would have

produced mast bumping. Typical pilot control inputs while reacting to flight dynamics were not

modeled.

Although the Georgia Tech mathematical model of the R22 was not developed sufficiently

to explain the divergent behavior of the rotor system, the researchers stated that the trends that the

model exhibited inside the validated flight envelope show that the R22 model is a useful tool in

analyzing rotor stability and transient response within the normal operating envelope and verifying

the theory that pushovers and rotor stall may lead to rotor/airframe contact. The Safety Board

believes that the mathematical model sufficiently simulates the R22 rotor system behavior in the

normal operating range, and that the results suggest that large, abrupt, and multiple control inputs

could lead directly to a mast bumping event or high blade angles-of-attack, either of which could

lead to loss of main rotor control. 
25

Chapter 5 - Summary Discussion

The Board examined various potential scenarios that could lead to an in-flight rotor/fuselage

contact, including the following: an unstable main rotor design; too rapid rotor rpm decay; 

mechanical failures; high blade angles (rotor stall); mast bumping; over-sensitive flight controls; or

sensitivity to multiple or large control inputs.

During this investigation, the Safety Board found no direct evidence of an unstable blade or

rotor system design. The extensive operational history, the wreckage evidence, flight tests, and

computer simulations indicate that a dynamically unstable main rotor system is unlikely. Although

the Board cannot conclusively eliminate the possibility of a deficient design, many professional

pilots continue to operate the R22 and R44 in difficult conditions without inducing loss of control

of the main rotor blade. For example, the R22 is used by police departments, pipeline spotters, and

the news media, often under challenging operating conditions.

The Safety Board originally determined that the probable causes of most of the past R22

accidents were the result of pilots allowing the main rotor rpm to decay or low-G maneuvering. 

The absence of preexisting material defects in the rotor system, the FAA's assurance that all

relevant certification standards had been met, and the belief that the certification standards were

adequate led the Safety Board to that determination. However, the Safety Board's investigation of

the Richmond, California, accident presented evidence of an in-flight loss of main rotor control

with normal rpm until the breakup began. In the other accidents, the rotor system rpm at the

moment of loss of control is unknown. Further, the majority of the evidence in most of the

accidents is not consistent with pilot maneuvers that would result in a low-G condition before the

loss of control. Therefore, the Safety Board has revised 13 of the Briefs of Accidents to more

accurately reflect the known and unknown factors in these accidents.

Indications of mast bumping were present in all of the loss of main rotor control accidents

discussed in the Board's special investigation report, although the mast bumping generally did not

result in significant damage at the point of contact with the mast. The Safety Board does not

believe that mast bumping was the precipitating causal event in the R22 and R44 loss of main rotor

control accidents.

A number of situations could have led to the mast bumping in the R22 and R44 accidents. 

Unstable blade or rotor system design has been considered, as were rotor blade stall; low-G

maneuvers; large, abrupt control inputs; and turbulence. Large, abrupt control inputs can lead

directly to mast bumping or induce blade stall, which, in turn, can lead to mast bumping.

Turbulence may produce blade stall or lead pilots to make large control inputs. Some low-G

maneuvers initially result from deliberate control inputs, but at times these may be followed by

larger control inputs during recovery from the low-G situation that may lead to a loss of main rotor

control.
26

Given that the R22 helicopter is very responsive to control inputs, the Board became

concerned that many of the loss of control accidents involved large control inputs leading to the loss

of main rotor control and subsequent in-flight airframe contact by the main rotor. The Safety Board

notes that large, abrupt control inputs when the R22 is in a steady-state condition with an already

existing teeter could cause the teeter limit (12o

) to be exceeded, followed by a mast strike and

subsequent loss of main rotor control.34 Although the Georgia Tech mathematical model of the

R22 was not developed sufficiently to demonstrate this conclusively, the results of the studies

suggest that large, abrupt cyclic movements can lead to blade stall and/or mast bumping.

The Board notes that this is not new information; an earlier study on teetering rotor

systems by Bell Helicopter also highlighted that large flapping amplitudes could be expected

with large, abrupt control inputs.35 The report stated that blade flapping can increase rapidly

from acceptable to excessive angles in only one or two revolutions. The Bell Helicopter report

further stated, in part:

Control inputs were a source of high flapping when made rapidly and with large

amplitude. If the cyclic control is applied faster than the helicopter will respond,

high flapping will result in proportion to this input....Most conditions which cause

excessive flapping result from rapidly developing phenomena such as blade stall,

landing loads, or abrupt, large control inputs. Blade flapping increases rapidly from

acceptable to excessive angles in only one or two rotor revolutions, leaving little, if

any, reaction time for the pilot to correct the situation.

The pilot is the ultimate controller of conditions that generate excessive flapping. 

The danger of large, abrupt control inputs in flight conditions near retreating blade

stall, or low g-levels, should be emphasized in training. Flight maneuvers which are

acceptable when the aircraft is held within its operational envelopes may result in

excessive flapping if any of those envelopes are exceeded.

Further, because R22s are used extensively for training, the Safety Board attempted to

determine if limited pilot experience and the training environment might have been common

factors in some of the loss of main rotor control accidents. Although many accidents involved

low-time pilots, the Board cannot be certain in many of the dual-instructional accidents whether

the student or the flight instructor was manipulating the flight controls immediately preceding the

loss of control. In many of these accidents, student pilots were probably operating the flight

controls, but instructor pilots may have been demonstrating maneuvers. Although flight

instructors should be able to prevent a student pilot from improperly manipulating the controls by

 

 34Again, the Safety Board is aware of only two incidents in which an R22 experienced a mast strike and

recovered. 

 35Rotor Blade Flapping Criteria Investigation. December 1976. Prepared for the Eustis Directorate,

U.S. Army Air Mobility Research and Development Laboratory by Bell Helicopter Textron, Ft. Worth,

Texas.
27

guarding the controls, the Board is concerned that flight instructors may not always or properly

guard the cyclic flight control during long periods of instructional flight because of the somewhat

awkward position of the cyclic-T handle for the nonflying pilot. Based on the Richmond,

California, accident, the Bell Helicopter study, and the Georgia Tech study, the Board concludes

that the low inertia main rotor blade can diverge from normal rotation to strike the body of the

helicopter in just a few revolutions of the blade. This would take less than 0.5 seconds when the

blade is operating at a normal rate of 530 rpm. Thus, unless the instructor is actually holding the

cyclic handle and preventing a large, abrupt input, there is insufficient time for the instructor to

react once a student makes such an input.

The Safety Board recognizes that all of the loss of control accidents may not have resulted

from a single scenario. Some may have involved low rotor rpm leading to blade stall, and some

may have involved turbulence. The high responsiveness of the helicopter to flight control input

combined with possible lack of pilot skills, knowledge, proficiency, or alertness could also offer

possible explanations for some of the subject accidents. Further, because of the high

responsiveness of the R22 to cyclic input and the rapidness with which the rotor blade could

diverge and strike the fuselage, it is possible that diversion of attention to tasks such as retrieving

charts, tuning radios, or turning to look at something could result in a control input and subsequent

change in aircraft attitude that requires corrective action to which even an experienced pilot may

inadvertently respond with a large, abrupt movement of the cyclic control.

Although the Board could not identify a particular cause that led to the in-flight rotor blade

contact with the fuselage of the R22s, during the investigation, the FAA did implement numerous

operational changes, primarily to ensure that pilots and flight instructors were more knowledgeable

of specific R22 operational hazards and were better trained, and that flights in adverse weather

conditions by low-experienced pilots were limited. There have been no in-flight rotor/fuselage

contacts of the R22 in the United States in the past year since the changes were implemented. 

Although the Safety Board cannot conclude that the operational changes will eliminate all in-flight

rotor strikes, the absence of such accidents since these actions were implemented suggests that they

have been effective. The absence of such accidents also supports the proposition that most of the

accidents were caused by large, abrupt control inputs and the corrective actions taken should help

prevent such accidents. Because the R22 appears to be more responsive to control inputs than other

helicopters normally used in training or routinely used by low-time pilots, the Board concludes that

there is a need to continue the special operating rules for flight instructors and student, lowexperience, and non-proficient pilots to ensure the safe operation of the helicopter. Therefore, the

Safety Board believes that the FAA should ensure that SFAR 73, the FSB specifications, and the

ADs applicable to the operation of the R22 and R44 are made permanent.

The Board is also concerned that in the future, other highly responsive helicopters are

likely to be designed and built that may have characteristics similar to the R22. Consequently,

the Safety Board believes that as a part of the certification process for highly responsive

helicopters, the FAA should establish operational requirements, student pilot training

requirements, and instructor pilot requirements, such as those imposed on the R22 and R44, to
28

ensure that pilots at all levels of qualification and skills can adequately operate the helicopter. The

Safety Board concludes that although the response rate of the R22 to cyclic input is not unsafe so

long as the special operating rules remain in place, there is a need for the FAA to consider the

responsiveness of helicopters (especially lightweight, high performance helicopters such as the

R22) as part of the certification process to determine if special operating rules or guidance are

necessary. Thus, the Safety Board believes that the FAA should require helicopter manufacturers

to provide data on the response of helicopters to large, abrupt cyclic inputs as a part of the

certification process and require operational limitations or other measures for those helicopters that

are more responsive, such as the R22.

The Safety Board is aware of the complexity, difficulty, and potential hazards associated

with flight tests and full-scale wind tunnel testing. However, further research into lightweight

helicopter behavior would benefit the helicopter industry and create a national resource tool that

would aid certification of future helicopter models, especially those that are lightweight and highly

responsive. Therefore, the Safety Board believes that the FAA, in conjunction with NASA, should

continue the development of the simulator model of lightweight helicopters, using flight tests and

whirl tower tests as needed to validate the model, to create a national resource tool for the study of

flight control systems and main rotor blade dynamics.

As indicated above, the FAA did ultimately implement several operational changes

regarding pilot and flight instructor knowledge and training in the R22. Although these changes

appear to have improved R22 safety, the Safety Board will continue to monitor any future R22 and

R44 accidents. 

Also, records made available to the Safety Board suggest that resolution of internal

recommendations made during the SCRs has not been documented and may not have been

accomplished in response to those SCRs. The Safety Board believes that the FAA should review

the process and procedures by which the FAA's aircraft certification offices and management

resolve or bring to closure safety recommendations that are presented in internal documents,

including SCRs, assuring that each recommendation is properly reviewed and that the results of the

review are properly documented.
29

Findings

1. The Federal Aviation Administration's special certification reviews concluded that the

Robinson Helicopter Company R22 met the certification standards of 14 CFR Part 27 when

it received its type certificate on March 16, 1979.

2. Between 1981 and 1994, the R22 experienced fatal accidents involving loss of control,

including accidents involving loss of main rotor control at rates much greater than other

helicopters.

3. Between 1981 and 1995, at least 31 R22 accidents and three R44 accidents (domestic and

foreign) have involved a loss of main rotor control. The investigations of these accidents

did not identify any precipitating progressive mechanical failures or material defects. The

cause of the loss of main rotor control in many of the accidents most likely stems from a

large, abrupt pilot control input to a helicopter that is highly responsive to cyclic control

inputs.

4. The median flight hours of the pilots-in-command, including flight instructors, were 180

hours helicopter and 127.5 R22 hours of flight experience when involved in fatal R22 loss

of main rotor control accidents. However, the median R22 flight experience for the lowest

time pilots who may have been manipulating the flight controls was 52.5 hours.

5. Because the R22 is likely more responsive to cyclic control inputs than other helicopters

normally used in training or by low-time pilots, special training requirements for both

student pilots and flight instructors are needed.

6. As part of the certification process, manufacturers have not been required to provide data on

the responsiveness of helicopters to cyclic inputs.

7. Flight instructors probably do not have sufficient time to react to R22 students' large, abrupt

flight control inputs; therefore, they must guard the cyclic closely to prevent such inputs.

8. There have been no in-flight main rotor loss of control accidents in the United States

involving the R22 or R44 helicopter since early 1995, when the Federal Aviation

Administration issued airman information alerts, airworthiness directives, a flight

standardization board report, and Special Federal Aviation Regulation 73, all of which

pertain to the operation of the R22 and R44 helicopters.

9. Mathematical modeling of the R22 main rotor system conducted by the Georgia Institute of

Technology suggests that large, abrupt cyclic control inputs may result in mast bumping or

blade angles-of-attack greater than the stall angle; however, the mathematical model was
30

not developed sufficiently to demonstrate that flight control inputs would lead to loss of

control of the main rotor.

10. Further research into lightweight helicopter behavior would benefit the helicopter industry

and create a national resource tool that would aid in the certification of future helicopter

models, especially those that are lightweight and highly responsive.

11. Federal Aviation Administration (FAA) staff made several internal recommendations related

to the R22 accidents and special certification reviews (SCRs). Based on documents

provided by the FAA under subpoena and in subsequent correspondence, the Safety Board

is concerned that although some actions were taken to address the safety concerns related to

the R22, the FAA could not show that those actions were taken as a direct result of the SCR

recommendations or that a process existed to ensure that the SCR recommendations were

followed up on.
31

Recommendations

As a result of this special investigation, the National Transportation Safety Board made the

following safety recommendations:

To the Federal Aviation Administration—

Ensure that Special Federal Aviation Regulation 73, the Flight Standardization

Board specifications, and the airworthiness directives applicable to the operation of

the R22 and R44 are made permanent. (Class II, Priority Action) (A-96-9)

Establish, for future certification of highly responsive helicopters, operational

requirements, student pilot training requirements, and instructor pilot requirements,

such as those imposed for the R22 and R44, necessary to ensure that pilots of all

levels of qualification and skills can adequately operate the helicopter. (Class II,

Priority Action) (A-96-10)

Require helicopter manufacturers to provide data on the response of helicopters to

flight control inputs to be used as part of the certification process, and require

operational limitations or other measures for those helicopters that are highly

responsive. (Class II, Priority Action) (A-96-11)

In conjunction with the National Aeronautics and Space Administration, continue

the development of the simulator model of lightweight helicopters, using flight tests

and whirl tower tests as needed to validate the model, to create a national resource

tool for the study of flight control systems and main rotor blade dynamics. If any

unusual main rotor blade system characteristics are found, ensure that the

information and data gathered are disseminated to the appropriate agencies and

industry. (Class II, Priority Action) (A-96-12)

Review the process and procedures by which the Federal Aviation Administration's

aircraft certification offices and management resolve and bring to closure safety

recommendations that are presented in internal documents, including special

certification reviews, and take appropriate action, if necessary, to ensure that each

recommendation is properly reviewed and that the disposition of the

recommendations is properly documented. (Class II, Priority Action) (A-96-13)

To the National Aeronautics and Space Administration:

In conjunction with the Federal Aviation Administration, continue the

development of the simulator model of lightweight helicopters, using flight tests

and whirl tower tests to validate the model, to create a national resource tool for

the study of flight control systems and main rotor blade dynamics. If any unusual
32

main rotor blade system characteristics are found, disseminate the information and

data gathered to the appropriate agencies and industry. (Class II, Priority Action)

(A-96-14)

BY THE NATIONAL TRANSPORTATION SAFETY BOARD

JAMES E. HALL

Chairman

ROBERT T. FRANCIS II

Vice Chairman

JOHN A. HAMMERSCHMIDT

Member

JOHN J. GOGLIA

Member

GEORGE W. BLACK, Jr.

Member

April 2, 1996
33

APPENDIX A-Summary Reports of R22 Loss of Main Rotor Control Accidents

Case Registration Robinson NTSB

 No. Date Location No. Serial No. Accident No.

 1. 11-11-81 Livermore, CA N9073Q 0227 LAX82FA012

 2. 09-25-82 Nashville, TN N9072V 0212 ATL82FA285

 3. 09-30-82 Paige, TX N9063Z 0147 FTW82FA402

 4. 10-06-82 Santa Ana, CA N8358B 0302 LAX83FUA01

 5. 03-14-83 St. Louis, MO N9024Z 0038 MKC83FA076

 6. 12-25-84 Huntsville, AL N8475K 0391 ATL85FA067

 7. 05-05-85 San Angelo, TX N83745 0320 FTW85FA207

 8. 01-05-86 Grenchen,

Switzerland HB-XOC 0327 None

 9. 03-22-86 Memphis, TN N9069S 0181 ATL86FA097

10 . 05-10-86 E. Fishkill, NY N8511Z 0415 NYC86FA127

11 . 03-16-87 Scottsdale, AZ N2256M 0498 LAX87FA147

12. 06-03-87 S. Windsor, CT N2287L 0512 MNYC87FA160

13. 11-03-87 Moraga, CA N8475A 0389 LAX88FA032

14. 08-01-89 Whitford Forest,

New Zealand ZK-HYX 0666 None

15. 11-23-90 Simi Valley, CA N80783 1319 LAX91FA037

16. 01-04-91 Hukerenui,

New Zealand ZK-HDC 1535 None

17. 07-05-91 Phoenix, AZ N23039 1846 LAX91FA288

18. 09-08-91 Welford, England G-BSHF 1382 None

19. 10-27-91 Obrigheim,

Germany D-HEXE 1156 None

20. 01-30-92 Malabar, FL N2313G 2015 MIA92FA072

21. 03-04-92 Maricopa, AZ N8413Q 0354 LAX92FA137

22. 05-06-92 Mt. Pleasant, TN N191KC 1818 ATL92FA096

23. 06-15-92 Julia Crk, Australia VH-HBK 0546 None

24. 06-29-92 Richmond, CA N83858 0337 LAX92FA267

25. 09-30-92 Martinez, CA N8069X 1364 LAX92FA410

26. 03-28-93 Wissen/Sieg,

Germany D-HUPS 1944 None

27. 08-10-93 Honolulu, HI N4017J 1443 LAX93FA318

28. 06-08-94 Martin, England G-PUDD 0863 DCA94RA060

29. 09-28-94 Knightdale, NC N83112 2446 ATL94FA179

30. 12-27-94 Zurich, Switzerland HB-XZW 2387 DCA95RA011

31. 07-17-95 Brighton Downs

Station, Australia VH-BEI 2494 None
Robinson R22 In-flight Breakup Accident Data

Location & Fatal PIC Student Prevailing Tailboom/ Aircraft Serial Number

Date Registration R22/Helo/All R22/Helo/All Flight Purpose Wind Cockpit Strike TT

Velocity/

Certificates Held/ Gusts Mast Contact2

Helo Rating (yes,

no)1

1 11/11/81 Livermore, CA 2 750/1,040/12,200 0/0/190 Personal Calm Tailboom 20.2 0227

N9073Q S, M, C, I, FI, ATP/ Tx-S

Yes

2 9/25/82 Nashville, TN 2 30/65/1,046 Personal 6 Kts Tailboom 648 0212

N9072V S, M, C, I, FI,/ Yes TX-S

3 9/30/82 Paige, TX 1 85/85/290 Personal 12 Kts Tailboom 34 0147

N9063Z S/Yes M-D

4 10/06/82 Santa Ana, CA 1 37/37/4,777 Solo X-Country 5 Kts Tailboom 13.5 0302

N8358B S, M, C, I,/ No M-S

5 03/14/83 St. Louis, MO 2 110/334/748 1/1/1 Instructional 8 Kts Tailboom 339 0038

N9024Z S, M, C, I, FI/ Yes None/No ND

6 12/25/84 Huntsville, AL 1 Unknown Personal 8 Kts Cockpit 377 0391

N8475K S, M, C, I, FI, ATP/ Tx-S

Yes

7 05/05/85 San Angelo, TX 2 115/133/2,433 Practice 11 Kts Tailboom 1478 0320

N83745 S, M, C, I, ATP/ Tx-S

Yes

l

Single-Engine Land: S; Multiengined Land: M; Private: P; Commercial: C; Instrument: I; Flight Instructor: FI; Helicopter: H, Airline Transport: ATP

2

M-S: Separation of upper main rotor mast; M-D Significant deformation of upper main rotor mast; TX-S: Separation of main rotor mast at the top of the transmission; ND:

Information not available.
Location & Fatal PIC Student Prevailing Tailboom/ Aircraft Serial Number

Date Registration R22/Helo/All R22/Helo/All Flight purpose Win d cockpit Strike TT

Velocity/

Certificates Held/ Gusts Mast Contact4

Helo Rating (yes,

no)3

8 01/05/86 Grenchen, Switzerland 2 75/75/75 Personal 10-15Kts Tailboom 0327

HB-XOC /30 Kts ND 658

9 03/22/86 Memphis, TN 2 229/306/2370 Personal 8 Knots Tailboom 1805 0181

N9069S S, M, C, I/ Yes Tx-S

10 05/10/86 E. Fishkill, NY 1 61/67/435 Personal 6 Knots Tailboom 984 0415

N8511Z S, M, C, I/ Yes Tx-S

11 03/16/87 Scottsdale, AZ 2 180/180/180 34/76/245 Instructional 4 Knots Tailboom 202 0498

N2256M S, C, FI/ Yes S / No ND

12 06/03/87 S. Windsor, CT 2 451/451/451 15/15/15 Instructional Calm Tailboom 1288 0512M

N2287L S, C, FI/ Yes M-D

13 11/03/87 Moraga, CA 2 456/582/772 118/142/265 Instructional 6 Knots Tailboom 371 0389

N8475A P, S, C, FI/ Yes S, C, FI/ Yes M-D & Tx-S

14 08/01/89 Whitford Forest New Tailboom

Zealand 2 276/282/286 Personal 10 Knots 727 0666

ZK-HYX P/ Yes Tx-S

15 11/23/90 Simi Valley, CA 1 37/37/160 Solo 18 Knots Tailboom & Cockpit 743 1319

N80783 P, S/ No

ND

16 01/04/91 Hukerenui, New

Zealand 2 47/153/213 Personal 20 Knots Cockpit 18 1535

ZK-HDC P/ Yes

M-S

3

Single-Engine Land: S; Multiengined Land: M; Private: P; Commercial: C; Instrument: I; Flight Instructor: FI; Helicopter H; Airline Transport: ATP

4

M-S: Separation of upper main rotor mast; M-D: Significant deformation of upper main rotor mast; TX-S: Separation of main rotor mast at the top of the transmission; ND:

Information not available.
Location & Fatal PIC Student Prevailing Tailboom/ Aircraft Serial Number

Date Registration R22/Helo/All R22/Helo/All Flight Purpose Wind Cockpit Strike TT

Velocity/

Certificates Held/ Mast Contact6

Helo Rating (yes,

no)5

17 07/05/91 Phoenix, AZ 1 500/500/6,656 Traffic Watch 8 Knots Tailboom 51 1846

N23039 S, C, I/ Yes ND

18 09/08/91 Welford, England 1 2/275/2,473 Practice Light/Vari Tailboom & Cockpit Unknown 1382

G-BSHF C / Yes - lapsed ND

19 10/27/91 Obrigheim, Germany 1 43/ 43/ Unknown Practice Calm Tailboom 1156 1156

D-HEXE Student ND

20 01/30/92 Malabar, FL 2 9/53/2,929 1/1/1,199 Instructional 15 Kts/ 20 Cockpit 85 2015

N2313G S, M, C, I, FI, ATP/ S, M, C, I, FI/ Kts Tx-S

Yes Yes

21 03/04/92 Maricopa, AZ 1 292/292/31,000 Personal 9 Kts/ 20 Tailboom 117 0354

N8413Q S, M, I, FI, ATP/ Kts Tx-S

Yes

22 05/06/92 Mt. Pleasant, TN 2 57/57/116 Personal 10 Kts/ 20 Cockpit 466 1818

N191KC P/ Yes Kts M-D

23 06/15/92 Julia Creek, Australia 1 772/?/1 ,035 Ferry Calm ND Unknown 0546

VH-HBK C/ Yes M-S

24 06/29/92 Richmond, CA 2 2,000/2,000/2,200 4/4/4 Instructional 11 Kts Tailboom 15 0337

N83858 S, C, FI/ Yes Tx-S

25 09/30/92 Martinez, CA 2 234/234/234 0/0/0 Flight 14 Kts Cockpit 787 1364

N8069X C, FI/ Yes Demonstration Tx-S

26 03/28/93 Wissen/Sieg, Germany 114/114/114 Personal 10 Kts Tailboom 206 1944

D-HUPS 2 Yes ND A

5

Single-Engine Land: S; Multiengined Land: M; Private: P; Commercial: C, Instrument I; Flight Instructor: FI; Helicopter H; Airline Transport: ATP

6

M-S: Separation of upper main rotor mast; M-D. Significant deformation of upper main rotor mast; TX-S: Separation of main rotor mast at the top of the transmission; ND:

Information not available.
Location & Fatal PIC Student Prevailing Tailboom/ Aircraft Serial Number

Date Registration R22/He1o/All R22/Helo/All Flight Purpose Wind Cockpit Strike TT

Velocity/

Certificates Held/ Gusts Mast Contact8

HeIo Rating (yes,

no)7

27 08/10/93 Honolulu, HI 2 140/140/4,350 Personal 16 Knots Cockpit 5 1443

N4017J S, M, C, I, ATP/ M-D

Yes

28 06/08/94 Martin, England 2 5,200/7,170/8,257 22/40/4,000 Instructional 15 Kts/20 Tailboom 1093 0863

G-PUDD ATP/ Yes Kts Tx-S

29 09/28/94 Knightdale, NC 1 305/373/790 Business Calm Tailboom 150 2446

N83112 S, M, C, I, FI/ Yes M-S

30 12/27/94 Zurich, Switzerland 2 30/91/91 Personal 18 Kts/36 Tailboom Unknown 2387

HB-XZW /Yes Kts Tx-S

31 07/17/95 Brighton Downs Station, 1 792/794/794 Business Strong Tailboom & Cockpit Unknown 2494

Australia /Yes Gusts Tx-S

VH-BEI

# PIC Median Flight Student Medians

Experience

R22/Helo/All R22/Helo/All

30 127.5/180/790 22/31/658

7

Single-Engine Land S; Multiengine Land: M; Private: P; Commercial: C, Instrument: I; Flight Instructor FI; Helicopter H; Airline Transport: ATP

8

M-S: Separation of upper main rotor mast M-D Significant deformation of upper main rotor mast TX-S: Separation of main rotor mast at the top of the transmission; ND:

Information not available.
38

1. Livermore, California

On November 11, 1981, about 1520 Pacific standard time, N9073Q, a Robinson R22HP,

operated by Cheyenne Aviation, Inc., crashed in Livermore, California, during an instructional

flight. The flight had departed Livermore Airport to the north and was last observed about 500 feet

above ground level (agl), 4 miles north-northwest of the airport. A witness reported that the

weather was visual meteorological conditions (VMC) with 15 miles visibility and calm winds. 

Witnesses observed the helicopter begin a shallow right turn when they heard a loud noise after

which the tailboom and one main rotor blade separated. According to the witnesses, the helicopter

then began turning rapidly, and crashed to the ground. The fuselage and landing skids were

destroyed by impact and postimpact fire damage. The tailboom and main rotor blade were located

along the wreckage path about 710 feet from the main wreckage.

The flight instructor held an airline transport pilot certificate and an instructor's certificate

with an endorsement for rotorcraft-helicopter. He had a total of 12,200 hours of flight experience,

1,040 of which were in helicopters and 750 in the R22 helicopter. The student held a private pilot

certificate, with an endorsement for multiengine land airplanes. He had no previous helicopter

experience. 

The Safety Board found the mast separated and bent approximately 10o

, 8 inches above the

transmission splines. One main rotor blade had separated about 18 inches outboard of the coning

hinge, and the outboard section of the blade was bent upward and aft. The other main rotor blade

was bent upward about 90o

, and the outboard spar was bent aft. The tailboom exhibited evidence

that a main rotor blade had intersected it just aft of the rotating beacon. The forward tailboom

section was crushed and deformed, and had separated at the forward and aft manufacturing rivet

lines. In addition, both tail rotor blades had separated from the hub about 3 inches outboard of the

attachment bolts. The directions of the fractures indicated that the blades had been contacted by a

main rotor blade during the aft tailboom separation sequence; the fractures were typical of overload

forces.

The main rotor assembly was examined. The swashplate assembly rotating star rotated

smoothly, and the rotating scissors were fractured at the mast clamp in overload. Both pitch change

links exhibited bending overload failures at the upper adjustment threads. The flight controls were

examined. All separations in the flight control system between the cockpit, main rotor head, and

tail rotor assembly were the result of impact damage. The flight controls exhibited no evidence that

a preimpact malfunction or failure had occurred. The Safety Board could find no evidence of the

specific event that caused or allowed the main rotor blades to diverge from their normal flightpath

plane and strike the airframe. Therefore, the probable cause of the accident is listed as

undetermined in the Board's brief of the accident.

2. Nashville, Tennessee
39

On September 25, 1982, about 1310 eastern standard time, a Robinson R22, N9072V,

registered to Aero International Helicopter, Inc., broke up in flight during a personal flight near

Nashville, Tennessee. Witnesses reported hearing a slapping and cracking sound followed by

pieces separating from the helicopter. The helicopter crashed into a residential area, and

investigators found wreckage scattered over an area 700 feet long and 500 feet wide. The pilot and

passenger were killed. The pilot held a flight instructor's certificate with an endorsement for

rotorcraft-helicopter, with a total of 1,046 hours of flight time, 65 of which were in helicopters and

30 in the Robinson R22 helicopter. The pilot had received a weather briefing from Nashville flight

service station at 1248, indicating that the local weather conditions were 8 miles visibility, winds

from 030o at 6 knots, the temperature was 68o F, and the dewpoint was 59o F.

The tailboom had separated into four sections and the tail rotor assembly was 200 feet from

the main wreckage. There was evidence that a main rotor blade had struck the tailboom three times

and severed the aft 3 feet of the tailboom. There was also evidence of contact between the main

rotor hub and mast. The hub damage was consistent with the rotor blades traveling beyond their

design limits in the up and down direction (flapping). Both main rotor blades were bent downward;

one blade was separated about 2 feet outboard of the coning bolt and exhibited an overload fracture.

 The main rotor mast also separated at the upper transmission attachment flange and exhibited

bending and torsional damage. The flight controls were examined for possible evidence of a

progressive failure. All fractures examined in the main and tail rotor control systems were typical

of overload and exhibited damage consistent with impact forces. The Safety Board could find no

evidence of the specific event that caused or allowed the main rotor blades to diverge from their

normal flightpath plane and strike the tailboom. Therefore, the probable cause of the accident is

listed as undetermined in the Board's brief of the accident.

3. Paige, Texas

On September 30, 1982, about 1307 central daylight time, N9063Z, a Robinson R22

operated by Tejas Helicopters crashed near Paige, Texas, during a personal cross-country flight. 

After the helicopter's annual inspection, the aircraft mechanic/pilot departed Clover Field in

Houston, Texas, about 1130 en route to the R22's home base at Tim's Airpark. The helicopter

was last seen in cruise flight traveling westbound over U.S. highway 290, when motorists heard a

loud noise then witnessed pieces of the helicopter falling from the helicopter. The pieces seen by

one witness were later identified as the tailboom and tail rotor assembly. The tailboom was

located on the north side of highway 290. The main wreckage, which included the fuselage,

engine, transmission, main rotor assembly, and landing skids, crashed on highway 290, and was

scattered about 117 feet to south side of highway 290. The pilot, holder of a private pilot

certificate with a helicopter rating, was killed. His logbook indicated a total of 290.1 hours of

flight experience, 84.6 of which were in the R22. Visual meteorological conditions were

reported, with 3,000 foot broken clouds, 15 miles visibility, and winds from 130o

 at 12 knots. 

Multiple electrical transmission lines crossed highway 290, near the crash site; the wires revealed

no evidence of having been struck by the helicopter.
40

The Safety Board's examination of the wreckage found that one main rotor blade had

separated at the attachment of the blade spar to the root fitting. A Safety Board metallurgist

reported that the blade separation resulted from gross overstress caused by excessive bending. 

The blade and highway surface exhibited evidence that showed that the blade contacted the

highway during the crash sequence. The other main rotor blade remained attached to the main

rotor hub and was bent upwards 180o about 18 inches outboard of the blade root fitting. The

tailboom exhibited evidence that a main rotor blade had struck the left side of the tailboom aft of

the rotating beacon. The main rotor blade also contained white paint transfer matching the white

paint on the tailboom. A forward section of tailboom also contained evidence that it had been

struck by a main rotor blade, and the other tailboom sections were separated at the manufacturing

rivet lines. Rotational scoring was noted on the interior tail rotor driveshaft and tailboom.

The main rotor mast was partially separated at the top of the transmission. The upper

mast between the swashplate and main rotor hub exhibited severe indentations. The inboard

edges of the main rotor hub contained indentations in positions corresponding to the tusks

normally attached to the main rotor spindles. The damage was consistent with the rotor blades

traveling beyond their design limits in the up and down direction (flapping). A complete

disassembly of the engine revealed no anomalies with the internal engine components. 

Examination of the flight controls revealed the collective control was in the "full up" position

with the friction lock applied and the throttle control was jammed in the "full open" position. 

The Safety Board could find no evidence of preimpact failure or evidence of a specific event that

caused or allowed the main rotor blades to diverge from their normal flightpath plane and strike

the tailboom. Therefore, the probable cause of the accident is listed as undetermined in the

Board's brief of the accident.

4. Santa Ana, California

On October 6, 1982, about 1008 Pacific daylight time, N8358B, a Robinson R22 operated

by Madison Manufactured Housing, crashed near Santa Ana, California, during a solo crosscountry flight. The flight had departed John Wayne Airport, Newport, California, at

approximately 1002 on the pilot's first solo flight in the R22 helicopter. Witnesses saw the

helicopter pitch nose up, then nose down, and then saw a main rotor blade strike the cockpit,

followed by the main rotor mast separating from the helicopter. The helicopter was observed to

descend nose low and hit the ground on the second fairway of the Riverview Golf Course, Santa

Ana, California. The helicopter burst into flames upon impact. The wreckage debris

encompassed an area that included the entire second hole of the Riverview Golf Course, an

equipment yard of a foundry (located south of the second green), an adjacent tennis court, and a

recreation field. Debris was also located on the roof of the foundry. The pilot, who held a

commercial certificate with an instrument rating but was a student in helicopters, was killed. A

review of the pilot's logbook revealed that he had accumulated a total of 4,777 hours of flight

time, 37 of which were in the Robinson R22 helicopter. The weather at John Wayne Airport (5
41

miles from the accident site), at 0945 hours was reported to be partially obscured, 1/2 mile

visibility with haze, winds from 160o at 5 knots.

The Safety Board's examination of the cabin windscreen and landing gear found that a

main rotor blade had entered the cockpit through the right side of the roof and struck the

windscreen retainer below the magnetic compass. The instrument panel, the left tail rotor control

pedals, the left cabin floor, and lower forward section of the cabin all exhibited damage

consistent with a main rotor blade strike, and were found separated from the main wreckage and

scattered throughout the wreckage path. The left skid and portion of the aft cross tube also

exhibited damage consistent with a main rotor blade strike, and were located approximately 307

feet from the initial impact area. A 20-inch section of the outboard end of a main rotor blade was

found 306 feet east of the main rotor assembly. The inboard leading edge of the adjoining

section of main rotor blade exhibited impact damage 89 inches from the blade horn, and blood

was found on the upper surface of the blade 58 inches from the blade horn. Examination of the

fractured section of main rotor blade by a Safety Board metallurgist revealed that the blade

separation was a result of gross overstress.

Examination of the main rotor assembly revealed that the upper main rotor shaft

separated between the main rotor hub and swashplate. The main rotor hub contained paint

transfer and semicircular gouge marks inside the hub, in positions corresponding to the tusks

normally attached to the main rotor spindles. The hub damage was consistent with the rotor

blades traveling beyond their design limits in the up and down direction (flapping). The cyclic

and collective control systems were examined. Both control assemblies exhibited impact and

postcrash fire damage; no evidence of an in-flight failure or malfunction was found. A complete

disassembly of the engine revealed no anomalies with the internal engine components. The

Safety Board found no evidence of a mechanical failure that allowed the main rotor blades to

diverge from their normal flightpath plane and strike the fuselage and, therefore, the probable

cause of the accident is listed as undetermined in the Board's brief of the accident.

5. St. Louis, Missouri

On March 14, 1983, about 1420 central standard time, N9024Z, a Robinson R22 

operated by Helicopters, Inc., crashed into the Mississippi River near St. Louis, Missouri, during

an instructional flight. The flight had originated at East St. Louis, Illinois, about 45 minutes

before the accident. The helicopter was seen in cruise flight at about 300 feet agl traveling north

over the river.

Several people witnessed the accident. One witness, located on the 22nd floor of an

office building near the accident site, said the helicopter wobbled as if buffeted by the wind. The

helicopter was then observed to roll to the right and begin spinning, followed by a loud noise and

the separation of the tail rotor. The witness then observed bluish/white-colored smoke as the

helicopter rapidly descended. The witness also reported that he observed the main rotor separate

before the helicopter fell into the river. A witness located on the 17th floor of an office building
42

near the accident site said the front of the helicopter pitched nose down and the tail flipped over. 

She said she saw a long, thin, shiny object separate from the helicopter. Another witness located

on the east side of the river said he observed the helicopter at about 200 to 300 feet when he

heard a loud pop and witnessed the main rotor and possibly the transmission and engine separate

from the helicopter. Several witnesses located on the west shore of the river heard a loud bang,

then saw pieces separating from the helicopter before the helicopter descended out of control into

the Mississippi River. A pilot and first mate of a tugboat operating near the vicinity of the crash

reported observing an object about the size of a small car that fell into the river several hundred

feet from their boat and sank immediately. They recovered a wallet and two seat cushions, which

were later identified as being from the helicopter. The river was searched by local authorities

until March 18, 1983, but the helicopter wreckage and the bodies of the victims were never

found. Visual meteorological conditions prevailed at the time of the accident. The closest

weather reporting station, 4 miles southeast of the accident site, reported 12 miles visibility and

winds from 290o at 8 knots.

The pilot held commercial pilot and flight instructor certificates with an endorsement for

rotorcraft-helicopter, with a total of 748 hours of flight time, 334 of which were in helicopters

and 110 in the Robinson R22 helicopter. According to the owner of N9024Z, the accident flight

was the student's second instructional flight; the student did not have a pilot certificate. The

Safety Board could not determine the probable cause of this accident.

6. Huntsville, Alabama

On December 25, 1984, approximately 1615 central standard time, N8475K, a Robinson

R22 Alpha, operated by Executive Air, Inc., broke up in flight near Huntsville, Alabama, during

a personal flight. The flight had originated at Decatur, Alabama, at 1330. Before the accident,

the helicopter was seen in cruise, at 400 to 600 feet elevation, heading west. One witness

reportedly heard a loud noise and observed the helicopter falling vertically. Another witness

stated he observed a puff of black smoke and saw parts separating from the helicopter. Both the

pilot and passenger were killed. The weather at the time of the accident was reportedly clear

with 15 miles visibility, winds from 010o at 8 knots, and no reported gusts.

The pilot held an airline transport pilot certificate with commercial pilot privileges in

rotorcraft. The pilot also held airman ratings for airplane multiengine land, sea, and glider, and

he held an airframe and powerplant mechanic certificate. The pilot's flight logs and reported

flight experience was so inconsistently reported that the Safety Board was unable to determine

his total flight experience or his relevant experience in helicopters.

The Safety Board's examination of the wreckage found parts of the helicopter scattered

over an area 400 by 700 feet. The pilot's body was found 141 feet from the fuselage. The pilot's

seat cushion and windscreen centerpost (with the compass attached) were also located 365 feet

and 85 feet east of the main wreckage, respectively. There was evidence that one main rotor

blade had struck the windscreen about 2 feet left of the centerpost and traveled aft to the rear of
43

the left passenger's seat. One of the main rotor blades exhibited downward bending and evidence

that the blade had contacted the fuselage. The main rotor mast separated at the top of the

transmission in an overload fracture, resembling the fracture of the transmission upper cap. The

upper main rotor mast was bent above the swashplate, where the main rotor hub contacts the

shaft. One of the droop stops was fractured and the other droop stop was deformed. The main

rotor shaft and droop stop damage was consistent with the rotor blades traveling beyond their

design limits in the up and down direction (flapping). Both pitch change links were fractured at

the top of the jam nut and upper adjustment threads and exhibited overload fractures.

The tail rotor driveshaft exhibited torsional twisting, and rotational scorings on the inside

of the tailboom were consistent with tail rotor rotation at impact. A complete disassembly of the

engine revealed no anomalies with the internal engine components. Examination of the flight

controls revealed that the collective control was found between 3/4 to the full-up position. No

indication of preimpact malfunction or failure was found throughout the control system. The

Safety Board could find no evidence of the specific event that caused or allowed the main rotor

blades to diverge from their normal flightpath plane and strike the airframe. Therefore, the

probable cause of the accident is listed as undetermined in the Board's brief of the accident.

7. San Angelo, Texas

On May 5, 1985, at approximately 1630 central daylight time, a Robinson R22, N83745,

operated by American Helicopters, Inc., broke up in flight during a practice flight 3 miles south

of San Angelo, Texas. The commercial helicopter pilot, who was preparing to take a certified

flight instructor (CFI) helicopter check flight, and a passenger were killed. The pilot had

accumulated 2,433 total flight hours, 133 of which were in helicopters and 115 in the R22.

The helicopter, including the fuselage, engine, transmission, and skids, came to rest on its

left side, with considerable crushing to the fuselage. Portions of the fragmented windshield and

tailboom came to rest in a soft plowed field 1,200 feet west of the main wreckage, along the

helicopter's flightpath. The tailrotor gear box was located approximately 600 feet west of the

main wreckage. Reconstruction of the tailboom showed that a main rotor blade had struck the

tailboom at three separate locations. The first strike occurred approximately 2 feet forward of the

tail rotor. The second and third strikes were measured at 4 feet from the tail rotor, and 2 feet

from the tailboom attachment to the fuselage. The tail rotor driveshaft and tail rotor push-pull

tubes were severely bent and exhibited a strike from a main rotor blade. Both main rotor blades

contained multiple areas of blue paint transfer, leading and trailing edge damage, and chordwise

scratches on the surface of the blades.

The main rotor mast separated in overload at the upper transmission cap. The upper main

rotor mast and hub assembly remained intact with the spindles attached to the hub and the blade

horns secured to the main rotor blades. The transmission and free-wheeling unit were free to turn

manually with no binding or anomalies. Disassembly of the engine revealed no evidence of

mechanical malfunction, and the carburetor throttle was observed in the full open position. 
44

Examination of the collective and cyclic control tubes revealed overload fractures due to impact

and no evidence of fatigue or preimpact failure. The Safety Board was unable to define the event

that caused the main rotor blades to divert from their normal plane of rotation and strike the

tailboom. Therefore, the probable cause of the accident is listed as undetermined in the Board's

brief of the accident.

8. Grenchen, Switzerland

On January 5, 1986, about 1551 local time, a Robinson R22 helicopter, registered in

Switzerland as HB-XOC, crashed near Grenchen, Switzerland, after an in-flight separation of the

tailboom. The pilot and his wife were on a personal flight and had planned to fly round trip from

Basel-Mulhausen to Grenich, Switzerland. Witnesses reported observing the helicopter in a wide

gradual turn and heard a loud noise, "like hearing a machine gun burst," and subsequently

witnessed pieces of the tailboom break apart in flight. The helicopter entered a steep descent as

it fell to the ground. Pieces of the tailboom and tail rotor were found about 150 meters from the

main wreckage. The private helicopter pilot and his wife were killed, and the helicopter was

destroyed. The pilot had accumulated 75 pilot flight hours, all of which were in the R22

helicopter.

The investigation by the Swiss Air Accidents Investigation Branch (AAIB) found that the

that the right and left cabin doors were located 10 and 15 meters from the fuselage. The winds in

the vicinity were reported from the southwest at 10 to 15 knots, gusting to 30 knots. A friend of

the pilot's stated that he had informed the pilot of gusty conditions over the Jura Mountains, west

of the intended flight route, before the pilot's departure. The pilot reportedly replied that if the

winds were too gusty he would divert his flight from his intended route to avoid adverse

conditions. AAIB reported that the reason for the loss of control and subsequent crash of the

R22 helicopter was that the main rotor blade struck and severed the tail assembly. It did not

determine what factors may have led to the accident.

9. Memphis, Tennessee

On March 22, 1986, about 1415 central standard time, a Robinson R22 helicopter,

registration N9069S, broke apart during a personal flight 8.5 nautical miles west of its point of

origin at Memphis International Airport. The flight had departed at 1247. Both occupants of the

R22 were killed, and the helicopter was destroyed. The pilot-in-command had accumulated

2,370 pilot flight hours, 306 of which were in helicopters and 229 in the R22. The main

wreckage (cockpit, skid assembly, and engine) came to rest on a level plowed field. The

tailboom and tail rotor assembly had separated from the fuselage, and pieces were located 412

feet northwest of the main wreckage. Visual meteorological conditions reportedly prevailed with

the sky clear, visibility 15 statute miles, winds from 230o at 8 knots, and the temperature 55o F at

the time of the accident.
45

The helicopter's paint scheme was white, blue, and yellow. Examination of the wreckage

revealed that the tailboom had separated at the second bay aft of the fuselage attachment point. 

The leading edges of the main rotor blades exhibited blue and yellow paint transfer. The

separated sections of tailboom structure exhibited diagonal impacts from left to right (viewed

from aft looking forward) at three locations. One main rotor blade droop stop and both pitch

change links were fractured in overload. The upper main rotor mast exhibited indentations

corresponding to hub contact. The base of the main rotor mast was fractured in overload and

separated at the top of the transmission. Physical evidence indicated that the bending of the

upper main rotor shaft occurred before the fracture of the transmission cap, and secondary to the

main rotor blades traveling beyond their normal flapping range. An instability of the main rotor,

rocking of the mast, and extreme pitch divergence of the main rotor blades appeared to precede

the fractures of the main rotor flight control system.

The helicopter's cyclic and collective controls were examined. There was no evidence of

fatigue or preimpact failure of control system components. A partial disassembly of the engine

revealed no evidence of engine internal mechanical malfunction. The Safety Board was unable to

define the event that caused the main rotor blades to divert from their normal plane of rotation and

strike the tailboom. Therefore, the probable cause of the accident is listed as undetermined in the

Board's brief of the accident.

10. East Fishkill, New York

On May 10, 1986, at approximately 1220 eastern daylight time, a Robinson R22 Alpha,

N8511Z, broke up in flight while on a visual flight rules (VFR) flight from Hyde Park, New

York, to Danbury, Connecticut. The pilot had departed Hyde Park about 1212. The helicopter

was destroyed and the commercial certificated pilot was killed. The last entry in the pilot's

logbook indicated that he had accumulated 435 total flight hours, 67 of which were in

helicopters and 61 in the R22. A helicopter pilot who had witnessed the R22's departure

requested the tower to inform N8511Z that the helicopter's cowl door was open in the back and to

land the helicopter. The pilot of N8511Z acknowledged but continued the flight.

Witnesses to the crash reported a loud "pop" and seeing pieces falling from the helicopter.

The severed section of tailboom was the first piece of the helicopter along the 584-foot long

wreckage path. The main wreckage exhibited evidence that a main rotor blade had struck the

tailboom at several locations. The main rotor assembly was separated from the mast at the top of

the transmission and was located next to the fuselage. A cabin door and pieces of plexiglass were

found 15 feet southwest of the main wreckage.

Parts of the wreckage were examined by a Safety Board metallurgist for evidence of

possible preimpact failure of a control system or flight component that might have initiated the

breakup. The examination of the top of the transmission case and lower main rotor mast revealed

signatures typical of overstress separation. The upper main rotor shaft also exhibited features typical

of a bending overstress separation. One of the pitch change links was fractured at the lower
46

rod end, and the other remained attached to the fractured swashplate arm. Both fracture surfaces

exhibited evidence of overstress separations. Examination of the main rotor spindles revealed

deformed tusks on each spindle consistent with the blades traveling beyond the design limits. 

The droop stops were also found crushed and deformed and exhibited the effects of repeated

pounding.

The Safety Board's metallurgist also examined the helicopter's collective and throttle

controls. The right collective stick was fractured at the crosstube adjacent to the weld that

attached the stick to the crosstube. The fracture surface exhibited a flat shiny region typical of

fatigue cracking; however, the fatigue crack did not propagate through the total fracture surface. 

A closer examination of the fatigue region with a scanning electron microscope revealed

intermittent patches of ductile dimples adjacent to the fatigue area, suggesting that the stresses

propagating the fatigue crack may have been relatively high. The fracture features beyond the

fatigue zone were typical of overstress separation.

The Safety Board could find no evidence of the specific event that caused or allowed the

main rotor blades to diverge from their normal flightpath plane and strike the airframe. 

Therefore, the probable cause of the accident is listed as undetermined in the Board's brief of the

accident.

11. Scottsdale, Arizona

On March 16, 1987, at 1117 mountain standard time, a Robinson R22 Alpha helicopter,

N2256M, operated by Arizona Wing and Rotor, broke up in flight during an instructional flight at

Scottsdale, Arizona. A witnesses reported observing the helicopter in hover flight at about 400

feet agl. The helicopter was then observed to turn slowly to the left and lose altitude, then to turn

360o

 as the nose lowered about 45o, and to rapidly descend in a spin. The witness subsequently

reported a loud noise and observed pieces separating from the helicopter as it crashed to the

ground. The helicopter was destroyed, and the certificated flight instructor and student were

killed. The flight had departed Scottsdale Airport about 30 minutes earlier and was operating in

the traffic pattern practicing touch-and-go landings. The pilot-in-command had accumulated 180

pilot flight hours, all of which were in the R22. The student pilot had accumulated 245 pilot

flight hours, 76 of which were in helicopters and 34 in the R22. Visual meteorological

conditions reportedly prevailed at the time of the accident. Winds near the accident site were

reported from 120o at 4 knots; visibility was 30 miles, and the temperature was 48o

 F.

Pieces of the helicopter wreckage were found scattered over a 270-foot area on a level

undeveloped field adjacent to a paved road, approximately 1/2 mile north of Scottsdale Airport. 

Sections of the red and white tailboom were located west and north of the main wreckage. Two

sections of the tail rotor assembly were located northwest of the main wreckage. The first

section measured 18 inches and was located at about 150 feet, and the second section included

the forward end of the tailrotor driveshaft and damper and was found 250 feet from the main
47

wreckage. The main wreckage consisted of the fuselage, engine, transmission, main rotor

assembly, and 24 inches of the tailboom that remained attached to the fuselage. Examination of

the main rotor blades showed evidence of impact damage and red paint transfer on the bottom

surface of the blades. Both main rotor blade pitch bearings were found to rotate freely, and

control continuity was established to the swashplate assembly. The cyclic and collective control

system exhibited no evidence of fatigue or preimpact failure. The tail rotor driveshaft turned

and exhibited no evidence of binding or preimpact failure. Disassembly of the engine revealed

no evidence of mechanical failure or anomalies before impact. The Safety Board could find no

evidence of the specific event that caused or allowed the main rotor blades to diverge from their

normal flightpath plane and strike the tailboom. Therefore, the probable cause of the accident is

listed as undetermined in the Board's brief of the accident.

12. South Windsor, Connecticut

 On June 3, 1987, at approximately 1120 eastern daylight time, a Robinson R22 Mariner,

N2287L, equipped with fixed flotation gear, operated by Northeast Helicopters, broke up in flight

during an instructional flight near South Windsor, Connecticut. A witness near the accident site

stated that he heard a loud ping, then observed pieces falling from the helicopter, and the main

rotor blades appear to fold upwards before the crash. The CFI and student pilot were killed. The

22-year old CFI had accumulated 451 total flight hours, all of which were in the R22. The

student pilot had accumulated 15 pilot flight hours, all of which were in the R22.

Fragmented portions of the tailboom were found 300 feet from the main wreckage. The

tailboom exhibited evidence of two severe strikes to its left side. The strikes were consistent

with the size and shape of the main rotor blade leading edge. Inspection of the main rotor blades

revealed evidence of yellow paint transfer along the main rotor blade leading edges. 

Examination of the engine revealed no evidence of mechanical malfunction or anomalies before

impact. The collective and cyclic control system was examined, and there was no evidence of

preimpact failure. Therefore, the probable cause of the accident is listed as undetermined in the

Board's brief of the accident.

13. Moraga, California

On November 3, 1987, at approximately 1338 Pacific standard time, N8475A, a Robinson

R22 Alpha operated by Helicopter Adventures, Inc., broke up in flight during a dual instructional

flight near Moraga, California. The CFI and commercial helicopter pilot student, who was

preparing to revalidate his CFI helicopter certificate, were killed. Witnesses reported observing the

helicopter hovering at 400 feet agl when they heard the engine noise suddenly decrease, and a loud

pop. The witnesses said they observed parts, "like sparklers," fall off the helicopter as it fell to the

ground. The flight had originated 8 miles southwest, in Concord, California, about 1230, after

having been refueled with 100LL fuel. The weather at Concord, 8 miles northeast of the accident
48

site, at 1245 was reported as 15,000 feet scattered clouds, 15 miles visibility, temperature 67o

 F,

dewpoint 48o F, winds from 060o at 6 knots, altimeter 30.02.

The pilot-in-command was a full-time flight instructor. On May 13 through May 16,

1987, the pilot had attended a Flight Instructor Safety Course given by the Robinson Helicopter

Company in Torrance, California. The RHC "Instructor/Pilot Evaluation" sheet for the pilot

reported that at the time of the course, the pilot's basic flying skills were good; however, his weak

areas included: high flares, poor heading control, and late pitch pulls during hovering autos. The

RHC instructor stated that some of these problems were worked out, and reported "I don't feel

he's unsafe." The RHC instructor also reported that the pilot's difficulty with proficiency was a

result of having accumulated only 150 flight instructor hours at the time of the evaluation. A

review of the instructor pilot's logbook revealed that, at the time of the accident, he had

accumulated 236 flight instructor hours in the R22. The pilot's logbook documented 772 total

flight hours, 582 of which were in helicopters and 456 in the R22. The other pilot's logbook

revealed that he had accumulated 265 total flight hours, 142 of which were accrued in helicopters

and 118 in the R22.

The main wreckage was found in the center of a dried-up cattle pond surrounded by a

grass pasture and rolling foot hills that were 150 to 300 feet higher than the center of the pond. 

The main wreckage included the fuselage, engine, transmission, tailboom, and both rotor

systems. The wreckage exhibited extensive vertical crushing and deformation. The left side of

the cockpit, including the door, lower half of the left windscreen, and both left and right side

cockpit windscreens, were separated from the main wreckage and found along the wreckage path.

 The most distant piece, identified as the cockpit procedures checklist, was found about 1,750

feet from the main wreckage. Portions of the fragmented windshield and left fuselage came to

rest in the pasture 500 feet from the main wreckage, along the helicopter's flightpath. 

Reconstruction of the fuselage wreckage revealed evidence that a main rotor blade intersected the

forward cockpit 12 inches left of the centerpost and continued through the left side of the cockpit

to the left rear door post. The left rudder pedal and left seat cyclic cross tube exhibited

indentations that matched the leading edge profile of the main rotor blade. An outboard section

of one of the main rotor blades was found wrapped around the left side of the cockpit with the

leading edge of the blade resting against the forward left skid crosstube. The blade exhibited

extensive leading edge damage, chordwise scoring, and deformation over the outboard half of the

blade.

The upper main rotor mast and hub assembly remained intact with the spindles attached

to the hub and the blade horns secured to the main rotor blades. Both main rotor blades remained

attached to the blade horns. The upper end of the main rotor driveshaft, between the hub and

swashplate, exhibited torsional twist and deformation of the shaft, at about a 90o

 angle (relative

to the vertical position of the shaft). The main rotor mast contained an overload fracture at a

point just above the attachment to the transmission. The internal rotor driveshaft remained intact

and attached to the gear system in the transmission. The engine was rotated and the accessory

gear and valve train continuity was established throughout the engine. Disassembly of the engine

revealed no evidence of mechanical malfunction or overspeed. Continuity was established

throughout the tail rotor drive system, and no unusual operating signatures were found. 
49

Examination of the collective and cyclic control tubes found control system continuity from the

cockpit throughout the helicopter, and no evidence of fatigue or preimpact failure. 

A toxicological analysis of urine specimen taken from the PIC revealed minute traces of

marijuana metabolites at a level of approximately 100 ng/ml; however, there was no evidence of

alcohol or drug substances found in the pilot's blood. The toxicologist concluded that there was

no evidence found of "current influence" of the drug. The student pilot's blood specimen

revealed less than 1 meg/ml of caffeine, and the urine specimen revealed cocaine metabolite. 

The Safety Board found that there was insufficient evidence to conclude that drugs had affected

either pilot's ability to operate the aircraft. The Safety Board was unable to define the event the

caused the main rotor blades to divert from their normal plane of rotation and strike the tailboom.

The Safety Board determined that the probable cause of this accident was a divergence of the

main rotor from its normal plane of rotation for an undermined reason, which resulted in main

rotor blade contact with the cockpit area. 

14. Whitford Forest, New Zealand

On August 1, 1989, at about 1522 local time, a Robinson R22 Beta helicopter, registered

in New Zealand as ZK-HYX, crashed near Whitford Forest, New Zealand, after an in-flight

separation of the tailboom. After departing from Ardmore Aerodrome, the pilot had informed

Ardmore Tower that he and his passenger were proceeding to Whitford Forest and indicated that

the duration of the flight would be about 20 minutes. The private helicopter pilot and passenger

were killed, and the helicopter was destroyed. The pilot had accumulated a total of 286 flight

hours, 282 of which were in helicopters and 276 in the R22 helicopter. The pilot had flown a

helicopter only twice in the 4 months preceding the accident. The weather at Ardmore

Aerodrome, 10 km south-south-west of the accident site at 1500 hours, was reported to be 2 octas

cumulus at 3,500 feet, 60 km visibility with haze, winds from 50o at 10 knots. Subsequent

analysis of the weather data suggested that a prevailing northeast wind may have produced

"moderate turbulence close the hills with down draughts in the lee of the ridges." It was believed

that the flight was conducted at low altitude.

The investigation by the New Zealand Transport Accident Investigation Commission

(TAIC) found that the tailboom, vertical and horizontal stabilizers, and the tail rotor assembly

were located in the tops of trees 80 to 100 meters from the fuselage. The severe "near vertical"

impact of the fuselage on a steep hillside bent the left skid of ZK-HYX upwards, and splayed the

right skid outwards. The under structure and engine were forced upwards and the mast assembly

was fractured at its base and was bent to the left. Both main rotor blades remained attached to

the rotor head. The TAIC reported that the damage to the blades was characteristic of low

rotational energy at the time of ground impact. One main rotor blade had revolved 180o

 in its

pitch bearing, and was bent "upwards" when positioned correctly in relation to the opposite

blade, which was bent "downwards." The TAIC reported that both pitch change links had bent

before failure. It also reported that there was no significant evidence of mast bumping but the

rotor mast was bent slightly at the top, and the metal droop stops (that limit the downward
50

movement of the main rotor blades) had sheared in overload. The examination found no defect

in the main rotor head or blades that was considered to have contributed to the cause of the

accident.

The TAIC's examination found that a main rotor blade had struck the tailboom dislodging

the anticollision beacon and caused the aft section of tailboom and driveshaft assembly to

separate. Severe damage to the inboard end of the middle section of the tailboom and matching

paint smears on the main rotor blades indicated that this section had been struck by the blades

while the helicopter was still in flight. The TAIC reported that is was evident that at the time of

separation, the tail rotor was being driven with considerable rotational energy. Heavy scoring

was on the internal skin in the area of the fuselage/tailboom attachment indicating continued

rotation of the transmission drive when the strike occurred. Both tail rotor blades were dented

over the final 125 mm of their outboard ends. The TAIC found that the relative symmetry of the

damage areas on each blade and the severity of denting, with the absence of "rotational" marking,

suggested that the outer portion of the tail rotor had struck a branch or tree trunk.

The TAIC reported that no evidence suggested that the cyclic, collective, or tail rotor

controls had been obstructed in any way, and established continuity of these control systems. 

The lower engine compartment was substantially damaged on impact. The carburetor bowl was

broken off but both floats were intact. The fuel and engine oil filters were free of contamination,

and the engine sump contained an ample supply of oil. The main and auxiliary fuel tanks were

ruptured; however, a quantity of fuel remained in the line to the fuel selector. The TAIC's

examination of the airframe and engine found no evidence of a preimpact mechanical defect or

failure in any component of ZK-HYX. 

The TAIC concluded that although there was evidence that at least one main rotor blade

had struck and separated the aft section of the tailboom in flight, it was unable to establish the

cause of the accident with certainty. The TAIC reported the probable cause of the accident was

"a decision by the pilot to descend the aircraft below the minimum approved height above the

ground in order to demonstrate maneuvers at which he had no recent practice." The TAIC

reported that contributing factors may have included turbulence and the effects of a glass of wine

that the pilot had consumed about 3 hours before the accident flight.

15. Simi Valley, California

On November 23, 1990, about 1603 Pacific standard time, N80783, a Robinson R22 Beta

operated by Orbic Helicopters, Inc., broke up in flight near Simi Valley, California, during a solo

flight. The flight had originated at Van Nuys Airport, Van Nuys, California, at approximately

1535. No one witnessed the in-flight breakup; however, one person reported hearing the rotor

blades make a "thwack-thwack-thwack" sound, and then observed a plume of smoke where the

wreckage was subsequently located. The helicopter was destroyed by impact forces and by

postimpact fire. The wreckage debris encompassed an oval-shaped area 300 feet long, on 25o

up-sloping hilly terrain. The private pilot (airplane) who was a "student" helicopter pilot, was
51

killed. The weather at Van Nuys Airport (15 miles from the accident site) at 1546 hours was

reported to be 25 miles visibility with winds from 340o

 at 18 knots.

The pilot's logbook recorded a total of 160 hours of pilot flight time, 37 of which were in

helicopters--all in the Robinson R22 helicopter. The student pilot's flight instructor had

reportedly authorized the student to fly from the Van Nuys Airport to a practice area, near the

crash site. The student pilot had been directed to practice takeoffs to a hover, traffic patterns, and

normal approaches to landings. According to the flight instructor, the pilot had been a "betterthan-average" student and had not experienced any unusual problems learning to fly the

helicopter.

The Safety Board's examination of the wreckage found that a main rotor blade had struck

and severed the tailboom. Pieces of the tailboom and the intact tail rotor assembly were located

approximately 165 feet from the main wreckage. Fragmented pieces of the helicopter's

plexiglass windscreen were located along the wreckage path, an estimated 100 yards from the

fuselage. The upper section of the left door frame was also found separated from the fuselage

and located about 135 feet from the main wreckage. The section of separated door contained a

smearing of black paint that matched the color of paint found on the leading edge of the main

rotor blades. Both main rotor blades were found bent in the aft direction and exhibited impact

damage to the blades, and leading edge and chordwise scoring. Examination of the collective

and cyclic control tubes, swashplate assembly, and main rotor pitch change links revealed no

evidence of preimpact failure. The examination of the engine revealed no anomalies or

preimpact damage to the internal engine components. The Safety Board could find no evidence

of the specific event that caused or allowed the main rotor blades to diverge from their normal

flightpath plane and strike the airframe. The Safety Board determined that the probable cause of

this accident was a divergence of the main rotor from its normal plane of rotation for an

undermined reason, which resulted in main rotor blade contact to the tailboom and cockpit. 

16. Hukerenui, New Zealand

On January 4, 1991, at about 1233 local time, a Robinson R22 Beta helicopter, registered

in New Zealand as ZK-HDC, broke up in flight during a personal flight near Hukerenui, New

Zealand. Witnesses about 150 meters from the accident site observed the helicopter flying

normally between 50 and 100 feet above the ground when they heard a loud noise. Immediately

afterward, several items, including the main rotor, separated from the helicopter, and the

helicopter descended to the ground and caught fire. The private helicopter pilot and passenger

were killed, and the helicopter was destroyed. The pilot had accumulated a total of 213 flight

hours, 153 of which were in helicopters and 47 in the R22 helicopter. The weather at the

accident site was determined to be a variable scattered cloud base of 2,500 to 3,000 feet, with

visibility in excess of 30 km, and winds from the southwest at 20 to 25 knots. Light or moderate

turbulence in the area was considered likely.

The investigation by the New Zealand TAIC found the accident site to the windward side of

a row of tall pine trees and downwind of an unobstructed expanse of rolling pasture land. The
52

wreckage of ZK-HDC was located in a flat paddock. The TAIC reported that the left door, pieces

of the canopy, the magnetic compass, the passenger's handbag, landing light fragments, and a

metal clip (used to retain the rubber boot of the main rotor blade) were among the debris found

some 150 meters before to the helicopter's ground impact location. The main rotor blades

remained attached to the main rotor head assembly and were located 50 meters north of the

burned fuselage. The upper section of the main rotor driveshaft was fractured and exhibited a

torsional overload failure. The TAIC reported that it found no significant damage to the leading

edge of either main rotor blade, and the blade damage was characteristic of low rotational energy

at the time of ground impact. One blade exhibited evidence of striking the cockpit canopy along

a line between a point immediately above the forward upper corner of the right door to a

midpoint on the forward edge of the left door's "transparency." The other main rotor blade had

rotated about 180o in pitch before it also struck the canopy with its trailing edge. Mast bumping

was evident on the pitch stops. The right door was found about 1 meter to the north of the rear

end of the tailboom.

The examination of the helicopter's tail rotor driveshaft revealed that it had failed in

overload with the tail rotor operating normally when impact occurred. The tail rotor blades had

been bent symmetrically at right angles, close to their hubs with no significant damage to their

leading edges.

The TAIC found most of the helicopter's instruments and cockpit area damaged by

postimpact fire; however, examination of the rotor and engine tachometer revealed an impactcaptured main rotor speed of 100%, and an airspeed indication of 52 knots. The positions of the

collective and throttle could not be established because they were destroyed by fire. The TAIC's

examination of the engine and transmission were impeded by fire damage. The examinations

revealed no defects that would have led to an in-flight power loss. 

The TAIC report discussed the possibility and consequences of abrupt pull-ups and pushovers and low-G maneuvering, citing the R22 Pilot Operating Handbook and RHC Safety Notice

"SN-11." However, the report contained no information that suggested that low-G maneuvering

was observed by anyone before the accident. The TAIC analysis speculated that the pilot may

have been flying low to avoid crosswinds and to look for friends who were driving the same

route. As the terrain rose and fell along the flightpath, the TAIC suggested that the pilot may

have followed the terrain and thus encountered a low-G condition, precipitating the accident.

The TAIC reported the probable cause of the accident was that "the pilot failed to

recognize that he had inadvertently entered a low-G flight regime which caused the aircraft to

roll. Consequently, he endeavored to right the aircraft by applying left cyclic without first

restoring positive loading to the main rotor blades." The TAIC found that the pilot's inexperience

in the R22 and his lack of awareness of the helicopter's vulnerability to low-G flight were factors

that contributed to the accident. Consequently, the TAIC recommended to the Air Transport

Division of the Ministry of Transport that it "review the flight test requirements for ratings on the

R22 aircraft to ensure appropriate knowledge of this peculiarity of the Robinson R22 aircraft's

behavior and the appropriate technique is understood by all applicants."
53

17. Phoenix, Arizona

On July 5, 1991, at approximately 1639 mountain standard time, N23039, a Robinson

R22 Beta, operated by Skyview Traffic Watch, Inc., broke up in cruise flight near Phoenix,

Arizona. The commercial helicopter pilot, who was conducting routine local traffic reporting,

was killed. Witnesses reported observing the helicopter cruising between 300 and 500 feet agl

when they heard a loud bang and observed pieces falling from the helicopter. One witness

observed the helicopter's main rotor blades fold upward as it fell to the ground. The flight had

originated about 1631, at the Scottsdale Municipal Airport, Scottsdale, Arizona. The weather at

Scottsdale Airport, 6 miles southeast of the accident site, at 1656 was 15,000 feet scattered

clouds, 40 miles visibility, with winds from 260o at 8 knots.

The pilot held a commercial pilot certificate, and was rated for airplane single-engine

land, instrument airplane, and rotorcraft-helicopter. The pilot also held a flight instructor

certificate for helicopters. On May 10, 1991, the pilot had attended a Flight Instructor Safety

Course given by the Robinson Helicopter Company in Torrance, California. The RHC

"Instructor/Pilot Evaluation" sheet for the pilot indicated that the pilot was "a good conservative

pilot" who "exercised good judgement throughout the flight." The pilot's logbook indicated

6,656 total flight hours, 500 of which were in helicopters, all in the R22.

N23039 had collided with a residence initiating a fire that gutted the interior of the

residence and severely burned the helicopter wreckage. The majority of the helicopter main

wreckage was located in the living room and included the fuselage, engine, transmission,

tailboom, and both rotor systems. The tailboom and tail rotor system were located in an adjacent

atrium, and were separated from the main wreckage by the living room wall. The left skid, the

windscreen bubble, and the left door were separated from the main wreckage, and found

scattered over a distance of about 1,400 feet to the northeast of the main wreckage. A fragment

of clear plastic, similar to the windscreen bubble material, was found near the accident site and

exhibited a red smudge on its surface. 

Remnants of model toy rockets identified as Estes B6-4 and A8-3 stages, also colored

red, were located in a vacant lot near the accident site. The plastic fragment and model rockets

were chemically examined by Truesdail Laboratories, Tustin, California, to determine if the

smudge matched the red surface of the rockets. The paint smudge on the plastic was determined

to be alkyd (multipurpose paint) and did not match the material of the rocket, which was found to

be polystyrene.

Following the examination at the accident site, the wreckage was recovered and moved to

the Robinson Helicopter Company where it was examined on two other occasions. No evidence

was found of any control system failures or malfunctions or of any material defects of the

components or systems destroyed by postimpact fire. The examination of the main rotor mast

and droop stops revealed indentations where the main rotor hub makes contact with the mast. 

The hub and droop stop damage was consistent with the rotor blades traveling beyond their design
54

limits in the up and down direction (flapping). The engine and accessories were examined. Fuel

was observed in the accelerator pump port of the carburetor, and normal electrode wear

signatures were on the spark plugs. No metal contamination was found in the engine oil screen. 

The engine crankshaft rotated and continuity of the gear and valve train was established. The

disassembly of the engine revealed no evidence of mechanical malfunction. 

The Safety Board was unable to define the event the caused the main rotor blades to

divert from their normal plane of rotation and strike the cockpit. The Safety Board determined

that the probable cause of this accident was a divergence of the main rotor from its normal plane

of rotation for an undermined reason, which resulted in main rotor blade contact to the cockpit. 

18. Welford, England

On September 8, 1991, about 1600 local time, a Robinson R22 Beta helicopter, registered

in England as G-BSHF, broke apart during a practice flight about 1,500 feet agl, over the village

of Weston-on-Avon. Witnesses reported observing the helicopter in straight-and-level flight

traveling southwest when it began to yaw suddenly to the right and left. A loud noise like a bang

or pop followed, and the helicopter was observed to pitch nose-down as the tail rotor assembly

and sections of the tailboom separated from the helicopter. The witnesses said they then

observed the helicopter's main rotor blades flailing and one of the blades separated as the

helicopter entered a vertical dive and crashed to the ground. The pilot was killed, and the

helicopter was destroyed. The pilot held commercial pilot licenses for both fixed wing and

helicopter, and a flight instructor's certificate. His flight experience was 2,473 total flight hours,

275 of which were in helicopters and 2 in the R22. The main wreckage (cockpit, skid assembly,

and engine) came to rest upright on level ground. The tailboom and one main rotor blade had

separated from the fuselage, and the rotor blade and tailboom pieces were located 200 meters

north of the main wreckage. The U. K. Air Accidents Investigation Board (AAIB) investigated

the accident.

Examination of the wreckage on site and later at the AAIB facility at Farnborough,

England, did not reveal any preimpact defects in the structure, flight controls, engine, or

transmission. The droop stops were distorted due to the downward bending of the main rotor

blades. Several strikes to the tailboom by a main rotor blade were evident. The initial strike was

in the region of the warning arrows and "DANGER" decal at the rear of the tailboom. 

Subsequent high energy strikes had also occurred with the blade contacting the tailboom at about

a 45o degree angle forward of the initial strike. There was also evidence of a main rotor blade

strike to the left side of the cabin area. The AAIB could find no evidence of the specific event

that caused or allowed the main rotor blades to diverge from their normal flightpath plane and

strike the airframe.

19. Obrigheim, Germany
55

On October 27, 1991, about 1421 local time, a Robinson R22 helicopter, registered in

Germany as D-HEXE and operated by Helicopter Service Mitte, crashed near Obrigheim,

Germany, after the tailboom separated in flight. The student pilot had been operating the

helicopter on a solo cross-country flight, from Egelsbach, Germany, via Baden-Baden and

Mosbach, and returning to Egelsbach, Germany. Witnesses reported that they observed the

helicopter flying over a shallow valley that had a power line suspended across the intended flightpath. Shortly before overflying the power line, the helicopter was observed to turn steeply to the

left and crash into an uphill sloping grassy area. The student pilot, who had logged a total of 43

flight hours in the R22, was killed, and the helicopter was destroyed. The weather at the time

and location of the accident was reported as calm winds, visibility greater than 10 kilometers, and

clear sky.

The R22 helicopter was equipped with an altitude scriber. The German Accidents

Investigation Bureau's examination of a plotted graph of D-HEXE's flightpath and altitude before

impact revealed an almost level flight, followed by a rapid descent to the final impact.

 

The German Accidents Investigation Bureau's examination of the wreckage revealed that

one of the helicopter's main rotor blades had struck and severed the tailboom. The examination

revealed that the actuator for the clutch was found in the full extend position; however, the clutch

actuator was not believed to have contributed to the in-flight breakup because the electric motor

for the clutch was found separated due to impact forces. The examination of the helicopter's

cockpit annunciator lights concluded that the lamps for "Low Rotor RPM," and "Clutch" were

probably not illuminated. The examination of the wreckage did not reveal any evidence of

mechanical failure or malfunction, and the German Accidents Investigation Bureau reported that

it closed the accident file without having determined a probable cause.

20. Malabar, Florida

On January 30, 1992, about 1355 eastern standard time, N2313G, a Robinson R22 Beta

operated by Melborne Helicopters, Inc., crashed in Malabar, Florida, during an instructional

flight. The flight had originated in Melborne, Florida, approximately 20 minutes before the

accident. Witnesses near the accident site stated that they heard a loud bang, looked up, and

observed one rotor blade broken and part of the cabin area missing. According to the witnesses,

the helicopter then yawed left and crashed into palm trees. The main wreckage came to rest at

the base of two palm trees; both occupants were killed. Witnesses on the ground reported that

the engine ran until impact. The pilot held airline transport pilot and flight instructor certificates,

with a helicopter rating. He had logged a total of 2,929 hours of flight time, 53 of which were in

helicopters and 9 in the R22. The R22 student held commercial pilot and flight instructor

certificates, was rated in airplanes, and had accumulated 1,199 hours of flight time, 1 hour of

which was in the R22. The closest weather station, 6 miles north, reported 7 miles visibility and

winds from 230o

 at 15 knots, gusting to 20 knots.
56

The main rotor mast had failed at the attachment to the top of the transmission. Both

fracture surfaces exhibited bending overload in the forward left direction, as viewed from above.

 The examination of the main rotor blades revealed that the outboard end of the blade was bent

downward about 100o and that an area of heavy abrasion was 28 inches inboard from the blade

tip. The forward door frame on the left side of the helicopter exhibited damage consistent with

the abrasion found on the blade's leading edge. Indentations on either side of the hub were

observed in positions corresponding to the tusks normally attached to the main rotor spindles. 

The hub damage was consistent with the rotor blades traveling beyond their design limits in the

up and down direction (flapping). The pitch change links remained attached to the upper

swashplate assembly and exhibited bending overload separations at the upper rod end bearings.

The blade horn flanges exhibited scoring and compression damage consistent with the lateral

impressions on the chord arm. The flight controls were examined, and control continuity was

established. The Safety Board determined that the probable cause of this accident was a

divergence of the main rotor from its normal plane of rotation for an undermined reason, which

resulted in a main rotor blade contact with the cockpit. 

21. Maricopa, Arizona

On March 4, 1992, at approximately 1535 mountain standard time, N8413Q, a Robinson

R22, broke up in flight while cruising about 5 miles northeast of Estrella Sailport near Maricopa,

Arizona. The helicopter was destroyed, and the airline transport pilot (a former airline captain)

was killed. The last entry in the pilot's logbook was dated January 1, 1992, and indicated that he

had accumulated 31,000 total flight hours, 292 of which were in helicopters--all in the R22. No

one witnessed the accident. 

The pilot was last seen departing Estrella Sailport in his R22. A CFI who had witnessed

the R22's departure stated that at approximately 1520 [the accident pilot] had completed his "bythe-book preflight" and departed for his home at Stellar Airpark. The crash site was located on

the direct course line between the Estrella and Stellar airports. The severed left door was the first

piece of the helicopter found along the 475-foot-long wreckage path. The main wreckage

exhibited evidence that a main rotor blade had entered the left side of the cockpit and severed a

portion of the left forward skid. One of the main rotor blades revealed damage approximately 48

inches from the tip. The left door and left forward airframe exhibited a severe strike and yellow

paint transfer consistent with the shape and painted surface of the main rotor blade. The damage

to the left forward skid was consistent with a main rotor blade strike while the blade was being

driven at a downward angle (as referenced with the main rotor hub) of approximately 70o.

The wreckage was examined for evidence of possible preimpact failures of the control

system or airframe that might have initiated the breakup; however, none were found. The top of

the transmission case was fractured (360o

) and exhibited signatures of both compression and

tension. The left rear transmission mount and right forward mount bolt made an impression in

the surface of the engine shroud consistent with a rocking motion of the transmission in the leftrear to right-forward direction. The main rotor shaft exhibited torsional twisting and bending
57

directly below the mount for the main rotor hub. The pitch change links exhibited bending

overload failures at the upper adjustment threads. The pitch change links and spindle tusks were

fractured in overload, consistent with damage resulting from the divergence of the main rotor

blades from their normal plane of rotation. The droop stops were found crushed and exhibited

the effects of repeated pounding. The main rotor hub was disassembled and examined at the

Robinson Helicopter Company in Torrance, California, under the supervision of a Designated

Manufacturing Inspector Representative (DMIR) and the Safety Board. Examination of the main

rotor hub and main rotor spindles revealed multiple indentations, adjacent to where the spindle

tusks are installed, consistent with the blades traveling to the design limits in the up and down

direction (flapping). The Safety Board's probable cause of this accident was destructive mast

bumping for an undetermined reason.

22. Mt. Pleasant, Tennessee

On May 6, 1992, at 1630 central daylight time, N191KC, a Robinson R22 helicopter

owned by Kansas Copter and Wings, broke up in flight about 3 miles south of Mount Pleasant,

Tennessee. Witnesses reported hearing a loud bang, and shortly afterwards, the helicopter came

to rest in a pasture. The pilot had notified Jackson Automated Flight Service Station that he

planned to depart Jackson, Tennessee, under VFR en route to Tullahoma, Tennessee, and that he

intended to fly at 3,000 feet mean sea level (msl). The pilot then obtained a preflight weather

briefing. Visual meteorological conditions prevailed at the time of the accident. Winds near the

accident site were reported between 7 and 15 knots, gusting to 20 knots. The pilot had

accumulated 116 total flight hours, 57 of which were in the R22.

The helicopter wreckage was scattered over an area 1,500 feet long and 100 feet wide. 

Sections of the left door frame were recovered from a field 1,100 feet southwest of the main

wreckage. Examination of the left door frame revealed paint transfer and rotor blade leading

edge indentations. Examination of the wreckage revealed the main rotor shaft exhibited about a

25o bend directly below the main rotor hub, corresponding with the full downward teetering of

the main rotor hub. Both spindle tusks were sheared, and indentations and chipped primer on

either end of the hub were observed in positions corresponding to the spindles, and consistent

with the rotor blades traveling beyond their design limits in the up and down direction (flapping).

 No evidence could be found of main rotor coning. The pitch change links remained attached to

the upper swashplate assembly and exhibited bending overload separations at the upper rod end

bearings. The examination of the main rotor blades revealed blue paint transfer on one of the

blades upper surface and leading edge from 43 to 46 inches from the tip of the blade. The left

door frame, left cyclic hand grip, and left side of the lower fuselage at the aft bulkhead exhibited

damage consistent with a strike from the main rotor blade and consistent with the abrasion found

on the blade's leading edge. In addition, the left skid cross tube (located directly below the

damaged left door frame and bulkhead) exhibited damage consistent with a strike from the main

rotor blade. The examination of the cyclic and collective control system revealed no evidence of

fatigue or a preimpact failure. The Safety Board could find no evidence of the specific event that

caused or allowed the main rotor blades to diverge from their normal flightpath plane and strike
58

the airframe. The Safety Board determined that the probable cause of this accident was a

divergence of the main rotor from its normal plane of rotation for an undermined reason, which

resulted in main rotor blade contact to the cockpit area. 

23. Julia Creek, Australia

On June 15, 1992, about 0705 local time, a Robinson R22 Beta helicopter, registered in

Australia as VH-HBK, crashed near Julia Creek, Australia, after of the main rotor hub and

tailboom separated during an intended ferry flight. The pilot was killed, and the helicopter was

destroyed. The pilot held a commercial helicopter pilot certificate and had accumulated 1,035

pilot flight hours, 772 of which were in the R22 helicopter. The weather at the time and location

of the accident was reported as fine and calm.

No one witnessed the accident. When the helicopter failed to arrive at its intended

destination a search commenced. The wreckage was found the following morning close to the

intended route for the ferry flight. The Australian Bureau of Air Safety Investigation (BASI)

established that the helicopter was in cruise flight when the R22 broke apart. The main rotor hub

with blades attached was found about 140 meters from the burnt-out fuselage.

BASI established that the main rotor mast exhibited a torsional overload separation

between the swashplate and hub assembly. One of the droop stops had been subjected to a

compressive force of sufficient magnitude to fracture its elastomeric retaining strap. 

Examination of the control linkages indicated that they had failed in overload.

No known aircraft were operating in the area at the time of the accident with which the

helicopter might have conflicted. Information from other local aircraft operators indicated that

large concentrations of birds were not uncommon in the area at that time of the year. However,

not withstanding the severe fire damage to the fuselage, no evidence of a bird strike was found. 

The drive train between the engine and transmission were examined at the accident site and later

in a workshop. The inspection revealed no abnormalities or faults that could have contributed to

the accident. BASI found the following factors relevant to the in-flight separation of the main

rotor hub and subsequent crash of the helicopter: "1. For reasons which could not be

determined, a mast bump occurred during flight; and 2. The main rotor mast failed due to

torsional overload as a result of the mast bumping." 

24. Richmond, California

On June 29, 1992, at 1242 Pacific daylight time, N83858, a Robinson R22 HP helicopter,

operated by the Sierra Academy of Aeronautics, Inc., broke apart in flight during an instructional

flight near Richmond, California. Witnesses reported observing the tailboom and main rotor

separate from the helicopter in flight. A CFI was providing a primary flight lesson to his student,

who was recording the lesson (cockpit interphone and radio communications) with a
59

microcassette tape recorder. The recording revealed no operational difficulties during the engine

start, ground checks, takeoff, or the 17-minute flight en route to a practice area. The low rotor

rpm warning horn was checked and operated normally on the ground. While en route, the CFI

instructed the student to perform a left turn. According to the recording, the student completed

the turn using a shallow bank. While cruising southbound at about 2,000 feet, the CFI began

talking, but in mid-word, with no prior indication of an anomaly, an undetermined event

interrupted the CFI's speech and culminated in the breakup of the helicopter. A wind-like

background noise then became evident on the tape and muffled the student's exclamation,

"Help." The helicopter rapidly descended and crashed into San Pablo Bay, 3 miles northwest of

Richmond, California. The CFI, who had accumulated about 2,000 hours of R22 flight time, and

the student pilot were killed.

The record of the flight provided by the audiotape showed that neither pilot voiced any

concern with the operation of the helicopter before the breakup. The low rotor warning horn did

not activate before or during the breakup sequence. The Safety Board's analysis of the audiotape

revealed that during most of the flight the main rotor sound signature was measured between

17.5 and 18 Hz, equivalent to a main rotor speed of 525 to 540 rpm.36 No unusual rotor system

noises were heard before the event that resulted in the in-flight breakup. The Safety Board's

sound spectrum analysis of the audiotape indicated that the main rotor rpm did not decay before

the breakup. Analysis of the recorded primary and secondary air traffic control (ATC) radar data

supported an in-flight breakup scenario with the initial breakup occurring at 2,000 feet msl. The

helicopter's indicated airspeed (IAS) was calculated from available radar data to have been about

85 knots (in cruise flight) when the main rotor blades suddenly departed from their normal

rotational plane and struck the tailboom.

After recovery from San Pablo Bay, the wreckage was examined for evidence of possible

preimpact control system or airframe failures that might have initiated the breakup, but none

were found. No evidence was found of control interference, and the swashplate, spindle

bearings, and engine exhibited no signs of preimpact damage. The main rotor mast assembly,

with the main rotor blades attached, was recovered about 970 feet north of the main wreckage. 

The assembly had separated from the upper portion of the helicopter's transmission housing. One

main rotor blade was found curled 39o upward and both main rotor blades exhibited multiple red

paint smears that appeared to match the tailboom paint. The aft portion of the tailboom (aft of

the first bay area) was not recovered. However, a main rotor blade had left its impression in the

crushed left side of the tailboom's first bay area. Both pitch change links exhibited bending

overload failures, and the tusks were fractured from each spindle, consistent with damage

resulting from the divergence of the main rotor blades from their normal plane of rotation. This

accident was unique among other R22 in-flight loss of main rotor control accidents in that the

audio recording documented the event, and analysis of the audiotape showed that the failure

occurred with main rotor rpm in the normal R22-powered operating range. The Safety Board

 

 36 0QTOCN 444"OCKP TQVQT URGGF HQT RQYGTGF HNKIJV KU"6;7"VQ"752"TRO0
60

determined the probable cause of this accident was a divergence of the main rotor from its

normal plane of rotation for an undetermined reason, which resulted in rotor contact to the

tailboom.

25. Martinez, California

On September 30, 1992, at approximately 1445 Pacific daylight time, N8069X, a

Robinson R22 Beta, operated by Helicopter Adventures, Inc., broke up in flight during a

demonstration flight near Martinez, California. A witness reported hearing five or six popping

and thumping sounds emanating from the helicopter, and heard the engine accelerate and then

quit. The witness looked up and observed the helicopter descending in a 45o

, nose-down attitude

with the rotor blades distorted and one main rotor blade appearing more vertical than the other. 

The CFI, who was conducting an intended 30-minute demonstration flight, and the prospective

student were killed. The 28-year-old CFI had accumulated 234 total flight hours, all in the R22,

and had completed the Robinson Helicopter Company's R22 Flight Instructor Safety Course on

February 15, 1992.

Fragments of the broken windshield were found 1,800 feet from the main wreckage. An

aircraft boom microphone and part of a headset were located about 450 feet east of the accident

site. The wreckage revealed evidence of a main rotor blade strike to the right front portion of the

cockpit windshield. The forward tip of the right skid tube also displayed damage consistent with

a glancing blow by the main rotor blade. One main rotor blade exhibited abrasion and black

paint transfer 29 inches from the blade tip on the blade's leading edge that matched the paint on

the helicopter's right skid. The tailboom had also suffered a rotor strike to its left side, 53 inches

from the where it attached to the fuselage. The 2-inch deep strike contained white paint similar

to the white paint on the main rotor blade. The dimensions of the damaged area were consistent

with the size and shape of the main rotor blade leading edge. Interior inspection of the tailcone

revealed circular scoring of the tail rotor driveshaft and scratches to the interior tailcone

structure, consistent with substantial rotation and operation of the tail rotor driveshaft at impact. 

The main rotor shaft exhibited an 8o

 bend near the top of the transmission and exhibited an

overload fracture at the transmission upper cap. The base of the mast also contained an overload

fracture (360o). Indentations on either side of the hub were observed in positions corresponding

to the tusks normally attached to the main rotor spindles. The hub damage was consistent with

the rotor blades traveling beyond their design limits in the up and down direction (flapping). The

pitch change links remained attached to the upper swashplate and exhibited bending overload

separations at the upper adjustment threads. The cyclic control yoke was found bent upwards

about 45o in a "V" shape at the connection to the vertical tube. Both collective controls and

transverse torque tube remained intact and exhibited no evidence of fatigue or separation. The

Safety Board was unable to find evidence of any preimpact airframe or engine malfunction. The

Safety Board determined that the probable cause of this accident was a divergence of the main

rotor from its normal plane of rotation for undetermined reasons, which resulted in rotor contact

to the fuselage and passenger.
61

26. Wissen/Sieg, Germany

On March 28, 1993, about 1750 local time, a Robinson R22 Beta helicopter, registered in

Germany as D-HUPS and owned by the pilot, crashed near Wissen/Sieg, Germany, after the

tailboom separated in flight. The intended flight was a local personal flight from an airfield in

Siegerland, Germany. Witnesses reported they observed the helicopter flying straight and level

over a power line before it lost several parts from the structure and hit the ground in a steep

impact angle. The pilot and passenger were killed. The pilot held a private pilot certificate and

was rated in helicopters, with a total of 114 hours of flight time, all of which were in the R22. 

The weather at the time and location of the accident was reported to be winds from 070o at 10

knots, visibility greater than 10 kilometers, and clouds 4/8 in more than 5,000 ft, temperature 2o

Celsius, dewpoint -14o Celsius.

The German Accidents Investigation Bureau's examination of the wreckage revealed that

a main rotor blade had struck and severed the tailboom about 19 inches forward of the tail rotor

but was unable to determine the reason for the main rotor divergence that led to the contact with

the tailboom. The Bureau was unable to recover the tailrotor and severed section of tailboom aft

of the strike and its examination revealed no evidence of engine failure or mechanical defect. 

The accident file remains open as the German Accidents Investigation Bureau is, "hoping to learn

more about this type of accident in the R22."

27. Honolulu, Hawaii

On August 10, 1993, about 1806 Hawaiian standard time, N4017J, a Robinson R22 Beta

helicopter, crashed into the Pacific Ocean about 8 miles southeast of Honolulu, Hawaii, during an

intended pleasure flight. The airline transport pilot and his wife were killed. The pilot had

logged 4,350 total flight hours and 140 hours in helicopters, all in the R22. An endorsement in

his logbook indicated that the pilot had successfully completed the Robinson Helicopter

Company Safety Course and biennial flight review in Torrance, California, on March 12, 1993.

According to a CFI who had instructed him, the pilot was proficient with emergency procedures

in the R22. 

A witness kayaking in the ocean approximately 1/4 mile offshore indicated that the

helicopter "appeared to be operating properly when all of a sudden it went down into the water." 

Another witness located aboard a catamaran said he saw "the front rotor blades' shaft bend

toward the right side of the helicopter" and hit the helicopter body. The helicopter crashed into

the water 50 to 75 feet from the catamaran. The last recorded radar data showed N4017J at an

altitude of 500 feet above the ocean, 1/4 mile off the southeast coast of Oahu, and the radar track

indicated that it was cruising at about 90 knots just before radar contact was lost. 

Examination of the retrieved wreckage revealed that one main rotor blade was bent

downward and had entered the left forward section of the cockpit. The main rotor hub exhibited

deep gouges where the droop stop tusks contacted the hub; the droop stop tusks were sheared. 

The upper transmission and lower mast remained intact; however, the upper main rotor shaft was
62

bent approximately 30o

, consistent with an aerodynamically divergent blade striking the body of

the helicopter during powered flight. The Safety Board determined that the probable cause of

this accident was a divergence of the main rotor from its normal plane of rotation for an

undermined reason, which resulted in rotor contact with the airframe. 

28. Martin, England

On June 8, 1994, about 1139 local time, a Robinson R22 helicopter, registered in England

as G-PUDD and operated by Bizzi-B Helicopters, broke apart during an instructional flight about

1,500 feet agl near Martin, England. A witness about 1 1/4 mile from the accident site reported

that he saw the helicopter flying normally and then heard a loud noise and observed the

helicopter falling vertically to the ground with the main rotor assembly separated from the

helicopter. The instructor pilot and student were killed, and the helicopter was destroyed. The

instructor had accumulated 8,257 pilot flight hours, 7,170 of which were in helicopters and 5,200

in the R22. The helicopter student held an airline transport pilot certificate (airplane) with

approximately 4,000 hours of total flight time, including 40 hours in helicopters and 22 in the

R22. The investigation established that the helicopter was cruising at about 80 knots (nautical

miles per hour) before the accident. The main wreckage (cockpit, skid assembly, and engine)

came to rest inverted on level ground. The tailboom had separated from the fuselage, and pieces

were located 300 feet south of the main wreckage. The main rotor mast and rotor assembly had

separated at the top of the transmission and were located about 100 feet from the main wreckage.

 The Safety Board and the FAA participated in the AAIB's investigation of the accident.

Examination of the wreckage revealed that the fourth tailboom bay aft of its fuselage

attachment point was struck twice by the main rotor blades. One of the main rotor blades

exhibited red paint transfer 10 inches from its tip that matched the red "DANGER" sign where

the tailboom was struck. The blade was fractured 22 inches from the blade horn and was bowed

approximately 8 inches downward. The other main rotor blade exhibited severe bending and

twisting, and was fractured 14 inches from the blade tip. Examination of the tail rotor drive

assembly showed no indications of preimpact failure.

The main rotor gear box (transmission), main rotor mast, and main rotor assembly were

examined, but no evidence of an initiating failure was found. The transmission upper cap and

lower mast exhibited multiple overload fractures indicative of the mast rocking in flight. The

mating main rotor shaft exhibited an overload bending failure, and the upper portion of the shaft

contained a 4o

 bend directly below the main rotor hub. Physical evidence indicates that the

bending of the upper main rotor shaft occurred before the fracture of the transmission cap, and

secondary to the main rotor blades traveling beyond their normal flapping range. One side of the

upper swashplate was fractured at the outer arm, and the corresponding pitch change link was

also fractured. Examination of the recovered pieces indicated overload failures, with the arm of

one main rotor blade horn striking the failed pitch change link. An instability of the main rotor,

rocking of the mast, and extreme pitch divergence of the main rotor blades appeared to precede

all of the fractures of the main rotor flight control system. The AAIB could find no engineering
63

reason that would account for the apparent main rotor blade divergence that resulted in the strikes

of the tailboom.

29. Knightdale, North Carolina

On September 28, 1994, about 0947 eastern daylight time, N83112, a Robinson R22 Beta

helicopter operated by Raleigh Helicopters, broke apart during a business flight near Knightdale,

North Carolina. Witnesses at Raleigh East Airport, Knightdale, North Carolina, reported that the

pilot's takeoff and initial climb out at 0945 were normal; however, seconds after the departure,

the pilot of N83112 radioed in an excited voice "I've got a," and no further transmission was

received. Witnesses near the accident site stated that they observed the helicopter flying west at

an altitude between 200 and 300 feet when it appeared to "fishtail" and a sputtering sound was

heard. The helicopter was then observed to disappear into trees, followed by a fireball rising

from the area where the helicopter was last viewed. The pilot was killed, and the helicopter was

destroyed. Visual weather conditions prevailed at the time and location of the accident. The

commercial pilot had accumulated 790 total flight hours, 373 of which in helicopters and 305 in

the R22.

The main wreckage was located 1 1/2 miles west of the Raleigh East Airport and was

adjacent to Norfolk and Southern Railroad tracks. The helicopter debris was scattered over an

area 850 feet long and 100 feet wide. Portions of the fragmented windshield were found 250 feet

from the main wreckage, and pieces of the tailrotor driveshaft were located about 600 feet from

the main wreckage. The main rotor assembly, with both blades attached, was located 31 feet

north of the main wreckage. The main wreckage consisted of the impact-damaged fuselage,

engine, transmission, main rotor assembly, and skids. The wreckage exhibited severe fire and

heat damage, and the postimpact fire had partially destroyed the cockpit section of the airframe

and engine compartment. Detailed examination of the wreckage revealed that a main rotor blade

had struck the cockpit windshield and the tailboom. The main rotor blades exhibited upward

bending. Main rotor blade S/N 8262C exhibited fractures 7 feet, 3 inches from the tip of the

blade. The fractures in the blade's skin and spar were typical of overstress separations. The other

main rotor blade, S/N 8246C, exhibited red and gray paint smears on the upper skin and along

the leading edge of the blade. The paint smears matched the paint scheme on the tailboom. 

The main rotor hub and spindles were examined. The tusk for the spindle from blade S/N

8262C was fractured in overstress shear; the tusk from blade S/N 8246C remained attached to the

spindle. The hub and spindles exhibited damage consistent with the rotor blades traveling

beyond their design limits in the up and down direction (flapping). The main rotor driveshaft

exhibited an overload separation between the swashplate and main rotor hub. The pitch change

links exhibited bending overstress separations at the upper adjustment threads.

On-site and followup examination of the engine did not reveal any evidence that would

indicate a loss of power before the tailboom separation and loss of control. The Safety Board

was unable to find evidence of any preimpact airframe or engine malfunction.
64

Following the on-site investigation, pieces from the main rotor blades, transmission, tail

rotor assembly, and main rotor head were sent to the Safety Board's materials laboratory for

examination. The main rotor blades were examined for bonding between the skin and

honeycomb structure. Samples from undamaged portions of the main rotor blades were

examined and showed no evidence of adhesive separation or voids. No evidence was found to

indicate a precipitating mechanical or material failure of any helicopter system. The Safety

Board determined that the probable cause of this accident was a divergence of the main rotor

from its normal plane of rotation for undetermined reasons, which resulted in rotor contact with

the tailcone.

30. Zurich, Switzerland

On December 27, 1994, about 1434 local time, a Robinson R22 Beta helicopter,

registered in Switzerland as HB-XZW and operated by B. B. Helikopter, crashed into an

apartment building in Zurich, Switzerland, after an in-flight separation of the tailboom. The pilot

had been operating the helicopter for pleasure. Witnesses reported that they observed the

helicopter roll right and then left, and the tail structure "wig-wagged." The witnesses then heard

a loud bang and observed pieces of the tail structure separate from the structure. The helicopter

was then observed to pitch forward and fall vertically onto the upper balcony of the apartment

building. Pieces of the tailboom and tail rotor were found about 1/4 mile from the accident site. 

The private pilot and passenger were killed, and the helicopter was destroyed. The Safety Board

and FAA participated in the Swiss AAIB's investigation of the accident.

The pilot's experience included 91 flight hours, all in helicopters, with 30 hours in the

R22. The pilot had received his type rating in the R22 on December 17, 1994, and had

accumulated 5 hours in the R22, 2 weeks before the accident. Zurich ATC had cleared HB-XZW

to the Katzensee VFR check point on the pilots' approach to Zurich airport. Before the crash,

witnesses observed the helicopter in level flight at about 1,000 feet agl, and stated that the

helicopter's engine sounded normal. Radar data indicate that the helicopter was traveling at

approximately 80 knots before the event. Winds at Zurich, at the time of the accident, were

reported from 250o, at 18 knots, gusting to 36 knots. The pilot had acquired a weather report

before his flight that indicated that winds were 12 knots; however, the report was a general

weather report for all of Switzerland, and not specific to Zurich.

Examination of the wreckage revealed that one of the main rotor blades exhibited red

paint transfer that matched the color of the tailboom, 1 inch from the blade tip and extending 52

inches inboard along the leading edge of the blade. The tailboom exhibited compression of bays

3-5 and a swipe on the right side of the boom, which resulted in missing and chipped red paint. 

The spindle tusks were fractured and exhibited overload separation fractures. Both pitch change

links were fractured at the upper adjustment threads, and one arm of the stationary swash plate

was fractured; however, the corresponding pitch change link remained attached to the fractured

arm. The pitch change link and swashplate arm fractures exhibited overload separations, and no

evidence of fatigue. A preliminary examination of the engine revealed that the flywheel exhibited
65

damage to the teeth consistent with the engine operating at impact. The governor switch (located

on the pilot's collective control) was in the "on" position, and the cyclic control was fractured at

the inboard side of the pilot's "T" handle control. Examination of the helicopter's control tubes

revealed no evidence of preimpact failure or fatigue. Small specimens of bird feathers were

found at two remote locations on the main rotor blade, which exhibited red paint transfer, and on

the engine casing. No other evidence of bird remains or bird blood was found adjacent to the

feathers or on any other location of the helicopter. In addition, there was no report of birds in the

vicinity at the time of the accident. The reason for the main rotor divergence that led to the

contact with the tailboom has not been determined and the investigation of the accident is

continuing. The Safety Board received the Swiss AAIB's draft factual report on April 2, 1996.

31. Brighton Downs Station, Australia

On July 17, 1995, about 1950 local time, a Robinson R22 Beta helicopter, registered in

Australia as VH-BEI, crashed near Brighton Downs Station, Australia, after one of the main rotor

blade contacted the cockpit and tailboom in flight. The flight had originated about 185 nautical

miles south-east of the accident site near Headingly Station, Australia. The pilot was positioning

the helicopter for cattle herding to be accomplished the following day. The pilot was killed, and

the helicopter was destroyed. The pilot held a helicopter pilot certificate and had accumulated

794 pilot flight hours, 792 of which were in the R22 helicopter. The weather at the time and

location of the accident was reported as strong gusting winds from the south/south east with wide

spread clouds.

The Australian Bureau of Air Safety Investigation (BASI) found the R22 helicopter

wreckage scattered over 1,000 meters and the tailboom was found 300 meters from the cabin. 

The tailboom exhibited evidence that it had been struck by a main rotor blade. The cabin landed

right side up and was compressed from impact forces to about a meter in height. The rotor

system and main rotor blades separated from the transmission, and were found next to the cabin.

 The Australian BASI is continuing their investigation of the accident. The reason for the

main rotor blade divergence has not been determined. 
66

APPENDIX B-Summary Reports of R44 Loss of Main Rotor Control Accidents

Case Registration Robinson NTSB

 No. Date Location No. Serial No. Accident No.

 1. 04-02-94 Hanover, Germany D-HTOP 0013

 2. 12-08-94 Speyer, Germany D-HPHS 0107 DCA95RA005

 3. 05-08-95 Riesa, Germany D-HFSD 0101 DCA95RA034
67

1. Hanover, Germany

On April 2, 1994, about 1345 local time, a Robinson R44 helicopter, registered in

Germany as D-HTOP, crashed about 8 miles east of Hanover, Germany, during an intended

pleasure flight. The private pilot and his wife were killed. The pilot was qualified in fixed-wing

airplanes and helicopters. His total flight experience was not known, but he had logged 110

hours of R22 flight time. This was the pilot's first unsupervised flight after receiving more than 5

hours of R44 instruction and his R44 type-rating checkout. The Safety Board and the FAA

participated in the German Accidents Investigation Bureau's investigation of the accident.

The investigation revealed that the main rotor blades struck the cockpit area of the

fuselage. The evidence indicates that the helicopter yawed sharply due to the blade strike, and

the structure of the tailboom wrinkled and then failed, resulting in separation of the tailboom. 

The main rotor mast shows evidence of being bumped by the main rotor blades, and the main

rotor system separated from the helicopter. No precipitating mechanical failure of the helicopter

was found. The investigation did not determine the reason for the main rotor blade divergence

that resulted in the rotor striking the body of the helicopter during powered flight.37

2. Speyer, Germany

On December 8, 1994, about 1405 local time, a Robinson R44 helicopter, registered in

Germany as D-HPHS and operated by Luftfahrt-Geseltschaft-Mannheim, broke apart during an

instructional flight about 2,000 feet agl, near Speyer, Germany. The flight was intended to be a

continuation of the second pilot's R44 type-rating training.38 Witnesses near the accident site

reported that they heard a loud noise and observed the helicopter falling to the ground with parts

of the helicopter separating from the structure as it fell. The instructor pilot and student were

killed, and the helicopter was destroyed. The instructor had accumulated 2,885 pilot flight hours

in helicopters, 123 of which were in the R44. The R44 student held a commercial pilot

certificate (airplane and helicopter) with flight time in the smaller, but similar, Robinson R22 and

several hours in the R44. The Safety Board and the FAA are participating in the German

Accidents Investigation Bureau's continuing investigation of the accident.

 Radar data and the history of flight indicate that the helicopter was cruising about 80

knots (nautical miles per hour) before the accident. The main wreckage (cockpit, skid assembly,

and engine) came to rest inverted on level ground. The tailboom had separated from the

fuselage, and pieces were located 1,400 feet north of the main wreckage. The main rotor mast and

 

 37 For more detailed information, refer to the German FUS Accident File 3x047-94.

 38 German regulations require that pilots obtain a minimum of 5 hours of flight time in the specific model

before acting as pilot-in-command.
68

rotor assembly remained attached to the transmission assembly. One main rotor blade had broken

chordwise, approximately 2 feet from the root, and the outer portion of the blade was located

about 1,200 feet south of the main wreckage.

Examination of the wreckage revealed that a main rotor blade had struck the front cockpit

structure of the helicopter and that the other main rotor blade had struck the second tailboom bay

causing the tailboom in the fourth tailboom bay aft of the fuselage to separate. One of the main

rotor blades exhibited scoring that matched the windshield attachment screws of the center

support in the nose of the fuselage. The other main rotor blade exhibited scoring that matched a

row of similarly scored rivets on the left side of the tailboom. One main rotor blade was

fractured about 2 feet from the blade horn and was found 1,400 feet from where the fuselage

came to rest. The other main rotor blade exhibited severe bending and twisting, and was

fractured in several places. Examination of the tail rotor drive assembly showed no indications

of preimpact failure.

The main rotor gear box (transmission), main rotor mast, and main rotor assembly were

examined. The main rotor shaft exhibited evidence of mast bumping but no evidence of an

initiating material failure was found. The evidence indicates that the mast bumping occurred

secondary to the main rotor blades traveling beyond their normal flapping range. The

transmission upper cap and entire mast assembly were integral to the transmission and helicopter

structure. Both sides of the upper swashplate were fractured at the outer arms, and the

corresponding pitch change links were also fractured. Examination of the recovered pieces of

pitch change links indicated overload failures. The structural damage of the Plexiglas and

cockpit structure indicated low blade momentum during the in-flight strike. An instability of the

main rotor, rocking of the mast, and extreme pitch divergence of at least one of the main rotor

blades appeared to precede all of the fractures of the main rotor flight control system. The reason

for the main rotor pitch divergence has not been determined.

3. Riesa, Germany

On May 8, 1995, about 1745 local time, a Robinson R44, registered in Germany as

D-HFSD, and operated by Herkules-Flugservice GmbH, experienced an in-flight separation of

the main rotor during a familiarization flight 50 kilometers west of Riesa, Germany. The pilot

occupying the left seat had completed the mandated R44 awareness training 1 week before this

flight. Witnesses near the accident site reported they heard a loud noise and observed parts of the

helicopter separate in flight before the helicopter crashed into a level plowed field. The

instructor helicopter pilot and three other commercial helicopter pilots were killed, and the

helicopter was destroyed. The pilot-in-command had accumulated approximately 10,000 hours

in helicopters, 115 hours of which were in the R44, and 52 in the R22. The right seat pilot held a

commercial helicopter pilot certificate and had accumulated about 1,850 hours in helicopters, 6

hours of which were in R22 helicopter, and 1,800 in the MI-8 helicopter. He did not have

previous R44 experience. Both passengers held commercial helicopter certificates and were
69

experienced with the Robinson helicopters. The Safety Board and the FAA are participated in

the German Accidents Investigation Bureau's investigation of the accident.

The flight had originated at Kassel Calden, home base of the operator, and flew to

Bradenburg and Nardt where the helicopter landed safely. The pilot had reported his intention to

land at the Riesa Airport at 1815 local time for fuel.

The main wreckage (cockpit shell, skid assembly, engine, and forward tailboom) came to

rest on its left side on level ground. The aft tailboom assembly came to rest about 30 meters

from the main wreckage. The main rotor had separated at the upper main rotor shaft and was

located 180 meters west of the main wreckage. Several pieces of the instrument panel and the

Plexiglas windscreen were located close to the main rotor assembly.

Examination of the wreckage revealed that a main rotor blade had struck the right side of

the cockpit windscreen centerpost and had sliced through the instrument panel and the lower left

side of the cockpit. One of the main rotor blades had separated in three sections and exhibited

scoring and blue paint transfer that matched the color of the paint on the helicopter and a row of

similarly scored rivets on the cabin centerpost. The lower left side of the fuselage structure was

severed 15 inches below the left forward door frame and from the nose of the cockpit to the

forward crosstube of the helicopter skid assembly, along the inboard side of the left passenger

seat. The left outboard rudder pedal and left cyclic arm also exhibited contact by the main rotor

blade.

Examination of the main rotor hub assembly revealed damage corresponding to a main rotor

blade pitch horn. The corresponding main rotor pitch horn exhibited severe scoring on the interior

surface of the horn. Its pitch change link was fractured in tension overload at the upper adjustment

threads. The upper main rotor hub revealed counter-clockwise smearing on both sides of the upper

hub where the blade horn had contacted the hub, as the main rotor blade rotated a minimum of 180o

about the hub. The main rotor blade's spindle tusk was found intact; however, the other main rotor

blade's spindle had fractured in overload at the base of tusk, and the hub exhibited a gouge where

the tusk had fractured as it gouged into the hub. The other main rotor blade horn was fractured at

the blade collar, and the upper hub exhibited severe gouges that corresponded to the main rotor

blade horn pivoting about the hub, resulting in a fracture of the blade horn. The droop stop

retaining bolt was also sheared. The main rotor mast exhibited a torsional overload failure 11

inches from the teetering hinge. One-sixteenth-inch deep indentations were observed on both sides

of the upper shaft 3 1/2 inches below the center of the teetering hinge, which corresponded to the

location of the spindles if they had traveled excessively and bumped the shaft. The structure that

houses the main rotor mast exhibited bending on the left (as viewed from aft looking forward) at

the point of separation of the main rotor shaft.

Examination of the flight control system revealed overload failures of the support tubing

for the jack shaft, the forward cyclic bracket, and the forward bellcrank integral to the rudder

pedal controls. All other control tubes showed impact damage but remained intact and attached

to their respective components. The examination of the main rotor gear box (transmission)
70

revealed no evidence of damage or preimpact failure. The examination of the cooling fan and

alternator revealed circular scoring consistent with the engine operating at impact. The

examination of the tail rotor blades revealed fragments of the helicopter's Plexiglas windscreen

imbedded in the aluminum honeycomb core of the blade.

On May 12, 1995, the German Accidents Investigation Bureau issued safety

recommendation 3X114-0/95 to the Luftfahrt-Bundesamt (LBA), the aviation regulatory agency

of Germany, to stop the operation of all R44 helicopters that are certified in Germany. After

discussions with the FAA, the LBA placed limitations on the R22 and R44 similar to those

placed by the FAA in early 1995.
71

APPENDIX C-Main Rotor Hub Teeter Angle and Rpm Decay Survey



75

National Transportation Safety Board

Washington, D.C. 20594

December 12, 1995

Frank Robinson

President

Robinson Helicopter Company

2901 Airport Drive

Torrance, California 90505

Dear Mr. Robinson:

As you know, the National Transportation Safety Board (NTSB)

is performing a special investigation on the Robinson R22

helicopters and will be issuing a report detailing its findings.

We plan to include in that report two tables from the October 26,

1982, Main Rotor Hub Teeter Angle and RPM Decav Survey, RTR-073,

submitted by your company to the Federal Aviation Administration

(FAA) in conjunction with the R22 certification process.

Specifically, the tables that we plan to include in the

report are Table V., Summary of Level Flight Conditions, page 10,

and Table XI., Azimuth Location of Maximum Teeter Angle, page 16.

Copies are enclosed for your convenience. This information is

needed to adequately explain several steady state and flight

control positions.

Included on the cover page of the Main Rotor Hub Teeter

Angle and RPM Decav Survey is a notation stating that the

document contains proprietary technical and commercial trade

secrets. Therefore, we are now providing Robinson Helicopter

with an opportunity to comment on the inclusion of this

information in the NTSB report. We would appreciate a response

within ten days and will give it careful consideration.

Thank you for your cooperation.

Sincerely,

!John B. Drake

Chief, Aviation

Engineering Division

Enclosures
76

ROBINSO N

December19, 1995

NATIONAL TRANSPORTATION SAFETY BOARD

Attn: John B. Drake

Chief, Aviation Engineering Division

Washington, DC 20594

Dear Mr. Drake:

Robinson Helicopter Company does not object to the NTSB

including Table V and Table XI from RTR 073 in the forthcoming NTSB

report on the R22 helicopter.

If we can be of any further assistance in this regard, please

let me know.

Yours truly,

ROBINSON HELICOPTER COMPANY

/--$ :;~ ,,Gm,

/’F&anb)fiobinsQn

FDR: map

President /

,
77

APPENDIX D-Applicable Safety Recommendations and Related Correspondence

%

National Transportation Safety Board

Washington, D.C. 20594

Safety Recommendation

Date: July 21, 1994

In reply refer to: A-94-143 through -145

Honorable David R. Hinson

Administrator

Federal Aviation Administration

Washington, D. C. 20591

On June 8, 1994, about 1139 local time, a Robinson R22 helicopter, registered in

England as G-PUDD and operated by Bizzi-B Helicopters, broke apart during an instructional

flight about 1,500 feet above ground level, near Martin, England. A witness about 1 1/4 mile

from the accident site reported that he saw the helicopter flying normalIy and then heard a loud

noise and observed the helicopter falling vertically to the ground with the main rotor assembly

separated from the helicopter. The instructor pilot and student were fatally injured and the

helicopter was destroyed. The instructor had accumulated 8,400 pilot flight hours, of which

5,200 hours were in the R22. The helicopter student held an airline transport pilot certificate

(airplane) with approximately 4,000 hours of total flight time, including 22 hours in the R22.

The investigation established that the helicopter was cruising at about 80 knots (nautical miles

per hour) before the accident. The main wreckage (cockpit, skid assembly, and engine) came

to rest inverted on level ground. The tailboom had separated from the fuselage and pieces were

located 300 feet south of the main wreckage. The main rotor mast and rotor assembly had

separated at the top of the transmission and were located about 100 feet from the main

wreckage. The Safety Board and the Federal Aviation Administration (FAA) participated in the

U. K. Air Accidents Investigation Board’s investigation of the accident.

Examination of the wreckage revealed that the fourth tailboom bay aft of its fuselage

attachment point was struck twice by the main rotor blades. One of the main rotor blades

exhibited red paint transfer 10 inches from its tip that matched the red “DANGER” sign where

the tailboom was struck. The blade was fractured 22 inches from the blade horn and was bowed

approximately 8 inches downward. The other main rotor blade exhibited severe bending and

twisting, and was fractured 14 inches from the blade tip. Examination of the tail rotor drive

assembly showed no indications of preimpact failure.

The main rotor gear box (transmission), main rotor mast, and main rotor assembly were

examined but no evidence of an initiating failure was found. The transmission upper cap and

lower mast exhibited multiple overload fractures indicative of the mast rocking in flight. The

6405
78

mating main rotor shaft exhibited an overload bending failure, and the upper portion of the shaft

contained a 4° bend directly below the main rotor hub. Physical evidence indicates that the

bending of the upper main rotor shaft occurred before the fracture of the transmission cap, and

secondary to the main rotor blades traveling beyond their normal flapping range. One side of

the upper swashplate was fractured at the outer arm and the corresponding pitch change link was

also fractured. Examination of the recovered pieces indicated overload failures, with the arm

of one main rotor blade horn striking the failed pitch change link. An instability of the main

rotor, rocking of the mast, and extreme pitch divergence of the main rotor blades appeared to

precede all of the fractures of the main rotor flight control system. The reason for the main

rotor pitch divergence has not been determined and the investigation of the accident is

continuing.

On August 10, 1993, about 1806 Hawaiian standard time, a Robinson R22 helicopter,

N4017J, crashed into the Pacific Ocean about 8 miles southeast of Honolulu, Hawaii, during an

intended pleasure flight. The airline transport pilot and his wife received fatal injuries. The

pilot had logged 4,350 total flight hours and 140 hours of R22 flight time. An endorsement in

his logbook indicated that the pilot had successfully completed the Robinson Helicopter Company

Safety Course and biennial flight review in Torrance, California, on March 12, 1993.

According to a certified flight instructor (CFI) who had instructed him, the pilot was proficient

with emergency procedures in the R22. A witness kayaking in the ocean approximately 1/4

mile offshore indicated that the helicopter “appeared to be operating properly when all of a

sudden it went down into the water. ” Another witness located aboard a catamaran said he saw

“the front rotor blades’ shaft bend toward the right side of the helicopter” and hit the helicopter

body. The helicopter crashed into the water 50 to 75 feet from the catamaran. The last

recorded radar data showed N4017J at an altitude of 500 feet above the ocean, 1/4 mile off the

southeast coast of Oahu, and the radar track indicated that it was cruising at about 90 knots just

before radar contact was lost. Examination of the retrieved wreckage revealed that one main

rotor blade was bent downward and had entered the left forward section of the cockpit. The

main rotor hub exhibited deep gouges where the droop stop tusks contacted the hub; the droop

stop tusks were sheared. The upper transmission and lower mast remained intact; however, the

upper main rotor shaft was bent approximately 30°, consistent with an aerodynamically

divergent blade striking the body of the helicopter during powered flight. The Safety Board was

unable to establish the exact cause of the main rotor blade divergence. 1

On June 29, 1992, at 1242 Pacific daylight time, a Robinson R22 helicopter, N83858,

operated by the Sierra Academy of Aeronautics, Inc., experienced an in-flight breakup during

an instructional flight near Richmond, California. Witnesses reported observing the tailboom

and main rotor separate from the helicopter in flight. A CFI was providing a primary flight

lesson to his student, who was recording the lesson (cockpit interphone and radio

communications) with a microcassette tape recorder. The recording revealed no operational

difficulties during the engine start, ground checks, takeoff, or the 17-minute flight en route to

a practice area. The low rotor revolutions per minute (rpm) warning horn was checked and

1For more detailed information, read Brief of Accident File #1420 (attached).
79

operated normally on the ground. While en route, the CFI instructed the student to perform a

left turn. According to the recording, the student completed the turn using a shallow bank.

While cruising southbound at about 2,000 feet, the CFI began talking, but in mid-word, with

no prior indication of an anomaly, an undetermined event interrupted the CFI’s speech and

culminated in the breakup of the helicopter. A wind-Iike background noise then became evident

on the tape and muffled the student’s exclamation, “Help. ” The helicopter rapidly descended

and crashed into San Pablo Bay, 3 miles northwest of Richmond, California. The CFI, who had

accumulated about 2,000 hours of R22 flight time, and the student pilot were fatally injured.

The record of the flight provided by the audiotape showed that neither pilot voiced any

concern with the operation of the helicopter before the breakup. The low rotor warning horn

did not activate before or during the breakup sequence. The Safety Board’s analysis of the

audiotape revealed that during most of the flight the main rotor sound signature was measured

between 17.5 Hz and 18 Hz, equivalent to a main rotor speed of 525 to 540 rpm.2 No unusual

rotor system noises were heard before the event that resulted in the in-flight breakup. The

Safety Board’s sound spectrum analysis of the audiotape indicated that the main rotor rpm did

not decay before the breakup. Analysis of the recorded primary and secondary air traffic control

(ATC) radar data supported an in-flight breakup scenario with the initial breakup occurring at

2,000 feet mean sea level (msl). The helicopter’s indicated airspeed (IAS) was calculated from

available radar data to have been about 85 knots in level, cruise flight when the main rotor

blades suddenly departed from their normaI rotational plane and impacted the tailboom.

After recovery from San Pablo Bay, the wreckage was examined for evidence of possible

preimpact control system or airframe failures that might have initiated the breakup, but none

were found. No evidence was found of control interference, and the swashplate, spindle

bearings, and engine exhibited no signs of preimpact damage. The main rotor mast assembly,

with the main rotor blades attached, was recovered about 970 feet north of the main wreckage.

The assembly had separated from the upper portion of the helicopter’s transmission housing.

One main rotor blade was found curled 39°upward and both main rotor blades exhibited multiple

red paint smears that appeared to match the tailboom paint. The aft portion of the tailboom (aft

of the first bay area) was not recovered. However, a main rotor blade had left its impression

in the crushed left side of the tailboom’s first bay area. Both pitch change links exhibited

bending overload failures and the tusks were fractured from each spindle, consistent with damage

resulting from the divergence of the main rotor blades from their normal plane of rotation. This

accident was unique among other R22 in-flight loss of main rotor control accidents in that the

audio recording documented the event, and analysis of the audiotape showed that the failure

occurred with main rotor rpm in the normal R22-powered operating range. The Safety Board

could find no evidence of the specific event that caused or allowed the main rotor blades to

diverge from their normal flightpath plane and strike the airframe.3

2Normal R22 main rotor speed for powered flight is 495 to 530 rpm.

3For more detailed information, read Brief of Accident File #1003 (attached).
80

In the three accidents described above, the in-flight breakups occurred while the

helicopters were being operated at cruise speeds well within the aircraft’s defined operating

envelope. In all cases the pilots were experienced and the investigation indicates that they had

been adequately trained in the R22. The Safety Board has found no evidence that the pilots were

improperly operating the helicopters. In addition to these three accidents, the Safety Board has

investigated 18 others that have occurred since 1981 involving an in-flight breakup of an R22

helicopter. In all of these, the breakup occurred when the main rotor blades diverged from their

normal plane of rotation and struck the airframe.

The R22 main rotor system is unique. The two-bladed, semi-rigid main rotor system

includes rotor blades that are connected to the main rotor hub through coning (flapping) hinges.4

The main rotor hub is connected to the main rotor shaft (mast) through an additional hinge so

that the hub teeters with influence from main rotor blade movement. In other two-bladed, semirigid systems, the advancing blade flaps up, causing the retreating blade to flap down; however,

each R22 main rotor blade flaps independently of the other blade’s vertical movement. The

chord and diameter of the main rotor blades measure 7 inches and 25 feet, 2 inches,

respectively, and each blade weighs approximately 26 pounds. The main rotor rpm is much

higher, and the rotor inertia is very low by comparison to other two-bladed rotor systems.

When in forward flight, the dynamic speed of the air over the rotor blade is the

rotational speed of the blade algebraically added to airspeed. Thus, the airflow over the

advancing blade is greater than the airflow over the retreating blade, and at a given pitch the

rotor would create asymmetrical lift. To compensate, the lift generated by the advancing blade

results in movement of the teetering hinge and tilting of the main rotor hub, such that the angleof-attack (AOA) of the advancing main rotor blade is reduced and the AOA of the retreating

blade is increased to balance the lift in the rotational plane. Thus, as the helicopter’s forward

airspeed increases, the advancing blade’s AOA decreases as the retreating blade’s AOA

increases. However, if the AOA on the retreating blade exceeds the critical AOA, the blade will

stall (retreating blade stall). The combination of large changes in the AOA of the main rotor

blades, high forward airspeed, and high gross weight (high gross weight requires more lift,

which increases the AOA of the main rotor blades) creates instabilities in the main rotor system

as the retreating blade becomes stalled. The Safety Board is concerned that these instabilities

are a potential contributing cause of blade divergence. Other aerodynamic characteristics (Mach

tuck, drag divergence, pitch moment oscillations, and negative blade damping) also could have

devastating effects on a low-inertia, high rpm rotor system. Therefore, the Safety Board is

concerned that adequate testing may not have been accomplished to resolve any potential adverse

aerodynamic characteristics of the rotor system.

The Safety Board is aware of other potential blade characteristics that this design would

be likely to encounter. The construction of the R22 main rotor blade is unlike most other

4Coming is the upward bending of the blades caused by the resultant forces of lift and

centrifugal force. Flapping is the vertical movement of the blade as a result of aerodynamic forces.
helicopter blades in that there is no mid-chord shear web. The main rotor blade is constructed

with a leading edge stainless steel D-shaped bar (spar), which is also designed to be the loadcarrying structure. The honeycomb and blade skin is adhesively attached to the leading edge

spar. The Safety Board is not aware of any wind tunnel testing using this blade design. The R22

main rotor blade was modified shortly after certification with weights in each main rotor blade

tip. The weight was designed to improve the low inertia problem and aid in autorotational

landings.

The R22 main rotor rpm will rapidly decay following a loss of power. The Robinson

Helicopter Company has reported to Safety Board staff that it attributes most R22 loss of main

rotor control accidents to pilot-induced low rotor rpm, or low-G maneuvering. The following

physical evidence refutes these theories: In all three of the above accidents, there was physical

evidence of main rotor blade strikes to the tailboom or cockpit under substantial operating power;

the overload fractures of the spindles, pitch change links, transmission cap, and bending of the

upper main rotor shaft all indicate that significant force was required to cause this damage; and

the location and angle at which the strikes occurred revealed that the blade was not at its normal

plane of rotation at the time of the strike to the helicopter body.

The Safety Board is aware that the R22 has demonstrated compliance with the certification

requirements and that previous certification reviews have not uncovered evidence of

noncompliance with certification standards or of a deficiency that would explain accidents such

as those discussed above. However, because of the violent nature of the accidents and the

evidence of possible main rotor involvement, the Safety Board believes that the FAA should, in

conjunction with the National Aeronautics and Space Administration (NASA) and Robinson

Helicopter Company, conduct further testing to evaluate the R22 main rotor and control system.

The testing should include wind tunnel and computer modeling to evaluate the main rotor design,

main rotor performance in cruise flight, rotor stability and other possible areas in which main

rotor divergence or instabilities may have occurred on accident flights. The Safety Board is

concerned that the unique design of the R22 may result in flight characteristics that are not

adequately addressed by Title 14 Code of Federal Regulations (CFR) Part 27 standards. The

Safety Board is concerned that the R22 main rotor control system may allow flight characteristics

that were not flight or ground-tested under 14 CFR Part 27 standards, allowing anomalies in the

main rotor system to go undetected during the original certification process.

Because the Richmond, California, accident occurred abruptly and with no apparent

warning to the flightcrew, it was of particular concern to the Safety Board. That accident and

the 20 other similar R22 in-flight breakup accidents examined by the Safety Board indicated that

there may be undesirable aerodynamic characteristics of R22 main rotor blades that can result

in one or both blades diverging from their normal plane of rotation (see Appendix A for the list

of accidents). The Safety Board is concerned that the stability of the R22 main rotor blades is

compromised by an inherent rotor system design deficiency that may allow loss of control of the

rotor system when operating the helicopter within the currently defined flight envelope and in a

manner that would seem normal in other light helicopters. The Safety Board is aware of the
82

importance of the R22 as a training and Iight utility helicopter. However, until the cause of the

accidents, like those cited above, is determined, the flight envelope should be restricted.

In each of the in-flight breakups described above, the helicopter was being operated at

a speed close to that recommended for cruise. The R22 flight manual indicates 83 knots as

maximum range airspeed, and the Robinson Helicopter Training Manual specifies 75 knots as

the recommended cruise speed. The FAA-approved never exceed airspeed (Vne) is 102 knots.

The Safety Board believes that, as an interim measure, while the cause of the in-flight breakup

accidents is being determined, the maximum R22 operating speed should be reduced to a speed

lower than the cruise speeds at which the accidents have occurred in the past.

The Safety Board has paid particular attention to the R22 main rotor blades and the rotor

head because its special investigation has revealed that the in-flight breakup accidents were more

likely caused by failures that initiated at the main rotor, rather than in the transmission, its

mounts, or the main rotor control system. Because of its investigative findings, the Safety Board

requested Material Review Records (MRRs) for the main rotor blades involved in the accidents

but has not yet received those records. The Safety Board’s review of an MRR of rotor blades

not involved in an accident caused the Board to become concerned with the disposition and

subsequent approval of blades containing defects, as illustrated by that MRR. The MRR

examined showed that the Designated Engineering Representative (DER) employed by the

Robinson Helicopter Company approved the use of main rotor blades for use on new helicopters

when those blades did not pass design inspection requirements. The proper design, manufacture,

testing, and approval of main rotor blades are crucial to the airworthiness of a helicopter.

Defects in main rotor blades should be carefully examined and any blade not meeting the

originaI design limits should be rejected. The Safety Board does not know the circumstances

under which the approval was granted by the DER but is concerned about the appearance of the

action.

A DER is the quality assurance link between the FAA and the manufacturer. The Safety

Board believes that to ensure product integrity and safety during the design and development of

an aircraft, the FAA must closely monitor the manufacturing process. The DER has the

authority, granted by the FAA, to approve deviations during the manufacturing of a component

that will be installed on an aircraft. The Safety Board was concerned to learn that the only

FAA-designated DER currently at the Robinson Helicopter Company was also the president of

the company. (A previously assigned DER left the company on September 3, 1993, and has not

been replaced.) The Safety Board is concerned that the potential exists for any senior company

officer, especially its president, to have a conflict of interest that could influence the

performance of his or her duties as a DER. The president of any company has a financial

interest in the success of the company and has other duties that could conflict with his or her

responsibilities as a DER. Therefore, the Safety Board believes that it is essential that the FAA

promptly review the appointment of any DER who is both a senior company officer and a DER.
83

Therefore, the National Transportation Safety Board recommends that the Federal

Aviation Administration:

Issue an immediate airworthiness directive to reduce the Robinson R22 helicopter

“never exceed airspeed” (Vne) to an airspeed that would provide an adequate

margin of operating safety below the airspeeds at which loss of main rotor control

accidents have occurred, until the reason for in-flight main rotor blade divergent

behavior is established and design changes are approved and implemented, as

necessary. (Class I, Urgent Action) (A-94-143)

In conjunction with the National Aeronautics and Space Administration and

Robinson Helicopter Company, conduct wind tunnel and modeling tests to

examine flight parameters of the R22 helicopter to determine the helicopter’s

design characteristics that are related to main rotor divergent behavior; and if any

abnormal rotor system performance characteristics are found, take the necessary

actions to assure proper dissemination of the information and to modify the R22

design. (Class I, Urgent Action) (A-94-144)

Examine the appropriateness of the Designated Engineering Representative (DER)

assignment at the Robinson Helicopter Company and at other small manufacturers

where senior executives are assigned DER responsibilities, and take necessary

actions to eliminate any conflict of interest with DER responsibilities. (Class II,

Priority Action) (A-94-145)

Acting Chairman HALL, and Members LAUBER, HAMMERSCHMIDT, and VOGT

concurred in these recommendations.

4ii!53

Appendix A, R22 Loss of Main Rotor Control Accidents
84

Date

11-11-81

09-25-82

10-06-82

12-25-84

05-05-85

03-22-86

05-10-86

03-16-87

06-03-87

11-03-87

11-23-90

07-05-91

09-23-91

01-30-92

03-04-92

05-06-92

05-08-92

06-29-92

09-30-92

08-10-93

06-08-94

APPENDIX A

R22 Loss of Main Rotor Control Accidents

Location

Livermore, CA

Nashville, TN

Santa Ana, CA

Huntsville, AL

San Angelo, TX

Memphis, TN

E. Fishkill, NY

Scottsdale, AZ

S. Windsor, CT

Moraga, CA

Simi Valley, CA

Phoenix, AZ

Point Judith, RI

Malabar, FL

Maricopa, AZ

Mt. Pleasant, TN

Anaheim, CA

Richmond, CA

Martinez, CA

Honolulu, HI

Martin, England

Registration No.

N9073Q

N9072V

N8358B

N8475K

N83745

N9069S

N8511Z

N2256M

N2287L

N8475A

N80783

N23039

N950CW

N2313G

N8413Q

N191KC

N8064E

N83858

N8069X

N4017J

G-PUDD

Robinson

Serial No.

0227

0212

0302

0391

0320

0181

0415

0498

0512M

0389

1319

1846

1637

2015

0354

1818

1264

0337

1364

1443

0863

NTSB

Accident No.

LAX82FA012

ATL82FA285

LAX83FUA01

ATL85FA067

FTW85FA207

ATL86FA097

NYC86FA127

LAX87FA147

NYC87FA160

LAX88FA032

LAX91FA037

LAX91FA288

NYC91FA254

MIA92FA072

LAX92FA 137

ATL92FA096

LAX92FA206

LAX92FA267

LAX92FA41O

LAX93FA318

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89

National Transportation Safety Board

z Washing-ton, D.C. 20594

Safety Recommendation

Date: January 6, 1995

Inreply refer to: A-95-1 through-8

Honorable David R. Hinson

Administrator

Federal Aviation Administration

Washington, D.C. 20591

On December 8, 1994, about 1405 local time, a Robinson R44 helicopter, registered in

Germany as D-HPHS and operated by Luftfahrt-Geseltschaft-Mannheim, broke apart during an

instructional flight about 2,000 feet above ground level, near Speyer, Germany. The flight was

intended to be a continuation of the second pilot’s R44 type-rating training.1 Witnesses near the

accident site reported that they heard a Ioud noise and observed the helicopter falling to the

ground with parts of the helicopter separating from the structure as it fell. The instructor pilot

and student were fatally injured, and the helicopter was destroyed. The instructor had

accumulated 2,885 pilot flight hours in helicopters, 123 hours of which were in the R44. The

R44 student held a commercial pilot certificate (airplane and helicopter) with flight time in the

smaller, but similar, Robinson R22 and several hours in the R44. The National Transportation

Safety Board and the Federal Aviation Administration (FAA) are participating in the German

Flugenfalluntersuchungsstelle (FUS) Accidents Investigation Board’s continuing investigation of

the accident.

Radar data and the history of flight indicate that the helicopter was cruising about 80

knots (nautical miles per hour) before the accident. The main wreckage (cockpit, skid assembly,

and engine) came to rest inverted on level ground. The tailboom had separated from the

fuselage, and pieces were located 1,400 feet north of the main wreckage. The main rotor mast

and rotor assembly remained attached to the transmission assembly. One main rotor blade had

broken chordwise, approximately 2 feet from the root, and the outer portion of the blade was

located about 1,200 feet south of the main wreckage.

Examination of the wreckage revealed that a main rotor blade had struck the front cockpit

structure of the helicopter and that the other main rotor blade had struck the second tailboom

bay causing the tailboom in the fourth tailboom bay aft of the fuselage to separate. One of the

1German regulations require that pilots obtain a minimum of 5 hours of flight time in the specific model before

acting as pilot-in-command.
90

main rotor blades exhibited scoring that matched the windshield attachment screws of the center

support in the nose of the fuselage. The other main rotor Made exhibited scoring that matched

a row of similarly scored rivets on the left side of the tailboom. One main rotor blade was

fractured about 2 feet from the blade horn and was found 1,400 feet from where the fuselage

came to rest. The other main rotor blade exhibited severe bending and twisting, and was

fractured in several places. Examination of the tail rotor drive assembly showed no indications

of preimpact failure.

The main rotor gear box (transmission), main rotor mast, and main rotor assembly were

examined. The main rotor shaft exhibited evidence of mast bumping but no evidence of an

initiating material failure was found. The evidence indicates that the mast bumping occurred

secondary to the main rotor blades traveling beyond their normal flapping range. The

transmission upper cap and entire mast assembly were integral to the transmission and helicopter

structure. Both sides of the upper swashplate were fractured at the outer arms, and the

corresponding pitch change links were also fractured. Examination of the recovered pieces of

pitch change links indicated overload failures. The structural damage of the plexiglass and

cockpit structure indicated low blade momentum during the in-flight strike. An instability of the

main rotor, rocking of the mast, and extreme pitch divergence of at least one of the main rotor

blades appeared to precede all of the fractures of the main rotor flight control system. The

reason for the main rotor pitch divergence has not been determined.

On April 2, 1994, about 1345 local time, another Robinson R44 helicopter, registered

in Germany as D-HTOP, crashed about 8 miles east of Hanover, Germany, during an intended

pleasure flight. The private pilot and his wife received fatal injuries. The pilot was qualified

in fixed-wing airplanes and helicopters. His total flight experience was not known, but he had

logged 110 hours of R22 flight time. This was the pilot’s first unsupervised flight after

receiving more than 5 hours of R44 instruction and his R44 type-rating checkout. The Safety

Board and the FAA are participating in the continuing FUS investigation of the accident.

The investigation has revealed that the main rotor blades struck the cockpit area of the

fuselage. The evidence indicates that the helicopter yawed sharply due to the blade strike, and

the structure of the tailboom wrinkled and then failed, resulting in separation of the tailboom.

The main rotor mast shows evidence of being bumped by the main rotor blades, and the main

rotor system separated from the helicopter. No precipitating mechanical failure of the helicopter

has been found. The investigation has not determined the reason for the main rotor blade

divergence that resulted in the rotor striking the body of the helicopter during powered flight.2

On December 27, 1994, about 1440 local time, a Robinson R22 helicopter, registered

in Switzerland as HB-XZW and operated by BB Helicopter AG, crashed onto the roof of an

apartment house near Zurich, Switzerland, after a loss of control in flight. The flight’s purpose

was not reported, and the pilot’s flight experience is not yet known. The weather was reported

2For more detailed information, refer to the German FUS Accident File 3x047-94.
91

to be good with gusting wind conditions.Witnesses saw the helicopter in cruise flight about

1,000 feet above the ground and heard the engine running normally before the accident. The

witnesses then heard a loud bang and saw parts of the tailboom separate from the helicopter

before the helicopter crashed onto the apartment house. Parts of the tailboom and tail rotor

assembIy were found about a quarter of a mile from the accident site, and there was evidence

of paint transfer from the tailboom to one of the main rotor blades. The pilot and passenger

received fatal injuries. The Swiss Aircraft Accident Investigation Bureau (AAIB) has requested

the assistance of the Safety Board and the FAA in the continuing investigation. The cause of

the main rotor divergence that led to the contact with the tailboom has not been determined.

On September 28, 1994, about 0947 local time, a Robinson R22 helicopter, registered

in the United States as N83112, crashed near Knightdale, North Carolina, after an in-flight

separation of the tailboom. The pilot was operating the helicopter for business purposes. The

pilot had accumulated 790 total flight hours, with 373 of those hours in helicopters and 305 in

the R22. A witness observed the helicopter about 200 feet above the ground when it appeared

to fishtail and began to lose parts. He additionally said he heard a sputtering sound, which has

not been identified. Radar data indicated that the helicopter was maneuvering at a moderate

speed before the accident. The pilot was fatally injured, and the helicopter was destroyed.

Following the on-site investigation, pieces from the main rotor blades, transmission,

taiIboom, and main rotor head were sent to the Safety Board’s materials laboratory for

examination; however, no evidence was found to indicate a precipitating mechanical or material

failure of any helicopter system. The engine did not exhibit any evidence that would indicate

a loss of power before the tailboom separation and loss of control. The investigation is

continuing and no determination has been made as to the cause of the accident.

In the four recent R44 and R22 accidents described above, the in-flight breakups are

believed to have occurred while the helicopters were being operated at speeds well within the

aircraft’s defined operating envelope. In these cases, the pilots-in-command were experienced,

and the investigations indicate that they had been adequately trained in the R44 and R22. The

pilots assumed to be manipulating the flight controls of the R44s had low R44 experience;

however, the investigations found no evidence that the pilots were improperly operating the

helicopters. In addition to these accidents, the Safety Board is investigating other Robinson

helicopter accidents involving over 20 in-flight breakups of the R22 helicopter. In all of these

accidents, the breakups occurred when the main rotor blades diverged from their normal plane

of rotation and struck the airframe in flight. The known circumstances of the above R44

accidents are very similar to the R22 accidents that have concerned the Safety Board since 1982.3

On September 30, 1982, a Robinson R22 was involved in an in-flight breakup accident

near Paige, Texas, The investigation determined that the tailboom of the helicopter was struck

in flight after the pilot maneuvered near power lines, possibly in an evasive maneuver.

3

For more information refer to the Safety Board’s safety recommendation letters to the FAA dated October 27,

1982, and July 21, 1994.
92

Following this accident, the Safety Board issued Safety Recommendations A-82-143 and -144

to the FAA on October 27, 1982, which stated:

Suspend the Airworthiness Certificate of the Robinson R22 model helicopter until

(1) The main rotor system stability/stall characteristics and the main rotor rpm

[revolutions per minute] decay rates are determined to provide adequate margins

of safety and to be compatible with normal pilot reaction times, and (2) the R22

main rotor system is determined to be in compliance with 14 CFR [Code of

Federal Regulations] 27.661.4 (A-82-143)

Conduct a study to verify that adequate engine torque is available to the Robinson

R22 model helicopter main rotor system to recover rpm should a rapid decay of

rpm occur during flight. (A-82-144)

On December 29, 1982, the FAA responded that it had completed a supplementary flight

test program and a critical design review of the R22 main rotor system in conjunction with the

Robinson Helicopter Company. The results reportedly indicated that the main rotor system

complied with 14 CFR Part 27 and that no unusual flight characteristics existed when the R22

helicopter was operated within its Flight Manual Limitations. The FAA also stated that the rpm

decay rates and helicopter recovery characteristics were evaluated during supplementary flight

tests. The tests indicated that adequate engine power is available to recover rpm should a rapid

decay occur. In addition, the FAA issued a telegraphic airworthiness directive (AD) T82-23-51

on October 29, 1982, which required that the low rotor warning indication be increased from

91% +1% to 95%+1% rpm. The AD required installation of a low rotor speed warning light

adjacent to the rpm indicator.

The FAA also prepared an operations bulletin to emphasize R22 flight instructor

responsibilities in student training. Also, additional analytical and simulation studies considered

relevant to the evaluation of the R22 rotor system were conducted by the National Aeronautics

and Space Administration (NASA) Ames facility, at the FAA’s request. The NASA studies

reportedly did not disclose any adverse or divergent characteristics associated with the

lightweight, low inertia rotor system of the R22. There was no NASA report of the study. On

April 7, 1983, the Safety Board classified Safety Recommendations A-82-143 and -144 “Closed--

Acceptable Action” and “Closed--Acceptable Alternate Action” respectively.

The Safety Board is aware of 339 R22 accidents that have occurred in the United States.

According to the FAA, there are 855 currently registered R22s in the United States.

5The Safety

4

14 CFR Part 27.661 provides for the minimum acceptable standards for certification of helicopters by

speci fying the minimum clearance between the main rotor blades and the structure of the helicopter during any

operation.

5According to the FAA there are three currently registered R44 helicopters in the United States. There are

approximately 142 R44s operating worldwide.
93

Board has found that R22 mechanical reliability problems have not contributed significantly to

the accident rate compared to other light utility helicopters, but the R22 has had an unusually

high number of accidents attributed to pilot performance or undetermined causes (including inflight rotor instability and breakup accidents) compared to other helicopters. The R22 is the

smallest helicopter of those compared. Its small size and relatively low operating cost result in

its use as a training and light utility aircraft and operation by a significant population of

relatively inexperienced helicopter pilots.

The R44 main rotor system has design features that are very similar to the R22. The

two-bladed, semi-rigid R44 and R22 main rotor systems include rotor blades that are connected

to the main rotor hub through coning (flapping) hinges. 6 The main rotor hub is connected to the

main rotor shaft (mast) through an additional hinge so that the hub teeters with influence from

main rotor blade movement. In other two-bladed, semi-rigid systems, the advancing blade flaps

up, causing the retreating bIade to fl ap down; however, each R44 and R22 main rotor blade

flaps independently of the other blade’s vertical movement. The flapping blade causes a change

in the main rotor hub (teeter), which causes an appropriate change in the opposite blade. In

each of the R44 and R22 in-flight breakup accidents described above, the evidence relative to

the sequence of breakup was similar to that found by the Safety Board in other R22 accident

investigations.

The main rotor rpm of both the R44 and the R22 is much higher, and the rotor inertia

is very low by comparison to other light utility two-bladed main rotor systems manufactured in

the United States. Such systems are affected to a much greater extent by abrupt control inputs,

external perturbations, and other factors causing rpm to droop. The Safety Board believes that

changes in rpm occur at a significantly higher rate in the R44 and R22 than in other helicopter

rotor systems.

The Robinson Helicopter Company has theorized that low main rotor rpm is contributing

to the stall and divergence of the main rotor blades in some of the R22 in-flight breakup

accidents in the United States, including those involving experienced instructor pilots. However,

none of the participants in the Safety Board’s investigations have adequately defined a sequence

of events leading to a critically low rotor rpm (and follow-on instabilities of the main rotor

system) or the factors that prevented experienced pilots from being able to apply corrective

action to recover when main rotor rpm is lost.

The Safety Board is concerned that in the above accidents and in other accidents

investigated by the Safety Board, qualified pilots were unable to recognize and correct low main

rotor rpm or anomalous main rotor behavior before uncontrollable blade pitch and excessive

blade divergence followed. The R22 and R44 rpm indicator and the low rpm warning light are

smaller and less conspicuous, unlike those found in many other helicopters, and may not provide

pilots adequate cues when immediate response is necessary.

6Coning is the upward bending of the blades caused by the resul tant forces of lift and centrifugal force.

Flapping is the vertical movement of the blade as a result of aerodynamic forces.
94

The Safety Board has found that in at least one relevant accident, sound spectrum analysis

of background rotor noise on a tape recording of the flight showed that loss of main rotor

occurred in the normal main rotor rpm operating range and within the normal operating envelope

of the R22.7 Other aerodynamic characteristics (Mach tuck, drag divergence, dynamic pitch

moment changes, and negative blade damping) could also have devastating effects on a lowinertia, high rpm rotor system. Data from FAA certification test reports and Robinson

Helicopter engineering reports indicate that no math modeling, computer simulation, or wind

tunnel testing was conducted before, during, or after the R22 helicopter was issued its certificate

of airworthiness by the FAA. The required flight tests were accomplished in prototype

helicopters, but rotor systems were not tested in anomalous conditions such as to-failure or in

areas beyond the prescribed normal flight envelope. The data from the flight tests do not

indicate whether external disturbances to the rotor system such as turbulence, wind gusts, or

other phenomena that could upset a low inertia rotor system were conducted. According to the

FAA, the R44 flight test program was conducted similarly to the R22 flight test program.

Therefore, the Safety Board is concerned that adequate testing may not have been accomplished

during certification to resolve possible adverse aerodynamic characteristics of the rotor and flight

control systems of both the R22 and the R44.

Because of its concerns regarding the R22 main rotor system, on July 21, 1994, the

Safety Board made two urgent recommendations and one priority recommendation to the FAA:

Issue an immediate airworthiness directive to reduce the Robinson R22 helicopter

“never exceed airspeed” (Vne) to an airspeed that would provide an adequate

margin of operating safety below the airspeeds at which loss of main rotor control

accidents have occurred, until the reason for in-flight main rotor blade divergent

behavior is established and design changes are approved and implemented, as

necessary. (A-94-143)

In conjunction with the National Aeronautics and Space Administration and

Robinson Helicopter Company, conduct wind tunnel and modeling tests to

examine flight parameters of the R22 helicopter to determine the helicopter’s

design characteristics that are related to main rotor divergent behavior; and if any

abnormal rotor system performance characteristics are found, take the necessary

actions to assure proper dissemination of the information and to modify the R22

design. (A-94-144)

Examine the appropriateness of the Designated Engineering Representative (DER)

assignment at the Robinson Helicopter Company and at other small manufacturers

where senior executives are assigned DER responsibilities, and take necessary

actions to eliminate any conflict of interest with DER responsibilities. (A-94-145)

7For more detailed informa tion, see Brief of Accident File #1003 (attached).
95

The FAA responded to the recommendations on October 7, 1994, stating that it had

convened a panel to research the R22 in-flight breakup accidents, to recommend a course of

action for the FAA to follow concerning testing, and to evaluate the causes of the breakups. The

FAA also resolved to change the DER at the Robinson factory when conditions were

appropriate. However, the FAA slected not to restrict R22 flight operations pending completion

of the work of the special research panel. On December 13, 1994, the Safety Board classified

the first two recommendations “Open--Unacceptable Response” and the third recommendation,

“Open--Acceptable Response. ” The Safety Board stated that it was disappointed that the FAA

did not respond to the urgency of the recommendations, which were intended to prompt

appropriate interim action to reduce the potential for continuing loss of main rotor control

accidents while the cause(s) of main rotor instability were further researched.

The Safety Board is aware that the R44 complies with the FAA’s certification

requirements and that, following the July 31, 1993, accident, a certification review related to

the unique cyclic control system was conducted and evidence of noncompliance with certification

standards or of a deficiency that would explain accidents such as those discussed above was not

uncovered. However, because of the catastrophic nature of the continuing accidents and the

evidence of possible main rotor involvement, the Safety Board believes that the FAA should, in

conjunction with the National Aeronautics and Space Administration (NASA) and Robinson

Helicopter Company, conduct further testing to evaluate the R44 main rotor and control system.

The testing should include flight testing as well as wind tunnel and computer modeling to

evaluate the main rotor design, including rotor stability, control responsiveness, main rotor

performance in cruise flight, and other possible areas in which main rotor divergence or

instabilities may have occurred on accident flights. The Safety Board is specifically concerned

that the unique design of the R22 and R44 rotor system may result in flight characteristics that

are not adequately addressed by 14 CFR Part 27 certification standards; In addition, the Safety

Board is concerned that the R44 main rotor control system, which includes the teetering cyclic

control8 in the cockpit, may have undesirable dynamic characteristics that are not adequately

addressed in the flight and ground testing under 14 CFR Part 27 standards. Of special concern

to the Safety Board, are the effects that turbulence may have on the main rotor control system

and ergonomic factors relative to the interaction between the pilots through the unique teetering

cyclic control systems in R44 and R22 helicopters. Anomalies in the main rotor system or

cyclic control in the cockpit may have gone undetected during the original certification process.

Because the recent German R44 accidents occurred abruptly and with no apparent

warning to the flightcrew, they are of particular concern to the Safety Board. Those accidents

and the other similar R22 in-flight breakup accidents examined by the Safety Board indicate that

undesirable aerodynamic characteristics of R44 and R22 main rotor blades can result in one or

both blades diverging from their normal plane of rotation during normal operation in the

approved flight envelope. The Safety Board is concerned that the stability of the R44 and R22

main rotor blades may be compromised by an inherent rotor system design deficiency that may

8

The Robinson R44, like the R22, has a cyclic flight control that teeters to allow a dual control system for two

pilots.
96

allow loss of control of the rotor system when operating the helicopter within the currently

defined flight envelope and in a manner that would seem normal in other light helicopters. The

Safety Board is aware of the importance of the R44 and R22 as training and light utility

helicopters. However, until the causes of the accidents cited above are determined, and

appropriate flight envelope restrictions and operating limitations are defined, the FAA should

prohibit further flight.

The Safety Board has paid particular attention to the R22 main rotor blades and the rotor

head during an ongoing special investigation because the in-flight breakup accidents under

investigation were found to be more likely caused by blade divergence that initiated failures at

the main rotor, rather than initiating failures in the transmission, its mounts, or the main rotor

control system. As a result of its scrutiny of the main rotor, the Safety Board requested Material

Review Records (MRRs) for the main rotor blades involved in those accidents. The Safety

Board’s review of several MRRs of rotor blades not involved in an accident caused the Board

to become concerned with the disposition and subsequent approval of blades containing defects

or not passing quality assurance testing. The Safety Board is concerned about the reported use

of main rotor blades on new R22 or R44 helicopters when those blades did not pass design

inspection requirements. The proper design, manufacture, testing, and approval of main rotor

blades are crucial to the airworthiness of a helicopter. Main rotor blades should be carefully

examined for defects, and any blade not meeting the original design inspection requirements

should be rejected unless modification of the design inspection requirements are specifically

approved by the FAA. The Safety Board believes that additional FAA oversight of the R44 and

R22 main rotor blade manufacturing quality assurance program is necessary to ensure that these

blades are properly inspected and approved; and if inadequacies in the approval process are

found, the FAA should modify and correct the approval process as necessary.

Therefore, the National Transportation Safety Board recommends that the Federal

Aviation Administration:

Prohibit further flight of the Robinson Helicopter Company R44 helicopter until

(1) adequate research and testing are accomplished to determine the cause of inflight main rotor bIade divergent behavior, and (2) modifications are made to the

helicopter or appropriate limitations are placed in the flight manual to preclude

divergent main rotor behavior and in-flight breakup accidents where pilots are

unable to prevent loss of main rotor control in the approved operating envelope.

(Class 1, Urgent Action)(A-95-1)

Prohibit further flight of the Robinson Helicopter Company R22 helicopter until

(1) adequate research and testing are accomplished to determine the cause of inflight main rotor blade divergent behavior, and (2) modifications are made to the

helicopter or appropriate limitations are placed in the flight manual to preclude

divergent main rotor behavior and in-flight breakup accidents where pilots are

unable to prevent loss of main rotor control in the approved operating envelope.

(Class I, Urgent Action) (A-95-2)
97

Conduct flight, ground, simulation,and modeling tests to determine the

responsiveness of the Robinson Helicopter Company R44 and R22 rotor systems

in all flight conditions to ensure that any qualified pilot, including students

approved for solo flight and low experienced but rated helicopter pilots, may be

expected to receive adequate warning of rotor system anomalous conditions and

be capable of recovering from rotor system revolutions per minute decay or rotor

system divergence safely when warned of anomalous conditions. (Class I, Urgent

Action)(A-95-3)

Determine if the Robinson Helicopter Company rotor system low revolutions per

minute (rpm) warning and indication systems in the R22 and R44 helicopters

adequately alert the pilot in time to initiate prompt control inputs to correct a low

rotor rpm condition, and require modifications to those systems if deficiencies are

found. (Class II, Priority Action) (A-95-4)

Examine the appropriateness of the teetering cyclic flight control used in the

Robinson R22 and R44 helicopters and make any design and modification changes

to the cyclic and collective control systems as necessary to ensure that pilots-incommand and flight instructors can respond in time to prevent loss of control of

the main rotor following in-flight main rotor anomalies initiated by low main

rotor revolutions per minute or turbulence encounters in flight. (Class H, Priority

Action) (A-95-5)

Conduct special studies and reviews of the Robinson R44 certification similar to

that being conducted now for the R22, to determine that the ‘flight control and

main rotor system may be safely operated in all modes of flight and throughout

the approved flight envelope by all pilots qualified to operate the helicopter.

(Class II, Priority Action) (A-95-6)

Conduct Robinson R44 main rotor blade design and manufacturing process

reviews and testing to determine if there are any main rotor blade construction

deficiencies, either in design or in the manufacturing process, that may be

contributing to main rotor divergence incidents or accidents, and modify the

design and structure of the blade as necessary. (Class II, Priority Action)

(A-95-7)
98

Conduct special reviews of the Robinson R44 and R22 main rotor blade

inspection criteria and practices to determine if blades not meeting quality

assurance inspections are inappropriately being approved by company

personnel, and if inadequacies in the approval processes are found, modify and

correct the approval process as necessary. (Class II, Priority Action) (A-95-8)

Chairman HALL, and Members HAMMERSCHMIDT and FRANCIS concurred in these

recommendations.

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101

Honorable David R. Hinson

Administrator

Federal Aviation Administration

Washington, D.C. 20591

Dear Mr. Hinson:

Thank you for the Federal Aviation Administrationts (FAA)

responses of March 27, 1995, and November 20, 1995, to the National

Transportation Safety Board’s Safety Recommendations A-94-145, and

A-95-1 through -8. The recommendations addressed issues raised

during the Safety Board’s investigation of a series of Robinson

Helicopter Company (RHC) helicopter accidents in which the main

rotors struck the helicopter airframe in flight.

Safety Recommendation A-94-145 asked the FAA to examine the

appropriateness of the Designated Engineering Representative (DER)

assignment at the RHC and at other small manufacturers where senior

executives are assigned DER responsibilities, and take necessary

actions to eliminate any conflict of interest with DER

responsibilities.

The Safety Board notes that the FAA examined the

appropriateness of the DER assignment at the RHC and at other small

manufacturers. The FAA found that the DER approvals at Robinson

were proper; however, the President of the RHC no longer serves as

a DER. Also, although the FAA found no conflict of interest when

it evaluated 30 senior executives who hold DER positions elsewhere,

the FAA will issue guidance to transition these DERs out of the

program or to provide for increased oversight of their performance

as DERs. The Safety Board finds the FAA action to be fully

responsive to the intent of the recommendation and classifies

Safety Recommendation A-94-145 ‘Closed--Acceptable Action.m

Safety Recommendations A-95-1 and-2 asked the FAA to prohibit

further flight of R44 and R22 helicopters until (1) adequate

research and testing are accomplished to determine the cause of inflight main rotor blade divergent behavior, and (2) modifications

are made to the helicopter or appropriate limitations are placed in

the flight manual to preclude divergent main rotor behavior and inflight breakup accidents where pilots are unable to prevent loss of

main rotor control in the approved operating envelope.

The Safety Board notes that while the FAA did not prohibit or

promptly restrict further flight of the R22 or R44 in response to

these recommendations, it did place limitations on operations in

certain wind conditions, require enhanced pilot training, and
102

establish operational limitations to prevent pilots from performing

certain low-G in-flight maneuvers in the R22 and R44.

A technical panel created by the FAA in response to earlier

Safety Board recommendations completed its research in March 1995

and recommended further research and design enhancements. Also,

the FAA and the RHC conducted flight testing of the R44 in July

1995 to evaluate its performance in the approved flight envelope,

and the FAA contracted with the Georgia Institute of Technology

(Georgia Tech) to perform computer simulation modeling of the R22

main rotor. The Georgia Tech research was concluded with a report

to the FAA in December 1995. In response to RHC initiatives and

technical panel recommendations, the FAA issued a notice of

proposed rulemaking in December 1995, which asked for comments on

a proposal to require modification to R22s to include installation

of a new rotor speed governor.

The actions taken by the FAA have been responsive to Safety

Board concerns generated from the 34 known R22 and R44 loss of main

rotor control accidents. Also, the RHC has introduced optional

modifications to the R22 that, combined with pilot awareness and

proficiency training, also reduce the potential for loss of main

rotor control accidents. The positive actions undertaken to inform

R22 and R44 pilots of how to avoid such accidents are commendable,

and the Safety Board is pleased with the responsiveness and efforts

of the FAA and RHC to address the issues raised by the Safety

Board. Although the Board is disappointed that the FAA did not

promptly restrict operations of these helicopters when the reasons

for the accidents were totally unknown, it acknowledges the

significant alternate steps taken by the FAA to resolve the safety

issues associated with the accidents. Therefore, the Safety Board

classifies Safety Recommendations A-95-1 and -2 ‘Closed--Acceptable

Alternate Action.n

Safety Recommendations A-95-3 through -8 asked the FAA to

conduct flight, ground, simulation, and modeling tests to determine

the responsiveness of the R44 and R22 rotor systems in all flight

conditions; to consider the abilities of students and low

experienced but rated helicopter pilots who might fly these

helicopters; to determine if the RHC rotor system low revolutions

per minute (rpm) warning and indication systems in the R22 and R44

helicopters adequately alert the pilot in time to correct a low

rotor rpm condition; to examine the appropriateness of the

teetering cyclic flight control used in the R22 and R44

helicopters; to conduct special studies and reviews of the R44

certification to determine if the flight control and main rotor

system may be safely operated throughout the approved flight

envelope; to conduct RHC R44 main rotor blade design and

manufacturing process reviews and testing to determine if there are

any main rotor blade construction deficiencies; and to conduct

special reviews of the RHC main rotor blade inspection criteria and

practices.
103

The FAA has conducted or sponsored research on many of the

issues addressed by these recommendations, some of which is

ongoing. A detailed assessment of the FAA response to Safety

Recommendations A-95-3 through -8 will be included in the Safety

Board's report on its special investigation of R-22 accidents

involving the loss of main rotor control.

Sincerely,

Jim Hall

Chairman

cc: Dr. Donald R. Trilling, Director

Office of Environment, Energy and Safety
104

APR - 2 1996

Honorable David R. Hinson

Administrator

Federal Aviation Administration

Washington, D.C. 20591

Dear Mr. Hinson:

Thank you for the Federal Aviation Administration’s (FAA) response of March 11, 1996,

to the National Transportation Safety Board’s Safety Recommendations A-94-143 through -145

and A-95-1 through -8. The recommendations addressed issues raised during the Safety Board’s

investigation of a series of Robinson Helicopter Company (RHC) helicopter accidents in which

the main rotors struck the helicopter airframe in flight.

Safety Recommendations A-94-145, A-95-1, and A-95-2 were closed previously by the

Safety Board in a letter dated February 22, 1996. Safety Recommendation A-94-145 was

classified “Closed--Acceptable Action” and Safety Recommendations A-95-1 and -2 were

classified “Closed--Acceptable Alternate Action.”

Safety Recommendation A-94-143 asked the FAA to issue an immediate airworthiness

directive (AD) to reduce the RHC R22 helicopter “never exceed airspeed” (Vne) to an airspeed

that would provide an adequate margin of operating safety below the airspeeds at which loss of

main rotor control accidents have occurred, until the reason for in-flight main rotor blade

divergent behavior is established and design changes are approved and implemented, as

necessary.

The FAA has issued ADs that imposed airspeed and operational restrictions on the R22

in high wind and turbulent conditions and an AD prohibiting intentional low-G maneuvers in R22

helicopters. The FAA had previously issued art airworthiness alert warning pilots to avoid

operation of the R22 in high speed cruise flight. The Safety Board believes that these specific

changes are in line with the intent of this recommendation and therefore classifies Safety

Recommendation A-94-143 “Closed--Acceptable Action.”

Safety Recommendation A-94-144 asked the FAA, in conjunction with the National

Aeronautics and Space Administration and RHC, to conduct wind tunnel and modeling tests to

examine flight parameters of the R22 helicopter to determine the helicopter’s design

characteristics that are related to main rotor divergent behavior; and if arty abnormal rotor system
105

performance characteristics are found, take the necessary actions to assure proper dissemination

of the information and to modify the R22 design.

Safety Recommendation A-95-3 asked the FAA to conduct flight, ground, simulation, and

modeling tests to determine the responsiveness of the RHC R44 and R22 rotor systems in all

flight conditions to ensure that any qualified pilot, including students approved for solo flight and

low experienced but rated helicopter pilots, may be expected to receive adequate warning of rotor

system anomalous conditions and be capable of recovering from rotor system rpm decay or rotor

system divergence safely when warned of anomalous conditions.

The FAA commissioned the Georgia Institute of Technology (Georgia Tech) to conduct

a simulation study of the R22 helicopter and provided the results of the study to the Safety Board

staff in March 1996. The Board is aware that the research was concluded before the

mathematical model was thoroughly validated by comparison to flight test data. However, the

Safety Board’s review of the data produced by the simulation revealed main rotor-to-static stop

contact in several of the simulations. The Safety Board also recognizes the complexity and

potential hazards associated with flight tests and full-scale wind tunnel testing. Nevertheless, the

Safety Board believes that the information obtained from further development of the simulation

of lightweight, low rotor inertia helicopters could be extremely valuable to the FAA and to

manufacturers seeking certification of similar designs in the future. Therefore, the Safety Board

has issued a new recommendation on this subject in its final report of its special investigation

of R22 helicopter accidents, and Safety Recommendations A-94-144 and A-95-3 are classified

“Closed--Acceptable Action/Superseded.”

Safety Recommendation A-95-4 asked the FAA to determine if the RHC rotor system low

rpm warning and indication systems in the R22 and R44 helicopters adequately alert the pilot in

time to initiate prompt control inputs to correct a low rotor rpm condition, and require

modifications to those systems if deficiencies are found.

The FAA has conducted tests of the low rpm warning systems of the R22 and R44 and

has required changes to these systems. The Safety Board is also aware that a new R22 rotor

speed governor has been introduced by the RHC, and that the FAA plans to issue an AD to

mandate its use. The proposed AD would increase the low rotor warning rpm threshold and

mandate the use of the governor except under certain situations. The FAA determined that the

volume of the R22 low rotor rpm warning horn was adequate. The Safety Board believes that

the FAA has met the intent of this recommendation and therefore classifies Safety

Recommendation A-95-4 “Closed--Acceptable Action.”

Safety Recommendation A-95-5 asked the FAA to examine the appropriateness of the

teetering cyclic flight control used in the RHC R22 and R44 helicopters and make any design

and modification changes to the cyclic and collective control systems as necessary to ensure that

pilots-in-command and flight instructors can respond in time to prevent loss of control of the

main rotor following in-flight main rotor anomalies initiated by low main rotor rpm or turbulence

encounters in flight.
106

The FAA has participated in two human factors evaluations of the R22 and R44 teetering

cyclic controls to determine their adequacy for flight instruction. The FAA has also evaluated

an alternative STC-approved cyclic control and reports that both cyclic control designs were

found to be satisfactory. Based on this action, the Safety Board classifies Safety

Recommendation A-95-5 “Closed--Acceptable Action. ”

Safety Recommendation A-95-6 asked the FAA to conduct special studies and reviews

of the RHC R44 certification similar to what was being conducted for the R22, to determine that

the flight control and main rotor system may be safely operated in all modes of flight and

throughout the approved flight envelope by all pilots qualified to operate the helicopter.

The FAA has accomplished flight tests of the R44 to assess rotor stability and control

within the approved flight envelope and believes that its flight testing of the R44 has

demonstrated that the R44 meets the requirements of 14 CFR Part 27. Because the FAA has met

the intent of this recommendation, the Safety Board classifies Safety Recommendation A-95-6

“Closed--Acceptable Action.”

Safety Recommendation A-95-7 asked the FAA to conduct RHC R44 main rotor blade

design and manufacturing process reviews and testing to determine if there are any main rotor

blade construction deficiencies, either in design or in the manufacturing process, that may be

contributing to main rotor divergence incidents or accidents, and modify the design and structure

of the blade as necessary.

The FAA’s review of the R44 main rotor blade design, manufacturing process, and quality

assurance system found no deficiencies or non-conformities. The Safety Board has received a

copy of the FAA’s main rotor blade design and manufacturing process review. Based on the

FAA’s action, the Safety Board classifies Safety Recommendation A-95-7 “Closed--Acceptable

Action.”

Safety Recommendation A-95-8 asked the FAA to conduct special reviews of the RHC

R44 and R22 main rotor blade inspection criteria and practices to determine if blades not meeting

quality assurance inspections are inappropriately being approved by company personnel, and if

inadequacies in the approval processes are found, modify and correct the approval process as

necessary.

The FAA conducted a review of the Material Review Board processes and Material

Review Records on March 2, 1995, and found no discrepancies. The review concluded that there

were no inappropriate approvals by RHC personnel or inadequacies in the approval process. The
107

Safety Board believes that the FAA has met the intent of this recommendation and therefore

classifies Safety Recommendation A-95-8 “Closed--Acceptable Action. ”

Sincerely,

Chairman

cc: Dr. Donald R. Trilling, Director

Office of Environment, Energy and Safety