Colleen A. Withrow and Mary S. Reveley

Glenn Research Center, Cleveland, Ohio

Analysis of Aviation Safety Reporting System 

Incident Data Associated With the Technical 

Challenges of the Atmospheric Environment 

Safety Technology Project

NASA/TM—2014-217898

August 2014
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Colleen A. Withrow and Mary S. Reveley

Glenn Research Center, Cleveland, Ohio

Analysis of Aviation Safety Reporting System 

Incident Data Associated With the Technical 

Challenges of the Atmospheric Environment 

Safety Technology Project

NASA/TM—2014-217898

August 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

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either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2014-217898 iii

Contents

Summary....................................................................................................................................................... 1

1.0 Introduction........................................................................................................................................... 1

2.0 Aviation Safety Reporting System Database........................................................................................ 2

3.0 Technical Challenge 2: Airframe Icing Simulation and Engineering Tool Capability......................... 3

3.1 Federal Aviation Regulations Parts...............................................................................................4

3.2 Phase of Flight ..............................................................................................................................6

3.3 Flight Crew and General Results..................................................................................................6

3.4 Carburetor Icing............................................................................................................................7

3.5 Anomaly Categories......................................................................................................................7

3.5.1 In-Flight Event or Encounter .............................................................................................8

3.5.2 Procedural Deviation..........................................................................................................9

3.5.3 Altitude Deviation..............................................................................................................9

3.5.4 Aircraft Equipment Problem............................................................................................10

3.5.5 Track/Heading Deviation and Speed Deviation...............................................................10

3.6 Flight Plan and Flight Conditions...............................................................................................10

3.7 Weather Element—Visibility......................................................................................................11

3.8 Air Traffic Control Results.........................................................................................................11

3.9 Route in Use................................................................................................................................12

3.10 U.S. Region.................................................................................................................................12

3.11 Technical Challenge 2—Conclusion ..........................................................................................14

4.0 Technical Challenge 3: Atmospheric Hazard Sensing and Mitigation Technology Capability ......... 14

4.1 Clear Air Turbulence ..................................................................................................................15

4.2 Wake Vortex ...............................................................................................................................19

4.3 Windshear...................................................................................................................................21

4.4 St. Elmo’s Fire ............................................................................................................................25

4.5 Volcanic Ash...............................................................................................................................27

4.6 Controlled Flight Toward Terrain or Controlled Flight Into Terrain..........................................27

4.7 Ground Event or Encounter ........................................................................................................30

4.8 Ground Incursions.......................................................................................................................32

4.9 Near-Midair Collision Hazard ....................................................................................................33

4.10 Technical Challenge 3—Conclusion ..........................................................................................35

5.0 Conclusions......................................................................................................................................... 36

Appendix A.—Acronyms........................................................................................................................... 39

Appendix B.—Aviation Safety Reporting System Category Descriptions................................................. 41

References................................................................................................................................................... 46

List of Tables

Table 1.—Update of Icing- and Weather-Related Incidents in the ASRSa................................................... 3

Table 2.—Summary of 2008 NASA Icing Paper Incidents and Update Through 2011 by FAR 

Part........................................................................................................................................................ 4

Table 3.—Update of Icing-Related Incidents by Phase of Flight for FAR Parts 121, 135, and 91 

Combined.............................................................................................................................................. 6

Table 4.—Carburetor Icing Incidents in the ASRS ...................................................................................... 7

Table 5.—Anomaly Incidents by Search Criteria......................................................................................... 8

Table 6.—Airframe-Icing-Related Incidents by In-Flight Event or Encounter Anomaly and FAR 

Part........................................................................................................................................................ 8

Table 7.—Airframe-Icing-Related Incidents by Procedural Deviation Anomaly and FAR Part ................. 9
NASA/TM—2014-217898 iv

Table 8.—Airframe-Icing-Related Incidents by Altitude Deviation Anomaly and FAR Part...................... 9

Table 9.—Airframe-Icing-Related Incidents by Aircraft Equipment Problem Anomaly and FAR 

Part...................................................................................................................................................... 10

Table 10.—Airframe-Icing-Related Incidents by Flight Conditions, Flight Plan, and FAR Part............... 10

Table 11.—Airframe-Icing-Related Incidents by Visibility Weather Element and FAR Part ................... 11

Table 12.—Airframe-Icing-Related Incidents by Air Traffic Control Results and FAR Part.................... 11

Table 13.—Airframe-Icing-Related Incidents by Route in Use and FAR Part .......................................... 12

Table 14.—Airframe-Icing-Related Incidents by U.S. Region and FAR Part............................................ 12

Table 15.—Clear Air Turbulence-Related Incidents by General Results and FAR Part............................ 17

Table 16.—Clear Air Turbulence-Related Incidents For Flight Crew Results and FAR Part.................... 17

Table 17.—Clear Air Turbulence-Related Incidents For In-Flight Event or Encounter Anomaly 

and FAR Part ...................................................................................................................................... 18

Table 18.—Clear Air Turbulence-Related Incidents by Flight Conditions or Flight Plan, and FAR 

Part...................................................................................................................................................... 18

Table 19.—Wake Vortex-Related Incidents by In-Flight Event or Encounter Anomaly and FAR 

Part...................................................................................................................................................... 19

Table 20.—Wake Vortex-Related Incidents By General Results and FAR Part........................................ 20

Table 21.—Number and Percentage of Windshear-Related Incidents by FAR Part .................................. 22

Table 22.—Windshear-Related Incidents by General Results and FAR Part............................................. 23

Table 23.—Windshear-Related Incidents by In-Flight Event or Encounter Anomaly and FAR Part ........ 25

Table 24.—Windshear-Related Incidents by Flight Conditions or Flight Plan and FAR Part ................... 25

Table 25.—CFTT/CFITa

-Related Incidents by In-Flight Event or Encounter Anomaly and FAR 

Part...................................................................................................................................................... 29

Table 26.—CFTT/CFITa

–Related Incidents by Environmental Light Conditions and FAR Part .............. 29

Table 27.—CFTT/CFITa

-Related Incidents by Visibility Weather Element and FAR Part....................... 29

Table 28.—Ground Event or Encounter-Related Incidents by Environmental Light Conditions 

and FAR Part ...................................................................................................................................... 31

Table 29.—Ground Event- Or Encounter-Related Incidents by Visibility Weather Element and 

FAR Part............................................................................................................................................. 31

Table 30.—Ground Incursion-Related Incidents by Environmental Light Conditions and FAR 

Part...................................................................................................................................................... 32

Table 31.—Ground Incursion-Related Incidents by Visibility Weather Element and FAR Part ............... 33

Table 32.—Near-Midair-Collision-Related Incidents by In-Flight Event or Encounter Anomaly 

and FAR Part ...................................................................................................................................... 34

Table 33.—Near-Midair-Collision-Related Incidents by Flight Conditions and FARa Part ...................... 35

List of Figures

Figure 1.—Total number of incidents in the Aviation Safety Reporting System......................................... 4

Figure 2.—Annual weather-related incidents. .............................................................................................. 5

Figure 3.—Annual airframe-icing-related incidents..................................................................................... 5

Figure 4.—Airframe-icing-related incidents compared to weather-related incidents per year, 

percent................................................................................................................................................... 5

Figure 5.—Flight crew results for airframe-icing-related incidents. ............................................................ 7

Figure 6.—General results for airframe-icing-related incidents. .................................................................. 7

Figure 7.—Airframe-icing incidents by U.S. region and FAR part. (a) West. (b) Midwest. (c) 

South. (d) Northeast............................................................................................................................ 13

Figure 8.—Annual clear air turbulence-related incidents by FAR part. ..................................................... 15

Figure 9.—Visibility weather elements for clear air turbulence-related incidents by FAR part. ............... 16

Figure 10.—Phase of flight for clear air turbulence-related incidents by FAR part................................... 16
NASA/TM—2014-217898 v

Figure 11.—Phase of flight for wake vortex encounters by FAR part. ...................................................... 20

Figure 12.—Flight crew results for wake vortex encounters by FAR part................................................. 21

Figure 13.—Windshear-related incidents by year and FAR part................................................................ 22

Figure 14.—Phase of flight for windshear-related incidents by FAR part. ................................................ 22

Figure 15.—Visibility weather elements for windshear-related incidents by FAR part............................. 23

Figure 16.—Flight crew results for windshear-related incidents by FAR part........................................... 24

Figure 17.—General results for St. Elmo's fire incidents. .......................................................................... 26

Figure 18.—Flight crew results for St. Elmo's fire incidents...................................................................... 26

Figure 19.—In-flight events or encounters for St. Elmo's fire incidents. ................................................... 27

Figure 20.—Visibility weather element for St. Elmo's fire. ....................................................................... 27

Figure 21.—Phase of flight for controlled flight toward terrain or controlled flight into terrain 

incidents.............................................................................................................................................. 28

Figure 22.—Flight crew results for controlled flight toward terrain or controlled flight into terrain 

incidents.............................................................................................................................................. 28

Figure 23.—Ground event or encounter incidents during four phases of flight by FAR part. ................... 30

Figure 24.—Phase of flight for ground incursion incidents........................................................................ 32

Figure 25.—Phase of flight for near-midair collision-related incidents. .................................................... 33

Figure 26.—Flight crew results for near-midair-collision-related incidents............................................... 34

NASA/TM—2014-217898 1

Analysis of Aviation Safety Reporting System Incident 

Data Associated With the Technical Challenges of the 

Atmospheric Environment Safety Technology Project

Colleen A. Withrow and Mary S. Reveley

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Summary

This study analyzed aircraft incidents in the NASA Aviation Safety Reporting System (ASRS) that 

apply to two of the three technical challenges (TCs) in NASA’s Aviation Safety Program’s Atmospheric 

Environment Safety Technology Project. The aircraft incidents are related to airframe icing and 

atmospheric hazards TCs. The study reviewed incidents that listed their primary problem as weather or 

environment-nonweather between 1994 and 2011 for aircraft defined by Federal Aviation Regulations 

(FAR) Parts 121, 135, and 91. The study investigated the phases of flight, a variety of anomalies, flight 

conditions, and incidents by FAR part, along with other categories.

The first part of the analysis focused on airframe-icing-related incidents and found 275 incidents out 

of 3526 weather-related incidents over the 18-yr period. The second portion of the study focused on 

atmospheric hazards and found 4647 incidents over the same time period. Atmospheric hazards-related

incidents included a range of conditions from clear air turbulence and wake vortex, to controlled flight 

toward terrain, ground encounters, and incursions.

1.0 Introduction

This analysis was conducted to support the Aviation Safety Program’s (AvSP’s) Atmospheric 

Environment Safety Technology (AEST) Project milestone AEST.3.2.SA.01 (Ref. 1), Identification of 

AEST-Related Trends. In particular, this is a review of incident data from the NASA Aviation Safety 

Reporting System (ASRS) (Ref. 2). The following three AEST-related technical challenges (TCs) were 

the focus of the incidents searched in the ASRS database:

TC1: Engine icing characterization and simulation capability

TC2: Airframe icing simulation and engineering tool capability

TC3: Atmospheric hazard sensing and mitigation technology capability

The database was searched for incidents that listed either weather- or environment-nonweather-related 

primary problems. It was difficult to find any TC1-related incidents that resulted from engine icing due to 

clouds with high ice water content (HIWC). Twenty-four engine-related icing incidents were found, but 

none appeared to be related to clouds with HIWC. There is no further discussion on TC1 in this report 

because most incidents were related to engine ice, snow ingestion, or carburetor icing. The primary 

problem of weather was analyzed for TC2 and TC3, and the environment-nonweather-related primary 

problem was also analyzed for TC3. Acronyms used in this report are defined in Appendix A.

The AvSP is primarily interested in Federal Aviation Regulations (FAR) Parts 121, 135, and 91 

aircraft operations. Part 121 applies to major airlines and cargo carriers that fly large transport category 

aircraft. Part 135 applies to commercial aircraft air carriers, also referred to as ‟commuter airlines.” Prior 

to March 1997, Part 121 operations included aircraft with 30 or more seats. In March 1997, the definition 

of Part 121 operations changed to include aircraft with 10 or more seats. Part 91 applies to general 

aviation and noncommercial operations. 
NASA/TM—2014-217898 2

2.0 Aviation Safety Reporting System Database

Information requested from the ASRS database included weather- and environment-nonweatherrelated incidents only. At the time of the search, 163 558 full-form reports were in the database. The 

search, which was restricted to January 1994 to November 2011 for FAR Parts 121, 135, and 91, resulted 

in 3526 incidents with weather listed as the primary problem and 1122 incidents with environmentnonweather-related listed as the primary problem. Weather-related incidents are excluded in this ASRS 

category. 

The ASRS database includes incidents only, not accidents. The following definitions are used for 

incidents and accidents in aviation and are listed in the International Civil Aviation Organization (ICAO)

Annex 13 (Ref. 3).

• An incident is an occurrence, other than an accident, associated with the operation of an aircraft 

which affects or could affect the safety of operation. 

• An accident is an occurrence associated with the operation of an aircraft which takes place 

between the time any person boards the aircraft with the intention of flight until such time as all 

such persons have disembarked, in which (a) a person is fatally or seriously injured or (b) the 

aircraft sustains damage or structural failure which: adversely affects the structural strength, 

performance or flight characteristics of the aircraft and would normally require major repair or 

replacement of the affected component (except for engine failure or damage, when the damage is 

limited to the engine, its cowlings or accessories; or for damage limited to propellers, wingtips, 

antennas, tires, brakes, fairings, small dents or puncture holes in the aircraft skin); or (c) the 

aircraft is missing or is completely inaccessible.

There are caveats to be aware of when using ASRS data: incidents are reported voluntarily, are 

subject to self-reporting biases, and are not corroborated by the Federal Aviation Administration (FAA) 

or the National Transportation Safety Board. Voluntary incident reports cannot be considered to be a 

representative sample of the underlying population of events they describe (Ref. 4). Also, only a fraction 

of the incidents reported are found in the public database because of the lack of resources for reviewing

and categorizing incidents as they are received. Even though the data cannot be used for statistical or 

trend analysis, they can be used to identify vulnerabilities and to gain a better understanding of the root 

causes of human error. Also, they should be considered to complement data generated by mandatory, 

statistical, and monitoring systems.

The data received from ASRS contained a variety of information for each incident including FAR 

part, phase of flight, flight conditions, and narrative. The following AEST search criteria categories were 

used in this analysis:

1. FAR part

2. Phase of flight

3. Flight crew results

4. General results

5. Anomalies

a. In-flight event or encounter

b. Ground event or encounter

c. Ground incursion

d. Conflict

e. Procedural deviation

f. Altitude deviation

g. Aircraft equipment problem

h. Track/heading deviation

i. Flight planning
NASA/TM—2014-217898 3

6. Flight conditions

7. Visibility weather element

8. Air traffic control (ATC) results

9. Route in use

10. U.S. region

ASRS category definitions used in this report are listed in Appendix B. Most category options are not 

mutually exclusive, which can cause an incident to have multiple values under a single category. For 

example, an icing-related incident by phase of flight could be counted twice if the incident occurred 

during both cruise and descent phases of flight. 

3.0 Technical Challenge 2: Airframe Icing Simulation and Engineering 

Tool Capability

TC2 consists of airframe ice accretion and aerodynamic performance degradation, including freezing 

rain and freezing drizzle conditions. Several data sets were used for the TC2 analyses. The weatherrelated data sets included factors such as icing conditions, thunderstorms, turbulence, windshear, runway, 

taxiway, or ramp icing, and deicing. The weather-related incident narratives and synopses were searched 

(free-string text) for “ic% OR rime OR rhime OR de-ice OR sleet OR frost” and reviewed to create the 

icing-related dataset as shown in Table 1.

TABLE 1.—UPDATE OF ICING- AND WEATHER-RELATED INCIDENTS IN THE ASRSA

Year Icing and weather incidents

2008 NASA icing paper 

(Ref. 5)

Update to NASA icing paper 1994 to 2011 data for FARb Parts 

121, 135, and 91 combined

Total in 

ASRS

WeatherrelatedIcingrelatedTotal in 

ASRS

WeatherrelatedIcingrelatedWeatherrelatedAirframeicingrelated

Airframe icing, percent

WeatherrelatedTotal in 

ASRS

1994 6 764 300 44 282 30 10.6 0.44

1995 9 125 352 55 343 38 11.1 0.42

1996 8 044 378 45 346 28 8.1 0.35

1997 8 018 332 52 8 017 332 45 307 27 8.8 0.34

1998 8 428 276 34 8 428 276 22 250 18 7.2 0.21

1999 9 866 245 29 9 868 245 22 233 17 7.3 0.17

2000 7 998 232 38 7 999 232 26 229 17 7.4 0.21

2001 8 612 197 21 8 618 197 17 140 11 7.9 0.13

2002 7 198 201 26 7 200 201 22 200 10 5.0 0.14

2003 8 143 266 32 8 143 226 24 226 11 4.9 0.14

2004 6 200 161 27 6 205 161 22 161 12 7.5 0.19

2005 3 524 109 25 3 534 109 19 108 12 11.1 0.34

2006 5 196 188 11 6 574 140 13 138 8 5.8 0.12

2007 5 131 80 8 79 4 5.1 0.08

2008 6 476 138 24 133 9 6.8 0.14

2009 5 303 137 102 129 8 6.2 0.15

2010 5 485 121 94 118 7 5.9 0.13

2011 2 877 112 82 104 8 7.7 0.28

Total 73 183 2207 295 123 791 3737 686 3526 275 a

Aviation Safety Reporting System.

b

Federal Aviation Regulations.
NASA/TM—2014-217898 4

Figure 1.—Total number of incidents in the Aviation Safety Reporting System.

TABLE 2.—SUMMARY OF 2008 NASA ICING PAPER INCIDENTS

AND UPDATE THROUGH 2011 BY FAR PART

FAR Part Incidents

1997 to 2006 1994 to 2011

2008 NASA 

icing paper

WeatherrelatedUpdate to NASA 

icing paper

Airframe-icingrelated

121 131 2190 332 51

135 25 245 67 42

91 129 1091 287 182

Total 285 3526 686 275

The Aviation Safety Systems Analysis team published a report entitled “Subsonic Aircraft Safety 

Icing Study” (Ref. 5) and is referred to in this report as the “2008 NASA icing paper.” At the time of the 

study, there were 73 183 incidents in the ASRS database from 1997 through 2006. The study found 2207 

weather-related incidents and 295 icing-related incidents for the same timeframe. 

A new search of the ASRS database from 1994 to 2011 was performed following the same methods

as the 2008 NASA icing paper. Section 3.1 through 3.4 presents an update to Section 2 of the 2008 

NASA icing paper and is hereafter referred to as the “update to the NASA icing paper.” At the time of the 

updated analysis, the ASRS online database contained 123 791 incidents, and the search resulted in 3737 

weather-related incidents and 686 icing-related incidents. Figure 1 shows the results of the weatherrelated incidents by year.

The updated weather-related data set was further reduced to 3526 incidents because the AEST project 

is only interested in FAR Parts 121, 135, and 91. To analyze airframe-icing-related incidents more 

precisely, the weather-related incident narratives were searched for “ice” or “icing.” The resulting data set

that contained 275 incidents will be referred to as the ‟airframe-icing-related” incidents.

3.1 Federal Aviation Regulations Parts

Table 2 shows the 2008 NASA icing paper results and the updated results for each FAR part. Of the

total 3526 weather-related incidents for 1994 to 2011, most were categorized as Part 121, followed by

Part 91. Only 245 weather-related incidents were reported for Part 135. The updated analysis resulted in 

686 icing-related incidents compared to 285 in the 2008 NASA icing paper. There were 332 Part 121 

incidents in the updated analysis compared to 131 in 2008. Finally, the updated results showed 275 total 

airframe-icing-related incidents; most were in Part 91 with 182 incidents, followed by Part 121 with 51,

and Part 135 with 42.

Figure 2 and Figure 3 show respective annual weather-related and airframe-icing-related incidents for each 

of the three FAR parts. Even though there has been a downward trend in reported weather and airframe-icing 

incidents over the last 18 yr, it is uncertain whether the trend is due to fewer actual incidents or to fewer 
NASA/TM—2014-217898 5

reported incidents. Because the ASRS is a voluntary reporting system and not all reports submitted actually 

make it into the searchable database, trend analysis is not recommended for ASRS data. 

Figure 4 compares the percentage of airframe-icing-related incidents to weather-related incidents 

for each year. Part 121 incidents stayed between 0 and 9 percent for the 18-yr period, and averaged 

2.3 percent. Part 91 incidents fluctuated between 11.4 and 24.4 percent, and averaged 16.4 percent. 

Part 135 incidents varied significantly, with 100 percent of the weather-related incidents being airframeicing-related in 2008, but they were not listed as weather-related incidents in 2000, 2003, 2006, and 2007. 

More airframe-icing-related incidents were reported for Part 91 each year than for Parts 121 and 135. 

Figure 2.—Annual weather-related incidents.

Figure 3.—Annual airframe-icing-related incidents.

Figure 4.—Airframe-icing-related incidents compared to weather-related incidents per year, percent.
NASA/TM—2014-217898 6

TABLE 3.—UPDATE OF ICING-RELATED INCIDENTS BY PHASE OF

FLIGHT FOR FAR PARTS 121, 135, AND 91 COMBINED

Phase of flight Incidents

Weather-related Update to NASA 

icing paper

Airframe-icingrelated 

Taxi 167 62 0

Takeoff 167 33 5

Initial climb 121 20 10

Climb 285 54 25

Cruise 1495 292 168

Descent 447 101 58

Initial approach 767 95 49

Final approach 32 24 4

Landing 644 75 23

Parked 168 62 3

Phase listed 3412 667 269

Phase not listed 114 19 6

3.2 Phase of Flight

Table 3 shows an update to the icing-related incidents by phase of flight. Of the total 3526 weatherrelated incidents, 3412 listed a phase of flight and 269 airframe-icing-related incidents listed a phase of 

flight. The largest number of incidents occurred during cruise, followed by initial approach, landing, and 

descent, respectively. 

3.3 Flight Crew and General Results

The flight crew and general results categories were labeled “Incidents by Resolutory Actions” in the

2008 NASA icing paper (Ref. 5). The current ASRS database keeps this type of data in the flight crew 

results and general results categories. The flight crew results show the types of actions taken by the flight 

crew or others to resolve the airframe-icing-related anomalies. There were 116 incidents that listed 1 or 

more flight crew results and 159 that did not. Under flight crew results, the largest result was regained 

aircraft control (39), followed by aircraft diverted (32), and took evasive action (22) as shown in Figure 5.

The flight crew overcame equipment problems in 17 incidents and returned to clearance in 16 incidents.

Figure 6 shows that the general results category included incidents in five of the nine possible results.

There were 119 incidents that listed 1 or more general results. Declare an emergency was the largest

result with 69 incidents. This is fewer than the 141 incidents in the 2008 NASA icing paper and is most 

likely because the category was narrowed down to airframe-icing-related incidents only. Many of the 

declare emergency incidents were due to icing on the aircraft, meaning that declaring an emergency was 

the only way the pilot could get clearance from ATC to maneuver out of the icing conditions to a 

different altitude.
NASA/TM—2014-217898 7

Figure 5.—Flight crew results for airframe-icing-related incidents.

Figure 6.—General results for airframe-icing-related incidents.

TABLE 4.—CARBURETOR ICING INCIDENTS IN THE ASRS

Year 1994 1995 1996 1998 2000 2001 2003 2010 2011

Incidents/year 1 1 1 2 1 1 1 1 1

3.4 Carburetor Icing

Table 4 shows the 10 carburetor icing incidents found between 1994 and 2011. The weather primary 

problems data set was searched for “ice” OR “icing” AND “carb” and the narratives were read to ensure 

that the incidents were related to carburetor icing.

3.5 Anomaly Categories

An anomaly is an abnormality that occurs during an incident. Table 5 shows the anomaly categories 

for the 2008 NASA icing paper, the updated results, and weather and airframe icing results for Parts 121, 

135, and 91. The 2008 NASA icing paper grouped ATC issues, procedural deviation, track/heading

deviation, and speed deviation into a nonadherence category.
NASA/TM—2014-217898 8

The greatest number of incidents listed was in the anomaly category of in-flight event or encounter, 

procedural deviation, altitude deviation, and aircraft equipment. Table 6 to Table 9 look more closely at 

each of the anomaly categories by FAR part for airframe-icing-related incidents.

3.5.1 In-Flight Event or Encounter

The in-flight event or encounter anomaly category covers problems such as loss of control (LOC), 

weather or turbulence, flying under visual flight rules in instrument meteorological conditions (VFR in 

IMC), and controlled flight toward terrain or controlled flight into terrain (CFTT or CFIT). Table 6 shows 

the in-flight event or encounter anomalies for airframe-icing-related incidents. The airframe-icing-related 

data listed an in-flight event or encounter anomaly for 268 incidents, with over two-thirds for Part 91. 

Weather or turbulence was the most frequently cited problem, followed by other or unknown, and LOC 

for all three FAR parts combined and individually.

TABLE 5.—ANOMALY INCIDENTS BY SEARCH CRITERIA

Anomaly Incident search criteria

2008 

NASA 

icing paper

Update to 

NASA 

icing paper

FAR Parts 121, 135, and 91 

combined

Weatherrelated Airframeicing-related

Aircraft equipment problem 172 142 407 73

Airspace violation 5 7 75 0

Altitude deviation 165 139 665 74

Conflict 28 28 249 5

Flight deck or cabin aircraft event 8 16 0 0

Ground event encounter 12 62 168 3

Ground excursion 156 19 65 2

Ground incursion 7 4 31 0

In-flight event or encounter 532 574 3103 268

Other 415 183 1402 96

Nonadherence 267

ATC issues 42 87 4

Procedural deviation 251 1532 111

Speed deviation 44 160 7

Track or heading deviation 66 475 23

Total 663 686 3526 275

TABLE 6.—AIRFRAME-ICING-RELATED INCIDENTS BY IN-FLIGHT

EVENT OR ENCOUNTER ANOMALY AND FAR PART

In-flight event or encounter Incidents

FAR Part Total

121 135 91

Bird or animal 0 0 0 0

CFTT or CFITa 0 2 6 8

Fuel issue 1 1 0 2

LOC 9 11 33 53

Object 0 0 0 0

Unstabilized approach 3 0 1 4

VFR in IMCb 0 0 10 10

Wake vortex encounter 0 0 0 0

Weather or turbulence 44 38 170 252

Other or unknown 16 11 55 82

Total 51 42 182 268 a

Controlled flight toward terrain or controlled flight into terrain.

b

Visual flight rules in instrument meteorological conditions.
NASA/TM—2014-217898 9

3.5.2 Procedural Deviation

Table 7 shows the procedural deviation anomalies for airframe-icing-related incidents by procedural 

deviation type and FAR part. Of the 111 total procedural deviation anomalies for all 3 FAR parts, 

clearance was listed in 55 percent and Federal Aviation Regulations was listed in 46 percent. Clearance 

problems usually involved aircraft changing flight direction because of weather in the area or by aircraft 

that could not maintain altitude because of icing. Many pilots requested different headings or altitudes but 

were told to wait by ATC, which caused the pilots to make changes without proper clearance. In some 

cases the pilots were unable to communicate with the ATC because of ice buildup on the aircraft 

antennas. Of the 61 clearance anomalies, 43 incidents were for Part 91 and 14 were in Part 121. The 

Federal Aviation Regulations category involves noncompliance with, or violation of, any regulation 

except during an emergency. Of the 51 total Federal Aviation Regulations incidents, 35 were for Part 91 

and 8 each were for Parts 121 and 135. The third largest category was published material or policy, with 

14 for Part 91, 11 for Part 121, and 4 for Part 135. Some of these incidents were related to the regulations 

covering takeoff or landing in known icing conditions.

3.5.3 Altitude Deviation

Table 8 shows the altitude deviation anomalies for airframe-icing-related incidents by altitude 

deviation type and FAR part. Of the total 74 incidents, 69 were excursions from assigned altitude, 5 were 

overshoots, 1 was an undershoot, and 1 crossing restriction was not met. Most of the excursions from 

assigned altitude were for Part 91 with 50 incidents, followed by Part 121 with 11, and Part 135 with 8. 

Many of these excursions took place when the aircraft could not maintain the assigned altitude because of 

icing in the area or ice buildup. General aviation aircraft (Part 91) are less likely to have the same deicing 

TABLE 7.—AIRFRAME-ICING-RELATED INCIDENTS BY

PROCEDURAL DEVIATION ANOMALY AND FAR PART

Procedural deviation Incidents

FAR Part Total

121 135 91

Clearance 14 4 43 61

Federal Aviation Regulations 8 8 35 51

Hazardous material violation 0 0 0 0

Landing without clearance 1 0 0 1

Maintenance 0 0 0 0

MELa 0 0 0 0

Published material or policy 11 4 14 29

Security 0 0 0 0

Weight and balance 0 0 0 0

Other or unknown 3 0 1 4

Total 27 13 71 111 a

Minimum equipment list.

TABLE 8.—AIRFRAME-ICING-RELATED INCIDENTS BY

ALTITUDE DEVIATION ANOMALY AND FAR PART

Altitude deviation Incidents

FAR Part Total

121 135 91

Crossing restriction not met 1 0 0 1

Excursion from assigned altitude 11 8 50 69

Overshoot 1 1 3 5

Undershoot 1 0 0 1

Total 13 9 52 74
NASA/TM—2014-217898 10

abilities that commercial aircraft have, and thus are more likely to have problems with maintaining or 

changing altitude because of icing conditions. Ten of the Part 91 incidents listed VFR in IMC and had 

either not filed a flight plan or were flying under VFR.

3.5.4 Aircraft Equipment Problem

The two types of aircraft equipment problem anomalies are critical and less severe. Table 9 shows the 

breakdown by type and FAR part. Of the 73 airframe-icing-related incidents that listed an aircraft 

equipment problem anomaly, 49 were critical and 24 were less severe. The largest number of critical 

equipment problems was for Part 91 with 31, followed by Part 121 with 11. Some of these incidents 

included deicing equipment not keeping up with ice accretion, loss of airspeed indicator, iced radio 

antennae, and propeller icing.

3.5.5 Track/Heading Deviation and Speed Deviation 

There were 23 track/heading deviation anomalies: 19 for Part 91 and two each for Parts 135 and 121. 

Most of these anomalies were caused by changes in aircraft direction to get out of icing conditions. There 

were seven speed deviation anomalies with four for Part 121, two for Part 91, and one for Part 135.

3.6 Flight Plan and Flight Conditions

Table 10 shows the flight conditions and the flight plan filed by the pilot by FAR part. The pilots flew 

under instrument flight rules (IFR) for all of Part 121 incidents, all but two Part 135 incidents, and the 

majority of Part 91 incidents.

TABLE 9.—AIRFRAME-ICING-RELATED INCIDENTS BY

AIRCRAFT EQUIPMENT PROBLEM ANOMALY AND FAR PART

Aircraft equipment problem Incidents

FAR Part Total

121 135 91

Critical 11 7 31 49

Less severe 9 3 12 24

Total 20 10 43 73

TABLE 10.—AIRFRAME-ICING-RELATED INCIDENTS BY

FLIGHT CONDITIONS, FLIGHT PLAN, AND FAR PART 

Flight conditions or plan Incidents

FAR Part Total

121 135 91

Flight conditions

IMCa 46 34 138 218

Marginal 0 2 4 6

Mixed 4 2 27 33

VMCb 1 4 8 13

Flight conditions listed 51 42 177 270

Flight plan

DVFRc 0 0 1 1

IFRd 51 38 164 253

VFRe 0 0 9 9

No flight plan filed 0 2 8 10

Flight plan listed 51 40 181 272 a

Instrument meteorological conditions. b

Visual meteorological conditions.

c

Defense visual flight rules.

d

Instrument flight rules.

e

Visual flight rules.
NASA/TM—2014-217898 11

3.7 Weather Element—Visibility

Table 11 shows the visibility weather element conditions for the airframe-icing-related incidents by 

FAR part. Of the 21 incidents for Part 121 that listed 1 or more weather element, all 21 incidents listed 

icing, 8 also listed snow, 5 rain, 6 turbulence, and 4 thunderstorms. Of the 18 airframe-icing-related 

incidents for Part 135, 16 incidents also listed icing, 6 listed rain, 3 thunderstorms, and 3 turbulence. Of 

the 95 incidents for Part 91, all 95 listed weather, 94 also listed icing, 24 rain, 20 turbulence, 9 snow, and 

8 thunderstorms.

3.8 Air Traffic Control Results

Table 12 shows the ATC results for the airframe-icing-related incidents by FAR part. Controllers 

issued new clearances in 71 incidents and provided assistance in 53. Most of the new clearances (55) and 

assistance (46) incidents were for FAR Part 91. These results included issuing new clearances to avoid

icing conditions or providing assistance to get around storms.

TABLE 11.—AIRFRAME-ICING-RELATED INCIDENTS BY

VISIBILITY WEATHER ELEMENT AND FAR PART 

Visibility weather element Incidents

FAR Part Total

121 135 91

Cloudy 0 0 0 0

Fog 1 1 5 7

Hail 0 0 0 0

Haze or smoke 0 0 0 0

Icing 21 16 94 131

Rain 5 6 24 35

Snow 8 1 9 18

Thunderstorm 4 3 8 15

Turbulence 6 3 20 29

Windshear 2 1 5 8

Weather conditions listed 21 18 95 134

Weather conditions not listed 30 24 87 141

TABLE 12.—AIRFRAME-ICING-RELATED INCIDENTS BY

AIR TRAFFIC CONTROL RESULTS AND FAR PART

ATC results Incidents

FAR Part Total

121 135 91

Issued advisory or alert 1 1 8 10

Issued new clearance 10 6 55 71

Provided assistance 3 4 46 53

Separated traffic 1 0 1 2

Total 11 9 89 109
NASA/TM—2014-217898 12

3.9 Route in Use

Table 13 shows the route used by the aircraft during airframe-icing-related incidents. The majority of 

the airframe-icing-related incidents (275 incidents of the 364 total) did not list the route in use. For Part 

121 aircraft that did list a route in use, 9 of the 10 total incidents listed using a vector route and 1 an 

oceanic route. For Part 135, five aircraft used a vector route and one a direct route. For Part 91, 73 

incidents listed a route: 54 aircraft used a vector route, 19 a direct route, and 2 a VFR route. The single 

oceanic route listed for Part 91 was a turbofan corporate jet that flew into icing conditions and made an 

excursion from the assigned altitude because it could not maintain altitude.

3.10 U.S. Region

Table 14 shows the occurrence of airframe-icing-related incidents in each major region of the United 

States. There were 236 airframe-icing-related incidents that listed the state where the incident took place. 

States were grouped into West, Midwest, South, and Northeast regions. 

The West had the most icing incidents (86), with 58 for Part 91, 18 for Part 121, and 10 for Part 135. 

Figure 7(a) shows the western U.S. breakdown by state. California had the greatest number of incidents in 

Parts 121 and 91 incidents (6 and 13, respectively), followed by Colorado (4 and 11, respectively), and 

Washington (2 and 10, respectively). Both New Mexico and Arizona each had two incidents for Part 135. 

TABLE 13.—AIRFRAME-ICING-RELATED INCIDENTS

BY ROUTE IN USE AND FAR PART

Route in use Incidents

FAR Part Total

121 135 91

Direct 0 1 19 20

Oceanic 1 0 1 2

Vector 9 5 54 68

VFRa route 0 0 2 2

Visual approach 0 0 1 1

None 0 0 0 0

Route listed 10 6 73 89

Route not listed 51 42 182 275 a

Visual flight rules.

TABLE 14.—AIRFRAME-ICING-RELATED INCIDENTS

BY U.S. REGION AND FAR PART

Region Incidents

FAR Part

Total

121 135 91

West 18 10 58 86

Midwest 11 11 44 66

South 11 7 28 46

Northeast 5 3 30 38

Total 45 31 160 236
NASA/TM—2014-217898 13

Figure 7.—Airframe-icing incidents by U.S. region and FAR part. (a) West. 

(b) Midwest. (c) South. (d) Northeast.

Figure 7(b) shows the Midwest’s 38 airframe-icing-related incidents. Ohio had 10 incidents for Part 

91, 3 for Part 121, and 4 for Part 135, followed by Indiana with 8 for Part 91, and 1 each for Parts 121

and 135. 

Figure 7(c) shows the South, which had 28 incidents for Part 91 across 12 states; 11 incidents for Part 

121 across 6 states, and 7 incidents for Part 135 across 5 states. For Part 121 the most incidents occurred 

in Florida with four, followed by Georgia and Texas with two each. For Part 135, two incidents each 

occurred in Georgia and Texas. West Virginia had the most: Part 91 incidents with five, followed by 

Texas with four, and Georgia, Maryland, and Virginia with three each. 

Figure 7(d) shows the Northeast, which had the fewest number of incidents. For Part 91 there were 30 

incidents across 7 states, with the most in New York (10) and Pennsylvania (9). For Part 121 there were 

only five incidents in three states, with New Jersey and New York each with two incidents. For Part 135 

only three incidents were reported in the Northeast: Pennsylvania with two and Connecticut with one.
NASA/TM—2014-217898 14

3.11 Technical Challenge 2—Conclusion

The ASRS analysis for TC2, airframe icing simulation and engineering tool capability, resulted in 275

incidents from January 1994 to November 2011 for FAR Parts 121, 135, and 91. The updated AEST analysis

studied the 2008 NASA icing paper (Ref. 5) incidents in more detail to confirm that they were in-flight 

airframe-icing-related incidents and not runway or deicing incidents. The current analysis determined that the 

number of incidents decreased over what was previously shown. Since 1994, fewer incidents were submitted 

or entered into the ASRS database from a high of 9868 in 1999 to a low of 3534 in 2005.

When the data were broken down by FAR part, there were 51 icing incidents for Part 121, 42 for Part 

135, and 182 for Part 91. For some years, no airframe-icing-related incidents were entered into the 

database for Parts 121 or 135. The cruise, descent, and initial approach phases of flight had the greatest 

number of incidents. The visibility weather element anomaly was listed in 131 of the 134 icing incidents, 

with rain listed in 35, and turbulence listed in 29, although weather or turbulence was an in-flight event in 

252 of the 275 incidents.

A majority of the flights filed an IFR flight plan (93 percent) and flew in IMC (80 percent). Of the 

109 incidents for ATC results, 71 were issued a new clearance, 53 were provided assistance, 10 were 

issued an advisory or alert, and 2 were separated traffic. Only 11 ATC results were for Part 121, but Part 

91 had 89. The general results had 69 incidents where the flight crew declared an emergency and the 

flight crew results had 32 aircraft that diverted to other airports, many of these due to the need to exit 

icing conditions. Sixty-nine incidents from the altitude deviation anomaly involved an excursion from the 

assigned altitude. Fifty-three incidents under in-flight event or encounter reported an in-flight LOC. 

Thirty-nine flight crew result incidents reported that the flight crew regained control. 

Most airframe-icing-related incidents, 86 of the 236, occurred in the West. California had 20 incidents, 

more than any other state; of these, 6 were for Part 121 and 13 were for Part 91. In the Midwest, Ohio, with

17 incidents, was second. Of these, 3 incidents were for Part 121, 10 were for Part 91, and 4 were for Part 

135. Surprisingly, Alaska had only four reported airframe-icing-related incidents.

4.0 Technical Challenge 3: Atmospheric Hazard Sensing and Mitigation 

Technology Capability

The objective of TC3 is to improve and expand the remote sensing and mitigation of hazardous 

atmospheric environments and phenomena (Ref. 1) by

• Maturing technologies for real-time sensing and measurement of icing, turbulence, and wake 

vortex

• Dealing with low-visibility conditions for safer runway operations

• Developing technologies for lightning-immune composite aircraft

More advanced, affordable, and improved methods must be developed to provide pilots with needed 

atmospheric information, as well as detecting, collecting, mitigating, and disseminating information.

To address TC3, the current analysis reviewed ASRS data in the following areas:

1. Clear air turbulence (CAT)

2. Wake vortex

3. Windshear

4. St. Elmo’s fire

5. Volcanic ash

6. CFTT or CFIT

7. Ground event or encounter

8. Ground incursion

9. Midair collision hazard

Turbulence and wake vortex from the first TC3 goal are addressed with the ASRS analysis on CAT, 

wake vortex, and windshear. 
NASA/TM—2014-217898 15

Although not specifically mentioned in the TC3 goals, volcanic ash and midair collision hazards were also 

analyzed for relevance to maturing technologies for real-time sensing and measurement. To address lowvisibility conditions for safer runway operation, ASRS data relating to CFTT or CFIT, ground event or 

encounter, and ground incursion were analyzed. The St. Elmo’s fire data analysis can be indirectly applied to

lightning-immune aircraft technology development because St. Elmo’s fire is thought to be a precursor to 

lightning strikes. Lightning was not included in this analysis because that work is planned for the future.

Current remote-sensing capabilities to detect CAT, wake vortex, icing, and low-visibility aircraft 

environments are insufficient. The AEST project is working on technologies to provide real-time 

information to pilots and controllers in the national airspace with a broader range of atmospheric 

hazards information.

4.1 Clear Air Turbulence

The AEST project is working on atmospheric hazard sensing of CAT that is not associated with 

clouds and thus cannot be detected visually or by conventional weather radar. CAT can be caused by 

terrain, thunderstorm complexes, and jet streams, especially at the edges of the jet stream. CAT can 

damage aircraft structures, injure crew or passengers, and impair flight crew performance by making it 

difficult to read instruments.

There were 4647 incidents in the ASRS database that listed the primary problem as either weather- or 

environment-nonweather-related. Of the total number, 916 listed turbulence as a visibility weather 

element and 986 mentioned CAT in the narrative. Only 177 incidents mentioned CAT in the narrative

and turbulence as a visibility weather element, meaning that 809 incidents mentioned CAT in the 

narrative but did not list turbulence, and that 739 incidents listed turbulence as a visibility weather 

element but did not mention CAT. 

Because the database does not have a specific option for CAT, it is hard to be certain that CAT has 

been reported properly. The only way to determine if CAT occurred is by reading the narrative and not all 

incidents contain enough detail. It should not be assumed that all turbulence visibility weather elements

are associated with CAT. It is also possible that weather caused some flights to experience CAT and 

turbulence at different times. For the remainder of this section, only the 986 incidents that specifically 

mentioned “CAT” or “clear air turbulence” in the narrative are discussed. Although the narratives

mention CAT, it does not necessarily mean that CAT was the primary cause of each incident. A more 

detailed study would be required to make that determination.

Sorting the CAT-related incidents by FAR part resulted in 621 incidents for Part 121, 68 for Part 135, 

and 297 for Part 91. Figure 8 shows the number of incidents that mentioned CAT in the narrative by year 

and FAR part. Comparing these data to the number of incidents in the database by year did not show any 

pattern, except a large decrease in the number of reported incidents from 2009 to 2011. This decrease 

could have been because fewer incidents mentioned CAT, fewer incidents were reported, or not all 

reported CAT incidents were entered into the database. Until 2009, an average of 0.91 percent of all 

incidents mentioned CAT.

Figure 8.—Annual clear air turbulence-related incidents by FAR part. 
NASA/TM—2014-217898 16

Figure 9 shows the 349 visibility weather element incidents that mention CAT in the narrative. Many 

more Part 121 incidents (132) mentioned CAT than Part 135 (91). Of the Part 121 incidents that 

mentioned CAT, the following weather elements were also listed: thunderstorms (47), rain (21), and 

windshear (14).

Figure 10 shows CAT-related incidents by phase of flight. The greatest number of incidents occurred 

during cruise, followed by initial approach. A phase of flight was reported for 949 incidents and 

sometimes more than 1 phase of flight was reported for an incident.

Figure 9.—Visibility weather elements for clear air 

turbulence-related incidents by FAR part.

Figure 10.—Phase of flight for clear air turbulence-related incidents by 

FAR part.
NASA/TM—2014-217898 17

As shown in Table 15, 578 CAT-related incidents reported 1 or more general result. The largest 

category was declare emergency with 173 incidents (117 in Part 121), followed by maintenance action 

with 119 incidents (95 in Part 121), and physical injury or incapacitation with 95 incidents (89 in Part 

121). Many of the physical injuries were to the flight attendants when the planes hit turbulence.

Table 16 shows the flight crew results for CAT incidents. There were 437 total CAT-related incidents 

that reported a flight crew result. The largest category was regained control of the aircraft, with 117 

incidents (77 for Part 121), followed by 96 incidents where the aircraft returned to clearance (71 for Part 

121), 90 incidents where the aircraft diverted to another airport (53 for Part 121), and 88 incidents where 

the flight crew took evasive action (62 for Part 121).

TABLE 15.—CLEAR AIR TURBULENCE-RELATED INCIDENTS

BY GENERAL RESULTS AND FAR PART

General results Incidents

FAR Part Total

121 135 91

Declare emergency 117 16 40 173

Evacuated 0 0 0 0

Flight canceled or delayed 11 0 3 14

Maintenance action 95 2 22 119

Physical injury or incapacitation 89 2 4 95

Police or security involved 0 0 0 0

Release refused or aircraft not accepted 0 0 0 0

Work refused 0 0 0 0

None reported or taken 223 29 91 343

Total 393 43 142 578

TABLE 16.—CLEAR AIR TURBULENCE-RELATED INCIDENTS

FOR FLIGHT CREW RESULTS AND FAR PART

Flight crew results Incidents

FAR Part Total 121 135 91

Became reoriented 11 1 8 20

Diverted 53 3 34 90

Executed go around or missed approach 49 1 12 62

Exited penetrated airspace 5 0 10 15

Complied with automation or advisory 0 0 0 0

Overrode automation 13 0 1 14

In-flight shutdown 0 0 0 0

Landed as precaution 14 2 21 37

Landed in emergency conditions 31 1 10 42

Overcame equipment problem 9 0 11 20

Regained aircraft control 77 3 37 117

Rejected takeoff 11 0 4 15

Requested ATC assistance or 

clarification 0 0 0 0

Returned to clearance 71 3 22 96

Returned to departure airport 0 0 0 0

Returned to gate 0 0 0 0

Took evasive action 62 6 20 88

Total 292 17 128 437
NASA/TM—2014-217898 18

The in-flight event or encounter results for CAT-related incidents are shown in Table 17. There were 

677 CAT incidents that reported weather or turbulence affected the flight, with 409 incidents for Part 121 

alone. There were 142 incidents that reported an LOC, with 80 incidents for Part 121.

Table 18 shows the flight conditions and the flight plan filed for the CAT-related incidents. For the 

3 FAR parts combined, 342 incidents were under VMC compared with 309 incidents under IMC;

52 aircraft flew in marginal flight conditions and 130 in mixed conditions. For Part 121 there were 

211 CAT incidents with VMC flight conditions and 186 in IMC. As would be expected for Part 121 

incidents, 617 were under an IFR flight plan and only 1 did not have a flight plan filed. Part 91 had 129 

IFR incidents, 53 VFR incidents, 112 without a flight plan filed, 116 incidents in VMC, and 93 in IMC.

TABLE 17.—CLEAR AIR TURBULENCE-RELATED INCIDENTS FOR

IN-FLIGHT EVENT OR ENCOUNTER ANOMALY AND FAR PART

In-flight event or encounter Incidents

FAR Part Total

121 135 91

Bird or animal 53 0 4 57

CFTT or CFITa 2 3 13 18

Fuel issue 1 0 0 1

LOCb 80 7 55 142

Object 0 0 0 0

Unstabilized approach 13 1 6 20

VFR in IMCc 4 11 72 87

Wake vortex encounter 17 0 4 21

Weather or turbulence 409 52 216 677

Other or unknown 109 11 49 169

Total 529 59 263 851 a

Controlled flight toward terrain or controlled flight into terrain. b

Loss of control.

c

Visual flight rules in instrument meteorological conditions.

TABLE 18.—CLEAR AIR TURBULENCE-RELATED INCIDENTS BY

FLIGHT CONDITIONS OR FLIGHT PLAN, AND FAR PART

Flight conditions or flight plan Incidents

FAR Part Total

121 135 91

Flight conditions

IMCa 186 30 93 309

Marginal 14 6 32 52

Mixed 67 14 49 130

VMCb 211 15 116 342

Flight condition total 478 65 290 833

Flight plan

DVFRc 0 0 2 2

IFRd 617 45 129 791

SVFRe 0 0 4 4

VFRf 0 10 53 63

No flight plan filed 1 10 112 123

Flight plan total 618 65 292 975 a

Instrument meteorological conditions.

b

Visual meteorological conditions.

c

Defense visual flight rules.

d

Instrument flight rules.

e

Special visual flight rules. f

Visual flight rules.
NASA/TM—2014-217898 19

There are a few other categories for CAT incidents that might be interesting to consider. Almost 

69 percent of the incidents (604) in the environmental lighting category occurred during daylight and 

25 percent (223) occurred at night. The aircraft results category listed 126 aircraft as receiving damage 

out of the 130 that reported an aircraft result, with 80 of those in Part 121. An altitude deviation was 

reported in 167 CAT incidents. An altitude excursion was reported in 139 incidents with 88 of them in 

Part 121; 217 CAT incidents reported their route, with 106 vector routes, 45 oceanic, 44 visual approach, 

31 direct, and 6 VFR.

4.2 Wake Vortex

Wake vortex—turbulence that is created by an aircraft in flight—is caused primarily by wingtip 

vortices. A wake vortex encounter occurs when an aircraft enters the wake vortex turbulence created by 

another aircraft. The decay of wingtip vortices can take several minutes and can cause a considerable 

rotational influence on the aircraft that passes through it. The strength of the vortex increases 

proportionately to the weight of the aircraft and is also influenced by the speed and wing shape of the 

aircraft generating it. Wake vortices are encountered most often during initial climb and final approach.

The ASRS only started listing the option for wake vortex encounter in 1999.

Table 19 shows the in-flight event or encounter anomalies. In the data set used for this analysis,

234 incidents reported a wake vortex encounter: 176 for Part 121, 3 for Part 135, and 55 for Part 91. An

LOC event occurred during 72 of the incidents: 47 for Part 121, 1 for Part 135, and 24 for Part 91. For 

Part 121, 7 unstabilized approaches and 8 weather or turbulence encounters were reported for the 176 

wake vortex encounters.

Figure 11 shows the number of reported wake vortex encounter incidents by phase of flight. For all 

three FAR parts the greatest number of incidents occurred during initial approach. Part 121 had the most 

incidents with 54 occurring during initial approach, 37 during descent, and surprisingly 29 during cruise.

TABLE 19.—WAKE VORTEX-RELATED INCIDENTS BY IN-FLIGHT

EVENT OR ENCOUNTER ANOMALY AND FAR PART

In-flight event or encounter Incidents

FAR Part Total

121 135 91

Bird or animal 0 0 0 0

CFTT or CFITa 0 0 0 0

Fuel issue 0 0 0 0

LOCb 47 1 24 72

Object 0 0 0 0

Unstabilized approach 7 0 0 7

VFR in IMCb 0 0 0 0

Wake vortex encounter 176 3 55 234

Weather or turbulence 8 0 1 9

Other or unknown 3 0 0 3

Total 176 3 55 234 a

Controlled flight toward terrain/controlled flight into terrain.

b

Loss of control. c

Visual flight rules in instrument meteorological conditions.
NASA/TM—2014-217898 20

Figure 11.—Phase of flight for wake vortex encounters by FAR part.

TABLE 20.—WAKE VORTEX-RELATED INCIDENTS

BY GENERAL RESULTS AND FAR PART

General results Incidents

FAR Part Total

121 135 91

Declare emergency 1 0 0 1

Evacuated 0 0 1 1

Flight canceled or delayed 0 0 0 0

Maintenance action 8 0 3 11

Physical injury or incapacitation 25 0 4 29

Police or security involved 0 0 0 0

Release refused or aircraft not accepted 0 0 0 0

Work refused 0 0 0 0

None reported or taken 44 0 11 55

Total 65 0 17 82

In the general results category for wake vortex encounters, there were 29 physical injuries or 

incapacitations, with 25 for Part 121. Eleven maintenance actions were required, as shown in Table 20.

Figure 12 shows the flight crew results for wake vortex encounters. There were a total of 142 

incidents for flight crew results: 101 for Part 121, 3 for Part 135, and 38 for Part 91. In the 79 incidents 

where the flight crew regained control of the aircraft, 49 were for Part 121, 1 was for Part 135, and 29 

were for Part 91. In the 32 incidents where the flight crew took evasive action, 25 were for Part 121, 1 for 

Part 135, and 6 for Part 91. The flight crew executed a go around or missed approach in 24 incidents, with 

21 for Part 121 and 3 for Part 91. Returned to clearance was the only other category with more than 20 

incidents, with 15 for Part 121, 1 for Part 135, and 5 for Part 91.
NASA/TM—2014-217898 21

Figure 12.—Flight crew results for wake vortex encounters by FAR part.

4.3 Windshear

Windshear is a rapid change in wind current, both wind speed and direction, over a short distance in 

the atmosphere. According to an FAA advisory, a severe windshear is a rapid change in wind direction or 

velocity causing airspeed changes greater than 15 knot or vertical speed changes greater than 500 ft per 

minute (Ref. 6). Windshear is most dangerous to aircraft at low altitudes and in particular, at takeoff and 

landing because of the short amount of time and distance for recovery. Windshear conditions can occur in 

thunderstorms, rain, strong surface winds, frontal systems, sea breezes, temperature inversions, and 

because of topographical conditions. Windshear is not always associated with rain.

The ASRS database added a windshear option under the weather search option in 1999. From 1999 

through 2011, 355 incidents that listed the primary problem as either weather- or environmentnonweather-related had a windshear element: 244 incidents were for Part 121, 9 for Part 135, and 102 for 

Part 91. Windshear incidents by year and FAR part can be seen in Figure 13 and Table 21. Windshear 

accounts for 8 to 16 percent of the weather or environmental primary problems. There is no pattern of 

windshear incidents decreasing.

Figure 14 shows the phases of flight for windshear incidents by FAR part. The greatest number of 

incidents occurred during initial approach (126) and landing (108). The majority of the incidents reported 

were for Part 121, with 97 during initial approach and 80 during landing. During cruise there were almost 

as many Part 91 incidents (25) as Part 121 incidents (27). 
NASA/TM—2014-217898 22

Figure 13.—Windshear-related incidents by year and FAR part.

TABLE 21.—NUMBER AND PERCENTAGE OF WINDSHEAR-RELATED INCIDENTS BY FAR PART

Category Incidents per year Total

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Weather or environmental 339 292 184 277 339 243 150 247 186 253 232 218 158 3118

Windshear-related 38 29 15 36 32 29 17 34 21 29 22 27 26 355

Windshear-related, percent 11 10 8 13 9 12 11 14 11 11 9 12 16 11

FAR Part

121 25 21 10 30 23 17 11 24 14 20 18 17 14 244

135 1 1 2 0 0 1 1 0 0 0 0 1 2 9

91 12 7 3 6 9 11 5 10 7 9 4 9 10 102

Figure 14.—Phase of flight for windshear-related incidents by FAR part.
NASA/TM—2014-217898 23

Figure 15 shows the visibility weather element for windshear-related incidents. Some of the incidents 

involved other weather elements: turbulence was reported in 60 percent of the incidents, thunderstorms in 

32 percent, and rain in 27 percent.

Table 22 shows the general results for windshear-related incidents. The most frequent general result 

was maintenance action with 73 total incidents: 58 for Part 121, 14 for Part 91, and 1 for Part 135. An 

emergency was declared in 31 incidents: 26 for Part 121, 4 for Part 91, and 1 for Part 135. Physical injury 

or incapacitation occurred in eight incidents: six for Part 121 and two for Part 91.

Figure 15.—Visibility weather elements for windshear-related incidents by FAR part.

TABLE 22.—WINDSHEAR-RELATED INCIDENTS

BY GENERAL RESULTS AND FAR PART

General results Incidents

FAR part Total

121 135 91

Declare emergency 26 1 4 31

Evacuated 0 0 1 1

Flight canceled or delayed 8 0 3 11

Maintenance action 58 1 14 73

Physical injury or incapacitation 6 0 2 8

Police or security involved 0 0 0 0

Release refused or aircraft not accepted 0 0 0 0

Work refused 0 0 0 0

None reported or taken 44 1 23 68

Total 109 3 37 149
NASA/TM—2014-217898 24

As shown in Figure 16, 264 incidents reported a flight crew result with 184 for Part 121, 8 for Part 

135, and 72 for Part 91. The greatest number of incidents (114) executed a go around or missed approach, 

with 99 for Part 121, 13 for Part 91, and 2 for Part 135. This was followed by 108 incidents that regained 

aircraft control (65 incidents for Part 121, 39 for Part 91, and 4 for Part 135). Diverted was third with 76 

incidents (57 for Part 121, 17 for Part 91, and 2 for Part 135) followed by took evasive action with 54 

incidents (37 for Part 121, 13 for Part 91, and 4 for Part 135). Eleven incidents rejected takeoff.

Table 23 shows the in-flight event or encounter anomalies for windshear-related incidents. LOC was 

reported in 116 total incidents with 65 for Part 121 and 47 for Part 91. Unstabilized approaches were 

reported for 51 incidents, and weather or turbulence for 302 incidents. The majority of the encounters 

were for Part 121 categories.

Table 24 shows the last areas analyzed for windshear-related incidents—flight conditions and flight 

plan. Flight conditions were reported for 267 incidents. Of these, 137 were in VMC: 80 for Part 121 and 

54 for Part 91. IMC was reported for 87 incidents: 57 for Part 121, 27 for Part 91, and 3 for Part 135. Of 

the 345 incidents that provided flight plan information, 305 had an instrument flight plan, 24 had not filed 

a flight plan (22 for Part 91), and 16 had a visual flight plan (15 for Part 91).

Figure 16.—Flight crew results for windshear-related incidents by FAR part.
NASA/TM—2014-217898 25

TABLE 23.—WINDSHEAR-RELATED INCIDENTS BY IN-FLIGHT

EVENT OR ENCOUNTER ANOMALY AND FAR PART 

In-flight event or encounter Incidents

FAR Part Total

121 135 91

Bird or animal 0 0 2 2

CFTT or CFITa 1 0 5 6

Fuel issue 7 0 0 7

LOCb 65 4 47 116

Object 0 0 0 0

Unstabilized approach 38 0 13 51

VFR in IMCc 0 2 3 5

Wake vortex encounter 4 0 0 4

Weather or turbulence 209 9 84 302

Other or unknown 70 1 25 96

Total 226 9 97 332 a

Controlled flight toward terrain or controlled flight into terrain. b

Loss of control. c

Visual flight rules in instrument meteorological conditions.

TABLE 24.—WINDSHEAR-RELATED INCIDENTS BY FLIGHT

CONDITIONS OR FLIGHT PLAN AND FAR PART

Flight conditions or flight plan Incidents

FAR Part Total

121 135 91

Flight conditions

IMCa 57 3 27 87

Marginal 5 2 5 12

Mixed 22 1 8 31

VMCb 80 3 54 137

Flight condition total 164 9 94 267

Flight plan

DVFR or SVFRc 0 1 2 3

IFRd 236 7 62 305

VFRe 0 1 15 16

No flight plan filed 1 1 22 24

Flight plan total 237 9 99 345 a

Instrument meteorological conditions. b

Visual meteorological conditions. c

Defense visual flight rules or special visual flight rules. d

Instrument flight rules. e

Visual flight rules.

4.4 St. Elmo’s Fire

St. Elmo’s fire is a weather phenomenon in which a coronal discharge lights up the aircraft surface 

where maximum electrical static discharge occurs (Ref. 7). It is an indication that thunderstorm activity is 

in the area and could be a precursor to a lightning strike.

A search of weather and nonweather primary problems for “st. elmo” or “saint elmo” found 13 St. 

Elmo’s fire incidents in the narratives, with 12 for Part 121 and 1 for Part 135. Nine incidents occurred 

during cruise, two each during descent and initial approach, and one each during climb and landing.
NASA/TM—2014-217898 26

In the general results category for St. Elmo’s fire shown in Figure 17, four incidents reported 

physical injury or incapacitation, four declared an emergency, and two required a maintenance action.

The flight crew results in Figure 18 show that the flight crew regained control in five incidents and the

aircraft returned to clearance in four incidents. 

Figure 19 shows the in-flight event or encounter anomalies for St. Elmo’s fire incidents. Eleven 

incidents reported weather or turbulence and four each reported an LOC and other or unknown. The 

visibility weather elements that occurred at the time of the St. Elmo’s fire incidents for FAR Part 121 is 

shown in Figure 20. Five incidents were involved with turbulence, four with thunderstorms, and three 

each icing and rain. Eleven of the St. Elmo’s fire incidents reported the time as night and one reported 

daylight. The only Part 135 St. Elmo’s fire incident did not report a visibility weather element.

Figure 17.—General results for St. Elmo's fire incidents. 

Figure 18.—Flight crew results for St. Elmo's fire incidents.
NASA/TM—2014-217898 27

Figure 19.—In-flight events or encounters for St. 

Elmo's fire incidents.

Figure 20.—Visibility weather element for St. Elmo's 

fire.

4.5 Volcanic Ash

Eight incidents mentioned volcano, volcanic, or ash with a primary problem of environmentnonweather-related, and all were for FAR Part 121. Three incidents required a maintenance action and 

one flight was canceled. Narratives mentioned that ash clouds were encountered but had not been 

reported; ash clouds had expanded from where they were predicted; pilots had tried to get rerouting 

information around ash clouds, but dispatch said there was no issue, even though the pilots could see the 

brown ash cloud in the distance; or that foreign weather services were not reporting on volcanic activity 

properly. There was no mention of actual damage to aircraft or engines, but a few incidents did report

brown specks on the windscreen.

4.6 Controlled Flight Toward Terrain or Controlled Flight Into Terrain

Most of the CFTT or CFIT incidents involved CFTT and spot checks of the narratives provided a 

variety of reasons. Some aircraft were too low on the glide slope, while others made visual approaches, or

were too close to low hills, which caused the terrain warning to sound. Other incidents were caused by

towers on the ground or on top of buildings. Some pilots reported flying into the warning area to try to

figure out what caused the warning. In other incidents, the flight crews complained about approach plates 

and the lack of navigation information only to discover that they were not the only ones having these issues.

There were 101 CFTT or CFIT incidents in the database for primary problems of weather or 

environment: 35 for Part 121, 8 for Part 135, and 58 for Part 91. Figure 21 shows the phases of flight in 

which the incidents occurred. Initial approach had the greatest number of incidents, followed by landing 

and cruise. Figure 22 shows the flight crew results for CFTT or CFIT incidents. The flight crew took 

evasive action in 19 incidents and executed a go around or missed approach in 13 incidents.
NASA/TM—2014-217898 28

Figure 21.—Phase of flight for controlled flight toward terrain or 

controlled flight into terrain incidents.

Figure 22.—Flight crew results for controlled flight toward terrain or controlled flight 

into terrain incidents.

Table 25 shows the 101 total in-flight event or encounter anomalies that occurred during CFTT or CFIT 

incidents; 4 of the 35 incidents for Part 121 also had an LOC and 14 involved weather or turbulence. Twelve 

of the 58 incidents for Part 91 also had an LOC and 36 involved weather or turbulence.

Table 26 shows the environmental light conditions reported during CFTT or CFIT incidents. Of the 

95 total incidents reporting light conditions, 65 occurred during daylight with 14 for Part 121, 7 for Part 

135, and 44 for Part 91. Only one Part 121 and one Part 91 occurred at dawn, and one Part 121 and five 

Part 91 incidents occurred at dusk. Thirteen incidents for Part 121, one for Part 135, and eight for Part 91 

occurred at night.

Table 27 shows the visibility weather elements that were listed for CFTT or CFIT incidents. Only 22 

of the 101 incidents listed a weather element, with rain and turbulence both at 7. Windshear and icing 

followed with six incidents each. 
NASA/TM—2014-217898 29

TABLE 25.—CFTT/CFITa-RELATED INCIDENTS BY IN-FLIGHT

EVENT OR ENCOUNTER ANOMALY AND FAR PART

In-flight event or encounter Incidents

FAR Part Total

121 135 91

Bird or animal 0 0 0 0

CFTT or CFITa 35 8 58 101

Fuel issue 0 0 0 0

LOCb 4 0 12 16

Object 0 0 0 0

Unstabilized approach 2 0 2 4

VFR in IMCc 0 1 8 9

Wake vortex encounter 0 0 0 0

Weather or turbulence 14 7 36 57

Other or unknown 1 0 11 12

Total 35 8 58 101 a

Controlled flight toward terrain or controlled flight into terrain. b

Loss of control. c

Visual flight rules in instrument meteorological conditions.

TABLE 26.—CFTT/CFITa

–RELATED INCIDENTS BY

ENVIRONMENTAL LIGHT CONDITIONS AND FAR PART

Light conditions Incidents

FAR Part Total

121 135 91

Dawn 1 0 1 2

Daylight 14 7 44 65

Dusk 1 0 5 6

Night 13 1 8 22

Total 29 8 58 95 a

Controlled flight toward terrain or controlled flight into terrain.

Table 27.—CFTT/CFITa

-RELATED INCIDENTS BY

VISIBILITY WEATHER ELEMENT AND FAR PART

Visibility weather element Incidents

FAR Part Total

121 135 91

Cloudy 0 0 0 0

Fog 1 0 3 4

Hail 0 0 0 0

Haze or smoke 0 0 0 0

Icing 0 0 6 6

Rain 3 0 4 7

Snow 0 0 2 2

Thunderstorm 1 0 1 2

Turbulence 2 1 4 7

Windshear 1 0 5 6

Total 6 1 15 22 a

Controlled flight toward terrain or controlled flight into terrain.
NASA/TM—2014-217898 30

During the CFTT or CFIT incidents 11 emergencies were declared; there were 5 maintenance actions 

and 2 physical injuries or incapacitations, and 20 aircraft were damaged. Obtaining a better understanding 

of the CFTT or CFIT incidents would require reading through all 101 narratives and trying to add more 

categorizations.

4.7 Ground Event or Encounter

The ground event or encounter and runway incursion data sets with the primary problem of weather 

or environment-nonweather are used to address TC3’s low-visibility conditions for safer runway 

operations. Both of these could occur during low visibility conditions on the runway. The ground event or 

encounter category includes

• Near misses or physical contact of such items as aircraft, vehicles, animals, people, and birds

within the aerodrome

• Foreign object damage (FOD)

• Gear-up landings

• Aircraft ground strikes

There were 315 ground events or encounters, with 174 for Part 121, 22 for Part 135, and 118 for Part 91.

By definition, a ground event or encounter only include incidents that occurred during taxi, takeoff, landing, 

and while parked when any part of the aircraft is in contact with the surface. The aircraft category, which is 

the aircraft striking another aircraft on the ground, had 176 incidents with 90 Part 121, 13 Part 135, and 73 

Part 91. LOC on the ground had 167 incidents, with 87 Part 121, 13 Part 135, and 67 Part 91.

Figure 23 shows ground event or encounter incidents by phase of flight and FAR part. The most 

frequent phase of flight cited was landing with 124 incidents, followed by taxi with 102, takeoff with 54, 

Figure 23.—Ground event or encounter incidents during four phases of flight by FAR part.
NASA/TM—2014-217898 31

and parked with 50 incidents. Aircraft (strikes another aircraft) and LOC were the most frequent incidents 

in the landing phase of flight for all FAR Parts, with 34 and 32 incidents for Part 121, respectively, 5 each 

for Part 135, and Part 91 had 44 and 39, respectively. The taxi phase of flight was the second largest 

group with 102 incidents and the majority in aircraft (strikes another aircraft) and LOC, with Part 121 

having 47 each, Part 135 having 6 each, and Part 91 having 19 each. The takeoff phase of flight had three 

top categories: aircraft (strikes another aircraft), LOC, and person/animal/bird.

Table 28 shows the environmental lighting conditions during the ground event or encounter incidents: 

181 incidents occurred during daylight and 78 at night.

Table 29 shows the visibility weather elements for ground event or encounter incidents. Snow was a 

factor in 44 of the incidents with 34 for Part 121. Icing—primarily on the runway or taxiway surface—

was a factor in 39 incidents. Fog had an impact on 11 incidents and rain, thunderstorms, and turbulence 

were also factors for incidents. Obtaining a full understanding of the impact that the weather had on 

ground event or encounters would require reading through each of the narratives. There was not sufficient 

time to do so for this study.

TABLE 28.—GROUND EVENT OR ENCOUNTER-RELATED INCIDENTS

BY ENVIRONMENTAL LIGHT CONDITIONS AND FAR PART

Light conditions Incidents

FAR Part Total

121 135 91

Dawn 5 3 2 10

Daylight 79 11 91 181

Dusk 20 0 4 6

Night 55 7 16 78

Total 141 21 113 275

TABLE 29.—GROUND EVENT- OR ENCOUNTER-RELATED INCIDENTS

BY VISIBILITY WEATHER ELEMENT AND FAR PART

Visibility weather element Incidents

FAR Part Total

121 135 91

Cloudy 0 0 0 0

Fog 6 4 1 11

Hail 2 0 0 2

Haze/smoke 0 0 2 2

Icing 31 3 5 39

Rain 22 3 11 36

Snow 34 0 10 44

Thunderstorm 13 1 6 20

Turbulence 14 0 14 28

Windshear 18 0 13 31

Total 82 8 39 129
NASA/TM—2014-217898 32

Other noteworthy ground event or encounter-related incident data that is not shown in tables or 

figures is summarized below: 

• Of the 66 ground excursions, 51 occurred on the runway, 14 on the taxiway, and 2 at ramps.

• Of the 10 ground incursions, 9 occurred on the runway and 2 on the taxiway.

• Of the 225 general results, 108 required maintenance actions, 24 flights were canceled or 

delayed, and 8 encounters resulted in physical injuries or incapacitations.

• Of the 127 aircraft results, 126 aircraft received damage, with 49 Part 121, 11 Part 135, and 66 

Part 91.

4.8 Ground Incursions

The ASRS considered a ground incursion to be the undesirable or unwanted entry of an aircraft, 

vehicle, person, or animal into a confined, marked, or identified standard movement area within the 

runway or taxiway. Ground incursions include collision with, risk of collision, or evasive action being 

taken. There were 53 ground incursions with a primary problem of weather or environment: 47 on a

runway and 8 on a taxiway; 25 incidents were for Part 121, 8 were for Part 135, and 20 were for Part 91.

Figure 24 shows the phase of flight results for ground incursion incidents: 35 incidents occurred 

during taxi and 12 during landing. As shown in Table 30, 47 total incidents reported environmental light

conditions during the incursion with 23 incidents occurring at night, 22 in daylight, and 1 each at dusk 

and dawn. 

Figure 24.—Phase of flight for ground incursion incidents.

TABLE 30.—GROUND INCURSION-RELATED INCIDENTS BY

ENVIRONMENTAL LIGHT CONDITIONS AND FAR PART

Light conditions Incidents

FAR Part Total

121 135 91

Dawn 1 0 0 1

Daylight 6 3 13 22

Dusk 0 0 1 1

Night 12 5 6 23

Total 19 8 20 47
NASA/TM—2014-217898 33

Table 31 shows the visibility weather elements that affected the ground incursions and could have 

affected the pilots’ visibility. Ten incidents involved snow, seven rain, and five thunderstorms. 

4.9 Near-Midair Collision Hazard

The ASRS incident database had 46 near-midair collisions (NMACs) that listed weather or 

environment as the primary problem. An NMAC is defined in the ASRS as an event that has less than 500 

ft of vertical and horizontal separation between two airborne aircraft. A total of 12 of the NMACs were 

for Part 121, 1 for Part 135, and 33 for Part 91. Only four NMAC incidents listed one or more visibility 

weather elements: one listed haze or smoke, two listed rain, three listed thunderstorms, three listed 

turbulence, and two listed windshear. Figure 25 shows the phases of flight for the NMAC incidents by 

FAR part—17 incidents occurred during cruise including 4 for Part 121, 1 for Part 135, and 12 for Part 

91. During initial approach there were nine incidents for Part 91 and two for Part 121. The single Part 121 

incident that occurred during the parked phase of flight also listed the taxi and landing phases of flight. It 

was actually on the ground, but was described in the ASRS as an NMAC on the ground in dense fog. 

TABLE 31.—GROUND INCURSION-RELATED INCIDENTS BY

VISIBILITY WEATHER ELEMENT AND FAR PART

Visibility weather 

element

Incidents

FAR Part Total

121 135 91

Cloudy 0 0 0 0

Fog 2 1 0 3

Hail 0 0 0 0

Haze or smoke 0 0 1 1

Icing 1 1 1 3

Rain 3 1 3 7

Snow 5 1 4 10

Thunderstorm 3 1 1 5

Turbulence 0 0 1 1

Windshear 1 0 1 2

Total 10 3 8 21

Figure 25.—Phase of flight for near-midair collision-related 

incidents.
NASA/TM—2014-217898 34

Figure 26 shows the flight crew results for NMAC-related incidents; 32 reported a flight crew result

with 28 incidents taking evasive action. The aircraft returned to clearance in seven incidents and the flight 

crew executed a go around or missed approach in three. Only 15 of the NMAC incidents reported an inflight event or encounter as shown in Table 32. Weather or turbulence was involved in the most in-flight 

event or encounters with nine.

Figure 26.—Flight crew results for near-midair-collision-related incidents.

TABLE 32.—NEAR-MIDAIR-COLLISION-RELATED INCIDENTS BY

IN-FLIGHT EVENT OR ENCOUNTER ANOMALY AND FAR PART

In-flight event or encounter Incidents

FAR Part Total

121 135 91

Bird or animal 0 0 0 0

CFTT or CFITa 0 0 1 1

Fuel issue 0 0 0 0

LOCb 1 0 0 1

Object 0 0 1 1

Unstabilized approach 1 0 0 1

VFR in IMCc 0 0 1 1

Wake vortex encounter 0 0 0 0

Weather or turbulence 3 0 6 9

Other or unknown 2 0 2 4

Total 5 0 10 15 a

Controlled flight toward terrain or controlled flight into terrain.

b

Loss of control. c

Visual flight rules in instrument meteorological conditions.
NASA/TM—2014-217898 35

TABLE 33.—NEAR-MIDAIR-COLLISION-RELATED INCIDENTS

BY FLIGHT CONDITIONS AND FARA PART

Flight conditions Incidents

FAR Part Total

121 135 91

IMCb 4 0 1 5

Marginal 0 0 1 1

Mixed 2 0 2 4

VMCc 4 1 29 34

Total 10 1 33 44 a

Federal Aviation Regulations.

b

Instrument meteorological conditions. c

Visual meteorological conditions.

Table 33 shows the flight conditions in which the NMAC-related incidents occurred. Of the 44 total 

incidents 34 occurred while the aircraft was flying in VMC. Only five incidents occurred in IMC, four in 

mixed conditions, and one in marginal meteorological conditions. Thirty-nine incidents occurred in 

daylight, four were at night, and two were at dusk.

4.10 Technical Challenge 3—Conclusion

The ASRS analysis for TC3, atmospheric hazard sensing and mitigation technology capability, 

reviewed the 4647 incidents in the database that involved the primary problems of weather- and 

environment- nonweather-related incidents. A variety of atmospheric hazards were analyzed, both in the 

air and on the ground. All categories could undergo a more detailed analysis in the future if the project is 

interested. That analysis would include reading all narratives to gain a better understanding of how the 

events affected each other.

CAT was specifically mentioned in 986 incident narratives, with two-thirds of the incidents for FAR 

Part 121. Most of the incidents occurred during cruise and initial approach. The CAT-related incidents

resulted in 95 physical injuries or incapacitations, 173 declared emergencies, and 119 maintenance 

actions. Of the 142 CAT incidents that reported an in-flight LOC, the flight crew regained control in 117 

incidents and the aircraft returned to clearance in 96. The number of CAT reports decreased from 1994 to 

2011, especially in 2001, 2009, and 2011. This could have been caused by fewer incidents or insufficient

ASRS analysis resources.

Of the 3118 weather or environmental incidents from 1999 through 2011, 355 were windshear-related

incidents. Although the percentage varied between 8 and 16 percent over the last 14 yr, windshear-related

incidents in the weather or environment categories have not declined. Windshear incidents occurred most 

frequently during initial approach and landing, especially for Part 121. Flight crews had to execute a go 

around in 114 of these incidents. As a result, 73 incidents reported a maintenance action and 31 declared

an emergency.

Wake vortex was reported in 234 incidents for all 3 FAR parts, with the greatest number in Part 121 

aircraft during initial approach and descent. Twenty-nine of these incidents caused physical injury or 

incapacitation. Following the wake vortex encounters, flight crew regained aircraft control (79 incidents), 

took evasive action (32), executed a go around or missed approach (24), or returned to clearance (21). 

The ASRS database had 101 CFTT or CFIT incidents that listed weather or environment as the 

primary problem: 58 for Part 91 and 35 for Part 121. These incidents occurred most frequently during 

initial approach, landing, and cruise. Weather or turbulence was reported as a factor in 57 incidents and 

65 occurred during daylight and 22 at night. The flight crews took evasive action in 19 incidents and 

executed a go around in 13.

NMAC incidents were reported in 46 incidents with a primary problem of weather or environment: 

33 for Part 91, 1 for Part 135, and 12 for Part 121. NMACs occurred most frequently during cruise and 
NASA/TM—2014-217898 36

initial approach; 34 of these incidents occurred in VMC, 5 in IMC, 4 in mixed conditions, and 1 in 

marginal conditions.

The analysis on St. Elmo’s fire resulted in 13 St. Elmo’s fire incidents along with 8 incidents that 

involved volcanic ash with 3 that required maintenance action.

Two areas were analyzed for low-visibility runway environments: ground event or encounters and 

ground incursions. There were 314 ground event or encounter incidents: 174 for Part 121 and 118 for

Part 91. Another aircraft was involved in 176 incidents, an LOC in 167, and a bird, animal, or person in 

33. The greatest number of encounters occurred during landing and taxi, with 181 incidents during 

daylight and 78 at night. A visibility weather element was reported for 129 of the incidents and aircraft

damage was reported for 126. Maintenance actions were required for 108 incidents and 8 incidents 

resulted in physical injury or incapacitation.

A total of 53 ground incursion incidents were found in the data set with 35 incursions during the taxi 

phase of flight; 23 incidents occurred at night and 22 during the day. A visibility weather element was a 

factor in 21 of the incidents.

5.0 Conclusions

The Aviation Safety Reporting System (ASRS) database was searched for any incidents related to the 

three Aviation Safety Atmospheric Environment Safety Technology (AEST) technical challenges (TCs) 

in support of milestone AEST3.2.SA.01: Identification of AEST-Related Trends. Of the 163 558 incident 

reports in the ASRS database, 3526 incidents were related to weather and 1122 incidents were related to 

the environment.

Airframe icing is the focus of TC2, airframe icing simulation and engineering tool capability. The

current study performed an update to the 2008 NASA icing paper as well as a search of incident data

through 2011. The search resulted in 275 incidents related to airframe icing from 1994 through 2011, with 

the majority (182 incidents) for Part 91 aircraft. Incidents are presented by FAR Parts 121, 135, and 91 

and by phase of flight. Airframe incidents occurred most often during cruise, descent, and initial 

approach. Flight crews declared emergencies in 69 incidents, 252 involved an encounter with weather or

turbulence, 53 involved a loss of control (LOC), and 218 flew in instrument meteorological conditions 

(IMC) under an instrument flight rules (IFR) flight plan. In addition, 61 incidents resulted in a clearance 

deviation, 69 resulted in an altitude deviation, and 49 involved critical equipment problems.

The focus of TC3, atmospheric hazard sensing and mitigation technology capability, includes realtime sensing and measurement of icing, turbulence, wake vortex hazards, runway safety under lowvisibility conditions, and lightning-immune composite aircraft. The airframe-icing-related analysis 

completed for TC2 also applies to TC3. The ASRS database was searched for incidents that involved 

1. Clear air turbulence (CAT)

2. Wake vortex

3. Windshear

4. St. Elmo’s fire (related to lightning risk)

5. Volcanic ash

6. Runway safety (ground event or encounters and ground incursions)

7. Low visibility (controlled flight toward terrain or controlled flight into terrain and near-midair 

collisions)

CAT was involved in a total of 986 incidents with 621 incidents for Part 121. Of the 986 incidents, 

677 resulted in an encounter with weather or turbulence (409 for Part 121), and 142 resulted in a LOC (80 

for Part 121). CAT incidents, which occurred primarily during cruise, resulted in 173 declared 

emergencies, 119 required maintenance actions, and 95 physical injuries/incapacitations. Aircraft control 

was regained in 177 incidents, aircraft were diverted during 90 incidents, and evasive action was taken for 

88 incidents. An IFR plan was filed for 791 of the flights and 309 aircraft flew in IMC. 
NASA/TM—2014-217898 37

Wake vortex was involved in 234 incidents (176 for Part 121). Of these, 72 incidents involved an 

LOC, 29 involved physical injury or incapacitation, and 11 required maintenance action. A majority of 

wake vortex incidents occurred during the initial approach and descent phases of flight.

Windshear was reported in 355 incidents (244 for Part 121). The majority of these incidents occurred 

during the initial approach and landing phases of flight, and they most frequently resulted in an executed 

go around/missed approach, regained aircraft control, and diverted. Maintenance action was required for

73 incidents and emergencies were declared in 31 incidents. An IFR flight plan was used in 305 of the 

incidents, and 137 incidents involved aircraft flying in VMC. 

St. Elmo’s fire was involved in 13 incidents and volcanic ash in 8 incidents. 

CFTT or CFIT was reported in 101 incidents (35 for Part 121). Of these, 47 incidents occurred during 

initial approach, 57 involved weather or turbulence, and 65 occurred during daylight. Most often CFTT or 

CFIT resulted in the flight crew taking evasive action and executing a go around. 

Ground events/encounters occurred in 314 incidents (174 for Part 121); 176 incidents involved 

encounters with other aircraft and 167 incidents involved LOC. The majority of these incidents occurred 

during the landing phase of flight during daylight hours.

To verify which of the incidents lightning, CAT, windshear, and other conditions were causal factors

would require an analyst to read each narrative. This could be done in a future AEST project study.

NASA/TM—2014-217898 39

Appendix A.—Acronyms

AEST Atmospheric Environment Safety Technology

ASRS Aviation Safety Reporting System 

ATC air traffic control

AvSP Aviation Safety Program

CAST Commercial Aviation Safety Team 

CAT clear air turbulence

CDL Configuration Deviation List

CFTT or CFIT controlled flight toward terrain or controlled flight into terrain

DVFR defense visual flight rules

FAA Federal Aviation Administration 

FAR Federal Aviation Regulations

FOD foreign object damage

HAT height above terrain

HIWC high ice water content

ICAO International Civil Aviation Organization 

IFR instrument flight rules

IMC instrument meteorological conditions

LOC loss of control or loss of aircraft control

MEL minimum equipment list

NEF nonessential function (list)

NMAC near-midair collision

SID standard instrument departure

STAR standard terminal arrival route

TC technical challenge

VFR visual flight rules

VMC visual meteorological conditions

NASA/TM—2014-217898 41

Appendix B.—Aviation Safety Reporting System Category Descriptions

Only some ASRS categories apply to Atmospheric Environment Safety Technology and if the 

category is discussed, all options are presented. Most of the descriptions and notes came from the ASRS 

Standard Operating Procedures (an internal document). Many of the terms and definitions are from the 

Commercial Aviation Safety Team (CAST)/ICAO web site for aviation common taxonomies (Ref. 5).

Phase of flight (10 options)

• Taxi

• Takeoff

• Initial climb

• Climb

• Cruise

• Descent

• Initial approach

• Final approach

• Landing

• Parked

Flight crew results (17 options or subcategories)

• Became reoriented: Overcame disorientation

• Diverted: Diverted to a filed alternate or any airport that is not a filed alternate. Does not include 

returning to departure airport

• Executed go around or missed approach: A pilot or controller-initiated action, typically to avoid a 

potentially hazardous situation/condition on a movement surface, that does not require adherence 

to a published missed approach procedure. A missed approach is a procedure that is formulated 

for each published instrument approach and it is initiated by either the pilot or the controller due 

to weather, unstable approach, “fouled” runway, etc. It allows the pilot to enter a new approach 

while remaining clear of obstacles.

• Exited penetrated airspace: Airborne maneuver to exit erroneously penetrated airspace

• Flight crew complied with automation or advisory: Flight crew followed what was recommended 

by what computer

• Flight crew overrode automation: Pilot or flight crew took action and overrode any aircraft 

programmed system

• In-flight shutdown: Pilot or flight crew shut down one or more engines

• Landed as precaution: May or may not be caused by an emergency event

• Landed in emergency conditions

• Overcame equipment problem: Pilot or flight crew restored function of malfunctioning 

equipment, or developed a successful ‟work-around” to deal with loss of equipment

• Regained aircraft control: Pilot or flight crew became aware of and took successful action to 

resolve an LOC

• Rejected takeoff: Pilot- or ATC-initiated takeoff abort

• Requested ATC assistance or clarification: Pilot or flight crew asked ATC for vectors or other 

assistance including filing an en route IFR clearance

• Returned to clearance

• Returned to departure airport: Diverted or returned to departure airport

• Returned to gate: While on ground, aircraft returned to gate

• Took evasive action: Evasive, typically an abrupt action taken, whether on the ground or 

airborne, to avoid another aircraft, object, terrain, weather, or environmental conditions (such as 

wake turbulence). Also includes precautionary avoidance action.
NASA/TM—2014-217898 42

General results (9 options)

• Declare emergency

• Evacuated: Personnel and/or passengers leave the aircraft

• Flight canceled or delayed: The flight did not proceed on a previously planned or scheduled flight 

or was delayed

• Maintenance action: The reporter has provided information that maintenance activity was or is to 

be conducted

• Physical injury or incapacitation: Injury or incapacitation has occurred as a result of the event or

incident, airborne or on the ground. If it “was” the event, it should be coded under anomaly.

• Police or security involved—As a result of the event or incident

• Release refused or aircraft not accepted—Usually pertaining to flight crew not accepting the 

aircraft release

• Work refused: As a result of the incident, the person refused to do their work. Can include flight 

assignment or any assignment refused by reporter

• Nonreported or taken: This can only be coded if nothing else can be coded under “result.” No 

result is reported and no action is taken can be for insufficient time, detection after the fact, 

reporter was unaware of the problem, or the reporter was aware but accepted the condition.

Of the 15 different event anomaly categories that can be searched in ASRS, only 8 were of value to 

this analysis and are listed below. Anomalies are coded to reflect what happened when those involved 

became aware of the problem. Some of the anomalies list deviations or violations. An anomaly deviation 

does not necessarily mean that a violation of rules, regulations, or procedures has occurred.

In-flight event or encounter (10 options)

• CFTT or CFIT: Pilot or crew has control of an airworthy, mechanically normal aircraft, but is 

unaware of in-flight proximity to dangerous or unsafe terrain or obstacles. An example is an 

aircraft in IMC vectored unacceptably close to terrain.

• Fuel issue: Event related to a fuel concern that occurred during flight

• LOC: Pilot is unable to effectively maintain control of the aircraft in flight, due to pilot error, 

environment, aircraft, or other reasons

• Object: Physical contact with any object during aircraft flight other than what is covered in the 

other categories 

• Person/animal/bird: Actual physical contact or near miss to a bird, animal, or skydiver when 

aircraft is in flight

• Unstabilized approach: Failure to establish and maintain a constant attitude, airspeed, or descent 

rate on approach; or making aircraft configuration changes at or below 500-ft height above terrain 

(HAT) (above ground level) on approach when conducting a precision approach in VMC; or at or 

below 1000-ft HAT on approach when conducting a precision approach in IMC. A nonprecision 

approach may also be considered unstabilized if there is a significant variance from appropriate 

speed, rate of descent, attitude, or configuration profiles.

• Visual flight rules in IMC: Flight in IMC without an ATC clearance is usually reported by a

general aviation pilot without IFR qualifications or not on an IFR clearance. Note: if a pilot is not 

instrument-qualified but enters IMC with ATC clearance, the event is a violation of Federal 

Aviation Regulations, and not considered VFR in IMC.

• Wake vortex encounter: Any encounter with another aircraft’s wake vortices in the terminal 

environment or en route (does not include jet or propeller blast events)

• Weather or turbulence: An encounter with weather or turbulence, including CAT, that is

commented on by the reporter in the narrative

• Other/unknown: No other fields in this section are appropriate
NASA/TM—2014-217898 43

Ground event or encounter (9 options)

Note: an aircraft is considered to be on the ground when any of the aircraft’s main gear, nosewheel, 

tailwheel, or all skids are on the ground at both takeoff and landing.

• Aircraft: Actual physical contact with another aircraft when aircraft is on the ground

• FOD: Hazard to an aircraft caused by unavoidable or unseen plant, animal, or object

• Gear-up landing: Aircraft is landed without landing gear extended (does not include gear 

collapse)

• Ground strike—aircraft: Physical contact by a portion of the aircraft such as a wingtip, prop, or 

tail strike to the ground

• Loss of aircraft control: the pilot is unable to effectively maintain control of the aircraft due to 

pilot error, environment, aircraft, or other reasons

• Object: Physical contact with any object (not alive) other than what is covered in the other 

categories when the aircraft is on the ground (runway lights, airport signs, fencing, etc.)

• Person/animal/bird: Actual physical contact or near miss with a person, animal, bird, or any “live 

object” when aircraft is on the ground. Includes encounters with birds or live objects during the 

takeoff roll.

• Vehicle: Actual physical contact with a vehicle (not aircraft) when aircraft is on the ground. A 

jetway, bridge, or baggage cart is considered an object, not a vehicle. A vehicle is defined as 

being equipped with an engine, is self-propelled, and can be driven, legally or illegally, to the 

store.

• Other/unknown: Everything not covered elsewhere in this section

Ground incursion (2 options)

Ground incursion is an undesirable or unwanted entry into a confined, marked, or identified standard 

movement area

• Runway: Uncoordinated, unauthorized, or improper entry to any active runway by an aircraft, 

vehicle, or person, but does include landing on a closed runway. Actual collision hazard or 

conflict does not need to not occur.

• Taxiway: Uncoordinated, unauthorized, or improper entry to a taxiway by an aircraft, vehicle, or 

person

Conflict anomaly (4 options)

• Near-midair collision: Conflict between two airborne aircraft with LESS than 500 ft vertical and

horizontal separation

• Airborne conflict: two or more airborne aircraft in conflict, which may be equal to, or less than 

legal standard separation, or incidents where the reporter claims a potential conflict. This also

includes a conflict that requires an evasive maneuver initiated by a controller, a pilot, or by 

cockpit equipment.

• Ground conflict, critical: Two or more aircraft (one may be airborne) in conflict, or an aircraft in 

conflict with a vehicle/aircraft/person/object in which the reporter or other involved took some 

evasive action to avoid a collision, or a collision that almost occurred in cases where evasive 

action was not taken or was not possible

• Ground conflict, less severe: Two or more aircraft (one may be airborne) in conflict, or an aircraft 

in conflict with a vehicle/aircraft/person/object in which the reporter or other involved may, or 

may not, have taken some precautionary avoidance action, and in which a collision hazard was 

not imminent
NASA/TM—2014-217898 44

Procedural deviation (10 options)

• Clearance—Noncompliance with an ATC clearance. Note: noncompliance with a clearance does 

not imply a pilot deviation has occurred. Certain conditions, such as an emergency, evasive 

action, ground proximity warning system escape maneuver, weather factors, etc., mitigate pilot 

violation of Federal Aviation Regulations.

• FAR—Noncompliance with, or violation of, any Federal Aviation Regulation, other than in an 

emergency

• Hazardous material violation—All illegally loaded or boarded hazardous materials. Also includes 

incorrect or improper paperwork associated with legal carriage of hazardous materials.

• Landing without clearance—Landing without an ATC clearance, any wrong-runway landing, or 

an unauthorized landing on a taxiway

• Maintenance—Failure to comply with a normal, standard, or required maintenance procedures.

Caution: This anomaly must be the event type—do not code contributory factors or results in this 

field.

• Minimum equipment list (MEL)—Any violation of an MEL and associated configuration 

deviation list (CDL) and/or nonessential function (NEF) requirement, including documentation 

issues

• Published material/policy—Noncompliance with a published operational procedure, policy, or 

practice, for example, published approach or departure procedure, published company operational 

requirement, or ATC Handbook. This also includes improper documentation issues, including but 

not limited to, maintenance logs, journey logs, and other associated documentation.

• Security—Any noncompliance with Federal, local, or company security requirements. Note: this 

anomaly must be the event type—not security as an anomaly if security was an event contributor 

or result

• Weight and balance—Flight crew, maintenance, dispatch, or ground crew noncompliance with 

weight and balance requirements or standards including improper loading of passengers, luggage, 

or freight that compromises standard or calculated center of gravity balance calculations. This 

also includes circumstances where weight and balance were improperly calculated or omitted.

• Other/unknown—this field can be used where it is determined that a procedural deviation has 

occurred, but no other fields in this section are appropriate

Altitude deviation (4 options)

This includes events where a specified altitude requirement is not met due to an emergency or other 

defensible operational reason, such as a traffic collision avoidance system resolution advisory or collision 

avoidance.

• Crossing restriction not met—charted or ATC-assigned altitude crossing restriction not met

• Excursion from assigned altitude—Aircraft departs from a level ATC-assigned altitude

• Overshoot—Aircraft climbs or descends through an ATC-assigned or published altitude

• Undershoot—Aircraft in climb or descent fails to reach an ATC-assigned altitude

Aircraft equipment problem (2 options)

• Critical—Equipment problem that is vital to the specific flight and circumstances, whose failure 

or malfunction could significantly impact the safety of flight

• Less severe—Equipment problem that is not vital to the specific flight and circumstances

Track/heading deviation (1 option)

Aircraft deviates from ATC-assigned heading, track, airway or published heading, track, routing or 

airway requirement, such as a standard instrument departure (SID) or standard terminal arrival route 

(STAR). Note: this includes track, route, or heading deviations due to weather avoidance or other 

defensible reasons.
NASA/TM—2014-217898 45

Flight planning (Ref. 8) (4 options)

• Instrument flight rules(IFR)—A type of flight plan that follows a set of FAA rules governing the 

conduct of flight under instrument meteorological conditions

• Visual flight rules(VFR)—Type of flight plan that follows FAA rules that govern the procedures 

for conducting flight under visual conditions

• No flight plan filed—Pilot did not file a flight plan

• Defense visual flight rules (DVFR)—All VFR aircraft are required to file a DVFR prior to 

entering into the air defense identification zone

Flight conditions (4 options)

• IMC—Meteorological conditions expressed in terms of visibility, distance from cloud, and 

ceiling less than the minima specified for visual meteorological conditions (Ref. 9)

• Marginal—Weather flight conditions that are on edge between VMC and IMC

• Mixed—Weather flight conditions that are a combination of VMC and IMC

• VMC—Meteorological conditions expressed in terms of visibility, distance from cloud, and 

ceiling equal to or better than specified minima (Ref. 9). The pilot is able fly by outside visual 

references under VFR.

Weather element—visibility (10 options)

• Cloudy

• Fog

• Hail

• Haze or smoke

• Icing

• Rain

• Snow

• Thunderstorm

• Turbulence

• Windshear
NASA/TM—2014-217898 46

References

1. Aviation Safety Program: Atmospheric Environment Safety Technologies Project Plan. Oct. 1, 2011. 

Unpublished.

2. NASA Aviation Safety Reporting System. http://asrs.arc.nasa.gov/ Accessed June 25, 2014.

3. Commercial Aviation Safety Team/International Civil Aviation Organization: Official Site for 

Aviation Common Taxonomies. 2007. http://intlaviationstandards.org/index.html Accessed January 

2011.

4. NASA Aviation Safety Reporting System: ASRS: The Case for Confidential Incident Reporting 

Systems 2001. NASA ASRS Publication 60.

http://asrs.arc.nasa.gov/docs/rs/60_Case_for_Confidential_Incident_Reporting.pdf Accessed March 

24, 2011.

5. Jones, Sharon Monica, et al.: Subsonic Aircraft Safety Icing Study. NASA/TM—2008-215107, 

2008. http://ntrs.nasa.gov

6. Federal Aviation Administration: Pilot Windshear Guide. Advisory Circular No. 00‒54, Nov. 25, 

1988. 

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/b3fb7dd63

6fb870b862569ba0068920b/$FILE/AC00-54.pdf Accessed Jan. 31, 2012.

7. Williams, Scott: Aviation Glossary. http://aviationglossary.com Accessed Jan. 31, 2012.

8. Federal Aviation Administration: Air Traffic Management Glossary of Terms. 

http://www.fly.faa.gov/Products/Glossary_of_Terms/glossary_of_terms.pdf Accessed Jan. 7, 2012.

9. Federal Aviation Administration: Pilot/Controller Glossary (P/CG). Effective April 14, 2014. 

http://www.faa.gov/air_traffic/publications/atpubs/PCG/index.htm Accessed Jan. 7, 2012.

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01-08-2014

2. REPORT TYPE

Technical Memorandum

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Analysis of Aviation Safety Reporting System Incident Data Associated With the

Technical Challenges of the Atmospheric Environment Safety Technology Project

5a. CONTRACT NUMBER

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6. AUTHOR(S)

Withrow, Colleen, A.; Reveley, Mary, S.

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WBS 648987.02.01.03.40

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National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

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E-18710

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14. ABSTRACT

This study analyzed aircraft incidents in the NASA Aviation Safety Reporting System (ASRS) that apply to two of the three technical

challenges in NASA’s Aviation Safety Program’s Atmospheric Environment Safety Technology Project. The aircraft incidents are related to

airframe icing and atmospheric hazards technical challenges. The study reviewed incidents that listed their primary problem as weather or

environment-nonweather between 1994 and 2011 for aircraft defined by Federal Aviation Regulations (FAR) Parts 121, 135, and 91. The

study investigated the phases of flight, a variety of anomalies, flight conditions, and incidents by FAR part, along with other categories. The

first part of the analysis focused on airframe icing-related incidents and found 275 incidents out of 3526 weather-related incidents over the

18-year period. The second portion of the study focused on atmospheric hazards and found 4647 incidents over the same time period.

Atmospheric hazards-related incidents included a range of conditions from clear air turbulence and wake vortex, to controlled flight toward

terrain, ground encounters, and incursions.

15. SUBJECT TERMS

Aircraft safety; Ice formation; Systems analysis; Aircraft control; Atmospheric composition

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