Mary S. Reveley and Colleen A. Withrow

Glenn Research Center, Cleveland, Ohio

Lawrence C. Barr

Volpe National Transportation Systems Center, Cambridge, Massachusetts 

Joni K. Evans

TEAMS 2, Hampton, Virginia

Karen M. Leone

Vantage Partners, LLC, Cleveland, Ohio 

Sharon M. Jones

Langley Research Center, Hampton, Virginia

Aviation Trends Related to Atmospheric 

Environment Safety Technologies 

Project Technical Challenges

NASA/TM—2014-217729

September 2014
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Mary S. Reveley and Colleen A. Withrow

Glenn Research Center, Cleveland, Ohio

Lawrence C. Barr

Volpe National Transportation Systems Center, Cambridge, Massachusetts 

Joni K. Evans

TEAMS 2, Hampton, Virginia

Karen M. Leone

Vantage Partners, LLC, Cleveland, Ohio 

Sharon M. Jones

Langley Research Center, Hampton, Virginia

Aviation Trends Related to Atmospheric 

Environment Safety Technologies 

Project Technical Challenges

NASA/TM—2014-217729

September 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

Thanks are extended to the managers and researchers of the NASA Aviation Safety Program, the National Transportation 

Safety Board and the Federal Aviation Administration for providing information key to this study.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

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—2014-217729 iii

Contents

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

1.0 Introduction .......................................................................................................................................... 2 

2.0 Background .......................................................................................................................................... 3 

3.0 Technical Challenge Descriptions ....................................................................................................... 3 

4.0 National Transportation Safety Board Data Accident Analysis........................................................... 4 

4.1 TC1: Engine Icing Characterization and Simulation Capability ................................................. 5

4.2 TC2: Airframe Icing Simulation and Engineering Tool Capability ............................................ 6

4.3 TC3: Atmospheric Hazard Sensing Mitigation Technology Capability ...................................... 7

4.3.1 Turbulence ....................................................................................................................... 7

4.3.2 Wake Vortex .................................................................................................................... 9

4.3.3 Lightning Strikes ............................................................................................................ 11

4.3.4 Low Visibility ................................................................................................................ 12

4.3.5 All Six Atmospheric Conditions .................................................................................... 13

5.0 Federal Aviation Administration Incident Data Analysis .................................................................. 15 

5.1 TC1: Engine Icing Characterization and Simulation Capability ............................................... 15

5.2 TC2: Airframe Icing Simulation and Engineering Tool Capability .......................................... 16

5.3 TC3: Atmospheric Hazard Sensing and Mitigation Technology Capability ............................. 17

5.3.1 Turbulence ..................................................................................................................... 17

5.3.2 Lightning ........................................................................................................................ 19

5.3.3 Low Visibility ................................................................................................................ 20

5.3.4 Atmospheric Conditions Summary ................................................................................ 22

6.0 Future Safety Risk Analysis ............................................................................................................... 23 

6.1 Approach and Landing Accident Reduction .............................................................................. 23

6.2 Icing and Ice Detection .............................................................................................................. 24

6.3 Loss of Control in Flight ............................................................................................................ 26

6.4 Runway Safety ........................................................................................................................... 27

6.5 Super Density Operations .......................................................................................................... 28

7.0 Discussion and Conclusion ................................................................................................................ 29 

7.1 National Transportation Safety Board Accident Data ............................................................... 29

7.2 Federal Aviation Administration Incident Data ......................................................................... 30

7.3 Future Safety Risk ..................................................................................................................... 31

References ................................................................................................................................................... 32 

NASA/TM—2014-217729 1 

Aviation Trends Related to Atmospheric Environment Safety 

Technologies Project Technical Challenges

Mary S. Reveley and Colleen A. Withrow

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

Lawrence C. Barr

Volpe National Transportation Systems Center

Cambridge, Massachusetts 02142 

Joni K. Evans

TEAMS 2 

Hampton, Virginia 23681–2199 

Karen M. Leone

Vantage Partners, LLC

Cleveland, Ohio 44135 

Sharon M. Jones

National Aeronautics and Space Administration 

Langley Research Center

Hampton, Virginia 23681–2199 

Summary

Current and future aviation safety trends related to the National Aeronautics and Space 

Administration (NASA) Atmospheric Environment Safety Technologies (AEST) Project’s three technical 

challenges (TCs) were assessed by examining accidents from the National Transportation Safety Board 

(NTSB) database, incidents from the Federal Aviation Administration (FAA) database, and literature 

from various industry and government sources. The three AEST TCs are engine icing characterization 

and simulation capability; airframe icing simulation and engineering tool capability; and atmospheric 

hazard sensing and mitigation (AHSM) technology capability. 

The NTSB accident data was examined for fixed-wing airplanes operating under Federal Aviation 

Regulation (FAR) Parts 121, 135, and 91 from 1989 to 2008 for atmospheric conditions related to 

airframe icing, ice-crystal engine icing, turbulence, clear air turbulence (CAT), wake vortex, lightning, 

and low visibility (fog, low ceiling, clouds, precipitation, and low lighting). Less than 5 percent of all 

accidents were affected by icing conditions, although when icing was involved in Parts 91 and 135 

scheduled operations, between 25 and 54 percent involved fatalities. No accidents involving ice crystal 

engine icing were found in the NTSB database. Accidents with turbulence were a cause or factor in 

24 percent of Part 121 accidents. Turbulence accidents usually did not involve fatalities. CAT was found 

to be involved in 19 percent of all turbulence accidents. Less than 1 percent of all accidents involved 

either wake vortex or lightning, although this may increase in the future due to denser en route and 

terminal area operations for wake vortex and the introduction of new aircraft with composite structures 

for lightning. Low visibility was involved in between 6 and 23 percent of all accidents depending on 

operational category. Nearly one-third of all accidents are caused by the combined atmospheric conditions 

studied in this analysis. The accident rate for accidents involving any atmospheric condition causal factor 
NASA/TM—2014-217729 2 

declined for Parts 91 and Part 135–NS, increased for Part 135–S, and remained steady for Part 121 

operations. 

The FAA Accident/Incident Data System (AIDS) database was examined for incidents that occurred 

during the years 1985 to 2006 related to airframe icing, ice crystal engine icing, turbulence, CAT, wake 

vortex, lightning, and low visibility for fixed-wing airplanes operating under FAR Part 121, 135, and 91. 

Engine icing was found to be involved in 14 of the incidents, 2 of which specifically cited as “ice 

crystals”. These incidents are often not thoroughly investigated and often lack sufficient detail. Less than 

2 percent of the incidents were affected by airframe icing, and less than 3 percent involved turbulence. 

Low visibility was a factor in 27 percent of the incidents and has declined over the time period. Close to 

one-third of all incidents in Parts 121 and 135 operations were affected by the six atmospheric conditions 

examined in this study. Icing, turbulence, wake vortex, lightning, low lighting and low visibility and have 

declined over the time period of this study. 

Five future aviation safety risk areas associated with the three AEST TCs were identified after an 

exhaustive survey of a variety of sources and include: approach and landing accident reduction, icing/ice 

detection, loss of control (LOC) in flight, super density operations, and runway safety. The focus of 

AEST research is on in-flight icing, both engine and airframe, as well as on investigations of other highpriority atmospheric hazards and sensor technologies required for their detection. The engine and airframe 

icing TCs will impact the future risk areas of icing/ice detection and LOC in flight. The third TC, AHSM

technology capability, will impact the future risk areas of approach and landing reduction, icing/ice 

detection, LOC in flight, runway safety, and super density operations. 

1.0 Introduction

NASA’s Aviation Safety Program (AvSP) includes three research projects. One of these, the 

Atmospheric Environment Safety Technologies (AEST) Project, focuses on three technical challenges

(TCs) (Ref. 1): 

• TC1: Engine icing characterization and simulation capability 

• TC2: Airframe icing simulation and engineering tool capability

• TC3: Atmospheric hazard sensing and mitigation (AHSM) technology capability

This report analyzes aviation safety trends related to these AEST TCs by reviewing the most current 

statistical and prognostic data available including accident and incident data as well as literature from 

academia, industry, and other Government agencies. This study evaluates possible impacts of the TCs on 

historic and future aviation safety hazards and how aviation safety risk areas change over time. The 

following data and published research were examined:

• Accidents (using the National Transportation Safety Board (NTSB) Aviation Accident and

Incident Data System) 

• Incidents (using the Federal Aviation Administration (FAA) Accident/Incident Data System 

(AIDS) database)

• Industry and government data and published research sources:

○ Decadal Survey of Civil Aeronautics, National Research Council

○ National Aeronautics Research and Development Plan

○ National Transportation Safety Board Most Wanted Aviation Safety Improvements 2010 

○ Joint Planning and Development Office Safety Working Group Issues Database
NASA/TM—2014-217729 3 

○ Future Aviation Safety Teams Areas of Change

○ Next Generation Air Transportation System (NextGen) Integrated Work Plan

○ Commercial Aviation Safety Team’s Safety Enhancements

○ NextGen Implementation Plan 

○ Icing Protection Harmonization Working Group 

2.0 Background

Public benefits derived from continued growth in passenger and cargo transport depend on intrinsic 

safety improvements on current and future air vehicles to operate in NextGen. The AvSP addresses this 

challenge by conducting cutting-edge, fundamental research to yield innovative algorithms, tools, 

concepts, and technologies from the discipline level up to subsystem and system levels. As part of the 

AvSP, the AEST Project focuses primarily on both engine and airframe in-flight icing. Research also 

includes investigations of other high-priority atmospheric hazards and sensor technologies for their 

detection. The AEST Project will investigate sources of risk and provide technology needed to help 

ensure safe flight in and around atmospheric hazards.

3.0 Technical Challenge Descriptions

The AEST Project focuses on the following three TCs: 

• TC1: Engine icing characterization and simulation capability: Develop knowledge bases, analysis 

methods, and simulation tools to address engine icing, particularly ice crystal icing. The goal of 

TC1 is to eliminate turbofan engine interruptions, failures, and damage caused by flights into high 

ice crystal content clouds. Research will enable new engine icing protection systems and methods 

and provide a verified basis for engine icing certification requirements. 

• TC2: Airframe icing simulation and engineering tool capability: Develop and demonstrate threedimensional simulations and experimental methods for modeling of airframe ice accretion and 

related aerodynamic performance degradation in an icing environment that expands to include 

freezing drizzle and rain for current and future aircraft configurations. The goal of TC2 is to 

achieve acceptance of simulation tools for design and certification of swept-wing configurations 

over an expanded range of icing conditions. Research will develop technology that enables safe 

flight operations in a supercooled large droplet (SLD) environment; enables aircraft 

manufacturers to perform reliable icing assessments; and builds in effective icing mitigation 

approaches for current and future aircraft.

• TC3: AHSM technology capability: Improve and expand remote sensing and mitigation of 

hazardous atmospheric environments and phenomena. The goals of TC3 are to develop mature 

technologies for sensing and measuring icing, turbulence, and wake vortex hazards to provide 

real-time information to pilots and operators in the National Airspace System (NAS); address low 

visibility conditions for safer runway operations; and advance technologies for lightning-immune 

composite aircraft. Research performed under this TC will provide flight crews with a greater 

ability to avoid hazards, share available information available with other aircraft and groundbased systems, and reduce the vulnerability of aircraft to lightning and other hazards. 

Research will be conducted in several associated subproject elements to address the three TCs shown 

in Figure 1. 
NASA/TM—2014-217729 4 

Figure 1.—AEST Project research framework. 

4.0 National Transportation Safety Board Data Accident Analysis

The NTSB, an independent Federal agency, investigates every civil aviation accident in the United 

States and significant accidents in other transportation modes. The NTSB also conducts special 

investigations, provides safety studies, and issues safety recommendations to prevent future accidents.

Reports produced by NTSB investigators provide details about accidents, analysis of factual information, 

conclusions and the probable cause, factors and findings of accidents, and related safety 

recommendations. Causes are the direct source of accidents while factors contribute to accidents. Each 

accident can have multiple causes and factors (Ref. 2). Findings are actions that occurred in conjunction

with the accident, but were not determined to have contributed to the accident. For example, an aircraft 

might have flown in or near a thunderstorm with lightning, but the lightning had no impact on the flight 

or the accident. Similarly, the flight might have taken place during the night, but lack of visibility in the 

dark had no effect on the flight. Reports are located in the NTSB Aviation Accident and Incident Data 

System.

The NTSB data was searched for AEST TC-related accidents including the following atmospheric 

conditions that were cited as “causes” or “factors,” but not findings: 

• Icing (ice, freezing rain, drizzle, and ice crystals)

• Turbulence (related to thunderstorm and clear air) 
NASA/TM—2014-217729 5 

• Wake vortex

• Lightning 

• Low lighting 

• Low visibility (fog, low ceilings, clouds, below approach/landing minimums, haze, smoke, 

obscuration, and whiteout)

Federal Aviation Regulations (FARs) are FAA regulations that govern all aviation activities in the 

United States and are contained in Title 14 of the Code of Federal Regulations. FARs are organized into 

sections called parts depending on specific types of activity. Other parts regulate certification of pilots, 

schools, and aircraft and their operation (Ref. 3). 

The NTSB database includes accident reports that involved airplanes, helicopters, hot air balloons, 

gliders, ultralight aircraft, etc., and operations conducted under the following operational categories: 

• Part 91: General Aviation (small noncommercial aircraft)

• Part 121: Commercial Air Carriers (aircraft with 10 or more seats)

• Part 129: Foreign Air Carriers 

• Part 135: Commuters and On-Demand Air Taxis (aircraft with fewer than 10 seats)

• Part 137: Agricultural Operations 

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

changed to include aircraft with 10 or more seats. (Ref. 4).

The NTSB considers each event to be either an accident or an incident using the following definitions 

(Ref. 5):

• Accident: 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 and all such persons disembark, 

and in which any person suffers death or serious injury, or in which the aircraft receives 

substantial damage. 

• Incident: an occurrence other than an accident, associated with the operation of an aircraft, which 

affects or could affect the safety of operations. 

Injuries or aircraft damage that occur when there was no intent for flight (high-speed taxi tests, 

aircraft movement around the airfield, maintenance run-ups, etc.) are, by definition, incidents. 

All recorded accidents involving commercially built fixed-wing airplanes operating under FAR Parts 

121, 135, and 91 regulations were included in this analysis, regardless of whether the investigations are

preliminary or finalized, and whether or not the events occurred in the United States. Amateur-built and 

experimental aircraft, helicopters, ultralight aircraft, gliders, and balloons were excluded. The only sky 

diving accidents that were included were those caused by the aircraft, not the parachute or parachutist. 

4.1 TC1: Engine Icing Characterization and Simulation Capability

A search of NTSB accident data did not yield propulsion system-related accidents that were impacted 

by ice crystals. There were accidents, however, that did involve carburetor and/or fuel system icing. 
NASA/TM—2014-217729 6 

4.2 TC2: Airframe Icing Simulation and Engineering Tool Capability

The database was searched for any accident caused by in-flight icing conditions, freezing rain, or 

drizzle and with or without confirmed ice accretion. Accidents caused by inadequate preflight frost, ice, 

and snow removal were not included in this group. A summary of the accidents caused by in-flight icing 

conditions are found in Table 1. Two types of fatal accident percentages are of interest: the percentage of 

all fatal accidents caused by icing conditions and the percentage of accidents caused by icing conditions 

with a fatality. The numerators in these percentages are identical (fatal accidents caused by icing 

conditions) but the denominators are different. 

Less than 5 percent of the accidents were related to icing conditions. Between 3 and 8 percent of fatal 

accidents and between 3 and 11 percent of fatalities were related to icing conditions. Percentages were 

highest in nonscheduled Part 135 (Part 135–NS) flights, except for the percentage of fatalities, which was 

highest among scheduled Part 135 (Part 135–S) flights. Although the percentage of accidents, fatal 

accidents, and fatalities that were related to icing conditions is low, the percentage of accidents related to 

icing conditions, in which there was at least one fatality, ranges from 25 percent for Part 135–S to 

54 percent for Part 91. 

Figure 2 displays the change in the accident rate (per million flight hours) for accidents that were 

related to icing conditions across four time periods. Rates have declined over time for every category of 

flight operations, except Part 135–S, which increased significantly between the 1994 to 1998 and the 

1999 to 2003 time periods. 

TABLE 1.—ICING-CONDITION-RELATED ACCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2008

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Accidents, total 738 219 1202 26,922

Accidents per million flight hours 2.321 8.959 19.465 53.047

Accidents caused by icing conditions 5 8 54 321

Accidents caused by icing conditions per million flight hours 0.015 0.327 0.874 0.632 

Accidents caused by icing conditions, percent 1%

(5/738)

4%

(8/219

5%

(54/1202)

1%

(5214/26922)

Fatal accidents, total 67 48 307 5214

Fatal accidents caused by icing conditions 2 2 23 172

Fatal accidents caused by icing conditions, percent 3%

(2/67)

4%

(2/48)

8%

(23/307)

3%

(172/5214)

Accidents caused by icing conditions with a fatality, percent 40%

(2/5)

25%

(2/8)

43%

(23/54)

54%

(172/5214)

Fatalities, total 1956 309 745 10 109

Fatalities caused by icing conditions 70 35 27 395

Fatalities caused by icing conditions, percent 4%

(70/1956)

11%

(35/309)

4%

(27/745)

4%

(395/10109)
NASA/TM—2014-217729 7 

Figure 2.—Icing-related accident rate by operational category across four time periods.

4.3 TC3: Atmospheric Hazard Sensing Mitigation Technology Capability 

The database was searched for accidents caused by turbulence, wake vortex, lightning, low lighting 

and low visibility conditions. Information on icing conditions is found in Section 4.2. 

4.3.1 Turbulence

A summary of the accidents that were caused by in-flight turbulence are found in Table 2. Turbulence 

related to thunderstorms is included but not terrain-induced or wake vortex turbulence. There are two 

types of fatal accident percentages that are of interest: the percentage of all fatal accidents that included a 

condition (such as turbulence) and the percentage of accidents marked by that condition in which there 

was a fatality. The numerators are identical (e.g., fatal turbulence accidents) but the denominators are 

different. Twenty-four percent of Part 121 accidents, compared with less than 1 percent of accidents in 

other flight operations, were caused by turbulence. Less than 3 percent of fatal accidents and fatalities

were caused by turbulence, regardless of flight operation. Turbulence was least likely to be a factor in 

Part 135–S accidents, and no fatalities were associated with turbulence in that flight category. Even in 

Part 121 the number of fatalities was very low. However, more than half of the Part 135–NS and Part 91 

accidents that were affected by turbulence included at least one fatality. 

Figure 3 displays the change in the accident rate related to turbulence across four time periods. Rates 

have declined over time for Parts 91 and 135–NS, while Part 135–S rates have increased. As shown in 

Table 2, the total number of accidents affected by turbulence was lowest in Part 135–S. Part 121 

turbulence caused accident rates to increase during the 1994 to 2003 time period before decreasing during 

the 2004 to 2008 time period.

The AEST Project is particularly interested in reducing the risk of encounters with clear air 

turbulence (CAT). Figure 4 displays the change in the accident rate related to CAT across the four time 

periods. The rate for Part 135–S has increased dramatically in the most recent time period. Table 3 

summarizes CAT-related accidents, which accounts for 19 percent of all turbulence-related accidents (64 

of 344 turbulence-caused accidents). CAT was involved in 6 percent of all Part 121 accidents and was 

more likely to be a factor in Part 121 operations than Parts 135 or 91. Most CAT accidents did not involve 

a fatality.
NASA/TM—2014-217729 8 

TABLE 2.—TURBULENCE-RELATED-ACCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2008

Data Category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Accidents, total 738 219 1202 26,922

Accidents per million flight hours 2.321 8.959 19.465 53.047

Accidents caused by turbulence 180 2 9 153

Accidents caused by turbulence per million flight hours 0.566 0.082 0.146 0.301 

Accidents caused by turbulence, percent 24%

(180/738)

1%

(2/219)

1%

(9/1202)

1%

(153/26922)

Fatal accidents, total 67 48 307 5214

Fatal accidents caused by turbulence 2 0 5 91

Fatal accidents caused by turbulence, percent 3%

(2/67)

0%

(0/48)

2%

(5/307)

2%

(91/5214)

Accidents caused by turbulence with a fatality, percent 1%

(2/180)

0%

(0/2)

56%

(5/9)

60%

(91/153)

Fatalities, total 1956 309 745 10,109

Fatalities caused by turbulence 2 0 8 201

Fatalities caused by turbulence, percent 0%

(2/1956)

0%

(0/309)

1%

(8/745)

2%

(201/10109)

Figure 3.—Turbulence-related accident rate by operational category across four time periods.
NASA/TM—2014-217729 9 

Figure 4.—Clear-air-turbulence-related accident rate by operational category 

across four time periods. 

TABLE 3.—CLEAR-AIR-TURBULENCE-RELATED ACCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2008

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Accidents, total 738 219 1,202 26,922

Accidents per million flight hours 2.321 8.959 19.465 53.047

Accidents caused by CATa 45 1 1 17

Accidents caused by CAT per million flight hours 0.142 0.041 0.016 0.033 

Accidents caused by CAT, percent 6%

(45/738)

1%

(1/219)

0%

(1/1202)

0%

(17/26922)

Fatal accidents, total 67 48 307 5214

Fatal accidents caused by CAT 1 0 1 3

Fatal accidents caused by CAT, percent 2%

(1/67)

0%

(0/48)

0%

(1/307)

2%

(17/5214)

Accidents caused by CAT, percent 2%

(1/45)

0%

(0/1)

100%

(1/1)

18%

(3/17)

Fatalities, total 1956 309 745 10,109

Fatalities caused by CAT 1 0 1 8

Fatalities caused by CAT, percent 0%

(1/1956)

0%

(0/309)

0%

(1/745)

0%

(8/10109) a

Clear air turbulence.

4.3.2 Wake Vortex 

A summary of accidents caused by wake vortex are found in Table 4. These numbers include aircraft 

that were impacted by the wake vortex, not the aircraft that caused the wake vortex encounter. Jet blast or 

propeller wash from engine run-up or movements from a standing position were not included either. In 

general, less than 1 percent of all accidents, fatal accidents, and fatalities were related to wake vortex, 

except that 2 percent of fatal accidents in Part 135–S were associated with wake vortex. This single 

accident resulted in three fatalities. Two of the three Part 135–NS wake vortex accidents included at least 

one fatality (67 percent). Almost 12 percent of the Part 91 wake vortex accidents were fatal.
NASA/TM—2014-217729 10

Figure 5 displays the change in the accident rate related to wake vortex across four time periods. 

Rates have remained fairly constant over time, except for Part 91, which has shown a consistent decline. 

Although the accident rate caused by wake vortex has been low and decreasing during this study, 

expected growth in the demand for air transportation will require denser en route and terminal area 

operations, resulting in an increase in likelihood of an encounter with a wake vortex. 

TABLE 4.—WAKE VORTEX-RELATED ACCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2008

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Accidents, total 738 219 1,202 26,922

Accidents per million flight hours 2.321 8.959 19.465 53.047

Accidents caused by wake vortex 5 1 3 61

Accidents caused by wake vortex per million flight hours 0.016 0.041 0.049 0.120 

Accidents caused by vortex, percent 1%

(5/738)

1%

(1/219)

0%

(3/1202)

0%

(61/26922)

Fatal accidents, total 67 48 307 5214

Fatal accidents caused by wake vortex 0 1 2 7

Fatal accidents caused by wake vortex, percent 0%

(0/67)

2%

(1/48)

1%

(2/307)

0%

(7/5214)

Accidents caused by vortex with a fatality, percent 0%

(0/5)

100%

(1/1)

67%

(2/3)

12%

(7/61)

Fatalities, total 1956 309 745 10,109

Fatalities caused by wake vortex 0 3 6 15

Fatalities caused by wake vortex, percent 0%

(0/1956)

1%

(3/309)

1%

(6/745)

0%

(15/10109)

Figure 5.—Wake-vortex-related accident rate by operational category across four 

time periods.
NASA/TM—2014-217729 11

4.3.3 Lightning Strikes

A summary of accidents caused by lightning are found in Table 5. In general, lightning was a cause or 

factor in less than one-half of 1 percent of all accidents, fatal accidents, and fatalities. The one exception 

was that 2 percent of the Part 121 fatal accidents were associated with lightning. This single accident in 

1989 resulted in one fatality when lightning struck an aircraft in the ramp area, and a ground crew 

member was connected to the aircraft via headset. There were no lightning-related fatal accidents in Part 

135, but 64 percent of the Part 91 lightning-related accidents were fatal. 

Figure 6 displays the accident rate related to lightning across four time periods. With the exception of 

Part 135–S, rates have been fairly consistent over time.

TABLE 5.—LIGHTNING-RELATED ACCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2008

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Accidents, total 738 219 1,202 26,922

Accidents per million flight hours 2.321 8.959 19.465 53.047

Accidents caused by lightning 3 0 2 14

Accidents caused by lightning per million flight hours 0.009 0.000 0.032 0.028 

Accidents caused by lightning, percent 0%

(3/738)

0%

(0/219)

0%

(2/1202)

0%

(14/26922)

Fatal accidents, total 67 48 307 5214

Fatal accidents caused by lightning 1 0 0 9

Fatal accidents caused by lightning, percent 2%

(1/67)

0%

(0/48)

0%

(0/307)

0%

(9/5214)

Accidents caused by lightning with a fatality, percent 33%

(1/3)

0%

(0/0)

0%

(0/2)

64%

(9/14)

Fatalities, total 1956 309 745 10,109

Fatalities caused by lightning 1 0 0 19

Fatalities caused by lightning, percent 0%

(1/1956)

0%

(0/309)

0%

(0/745)

0%

(19/10109)

Figure 6.—Lightning-related accident rate by operational category across four time 

periods.
NASA/TM—2014-217729 12

4.3.4 Low Visibility

In this report, low visibility refers to fog, low ceiling or clouds, below approach minimums, haze, 

smoke, obscuration, whiteout conditions, sand/dust storms, sun glare, precipitation, low lighting and 

unspecified visibility issues. “Visibility is the farthest distance at which an observer can distinguish 

objects. Ceiling is the altitude to the nearest cloud layer that obscures at least half of the sky” (Ref. 6). 

Low lighting conditions at dusk, dawn, and nighttime have a profound impact on safety. Low lighting 

conditions occur, for example, when the Moon is not present, there is extensive cloud cover, or manmade 

lighting (from around a city) is limited or nonexistent. Narratives from some accidents mention lack of 

visibility without being specific for example, instrument meteorological conditions (IMC) or some sort of 

precipitation, usually rain or snow. 

A summary of the accidents that were related to low visibility are found in Table 6. The frequency of 

accidents caused by low visibility conditions varied across the four flight operation categories. In Part 

121, low visibility caused 6 percent of all accidents and 6 percent of fatal accidents. In Part 91, low 

visibility caused 10 percent of accidents and 31 percent of fatal accidents. Part 135 flights were affected 

most by low visibility (19 to 23 percent of accidents and 37 to 40 percent of fatal accidents). 

Figure 7 displays the change in the accident rate related to low visibility conditions across four time 

periods. The rate for Part 121 remained steady across the time periods, while Parts 91 and 135–NS rates

declined. 

TABLE 6.—LOW-VISIBILITY-RELATED ACCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2008

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Accidents, total 738 219 1202 26,922

Accidents per million flight hours 2.321 8.959 19.465 53.047

Accidents caused by low visibility 41 51 227 2689

Accidents caused by low visibility per million flight hours 0.129 2.086 3.676 5.298 

Accidents caused by low visibility, percent 6%

(41/738)

23%

(51/219)

19%

(227/1202)

10%

(2689/26922)

Fatal accidents, total 67 48 307 5214

Fatal accidents caused by low visibility 4 19 114 1606

Accidents caused by low visibility with a fatality, percent 6%

(4/67)

40%

(19/48)

37%

(114/307)

31%

(1606/5214)

Fatal accidents caused by low visibility, percent 10%

(4/41)

37%

(19/51)

50%

(114/227)

60%

(1606/2689)

Fatalities, total 1956 309 745 10,109

Fatalities caused by low visibility 22 92 279 3238

Fatalities caused by low visibility, percent 0%

(1/1956)

0%

(0/309)

34%

(279/745)

32%

(3238/10109)
NASA/TM—2014-217729 13

Figure 7.—Low-visibility-related accident rate by operational category across four 

time periods.

Accidents with low visibility as causes or factors were further sorted by the following types of low 

visibility to include 

• Fog

• Low ceiling or clouds 

• Precipitation

• Low lighting (dusk, dawn, or nighttime) 

• Unspecified

Accidents caused by these types of low visibility conditions are summarized in Table 7. More than 

one condition might have affected particular accidents. In Parts 121, 135–NS and 91, the most likely low 

visibility condition was low lighting (39 to 80 percent). Low lighting was involved in 80 percent of Part 

121 low-visibility-related accidents. In contrast, low ceiling and clouds were a cause in 42 to 53 percent 

of Parts 135 and 91 accidents, yet rarely a cause in Part 121 accidents.

4.3.5 All Six Atmospheric Conditions

A summary of accidents related to any of the atmospheric conditions previously discussed (icing, 

turbulence, CAT, wake vortex, lightning, and low visibility) are found in Table 8. Nearly one-third of Part 

121 accidents had one or more of these six atmospheric conditions as a cause or factor, with only 

4 percent involving a fatality. The lowest percentage of accidents caused by any of the atmospheric 

conditions was in Part 91, yet 58 percent involved fatalities.

Figure 8 displays the change in the accident rate related to the following atmospheric conditions 

across four time periods: icing, turbulence, CAT, wake vortex, lightning, and low visibility. Rates 

declined in every time period for Parts 91 and 135–NS, while Part 135–S operations increased. Part 121 

operations remained somewhat steady over the time periods. 
NASA/TM—2014-217729 14

TABLE 7.—LOW VISIBILITY-RELATED ACCIDENT FREQUENCY BY TYPE 

AND OPERATIONAL CATEGORY, 1989 TO 2008

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Total 41 51 227 2689

Fog 4 (10%) 15 (29%) 58 (26%) 677 (25%) 

Low ceiling/clouds 1 (2%) 27 (53%) 96 (42%) 1203 (45%)

Precipitation 7 (17%) 13 (25%) 45 (20%) 329 (12%) 

Low lighting 33 (80%) 20 (39%) 116 (51%) 1427 (53%)

Unspecified 2 (5%) 13 (25%) 48 (21%) 382 (14%)

TABLE 8.—ATMOSPHERICa CONDITION-RELATED ACCIDENT DATA 

BY OPERATIONAL CATEGORY, 1989 TO 2008

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Accidents, total 738 219 1202 26,922

Accidents per million flight hours 2.321 8.959 19.465 53.047

Accidents caused by atmospheric conditions 234 61 279 3102

Accidents caused by atmospheric conditions per million 

flight hours 0.736 2.495 4.518 6.112 

Accidents caused by atmospheric conditions, percent 32%

(234/738)

28%

(61/219)

23%

(279/1202)

12%

(3102/26922)

Fatal accidents, total 67 48 307 5214

Fatal accidents caused by atmospheric conditions 9 22 139 1783

Fatal accidents caused by atmospheric conditions, percent 13%

(9/67)

46%

(22/48)

45%

(139/307)

34%

(1783/5214)

Accidents caused by atmospheric conditions with a fatality, 

percent

4%

(9/234)

36%

(22/61)

50%

(139/279)

58%

(1783/3102)

Fatalities, total 1956 309 745 10,109

Accident fatalities caused by atmospheric conditions 95 130 312 3649

Fatalities caused by atmospheric conditions, percent 5%

(95/1956)

42%

(130/309)

42%

(312/745)

36%

(3649/10109) a

Icing, turbulence, CAT, wake vortex, lightning, and low visibility combined.
NASA/TM—2014-217729 15

Figure 8.—Atmospheric-condition-related accident rate (icing, turbulence, CAT, 

wake vortex, lightning, and low visibility combined) by operational category 

across four time periods.

5.0 Federal Aviation Administration Incident Data Analysis

Each incident and accident is reported to the FAA by pilots, airport personnel, and private citizens

although the NTSB does not investigate all incidents. The FAA maintains the Accident/Incident Data 

System (AIDS) database with the information contained in these reports and collected in their 

investigations. 

A copy of the FAA’s AIDS database was obtained from the Aviation Safety Information Analysis and 

Sharing (ASIAS) System in October 2007—late enough in the year that nearly all incidents from 1985 to 

2006 had been investigated. 

FAA incident data are far less detailed than NTSB accident data. Weather and visibility conditions 

are mentioned without specifying causality. All mentions of fog, low ceilings, dusk or night flight, or less 

than one mile visibility were included in the low visibility results, as were those mentioned in the 

narratives of Visual Flight Rules (VFR) flight into IMC conditions and other unspecified, restricted 

visibility conditions. These unspecific visibility conditions are referred to as “unspecified” later in the 

report. Precipitation-related incidents were evaluated to determine whether the precipitation impacted the 

visibility or runway conditions. 

Among the incidents in this analysis were some midair and ground collisions between multiple 

aircraft. The database includes an incident record for each aircraft involved, unless the aircraft was parked 

and unoccupied. In order to reduce the analysis data set to one record for each incident, incidents that 

involved multiple aircraft were reviewed and reports for passive aircraft (i.e., aircraft that was hit during 

the collision) were eliminated. 

5.1 TC1: Engine Icing Characterization and Simulation Capability

A search of the FAA incident database showed that 38 incidents were caused by the propulsion 

system. Of the 38 incidents, 14 were caused by icing conditions or ice ingestion. Only two incidents 

specifically cited ice crystals in the propulsion system as a cause. Both of these incidents resulted in 

flameouts of all engines, cited a lack of training on the hazards of high-altitude ice crystals to turbine fan

engines, and occurred in Florida. Of interest is a third incident that involved multiple-engine flameouts 

after passing through thunderstorms near Miami, Florida, but the report did not cite engine icing. Because 

a majority of the propulsion system-related incidents were not thoroughly investigated, clear causes of the 
NASA/TM—2014-217729 16

failures were not provided. Table 9 summarizes incidents that involved any form of engine icing. Most of 

the engine icing incidents occurred during Part 121 operations. Figure 9 provides the incident rate related 

to engine icing across four time periods. 

5.2 TC2: Airframe Icing Simulation and Engineering Tool Capability 

This section examines incidents that were related to in-flight icing conditions, freezing rain, or 

drizzle, with or without confirmed ice accretion. Incidents caused by inadequate preflight frost, ice, or 

snow removal were not included in this group. A summary of the incidents that were affected by in-flight 

icing conditions is found in Table 10. Less than 1 percent of incidents were affected by icing conditions. 

Figure 10 displays the incident rate related to icing conditions across four time periods. Rates have 

declined over time for each flight operations category except for Part 135–S. Rates for 2004 to 2008 are 

underestimated because they do not include 2 years of incident data. 

TABLE 9.—ENGINE-ICING-RELATED INCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2006

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Incidents, total 7647 1921 2189 27,919

Incidents per million flight hours 24.047 78.578 35.449 55.011

Incidents caused by engine icing 7 3 1 2 

Incidents caused by engine icing per million flight hours 0.022 0.123 0.016 0.004

Incidents caused by engine icing, percent 0%

(7/7647)

0%

(8/21921)

1%

(28/2189)

0%

(2/27919)

Figure 9.—Engine icing–related incident rate by operational category across four 

time periods.
NASA/TM—2014-217729 17

TABLE 10.—ICING-RELATED INCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2006

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Incidents, total 7647 1921 2189 27,919

Incidents per million flight hours 24.047 78.578 35.449 55.011

Incidents caused by icing conditions 32 8 28 95

Incidents caused by icing conditions per million flight hours 0.101 0.327 0.453 0.187

Incidents caused by icing conditions, percent

0%

(32/7647)

0%

(8/21921)

1%

(28/2189)

0%

(95/27919)

Figure 10.—Icing-related incident rate by operational category across four time 

periods.

5.3 TC3: Atmospheric Hazard Sensing and Mitigation Technology Capability 

This section examines incidents that were related to turbulence, wake vortex, lightning, low lighting 

and low visibility conditions. Information on icing conditions is found in Section 5.2. 

5.3.1 Turbulence

A summary of the incidents that were affected by in-flight turbulence is found in Table 11. Terraininduced and wake vortex turbulence are not included here, but turbulence related to thunderstorms is 

included. Less than 1 percent of all incidents in the FAA AIDS database were caused by turbulence. 

Figure 11 displays the incident rate related to turbulence across four time periods. The incidence rate has 

declined for all operation categories. 

Of particular interest to the AEST Project is reducing the risk of encounters with CAT. Table 12

summarizes CAT-related incidents, which accounted for 3 percent of all turbulence-related incidents (10 

of 295). Figure 12 displays the change in the incident rate related to CAT across four time periods. The 

rate has declined over time for Part 121 operations. 
NASA/TM—2014-217729 18

A summary of the incidents that were related to wake vortex are found in Table 13. These numbers 

include only aircraft that were affected by the wake vortex encounter, not the aircraft responsible for the 

wake vortex. Jet blast or propeller wash from engine run-up or movement from a standing position was 

not included either. Less than 1 percent of all incidents were influenced by wake vortex. Figure 13 

displays the change in the incident rate related to wake vortex across four time periods. These rates 

increased during 1994 to 1998 and then decreased in the last two time periods. Although the incident rate 

related to wake vortex has been low and decreasing in the time period of this study, expected growth in 

the demand for air transportation will require denser en route and terminal area operations resulting in the 

need for wake vortex mitigation. 

TABLE 11.—TURBULENCE-RELATED INCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2006

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Incidents, total 7647 1921 2189 27,919

Incidents per million flight hours 24.047 78.578 35.449 55.011

Incidents in turbulence 190 8 17 80

Incidents caused by turbulence per million flight hours 00.597 0.327 0.275 0.158

Incidents caused by turbulence, percent 2%

(190/7647)

0%

(8/21921)

1%

(17/2189)

0%

(80/27919)

Figure 11.—Turbulence-related incident rate by operational category across four 

time periods.

TABLE 12.—CLEAR-AIR-TURBULENCE-RELATED INCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2006

Data category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Incidents, total 7647 1921 2189 27,919

Incidents per million flight hours 24.047 78.578 35.449 55.011

Incidents caused by CATa 8 1 0 1

Incidents caused by CAT per million flight hours 0.025 0.041 0.000 0.002

Incidents caused by CAT, percent 0%

(8/7647)

0%

02/21921)

0%

(3/2189)

0%

(1/27919) a

Clear air turbulence.
NASA/TM—2014-217729 19

Figure 12.—Clear-air-turbulence-related incident rate by operational category 

across four time periods.

TABLE 13.—WAKE-VORTEX-RELATED INCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2006

Incident data Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Incidents, total 7647 1921 2189 27,919

Incidents per million flight hours 24.047 78.578 35.449 55.011

Incidents caused by wake vortex 18 2 3 38

Incidents caused by wake vortex per million flight hours 0.057 0.082 0.049 0.075

Incidents caused by wake vortex, percent 0%

(18/7647)

0%

(2/21921)

0%

(3/2189)

0%

(38/27919)

Figure 13.—Wake-vortex-related incident rate by operational category across four 

time periods.

5.3.2 Lightning

A summary of the incidents that were related to lightning are found in Table 14. Less than 1 percent 

of all FAA database incidents cited lightning as a cause or factor. Figure 14 displays the change in the 

incident rate related to lightning across four time periods. Generally, the incident rate has decreased for all 

operational categories over time. 
NASA/TM—2014-217729 20

TABLE 14.—LIGHTNING-RELATED INCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2006

Incident data Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Incidents, total 7647 1921 2189 27,919

Incidents per million flight hours 24.047 78.578 35.449 55.011

Incidents caused by lightning 38 5 2 16

Incidents caused by lightning per million flight hours 0.199 0.205 0.032 0.032

Incidents caused by lightning, percent 1%

(38/7647)

0%

(5/21921)

0%

(2/2189)

0%

(16/27919)

Figure 14.—Lightning-related incident rate by operational category across four time 

periods.

5.3.3 Low Visibility

A summary of the incidents that were related to low visibility are found in Table 15. Low visibility 

conditions affected the flight operations categories differently. In Part 121, low visibility affected 

27 percent of all incidents, while in Part 135, low visibility impacted between 31 and 34 percent of 

incidents. Figure 15 displays the change in the incident rate related to low visibility across four time 

periods. The incident rate related to low visibility declined for all operational categories over time. 

Incidents with low visibility as a cause or factor were further sorted by the type of low visibility to 

include 

• Fog

• Low ceiling or clouds 

• Precipitation

• Low lighting (dusk, dawn, or nighttime) 

• Nonspecific 

These low visibility conditions are summarized in Table 16. More than one condition might affect a 

particular incident. In all operational categories the most frequent condition was low lighting ranging 

from 13 to 30 percent. 
NASA/TM—2014-217729 21

TABLE 15.—LOW-VISIBILITY-RELATED INCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2006

Incident data Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Incidents, total 7647 1921 2189 27,919

Incidents per million flight hours 24.047 78.578 35.449 55.011

Incidents caused by low visibility 2056 592 733 4329

Incidents caused by low visibility per million flight hours 6.465 24.216 11.870 8.530

Incidents caused by low visibility, percent 27%

(2056/7647)

31%

(592/21921)

34%

(733/2189)

16%

(4329/27919)

Figure 15.—Low-visibility-related incident rate by operational category across four 

time periods.

TABLE 16.—LOW VISIBILITY-RELATED INCIDENT FREQUENCY

BY TYPE AND OPERATIONAL CATEGORY, 1989 TO 2008

Low visibility type Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Low visibility, total 7647 1921 2189 27919

Fog 167 (2%) 80 (4%) 69 (3%) 407 (2%)

Low ceiling or clouds 194 (1%) 20 (1%) 46 (2%) 231 (1%)

Precipitation 133 (2%) 59 (3%) 69 (3%) 251 (1%)

Low lighting 1879 (25%) 512 (27%) 663 (30%) 3743 (13%)

Unspecified visibility 64 (5%) 38(2%) 26 (1%) 270 (1%)
NASA/TM—2014-217729 22

5.3.4 Atmospheric Conditions Summary

A summary of the incidents affected by any of the six atmospheric conditions (icing, turbulence, 

CAT, wake vortex, lightning, and low visibility) are found in Table 17. Close to one-third of all incidents 

involved icing, turbulence, wake vortex, lightning, or low lighting/visibility for all operational categories 

except Part 91. Figure 16 displays the change in the incident rate (per million flight hours) for incidents 

affected by any of the six atmospheric conditions across four time periods showing a decline across all 

operational categories over time.

TABLE 17.—ATMOSPHERICa CONDITION-RELATED INCIDENT DATA BY OPERATIONAL CATEGORY, 1989 TO 2008

Incident Data Category Flight operation category

Part 121 Part 135–S Part 135–NS Part 91

Flight hours, total 317,999,117 24,446,927 61,751,000 507,516,000

Incidents, total 7647 1921 2189 27,919

Incidents per million flight hours 24.047 78.578 35.449 55.011

Incidents caused by atmospheric conditions 2260 606 758 4508

Incidents caused by atmospheric conditions per million flight hours 7.107 24.788 12.275 8.882

Percentage of incidents caused by atmospheric condition, percent 30%

(2260/7647)

32%

(606/1921)

35%

(758/2189)

16%

(4508/27919) a

Icing, turbulence, CAT, wake vortex, lightning, and low visibility combined.

Figure 16.—Atmospheric-condition-related incidents rate (icing, turbulence, CAT, 

wake vortex, lightning, and low visibility combined) by operational category 

across four time periods.
NASA/TM—2014-217729 23

6.0 Future Safety Risk Analysis

This section assesses the projected impact of the AEST TCs on the future aviation safety risks listed 

below. These future aviation safety risks were previously identified by the Aviation Safety Program’s 

Systems Analysis Team after an exhaustive survey of a variety of sources (Ref. 7). Ten critical future 

safety risk areas were identified and include (in alphabetical order, not priority): 

(1) Aircraft mixed fleet equipage

(2) Approach and landing accident reduction 

(3) Enhanced survivability in the event of an accident

(4) Human fatigue

(5) Icing and ice detection

(6) Inadequate protection, analysis, and dissemination of safety data

(7) Increasing complexity and reliance on automation 

(8) LOC—in flight

(9) Runway safety

(10) Super density operations 

The AEST Project could have potential impact on 5 of the 10 critical future safety risk areas. A 

description of each of those five areas and applicable AEST TC research is provided in Section 6.1. 

6.1 Approach and Landing Accident Reduction

Approach and landing is the most demanding phase of flight and carries the highest safety risk. This 

risk area includes unstabilized approaches, that is, approaches where airspeed, rate of descent, aircraft 

attitude, configuration, or power settings do not meet stabilized approach criteria at the prescribed 

approach point. These types of accidents often are manifestations of deficiencies that begin in or prior to 

the approach phase. 

Approach and landing accidents often involve high-energy approaches. The most significant threats 

during approach are fast airspeeds, high groundspeeds (not appreciating wind effects), and high and/or 

steep approach above the desired flight path. Early control of high energy, the combination of these 

conditions, can reduce these threats. Situations should be assessed based on the following parameters:

airspeed, altitude, runway length, runway surface conditions, wind, and visibility. Stabilized approaches

provide bases for good landings. Monitoring these flight parameters to detect anomalies is crucial to 

understanding causes of unstabilized approaches and, therefore, to discovering precursors to potential 

approach and landing accidents. Adverse environmental conditions in the terminal area that cause external 

hazards and disturbances, such as icing, freezing rain, and strong wake turbulence, are also precursors to 

approach and landing accidents.

The AEST Project is conducting research in airframe icing to simulate and model airframe and sweptwing ice accretion and related aerodynamic performance degradation for current and future aircraft 

configurations in an expanded icing environment that includes freezing drizzle and rain. A significant 

outcome of this research will be the ability to accurately predict the ice-shape profile that could result 

from an icing encounter in supercooled large droplet (SLD) conditions. This capability will allow analysis 

of the SLD ice accretion impact on aircraft performance, ice protection systems, and ice detection 

systems. With the limited SLD-experimental simulation capabilities that are currently available it is 

critical that computational-SLD ice-accretion simulation tools be developed, especially for freezing rain 

conditions. Airframe icing research will develop the experimental and computational simulation 
NASA/TM—2014-217729 24

capability to prevent and mitigate adverse events during the approach phase of flight using reliable icing 

assessments and effective design approaches. 

The National Research Council (NRC) recognizes the need to develop techniques, including lidar and 

laser-acoustic systems, and to predict and measure the formation, trajectory, and decay of upstream wake 

vortices. In addition, affordable new sensors, system technologies, and procedures must be developed to 

improve prediction and measurement of wake strength, location, motion, and aircraft upset risk in 

terminal and en route airspace to enable safe flight with reduced in-trail wake separation (Ref. 8). AHSM

research is focused on developing mature technologies to sense and measure icing, turbulence, and wake 

vortex hazards to provide aircraft with a greater ability to avoid external hazards including icing 

conditions, kinetic air hazards, and convective weather. 

The goal of AEST icing weather systems research is to enable safe flight through hazardous 

environmental conditions during the critical approach and landing phase of flight. Milestone AEST3.3.27 

will allow development and assessment of icing hazard detection algorithms for terminal area coverage. 

Milestones AEST4.3.05 and AEST4.3.09 in the lidar electro-optical research element are aimed at 

conducting field tests to determine the imaging capability of real-time observation of wake vortices and to 

characterize wake vortex hazards in the terminal area.

6.2 Icing and Ice Detection

Adverse weather conditions, including storms and icing conditions, significantly reduce the capacity 

and reliability of the air transportation system. Adverse weather also degrades system safety. In-flight or 

ground (i.e., deicing-related) accumulation of snow, ice, freezing rain, or frost on aircraft surfaces and 

sensors adversely affects aircraft control and performance. This issue is important to both civil and 

military aviation, and is critical for all types of aircraft, particularly for turboprop aircraft. Research is 

needed to improve the ability to predict and monitor environmental conditions and to develop robust 

aerodynamic designs and techniques for adverse conditions. Currently, icing hazards are handled by 

regulatory constraints on flight operations, and prediction techniques are largely empirical. Low-cost 

techniques to measure upstream environmental conditions should be developed. Techniques to predict 

and mitigate the impact of adverse environmental conditions on the aircraft operation, including 

validation of icing prediction capabilities, should be improved (Ref. 8).

AEST research conducted under the TC2, airframe icing simulation and engineering tool capability, 

will produce ice accretion models that will be validated over the freezing drizzle and freezing rain SLD 

icing regime. This capability will allow regulators and aircraft manufacturers to use computational codes 

for more effective designs and as a basis for certification. Research will be conducted to develop and 

demonstrate a validated computational-simulation capability for ice buildup on three-dimensional aircraft 

surfaces that are subjected to SLD icing conditions. A significant outcome of this research will be the 

ability to accurately predict the ice-shape profile resulting from an icing encounter in SLD conditions. 

This capability will allow analysis of the impact of SLD ice accretions on aircraft performance, ice 

protection systems, and ice detection systems. With the limited experimental SLD simulation capabilities 

that are currently available, it is critical that computational SLD ice-accretion simulation tools be 

developed, especially for freezing rain conditions. 

The consequences of operating an airplane in icing conditions without first having thoroughly 

demonstrated adequate handling and controllability characteristics in those conditions are sufficiently 

severe that they warrant a thorough certification test program, including application of revised standards 

to airplanes that are currently certified for flight in icing conditions. The airframe icing safety issue is well 

known and has been confirmed by several studies and by the international icing community. The aviation 

community, including airframe manufacturers, airlines, pilots, and the flying public, is aware of the 
NASA/TM—2014-217729 25

problem and is calling for it to be addressed. Their concerns have been well documented in expert venues 

including the FAA Ice Protection Harmonization Working Group (Refs. 9 and 10) and the NASA 2009 

Airframe Icing Workshop. In addition, the joint government/industry Commercial Aviation Safety Team 

has developed a safety enhancement plan to encourage manufacturers of new turboprop-type designs to 

adapt and implement systems that automatically detect the presence of icing conditions that exceed those 

for which the aircraft has been certified including an estimate of accretion rate for advisory purposes and 

to provide annunciation to the flight crew if feasible (Ref. 11). 

Finally, the NTSB’s “Most Wanted List” includes “Reduce Dangers to Aircraft Flying in Icing 

Conditions” as one of their top 10 transportation improvements. The NTSB’s specific recommendations 

for reducing these dangers are to use current research on freezing rain and large water droplets to revise 

the way aircraft are designed and approved for flight in icing conditions; to apply revised icing 

requirements to currently certificated aircraft; and to require that airplanes with pneumatic deice boots 

activate the boots as soon as the airplane enters icing conditions (Ref. 12). Research to address TC2 is 

focused on assessing current ice protection system simulation methods and on developing strategies for 

improving the experimental capability for ice protection systems in ground-based icing facilities. 

Strategies will also be recommended for improving experimental ice-accretion simulation capabilities for 

ice buildup on aircraft surfaces that are subjected to SLD icing conditions. 

There is a significant need for increased detection, information collection, and mitigation capabilities 

for atmospheric hazards, including icing conditions. Terminal area ground remote sensing to monitor and 

enforce pending regulations for SLD icing certification is not currently available. Operations will require 

real-time atmospheric diagnostic capabilities beyond those being used today. Research conducted under 

the icing weather systems development element of TC3, AHSM technology capability, will address this 

need by improving the detection and measurement capabilities for in-flight icing hazards and also by 

further developing NASA AvSP remote sensing technologies research. An important Level 1 milestone 

(AEST3.1.06) will focus on conducting a terminal area icing algorithm and remote sensing system 

assessment to meet joint FAA and NASA standards. The resulting operational capabilities and constraints 

of the terminal area icing remote sensing systems will either define additional development work required 

or enable installation of these systems into the NextGen airspace system.

The icing and ice detection critical future safety risk area encompasses engine icing as well as the 

icing hazards encountered on the airframe and aircraft surfaces. The term “engine icing” has been used 

historically to describe ice accreting on exposed engine surfaces as an aircraft flies through a cloud of 

SLDs. Liquid droplets freeze on impact, or shortly thereafter, and form obstructions and distortions to air 

flowing into engines. Engine icing caused by ice accretion inside the normally hot core of jet-based 

engines that fly in clouds containing highly concentrated ice crystals is a recently identified problem and 

occurs with alarmingly high frequency worldwide. This counterintuitive and complex phenomenon has 

produced numerous calls for research to better understand and effectively address engine icing. The 

complex aerothermodynamic processes that permit ice to accrete inside the core of an engine in flight are 

not understood to a level that allows for effective analysis and prevention or mitigation techniques to be 

employed with confidence. Therefore, the AEST Project is planning to invest its engine-icing resources in 

HIWC icing research.

Evidence indicates that engine icing incidents caused by ice accreting inside the core of turbojet 

engines have been occurring for more than two decades. Since the early 1980s, studies of worldwide 

aircraft engine incidents have identified over 140 engine power loss events caused by engine core icing 

(Ref. 13). These events include compressor surge and stall, combustor flameout, engine rollback, and 

engine damage during and after flight in high ice water content (HIWC) conditions. Both commuter and 

large transport airplanes were involved. Project level milestone AEST3.1.01 is aimed at developing a 
NASA/TM—2014-217729 26

suite of enhanced icing instruments to withstand the particularly harsh HIWC environments to 

characterize the atmospheric environment associated with engine icing power loss events. 

Research will attempt to determine which environmental conditions can lead to engine core icing; the 

location inside the engine where it will occur; and how much ice will accrete. Additionally, NASA 

intends to conduct a flight campaign using a specially instrumented research aircraft to characterize 

natural HIWC environments. Advanced radar enhancement research efforts and post-flight analyses will 

focus on discovering a signature for HIWC conditions; providing data for icing researchers that 

characterizes HIWC conditions; and evaluating designs and making performance estimates for future 

commercial detection systems.

The NRC (Ref. 8) has indicated the need for better analytical models to predict adverse weather (e.g., 

rain, ice, hail, and crosswinds) impact on fans, compressors, and combustor stability. These models 

should be physics-based and validated with experimental data. Detailed weather data, as a function of 

altitude, are needed for altitudes up to 20,000 ft. Data on water concentration and droplet size are 

especially important. Analyses of the impact of ingested freezing rain and ice as they traverse the 

propulsion system need to be improved. Better models will lead to more robust engine designs and 

improved operational procedures. 

A significant segment of TC1, engine icing characterization and simulation capability, is focused on 

development and application of advanced numerical simulation codes to address the engine core icing 

hazard. Three key AEST Level 2 milestones are planned to target engine icing simulation capability. 

Milestone AEST3.2.02 will develop a tool to generate compression system performance maps with 

assumed simulated and computed blockage effects caused by ice crystal engine icing for use in an engine 

system modeling code. Milestone AEST3.2.04 will develop a tool to calculate mixed-phase ice accretion 

on engine components and assess the sensitivity of an engine to mixed-phase icing. The tool will predict 

ice accretion rates and locations by comparing the calculated results with results from engine icing 

laboratory tests. Finally, milestone AEST3.2.05 will produce an engine icing validation test to assess the 

Propulsion Systems Laboratory (PSL) icing test cell capability at the NASA Glenn Research Center and 

to simulate an engine icing event.

6.3 Loss of Control in Flight

LOC in flight involves accidents that occur during airborne phases of flight where aircraft control is 

lost. LOC can occur during either IMC or visual meteorological conditions. LOC in flight may be caused 

by a stall, an icing-related event, a severe atmospheric turbulence or wake vortex encounter, or a flightcritical system or component malfunction or failure. Aircraft energy management systems (excessive 

airspeed, altitude, and rate of descent) and aircraft configuration (flaps, slats, speed brakes, and onboard 

systems) can also result in LOC accidents or incidents. Analyses of NTSB accident and FAA incident 

data established that both powerplant and nonpowerplant system or component failures and malfunctions 

are significant contributing factors to aviation safety risk (Ref. 14). 

The National Aeronautics Research and Development Plan states that research is needed to facilitate 

implementation of advanced systems logic and architectures to avoid, detect, and resolve upset conditions 

that can lead to loss of stability and maneuverability caused by hazardous convective environments or 

icing (Ref. 15). HIWC atmospheric conditions capable of causing ice accretion on vital aircraft sensors 

and inside jet engines are representative of the types of hazardous weather environments that are potential 

contributors to upset conditions that can lead to LOC accidents or incidents.

Portions of the AEST project are addressing icing-related events and powerplant system failures and 

malfunctions as causal factors in LOC accidents through current work on airframe and engine icing. 

Research conducted under TC2 will increase flight safety in icing conditions by introducing validated and 
NASA/TM—2014-217729 27

accepted icing simulation tools earlier in the design cycle. Reliable assessments and effective mitigation 

design approaches will be provided, and technology to enable safe flight operations in SLD environments 

will be developed.

TC1 focuses on developing engine core icing simulation tools and techniques to predict ice accretion 

inside engines. Research will determine the environmental conditions in which icing can occur, the 

location inside the engine where it will occur, and how much ice will accrete. Another key component or 

research element of TC1 is the development of automatic engine control methods to mitigate the effects 

of, or prevent engine core icing altogether. The ultimate goal—reducing engine core icing events—

depends on discovering how it occurs and affects engine operation, as well as finding effective means to 

prevent it. A sophisticated engine simulation and control methodology was developed, validated, and 

verified under the NASA AvSP’s Integrated Resilient Aircraft Control project called Vehicle Systems 

Safety Technology (VSST). The AEST and VSST projects are working to capitalize on this technology 

by applying it to this TC. 

Finally, one of the objectives of TC3 is to develop mature technologies to remotely sense and 

measure icing conditions, and to provide real-time information to the pilot and operators in the NAS. This 

will provide aircraft with a greater ability to avoid the hazards of icing, convective weather, and wake 

vortices that are precursors to LOC accidents and incidents. The icing weather systems development 

element of TC3 includes research activities that will expand detection and measurement capabilities for 

inflight icing hazards for long-term adoption in the NextGen airspace system. Research will focus on 

mapping ground-based and airborne remote sensing of meteorological conditions that are conducive to inflight icing hazards. In addition, the advanced radar enhancements element will support development of 

hazard characterization studies for aircraft engine and airframe icing conditions and aircraft wake vortex 

investigation and modeling. The need for icing remote sensing technologies has long been recognized 

within NASA, and an FAA-sponsored international review of in-flight icing issues concluded that remote 

sensing could be useful in avoiding severe icing conditions by all aircraft (Refs. 16 and 17). 

6.4 Runway Safety

Runway safety encompasses runway incursions, excursions, and confusion (takeoffs and landings on 

wrong runways or taxiways). The number of runway and taxiway incursions remains unacceptably high 

despite recent efforts to minimize their occurrence. Runway incursions are expected to remain as a critical 

safety risk in the future. Flight deck workload and distractions preflight are contributing factors. The 

NTSB considers runway incursions one of its top three aviation safety issues, noting that “these incidents 

continue to occur with alarming, and increased, frequency” (Ref. 12). Implementing the NTSB’s

recommended safety system for ground movement will ensure the safe movement of airplanes on the 

ground and provide immediate warnings of probable collisions and incursions directly to flight crews in 

the cockpit. 

The Flight Safety Foundation conducted the Runway Safety Initiative (RSI) to address safety 

concerns and events involving runway incursions, excursions, and confusion. The RSI project team 

consisted of about 20 representatives from worldwide organizations, including operators, manufacturers, 

air navigation service providers, pilot groups, and other industry associations. After reviewing all these 

areas, the RSI team focused primarily on reducing runway excursion risks since 97 percent of runway 

accidents were found to have been caused by excursions (Ref. 18). The team also pointed out that runway 

incursion and confusion interventions would be useful to improve runway safety, including moving map 

displays and vision enhancement technologies. In the NextGen Integrated Work Plan (IWP), the Joint 

Planning and Development Office outlined a set of research and development activities that are required 

to support the proposed NextGen operational concepts (Ref. 19). Key activities identified in the IWP were
NASA/TM—2014-217729 28

applied research and development of low visibility and surface operations technologies to increase 

operator situational awareness in poor visibility terminal and airport surface conditions.

Higher numbers of aircraft in the system increases air and ground aircraft density. To address this 

increased demand, research needs to develop systems that improve pilot and controller awareness of 

airport surface conditions (aircraft and ground vehicle locations, runway occupancy, and pavement 

conditions), particularly in low visibility situations. Improving flight crew and ground controller 

situational awareness is critical to reducing incidents and accidents on the ground (Ref. 15). 

Poor visibility conditions are also a significant contributing factor to runway and taxiway surface 

incidents. AEST research is addressing runway safety through the smart visual awareness element of 

TC3. Level 2 milestones AEST4.2.14 and AEST4.2.15 plan to complete both a laboratory and flight 

demonstration of a real-time testbed of a basic pattern recognition engine for enhanced vision. These 

demonstrations are the culmination of the development of the pattern-recognition engine known as the 

streamlined spatial vision tree. The resulting smart avionics imaging sensor is a compact flight test unit 

that uses both image enhancement and pattern recognition. The research focuses on the low-visibility 

hazard and on terrain detection, as well as takeoff, landing, and taxiing hazards in poor visibility 

conditions. Moreover, Laser Imaging Through Obscurants, an active imaging system, and ForwardLooking Interferometry, a hyperspectral passive imaging system, are both being pursued under the AEST 

Project to enable equivalent VFR operations under low visibility conditions. 

6.5 Super Density Operations

Expected growth in the demand for air transportation will require more efficient and dense en route 

and terminal area operations. New procedures will be needed to reduce minimum spacing requirements 

during all phases of flight and in all weather conditions, through an integrated approach that leverages a 

suite of emerging technologies such as performance-based navigation and automatic dependent 

surveillance broadcast. Performance-based navigation procedures such as required navigation 

performance, area navigation, optimized profile descents, and tailored arrivals for oceanic flights, are 

being developed to increase the capacity and efficiency of the NAS, as well as provide environmental 

benefits such as reductions in fuel emissions and aircraft noise. 

Higher levels of air traffic will require simultaneous arrival procedures at airports with closely spaced 

parallel runways to achieve the capacity and efficiency gains needed in the NextGen environment. 

NextGen Operational Improvements 0400 and 0401 specify wake turbulence mitigation procedures for 

departures and arrivals to allow more closely spaced arrival and departure operations to maintain airport 

and runway capacity (Ref. 19). The National Aeronautics Research and Development Plan states that 

“reduced aircraft separation will require a move to trajectory-based operations, performance-based 

navigation, and a new allocation of responsibilities between air and ground and between humans and 

automation. In addition, planned advanced airspace design concepts that can be dynamically adjusted to 

meet demand requirements and avoid hazardous weather conditions must be developed with safety in 

mind.” (Ref. 15). 

The introduction of increased operational complexity and new technologies in the NextGen will likely 

give rise to unanticipated consequences that could reduce the safety profile of aircraft and the NAS. 

Atmospheric hazard information presented to flight deck crews is currently limited and must be improved 

to provide greater safety, especially as NextGen operations evolve. Thus, there is a significant need for 

increased atmospheric hazard detection, information collection, and mitigation capabilities. A remote 

sensing capability does not currently exist on aircraft to detect CAT, wake vortex, icing conditions, and 

ice-crystals, or to see through low-visibility aircraft environments. Current atmospheric hazard sensing, 

lightning sensing, and mitigation deficiencies have been confirmed by several independent studies and 
NASA/TM—2014-217729 29

aviation community feedback. The FAA’s NextGen Reduce Weather Impact solution set supports 

integration of a broad range of weather information into air traffic decisionmaking and the 

implementation of remote atmospheric hazard sensor technologies to mitigate the effects of weather 

resulting in safer operations in the NAS (Ref. 20). Also, several published reports including the “Decadal 

Survey of Civil Aeronautics,” confirm the need for continued remote sensing research and development. 

In this report the NRC calls for improved onboard weather systems and tools to minimize the impact of 

weather on the control of flight in heavy traffic. The report identifies affordable new sensors, system 

technologies, and procedures to improve wake turbulence prediction and measurement as an important 

research technology and challenge. It states, “Existing wake vortex separation standards reduce system 

capacity during takeoff and landing operations and instrument approaches. Research by the FAA and 

NASA has been focused on procedural enhancements that take advantage of wake transport by winds. 

However, the relaxation of in-trail wake separation standards awaits improved measurement and 

prediction of wake behavior. Existing sensors and models do not adequately characterize wake decay 

phenomena, especially at typical final approach altitudes. Improved sensors, including coherent pulsed 

lidars, capable of directly measuring wake rotational momentum, are needed to support phenomenological 

studies and enable more accurate predictions of wake magnitude and decay in various atmospheric 

conditions. Those predictions, combined with models of aircraft upset risk, should allow reduced wake 

separation standards without degrading safety” (Ref. 8). 

AEST research activity that addresses TC3 will enable pilots and controllers to obtain real-time 

accurate information on a broad range of atmospheric hazards, including CAT, wake vortex, icing, ice

crystals, and low visibility aircraft environments. Research will be conducted to develop advanced 

airborne radar technology for safety applications such as windshear detection and enhanced turbulence 

detection. Milestone AEST 4.2.12 will result in ground-based testing and verification of a new fiber 

optics-based lidar system that will be used to detect kinetic air hazards (e.g., turbulence, wake vortices, 

and windshear) on commercial transport and business jet aircraft. In addition, two key TC3 Level 3 

milestones are focused on: (1) a forward-looking interferometer field test for wake vortex hazard 

observation (AEST 4.3.05) and (2) characterization of wake vortex hazards in terminal area and en route 

airspace to provide models and hazard signatures for sensor algorithm development and validation (AEST 

4.3.09). 

7.0 Discussion and Conclusion

7.1 National Transportation Safety Board Accident Data 

The NTSB accident data was analyzed to identify trends related to the AEST Project TCs. Four types 

of NTSB operational accident data were examined across four time periods (1989 to 2008): Parts 121, 

135–S, 135–NS, and 91. The specific accidents types were engine icing related to ice crystals (TC1), 

icing/freezing rain/drizzle (TC2), and turbulence, wake vortex, lightning, low visibility, and low lighting 

(TC3). 

TC1’s focus is the characterization and simulation capability related to ice-crystal icing. A search of 

the NTSB accident data set did not yield accidents related to propulsion systems that were impacted by 

ice crystals and less than 5 percent of all accidents were affected by icing conditions (TC2). When icing 

was involved in Part 91 and 135–S operations, however, between 25 and 54 percent involved fatalities. 

Turbulence (excluding terrain-induced and wake vortex) was a cause or factor in 24 percent of Part 

121 accidents compared with less than 1 percent of accidents in other flight operations. Less than 

3 percent of fatal accidents and fatalities, regardless of flight operation, were affected by turbulence. 

Turbulence accidents usually did not involve any fatalities. Turbulence-related accident rates have 
NASA/TM—2014-217729 30

declined over time for Parts 91 and 135–NS, while the accident rate for Part 121 has fluctuated over time. 

CAT was involved in 19 percent of all turbulence accidents. In general, less than 1 percent of all accidents 

involved wake vortex and have decreased over time, except for Part 91 operations. Although the accident 

rate related to wake vortex has been low and decreasing during this study, expected growth in the demand 

for air transportation will require more dense en route and terminal area operations resulting in the need 

for wake vortex mitigation. 

Lightning was a cause or factor in less than 1 percent of all accidents. With the exception of Part 135–

NS, the rate of lightning incidents has remained fairly constant over time. Even though the rate is low, 

there is concern that future aircraft, with more composite structures, will be more impacted by lightning. 

Low visibility/lighting was involved in 6 to 23 percent of all accidents depending on operational 

category. While only 10 percent of Part 91 operations involved low visibility/lighting, 60 percent of these 

involved fatalities. Generally, the rates of low visibility/lighting accidents have been declining over time, 

except for Part 121 operations where the rate has remained steady. Most low visibility/lighting accidents 

involved low lighting. 

All icing, turbulence, wake vortex, lightning, low visibility, and low lighting accidents combined 

(account for nearly one-third of all accidents. The lowest percentage of accidents affected by any of these

atmospheric conditions was in Part 91, yet 58 percent involved fatalities. The combined rate of accidents 

for all atmospheric conditions declined for Parts 91 and 135–NS, increased for Part 135–S, and remained 

steady for Part 121 operations. 

7.2 Federal Aviation Administration Incident Data 

The FAA’s Accident/Incident database was analyzed to identify trends related to the AEST Project 

TCs. Four types of FAA operational incident data were examined across four time periods (1989 to 

2006): Parts 121, 135–S, 135–NS, and Part 91. The specific incident types were engine icing related to ice 

crystals (TC1), icing/freezing rain/drizzle (TC2), and turbulence, wake vortex, lightning, low visibility, 

and low lighting (TC3). 

TC1’s focus is the characterization and simulation capability related to ice crystal icing. A search of 

the FAA database set yielded 38 incidents related to propulsion system failures. Of those, 14 were caused 

by engine icing; two specifically cited ice crystals in the propulsion system as a cause for engine 

flameout. Most of the engine icing incidents occurred during Part 121 operations. The incidents were 

often not thoroughly investigated; the report giving little information as to the cause of the engine issues. 

Less than 2 percent of incidents were affected by airframe icing. Reported icing-related incidents 

declined during this study, except for Part 135–S operations.

Turbulence caused less than 3 percent of incidents in the FAA incident database, and the rates 

declined during this study. CAT was found in only 3 percent of the turbulence-related accidents. Wake 

vortex and lightning were each involved in less than 1 percent of the reported incidents and the rate of 

these types of incidents has declined over time as well. Low visibility/lighting was a factor in 27 percent 

of the incidents and declined during the study. Part 135 operations were most affected by low 

visibility/lighting conditions. 

Close to one-third of all incidents in Part 121s and 135 operations were affected by one or more of the 

six atmospheric conditions reviewed in this study (icing, turbulence, wake vortex, lightning, low lighting, 

and low visibility). 
NASA/TM—2014-217729 31

7.3 Future Safety Risk

A summary of the evaluation of research in the three TC areas’ projected impacts on critical future 

safety risk areas is provided in Table 18 showing how each research area maps to the critical future safety 

risk areas. 

An evaluation of the AEST Project research and technology development activities against several 

critical areas of future safety risk in the air transportation system was completed. These high-level 

qualitative future safety challenges were identified from multiple literature sources and are documented in 

the Systems Analysis of NASA AvSP—Final Report (Ref. 7). As stated in the AEST Project Plan 

(Ref. 1), the AvSP, including the AEST project, will “identify and develop tools, methods, and 

technologies for improving overall aircraft safety of new and legacy vehicles operating in the Next 

Generation Air Transportation System.” More specifically, the AEST Project will investigate sources of 

risk and provide needed technology to ensure safe flight in, and around atmospheric hazards. 

The focus of AEST research is on in-flight icing, both engine and airframe, as well as investigating 

other high-priority atmospheric hazards and sensor technologies required for their detection. These 

include wake vortex encounters, convective weather, turbulence, reduced visibility conditions, and 

lightning. The research is relevant to the safety needs of the aviation community and is crucial to the 

mitigation of future aviation safety risks associated with icing and atmospheric hazards for several 

reasons. First, icing is still a safety concern across all categories of subsonic aircraft and engine icing 

incidents are occurring on a frequent basis with the aviation community calling for action. Secondly, new 

aviation regulatory requirements necessitate development of new and enhanced icing simulation tools. 

Finally, operations in the NextGen environment will demand precise information about the atmosphere 

and awareness of weather hazards.

The engine icing characterization and simulation capability (TC1) will explicitly address the future 

safety risk area of icing/ice detection through its emphasis on developing analytical methods and both 

computational and experimental simulation tools to prevent ice crystal icing that can cause turbofan 

engine interruptions and failures. It will also target engine icing-related events as causal factors that can 

lead to in-flight LOC accidents and incidents. 

TABLE 18.—PROJECTED IMPACT OF AEST TECHNICAL CHALLENGE (TC) 

RESEARCH ON CRITICAL FUTURE SAFETY RISK AREAS

Critical future safety risk areas TC areas

TC1 engine 

icing

TC2 airframe 

icing

TC3 AHSMa

Aircraft mixed fleet equipage

Approach and landing accident reduction  

Enhanced survivability in the event of an accident

Human fatigue

Icing/ice detection   

Inadequate protection, analysis, and dissemination of safety data

Increasing complexity and reliance on automation

Loss of control—in flight   

Runway safety 

Super density operations  a

Atmospheric hazard sensing and mitigation
NASA/TM—2014-217729 32

The airframe icing simulation and engineering tool capability (TC2) will develop and demonstrate the 

capability to simulate and model airframe ice accretion and related aerodynamic performance degradation 

for current and future aircraft configurations over an expanded range of icing conditions. In addition, this 

research will have a positive impact on solving the problem of icing effects that can lead to LOC and 

approach and landing accidents by disrupting the aircraft’s aerodynamic lift characteristics. 

Research carried out under the AHSM technology capability (TC3) will address several areas of 

future aviation safety risk. The major focus of this TC is to develop advanced technologies for sensing 

and measurement of icing, turbulence, and wake vortex hazards for real-time information in the cockpit 

and throughout the NAS. As a result, risks associated with icing and ice detection, LOC in flight, and 

approach and landing accidents will be reduced. In addition, research to develop new technologies for 

enhanced vision systems and advanced imaging sensors will enable automatic detection of aviation safety 

hazards, in particular runway, terrain, and midair collisions during poor visibility flight conditions. This 

smart visual awareness research will bring about safer runway operations and improvements in safety as 

capacity increases in the NextGen environment. Finally, Super Density Operations, a foundational 

NextGen capability, will significantly increase the capacity in the NAS in response to the projected 

growth in demand. Enabling technologies required to achieve this NextGen operational concept are 

defined in the NextGen IWP (Ref. 20), and include wake vortex advisory systems to mitigate inadvertent 

wake turbulence encounters; enhanced onboard vision systems to allow VFR operations in low visibility 

conditions; and onboard turbulence mitigation systems to allow aircraft to safely travel through greater 

intensities of adverse weather. Research in this TC will produce technological advances in all of these 

areas, and allow for a safe reduction in the longitudinal and lateral spacing requirements needed in the 

super density environment. 

References

1. R&T Portfolio Project Plan. Atmospheric Environment Safety Technologies (AEST) Project Plan, 

Draft Version 5.0, NASA, Oct. 1, 2011. 

2. https://asafe.larc.nasa.gov/DOC/definitions.html

3. Code of Federal Regulations: Aeronautics and Space. 14 CFR: Office of the Federal Register 

National Archives and Records Administration, Washington, DC, 2004. 

4. National Transportation Safety Board: U.S. Air Carrier Operations Calendar Year 1999, Annual 

Review of Aircraft Accident Data. NTSB/ARC‒02/03, PB2002‒109241, 2003.

5. National Transportation Safety Board: Government Information Locator Service (GILS): Aviation 

Accident Synopses. 2002. http://ntsb.gov/Info/gils/gilssyn.htm Accessed Jan. 14, 2013.

6. National Aviation Weather Program Council: FAA Service Areas: Ceiling and Visibility. Ch. 3, p. 3‒1.

7. Jones, Sharon Monica, et al.: Systems Analysis of NASA Aviation Safety Program—Final Report. 

2010. 

8. Decadal Survey of Civil Aeronautics: National Research Council, 2006 

9. Ishimaru, K.: Ice Protection Harmonization Working Group (IPHWG) Icing Rulemaking 

Recommendations. Presentation to Joint Implementation Measurement and Data Analysis Team, 

Federal Aviation Administration, July 8, 2009. 

10. Federal Aviation Administration: Activation of Ice Protection: Final Rule. Federal Register, vol. 76, 

no. 162, Docket No. FAA–2009–0675, Amendment No. 121‒356, 2011.

11. Commercial Aviation Safety Team: Safety Enhancements. Joint Implementation Measurement Data 

Analysis Team (JIMDAT) Meetings and Aviation Safety Information Analysis and Sharing (ASIAS) 

System, Directed Studies, 2011. 

12. National Transportation Safety Board: Most Wanted Aviation Safety Improvements, 2010. 

http://www.ntsb.gov/safety/mwl.html Accessed Jan. 14, 2013. 
NASA/TM—2014-217729 33 

13. Mason, Jeanne G.; Strapp, J. Walter; and Chow, Philip: The Ice Particle Threat to Engines in Flight. 

AIAA 2006‒206, 2006. 

14. Reveley, Mary S., et al.: Commercial Aircraft Integrated Vehicle Health Management Study. 

NASA/TM—2010-215808, 2010. http://ntrs.nasa.gov/

15. National Aeronautics Research and Development Plan, Biennial Update, National Science and 

Technology Council, Feb. 2010. 

16. Reehorst, Andrew, et al.: Progress Towards the Remote Sensing of Aircraft Icing Hazards. 

NASA/TM—2009-215828, 2010. http://ntrs.nasa.gov/

17. Mead, James B.; Pazmany, Andrew; and Goodberlet, Mark: Evaluation of Technologies for the 

Design of a Prototype In-Flight Remote Aircraft Icing Potential Detection System. DOT/FAA/AR‒

98/72, 1998.

18. Flight Safety Foundation Safety Initiatives, 2010. 

19. Joint Planning and Development Office: Next Generation Air Transportation System Integrated Work 

Plan: A Functional Outline. Version 1.0, Sept. 30, 2008. 

20. Federal Aviation Administration: NextGen Implementation Plan. March 2010 
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Aviation Trends Related to Atmospheric Environment Safety Technologies Project

Technical Challenges

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14. ABSTRACT

Current and future aviation safety trends related to the National Aeronautics and Space Administration’s Atmospheric Environment Safety

Technologies Project’s three technical challenges (engine icing characterization and simulation capability; airframe icing simulation and

engineering tool capability; and atmospheric hazard sensing and mitigation technology capability) were assessed by examining the National

Transportation Safety Board (NTSB) accident database (1989 to 2008), incidents from the Federal Aviation Administration (FAA)

accident/incident database (1989 to 2006), and literature from various industry and government sources. The accident and incident data were

examined for events involving fixed-wing airplanes operating under Federal Aviation Regulation (FAR) Parts 121, 135, and 91 for

atmospheric conditions related to airframe icing, ice-crystal engine icing, turbulence, clear air turbulence, wake vortex, lightning, and low

visibility (fog, low ceiling, clouds, precipitation, and low lighting). Five future aviation safety risk areas associated with the three AEST

technical challenges were identified after an exhaustive survey of a variety of sources and include: approach and landing accident reduction,

icing/ice detection, loss of control in flight, super density operations, and runway safety.

15. SUBJECT TERMS

Flight safety; Aircraft safety; Turbulence; Wakes; Vortices; Clear air turbulence; Ice formation; Lightning; Low visibility

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