Van B. Nakagawara 

Kathryn J. Wood 

Ron W. Montgomery

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

May 2003

Final Report

This document is available to the public

through the National Technical Information

Service, Springfi eld, Virginia 22161.

 

Offi ce of Aerospace Medicine

Washington, DC 20591

Natural Sunlight and Its 

Association to Aviation Accidents: 

Frequency and Prevention

DOT/FAA/AM-03/6
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

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1

NATURAL SUNLIGHT AND ITS ASSOCIATION TO AVIATION ACCIDENTS: 

FREQUENCY AND PREVENTION

INTRODUCTION

Glare is a temporary sensation produced by luminance 

(brightness) within the visual fi eld that is signifi cantly 

greater than that to which the eyes are adapted (1) and 

is not associated with biological damage. The effects on 

vision from glare only last as long as the light source is 

present within the individual’s fi eld of vision. Glare has 

been classifi ed as either “discomfort” or “disability” (2,3). 

Discomfort glare is the subjective response of annoyance 

caused by a light source without any measurable effect 

on visual performance. Disability glare is a loss of visual 

performance by the apparent scattering of light within 

the eye (1). In some instances, after exposure to a bright 

light, fl ashblindness (a visual interference effect that 

persists even after the source of illumination has been 

removed) or afterimage (a transient image left in the 

visual fi eld after exposure to a bright light) may occur. 

These effects can result in prolonged visual impairment 

and be extremely hazardous to individuals that require 

optimum vision, such as a pilot in fl ight.

In normal young eyes, approximately 10 to 20% 

of the light incident on the corneal stroma is scattered, 

causing a reduction in the contrast of the retinal image 

(4). This reduction in retinal image quality can be particularly debilitating when viewing objects of low contrast 

and while looking through a compromised optical media (e.g., scratched or dirty windscreens, eyeglasses, and 

contact lenses) or certain atmospheric conditions (e.g., 

haze, fog, and mist). Additionally, individuals with lightly 

pigmented eye color may be less tolerant of bright light 

than those with darker pigmentation (5,6). This may be 

due to lighter pigmentation of the retinal epithelium, 

which absorbs less of the scattered light rays and results 

in visual noise (7).

Some abnormal ophthalmic conditions can increase 

glare sensitivity. Cataract (opacifi cation of the eye’s crystalline lens) is a source of intraocular scattering that results 

in “ghost images” (halos around lights) or a “dazzling” 

sensation, which can be caused by bright car headlights 

or intense sunlight (8,9,10,11). Other ophthalmic conditions that can result in discomfort or disability glare 

include age-related macular degeneration (12,13,14), 

pterygium (15), corneal scarring, corneal edema from 

contact lens wear (16,17), aphakia (18,19), intraocular 

lens implants (20,21,22), lens capsule opacifi cation after 

cataract surgery (23,24,25), radial keratotomy (26,27), 

and laser refractive surgery (28,29,30,31).

Certain medications and drugs can affect an individual’s ability to tolerate illumination by high-intensity light 

sources. Photosensitizing medications include: antibiotics (tetracycline, sulfonamide) (32), oral contraceptives 

(estrogen, progesterones) (33), or acne medications (AccutaneTM) (34). Additionally, alcohol use can result in an 

increase in glare recovery time. Relatively low doses of 

alcohol can produce a signifi cant increase in glare recovery time, which may last for several hours after ingestion 

(35). Marijuana signifi cantly delays the time course of 

glare recovery after intense light exposure (36) and may 

have additive effects when used with alcohol or other 

drugs that can further reduce glare tolerance and increase 

recovery time.

Pilots are exposed to various meteorological conditions while in-fl ight that may increase glare and limit 

visibility and contrast. For example, aviators are often 

subjected to direct and indirect sunlight, which can act 

as an intense source of glare. Furthermore, airmen fl ying 

at high altitude may be exposed to darkened skies above 

and bright refl ected light from the clouds beneath. The 

contours of the human face serve to protect the eyes from 

bright light coming from above, but not from below 

(37). At 10,000 feet above ground, an aviator is exposed 

to approximately 11,800 foot-candles, while at sea level 

the exposure is approximately 10,000 foot-candles (38). 

Finally, an aviator may be temporarily visually disabled from sunlight scattering off dirty or damaged 

windscreens when flying out from behind the shadow 

of a mountain or flying out of cloud cover into a 

brightly-lit environment.

Harsh environmental lighting conditions may seriously compromise an aviator’s ability to “see-and-avoid” 

other aircraft in the adjacent airspace or complete critical fl ight operations (landings, takeoffs). Life-threatening 

situations may develop in an instant if a pilot is visually 

impaired due to glare at a critical moment. At lower altitudes, where these operations are often executed, there 

is less time to react, and the risk of an aviation accident 

increases dramatically.

The FAA’s Vision Research Team has an ongoing 

research program that investigates issues involving lasers 
2

and other high-intensity light sources and their effects on 

pilot vision and performance. The purpose of this study 

is to investigate the relationship between visual impairment from natural sunlight and aviation accidents.

METHODS

The National Transportation Safety Board (NTSB) 

Aviation Accident/Incident Database was queried for 

terms related to glare from January 1, 1988, to December 

31, 1998. Terms used in this search included, sun, glare, 

vision, blinded, and refl ections. All reports annotated with 

one or more of these terms were reviewed to determine 

whether some form of glare from natural sunlight was 

considered a direct or contributing factor in the aviation accident. In this study, accidents and operational 

problems that did not involve the pilot-in-command of 

an air transport or general aviation aircraft were omitted 

from all search results. The remaining records were then 

organized by time of day, visual conditions, phase of fl ight, 

and type of operational error and analyzed.

RESULTS

For the study period, there were a total of 25,226 

accidents in the NTSB Aviation Accident/Incident 

Database. Of these, there were 130 accidents in which 

direct or refl ected glare from the sun was found to be a 

contributing factor in the event. Table 1 summarizes the 

ambient lighting (position of sun) and visual conditions 

(weather, atmospheric, or optical media) that were present 

during the mishaps.

Table 2 summarizes the accidents by phase of fl ight and 

the type of error that resulted from or was exacerbated 

by exposure to glare from natural sunlight.

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3

DISCUSSION

The results of the study confi rm that glare from 

natural sunlight has caused visual impairment of pilots 

while operating aircraft and has contributed to aviation 

accidents. The majority of mishaps (85%), with glare 

mentioned as a contributing factor in the report narratives, occurred under optimal atmospheric and visual 

conditions (clear). In addition, most accidents occurred 

during daytime hours (82%), rather than in the early 

morning or evening hours (18%) when the sun was lower 

in the sky. It is important to note that there were several 

accidents in which other visual stressors were mentioned 

as contributing to the glare conditions. For example, atmospheric conditions (e.g., smoke, haze, fog, and dust) 

added to the pilot’s diffi culty in coping with disability 

glare (8.5%). Other confounding factors mentioned in 

accident reports included damaged or dirty windscreens 

(5.4%) and an inability of the pilot to view the cockpit 

instruments due to glare (2.3%).

Temporary visual impairment due to glare can have 

serious consequences for pilots during critical maneuvers 

performed at low altitude. The study results indicate that 

55% (72/130) of the accidents involving glare occurred 

during the approach/landing and takeoff/departure phases 

of fl ight. The majority of these accidents (72%) involved 

collisions with objects or terrain. Approximately 75% 

of these collisions were due to under/overshooting the 

runway or failing to maintain alignment with the runway, 

suggesting an induced perceptual problem. Furthermore, 

of those who lost of control of the aircraft during the 

approach/landing or takeoff/departure phase of fl ight 

(14%), the accident reports described the pilots’ inability 

to judge altitude and/or distance, which resulted in hard 

landings from fl aring too early (or late), and unsuccessful 

attempts to abort landings or takeoffs, resulting in collisions with trees, power lines, utility/fence poles, or other 

structures near the runway. Two accidents (2.8%) on 

takeoff/departure and 6 (8.3%) during approach/landing 

were midair collisions that resulted from one or both pilots’ 

failure to “see-and-avoid” the others’ aircraft due to glare 

disability. Two (2.8%) pilots crashed on approach after 

losing power and being distracted by glare.

Thirty-seven of the 50 accidents (74%) categorized as 

“in-fl ight” involved collisions with objects or terrain. Of 

these, about 62% (23/37) involved aerial application of 

agricultural products (i.e., crop dusting). Although risky 

by nature, this activity is even more dangerous when the 

aircraft’s windscreen becomes contaminated with agricultural spray products that exacerbate the effects of glare 

and further limits the pilot’s outside view. Seven of the 

nine accidents (78%) that were listed as “midair collisions” 

were described as the pilot(s) failing to “see-and-avoid” 

the other aircraft due to glare from the sun. Four of the 

50 “in-fl ight” accidents (8%) occurred when the aircraft 

lost power due to mechanical failure or fuel exhaustion, 

which forced the pilot to perform an emergency landing. The stress of the emergency landing, often in an 

unfamiliar location and complicated by glare, resulted 

in collisions with objects or the terrain.

Taxiing an aircraft around the airport can be made 

more diffi cult when glare is present. About 6% (8/130) 

of all accidents reviewed occurred while the aircraft was 

taxiing, either to takeoff or after landing. In several instances, the glare effects were exacerbated due to neglected 

windscreens (dirty, scratched, and pitted), which further 

scattered the sunlight.

As these accident reports illustrate, glare from natural 

sunlight can be visually debilitating and lead to operational errors that can result in mishaps. In some cases, 

use of appropriate sunglasses would have minimized 

the affects of glare on vision performance. However, 

when using sunglasses there should be a proper balance 

between visibility of objects inside and outside of the 

cockpit environment. Proper sunglasses include lenses 

that are free from distortions and imperfections, have 

adequate light transmissivity (approximately 15% overall 

light transmission), and do not alter color perception 

(e.g., neutral gray). Additionally, the use of larger lens 

sizes and wrap-around frame styles can prevent sunlight 

from entering peripherally and affecting the pilot’s vision. 

Furthermore, the use of polaroid sunglasses should be 

discouraged, since they can reduce or effectively eliminate 

the visibility of instruments that incorporate anti-glare 

fi lters or can interfere with visibility through an aircraft 

windscreen due to striations in some laminated materials (39). Polaroid sunglasses can also mask the sparkle 

of light that refl ects off shiny surfaces, such as another 

aircraft’s wings, fuselage, or windscreen, which could 

reduce a pilot’s reaction time in a “see-and-avoid” traffi c 

situation.

Additional techniques that could help prevent operational errors resulting from glare exposure include:

Q Enlist the assistance of a co-pilot or passenger to help 

read important instruments and/or printed fl ight 

documents so the pilot-in-command can focus his/

her attention on overcoming glare conditions related 

to the exterior view;

Q Deploy the aircraft’s sun visor or use a brimmed hat 

to shield the pilot’s eyes from exposure to glare;

Q Avoid wearing light colored clothing that can create 

a refl ection on the windscreen or instrument panel;

Q Do not place light colored or refl ective materials on the 

glareshield that can refl ect light off the windscreen;
4

Q Clean the windscreen thoroughly to prevent additional 

light scatter (Note: Preventive maintenance should 

include repair or replacement of the aircraft windscreen 

once it becomes scratched or pitted.);

Q Be cautious about the use of medications that can be 

photosensitizing;

Q Use navigation lights during the day while performing 

takeoff/departure and approach/landing maneuvers 

to allow other pilots to “see-and-avoid” your aircraft; 

and,

Q Pilots with eye pathologies that may increase glare 

sensitivity (incipient cataract, age-related macular degeneration, etc.) should obtain appropriate sunglasses 

for use while fl ying.

In conclusion, glare from natural sunlight has contributed to aviation accidents. The use of appropriate 

ophthalmic lenses, personal protection devices, available human resources, proper aircraft maintenance, and 

other techniques to minimize the effects of disability 

glare discussed in this paper may have prevented some 

of the accidents identifi ed in this study. Reviewing these 

events provides pilots, crewmembers, aviation medical 

examiners, and eyecare specialists with important facts 

and recommendations that can help prevent future 

operational mishaps associated with glare and improve 

aviation safety.

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