When the Eyes Deceive the Pilot: Accidents in Which Vision and Eyewear Affected Flight Safety

 

Good eyesight is essential for flying, but aviation vision cannot be reduced to reading letters on an eye chart. Pilots must interpret distance, contrast, color, movement, external references, and cockpit instruments in an environment that can change rapidly.

During a nighttime approach, visual acuity and color perception decrease while the brain attempts to estimate altitude and distance using limited external references. It is within this interaction between ocular anatomy, optical quality, the operating environment, and brain interpretation that dangerous errors may develop.

Official accident reports document events involving scratched lenses, color vision deficiency, eye disease, glare, and visual illusions. These cases demonstrate that eyewear is not merely an accessory and that satisfactory daytime vision does not necessarily guarantee the same performance at night.

Vision is more than visual acuity

Visual acuity measured during an eye examination represents only one part of visual performance. Inside the cockpit, pilots also depend on:

• contrast sensitivity;
• color discrimination;
• adaptation to changes in illumination;
• peripheral vision;
• depth perception;
• binocular coordination;
• rapid changes of focus between instruments and the outside environment;
• resistance to glare.

Under low-light conditions, retinal cones—which provide detailed vision and color discrimination—become less effective. Rods assume a larger role, offering greater light sensitivity but reduced detail and virtually no color perception.

This helps explain why a runway, obstacle, slope, or terrain feature may be perceived differently at night. It also explains why scratches, dirt, reflections, and glare on ophthalmic lenses become more significant when the available contrast is already limited.

The accident in which scratched glasses were a significant factor

On December 21, 1993, a Piper PA-25 glider tug and a Glasflugel Mosquito glider collided near Benalla, Australia.

The glider was approaching the traffic pattern while the tug was flying on an almost opposite track. The sun was positioned behind the Piper, making it difficult for the glider pilot to detect the approaching aircraft.

The glider pilot was wearing scratched corrective lenses with clip-on sunglasses. An optometrist who assisted the investigation explained that scratches could significantly degrade vision when the pilot looked toward the sun. The clip-on sunglasses added two more optical surfaces that could introduce additional scratches, dirt, reflections, and light scatter.

The Australian Transport Safety Bureau identified several significant factors, including:

• the tug’s position against the sun;
• limitations in the glider pilot’s visual acuity;
• scratched corrective lenses;
• clip-on sunglasses worn over those lenses.

Communication and traffic-pattern entry problems also contributed to the occurrence. The glasses did not explain the accident by themselves, but they formed part of the chain of factors that reduced the pilot’s ability to detect the other aircraft. ATSB Investigation 199303898.

FedEx Flight 1478: when the PAPI colors were not correctly interpreted

On July 26, 2002, a FedEx Boeing 727 was conducting a nighttime visual approach to Runway 09 at Tallahassee, Florida.

The first officer, who was flying the aircraft, had a significant color vision deficiency. During the approach, the airplane descended below the proper glidepath. The Precision Approach Path Indicator, or PAPI, was providing red-and-white visual guidance, but that information was not correctly interpreted.

The National Transportation Safety Board concluded that the first officer’s color vision deficiency made it difficult for him to identify the PAPI indications correctly. The investigation also identified fatigue, failure to follow company procedures, and inadequate approach monitoring.

The probable cause was the flight crew’s failure to establish and maintain a proper glidepath during the nighttime visual approach. The first officer’s color vision deficiency was officially listed as a contributing factor.

This accident demonstrates that a pilot may have excellent near and distance acuity while still experiencing difficulty with tasks requiring rapid color discrimination. In aviation, those tasks include interpreting PAPI and VASI systems, navigation lights, airport markings, weather radar, electronic displays, and warning lights. NTSB Aircraft Accident Report AAR-04/02.

Macular degeneration and the decision to continue flying

Another relevant accident occurred on June 8, 2008, during an approach to Fremont Airport in Ohio. A Cessna U206C entered an inadvertent stall following a loss of control.

The investigation found that the pilot had a severe visual deficiency associated with macular degeneration. This condition primarily affects the central portion of the retina responsible for detailed vision.

The NTSB could not conclusively determine whether the eye disease directly initiated the loss of control. Other possible medical factors could not be completely excluded. However, the pilot’s decision to continue flying with a severe visual deficiency was classified as a contributing factor.

The aviation medical examiner’s failure to assess and report the visual deficiency accurately was also considered contributory.

The safety lesson concerns both the medical condition and the barriers intended to manage it. Corrective lenses can compensate for refractive errors such as myopia, hyperopia, and astigmatism, but they cannot restore retinal tissue affected by certain eye diseases. NTSB Aviation Investigation CHI08FA156.

When the eyes are healthy but the brain interprets the scene incorrectly

Not every vision-related accident is caused by eye disease or inappropriate eyewear. Pilots with clinically normal vision can still be deceived by runway dimensions, slope, lighting, dark terrain, or the absence of a visible horizon.

Visual perception is not a photographic reproduction of the outside world. The brain interprets information received from the eyes and compares it with previous experience. When important references are missing, it may create a convincing but inaccurate mental picture.

Canadian Airlines Boeing 767: the upslope-runway illusion

On March 8, 1996, a Canadian Airlines Boeing 767 sustained a tail strike while landing at Halifax, Nova Scotia.

The runway’s upslope created the impression that the aircraft was higher than it actually was. In response to this visual impression, power was reduced during the final part of the approach.

The investigation found that the pilots did not respond adequately to the PAPI, which indicated that the aircraft was below the proper glidepath. The captain’s concern about stopping on a slippery runway and other operational conditions also contributed.

The Transportation Safety Board of Canada concluded that the crew responded to a visual illusion. No eye disease was required for the perceptual error to occur. TSB Aviation Investigation Report A96A0035.

Air Canada Flight 630: black-hole conditions and an upslope runway

On September 24, 1999, an Air Canada Airbus A320 conducted a nighttime approach to Runway 29 at St. John’s, Newfoundland. The runway threshold had been temporarily displaced because of construction.

The aircraft touched down before the relocated threshold. Investigators found that the area surrounding the runway provided few visual references and was conducive to a black-hole illusion. The runway’s upslope also affected the pilots’ perception of altitude.

The TSB considered it probable that the lack of visual cues and the upslope-runway illusion contributed to a flightpath below the nominal three-degree approach angle. TSB Aviation Investigation Report A99A0131.

Astra SPX: only the runway lights were clearly visible

On March 22, 2000, an Astra SPX was conducting a nighttime visual approach to a private airfield at Fox Harbour, Nova Scotia.

The surrounding terrain was dark and nearly featureless. The runway had edge lights but no approach-lighting system or visual approach slope indicator. During the approach, the aircraft contacted trees before the runway. The crew initiated a go-around and subsequently landed at another airport.

The TSB concluded that conditions conducive to a black-hole illusion were present. The crew did not recognize the hazard or compensate adequately for it. The preparation and execution of the approach also did not fully comply with the expected operating procedures. TSB Aviation Investigation Report A00A0051.

Robinson R44: nighttime approach over dark water

On December 3, 2008, a Robinson R44 approached an illuminated landing area near Lac Simon, Quebec. The final approach was conducted over the dark surface of a lake.

Only a few lights were available, and the water provided almost no useful depth or height references. The helicopter descended below the intended flightpath and contacted the surface before reaching the landing area.

The TSB considered it likely that a black-hole illusion caused the pilot to believe that the helicopter was higher than it actually was. The aircraft was under control, but it was following a flightpath based on an incorrect visual perception. TSB Aviation Investigation Report A08Q0231.

What these accidents teach us about pilots’ eyewear

The cases do not establish that a particular lens brand or design can prevent accidents. The research also found no official accident report identifying ready-made reading glasses, incorrect pupillary distance, or poorly centered progressive lenses as the sole cause of an aviation accident.

That does not mean these issues are operationally irrelevant.

When the pupillary distance used for lens fitting does not correspond to the wearer’s actual measurements, the eyes may look through areas displaced from the intended optical centers. Depending on lens power and the amount of decentration, unwanted prismatic effects may result.

The visual system attempts to compensate through vergence, the coordinated movement that keeps both eyes aligned on the same object. Because accommodation and vergence are functionally linked, improper fitting can increase binocular effort, cause visual discomfort, or reduce efficiency when changing between different viewing distances.

This does not mean the crystalline lens is necessarily damaged. The concern is visual performance, comfort, and the additional workload imposed on the visual system.

Progressive lenses require even greater care. Monocular pupillary distance, fitting height, vertex distance, pantoscopic tilt, and frame wrap influence the position and usability of the distance, intermediate, and near zones.

Inside the cockpit, those zones correspond to different operational tasks:

• the runway, horizon, and outside traffic;
• the primary instrument panel;
• multifunction displays and side-mounted equipment;
• checklists, charts, and nearby devices.

A lens intended mainly for occasional reading is therefore not automatically suitable for flying.

Scratched, polarized, and excessively dark lenses

The Australian accident demonstrated the risk associated with scratched lenses when facing an intense light source. Scratches scatter light across the lens, increase veiling glare, and reduce contrast. At night, similar effects can intensify halos around runway lights, headlights, and cockpit illumination.

The FAA advises pilots to avoid polarized lenses because they may interact with liquid-crystal displays and other cockpit materials, causing instruments to appear dim, distorted, or temporarily unreadable from certain angles.

The FAA also recommends lens tints that reduce glare without significantly altering color perception. Photochromic lenses require careful evaluation because aircraft windshields may block some of the ultraviolet radiation needed to activate darkening, while temperature and other conditions can affect their response. FAA — Sunglasses for Pilots: Beyond the Image.

Safety defenses must work together

Visual safety does not depend on a single barrier. It results from the combination of:

• comprehensive periodic eye examinations;
• an updated prescription;
• color vision and contrast assessment;
• lenses free from significant scratches or deterioration;
• accurate individual measurements;
• a stable frame compatible with the headset;
• high-quality anti-reflective treatment;
• readily available backup glasses;
• training in nighttime visual illusions;
• disciplined instrument cross-checking;
• compliance with stabilized-approach criteria;
• an immediate go-around when visual references become unreliable.

Even with perfect eyesight, a pilot should not attempt to defeat a visual illusion simply by looking harder. The primary defense is to compare outside perception continuously with altitude, vertical speed, distance, the published approach path, PAPI or VASI indications, and every other reliable instrument available.

Conclusion

The eyes provide information, but the brain determines what that information means. Scratched lenses can scatter light. A color vision deficiency can interfere with PAPI interpretation. Retinal disease can compromise central vision. A brightly illuminated runway surrounded by darkness can create a false perception of height.

Official accident reports show that vision affects flight safety in different ways. Sometimes the limitation originates in the eye. In other cases, it involves eyewear. Frequently, the threat comes from the visual environment and the brain’s interpretation of incomplete references.

Pilots and crewmembers who require visual correction should use an updated prescription, quality lenses, accurate individual measurements, and professionally fitted eyewear. However, no pair of glasses can eliminate the physiological limitations of human vision or replace procedures, training, and instrument monitoring.

In aviation, seeing is not always believing. A safe pilot must recognize when the picture outside may not represent reality.

Marcuss Silva Reis
Fixed-Wing Commercial Pilot | Former Civil Aviation Flight Instructor | University Professor of Aeronautical Sciences | Aviation Expert Witness | Economist | Postgraduate Specialist in Aeronautical Sciences, Civil Aviation Safety, and Higher Education | Optical Technician | Undergraduate Student in Optics and Optometry
Founder of Instituto do Ar

References and Sources

1. Australian Transport Safety Bureau — ATSB. Piper PA-25-235 VH-AYB and Glasflugel Mosquito VH-GMN: Mid-air Collision Near Benalla, Victoria, December 21, 1993. Investigation 199303898.
   Official investigation report
2. National Transportation Safety Board — NTSB. Collision With Trees on Final Approach: Federal Express Flight 1478, Boeing 727-232, N497FE, Tallahassee, Florida, July 26, 2002. Aircraft Accident Report NTSB/AAR-04/02.
   Official accident report
3. National Transportation Safety Board — NTSB. Cessna U206C, N29122, Fremont Airport, Ohio, June 8, 2008. Aviation Investigation CHI08FA156.
   Official investigation report
4. Transportation Safety Board of Canada — TSB. Tail Strike on Landing: Canadian Airlines International, Boeing 767-375, C-FOCA, Halifax, Nova Scotia, March 8, 1996. Report A96A0035.
   Official investigation report
5. Transportation Safety Board of Canada — TSB. Landing Short: Air Canada Airbus A320-211, C-FKCO, St. John’s, Newfoundland, September 24, 1999. Report A99A0131.
   Official investigation report
6. Transportation Safety Board of Canada — TSB. Collision With Trees: Jetport Inc. Astra SPX, C-FRJZ, Fox Harbour, Nova Scotia, March 22, 2000. Report A00A0051.
   Official investigation report
7. Transportation Safety Board of Canada — TSB. Controlled Flight Into Water: Robinson R44 Raven I, C-GSVX, Lac Simon, Quebec, December 3, 2008. Report A08Q0231.
   Official investigation report
8. Federal Aviation Administration — FAA. Pilot Vision. Aerospace Medical Education Division, Civil Aerospace Medical Institute. Publication OK-17-2021.
   FAA Pilot Vision brochure
9. Federal Aviation Administration — FAA. Airplane Flying Handbook: Night Operations. FAA-H-8083-3C, Chapter 11.
   FAA Night Operations chapter
10. Federal Aviation Administration — FAA. Pilot’s Handbook of Aeronautical Knowledge: Aeromedical Factors. FAA-H-8083-25B, Chapter 17.
    FAA Aeromedical Factors chapter
11. Federal Aviation Administration — FAA. Montgomery, Ronald W.; Nakagawara, Van B. Sunglasses for Pilots: Beyond the Image. Civil Aerospace Medical Institute.
    FAA sunglasses guidance
12. Federal Aviation Administration — Civil Aerospace Medical Institute. Natural Sunlight and Its Association With Aviation Accidents.
    FAA/CAMI sunlight and glare study