DOT/FAA/AM-00/18

Van B. Nakagawara

Stephen J.H. Véronneau

Civil Aeromedical Institute

Federal Aviation Administration

Oklahoma City, Oklahoma 73125

May 2000

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, Virginia 22161.

A Unique Contact

Lens-Related Airline

Aircraft Accident Office of Aviation Medicine

Washington, D.C. 20591

U.S. Department

of Transportation

Federal Aviation

Administration
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

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-00/18

4. Title and Subtitle 5. Report Date

A Unique Contact Lens-Related Airline Aircraft Accident May 2000

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Nakagawara, VB, and Véronneau, S.J.H.

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

FAA Civil Aeromedical Institute

P.O. Box 25082

Oklahoma City, OK 73125 11. Contract or Grant No.

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered

Office of Aviation Medicine

Federal Aviation Administration

800 Independence Ave, S.W.

Washington, D.C. 20591 14. Sponsoring Agency Code

15. Supplemental Notes

This work was performed under task AM-A-99-TOX-203.

16. Abstract

INTRODUCTION. The use of contact lenses to satisfy the distant visual acuity requirements for obtaining a civil airman

medical certificate has been permitted since 1976. According to the Federal Aviation Administration's "Guide for Aviation

Medical Examiners," the use of monovision contact lenses is not considered acceptable for aviation duties. An aviation accident

involving the use of monovision contact lenses will be reviewed.

METHODS. A case report is presented utilizing information from a National Transportation Safety Board (NTSB) aircraft

accident report (NTSB/AAR-97/03) of a nonfatal scheduled airline accident. Past studies that examined the use of contact

lenses in the aviation environment are reviewed.

RESULTS. On October 19, 1996, a McDonnell Douglas MD-88 aircraft, Delta Airlines Flight 554, was substantially

damaged in an undershoot approach while landing at LaGuardia Airport, Flushing, NY. Weather observations indicated a

broken cloud layer at 800 feet, visibility between 1/2 and 1 mile in heavy rain and fog or mist, and easterly winds at 12 to 14

knots. The approach was over water to Runway 13 and the flight crew transitioned to visual references just above the decision

height. As the airplane continued to descend, it struck an approach light structure and the end of the runway deck, shearing off

the main landing gear and slid 2,700 feet down the runway. During an emergency evacuation, 3 passengers received minor

injuries.

CONCLUSION. The NTSB determined that the probable cause of this accident was the inability of the pilot to overcome his

misperception of the airplane's position relative to the runway, due to the use of monovision contact lenses. The adverse effects

of wearing contact lenses in the aviation environment are discussed. Research is recommended to better understand the effects

of environmental conditions on monovision to validate the current policy on such corrections.

17. Key Words 18. Distribution Statement

Document is available to the public through the

National Technical Information Service,

Aviation; Vision; Contact Lenses; Aeromedical

Certification; Aviation Accidents.

Springfield, Virginia 22161

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 9

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

1

A UNIQUE CONTACT LENS-RELATED AIRLINE AIRCRAFT ACCIDENT

INTRODUCTION

The use of corrective ophthalmic lenses is more

prevalent in older pilots. In a study of U.S. Air Force

pilots, about 50% of aviators between the ages of 41

and 45 wore eyeglasses, while approximately 90% of

those over age 45 wore spectacles (1). In 1977,

airmen ≥ 40 years of age comprised only 38.8% of the

civilian airman population, and about 42.8% had a

vision restriction. By 1997, airmen age 40 and over

comprised more than 59% of the population and 56.0%

of airmen had a vision restriction for flying (2).

The use of eyeglasses to correct refractive error

may have disadvantages for pilots. Spectacle frames

can reduce the field of vision, be uncomfortable when

not properly fit, displace during flight maneuvers (Gforces), and be incompatible with headsets and other

communication devices as well as protective breathing equipment. Spectacle lenses may also become

dislodged inflight and fogging can occur with changes

in air temperature. Additionally, adapting to multifocal spectacle lenses may be difficult for the pilot, as

the older aviator often requires special prescriptions

for the unique visual demands of the cockpit.

Prior to 1976, civilian airmen were not allowed to

use contact lenses while flying unless a waiver, i.e.,

Statement of Demonstrated Ability (SODA), was

issued by the Federal Aviation Administration (FAA).

As of December 21, 1976, Amendment 67-10 to the

Federal Aviation Regulations permitted the routine

use of contact lenses to satisfy the distant visual

acuity requirements, eliminating the need for the

SODA process (3). Contact lenses have inherent

advantages for pilots, including: more natural vision,

full field of vision, no lens fogging or water droplet

accumulation, no discomfort due to weight, and no

annoying obstruction from the frame or distracting

reflections from the lenses. Military pilots have used

contact lenses in harsh wartime environments and

have reported them to be operationally superior to

spectacles (4).

In 1998, within the United States, there were

about 32.1 million contact lens wearers, about 19%

of the 173 million eyewear users in this country (5).

As the “baby-boomers” reach the age when presbyopia (an age-related reduction in accommodative ability) becomes more prevalent, interest in near vision

correction contact lenses is expected to increase.

Currently, monovision (one lens for distant vision

and one lens for near vision) is the most successful

method to correct presbyopia with contact lenses and

is used in approximately 80% of all such presbyopic

corrections (6). However, under Federal Aviation

Regulations (7), the use of monovision contact lenses

is prohibited since one eye would not meet the visual

acuity standard. For example, first- and second-class

airmen are required to have 20/20 or better Snellen

visual acuity at distance in each eye separately, with

or without corrective lenses. A pilot wearing

monovision contact lenses in the near correcting eye

may not meet this distant vision standard. According

to the FAA’s “Guide for Aviation Medical Examiners

(3),” the use of monovision contact lenses is not

considered acceptable for aviation duties (Note: Additionally, bifocal contact lenses are not considered

acceptable for aviation duties.). In this paper, a case

report of an aircraft accident is presented in which the

use of monovision contact lenses was found to be a

contributing factor.

METHODS & RESULTS

The National Transportation Safety Board (NTSB)

aircraft accident report of the October 1996 Delta

Air Lines Flight 554 was reviewed (8). The following

is a summary of that report.

On October 19, 1996, Delta Air Lines Flight 554

departed Atlanta’s Hartsfield International Airport

bound for LaGuardia Airport at Flushing, NY. The

crew consisted of two pilots and three flight attendants. There were 58 passengers on board the aircraft.
2

The McDonnell-Douglas MD-88, N914DL, was on

approach to the ILS/DME-equipped (Instrument

Landing System with Distance Measuring Equipment) Runway 13. Weather observations at LaGuardia

indicated a broken cloud layer at 800 feet, visibility

between 1⁄2 and 1 mile in heavy rain and fog or mist,

and easterly winds at 12 to 14 knots. The flight

encountered light to moderate precipitation as it

descended below 3,000 feet. The crew was flying an

approach that called for them to cross over Bowery

Bay to the right of the extended runway centerline.

The approach end of Runway 13 is built on a concrete

deck about 15 feet above Bowery Bay. Approach

lights are installed on a narrow pier extending out

over the bay. The runway glideslope is unusable

below 200 feet, and the runway has a decision height

(altitude at which a decision is made to land or goaround during a precision approach) of 250 feet.

When the flight reached the decision height, the

approach continued with the crew transitioning to

visual references. The airplane then landed short,

resulting in the right wing striking the approach light

structure and the runway deck closest to the approach

end (see figure 1). The main landing gear assemblies

were sheared off; the aircraft proceeded to slide 2,700

feet down the runway and rotated 180 degrees (see

figure 2). With the odor of jet fuel present, an

emergency evacuation was ordered, during which

three passengers received minor injuries.

FIGURE 1. An aerial view of LaGuardia Airport: The darkest areas are water. The stars

represent the points where Delta 554 first impacted Runway 13 and where it skidded to a

stop 2,700 ft. down the runway and facing 180o from the direction of approach.

Appro ach Path

Impac t Point

Re s ting Point
3

FIGURE 2. This photo shows the wreckage of Delta 554 as it sat aside Runway 13 at LaGuardia

Airport. Three passengers suffered minor injuries during the emergency evacuation that was ordered by the pilot after a report of the smell of jet fuel.

The 48-year-old captain, with about 9,000 total

flight hours including 3,755+ flight hours in the

MD-88, was the pilot-in-command. He had no previous aviation accidents or incidents and possessed a

current FAA first-class medical certificate with the

requirement to have glasses available for near vision.

The first officer, a 38-year old with about 6,800 total

flight hours including 2,200+ flight hours in the

MD-88, had a current FAA first-class medical certificate with no visual restrictions. The captain had

flown the approach into LaGuardia several times

before, always under visual meteorological conditions. The pilot reportedly used his progressive addition lenses about 25% of the time and his monovision

contact lenses about 75% of the time while flying.

According to his eye doctor, the pilot was issued

contact lenses in 1990 with his dominant right eye set

for distance vision and the left eye set for near vision.

The right contact lens corrected distant vision to 20/

20 and the near vision to 20/60, while the left contact

lens corrected distant vision to 20/80 and near vision

to 20/20. The captain was wearing monovision contact

lenses at the time of the accident. The prescribing eye

doctor was unaware the patient was a pilot and that

monovision contact lens use was prohibited by the

FAA. Additionally, he could not remember discussing

monovision limitations with this patient and had not

performed a depth perception test. A post-accident

evaluation conducted at Emory University revealed

that the pilot had a substantial reduction in depth

perception when using his monovision contact lenses.

According to the NTSB report, several factors may

have contributed to the Delta 554 accident. The

lights on Runway 13 were spaced 150 feet apart,

while the lights on most runways are 200 feet apart.

To pilots accustomed to 200-foot spacing, this difference could result in an overestimation of their

altitude by 70 feet. Runway 13 had an infrequently

used ILS approach that was made more difficult by

the ILS localizer being offset to the right of the

runway centerline by several degrees of arc. The ILS

approach required the pilot to turn left slightly after

transitioning to visual references. In an approach

over water under reduced lighting conditions, there
4

are fewer visual cues for judging altitude than would

be found in detailed terrain. The absence of visible

ground features, the presence of rain and fog and the

irregular spacing of the runway lights may have

combined to create an optical illusion that caused the

pilot to believe he was higher and farther away from

the runway. Additionally, since the aircraft was not

equipped with an instantaneous vertical speed indicator (VSI), the lag time of the VSI limited the

usefulness of the information it provided during the

final critical seconds of the approach.

DISCUSSION

Depth perception is enhanced when viewed binocularly. The sense of depth mediated by binocular

vision is referred to as stereopsis, which is the disparity between images from each eye that is used to judge

angular difference and interpreted for depth perception. Stereopsis differs from monocular depth perception cues in that it is physiological rather than

learned. Therefore, stereopsis provides a more convincing stimulus for judging depth than does monocular cues. From the stereoacuity measurement

(arc min), the threshold of stereopsis (maximum

distance at which stereopsis is useful) may be calculated. Some scientists have measured the threshold of

stereopsis at over 300 feet, which is within the range

of many flight tasks, including aerial refueling, formation flight, and low altitude maneuvers (9). Other

vision specialists say that stereopsis is useless beyond

20-30 feet. However, depth perception is not exclusively dependent on stereopsis. Monocular depth

perception cues include linear perspective, interposition (an overlaid object appears farther away), clarity,

and motion parallax. Since these cues are learned,

they are more susceptible to fictitious information

from visual illusions.

In the Delta 554 accident, visual cues were obscured by the rain and fog, creating a visually deprived environment that could have induced visual

illusions. For example, the dim (low contrast) lights

on the runway could produce an illusion of being

farther away and higher. The “black hole” illusion,

which results when a pilot has little peripheral detail,

can also generate an illusion of being too high. The

pilot’s diminished distant vision and stereopsis may

have exaggerated these illusions. Additionally, the

offset or decentered approach to Runway 13 complicated the landing attempt, since it would have required

a precise readjustment by the pilot close to the runway. The lack of monocular cues may have reduced

the time the pilot had to react.

Contact lenses have limitations in the aviation

environment. Corneal edema has been reported in

well-fit contact lens wearers when exposed to altitude

hypoxia. Low barometric pressure and low relative

humidity are indigenous to the aviation environment. During decompression, nitrogen gas may form

bubbles beneath a contact lens affecting vision. Low

humidity, such as the 10-15% relative humidity of an

aircraft, can dehydrate hydrophilic (soft) contact

lenses, reduce lens movement, and increase conjunctival injection. Lens dehydration has been associated

with visual performance (low-contrast acuity) loss

(4,10). In a survey of 31,205 aircrew members of the

Active, Reserve, and Air National Guard Air Force

(pilots, navigators, and flight surgeons), 26% of

those who wore contact lenses experienced displacement and 10% experienced a loss of contact lenses inflight (11). This may be due to lens dehydration,

abrupt head and eye movements, and aggressive flight

maneuvers. About 24% of the aircrew reported removing a contact lens inflight, primarily for dryness

or a foreign body beneath the lens (11). It is important to note that about 98% of the military aircrew

members who wore contact lenses felt that their use

afforded them an operational advantage. A study

conducted by the U.S. Army found that presbyopic

helicopter pilots preferred bifocal contact lenses to

bifocal spectacles in actual flight operations. Of particular note was that over half (10 of 17) of these

aviators preferred the monovision or modified

monovision (a distance vision lens on one eye and a

bifocal contact lens on the other eye) contact lens

design as their corrective modality of choice (12).

Normal anatomical and physiological changes with

aging can affect the wearer’s visual performance when

using contact lenses. These changes include: flaccid

eyelids, reduced tears, diminished corneal sensitivity,

senile miosis, and loss of accommodation and lens

transparency. Contact lens use by women may also be

adversely affected by changes in hormone levels from

taking oral contraceptives or from menopause (13).

Additionally, individuals with compromised near

vision may have problems properly cleaning and

caring for their contact lenses. A soiled contact lens

can result in a water-repellant (hydrophobic) lens

surface, changing the fit and physical properties (water content, oxygen transmissibility) of the lens.
5

Furthermore, as people grow older, they are more

prone to use prescription medications that can alter

tear production and quality, and disrupt contact lens

wear. These medications include: anticholinergics,

antihistamines, antihypertensives, dermatological

medications, and antidepressants (14).

The Food & Drug Administration has recognized

monovision as an acceptable use for contact lenses.

The use of monovision has been a frequently prescribed modality to correct presbyopia for 30+ years

with no reported ill effects. Additionally, no scientific data demonstrate a higher incidence of motor

vehicle accidents with monovision contact lenses,

compared with traditional ophthalmic lens correction for presbyopia. It is interesting to note that the

pilot of Delta 554 had used monovision contact

lenses successfully for about six years while flying

with an exemplary record. During this period, the

pilot also appears to have passed check flight tests

performed by the FAA and Delta Air Lines while

wearing monovision correction. Therefore, this may

suggest that external factors, such as the marginal

visual conditions, lack of monocular cues, irregular

approach configuration, and the lag in the vertical

speed indicator overwhelmed the pilot’s visual capabilities, and could have affected even a pilot with

normal uncorrected or optimally corrected vision.

Further research is required to explore this possibility.

Eye care practitioners should always ask patients

about their work and recreational activities when

dispensing monovision contact lenses. Although they

may not be legally responsible for knowing the vision

standards and regulations for their patients’ occupations or avocations, such information would be invaluable to the doctor when consulting with the

patient on the limitations of monovision as a corrective modality while performing certain activities.

Patients with challenging occupational and

avocational distant vision demands may not be appropriate candidates for monovision lenses. If

monovision contact lenses are prescribed, ancillary

vision tests, such as depth perception, should be

performed to quantify any reduction in visual performance. To ensure the safety of the operator and the

general public, eye care practitioners may wish to

suggest that another qualified individual accompany

the patient after prescribing changes in strength or

type of refractive correction to aid in the adjustment

to any potentially adverse visual perception changes.

Eye care practitioners should consider the following before dispensing monovision contact lenses to

any patient:

1. Monovision can reduce second degree fusion in

10-20% of patients. Clinically measured near

stereoacuity (fine depth perception) can be expected to diminish 40-50 arc sec (15). Monovision

may be more problematic when the anisometropia (one eye requiring a different lens correction

than the other) reaches 1.25D (16). Although

monovision contact lenses may not cause a visual

illusion, it may compromise the quality of the

overall visual scene and binocular function.

2. Stereopsis is more sensitive to monocular blur

than to similar amounts of binocular blur. Additionally, residual astigmatic error, particularly in

the dominant distance-corrected eye at oblique

axis, causes significantly greater binocular visual

acuity loss in monovision (15).

3. Monovision contact lenses produce a slight reduction in high contrast visual acuity, compared

with traditional binocular distance and near correction (15).

4. Deficits of visual acuity and contrast sensitivity

from monovision at threshold conditions may

reduce target identification.

5. Monovision may result in both distance and near

“ghosting,” which is due to incomplete suppression of interocular blur (15). The ability to suppress the blur may improve with adaptation, but

equivalent performance levels to normal binocular vision may never be achieved.

6. Monovision patients may experience hazy vision

and occasional loss of balance during the adaptation period (15).

7. Monovision has been shown to decrease occupational task performance speed by 3 to 4%, compared with distance contact lenses combined with

reading glasses (17).

In conclusion, under current FAA regulations,

pilots are prohibited from using monovision contact

lenses to correct refractive error and/or presbyopia.

Although a pilot is well adapted to monovision, the

reduction in visual performance from using such

lenses may be exacerbated under marginal visual

conditions and high workloads. The limitations of

monovision contact lenses, including the voluntary
6

relinquishing of binocular cues to depth perception,

should be carefully discussed with the patient, especially if he/she operates an aircraft. In the case of the

Delta 554 accident, the NTSB concluded that the

inability of the pilot to overcome his misperception

of the airplane’s position due to degraded stereopsis

and increased reliance on monocular cues, was a

contributing factor. In a related issue, the use of

refractive surgery or intraocular lenses after cataract

surgery, with intentional unilateral undercorrection

for best uncorrected far and near visual acuity, could

result in similar visual performance decrements while

flying. Further research is recommended, perhaps in

a full-motion flight simulator and under a variety of

environmental conditions, with pilots wearing

monovision contact lenses. This research could provide additional understanding of the limitations of

monovision correction in an operational aviation

environment and how those limitations could influence flight performance under adverse conditions.

Additionally, the research may be used to review the

current policy regarding the use of monovision refractive correction.

REFERENCES

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4. Moore RJ, Green RP Jr. A survey of U.S. Air

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Delta Air Lines flight 554, McDonnell Douglas

MD-88, N914DL, LaGuardia Airport, New York,

October 19, 1996. NTSB/AAR-97/03. 1997.

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