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Guest Editorial:

 THE CRASH OF DELTA FLIGHT 554:

 Was Monovision Contact Lens use the Cause?

By Robert Baron (robert@aviationcrm.com)

 

THE ACCIDENT

October 19,1996. 1638 EDT: Delta Airlines Flight 554, a McDonnell Douglas MD-88, descended below the visual glidepath and collided with terrain on approach to New York's LaGuardia Airport. There were 3 minor injuries and 60 uninjured. In its official investigation of the crash, the NTSB (National Transportation Safety Board) reached the following conclusions as to the probable cause:

The airplane had struck the approach light structure and the end of the runway deck during the approach. Because of the captain's use of monovision contact lenses, he was unable to overcome the visual illusions resulting from the approach over water in limited light conditions (absence of visible ground features), the irregular spacing of the runway edge lights at shorter-than-usual intervals, the rain, and the fog, and that these illusions led the captain to perceive that the airplane was higher than it was during the visual portion of the approach, and thus, to his unnecessarily steepening the approach during the final 10 seconds before impact. Aviation medical examiners (AMEs) need to know if pilot examinees are using contact lenses, and currently no process is in place to ensure that AMEs are provided with that information. The lag time in the display of vertical speed information in the vertical speed indicator installed in the accident airplane limited the first officer's ability to provide the captain with precise vertical speed information during the critical final seconds of the approach, and therefore contributed to the accident.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows.

The inability of the captain, because of his use of monovision contact lenses, to overcome his misperception of the airplane's position relative to the runway during the visual portion of the approach. This misperception occurred because of visual illusions produced by the approach over water in limited light conditions, the absence of visible ground features, the rain and fog, and the irregular spacing of the runway lights. Contributing to the accident was the lack of instantaneous vertical speed information available to the pilot not flying, and the incomplete guidance available to optometrists, aviation medical examiners, and pilots regarding the prescription of unapproved monovision contact lenses for use by pilots. (NTSB Report AAR-97/03)[1]

This accident falls into a very distinct realm. There are very few, if any, documented aviation accidents where the root cause was attributed to the use of monovision contact lenses by a pilot. The use of monovision contact lenses is not approved by the FAA (Federal Aviation Administration); therefore the pilot of this flight should not have been using them to begin with. The result was not necessarily an attempt to violate regulations, but more of a “checks and balances” problem within the aeromedical/optometry field.

AGE AND VISION

As we age, our visual acuity will inevitably become worse. There are numerous factors that affect the speed and severity of this process. Some of these factors may include a genetic pre-disposition, an extended exposure to a hostile environment (ie: working behind a CRT for many years), or medical problems such as diabetes.

For simplicity, we will concentrate on near and distant vision problems as a result of age. For most people, but certainly there are many exceptions, we start out in life with relatively good vision. Relatively good implies that our visual acuity is 20/20. As we age, the first noticeable difference generally occurs with distant vision. It becomes harder to focus on objects in the distance. However, our near-vision acuity remains intact. This is known as Myopia, or near-sightedness. The opposite of this would be Hyperopia, or far-sightedness, where distant objects would remain focused, but near objects become blurry. We compensate for these deficiencies by using contact lenses, glasses, or perhaps laser surgery. As we continue to age, it becomes increasingly more difficult to focus on near objects, as well as distant objects. This begins to occur at around 40 years of age. At this point, a correction needs to be made for both near and distant vision. You may be familiar with bifocals. Bifocals provide this double correction, however some people do not like the unsightly “lines” on their glasses that can be a telltale giveaway of a person's vision problems.

There is one other age-related vision problem worth mentioning at this point. It is related to, but differs from Myopia and Hyperopia. It is called Presbyopia. Presbyopia is the decreasing ability to accommodate as our eyes age. Upon reaching the mid-forties, the average person can no longer accommodate sufficiently to bring very close objects into focus. Reduced accommodation arises from various sources, including sclerosis of the lens and reduced elasticity of the lens's capsule (Koretz and Handelman, 1988). An old lens is very sluggish in executing even the small shape changes of which it is still capable. This lengthens the time required to change gaze from near to distant objects, and vice versa, causing potential problems in driving and similar tasks.[2]

MONOVISION CONTACT LENSES

One solution to correct near and far visual acuity deficiencies can be found by using one contact lens for distant vision and the other contact lens for near vision. This is called Monovision. Compared to bifocal glasses, the results are much more satisfactory for patients as can be seen from the pictures below. 

        

            Field of vision with bifocals                     Field of vision with Monovision

 

Obviously, if you had a choice between the picture on the left and the picture on the right, you would choose the latter. Unfortunately, if you are a pilot, the FAA does not allow the use of monovision contact lenses for flight. This is where the problem begins. Apparently, there is a lack of dissemination of this fact through the aeromedical world. At this time, pilots are not required to let an Aviation Medical Examiner know they are using monovision lenses when they get their medicals. Additionally, many AME's do not query the pilots for this information. This is where a potential gap can be created. However, the shift will be towards education on both sides as well as divulgence of this critical information in the future.

WHAT WENT WRONG?

Now that you are aware of the visual acuity problems as we age and how we can combat the problem with monovision lenses, let's take a look at what may have gone wrong in the cockpit of Delta Flight 554 during landing at LaGuardia Airport. In its official findings, the National Transportation Safety Board, amongst other findings, found the following as contributory to the accident:

o The captain gradually reduced the engine power because he perceived a need to slightly increase the airplane's rate of descent; however, the descent rate increased beyond what the captain likely intended to command.

o Irregular and shortened runway edge light spacing and degraded weather conditions can result in a pilot making an unnecessarily rapid descent and possibly descending too soon, especially in the absence of other visual references or cues.

o The captain's use of monovision contact lenses resulted in his (unrecognized) degraded depth perception, and thus increased his dependence on monocular cues (instead of normal three dimensional vision) to perceive distance.

o Because of the captains' use of monovision contact lenses, he was unable to overcome the visual illusions resulting from the approach over water in limited light conditions (absence of visible ground features), the irregular spacing of the runway edge lights at shorter-than-normal intervals, the rain, and the fog, and that these illusions led the captain to perceive that the airplane was higher than it was during the visual portion of the approach, and thus, to his unnecessarily steepening the approach during the final 10 seconds before impact.[3]

 picture of runway 13 at LaGuardia as would be seen from the cockpit on a day with good weather. Note the path over water, which in itself can make this a tricky landing, particularly in bad weather or at night. Visual illusions may occur.

Another picture of runway 13 at LaGuardia. Although this picture is very grainy, you can still discern the complexity of an approach over water. Height judgment is the biggest problem. The Delta jet crashed into the approach light array at the beginning of the runway.

 

LEADING EXPERTS DISAGREE WITH THE NTSB

After the NTSB posted their findings, a few leading experts in the field countered with dissenting points of view. These opinions date back to an AvWeb online article from 1998:

Brent Blue, M.D., a Senior Aviation Medical Examiner based in Jackson Hole, Wyoming, and medical advisor to the U.S. Aerobatic Team says "Monovision" contact lenses, actually a misnomer, was listed as a probable cause of the MD-88 accident that occurred at LaGuardia October, 1996, by the NTSB. Of course, the NTSB also casually mentioned that pilot's vision was "also" affected by "visual illusions caused by the light conditions, irregular spacing of runway lights, rain, and fog." With all that going on, I am not sure how they focused on the pilots contact lenses since he had made hundreds of normal landings up to that time.”

“Monovision contact lenses is the use of a lens in one eye for distant vision and the use of a near vision contact in the other eye. Under FAA regulations, it is illegal for a pilot to use monovision contacts only because when in use, the opposite eye correction is not up to visual acuity standards. For instance, if one eye is 20/100 for distant vision and 20/70 for near vision, and the eye is corrected with a near vision lens, it will not meet standards for far vision at that time.”

“Of course, the NTSB and FAA's concern ignores the fact that hundreds of one eyed pilots have operated for years without difficulty and that depth perception based on stereoscopic vision is only good to about 50 feet. Monovision pilots have no vision in one eye.”[4]

NOTE: In a follow-up interview I conducted in October of 2002 with Dr. Blue in reference to a change of opinion, if any, to the use of monovision lenses, I quote the following:

“If anything, I am even more convinced that the monovision lenses had nothing to do with the Delta crash. It was just an easy excuse for the FAA and they will not back down.”

“Since the FAA now approves monovision with LASIK after a six-month break in period, their attribution concerning near vision contact lenses rings even more hollow.”

Dr. Neil Murray, another Aviation Medical Examiner and Ophthalmologist for the Civil Aviation Safety Authority in Australia says "The NTSB stance on monovision seems somewhat odd, basing their premise on the use of stereoscopic vision for approach and landing procedures. While stereopsis does need good clear vision in both eyes for the distance at which the task is done, most authorities within the vision sciences area will admit that stereopsis is not the vision ability that is used for approach and landing tasks."

"Another visual ability or skill called "Retinal Image Flow" is used to ascertain where one's direction of movement is in a three dimensional world. Retinal image flow relies on the relative angular movement of objects across the retina, with those objects further to the side from the centre of direction having a faster angular rate of movement."

"Retinal Image Flow is used for higher speed tasks such as occur in aviation, whereas stereopsis is thought not to provide adequate information above about 50 km/h (30 mph)."

"Thus the NTSB comment on reduced stereopsis from Monovision seems based on incorrect and irrelevant information."

NOTE: In a follow-up interview I conducted in October of 2002 with Dr. Murray in reference to a change of opinion, if any, to the use of monovision lenses, I quote the following:

"In the AvWeb article, there was one paragraph that I had submitted that was left out, and does qualify the conditions and pilots for whom monovision may be considered."

"As you would understand with monovision, it is normally the "non-Dominant" eye which has the reading correction placed on it, so effectively rendering it myopic (Short-sighted). The sentence or two which was left out of the AvWeb article was that this "near-corrected" eye does also still need to meet the relevant visual standard for the required class of licence. This means that the monovisioned pilot is no poorer than a binocular pilot who has slightly reduced vision in one eye, but has both eyes still passing the standard."

"With presbyopia and the onset of near focus problems, the strength of near correction will increase with time, so that the correct near-focus contact lens will gradually blur the distance vision more and more as the prescription is changed over the 43-55 age group."

"I would normally expect that with the loss of accommodation (close focus ability) that happens with age, I would anticipate that a pilot (Caucasian, as there are racial changes in the age at which presbyopia occurs) would still be able to pass the required Class 1 standard with monovision until about the age of 48-49, but beyond that the reading prescription required would cause too much distance blur for them to continue to pass the standard, and at this stage I would not only advise strongly against monovision, but would fail any applicants who presented to me for our CASA (your FAA) examinations."

"So, the situation with suitability with monovision being acceptable for flying (commercial or private) is really a temporary one, until the strength of the near reading correction is such that the pilot cannot pass the relevant standard required of the second eye for their licencing (either 20/30 or 20/40 depending on licence held)."

"I hope that this clears up the situation a bit better for you, but please feel free to contact me if you should wish any further details or comments. The mention I made of different ages of the onset of presbyopia depending on race (and relevant nutrition) was made very evident in 2000 when I was Director of Eye Services for the Sydney 2000 Olympic and Paralympic Games, at which we were looking after everyone in the Olympic Village - athletes, coaches, team officials, etc. We were amazed at the different ages that presbyopia was showing up across the different nationalities. I had some idea of what we were going to see though, as I had worked at the 1996 Atlanta Olympics for 3 weeks, and so ended up treating a number of patients I had seen at Atlanta, again in Sydney."

“Dr. Robert Liddell, past Director of Aviation Medicine in Australia, stated "I am astounded that the (US) aviation community let's the NTSB get away with some of their comments and flawed conclusions.... Blaming monovision for the MD-88 accident immediately implies that all monocular pilots can no longer be expected to operate safely.... Equally it negates the hundred of successful landings under all sorts of conditions...."

“Unfortunately, the NTSB tends to blame accidents on any misdemeanor possible. In this situation, they have recommended to the OKC FAA Civil Aeromedical Institute the publication of "the hazards of monovision contact lenses and that the Application for airman Medical Certification (FAA Form 8500-8) be revised to elicit contact lens usage information...."

“The 8500 form will take years to change even if the NTSB gets it way, but Jon Jordan, M.D., the Federal Air Surgeon, has already urged AMEs to remind pilots that monovision contacts are not kosher in his recent newsletter.”

“What's a pilot to do? I personally use bifocals but I think that distant vision contacts combined with near vision glasses probably offer the best compromise.”

“The best solution is make sure you do not have an accident and the issue will never arise!”[4]

IN YOUR OPINION?

This article served two purposes: To educate pilots to the fact that monovision lenses are NOT approved for flying. And, according to the NTSB, for good reason. The second purpose was to elicit contrasting points of view in order to let you come up with your own opinion. Was the crash of Delta Flight 554 due to the use of monovision lenses by the captain, or could that crash have occurred by a pilot with perfect vision but subjected to the same visual illusions during landing?

REFERENCES

[1] National Transportation Safety Board Website (2002). www.ntsb.gov. Aircraft Accident Report. File # NYC97MA005. Washington, DC.

[2] Sekular, Robert, and Blake, Randolph (1994) Perception: Third Edition. Presbyopia. pp. 51-52

[3] National Transportation Safety Board (1997). Descent Below Visual Glidepath and Collision With Terrain. Executive Summary Report on the Crash of Delta Airlines Flight 554.

[4] AvWeb Online Web Article (1998) Various Contributors. www.avweb.com/articles/monovisi.html. The Controversy Over "Monovision" Lenses.

CREDITS

Picture of Delta 554 at top of article: Courtesy Air Safety Online

Field of Vision Pictures: Courtesy WebMD

LaGuardia Airport Runway 13 Pictures: Courtesy David Byrd

Cockpit Voice Recording Transcript: Courtesy Aviation Safety Network

CVR (COCKPIT VOICE RECORDING TRANSCRIPT)

Flight 554 CVR (Cockpit Voice Recording Transcript)

October, 2002


Robert Baron is the president and chief consultant of The Aviation Consulting Group, an aviation consulting firm with a core specialization in CRM/Human Factors training and research, and expert witness support for aviation law firms. He holds an Airline Transport Pilot Rating and has over 16 years of aviation experience, including a Line Captain, Instructor and Check Airman in Learjet aircraft. He's also type-rated in the Cessna Citation and holds a Flight Engineer Rating for Turbojet aircraft. His academic achievements include a Bachelor's Degree in Professional Aeronautics/Aviation Safety, a Master's Degree in Aeronautical Science with dual specializations in Aviation Safety/Human Factors, and is currently working towards a PhD in General Psychology with an emphasis on Aviation/Aerospace Psychology. Mr. Baron is also an adjunct professor at Everglades University, where he teaches Graduate and Undergraduate courses in Aviation Safety and Human Factors.  Mr. Baron can be reached at 1-954-803-5807. Company website is http://www.tacgworldwide.com.


Robert J. Boser    
Editor-in-Chief 
AirlineSafety.Com

 

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