The Controversy Over "Monovision" Lenses

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NOTE: AVweb includes the full text of the NTSB's probable cause report on Flight 554 in Adobe's PDF format (if you don't already have one, PDF readers are available from http://www.adobe.com).

"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 lense, 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.

Dr. Robert Liddell (rob_liddell@msn.com), 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!

About the author...
Brent Blue, M.D. (BrentBlue@Dial-A-Doc.com) is a Senior Aviation Medical Examiner based in Jackson Hole, Wyoming, medical advisor to the U.S. Aerobatic Team, and AVweb's chief aeromedical consultant. Brent is an ATP and owns a Cessna 340.


A Physician's Opposing View...

Dr. Blue/Dr. Liddell,

Sirs, I'm sure that you have a great deal of expertise in aviation medicine, but I feel that you have missed the point of the NTSB's report on this mishap. While there are multiple truely monocular pilots (all of which must have a certificate of ability from the FAA) and some may have "operated without difficulties for years", there are concerns about their abilities to operate in decreased visibility environments (IFR landings at weather mins). The US Navy did research on this (Aerospace Medicine, Sep 69). While there were factors in this particular mishap which could have caused visual illusuions, these illusions were greatly exacerbated by the pilot's substandard distance vision and lack of stereopsis.

In your brief discussion on AVweb, it is clear that there is some confusion about monocular cues and true stereopsis (depth perception). True stereopsis requires 20/20 or better vision in both eyes (in the same focal plane, distance and/or near) and for the visual axis of each eye to be directed at the object/scene of interest. It is the disparity between the images that each eye "sees" that allows the brain to judge the angular difference and interprate these combined images as depth perception or stereoscopic image. Unlike what you quoted in your article, stereopscopic vision in flight is useful for much further than 50 feet (approximately 1,000 feet); but it is this loss the sense of closure rates and the space of manuvering which is nearly instantanously used in situations where monocular cues will fail you which should be your concern as AMEs and representatives of the medical community in Aviation. [The 50-foot figure came from the FAA's Civil Aeromedical Institute (CAMI) in Oklahoma City. --ed.]

Monocular cues are a learned phenomenon, which life's experiences teach us (i.e. object which are closer, are bigger), but which are easily fooled by visual illusions. While individuals who have long-standing monocularity have an ability to adapt to their permanent situation, individuals who give up stereopsis on an intermittent basis, like wearing monovision lenses, are at increased risk in environments which require all the "right stuff" to make approprate decisions in a timely manner to prevent mishaps.

In the future, please consider what science could be driving safety decisions and at least consider talking to a vision expert prior to expresing an opinion in an open forum among individuals who might consider you an expert. Not all decisions by the NTSB are purely political ones. The NTSB utilized the Aeromedical Consultation Service, Ophthalmology Branch (largely considered the world expert opinion in aviation ophthalmology) as consultants on this mishap.

Bryan Angle, M.D. (angle@alaoc.brooks.af.mil)


Yet Another View from an Opthalmologist/AME...

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.

Neil Murray (nbmurray@ozemail.com.au), B.Optom.(Hons.), F.A.A.O.
Optometrist, Designated Aviation Medical Examiner (Ophthalmology)
for Civil Aviation Safety Authority in Australia.