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 probablecause of the MD-88 accident that occurred at LaGuardia October, 1996, bythe NTSB. Of course, the NTSB also casually mentioned that pilot’s visionwas “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 hadmade hundreds of normal landings up to that time.

Monovision contact lenses is the use of a lens in one eye for distant visionand 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 inuse, the opposite eye correction is not up to visual acuity standards. Forinstance, 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 standardsfor far vision at that time.

Of course, the NTSB and FAA’s concern ignores the fact that hundreds of oneeyed pilots have operated for years without difficulty and that depth perceptionbased on stereoscopic vision is only good to about 50 feet. Monovision pilotshave no vision in one eye.

Dr. Robert Liddell([email protected]),past Director of Aviation Medicine in Australia, stated “I am astounded thatthe (US) aviation community let’s the NTSB get away with some of their commentsand flawed conclusions….Blaming monovision for the MD-88 accident immediatelyimplies that all monocular pilots can no longer be expected to operatesafely….Equally it negates the hundred of successful landings under allsorts of conditions….”

Unfortunately, the NTSB tends to blame accidents on any misdemeanor possible.In this situation, they have recommended to the OKC FAA Civil AeromedicalInstitute the publication of “the hazards of monovision contact lenses andthat the Application for airman Medical Certification (FAA Form 8500-8) berevised to elicit contact lens usage information….”

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

What’s a pilot to do? I personally use bifocals but I think that distantvision contacts combined with near vision glasses probably offer the bestcompromise

The best solution is make sure you do not have an accident and the issuewill never arise!

About the author…
Brent Blue, M.D. ([email protected]) 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 havea certificate of ability from the FAA) and some may have “operated withoutdifficulties for years”, there are concerns about their abilities to operatein decreased visibility environments (IFR landings at weather mins). TheUS Navy did research on this (Aerospace Medicine, Sep 69). While therewere factors in this particular mishap which could have caused visual illusuions,these illusions were greatly exacerbated by the pilot’s substandard distancevision and lack of stereopsis.

In your brief discussion on AVweb, it is clear that there is someconfusion about monocular cues and true stereopsis (depth perception). Truestereopsis requires 20/20 or better vision in both eyes (in the same focalplane, distance and/or near) and for the visual axis of each eye to be directedat the object/scene of interest. It is the disparity between the images thateach eye “sees” that allows the brain to judge the angular difference andinterprate these combined images as depth perception or stereoscopic image.Unlike what you quoted in your article, stereopscopic vision in flight isuseful for much further than 50 feet (approximately 1,000 feet); but it isthis loss the sense of closure rates and the space of manuvering which isnearly instantanously used in situations where monocular cues will fail youwhich should be your concern as AMEs and representatives of the medical communityin Aviation. [The 50-foot figure came from the FAA’s Civil AeromedicalInstitute (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 byvisual illusions. While individuals who have long-standing monocularity havean ability to adapt to their permanent situation, individuals who give upstereopsis on an intermittent basis, like wearing monovision lenses, areat increased risk in environments which require all the “right stuff” tomake approprate decisions in a timely manner to prevent mishaps.

In the future, please consider what science could be driving safety decisionsand at least consider talking to a vision expert prior to expresing an opinionin an open forum among individuals who might consider you an expert. Notall decisions by the NTSB are purely political ones. The NTSB utilized theAeromedical Consultation Service, Ophthalmology Branch (largely consideredthe world expert opinion in aviation ophthalmology) as consultants on thismishap.

Bryan Angle, M.D. ([email protected])


Yet Another View from an Opthalmologist/AME…

The NTSB stance on monovision seems somewhat odd, basing their premise onthe use of stereoscopic vision for approach and landing procedures. Whilestereopsis does need good clear vision in both eyes for the distance at whichthe task is done, most authorities within the vision sciences area will admitthat stereopsis is not the vision ability that is used for approach and landingtasks.

Another visual ability or skill called “Retinal Image Flow” is used to ascertainwhere one’s direction of movement is in a three dimensional world. Retinalimage flow relies on the relative angular movement of objects across theretina, with those objects further to the side from the centre of directionhaving 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 about50 km/h (30 mph).

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

Neil Murray ([email protected]), B.Optom.(Hons.), F.A.A.O.
Optometrist, Designated Aviation Medical Examiner (Ophthalmology)
for Civil Aviation Safety Authority in Australia.

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