January 29, 1998 The Controversy Over "Monovision" Lenses |
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January 29, 1998
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.
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