by Dr. Bill Blank, MD
© Copyright 2022. All rights reserved!
Published In Midwest Flyer Magazine February/March 2022 Online Issue
Our eyes are usually about the same: both nearsighted, far sighted or neither. Having one near sighted and the other far sighted is quite uncommon. Both eyes being similar simplifies fitting glasses when needed. Both eyes being focused simultaneously on the same object permits us to have depth perception or stereopsis and binocular vision.
Because our eyes are about 2 inches apart, each eye sees the same thing from a different angle. To see things from very close to about 20 feet requires each eye to look in. This is called “convergence.” The closer the object we are looking at, the more convergence required. Closer objects require more focusing effort to be seen clearly. Our brain automatically and quickly processes the amount of focusing effort and convergence used to give us a very accurate estimate of how far away something is. This is the basis of our depth perception and 3D vision.
Our brain automatically merges the two slightly different images into one in-depth 3D image. Binocular depth perception functions up to about 60 feet but is much more accurate closer. Beyond that we judge depth based on visual clues such as relative size and our knowledge of the size of common objects such as people, trees, cars, etc. People with excellent depth perception can tell the difference in the distance of objects a few inches apart out to about 20 feet.
As we age, our ability to see near decreases. People who never needed glasses for anything start needing reading glasses. Nearsighted people discover they need to take their distance glasses off to read. Previously, that was unnecessary. This is a nuisance. Contact lens wearers looked for an alternative. Fitting one eye with a contact lens for distance and the other with a contact lens for near is sometimes done. This means that the eye with the distant contact lens cannot see up close and the other eye cannot focus things clearly far away.
Some people only need a near contact lens in one eye and nothing in the other eye. This is what is called “monovision.” Some people tolerate this well. People who have occupational needs for excellent vision usually don’t. I would have never been able to do eye surgery that way. One tradeoff is that depth perception is greatly diminished because both eyes are focused on different distances. So far, I have only covered monovision from contact lenses. The same thing is frequently accomplished with refractive surgery and cataract surgery.
A Delta Airlines MD-88 landing mishap at LaGuardia Airport in New York in October 1996 (NTSB Accident Report AAR 97-03) caused the FAA to become interested in monovision. The captain was wearing one contact lens for distance and the other for near vision. U.S. Air Force (USAF) studies demonstrated poorer landing performance when pilots were wearing monovision contact lenses. As a result, the FAA banned monovision for all classes of medical certification and added question 17b to the 8500-8 form which all applicants for an FAA medical certificate are required to answer. It is poorly worded but is asking if an airman wears a contact lens in one eye which is only for near vision while flying. If the airman answers yes, the Aviation Medical Examiner (AME) is supposed to tell the applicant he may not do that while flying. Pilots requiring correction for distance vision must wear prescription glasses or non-mono vision contact lenses while flying.
I decided to write about this topic after completing a flight physical on an airman who wears monovision contact lenses and was not aware of this limitation. This limitation does not apply to people flying under “Basic Med.”
People drive their cars all the time with monovision, although depending on your state, they may need a waiver to do so.
They can generally meet the minimum vison standards for vision in the poorer eye (the eye corrected for near vision). However, federally licensed commercial drivers are prohibited from using monovision correction. Some studies indicate that it is harder to drive at night using monovision.
Does the FAA ever approve flying with monovision? They do certify someone who has lost one eye. That gives us a clue. A 6-month period to adapt is required in both cases. Then a Medical Flight Test (MFT) must be passed. If successful, a Statement of Demonstrated Ability (SODA) will be issued. A SODA has an unlimited duration unless the condition changes. So, you can fly with monovision under certain circumstances. There is a procedure to get approval.
Happy flying!
EDITOR’S NOTE: Columnist William A. Blank is a physician in La Crosse, Wisconsin, and has been an Aviation Medical Examiner (AME) since 1978, and a Senior AME since 1985. Dr. Blank is a retired Ophthalmologist, but still gives some of the ophthalmology lectures at AME renewal seminars. Flying-wise, Dr. Blank holds an Airline Transport Pilot Certificate and has 5600 hours. He is a Certified Instrument Flight Instructor (CFII) and has given over 1200 hours of aerobatic instruction. In addition, Dr. Blank was an airshow performer through the 2014 season and has held a Statement of Aerobatic Competency (SAC) since 1987.
DISCLAIMER: The information contained in this column is the expressed opinion of the author only, and readers are advised to seek the advice of others and refer to the Federal Aviation Regulations and FAA Aeronautical Information Manual for additional information and clarification.
