Near Vision Contact Lens(es) and the Flight Environment
Number 17.b. is one of the more confusing items on the MedXpress form pilots must fill out for the FAA medical certification application.
You might ask what is the FAA getting at with this question? Pilots are allowed correction of their vision for both near and far visual abnormalities, right? Those pilots who use glasses are certainly aware of the restrictions placed on their medical certificate - "Must wear corrective lenses".
In general, near vision contact lenses are used to correct for hyperopia (farsighted vision) as well as aid the vision for people who have lost the ability to accommodate (near visual focus of the eye's lens). As we get older, our connective tissues become dehydrated making them less flexible and more brittle. This can lead to injury and/or decreased functionality. How many middle aged or older people do you know who have suffered a rotator cuff tear? In that regard, the loss of hydration of the lens causes a loss of flexibility and thus accommodation begins to fade typically in someone's mid to late 40's and can progress to full-blown presbyopia (poor near visual acuity) in their 50's or earlier.
In a patient who is historically near sighted or myopic (can't see clearly far away), the loss of accommodation presents the need for both near and distant vision correction. This commonly results in the use of bifocal spectacles or possibly multi-focal contact lenses. I have seen many professional pilots in this category who function without issue using bifocal spectacles.
This scenario becomes significant for the FAA medical certification exam when eye-care specialists offer a pilot the option to use monovision or multi-focal contact lenses. Monovision contact lenses utilize a near vision contact lens for one eye, while using a distant vision contact lens for the opposite eye. Multi-focal contact lenses use several different designs to divide the visual field into different focal areas. One portion would be in near focus while the other would be in distant focus. Division of the visual field using monovision or multi-focal contact lenses degrades the visual field and is not desirable for flight safety.
Regarding monovision contact lenses, as one can imagine the different focus of the lenses would initially create a visual conflict: one eye would see distant objects clearly, while the other would be blurry and vice-versa for near objects. This conflict would continue until the pilot's brain adjusts to the difference in their visual acuity and learns to pay attention only to the eye with the clear image. But even when the brain adjusts, there are several effects that are significant and negatively impact the pilot's ability to operate in the flight environment. This is what lies at the heart of question 17.b. on the MedXpress form.
First, the pilot would not be able to meet the FAA visual acuity requirements for distant vision in the eye using the near vision contact lens. Equally, they would not be able to pass the requirements for near vision with the eye using the distant vision contact lens. The pilot would essentially be a monocular pilot, meaning they only have one good eye for interpreting their now limited distant or near visual fields. As one can imagine, pertinent flight tasks such as performing the visual scan for distant aircraft would be limited to what only one eye could see.
Second, the pilots ability to notice moving objects within their peripheral visual fields would be compromised. Our peripheral visual system is tuned to notice moving objects which helps us to react to the surrounding environment. Think of the hiker in the woods who catches a glimpse of a bear moving out the corner of their eye. In the flight environment, not being able to clearly see your full peripheral visual field would present a significant compromise of flight safety.
Finally, the use of monovision contact lenses creates difficulty with depth perception. Our visual system uses multiple visual cues to determine the distance of an object. Slight differences in the image generated by the position of two separate eyes (stereoscopic vision) aids in depth perception. Although this is not the only cue associated with perceiving an object's distance, in the flight environment, monovision contact lenses create an undesirable compromise for a pilot who otherwise has the capability of stereoscopic vision.
There is evidence from the flight environment that demonstrates the negative effects of monovision contact lenses. The basis of question 17.b. relates to the NTSB accident investigation and subsequent safety recommendation (goo.gl/vEwspx) of Delta flight 554, which in 1996 landed short of runway 13 at LaGuardia Airport. At the time of the accident, the NTSB concluded there were multiple factors that may have created an illusion of the aircraft being too high on approach. They also noted the Captain was using monovision contact lenses during the flight. The concern was that the use of monovision contact lenses degraded the Captain's stereoscopic vision, which may have contributed to the perception of excess altitude.
A subsequent study performed by the FAA Civil Aerospace Medical Institute determined there were several more accidents associated with the general use of contact lenses (goo.gl/L7uACH). The bottom line regarding near vision correction is that the current FAA regulation does not allow monovision or multi-focal contact lenses as both types would tend to degrade the pilots visual capabilities. Although spectacles present their own challenges in the flight environment, the best option is to use bifocal spectacles or use distant vision contact lenses and have near vision spectacles available in the cockpit.