Eye Stress Can Affect Ability to "See and Avoid"

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Editor's Note: This is the second in a four-part article series on business aviation mid-air collisions. Here is the first part.

Ground-school lessons often discuss the eye’s rods and cones, the blind spot and empty field myopia. If you need a refresher on that topic there are plenty of good sources available. Let’s progress into a discussion of additional, lesser-discussed but equally important issues that decrease an eye’s ability to accurately detect another aircraft on a collision course.

The human eye is brought into focus by muscle movements that change the shape of the eye lens. Important changes to our eyesight as we grow older negatively affect our performance and safety. Somewhere around age 40, your ability to focus on near and far objects will start to go away as the natural lenses in your eyes become less flexible. Your eye doctor will call this condition presbyopia.

According to Dr. Stanley Mohler, director of aerospace medicine at Wright State University School of Medicine, “Aging brings gradual changes to the structure of the eye and quality of vision. These changes can result in problems ranging from difficulty focusing on close objects, to dry eyes, to diseases such as glaucoma and macular degeneration.”

One of the most-common ailments in aging eyes are cataracts. They are caused by a change in the protein structure of the eye’s lens that generally occur after age 50. Cataracts often form slowly without any symptoms. Symptoms include blurred vision, a loss of color perception acuity, glare, double vision, halos around objects and a need for more light to see clearly.

It takes a finite amount of time for the eyes to refocus their focal length from inside the cockpit to outside the cockpit. This process is called “accommodation.” A young person will typically require about 1 sec. to accommodate to a stimulus. However, with increasing age, the eye lens tends to harden, making the task of the ciliary muscles to change the shape of the lens more difficult. As a result, the speed and degree of accommodation decreases with age. Furthermore, increased time is required for accommodation as a pilot becomes fatigued.

By the way, the design team for the first model of the Cessna Citation did an analysis of the “typical” pilot who would operate their revolutionary jet. The market for the proposed design would be a mid- to later-age businessman who likely did not have young eyes with perfect 20/20 vision. Instead, it was more likely their prospective client would wear bifocals. If you are on the “mature” side of 30 or 40, you’ve experienced the frustration of trying to read the overhead panel, and with bifocals this means tilting your head or glasses to find the right focal length so that you can read the tiny gauges. Cessna realized that it would be difficult for “mature” pilots to see an overhead panel without difficulty. They also worried about the subsequent threat from a “mature” pilot’s eyes being adjusted to this short distance and then attempting to look outside the aircraft for traffic due to the problem of eye accommodation. This is why the Cessna design team deliberated excluding any overhead panels in the cockpit.

The extensive amount of flight time that we spend cruising at stratospheric altitudes also has a direct effect on our ability to “see and avoid.” Flight crews of high-performance aircraft are exposed to elevated levels of ultraviolet radiation due to operating altitudes in the upper reaches of the troposphere and lower stratosphere. The air density in that portion of the earth’s atmosphere is thinner than at sea level and therefore it filters less of the dangerous solar radiation. Ultraviolet radiation increases with altitude at about 6-10% every 1,000 ft. Between 31,000 and 41,000 ft., where most commercial aircraft fly, UV radiation exposure doubles.

Research conducted by Dr. Adrian Chorley between 2008-15 studied the effects of ultraviolet radiation on a pilot’s eyesight. The study showed that although UVA is the least energetic of UV radiation, it is the most harmful to a pilot’s eyes and eyesight because a higher percentage of it penetrates the cockpit and cabin of an aircraft. He goes on to point out that “There is good evidence that long-term exposure to solar radiation, especially the ultraviolet and blue light components, is a risk factor for cataracts and, to a lesser extent, age-related degeneration of the retina.”

Glare directly affects a pilot’s ability to accurately detect other aircraft, especially when that aircraft is in the same direction as the glare. Glare can come directly from the light source or can take the form of veiling glare, reflected from crazing or dirt on the windscreen. The classic book Human Factors in Flight, by Frank H. Hawkins and Harry W. Orlady, informs pilots that when the source of the glare is only 5 deg. away from the direction you are looking, the loss of visual effectiveness is an astounding 84%. When glare is 40 deg. from the line of sight, the loss of visual effectiveness is 42%.

Has glare actually contributed to mid-air collisions? Optometrist Van B. Nakagawara, along with associates Kathryn J. Wood, CPOT (certified paraoptometric technician) and Ron W. Montgomery at the FAA’s Civil Aerospace Medical Institute, conducted a study titled “Natural Sunlight and its Association to Aviation Accidents: Frequency and Prevention” (DOT/FAA/AM-03/6, Office of Aerospace Medicine, May 2003) that searched 25,226 accidents in the NTSB’s accident database for terms including sun, glare, vision, blinded and reflections. The researchers found 130 accidents in which direct or reflected glare was found to be a contributing factor. Glare was cited as a contributing factor in seven mid-air collisions.

Thomas C.D. Whiteside, Ph.D., of the Royal Air Force Institute of Aviation Medicine, authored an authoritative text titled “The Problems of Vision at High Altitude.” Whiteside points out that “The frequency with which the pilot’s eyes must alternate from cockpit to exterior, from near to far, from dark to light, combines with all these factors to produce a visual environment that most readily results in fatigue and difficulty in seeing.”

The average person has a visual field of about 190 deg., although field of vision varies from person to person and is generally greater for females than males. The field of vision begins to contract after age 35, and in males, this reduction accelerates markedly after 55 years of age. A number of transient and psychological conditions can cause the effective field of vision to contract even further, such as vibration, fatigue, hypoxia or, more than likely, cockpit workload.

One factor involved with the difficulty of seeing other aircraft when on approach to airports in urban areas is that the background comprises a mosaic mixture of colors, but particularly a lot of gray. That background makes it more difficult to detect another aircraft. Furthermore, the light scatter from haze decreases the color contrast, which further decreases the likelihood of spotting other aircraft. 

On your next approach into a busy airport such as Van Nuys, California (KVNY), look at your TCAS screen, which will likely be filled with numerous blue diamonds. Then glance outside to see how easy it is to quickly detect most of those nearby aircraft. The sun’s angle and glare through the haze greatly diminishes an eye’s ability to detect color contrast and movement. These environmental viewing conditions are not conducive to quick and easy detection of other aircraft, especially aircraft on a collision course.

Patrick Veillette, Ph.D.

Upon his retirement as a non-routine flight operations captain from a fractional operator in 2015, Dr. Veillette had accumulated more than 20,000 hours of flight experience in 240 types of aircraft—including balloons, rotorcraft, sea plans, glides, war birds, supersonic jets and large commercial transports. He is an adjunct professor at Utah Valley University.