Vision and Physiological Factors

I am here today to talk to you about why vision is important in flight, specifically relating to the illusions that can be experienced and how we can mitigate some of those illusions.

Of all the body senses, vision is the most important for safe flight. Starting out, obviously we need to be able to see where we are going, look out for other aircraft, keep an eye out for terrain and obstacles, and judge and determine our altitude and distance based off visual references to keep us safe. According to regulation 14CFR 91.13B, despite who you are in radar contact or communication with, and when weather conditions permit, it is the pilot’s responsibility and the duty of each person operating the aircraft to always see and avoid oncoming traffic. Scanning the sky for other aircraft is a key factor in collision avoidance. Pilots must develop an effective scanning technique which maximises one’s visual capabilities. The probability of spotting a potential collision threat obviously increases with the time spent looking outside the cockpit. Thus, one must use time sharing techniques to effectively scan the airspace while monitoring their instruments as well.

Day and night vision

Because the eyes can only focus on a narrow viewing area during the day, called the cone of our eye, which are active at higher light levels capable of colour vision and are responsible for high spatial acuity, effective scanning is accomplished with a series of short, regularly spaced eye movements that bring successive areas of the sky into our central visual field, or cone, of our eyes. Though the cones and rods, which are peripheral vision, are both used during the day, each movement should not exceed 10 degrees and should be observed for at least one second to enable detection, usually from left to right.

Keep in mind that your eyes may take several seconds to adjust when refocusing or switching views between the cockpit and instrument panel and outside.

At night, or in conditions of dim illumination, small prints and colours on aeronautical charts and aircraft instruments become unreadable unless adequate lighting is used. In darkness, vision becomes more sensitive to light, a process called dark adaptation. Although exposure to total darkness for at least 30 minutes is required for complete dark adaptation, a pilot can achieve a moderate degree of dark adaptation using dim, red cockpit lighting. Keep in mind that white lighting must only be used when necessary for map or instrument reading. Since any degree of dark adaptation is lost within a few seconds of viewing a bright light, a pilot should close one eye when using a light to preserve some degree of night vision.

It is estimated that once fully adapted to darkness, the rods of the eye, which are ten thousand times more sensitive to light than the cones, make them the primary receptors for night vision. To see an object clearly at night, the pilot must expose the rods to the image which can be done by looking five to ten degrees off center of the object being seen and using your peripheral vision in a slow, constant scan to look for traffic. Keep in mind that a significant deterioration in night vision can occur at cabin altitudes as low as 5,000 feet, so it is recommended to use supplemental oxygen, if available, when night flying.

Effects of altitude on vision

Now we will talk about the effects of altitude on our vision. Altitude can have a major effect on our vision and other cognitive abilities as well. As we go up in altitude, the concentration of oxygen becomes much less and this state of oxygen deficiency in the body sufficiently impairs functions of the brain and other organs. This is referring of course to hypoxia.

Though significant effects usually do not occur in a normal, healthy pilot below twelve thousand feet, from twelve thousand to fifteen thousand feet of altitude, judgment, memory, alertness, coordination and the ability to make calculations are impaired, and headache, drowsiness, dizziness and a sense of well-being known as euphoria, or belligerence, occur. Above 15,000 feet, the periphery of the visual field grazed out to a point where only the sensual vision remains or called tunnel vision and cyanosis which is a bluing of the lips and fingertips, occur followed soon thereafter by unconsciousness, if corrective action, either descending to a lower altitude, or the use of supplemental oxygen, is implemented. There is also an assessment for that we can use as pilots to ensure we are all fit to fly, to be confident that we are not only physically and mentally safe to fly, but that there is nothing contributing to the impairment of our bodies and our vision. This is the I am SAFE checklist, consisting of illness, medication, stress, alcohol, fatigue and emotion.

I am safe checklist

Even a minor illness suffered in day-to-day living can seriously degrade performance of many piloting tasks vital to safe flight. Illness can produce fever and distracting symptoms that can impair our cognitive abilities. The safest rule is not to fly when suffering from any illness and to contact an AME if you have any questions. Pilot performance can be seriously degraded by both prescribed and over-the-counter medications, as well as by medical conditions for which they are taken, many medications such as tranquilisers, sedatives, strong pain relievers and cough suppressant preparations have primary effects that may impair judgment, memory, alertness, coordination, our vision and the ability to make calculations. These medications have side effects that may impair your body’s critical functions and can make a pilot much more susceptible to hypoxia.

Now we need to address stress. Stress from the pressures of everyday living can impair pilot performance often in very subtle ways. Difficulties, particularly at work, can occupy the thought process enough to markedly decrease alertness. With respect to alcohol, extensive research has provided several facts about the hazards of alcohol consumption and flying. As little as one ounce of liquor, one bottle of beer, or four ounces of wine can impair flying skills and affect your vision. Prolonged alcohol use can cause involuntary, rapid eye movement and slow down the communication between the eyes and the brain. This can cause double vision, decrease reaction time of the pupils and impair the ability to see colour shade. Alcohol also renders a pilot much more susceptible to distant orientation and hypoxia.

What about fatigue? Fatigue continues to be one of the most treacherous hazards to flight safety as it may not be apparent until serious errors are made to the pilot. Blurring, intolerance to light, headaches from reading, those are just a few of the problems with vision that can come with fatigue.

Finally, with emotion, certain tragic or upsetting events can render a pilot unable to fly the aircraft safely. The emotions of anger, depression and anxiety from such events not only decrease alertness but may also lead to taking risks that border on selfdestruction. It has been studied that our emotions affect the function of our eyes, particularly our pupils and our vision may be impaired. Any pilot who experiences an emotionally upsetting event should not fly until satisfactorily recovered from that upsetting experience.

Alex Johnson is a former student with AeroGuard and currently a flight instructor, a check instructor, and a training manager operating out of Punta Gorda campus in Florida. USA.