January 2004Instructor Report

Human factors in general aviation


Knowledge of our weaknesses is power

Have you ever read an article about an aircraft crash and asked, "How could that have happened?" After human error caused the ground collision of Pan Am and KLM Boeing 747s at Tenerife in March 1977, people realized the consequences of advancing the mechanical element while neglecting the human one. After that accident, human factors began to receive more scrutiny.

Many aircraft accidents are attributed to "pilot error," as if there is a difference between pilot error and all other forms of human error, such as a mistake by a surgeon or a police officer.

An individual can make an error by taking action when he shouldn't have, by not taking an action when he should have, or by doing the wrong thing. Examples of taking action at the wrong time would be a student lowering flaps when the aircraft is above VFE. Not taking an action when required might involve a pilot who has become fatigued after a long flight and forgets to lower the landing gear on approach, or the pilot who fails to stop for fuel and attempts to conclude a long cross-country flight with a headwind. Any multiengine pilot should be familiar with the consequences of taking one action when another is required; there's a good reason why the mnemonic "dead foot, dead engine" is taught in multiengine training.

In reviewing NTSB accident reports, it appears that inaction is far too common. Why? A few guesses: The pilot missed the clues that a problem was developing, underestimated the problem and action required, or was simply indecisive.

Another concept in human error is the error chain, well-documented by the NTSB. The principle involves snowballing mistakes; one error contributes to other problems until the demise of the flight. Take, for example, a pilot attempting VFR flight into IMC, then experiencing a vacuum pump failure and losing spatial orientation. Had the pilot avoided IMC, the vacuum pump failure would have been only an inconvenience. Therefore, don't take one chance assuming that it will be the only one you take in the flight.

The pilot's physical condition can add to the problem. Overall health influences a career pilot's decision-making as much as, if not more than, corporate pressures. Those who exercise and stay healthy can naturally expect to be able to push a little further than those who don't. However, no one is immune from physical factors. Most pilots today have been taught acronyms such as I'M SAFE for evaluating our physical state (illness, medication, stress, alcohol, fatigue, and emotion), but pilot training almost never explores this subject in depth.

One of the more common physical factors is fatigue, whether caused by a poor night's sleep or too many hours awake. Research has shown that after being deprived of rapid eye movement sleep for more than 16 hours, the mental acuity of the average person will be the same as if he had a blood alcohol level of 0.10 percent. Considering the degree of concentration and decision-making that flying requires, fatigue of this magnitude should dictate that additional thought is given to the go/no-go decision.

Even with a good night's sleep, disturbed body rhythm activity or intense physical and mental activity can interfere with concentration. Most of us have experienced such effects after a long day at work; those who fly long distances may experience jet lag. Many people believe that drowsiness, irritability, lack of motivation, and in some cases mild depression after a long flight are caused by the distance traveled; they are actually the result of a change in time zones.

Hypoxia-a state of oxygen deficiency in the body-also affects a pilot's ability to fly safely. Hypoxic hypoxia, the most common form among pilots, is a lack of oxygen to the brain at altitude. The decrease in air density is exponential with altitude, with available oxygen cut in half at only 18,000 feet mean sea level. At this altitude, the average person loses the ability to perform pilot-in-command duties within about 20 minutes. Twenty minutes may not seem short as you read this article, but time goes by quickly when you're flying a high-performance aircraft at the threshold of Class A airspace. At 24,000 feet msl the average person has approximately three to five minutes of useful consciousness. In the event of a slow cabin decompression or pressurization failure, the problem could be detected too late. This is why the federal aviation regulations require supplemental oxygen for the crew above 12,500 feet cabin pressure altitude after 30 minutes, and for all occupants above 14,000 feet cabin pressure altitude.

In any aircraft operation, the key is the human operator. We pilots-and our students-should try to understand not only what makes the airplane tick, but also how human decision-making works. With understanding and improvement come increased confidence and control, two things a pilot cannot fly without.

By Bruce Anzalone

Bruce Anzalone is an intern with the AOPA Air Safety Foundation. A recent graduate of Embry-Riddle Aeronautical University, he is a commercial pilot with instrument and multiengine ratings. Links to additional information about this subject may be found on AOPA Online.


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