As important as visual input is for a pilot, eyes can lie. For example, when flying above a cloud deck, there is a natural tendency to perceive any relatively straight line in the visual field as a horizon—which can lead to very undesirable results in a fast-moving aircraft.
False horizons are everywhere around you in the clouds. Your aircraft’s attitude may seem level, even if you are tilted and in a turn. Mountain ridges might lead you astray as well, and at night the combination of clouds, stars, mountains and lights on the ground can produce impossibly confusing percepts that lead the aircraft away from the safety of true straight and level flight.
Don’t think that you are safe from your own perception, however, just because you are flying above water on a clear day. A fixed horizon can still put you in the drink. Consider what may happen if you approach the beach from over the horizon. You may line up the beach in your sights and then keep it there in anticipation of going “feet dry” (flying from over sea to over land), but if so you will never reach land: the beach is fixed, unlike a true horizon, and the only way to keep it stationary in your sight is to point your aircraft progressively downward.
Choosing a fixed horizon in proximity to wires or cables stretched across a valley is especially problematic. As you approach the fixed horizon (such as where a valley floor and mountain wall meet), you slowly and unnoticeably nose down. As you descend, the approaching wires will appear to rise as if they will pass well above you, whereas in fact they remain well below the aircraft. If you don’t spot the wires until they are very close (because of mountain haze or the fog of war), your natural reaction may be to push the stick forward to dive under the wires. This reaction is what happened to the U.S. Marine pilots of an EA-6B Prowler aircraft on a training mission in 1998 near an active Italian ski resort in the Alps. The aircraft sliced through two wires, which held a cable car holding 20 skiers 370 feet above the ground. None survived.
One might think that objective information from instruments is the logical solution to subjective sensory illusions. The proliferation of instrumentation is part of the problem, however, because of mounting attentional demands on the pilots, which cause cognitive overload during combat and other stressful flight scenarios. This kind of mental distress is an important contributor to spatial D. New avionics are designed with simplicity, not complexity, in mind, and pilots learn how to scan their instruments at just the right times, under conditions of simulated duress. Systematic instrument scanning demands discipline, which may be one of the first casualties of battle, but until we learn better ways to overcome insidious in-flight illusions, it is one of the main techniques that keep pilots and crews safe.
We would like to thank LCDR Brian Swan, USN (Ret.), CDR Tyson Brunstetter, USN, and CDR Fred R. Patterson, USN (Ret.), for their ideas and contributions to this article.
This article was published in print as "The Aviator's Dilemma."