As Keenan gently turned the chair, he said to the group, “Your body keeps you alive. We learn to recognize cues from our environment.” The problem, he added, “is that our bodies are meant to walk on Earth.” As he spoke, the chair seemed to quit spinning. I moved the stick to the neutral position. Keenan then rattled off a litany of phenomena ready to befall pilots: the leans, the graveyard spiral, the inversion illusion, the elevator illusion, false horizons (see “Vertigo: A Primer,” below). As he spoke, the chair then seemed to reverse direction, spinning to the right. I moved the stick to the right. Some in the group tittered. Keenan then pulled off the blindfold, and I saw that the chair had stopped. “Get up carefully,” warned Keenan, helping me to my feet. “You’re still spinning.” Three other volunteers followed my lead. In each case, Keenan first got the chair spinning. After a bit, he gently brought the chair to a stop. In each case, the volunteer moved the stick exactly as I did.
The inner ear is designed to detect motion, or rather, acceleration. Thus, when the chair began to turn, I sensed it. However, once the turn rate was constant, the fluid in my inner ear returned to equilibrium, and without the benefit of visual cues, I could not tell the difference between turning and sitting still. So when the chair stopped turning, I sensed that as a turn in the opposite direction.
Vestibular illusions fall into two categories: somatogyral, for spinning illusions (“somato” is Greek for “body”), and somatogravic, for acceleration illusions. The Barany chair demonstrates a basic somatogyral illusion. An airplane in a stable, level turn will feel the same as an airplane in straight-and-level flight. If the airplane is returned to straight-and-level flight, or if the bank is decreased, a pilot’s natural reaction would be to make a correction that would steepen the actual turn. If at the same time the pilot’s head were tilted—if he were reading a map or picking up a pencil—the deception to the vestibular system would be compounded along a third axis, meaning that when the airplane returned to straight-and-level flight or the pilot lifted his head, he would sense not only a turn in the opposite direction but a feeling of pitching up or down.
Somatogravic illusions refer to situations in which an airplane that begins accelerating will feel the same as one climbing, and an airplane decelerating will feel the same as one descending. Because we live on the surface of the Earth, where the force of gravity pulling us toward the ground is more or less constant, or 1 G, our vestibular system cannot
distinguish the difference between pitch and acceleration. Today’s full-motion simulators take advantage of this fact to create the illusion of flight. For example, inside the simulator pod, as the pilot moves the throttles forward for takeoff and sees and feels the “airplane” accelerating down the runway, the pod itself begins to tilt up. The motions created by the simulators are so realistic that students have become airsick in them.
In 1917, Elmer Sperry invented the gyroscopic turn indicator, based on a similar device he had invented for ships. The indicator joined Sperry’s gyroscopic compass to make up what would later be the core of the panel for instrument flight. But as late as 1928, the idea of flying solely by reference to instruments—“flying blind”—remained as foreign as travel to other planets. Pilots were convinced that their most valuable tools were skill and instinct.
In 1926, Army Air Corps Captain William Ocker, who had been experimenting with Sperry’s turn indicator, took a medical exam that included a spin in a Barany chair to test his vestibular system. Experiencing the same spinning illusion, he had the revelation, writes William Langewiesche in Inside the Sky (Pantheon, 1998), “that instinct is worse than useless in the clouds, that it can induce deadly spirals, and that as a result having gyroscopes is not enough, that pilots must learn against all contradictory sensations the difficult discipline of an absolute belief in their instruments.” Ocker, with the zeal of a fundamentalist minister, began preaching the necessity of developing procedures and instructional programs in instrument flight. He was unable, however, to convert his superiors. Twice the Army had Ocker hospitalized to test his sanity. (In 1932, a vindicated Ocker coauthored the first treatise on instrument flying, Blind Flying in Theory and Practice.)
In 1927, a group of scientists and pilots that included Sperry and Army Air Corps Lieutenant James Doolittle built an artificial horizon, a gyroscopic device that gives the pilot a graphic representation of the airplane’s attitude in relation to the horizon. Doolittle used it in 1929 to make the first flight and landing solely by reference to the aircraft’s instruments, proving the feasibility of instrument-only flight.
Making trustworthy instruments was one thing, but making pilots trust them was another. At first, pilots reported the instruments seemed to work only in clear weather, that in clouds the devices went haywire, indicating turns the pilot was certain the airplane was not making. The instruments worked just fine; the pilots had to be taught to resist the instinct to fly “by the seat of their pants”—that is, by sensation alone.
Today, primary flight training for all pilots requires instruction in flight based on instruments and recovery from unusual attitudes, in which the flight instructor has the student close his eyes while the aircraft goes through a disorienting series of turns, climbs, and descents, then has the student return the airplane to straight-and-level flight. Military aviators, in addition to being subjected to periodic proficiency reviews, are required to attend, every five years, refresher courses in human physiology that include a section on spatial disorientation.