Under these conditions, bailing out often became the best option. Those who didn’t often joined their plane as it crashed into the ground.

Lost in the clouds, these pilots had fallen prey to a form of sensory disorientation known as a death spiral, or, more commonly, a graveyard spiral. The term describes an almost instinctive set of maneuvers pilots undertake when they lose sight of the horizon. The graveyard spiral begins when a plane flying in these conditions enters a gentle turn. As it turns, the plane will begin to descend, picking up speed.

Death spirals occur because the pilot feels the descent but not the turn. That has to do with the way the human body relies both on the visual and vestibular systems to perceive its orientation in space. As fluid moves through the small canals in the inner ear, the brain registers the body’s shifts in position. The fluid moves when the head turns, creating the sensation that the vessel under control is doing the turning. In mid-flight, though, the fluid can settle in place. If this happens, a turn can feel like level flight. In this situation, a pilot who follows the instruments and levels the plane’s wings feels, with absolute certainty, that the craft is turning in the opposite direction.

A pilot who recorrects to what feels level in his or her body simply reinitiates the spiral dive. Likewise, pulling back on the yoke to gain altitude without leveling the wings only tightens the plane’s downward spiral. Without a clear view of the horizon to correct against, the pilot can become so disoriented that a total loss of control results, ending in a crash. An Air Safety Institute scare-tactic training video, “178 Seconds to Live,” follows a pilot through the disorientation of a classic graveyard spiral.

Once it became clear that aviators were becoming disoriented in the clouds, they set themselves to the task of figuring out how to avoid it. This was the birth of what is known as “instrument flight.” Planes already carried basic instruments, such as turn and bank indicators, but these were primarily seen as navigational devices—implements that helped pilots reach a destination rather than keep the plane in the air. To tame the death spiral, these devices had to become part of how aviators kept control of the plane.

In 1932, William Ocker and Carl Crane published Blind Flight in Theory and Practice, a detailed guide to flying by instruments through darkness and fog. Ocker and Crane’s method relied on giving the pilot a visual reference against which to double-check the body’s fallible sensations. A turn and bank indicator shows the wings’ departure from level flight, and an artificial horizon visually represents the plane’s relation to the ground.

But designing and implementing instruments was the easy part. It was harder to teach pilots to believe what their instruments reported over (and against) the persuasive sensations they felt in their bodies. Here Ocker and Crane ran up against aviators’ long-standing belief that they controlled the plane, at least in part, through their superior “air sense”—their body’s special ability to maintain its equilibrium in flight. The idea that skillful flight depended on the body’s perception of its own weight and relative position, sometimes called “deep sensibility” or kinesthesia, was a truism among pilots. (Aviators referred to this skill as their ability to “fly by the seat of the pants,” a phrase that connoted, perhaps falsely, skill more than luck.)