On June 26, 2007, while on a training exercise off the Oregon coast, Major Gregory D. Young of the Air National Guard flew his F-15A fighter into the Pacific Ocean. The $32 million aircraft was destroyed and the pilot killed. There was no distress call, no attempt to eject, and no apparent aircraft malfunction. Young, 34, had 2,300 hours of flight time, more than 750 hours of it in F-15s.

As investigators sifted through the wreckage—what little was left—colleagues, family, and friends were left to wonder: What caused Young to guide his airplane right into the ocean at more than 600 mph? The answer, revealed in an investigative report two months later, was both profoundly unsettling and all too familiar. Young, in the prosaic terminology of the report, “experienced unrecognized (Type 1) spatial disorientation (SD), which caused him to misperceive his attitude, altitude, and airspeed. As a result, [he] was clearly unaware of his position and impacted the water.”

In other words: Young never knew what hit him.

Despite training, experience, and technology, all based on knowledge of how flight affects human physiology, Young had no idea that he was racing downward.

Once called pilot vertigo or aviator’s vertigo, spatial disorientation is a persistent killer. Federal Aviation Administration statistics show that the condition is at least partly responsible for about 15 percent of general aviation accidents, most of which occur in clouds or at night, and 90 percent of which are fatal. According to a 2004 study, the average life expectancy of a non-instrument-rated pilot who flies into clouds or instrument conditions is 178 seconds.

A U.S. Air Force review of 633 crashes between 1980 and 1989 showed that spatial disorientation was a factor in 13 percent, resulting in 115 deaths. Among crashes of high-performance aircraft, the rate was higher: 25 to 30 percent. A U.S. Navy study found that in contrast to general aviation accidents, a majority of accidents in high-performance aircraft occurred in daylight and in visual flight conditions. The pilots were an average of 30 years old, with 10 years in the cockpit and 1,500 hours of pilot-in-command or instructor time, and in the prior three months they had flown an average of 25 times—all of which shows that no amount of expertise, training, or experience immunizes against spatial disorientation.

Humans maintain orientation and posture through a system of senses: vision; the vestibular system (the labyrinthine series of ducts and canals in the inner ear); muscle-sense or proprioception, sensors in muscles and joints that inform us of our body’s position (standing versus sitting, for example); and the sense of gravity, or what we perceive as up and down. The system has evolved over eons, and is well adapted for Earth. But it is easily fooled. When you’re sitting on a stopped train and the train on an adjacent track begins to move, you’ll think that you’re the one who is moving.

In the air, things get more complicated. Early aviators were confronted by an assault on their senses, or “disturbances of equilibrium,” as Orville Wright described it. Until World War I, most flights were made during the day and limited to short, straight-and-level hops. Few risked flying at night, and fewer still flew into clouds, or at least did and lived to tell about it.

Research in the 19th and early 20th centuries helped shed light on the vestibular system and how it maintains equilibrium. In 1906, Robert Barany devised a swivel chair to simulate the effects of spatial disorientation on pilots.

At an FAA-sponsored safety seminar in Rhode Island recently, program manager Jack Keenan offered me a seat in a Barany chair, a device not unlike a barber’s chair, with an ersatz control stick. As a group of other pilots stood around, he blindfolded me and began to spin the chair to the left, telling me to move the control stick in the direction of the spin. I dutifully moved the stick to the left.