Killed by Adaptive Control

In 1968, pilot Mike Adams was flying the X-15 with the adaptive control installed, zooming past 250,000 feet. Once out of the atmosphere, instead of flying upward like an arrow, the nose began to fall off to the side. This change did not affect the trajectory;through the vacuum of space, the aircraft flew sideways as well as it flew straight. But Adams corrected for the change not by yawing to the left, but to the right—which exacerbated the error, and the vehicle continued to drift off to the right. He may have been distracted by an electrical arcing in one of the instruments the aircraft was carrying, or he may have been experiencing a fit of vertigo, which he had been prone to on previous flights. Unbeknownst to Adams, noise from the disturbance on the instrument forced the adaptive systems gains way down, essentially disabling the reaction controls. Flight controllers on the ground, commented, ''A little bit high____real good shape'' was their comment.40 They had no idea that the aircraft had turned completely around.

But as the X-15 arced over the top of its trajectory and began to reenter the atmosphere the attitude mattered a great deal. Becoming an airplane again, it needed to fly, and the X-15 wouldn't fly sideways or backwards. Soon Adams recognized these troubling effects. ''I'm in a spin, Pete,'' he radioed to the ground. Spins are dangerous at any altitude and airspeed, although recoverable and not inherently fatal. Nobody had ever entered a spin, however, at Mach 5 and 200,000 feet. Supersonic shockwaves could impose any kind of damage on the aircraft.

Miraculously, at Mach 4.7 and 120,000 feet, the aircraft recovered from the spin— due to the SAS, the pilot's inputs, the X-15's inherent stability, or some combination of all three. Yet here Adams lost control to the computers. During the spin, the adaptive controller drove the gain to the highest extreme in pitch and did not reduce it as the vehicle entered the atmosphere. With hypersensitive controls, the aircraft now became unstable and began oscillating while the vehicle was falling 160,000 feet per minute. The nose pitched up and down like a porpoise, forty to sixty degrees every few seconds. The pilot could have manually reduced the system gain, but he did not—an understandable omission given the stress he was experiencing. Within a few of these violent cycles, after about a minute, the X-15 broke apart at about 60,000 feet and fell to the desert floor in several pieces, killing the pilot. It was the only fatality of the nine-year X-15 program.

The accident report blamed a combination of pilot error and the MH-96 oscillation. ''The system functioned as designed,'' Thompson wrote, ''but the design did not consider this particular and unique combination of conditions and pilot response.''41 An automatic pilot with complex, adaptive loops could behave in unpredictable, even dangerous ways as it encountered unforeseen circumstances. The X-15 stopped flying soon after.

The Adams tragedy highlighted the discomfort with which the pilots adapted to the new world of computers and automatic controls. ''When first exposed, the pilots tended to distrust the systems and occasionally have the feeling that the system, rather than the pilot, is controlling the aircraft.'' Some pilots gradually learned to appreciate the benefits offered by the automatic control—the hold modes, the improved damping on reentry. Others had been ''bitten'' by it and came to distrust adaptive control.42 Similar problems would arise in Apollo, whose control systems depended on digital computers and software with similar complexity, adaptability, and potentially unpredictable behavior.

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