Bell X-1ABell X 1A airplane model. The Bell X 1A was a slightly larger, much modified version of the first generation X 1. On December 12, 1953, Maj. Chuck Yeager piloted the rocket plane to a speed of Mach 2. 44 (1,650 mph) in level flight at an altitude of approximately 76,000 feet. As he attained top speed, however, the X 1A tumbled violently out of control. He was encountering something new something aerodynamicists called "inertia coupling." The airplane