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 tumbled violently--about all three axes--for more than 40,000 feet before Yeager was able to begin to recover to wings-level, stable flight. When landed safely on Rogers Dry Lake he was, once again, the "fastest human alive" but it was quite apparent that there were still many mysteries to be solved concerning supersonic flight. Nine months later, on August 26, 1954, Maj. Arthur "Kit" Murray flew the same aircraft to a new altitude record of 90,440 feet. On July 20, 1955, the X-1A was lost just before its first NACA test flight when it had to be jettisoned from the launch aircraft following an onboard explosion.