<P> Later variants of the X-1 were built to test different aspects of supersonic flight; one of these, the X-1A, with Yeager at the controls, inadvertently demonstrated a very dangerous characteristic of fast (Mach 2 plus) supersonic flight: inertia coupling . Only Yeager's skills as an aviator prevented disaster; later Mel Apt would lose his life testing the Bell X-2 under similar circumstances . </P> <P> Ordered by the Air Force on 2 April 1948, the X-1A (serial 48 - 1384) was intended to investigate aerodynamic phenomena at speeds greater than Mach 2 (681 m / s, 2,451 km / h) and altitudes greater than 90,000 ft (27 km), specifically emphasizing dynamic stability and air loads . Longer and heavier than the original X-1, with a stepped canopy for better vision, the X-1A was powered by the same Reaction Motors XLR - 11 rocket engine . The aircraft first flew, unpowered, on 14 February 1953 at Edwards AFB, with the first powered flight on 21 February . Both flights were piloted by Bell test pilot Jean "Skip" Ziegler . </P> <P> After NACA started its high - speed testing with the Douglas Skyrocket, culminating in Scott Crossfield achieving Mach 2.005 on 20 November 1953, the Air Force started a series of tests with the X-1A, which the test pilot of the series, Chuck Yeager, named "Operation NACA Weep". These culminated on 12 December 1953, when Yeager achieved an altitude of 74,700 feet (22,800 m) and a new airspeed record of Mach 2.44 (equal to 1620 mph, 724.5 m / s, 2608 km / h at that altitude). Unlike Crossfield in the Skyrocket, Yeager achieved that in level flight . Soon afterwards, the aircraft spun out of control, due to the then not yet understood phenomenon of inertia coupling . The X-1A dropped from maximum altitude to 25,000 feet (7,600 m), exposing the pilot to accelerations of as much as 8g, during which Yeager broke the canopy with his helmet before regaining control . </P> <P> On 28 May 1954, Maj . Arthur W. Murray piloted the X-1A to a new record of 90,440 feet (27,570 m). </P>

When did planes first reach speeds of 300 to 400 miles per hour