Chuck Yeager in the context of "Hero"

A test pilot is an aircraft pilot with additional training to fly and evaluate experimental, newly produced and modified aircraft with specific maneuvers, known as flight test techniques.

The Bell X-1 (Bell Model 44) is a rocket engine–powered aircraft, designated originally as the XS-1, and was a joint National Advisory Committee for Aeronautics–U.S. Army Air Forces–U.S. Air Force supersonic research project built by Bell Aircraft. Conceived during 1944 and designed and built in 1945, it achieved a speed of nearly 1,000 miles per hour (1,600 km/h; 870 kn) in 1948. A derivative of this same design, the Bell X-1A, having greater fuel capacity and hence longer rocket burning time, exceeded 1,600 miles per hour (2,600 km/h; 1,400 kn) in 1954. The X-1 aircraft #46-062, nicknamed Glamorous Glennis and flown by Chuck Yeager, was the first piloted airplane to exceed the speed of sound in level flight and was the first of the X-planes, a series of American experimental rocket planes (and non-rocket planes) designed for testing new technologies.

The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approached the speed of sound, these effects were seen as constituting a barrier, making faster speeds very difficult or impossible. The term sound barrier is still sometimes used today to refer to aircraft approaching supersonic flight in this high drag regime. Flying faster than sound produces a sonic boom.

In dry air at 20 °C (68 °F), the speed of sound is 343 metres per second (about 767 mph, 1234 km/h or 1,125 ft/s). The term came into use during World War II when pilots of high-speed fighter aircraft experienced the effects of compressibility, a number of adverse aerodynamic effects that deterred further acceleration, seemingly impeding flight at speeds close to the speed of sound. These difficulties represented a barrier to flying at faster speeds. In 1947, American test pilot Chuck Yeager demonstrated that safe flight at the speed of sound was achievable in purpose-designed aircraft, thereby breaking the barrier. By the 1950s, new designs of fighter aircraft routinely reached the speed of sound, and faster.