The Space Shuttle prototype Enterprise rises from NASA's 747 Shuttle Carrier Aircraft (SCA) to begin a powerless glide flight back to NASA's Dryden Flight Research Center, Edwards, California, on its fourth of the five free flights in the shuttle program's Approach and Landing Tests (ALT), 12 October 1977. The tests were carried out at Dryden to verify the aerodynamic and control characteristics of the orbiters in preparation for the first space mission with the orbiter Columbia in April 1981. The Space Shuttle Approach and Landings Tests (ALT) program allowed pilots and engineers to learn how the Space Shuttle and the modified Boeing 747 Shuttle Carrier Aircraft (SCA) handled during low-speed flight and landing. The Enterprise, a prototype of the Space Shuttles, and the SCA were flown to conduct the approach and landing tests at the NASA Dryden Flight Research Center, Edwards, California, from February to October 1977. The first flight of the program consisted of the Space Shuttle Enterprise attached to the Shuttle Carrier Aircraft. These flights were to determine how well the two vehicles flew together. Five "captive-inactive" flights were flown during this first phase in which there was no crew in the Enterprise. The next series of captive flights was flown with a flight crew of two on board the prototype Space Shuttle. Only three such flights proved necessary. This led to the free-flight test series. The free-flight phase of the ALT program allowed pilots and engineers to learn how the Space Shuttle handled in low-speed flight and landing attitudes. For these landings, the Enterprise was flown by a crew of two after it was released from the top of the SCA. The vehicle was released at altitudes ranging from 19,000 to 26,000 feet. The Enterprise had no propulsion system, but its first four glides to the Rogers Dry Lake runway provided realistic, in-flight simulations of how subsequent Space Shuttles would be flown at the end of an orbital mission. The fifth approach and landing test, with the Enterprise landing on the Edwards Air Force Base concrete runway, revealed a problem with the Space Shuttle flight control system that made it susceptible to Pilot-Induced Oscillation (PIO), a potentially dangerous control problem during a landing. Further research using other NASA aircraft, especially the F-8 Digital-Fly-By-Wire aircraft, led to correction of the PIO problem before the first orbital flight. The Enterprise's last free-flight was October 26, 1977, after which it was ferried to other NASA centers for ground-based flight simulations that tested Space Shuttle systems and structure.
