Shuttle (STS) Approach and Landing Test (ALT)
The Space Shuttle Enterprise never flew in space. It was the first Space Shuttle built (completed on September 17, 1976), and was used only for aeronautical flight testing. The Enterprise arrived at Dryden in January 1977 for a flight program involving a Boeing 747 airliner that had been modified for use as a shuttle carrier aircraft (SCA).
The first flights with the Space Shuttle attached to the SCA were done to find out how well the two vehicles flew together. Five "captive-inactive" flights were made during this test phase; there was no crew in the Enterprise. The next series of tests were done with a flight crew of two onboard the Space Shuttle during three captive flights, with the Enterprise piloted and its systems activated. All of this led to the Space Shuttle Approach and Landing Tests (ALT), which began on August 12, 1977.
The ALT program allowed pilots and engineers to learn how the Space Shuttle handled during low-speed flight and landing. The Enterprise was flown by a crew of two after it was released from its pylons on the SCA at an altitude of 19,000 to 26,000 feet.
The Enterprise did not have a 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 on October 26, 1977. The following spring it was ferried to other NASA Centers for ground-based flight simulations that tested the Space Shuttle systems and structure.