The Shuttle Carrier Aircraft (SCA) are two extensively modified Boeing 747 airliners that NASA formerly used to transport Space Shuttle orbiters. One is a 747-100 model, while the other is a short range 747-100SR.
The SCAs were used to ferry Space Shuttles from landing sites back to the Shuttle Landing Facility at the Kennedy Space Center, and to and from other locations too distant for the orbiters to be delivered by ground transport. The orbiters are placed on top of the SCAs by Mate-Demate Devices, large gantry-like structures that hoist the orbiters off the ground for post-flight servicing, and then mate them with the SCAs for ferry flights.
In approach and landing test flights conducted in 1977, a test shuttle was released from an SCA during flight and glided to a landing under its own control.