NASA's four Gulfstream II business jets that were used to train space shuttle pilots in the skills of approach and landing. Pilots assigned to a shuttle mission received about 100 hours of STA training, which was equal to 600 shuttle approaches. Each of the aircraft had been modified to fly like a space shuttle during landing. The STA also simulated the shuttle's high speed and steep angle of landing by putting its engines in reverse thrust and lowering the main landing gear.