The aircraft was extensively modified for NASA by Boeing in 1976. While first-class seats were kept for NASA passengers, its main cabin and insulation were stripped, and the fuselage was strengthened. Mounting struts were added on top of the 747, located to match the fittings on the Shuttle that attach it to the external fuel tank for launch. With the Shuttle riding on top, the center of gravity was altered. Vertical stabilizers were added to the tail to improve stability when the Orbiter was being carried. The avionics and engines were also upgraded.