SCA stands for Shuttle Carrier Aircraft, referring to two extensively modified Boeing 747 airliners used by NASA. These planes were reinforced with mounting struts on the fuselage and fitted with vertical stabilizers on the tail to carry the Space Shuttle orbiter piggyback. They were essential for ferrying the shuttle from its landing sites in California back to the launch facilities in Florida.
Heard on the job
The orbiter is mounted to the SCA and they're cleared for the ferry flight back to the Cape.