Certain Boeing 747 aircraft were modified to carry a fifth engine mounted on a stub wing, creating a rare configuration used for engine testing.

 

 

Certain Boeing 747 aircraft were modified to carry a fifth engine mounted on a stub wing, creating a rare configuration used for engine testing.

The aircraft are operated by Pratt & Whitney and are used to flight-test new jet engines rather than provide additional thrust.

The extra engine is mounted in an inactive state during most flights and can be started midair for real-world performance evaluation.

These test aircraft differ internally from standard 747s, featuring workstations and monitoring equipment instead of passenger cabins.

The configuration allows engine makers to gather critical data without building dedicated test aircraft.