Pratt & Whitney's Unique Boeing 747SPs: A Fifth Engine and a Secret Life
Two Boeing 747SP aircraft, owned by Pratt & Whitney, are still in operation as of September 2026. These unique planes have a fifth engine mounted on top of their fuselage, which might look like an oddity but serves a crucial purpose.
The fifth engine is part of a testbed setup used to evaluate conceptual engine designs before they enter full production. The aircraft are designed to gather real-world flight data during test flights, pushing the extra engine to its limits in various uncontrollable conditions such as extreme temperatures and weather.
Each test can take up to eight hours to complete, simulating a commercial flight. Inside the 747s, Pratt & Whitney has set up workstations for flight engineers to monitor the fifth engine's performance under different configurations.
The Boeing 747SP is the smallest version of the 747 ever produced, with only 45 units made by Boeing. General Electric and Rolls-Royce also use testbed aircraft for similar purposes, but the Pratt & Whitney Boeing 747SPs are particularly striking due to their unique design.