A small number of Boeing 747 aircraft around the world carry one of the most unusual configurations ever seen on a commercial airframe: a fifth jet engine mounted on a short stub wing beneath the fuselage.

These rare aircraft are not designed for extra power or emergency backup. Instead, they serve a highly specialized role in the aviation industry — real-world flight testing of next-generation jet engines.

The modified aircraft are operated by Pratt & Whitney and are used as airborne test platforms to evaluate new and experimental engines long before those engines ever enter airline service.

Unlike standard 747s, which carry four operational engines, these test aircraft feature an additional engine mounted in a central position, attached to a reinforced pylon and stub wing structure beneath the fuselage. The mounting point allows engineers to expose a prototype engine to true operational conditions while keeping the main propulsion system unchanged.

During most flights, the extra engine remains inactive. It is carried purely as test hardware. Once the aircraft reaches a planned altitude and flight profile, engineers can start the test engine in midair and monitor its behavior in real time.

This setup allows manufacturers to evaluate performance during actual climb, cruise, descent and high-altitude conditions — something ground test facilities cannot fully replicate.

The aircraft themselves are dramatically different inside from a normal passenger 747.

Instead of rows of seats, the interior is filled with engineering workstations, instrument racks and monitoring consoles. Flight test engineers sit behind screens tracking vibration, temperatures, pressures, airflow, fuel systems and digital engine controls while the test engine operates just outside the fuselage.

Thousands of data channels are recorded during each flight.

The goal is not propulsion assistance. The aircraft’s primary four engines handle all routine flight operations. The fifth engine is strictly a flying laboratory, used to validate design performance, reliability, and safety before certification.

According to engineers familiar with the program, this approach offers a major advantage for engine manufacturers.