Boeing 747History, variants & specifications

The 747 as a flying testbed

Engine makers fly new engines on converted 747s, because three known engines can bring the aircraft home if the fourth misbehaves.

Aerial view of a Boeing 747 engine testbed aircraft parked outside a Rolls-Royce facility, with crowds of people around it
A 747 testbed outside Rolls-Royce. One position carries the engine under test; three known engines fly the aeroplane home.

Every major engine manufacturer has flown new engines on a converted 747, and the reason is arithmetic rather than sentiment.

Why four engines

A new engine has to be flown before it can be certified, and early in a test programme it may stop without warning.

On a twin, losing the test engine means losing half the aircraft’s thrust. On a four-engined aircraft it means losing a quarter — and the three remaining engines are proven units that will bring the aeroplane home. The test engine can be shut down, restarted, stalled deliberately, run to its limits and generally abused, without the aircraft ever being in danger.

That is why the flying testbed of choice has been a 747 for decades, long after airlines stopped buying them.

What gets carried

The test engine is usually hung on the number 2 position, inboard on the port wing, where it is easiest to instrument and observe. Engines far larger than anything the 747 was designed for have flown there — the GE90, which is wider in diameter than a 737’s fuselage, was flown on a 747 during its development.

The aircraft themselves are heavily modified: extra instrumentation, data recording, engineering stations in the cabin, and reinforced structure at the test pylon.

Who flies them

General Electric has operated 747 testbeds for many years, including work on the GE90 and the GEnx — the family the 747-8’s own GEnx-2B comes from.

Rolls-Royce has used a 747 for Trent development.

Pratt & Whitney likewise flew engines on a 747, having supplied the JT9D that powered the original aircraft.

There is a neatness to it: the 747-8’s engine was partly developed on an earlier 747, and engines for the twins that replaced the 747 were tested on 747s too.

The fifth pod is a different thing

Do not confuse a testbed with the fifth-pod ferry configuration. Every 747 has a hardpoint between the fuselage and the inboard engine for carrying a spare engine, inert, to an aircraft stranded without one. That engine is not running and usually has its fan removed.

A testbed engine is live, instrumented, and doing the work.

A long afterlife

Testbeds are among the last 747s that will fly. They are low-cycle aircraft, meticulously maintained, and there is no obvious replacement — a twin cannot do the job safely, and nobody is building four-engined airliners any more.

More in Special Boeing 747s