Boeing-747.comIndependent history, variants & specifications

The small gas turbine that keeps the big one flying

The 747's APU is a gas-turbine generator in the tail cone running while main engines are off. The 747-400 replaced the classic unit with the far more economical Pratt & Whitney Canada PW901A (two 90 kVA generators); the 747-8's PW901C ups that to two 120 kVA units.

By , Boeing 747 EnthusiastPublished Reviewed

While the 747 sits at the gate with engines shut down, a small gas turbine in the tail cone - the auxiliary power unit - drives bleed air for cabin conditioning and engine starts, and powers generators for the electrical system. It can also supply emergency backup power in flight.

Key facts
747-400 APUPratt & Whitney Canada PW901A
747-8 APUPW901C (derived from the PW901A)
Electrical output747-400: two 90 kVA generators; 747-8: two 120 kVA generators
RoleBleed air for cabin conditioning and engine start; ground and in-flight electrical backup

The -400's PW901A was a new APU for the Series 400 with roughly 40 percent lower fuel burn than the unit it replaced. Its load compressor delivers bleed air for cabin air conditioning and main-engine starting, while the gearbox-mounted generators supply electrical power.

The 747-8's PW901C keeps the same design but doubles the electrical rating to two 120 kVA generators. Compared with its predecessor, the 1970s-era classic 747 APU burned much more fuel for the same output.

A ground utility that became an in-flight system

The APU is usually described as a ground device, and on a modern airliner that is mostly true. But the 747's APU also matters in flight, and the reason is that it is the only independent source of both bleed air and electrical power once the engines are running. On one engine inoperative, or after a shutdown, the APU can carry the aircraft's utilities - cabin conditioning, and the electrical load that keeps the flight deck and essential systems alive - until it lands.

The -400 and -8 units

The 747-400's PW901A was a new auxiliary power unit for the Series 400, roughly 40 percent lower in fuel burn than the classic unit it replaced while producing more useful output. Its load compressor supplies bleed air for cabin air conditioning and main-engine starting, and its gearbox-mounted generators supply electrical power.

The 747-8's PW901C keeps essentially the same design but raises the electrical output to two 120 kVA generators, against the -400's two 90 kVA units. That increase is one of the clearest systems-level statements of what changed between the two types: more electrical capability, on a still-conventional pneumatic airframe.

Frequently asked questions

What does the APU do?
It supplies bleed air for cabin conditioning and engine start, and electrical power, while the engines are off.
Which APU is on the 747-400?
The Pratt & Whitney Canada PW901A; the 747-8 uses the uprated PW901C.

Sources

Facts on this page are checked against the primary and institutional references below.

  1. Boeing 747-400 guide - Aircraft Commerce
  2. Commercial auxiliary power units - Pratt & Whitney
  3. PW901A airworthiness directive - US Federal Register

Related topics

More in Aircraft Systems