Auxiliary Power Unit (APU) (APU)

What Is an APU?

An Auxiliary Power Unit (APU) is a small gas turbine engine installed in the tailcone of most commercial aircraft. Unlike the main propulsion engines, the APU is not used for thrust; instead, it generates electrical power and compressed air (bleed air) to operate aircraft systems — air conditioning, avionics, and engine starting — independently of the main engines and without relying on ground support equipment.

Function and Purpose

The APU fulfils three core functions. First, it provides electrical power: the APU drives generators that supply 115V AC power (typically 90–120 kVA) to the entire aircraft electrical bus when main engines are shut down at the gate, allowing cabin lighting, galley equipment, entertainment systems, and avionics to operate normally during boarding and deplaning.

Second, it supplies bleed air — high-pressure compressed air extracted from the APU compressor — to the aircraft's pneumatic system. This air powers the air conditioning packs that cool or heat the cabin before engine start and, critically, drives the main engine pneumatic starters, spinning the high-pressure spool up to ignition speed.

Third, the APU serves as an emergency backup: if both main engine generators fail in flight, the APU can be started to restore electrical power. On the Boeing 787, which uses an all-electric architecture (no bleed air from engines), the APU still provides electrical power but does not supply pneumatic bleed air to the cabin systems.

Types and Variations

• Standard bleed-air APU: Used on the vast majority of commercial jets (Boeing 737, 747, 777; Airbus A320, A330, A340). Supplies both electrical and pneumatic power.
• No-bleed APU (Boeing 787): The 787's APU (Honeywell HGT1700) provides only electrical power; the all-electric architecture uses electric compressors for cabin pressurization and engine starting motors instead of bleed air.
• Integrated drive generator (IDG) APU: Some regional jets integrate the APU function more closely with the main electrical system architecture.

Notable Examples

The Honeywell 131-9(A) APU powers the Boeing 737 Classic and Next Generation families, producing 90 kVA of electrical power and sufficient bleed air to start both CFM56 engines. The Pratt & Whitney Canada APS3200 serves the Airbus A320 family. The Boeing 787's APU is housed in the composite tailcone of the empennage, accessed through a distinctive louvered exhaust vent visible on the tail.

Related Components

The APU's bleed air output feeds directly into the same pneumatic manifolds used by the main engines, making it a transparent substitute for ground air carts. The APU exhaust exits through the tail of the aircraft via the empennage tailcone structure. The APU is automatically governed by a dedicated control unit and can be started and stopped from the overhead panel of the cockpit, or in some cases remotely from the ground via maintenance interfaces. Paired with turbofan engine starters, the APU forms a self-sufficient system that eliminates the need for ground starting equipment.