Auxiliary Power Unit (APU) bleed air is compressed air bled from the compressor section of the small turbine engine that sits in the aircraft tail. Because the APU can run while the main engines are shut down, this medium-to-high-pressure air lets the aircraft operate autonomously from any ground power unit, external air-conditioning unit or high-pressure air-start cart. On the ground the APU air-bleed system supplies limited airflow for cabin air-conditioning and pressurization as well as for starting the engines.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is APU bleed air in aircraft?
APU bleed air is compressed air taken from the compressor stage of a gas turbine engine upstream of its fuel-burning sections, which is medium to high pressure air bled from the compressor section of the APU. Bleed air extracted from either the Auxiliary Power Unit or another operating engine is used to power an air turbine starter motor.
APU bleed air is compressed air extracted from the compressor of the auxiliary power unit, a small gas turbine located typically in the tail cone of the aircraft. This air is conducted through a network of ducts, valves and regulators and is delivered at approximately 35-45 psi (2.41-3.10 bar) and 25-45 lb/min (11.34-20.41 kg/min) flow, values below those demanded by wing anti-ice systems. On the ground the bleed air is used to drive the air-turbine starter motor for main-engine start, to power the environmental control system packs for cabin air-conditioning, and to provide pressurization before the main engines are running. Once the engines are started the APU bleed air is isolated from the main engines. Thereafter it can remain available in flight as a backup pneumatic source for air-conditioning, pressurization or starter-assist during an inflight engine relight, thereby allowing the aircraft to operate autonomously from any ground support equipment.
