How It Works: Bleed Air

Engine bleed air has a variety of uses on modern aircraft.

For decades, aircraft have used engine bleed air for a variety of purposes, spanning everything from engine starting to cabin pressurization to anti-icing. Here’s how engine bleed air works.

Hot, High-Pressure Air

When air enters a turbine engine, it goes through a series of compressors, which significantly increase the air temperature and pressure before mixing that air with fuel and igniting it. A small portion of that compressed air, however, does not enter the combustion chamber and instead is redirected from the engine via valves, ducting and manifolds to various other areas of the aircraft. This engine bleed air is very hot, between 200 to 250 degrees C, and very high in pressure, around 40 psi.

Cabin Pressurization and Air Conditioning

Because the air at high altitudes is too thin to meet human oxygen needs, engine bleed air is used to provide ­appropriate cabin pressurization as well as air ­conditioning. After leaving the engine and passing through the ­air-conditioning pack, where it is cooled, this bleed air is combined with recirculated cabin air before it enters the cabin.


High-temperature bleed air is routed to the leading edges of the wing and empennage, as well as to key engine components such as the inlet guide, where it serves as a safeguard against ice accumulation.

Water and Hydraulic System Pressurization

Engine bleed air is used to provide the pressurization needed for the water and hydraulic system reservoirs. Such pressurization propels drinking water from the holding tank to the cabin and ensures the smooth flow of hydraulic fluid to the pump inlet in the absence of sufficient atmospheric pressure at high altitudes.

Engine Start

High-pressure bleed air from the auxiliary power unit, a non-thrust-producing engine often located in the rear of the aircraft, provides the pneumatic energy required to start the blade rotation in a main engine.