Engine bleed air: a primer

Use of Bleed Air in Aircraft Pneumatic Systems: A Primer

(taken from Chapter 6 on Pneumatic Systems from the 3^rd Edition of the book “Aircraft Systems” by Ian Moir and Allan Seabridge)

The use of aircraft engines as a source of high pressure, high temperature air can be understood by examining the characteristics of the turbofan engine.  Modern engines “bypass” a significant portion of the mass flow past the engine and increasingly a small portion of the mass flow passes through the engine core or gas generation section.  The ratio of bypass air to engine core air is called the bypass ratio and this can easily exceed 10:1 for the very latest civil engines; much higher than the 4 or 5:1 ratio for the previous generation.

The characteristics of a modern turbofan engine are shown in figure 6.1.  This shows the pressure (in psi) and the temperature (in degree centigrade) at various points throughout the engine for three conditions: ground idle, take off power and in the cruise condition.

It can be seen that in the least stressful condition – ground idle – the engine is in a state of equilibrium but that even at this low level the compressor air pressure is 50 psi and the temperature is 180 ^oC.  At take-off conditions the compressed air soars to 410 psi / 540 ^oC.  In the cruise condition the compressor air is at 150 psi / 400 ^oC.  The engine is therefore a source of high pressure and high temperature air that can be “bled” for the engine to perform various functions around the aircraft.  The fact that there are such considerable variations in air pressure and temperature for various engine conditions places an imposing control task upon the pneumatic system.  Also the variations in engine characteristics between similarly rated engines of different manufacturers poses additional design constraints.  Some aircraft such as the Boeing 777 offer three engine choices, Pratt & Whitney, General Electric and Rolls-Royce, and each of these engines has to be separately matched to the aircraft systems, the loads of which may differ as a result of operator specified configurations.

As well as the main aircraft engines the Auxiliary Power Unit (APU) is also a source of high pressure bleed air.  The APU is in itself a small turbojet engine, designed more from the viewpoint of an energy and power generator than a thrust provider which is the case for main engines.  The APU is primarily designed to provide electrical and pneumatic power by a shaft driven generator and compressor.  The APU is therefore able to provide an independent source of electrical power and compressed air while the aircraft is on the ground, although it can be used as a backup provider of power while airborne.  Some aircraft designs are actively considering the use of in-flight operable APUs to assist in in-flight engine re-lighting and to relieve the engines of offtake load in certain areas of the flight envelope.

It is also usual for the aircraft to be designed to accept high pressure air from a ground power cart, for aircraft engine starting.

The three sources of pneumatic power provide the muscle or means by which the pneumatic is able to satisfy the aircraft demands.  In a simplified form the pneumatic system may be represented by the interrelationships shown in figure 6.2 below:

This simplified drawing – the ground air power source is omitted – shows how the aircraft High Pressure (HP) air sources provide bleed air which forms the primary source for the three major aircraft air related systems:

·      Ice protection: the provision of hot air to provide anti icing of engine nacelles and the wing, tailplane or fin leading edges; or to dislodge ice that has formed on the surfaces

·      ECS and cooling: the provision of the main air source for environmental temperature control and cooling

·      Pressurization: the provision of a means by which the aircraft may be pressurized, giving the crew and passengers a more comfortable operating environment.

A simplified representation of this relationship is shown in figure 6.3.  This example shows a twin-engine configuration typical of many business jets and regional jet transport aircraft.

Bleed air from the engines is passed through a Pressure-Reducing Shut-Off Valve (PRSOV) which serves the function of controlling and, when required, shutting off the engine bleed air supply.  Air downstream of the PRSOV may be used in a number of ways:

·      By means of a cross flow Shut-Off Valve (SOV) the system may supply air to the opposite side of the aircraft during engine start or if the opposite engine is inoperative for any reason

·      A SOV from the APU may be used to isolate the APU air supply

·      SOVs provide isolation as appropriate to the left and right air conditioning packs and pressurization systems

·      Additional SOVs provide the means by which the supply to left and right wing anti-icing systems may be shut off in the event that these functions are not required

This is a simplified model of the use of engine bleed air in pneumatic systems (ATA Chapter 36).  A more comprehensive list of those aircraft systems with which bleed air is associated are listed as follows  with the accompanying civil ATA chapter classification:

·      Air conditioning (ATA Chapter 21)

·      Cargo compartment heating (ATA Chapter 21)

·      Wing and engine anti-icing (ATA Chapter 30)

·      Engine start (ATA Chapter 80)

·      Thrust reverser (ATA Chapter 78)

·      Hydraulic reservoir pressurization (ATA Chapter 29)

·      Rain repellent nozzles – aircraft windscreen (ATA Chapter 30)

·      Water tank pressurization and toilet waste (ATA Chapter 38)

·      Air driven hydraulic pump (ADP) (ATA Chapter 29)