Bleed Air and Pneumatic Systems

Bleed air is compressed air taken, or bled, from the compressor of a gas turbine engine before it reaches the combustion chamber. In a jet transport it feeds the pneumatic system, a network of ducts and valves that supplies the air conditioning packs and with them cabin pressurisation, thermal anti-icing, engine starting, and the pressurisation of the hydraulic reservoirs and the potable water tank. The same ducts carry air from the auxiliary power unit (APU) and from a ground air cart.

Air leaving a compressor is hot, and its pressure varies widely with engine power. The system must therefore regulate it, cool it, share it between the two sides of the aircraft and contain it if a duct fails. Each of these jobs has its own valve, protection and failure mode, which is why the bleed panel features so often in type rating checks and in the EASA ATPL airframe and systems syllabus. Bleed air is also not free: whatever the compressor delivers to the pneumatic system is lost to the engine's thrust.

Pneumatic system overview

A modern transport aircraft uses a low-pressure, high-volume pneumatic system. ATPL texts give 25 to 50 psi as a typical manifold pressure, far below the 3,000 psi or more of a hydraulic system. A few older aircraft, the Fokker F27 being the classic example, used a high-pressure pneumatic system for the landing gear, brakes and flaps; on current types these services are hydraulic.

The A320 FCOM names three high-pressure air sources: the engine bleed systems, the APU load compressor and the HP ground connection, through which a ground air start unit blows air into the ducts. A crossbleed duct joins the two engine bleed systems and also receives the APU and ground air. Typical consumers are:

• the air conditioning packs, which also supply the air that pressurises the cabin;
• wing anti-ice and, on most types, engine intake anti-ice (see thermal anti-icing);
• the air turbine starters (see gas turbine engine starting);
• hydraulic reservoir and potable water tank pressurisation;
• on certain 737 variants, the aspirated TAT probe and the nitrogen generation system.

ATPL texts add air turbine motors for flap drives and thrust reversers, pneumatic rams, and cargo heating. The details vary by type. The A320 takes engine anti-ice air independently of the engine bleed valve, so intake anti-ice remains available with that engine's bleed selected off, while the E190-E2's hydraulic reservoirs are not bleed consumers at all.

Each engine normally feeds its own side. On the A320, engine 1 supplies pack 1 through the left duct and engine 2 supplies pack 2, with the crossbleed valve closed; on the 737 NG the left pack likewise uses engine 1 air. Keeping the sides apart confines a leak or a failed bleed to one side.

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Low- and high-stage bleed ports

The pressure at any one compressor stage rises and falls with engine speed. A port that gives enough pressure at idle would deliver far too much at take-off power, and one sized for cruise would give too little at idle. Most turbofans therefore have two bleed ports:

• a low-stage or intermediate pressure (IP) port, used whenever the engine produces useful thrust, because air that has been compressed less costs less fuel;
• a high-stage or high pressure (HP) port, opened only when the low stage cannot supply enough pressure: at low power, in an idle descent and on the ground.

ATPL texts speak of the LP and HP compressor bleed stages; on the A320's CFM56 both ports lie on the high-pressure compressor, hence Airbus's term intermediate pressure.

The valve on the high-stage port is the high pressure bleed valve (HPBV) or HP valve to Airbus, the high stage valve to Boeing and the high pressure shut-off valve (HPSOV) in ATPL texts. It is pneumatically operated. On the A320 it regulates HP air to 36 ± 4 psi and closes pneumatically when the upstream pressure is too low to be useful or too high. On the 737 NG the high stage valve modulates open when 5th-stage air is insufficient and stays closed during take-off, climb and most cruise conditions. In ATPL texts the HPSOV opens slowly, to avoid a pressure surge, and closes quickly, so that an engine fire cannot drive fumes into the cabin; it is also scheduled open when airframe anti-icing is selected, because the higher stage delivers hotter air.

When HP air is flowing, a check valve, the non-return valve of ATPL texts or the A320's IP check valve, stops it flowing back into the low stage.

Bleed valves and pressure regulation

Downstream of the ports, each engine has a bleed valve that both regulates pressure and shuts off the supply. ATPL texts call it the bleed air control valve: the point where the engine's own bleed circuit ends and the aircraft's pneumatic manifold begins. Manufacturers use other names: pressure regulating and shutoff valve (PRSOV), pressure regulating valve (PRV), engine bleed air valve or simply engine bleed valve.

On the A320 the bleed valve is pneumatically operated and electrically controlled by its side's bleed monitoring computer (BMC). It regulates delivery pressure to 45 ± 5 psi at the precooler inlet; fluctuations between 38 and 56 psi are normal at high power up to FL100. It closes pneumatically if the upstream pressure falls below 8 psi or the flow reverses, and the BMC closes it electrically for an overtemperature, an overpressure, a leak, an open starter valve or APU bleed in use. It also closes with the ENG BLEED pushbutton OFF or the ENG FIRE pushbutton pushed.

On the 737 NG the engine bleed air valve acts as a pressure regulator and shutoff valve; with the BLEED switch ON it is DC activated and pressure operated. Pulling an engine fire switch closes it, which removes wing anti-ice on that side and closes the bleed-air-operated pack valve.

Further downstream, each pack has its own flow control valve, which holds the mass flow to the pack constant whatever the duct pressure (see air conditioning packs).

Overpressure and overheat protection

A valve that sticks open at high power can over-pressurise the ducts. ATPL texts describe layered protection: a pressure relief valve on the engine bleed duct; then, if the overpressure persists, a sensor that bleeds off the HPSOV's operating pressure and forces it closed; and a warning light. The crew can then select that bleed off and open the isolation valve to supply the side from the other engine. Against overheat, an electrical temperature switch downstream of the bleed air control valve closes the valve if the air becomes too hot.

The A320 also has an overpressure valve (OPV), which closes if pressure regulation fails and the pressure exceeds 85 psi. Before that is reached, the BMC triggers ENG BLEED FAULT at 57 psi. The 737 NG combines both protections in its amber BLEED TRIP OFF light: excessive bleed temperature or pressure closes that engine's bleed air valve, which stays closed until the fault has cleared and the TRIP RESET switch is pressed.

Precooler and fan air valve

Air from the high stage can be at several hundred degrees Celsius, too hot to send through the aircraft's ducts. The precooler is an air-to-air heat exchanger that cools the bleed air with cold air taken from the engine's fan stream. The fan air valve (FAV) meters that fan air and so sets the temperature; on the A320 it regulates the precooler outlet to about 200 °C, and a spring holds it closed when there is no air pressure. The bleed valve sets the pressure upstream; the precooler removes only heat.

The A320's BMC treats a precooler outlet temperature of 257 °C for 55 seconds, 270 °C for 15 seconds or 290 °C for 5 seconds as an overheat: the bleed valve closes and ENG BLEED FAULT is shown. At the other end of the scale, an ENG BLEED LO TEMP caution appears if the temperature falls below 150 °C with wing anti-ice on, because the air may then be too cool to protect the wing.

Crossbleed valve and duct

Between the two sides lies the crossbleed duct, with a crossbleed valve, called the isolation valve on Boeing aircraft and in ATPL texts. Closed, it keeps the sides independent; open, it lets one source supply both: APU air for both packs, a running engine's air for a crossbleed start, or one engine's air after the other bleed has failed.

• A320: a single crossbleed valve with two electric motors, one for automatic and one for manual control. With the X BLEED selector at AUTO it opens when the APU bleed valve is open, closes otherwise, and closes automatically on a leak signal except during an engine start. Selecting APU BLEED ON opens the crossbleed valve and closes the engine bleed valves, so APU air reaches both packs. After the loss of one engine bleed, X BLEED is selected OPEN so that the other engine supplies both packs.
• 737 NG: the AC-operated isolation valve, in AUTO, closes when both engine BLEED switches are ON and both PACK switches are in AUTO or HIGH, and opens if any of them is OFF; the APU BLEED switch does not affect it. The APU feeds the left side. If APU air and engine bleed air can meet, the amber DUAL BLEED light comes on and thrust must be limited to idle; with both supplies open at idle, APU air can also back-pressure the engine's 9th-stage valve and close it. Boeing's guidance is not to operate more than one pack from one engine.
• E190-E2: the crossbleed valve opens automatically when one side has pressure and the other has no bleed air.

APU bleed has its own altitude limits (see auxiliary power unit).

Bleed leak detection and duct overheat

Air escaping from a ruptured duct is hot enough to damage composite structure, wiring insulation, fuel and hydraulic lines and control cables, which is why ducts are insulated and routed clear of them. Bleed leak detection uses sensing elements along the duct routes in the pylons, wing leading edges and fuselage, which detect the heat of escaping air and signal a duct overheat or bleed leak. The response is always to isolate the leaking section by closing the valves that feed it (see also fire and overheat detection).

• A320: the BMCs watch sensing loops, a single loop in each pylon and along the APU duct and a double loop, A and B, in each wing. A wing leak is declared when both loops detect one, or one does while the other is inoperative. The thresholds are 124 °C for the wings and the APU duct and 204 °C for the pylons, where the engine must also be running. A wing leak closes that side's bleed valve, lights the FAULT light, closes the crossbleed valve except during an engine start, and closes the APU bleed valve if the leak is in the left wing. Leak detection is disregarded during an engine start.
• 737 NG: the left WING-BODY OVERHEAT light covers the left engine strut, left inboard wing leading edge, left air conditioning bay, keel beam and APU bleed duct; the right light covers the right strut, inboard leading edge and air conditioning bay.
• E190-E2: BLEED 1 (2) LEAK is a master caution above 125 °C, and the BLEED button may be cycled once the temperature is below 80 °C.

Bleed air and engine performance

Bled air no longer passes through the combustion chamber and turbine, so to keep the thrust the fuel control adds fuel: ATPL texts list a rise in rpm, exhaust gas temperature and specific fuel consumption, with a fall in EPR. Performance data are given for each bleed configuration, and some take-offs are made with the packs off or on APU air (see air conditioning packs).

Fume events and contaminated bleed air

Bleed air comes straight from the compressor, so anything that enters the compressor air can reach the cabin. The best-known case is engine oil leaking past a degraded bearing seal, which can bring a chemical smell and eye irritation to the cabin: a fume event, or bleed air contamination. De-icing fluid and engine exhaust can contaminate the supply in the same way. ATPL texts therefore close the air conditioning bleeds from the engines and APU during ground de-icing, and the A320's parking procedure selects APU BLEED on just before engine shutdown to keep exhaust fumes out of the air conditioning.

In flight, the crew protect themselves first, with oxygen masks at 100 per cent, then identify and isolate the source, typically the affected engine bleed and its pack, and divert if the fumes persist. The contaminants and their effects are covered in carbon monoxide and cabin air hazards.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.