Pneumatics (A320)

These notes cover the A320 pneumatic system, the part of the aircraft that supplies high pressure air. They were written as questions and answers for crews and engineers. Your FCOM is always the final reference if anything conflicts.

What it does and where the air comes from

The pneumatic system supplies high pressure air for air conditioning, engine starting, wing anti icing, water pressurisation and hydraulic reservoir pressurisation. The air can come from three places: the engine bleed systems, the APU load compressor, or a high pressure ground connection. The ground connection is on the left side of the cross bleed valve. External high pressure air may be used for air conditioning.

Engine bleed valves

The engine bleed valve keeps delivery pressure at 45 plus or minus 5 psi. In climb and cruise the air normally comes from the intermediate pressure stage of the engine high pressure compressor, which saves fuel. At low engine speed the intermediate air is too weak and too cool, so the system takes air from the high pressure stage and holds it at 36 plus or minus 4 psi.

The bleed valve closes by itself in two ways. It closes pneumatically if the upstream pressure falls below 8 psi or if there is a return flow. It closes electrically when:

• the BLEED pushbutton is switched off,
• the ENG FIRE pushbutton is pushed, or
• the bleed air monitoring computer (BMC) sees over temperature, over pressure, a leak, an open starter valve, or APU bleed being on.

When the APU bleed valve is selected ON, the engine bleed valves close automatically. If the cross bleed valve is shut and you select APU bleed on, the number 2 engine bleed valve stays open.

Leaks and the BMC

Leak detection loops watch for overheating near the hot air ducts in the fuselage, the pylons and the wings. A wing leak signal on the left side closes the bleed valve on that side, lights the FAULT light on the AIR COND panel, closes the cross bleed valve (except during engine start) and closes the APU bleed valve if it is open (except during engine start). On the right side the effects are the same, except that the APU bleed valve is not closed. The crew also see a FAULT light on the matching ENG BLEED pushbutton on the overhead panel and an ECAM warning. After a wing or engine bleed leak, the crew should avoid icing conditions for the rest of the flight.

If BMC 1 fails, the FAULT light on the AIR COND panel is lost and the bleed valve no longer closes automatically. The engine bleed leak warning for that engine is lost, and so is the APU bleed leak warning. If BMC 2 fails, the same two things are lost for its engine, but nothing about the APU. A reader later pointed out that leak detection is disregarded during an engine start, which matches the FCOM, so the original answer of "no" was wrong on that point.

Fault lights and warnings

The ENG 1 BLEED FAULT light comes on with an overpressure downstream of the bleed valve, a bleed air overheat, a wing or engine leak on that side, a bleed valve that is not closed during engine start, or a bleed valve that is not closed with APU bleed on. The APU BLEED FAULT light is amber, and an ECAM caution appears, when the system detects an APU leak.

If the cross bleed valve disagrees with its selected position, or fails to open, or fails to close, the warnings are the same: a single chime, the master caution light, and an amber cross bleed valve symbol on the BLEED system display page. The cross bleed selector has three positions: AUTO, OPEN and SHUT. The BLEED page shows the high pressure bleed valve, the engine bleed valve and the wing anti ice valve. The engine number on the BLEED page turns amber when the engine N2 is below idle.

Procedures

After an ECAM AIR ENG 1 BLEED FAULT caution, the bleed should close by itself. If it does not, the crew switch off the matching engine bleed switch. With that fault and wing anti ice on, only one pack may be used, because one pack must be closed for pre cooler performance when wing anti ice is in use.

The AIR DUAL BLEED FAULT checklist sends the crew to different altitudes, FL100 in one case and FL200 in another. The difference is whether APU bleed can feed the pressurisation system, which keeps the cabin altitude within limits. This works because the APU bleed source sits on the left of the cross bleed valve. The maximum altitude for APU bleed use is FL200.

Part of the A320 systems series. See also Airbus Tech.