Pneumatics

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Contents

Contents

• General
• Differences
• Schematic
• Limitations

All of the information, photographs & schematics from this website and much more is now available in a 374 page printed book or in electronic format.

*** Updated 05 Aug 2023 ***

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737-3/500 Pneumatics Panel

See also Air Conditioning & Pressurisation

General

The pneumatic system can be supplied by engines, APU or a ground source. The manifold is normally split by the isolation valve. With the isolation valve switch in AUTO, the isolation valve will only open when an engine bleed air or pack switch is selected OFF.

Air for engine starting, air conditioning packs, wing anti-ice and the hydraulic reservoirs comes from their respective ducts. Air for pressurisation of the water tank and the aspirated TAT probe come from the left pneumatic duct. External air for engine starting feeds into the right pneumatic duct. Ground conditioned air feeds directly into the mix manifold.

The minimum pneumatic duct pressure (with anti-ice off) for normal operation is 18psi.

On the Max, the pneumatic bleed air system now has an electronic controller. This allows the aircraft to digitally tune the amount of air that is needed in whatever flight regime you’re in. This is different to the previous "all or nothing" system which would often take more bleed air from the engines than necessary thereby reducing performance.

If engine bleed air temperature or pressure exceed limits, the BLEED TRIP OFF light will illuminate and the bleed valve will close. You may use the TRIP RESET switch after a short cooling period. If the BLEED TRIP OFF light does not extinguish, it may be due to an overpressure condition. Bleed trip off's are most common on full thrust, bleeds off, take-off's. The reason is excessive leakage past the closed hi stage valve butterfly which leads to a pressure build up at the downstream port on the overpressure switch within the hi stage regulator. The simple in-flight fix is to reduce duct pressure by selecting CLB-2 and/or using engine and/or wing anti-ice.

WING-BODY OVERHEAT indicates a leak in the corresponding bleed air duct. This is particularly serious if the leak is in the left hand side, as this includes the ducting to the APU. The wing-body overheat circuits may be tested by pressing the OVHT TEST switch; both wing-body overheat lights should illuminate after a minimum of 5 seconds. This test is part of the daily inspection.

737-400 Pneumatics Panel

Differences

1/200's - The PACK switches are simply OFF/ON, rather than OFF/AUTO/HIGH on all other series.

4/8/900's - Have two recirc fans for pax comfort and PACK warning lights instead of PACK TRIP OFF. See Air conditioning for an explanation. There are also two sidewall risers either side instead of one on all other series, this is why there appear to be two missing windows forward of the engine inlet.

MAX - BLEED TRIP OFF lights renamed BLEED. This is to indicate that either a bleed has tripped off due to excessive bleed air temperature, pressure or under-pressure, OR there is a failure in the bleed air system OR that there is an incorrect bleed config after take-off or go-around.

The MAX pneumatic bleed air system now has an electronic controller. This allows the aircraft to digitally tune the amount of air that is needed in whatever flight regime you’re in. This is different to the previous "all or nothing" system which would often take more bleed air from the engines than necessary thereby reducing performance.

One of the faults detected by the system is an underpressure. If the duct pressure drops below 13psi with engines running, the controller closes the suspect valve to protect the affected system.

Note that the system conducts a series of self tests after landing known as Post-Flight Built In Tests (PFBIT). This is a controller initiated, non-interactive test sequence that checks the capability of the PRSOV solenoid to close the valve, independent of the torque motor. These can be observed as duct pressure splits and are normal on the MAX.

737-400 Sidewall risers

Schematic

Schematic courtesy of Derek Watts

Click here to see a larger air conditioning & pneumatics schematic diagram for the 300/500 or 400.