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Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Study notes and technical information Close up photos of internal and external components A compilation of links to major 737 news stories with a downloadable archive Illustrated history and description of all variants of 737 Detailed tech specs of every series of 737 Databases and reports of all the major 737 accidents & incidents General flightdeck views of each generation of 737's Description & news reports of Advanced Blended Winglets Press reports of orders and deliveries A collection of my favourite photographs that I have taken of or from the 737 Details about 737 production methods A compilation of links to other sites with useful 737 content History and Development of the Boeing 737 - MAX A quick concise overview of the pages on this site



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*** Updated 08 Jun 2017 ***

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737-3/500 Pneumatics Panel

See also Air Conditioning & Pressurisation


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



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. These now indicate that either a bleed has tripped off, OR there is a fault in the bleed air system OR that there is an incorrect bleed config after take-off or go-around.

737-400 Sidewall risers


The hydraulic reservoirs are pressurised to ensure a positive flow of fluid reaches the pumps. A from the left manifold and B from the right. See wheel-well fwd.


Schematic courtesy of Derek Watts

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


Max external air pressure:

60 psig

Max external air temp:

450F / 232C

One pack may be inoperative provided maximum altitude is: FL250


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