Can someone explain what the HP ground unit is used for? And what damage happens to the bleed system if it is used?

  • $\begingroup$ Sure I'll do it here on out $\endgroup$
    – Jai
    Sep 25, 2017 at 7:10

1 Answer 1


The uses of the HP air of the pneumatic system are in the FCOM:

  • air conditioning
  • engine starting
  • wing anti-icing (see here)
  • water pressurization
  • hydraulic reservoir pressurization

The HP air sources are engine bleed, APU, and HP ground connection. The Bleed Monitoring Computers (BMC) do not recognize if the ground cart is connected or not.

The BMC of the engines can detect overpressure and overheat and act accordingly (check FCOM for logic schematic). However, the APU logic does not check for overpressure and overheat (see below).

Having APU and ground as dual HP sources can lead to overheat and/or overpressure of the pneumatic system.

Even when starting an engine with external pneumatic power, engine bleeds should be selected off (no need letting the BCM catch an error for you).

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  • $\begingroup$ Ohh great so when we start the engines using external pneumatic power(jet starter) if we don't switch of the engine bleeds the BMC will detect an overpressure or overheat? $\endgroup$
    – Jai
    Sep 1, 2017 at 6:34
  • $\begingroup$ @Jai - correct, due to the over supply of HP air. $\endgroup$
    – user14897
    Sep 1, 2017 at 7:36

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