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I had come across this statement will reading the limitations chapter in the FCOM. But once again there is no reasoning given for it I have also read the air conditioning systems chapter but can't seem to find an explanation.

Can someone explain the reasons?

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Latest FCOM, after revision 24 of section 3.01.21, contain the explanation:

AIR CONDITIONING WITH LP GROUND UNIT

  • Do not use conditioned air simultaneously from packs and LP ground unit (to avoid chattering of the non return valves).

The non-return valves prevent air in the mixer unit to return into any source in the case this source delivers air at a lower pressure, or doesn't deliver air. Valves are the one-way separation between the air generation section and the air distribution section:

enter image description here Source

If the LP ground connection is used while the packs are ON, pressure will be comparable on both sides of the valves. Valves can start chattering instead of being stable in the open position. While pressures compete, air from the mixer unit can return to the packs.

Damage can occur, as described in this 1991 incident (air conditioning duct and non-return valve damaged).

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  • $\begingroup$ "In some FCOM" or "In the same FCOM"? I'm not sure if your quote & picture are specific to the A320 or are generic. (Also, please link sources! :) $\endgroup$
    – FreeMan
    Commented Sep 11, 2017 at 19:09
  • $\begingroup$ More than good enough for me, sir! You might want to note that in the answer, just in case someone gets carried away with comment clean up. $\endgroup$
    – FreeMan
    Commented Sep 11, 2017 at 19:16

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