When on ground with APU bleed on and both engines off I have noticed a number of times that the pack flow isn't at the HIGH setting why?

As per the FCOM it states that when APU bleed is on the PACK FLOW automatically goes to the HIGH position regardless of the PACK FLOW selector position. But a number of times I have noticed on the BLEED SD it doesn't indicate HI even though both PACKS are on why? Is there any reason for this or some system particularly I don't understand?


From FCOM §Air Conditioning - Controls and Indicators:

The pack flow indication can be up to 30% below the actual flow rate.

That's why the needle may not be at the HI position. LO to HI is 80-120%, with normal being 100%.

enter image description here
The aforementioned needle (YouTube).

The sensor responsible for sending this data is the delta P (differential pressure) sensor at the inlet of the FCV (flow control valve). My guess is that it is not designed for a precise reading, but only an indicative reading.

  • $\begingroup$ Ahh okay i had read this line but there isn't a clear connection in the FCOM stating that this is the reason. One side it just says with single pack operation or with APU bleed supply the pack flow goes to HIGH regardless of the selectors position. Thank you for clarifying this point that this the reason 👍👌 $\endgroup$
    – Jai
    Feb 16 '18 at 10:09
  • $\begingroup$ @Jai - yes it runs in HI but may indicate lower than HI. $\endgroup$
    – ymb1
    Feb 16 '18 at 11:33

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