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B737 MAX/NG APU Bleed: Packs and recirculation fans airflow

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B737 MAX/NG APU Bleed: Packs and recirculation fans airflow

Old 10th May 2023, 07:10
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B737 MAX/NG APU Bleed: Packs and recirculation fans airflow

Hi everyone, I have a question concerning the amount of airflow available in the cabin with APU air bleed on ground.
We have a MAX8 in for completion and during some air conditioning testing we noticed that we have more air conditioning airflow when we have only Left Pack ON than when we have both packs ON. This is confirmed by airflow measurements in the main ducting and also by the different noise coming from the air outlet.

On the MAX we can see on the MFD that on ground with APU bleed both packs always operate in HI FLOW.
The total quantity of airflow coming from the APU air bleed is always the same in both configurations: 95 lb/min from L pack when R pack is off and 50+50lb/min coming from each pack when both are on. So this difference of airflow is not coming from the packs nor the APU air bleed.

Can someone please confirm that this airflow difference comes from the recirculation fans? I've read on a 737-800NG AFM that on ground with one pack in high flow both recirculation fans are on, but with both packs in high flow the Left recirculation fan goes off. Is it the same for the MAX8?

Is there any reference to this fans logic on the AMM or any other tech doc?

Thank you in advance
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Old 12th May 2023, 14:28
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For anyone interested in the subject, I found the confirmation I was looking for in the wiring diagrams.
So it's confirmed, on the MAX8 at least, that when on ground with APU air bleed on, if you switch on both packs (auto or high it doesn't matter as they are always in high mode with apu bleed) the left circulation fan goes automatically off.
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