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On the A321neo, the QRH has a procedure in the case of a ACT (Additional Center Tank) System Fault (Found in the QRH under Abnormal and Emergency Procedures, Fuel).

When such a fault occurs, you must land within the stated time in the following table, depending on the number of refueled ACTs at dispatch. My understanding is that ACTs are refueled in the following order: AFT1, AFT2 and FWD. They are then used, in flight, in the reverse order: FWD, AFT2 and AFT1.

According to the table, if you have refueled just 1 ACT (which implies you only refueled AFT1), you need to land within 120 minutes of the fault occurring. If you refueled all 3 ACTs, and AFT1 ACT XFR fails (which implies FWD and AFT2 ACTs are empty, due to the order of fuel consumption), you need to land within 200 minutes of the fault occurring.

Where does this difference of 80 minutes come from? In both scenarios, the only ACT with fuel in it would be AFT1. So why would you be given more time to land the aircraft in the latter scenario?

QRH ACT System Fault table

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  • $\begingroup$ If AFT1 ACT XFR fails, it does not mean that also AFT2 and FWD XFR are not working, indeed you can still use them in manual mode (even if not in the normal order). However, I don't think that the flight time depends on the fuel quantity itself, because the procedure requires you to use only wing tanks (so the fact that AFT2 and FWD are still usable are not relevant for the flight time). $\endgroup$
    – ocirocir
    Commented Aug 13, 2023 at 5:51

1 Answer 1

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The reason for that flight time is due to the CG limitations.

I don't have data from the 3 ACTs, but I found this revision of the AFM document of the A321 with 2 ACTs from FAA regarding this (link). Shortly, the flight time roughly keeps the ZFCG (Zero Fuel Center of Gravity) inside the allowed region. This region depends on how many ACTs are not empty.

ACT1 refueled

ACT2 refueled

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