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Difference in N1 values at EFB and FMC

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    Difference in N1 values at EFB and FMC


    this thing has been bugging me for a long time, so I have decided to ask here. It's a very small nuisance.

    When I calculate my takeoff performance using the EFB, and I get a certain Assumed temperature value and it's correlating N1 value, let's say 83% and assumed temperature 40 degrees with TO-2 (I am just guessing these values), and then I set TO-2 at the N1 LIMIT page of the FMC and then insert the assumed temperature of 40 degrees, FMC will say that the takeoff N1 is 85% for example.

    So to sum it up, I always get a slightly higher N1 in the FMC than EFB tells it should be. Am I doing something wrong, or is this a known issue?

    Thanks so much!
    Jan Podlipsky

    Check if you have PACKs or ENG BLEED switches off (PACKs are off with no air even if switches are on), that would bring about 0.7~1.0% of difference IRL.

    BTW PMDG still not get it completly corrct, it should be higher when PACK, or when ENG BLEED switches off, and it's also left to left and right to right, butit seems on PMDG, it dropes once there are bleed air and ENG BLEED switched are ON(even when it was came from APU or ground with engines off), and when PACKs are switched off. It's OK when performing both non-bleed and non-pressuized procedure, but it's just wrong systemwise
    Last edited by AngelofAttack; 22Nov2020, 11:23.
    ZHU Hai
    B737 Ground instructor


    • Dzosef
      Dzosef commented
      Editing a comment
      APU bleed switch position affects fmc n1.

    • Emi
      Emi commented
      Editing a comment
      Matthew, in fact it's whether you have pressure in the system or not, not just the APU Bleed switch.

    Check fmc n1/efb just before takeoff and see if there's still a difference.
    If you configured your aicraft correctly then there should be none or +/-0.1%
    Matthew Chalupniczak