737ng n1 change while putting APU online

[Patryk]

Hi there

I have been undergoing type rating training for 737. I have one question namely why while putting APU online, the FMC computed takeoff N1 value changes? Thanks in advance for yours replies guys.

[FlyingStone]

FMC takes the current bleed configuration into account when calculating N1 values. If the packs are running, it will give you bleeds ON N1 values, otherwise you get bleeds OFF values.

[Patryk]

Ok so correct me if I am wrong; when I have APU bleed switch ON, the FMC "thinks" that the packs are running and hence lower N1 takeoff value?

[RAT 5]

FMC bleeds ON/OFF N1% is taken from the pack/bleed switches. The packs do not need to be running. Thus it could be affected by the position of one of those switches at the time you fiddle with the APU.

[Patryk]

Thanks to You all for the responses. Surely it helped me a lot. Now I get it.
Thank You once again!