737-800 N1 limit changes with pack / bleed switches

[Driver 170]

Hi,

When on the ground and with engines off and APU, if you turn either pack / bleed switch AUTO / ON this should decrease the FMS N1 by a set amount ie .8%

But i’ve also read its the ONLY the position of the APU BLEED SW will have the effect on the FMS N1%

Anyone care to share the Boeing AMM referance for this please?

[FlyingStone]

The FMC gets the signal from the pack flow valves, engine and APU bleed valves to calculate target N1 for the engines.

With engines shutdown, pack valves will be closed (electrically controlled, pneumatically operated), until some bleed air is introduced. Moving the APU bleed air switch to ON will provide air to both packs, hence target N1 value will decrease, to simulate the same condition when the engines are running with packs in auto.

[Driver 170]

Ah makes more sense now. Does there need to be duct pressure for this N1 decrease to happen?

[FlyingStone]

Yes. The pack valves won't open without pneumatic pressure, even if the switches are in AUTO position. FMC senses the position of the pack valves, not the pack switches.