Originally Posted by barit1
Perhaps a 747 classic pilot can confirm my memory:
The baseline TO performance in the Boeing AFM is "APU ON", meaning that the few Newtons worth of APU thrust is tallied in with the four real engines for TO and climb. And if APU is inop, a small TOGW penalty is taken.
Or - am I losing it?
You sure it's not because with the APU on, the ECS bleed is not coming off the engines, thus they produce more thrust?