"If the take-off has to be achieved without air bleed fed from the engines for performance reasons, but air conditioning desired, the APU bleed may be used
with packs ON, thus maintaining engine performance level and passenger
comfort. In case of APU auto shut down during take-off, the engine thrust is
frozen till the thrust is manually reduced. The packs revert to engine bleed which causes an increase of EGT to keep N1/EPR".