Does air for the cabin come from the LP or HP compressor? Thanks

Depends on phase of flight. At low or idle thrust the HP bleed ps should open as LP bleed air will be insufficient.

Even simpler - bleed air is too hot. Packs cool it down.


Temperature and pressure from bleed air varies depending upon engine speed.


Packs can regulate the output air to an acceptable temperature and pressure.

Don't forget the pre cooler cools the air first and the pylon bleed valve (PRSOV) regulates the pressure.