Hi Gards,
WRT ur question abt the 767. We handle an airline here in india tht operates a 767. The airline recommends to operate only 1 pack on ground and the reason same being TO SAVE FUEL.But if the airline wnts to save fuel why not shut of the APU itself and use the GPU and ground AIRCON. I did not see much diff in fuel consumption when only 1 pack was operating.
PSST: I am not a pro in maintenence,just started my career.