Hi,All checklists I've seen for commercial aircraft require ground power to be connected and switched ON before starting engines. I thought that ground power is only required if the APU has malfunctioned, or is otherwise inoperative? In the normal case, isn't it possible to start up using the battery to start the APU, and the APU to start the engines, without the application of ground power?By the way, does ground power provide both an electrical source and an air source?Thanks,Frank

Hello Frank, So far in my career I have not flown an airplane large enough to require ground power but I have worked the ramp for 4 years so I can shed a bit of light on the topic. Ground power is used when most aircraft come into the gate on turn arounds. It provides electrical power only to the aircraft. Aircraft can usually start the APU with the batteries but in order to take the load off the batteries ground power is normally used. On aircraft such as the Dash-8 battery starts are possible but a minimum voltage of 26V must be present. I had a captain show me how long it takes the battery to go from 28V to 26V with no external power and I can't see how a batt start would work. Beech 1900's and smaller turbines do not not need ground power to start but to save the wear and tear on the batteries they use ground power. Now if an aircraft such as a 767 has an out of service APU, ground power is needed to supply the electric's and an air start unit to provide the pneumatic pressure to spin up the engine. Once the one engine is started they can either start up the opposite side but in the case of the 767 the air start unit is normally in front of the #2 engine so it is best to disconnect and let them do a cross bleed start ;) I apologize for the lack of technical information but that's a few reasons why ground power is needed.CheersAdam V

Thank you, Adam! Your comments are very helpful. I understand better now :-)Frank

I just want to add that many airports (especially big ones) restrict the use of APU (e.g. "Normally the APU shall not be started earlier than 5MIN before estimated time for push-back...")And of course the APU consumes fuel, which means money. (BTW, does the airline have to pay the electrical/air bill? Or is there a flat-rate on this service?)Martin767 fetishistIt's a lot like life and that's what's appealing