When is power changed from external power to the APU?

Question

is the APU just started secondes before push-back, to then allow engines to start, or do you start it quite before, like 15 min. before push-back?

Answer 1

15 to 20 minutes prior to pushback, generally. It all depends upon company procedure, how long the airplane has been, or will sit, availability of ground power, or other factors.

Just make sure that it's up and supplying power before the ground guys pull the electrical power from the plane.

Answer 2

APU's supply 2 kinds of power to aircrafts. 1.) is electrical power, which can be supplied by a GPU (ground power unit)or the external power, which is basically generator fo aircrafts. number 2.) is pneumatic power, which supplies the airconditioning and the engine starting system (in mondern airliners). Each purpose of which (aircon and starting) can be supplied by 2 different types of ground equipment. The airconditioning unit (ACU) and the Air start unit (ASU) respectively.

Now the decision when to start the APU depends on 1.) the condition of the aircraft, as in,  APU condition (operative/inoperative), eletrical system condition, etc. 2.) operator(company or person who owns or lessor of the aircraft) policy. 3.) terminal policy.

And since in most airlines the SOP is to use Ground power while doing maintenance work. The APU is started/operated when aircon, electricity or engine starting is needed. and that is usually immediately before Crew/passenger boarding.

And yes you save on Jet fuel (Jet a-1), which is cheaper than gasoline per liter. but you have to operate 3 ground equipment, which is gasoline engine operated, to supply all the APU power consumers.

Note: Airliners can still fly a revenue flight with out an operative APU for a maximum number of flying hours or cycle,  as per FAR.

Note 2: Aircraft engine or powerplant can/will replace all APU functions at or above engine idle.

- A &P guy

Answer 3

On short turnovers the APU is usually always running until main engine startup.  On longer layovers they will power it down if they have power to plug into it at a gate or a portable unit.

Answer 4

American airlines doesn't even use the APU anymore in the interest of saving fuel.

Answer 5

Usually in Asian airports Narita, Singapore , Bangkok that would be appropriate the 10-15 mins prior push back.esp. if you're in the gate where ext. power is connected.