What starts an airliner APU?

There are two main methods, usually electric starters using aircraft batteries or ground power, although some aircraft use hydraulic or pneumatic motors run off an accumulator.

Most starter motors on large jet engines are pneumatic, using high pressure air from the APU, although there are some aircraft such as the BAE 146 and the RJs use electric starters.

Interestingly, the B787 uses all electric starters which double as electricity generators when the engine is running. The APU drives a couple of very large generators to provide starting power.

Battery - airliners and business jets have 28V batteries to start APU -

APU all have a 28V electric starter -

All airplanes can also use an external (electrical) power source -

Airliners generally use a 115V AC external power -

Business jets generally use a 28V DC external power -