What is typically used to start an aircraft APU?

• A. Manual hand-crank
• B. An electric starter motor powered by aircraft power, and in some designs an air turbine starter driven by bleed air.
• C. Hydraulic pressure alone
• D. Solar-powered starter

Answer: (B)

Starting an aircraft APU requires giving its turbine enough initial rotation so fuel can ignite and the unit comes to life. The most common way is with an electric starter motor that is powered by the aircraft’s electrical system, drawing from batteries or ground/essential power to spin the APU until it reaches self-sustaining speed. In some designs, an air turbine starter driven by bleed air serves the same purpose, using compressed bleed air to turn the turbine until ignition occurs. This combination covers the practical ways APUs are actually started in practice: electric starts when electrical power is available, and bleed-air driven starts in designs where an air-start system is preferred or required. Hydraulic pressure alone doesn’t provide the necessary initial rotation by itself, and solar power isn’t used for aircraft APUs due to the lack of a reliable on-board solar source.