Page "Auxiliary power unit" Paragraph 6

APUs fitted to extended-range twin-engine operations ( ETOPS ) aircraft are a critical safety device, as they supply backup electricity and compressed air in place of the dead engine or failed main engine generator.

While some APUs may not be startable in flight, ETOPS-compliant APUs must be flight-startable at altitudes up to the aircraft service ceiling.

Recent applications have specified starting up to from a complete cold-soak condition such as the Hamilton Sundstrand APS5000 for the Boeing 787 Dreamliner.

If the APU or its electrical generator is not available, the aircraft cannot be released for ETOPS flight and is forced to take a longer non-ETOPS route.