In some unusual scenarios the APU may be used during flight. If one of the engine's generators fails the APU might be used to help provide power, and below som

In some unusual scenarios the APU may be used during flight. If one of the engine's generators fails the APU might be used to help provide power, and below some altitude the APU in the 777 can provide additional bleed air. In the 777 the left and right hydraulic systems are powered by the left and right engines, but the center hydraulic system has electric and pressurized air driven pumps that I believe can be run off the APU: http://www.flight-mechanic.com/wp-content/uploads/2017/11/12-66.jpg http://www.flight-mechanic.com/wp-content/uploads/2017/11/12... If MH370 lost engine power, maybe it was using the APU to keep the control surfaces active? Who knows. In that sort of situation the 777 would be a glider though.