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Although the accepted answer is basically correct, as an additional backup source of power (for hydraulics or electric, depending on the aircraft), there can be a Ram air turbine (RAT - that can be deployed in a failure of both primary and secondary energy sources. It uses the airflow from the airplane's speed ...


This is somewhat dependent on the aircraft of course, so I will focus on the Boeing 737 NG series as an example of a typical airliner. Details will differ with other aircraft, but the general concept should be the same. When there is loss of thrust on both engines, both engine driven generators will stop providing power. The battery will take over powering ...


Actual details vary between aircrafts. But in general, the battery can supply essential systems for some time and then APU/RAT/... should kick in.


@Jpe61 is correct in his comment that it's likely an electronics issue, since indeed a VSCF is mechanically simpler. VSCF is also used on the MD-90, with poor initial reliability$^1$ – MDC went back to IDGs for the MD-95 (717).$^1$ It's also used on the 777 (only for backup, also with poor initial reliability$^2$), and on the F/A-18.$^3$ Capacitors The F/...

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