Definition
A condition in which the aircraft's primary electrical generating device — either an alternator or a generator — stops producing electrical power. When this occurs, the battery becomes the sole source of electrical energy and will deplete within a limited time, after which all electrically powered equipment will cease to function unless load is reduced or power is restored.
Plain English
The part of the aircraft that makes electricity has stopped working. The battery is now the only source of electrical power, and it will run out unless the pilot turns off equipment that is not essential.
Context Anchor
Encountered during instrument flying, abnormal procedures, and electrical system monitoring, especially when a low-voltage light, discharge indication, or loss of electrical equipment appears.
Derivation
Alternator and generator both come from Latin: 'alternare' (to do by turns) and 'generare' (to produce or bring forth). Both devices produce electrical power while the engine runs — the alternator by turning a magnetic field through coils, the generator by turning coils through a magnetic field. 'Failure' simply means it has stopped doing its job.
Why Pilots Care
Electrical instruments, radios, and navigation equipment will continue operating only until the battery is exhausted, after which the pilot must rely on backup instruments and land as soon as practical.
Grounding Statement
After this failure, the airplane is running on stored battery power instead of a continuous supply of new electrical power.
Intuition Check
Do not assume alternator/generator failure means everything electrical stops instantly. It usually means the aircraft has stopped recharging the system, and the battery is now counting down.
Example Sentence 1
After the low-voltage light illuminated, the pilot recognized an alternator/generator failure and immediately shed nonessential electrical loads to preserve the battery.
Example Sentence 2
After confirming alternator/generator failure, the pilot switched off all non-essential avionics to conserve battery power for the approach.