Definition
An engine-driven electrical component that converts mechanical energy from the engine into electrical energy to power the aircraft's electrical system and recharge the battery during flight. An alternator produces alternating current that is then rectified to direct current, while a generator produces direct current directly. Most modern light aircraft use alternators because they produce useful output at lower engine speeds and weigh less than equivalent generators.
Plain English
The part driven by the engine that makes electricity for the aircraft and keeps the battery charged while flying. Alternators and generators do the same job in slightly different ways, with alternators being the more common modern choice.
Context Anchor
Seen during discussions of the airplane electrical system, preflight checks, engine run-up, and electrical failure procedures.
Derivation
Alternator comes from 'alternate' (Latin alternare, to do by turns), referring to alternating current that switches direction many times per second. Generator comes from Latin generare, to produce or bring forth. Knowing this helps remember that the alternator's output naturally alternates and must be converted, while a generator simply produces electricity in one direction from the start.
Why Pilots Care
Failure leaves the aircraft on battery power alone, which lasts only a limited time and can cause loss of instruments and lights.
Intuition Check
Do not think of the alternator/generator as the battery. The battery stores electrical power; the alternator/generator makes electrical power while the engine is turning.
Example Sentence 1
After starting the engine, the pilot checked that the alternator/generator was online by confirming a positive charge on the ammeter.
Example Sentence 2
After an alternator/generator failure, the pilot turned off nonessential electrical loads to save battery power.