Definition 1 of 2
Definition
An electric current that periodically reverses direction, with its voltage rising and falling in a regular pattern (typically a sine wave). In aircraft, alternating current is commonly produced by an alternator and is used to power equipment that requires it directly or through transformers and rectifiers.
Plain English
Electricity that keeps switching the direction it flows, back and forth, many times per second.
Context Anchor
Seen in aircraft electrical system discussions, maintenance checks, power supplies, and equipment that requires a specific type of electrical power.
Derivation
From Latin alternare, meaning 'to do one thing and then another by turns.' The current literally takes turns flowing one way, then the other.
Why Pilots Care
Many aircraft instruments and avionics run on alternating current. Knowing whether a system needs AC or direct current matters when troubleshooting electrical problems, understanding inverter and alternator failures, and reading electrical schematics.
Analogy
Think of water in a pipe being pushed first one way, then the other, over and over. The water is still moving, but the direction keeps reversing.
Intuition Check
“Current” here does not mean “present time” or a flow of air or water. It means the flow of electricity, and “alternating” means that electrical flow reverses direction repeatedly.
Example Sentence 1
The aircraft's inverter converts direct current from the battery into alternating current to power the attitude indicator.
Example Sentence 2
Maintenance checks include verifying that the alternating current output stays within the specified voltage range.