Definition
Electrical current that periodically reverses direction, flowing first one way through a circuit and then the other, typically in a smooth back-and-forth cycle measured in cycles per second (hertz). In aircraft electrical systems, AC is used to power certain instruments, avionics, and equipment that require it, and is generated either by an alternator or produced from the aircraft's DC supply through an inverter.
Plain English
An electric current that keeps switching the direction it flows, back and forth, many times each second. This is different from current that flows steadily in one direction.
Context Anchor
Seen in aircraft electrical system discussions, especially when comparing AC power with battery-type one-way power.
Derivation
From Latin alternare, meaning 'to do by turns.' The current takes turns flowing one way, then the other — which is exactly what 'alternating' describes.
Why Pilots Care
Aircraft alternators produce alternating current that must be understood for proper system operation and troubleshooting.
Intuition Check
Do not read “current” here as present time or moving water; it means the flow of electricity. “Alternating” does not mean choosing between two power sources; it means the electric flow reverses direction repeatedly.
Example Sentence 1
The attitude indicator in this aircraft runs on alternating current supplied by the inverter.
Example Sentence 2
Before conversion to direct current, the generated power exists as alternating current.