Definition
An electric current that periodically reverses direction, with voltage and current rising and falling in a regular cycle. In aircraft electrical systems, A.C. is typically generated by an alternator or produced from D.C. through an inverter, and is used to power instruments and equipment that require it, such as certain gyroscopic instruments and avionics.
Plain English
Electricity that flows back and forth in a repeating pattern, instead of flowing steadily in one direction. Some aircraft instruments need this kind of power to work.
Context Anchor
Seen in aircraft electrical system descriptions, especially when explaining how electrically powered gyroscopic instruments receive power.
Derivation
The term comes from the behavior of the current itself: it 'alternates' — switches back and forth — rather than flowing in a single steady direction. Knowing this makes the contrast with D.C. (direct current, which flows one way only) immediate and easy to remember.
Why Pilots Care
Many attitude and heading gyros require AC power; understanding the source helps diagnose instrument failures during flight.
Analogy
A.C. is like water in a pipe being pushed back and forth repeatedly, rather than flowing steadily in one direction.
Intuition Check
A.C. does not mean air conditioning in this context. Here it means alternating current, a type of electrical power.
Example Sentence 1
The attitude indicator in this aircraft runs on A.C. supplied by an inverter.
Example Sentence 2
Loss of A.C. power caused the heading indicator to slowly drift during the instrument approach.