Definition
An electric current that periodically reverses direction, flowing first one way through a circuit and then the other, typically in a smooth sinusoidal pattern. In aircraft electrical systems, alternating current is generated by alternators or produced from direct current using inverters, and is used to power instruments and equipment that require AC, such as certain gyroscopic instruments, autopilot servos, and navigation receivers.
Plain English
Electricity that keeps switching the direction it flows, back and forth, many times per second. It is the kind of power some aircraft instruments need to run.
Context Anchor
Seen in aircraft electrical system descriptions, especially when a system or instrument depends on alternating-current power.
Derivation
Alternating' comes from the Latin 'alternare', meaning 'to do by turns'. The current literally takes turns flowing one way, then the other. 'Current' comes from Latin 'currere', 'to run' — the running or flow of electricity.
Why Pilots Care
Aircraft alternators produce A.C. which is converted to D.C. for batteries and most avionics; understanding the distinction supports electrical troubleshooting and system awareness.
Analogy
Think of DC as water flowing steadily through a pipe in one direction. AC is more like a tide that pushes water in, then pulls it back out, over and over — the water still does work, but it never settles into a one-way flow.
Intuition Check
Alternating does not mean the power simply turns on and off. Here it means the electrical flow reverses direction repeatedly.
Example Sentence 1
If the inverter fails, the instruments that rely on alternating current will drop offline, even though the battery is still good.
Example Sentence 2
A loss of A.C. output from the alternator will eventually drain the battery even if the engine keeps running.