Definition
An alternating current whose voltage and amperage rise and fall in the smooth, repeating pattern of a sine wave. The current builds from zero up to a peak in one direction, falls back through zero, builds to a peak in the opposite direction, and returns to zero — repeating this cycle at a constant frequency. It is the standard waveform produced by rotating electrical generators and is the form of AC used in most aircraft and ground electrical systems.
Plain English
Electricity that flows back and forth in a smooth, even rhythm — strengthening, weakening, reversing, and repeating in a clean wave pattern.
Context Anchor
Seen in aircraft electrical system discussions, especially when describing alternator output, inverter output, and power supplied to equipment that needs alternating current.
Derivation
‘Sine’ comes from the Latin sinus, meaning a curve or fold, used in mathematics for the smooth wave produced by circular motion. Because AC generators work by rotating a coil through a magnetic field, the voltage they produce naturally traces this same curve — hence ‘sine-wave’ alternating current.
Why Pilots Care
Aircraft alternators produce this form of current; any distortion can affect the quality of power delivered to avionics and instruments after rectification.
Analogy
Think of a child on a swing moving smoothly forward and back, passing through the bottom each time. Sine-wave AC moves through zero and reverses in the same smooth, repeating way.
Intuition Check
Do not read “alternating” as random or on-and-off. Here it means the current reverses direction in a smooth, repeating pattern.
Example Sentence 1
The aircraft’s inverter converts DC from the battery into 115-volt sine-wave alternating current for the avionics.
Example Sentence 2
Technicians check the waveform to confirm it remains a clean sine-wave alternating current under load.