Definition
The average value of a sine wave alternating current is 0.637 times its peak (maximum) value. This figure is derived by averaging the instantaneous current values across one half-cycle of the waveform, since averaging across a full cycle would yield zero (the positive and negative halves cancel out).
Plain English
If you take a sine-shaped AC current and average all its values across half of one wave, the result is about 64% of the highest value the current reaches. The full wave averages to zero because it spends equal time positive and negative, so the average is always taken over a half-cycle.
Context Anchor
Seen in aircraft electrical-system study, especially when comparing alternating current values, generator or alternator output, and meter readings.
Derivation
‘Sine wave’ comes from the Latin sinus, meaning ‘curve’ or ‘bend’ — describing the smooth curving shape of the waveform. ‘Alternating’ comes from the Latin alternare, ‘to do by turns,’ because the current reverses direction in regular turns. The ‘average’ here is a true mathematical mean, not an estimate.
Why Pilots Care
Pilots and technicians working with aircraft AC systems need to understand that AC values can be expressed in different ways — peak, average, or RMS (effective). Confusing them leads to incorrect interpretation of meter readings, system specifications, and troubleshooting data.
Analogy
If a swing moves equally forward and backward, its average position over the whole motion is the center. But if you average only the forward half of the motion, you get a real positive amount. A sine wave alternating current works the same way.
Grounding Statement
A sine wave alternating current spends equal time flowing one way and the other, so the part of the wave being averaged matters.
Intuition Check
Do not assume average means the average of the entire alternating-current wave. In this usage, it usually means the average of one half-cycle; over a full cycle, the average is zero.
Example Sentence 1
When calculating the average value of a sine wave alternating current, multiply the peak value by 0.637.
Example Sentence 2
Knowing the average value of sine wave alternating current helps size wiring correctly for AC systems in the aircraft.