Definition
A mathematical method of expressing the effective value of an alternating current (AC) or voltage. It is calculated by squaring each instantaneous value over one cycle, taking the average of those squared values, then taking the square root of that average. For a pure sine wave, the RMS value equals the peak value multiplied by 0.707.
Plain English
A way of stating how much real work an alternating current can do, since AC is constantly changing in size and direction. The RMS value is the steady DC equivalent that would deliver the same heating or working power.
Context Anchor
Seen in aircraft electrical theory, generator and alternator output discussions, and meter readings for alternating current.
Derivation
The name describes the math itself: take the Root of the Mean (average) of the Squared values. Squaring removes negative numbers (since AC swings positive and negative), averaging finds the middle, and the square root brings the result back to the original units.
Why Pilots Care
Technicians use it to confirm that AC power systems deliver the correct usable voltage and current to aircraft equipment.
Grounding Statement
RMS is the AC value that does the same work as a steady DC value of the same number — it lets you compare apples to apples.
Intuition Check
Root mean square is not the same as the highest value in the wave. It is also not a simple average; it is the effective working value.
Example Sentence 1
The aircraft's AC bus is rated at 115 volts RMS, which corresponds to a peak voltage of about 163 volts.
Example Sentence 2
Using a true RMS meter, the technician verified the generator output before flight.