Definition
A mathematical method of expressing the effective value of an alternating current (AC) or voltage. The rms value of an AC waveform equals the value of a direct current (DC) that would produce the same heating effect in a resistive load. For a sine wave, the rms value is approximately 0.707 times the peak value.
Plain English
A way of giving alternating current a single steady number that represents how much real work it can do. Because AC voltage rises and falls many times a second, you can't just use its peak — rms gives you the steady value that does the same job as a battery (DC) of that same number.
Context Anchor
Seen in aircraft electrical-system descriptions, test procedures, and meter readings for alternating-current voltage or current.
Derivation
Root mean square describes how the value is calculated: take each instantaneous value, square it, find the mean (average) of those squares, then take the square root. The name is the math recipe spelled out. Knowing this helps explain why rms isn't just the average — squaring removes the negative half of the AC cycle so the calculation captures the true working value of the current.
Why Pilots Care
Aircraft AC system voltages (such as the 115-volt 400-Hz systems on many transport aircraft) are stated as rms values. Mistaking rms for peak voltage leads to wrong assumptions about what the system is producing and what equipment can handle.
Grounding Statement
When an electrical value rises and falls, Rms gives the steady-value equivalent of its real power effect.
Intuition Check
Rms is not the same as a simple average. A changing electrical wave can average near zero, while its Rms value still shows the useful power it can deliver.
Example Sentence 1
The aircraft's AC bus is rated at 115 volts rms at 400 Hz.
Example Sentence 2
Technicians use an RMS-calibrated meter to verify that the AC output from the inverter meets specifications.