Definition
A form of alternating electrical current that reverses direction 400 times per second, used as the standard electrical supply for many aircraft instruments and systems, including the flux gate compass. It is generated by the aircraft's engine-driven alternator or by an inverter that converts DC into AC at this frequency.
Plain English
Electrical power that flips back and forth 400 times every second. Aircraft use this kind of power instead of household power because it allows the equipment that runs on it to be smaller and lighter.
Context Anchor
Seen in aircraft electrical and instrument system discussions, including the flux gate compass system.
Derivation
Hz stands for hertz, named after the German physicist Heinrich Hertz, and means cycles per second. So 400 Hz simply means 400 cycles per second. 'Alternating' refers to the current alternating, or switching direction, repeatedly -- as opposed to direct current (DC), which flows in one direction only.
Why Pilots Care
Using 400 Hz AC reduces the size and weight of electrical components compared with standard household power.
Analogy
Household power in most countries runs at 50 or 60 Hz -- it switches direction 50 or 60 times a second. Aircraft power runs at 400 Hz, switching direction much faster, which lets the motors and transformers it powers be much smaller and lighter -- a big advantage in an aircraft.
Intuition Check
AC here does not mean air conditioning; it means alternating current. Also, 400 Hz is the rate of the electrical cycling, not the amount of voltage.
Example Sentence 1
The flux gate compass is powered by 400 Hz alternating current supplied by the aircraft's inverter.
Example Sentence 2
Aircraft generators supply 400 Hz alternating current (AC) to run multiple instruments without adding extra weight.