Definition
A form of alternating electrical current that reverses direction 400 times per second, used as the standard power supply for many aircraft electrical and electronic systems, including the flux gate compass system. It is generated onboard by an aircraft alternator or inverter and is preferred over the 60-Hz current used in homes because the higher frequency allows transformers, motors, and other electrical components to be made much smaller and lighter.
Plain English
Electrical power that switches direction 400 times every second. Aircraft use this instead of household-style power because it lets electrical equipment be smaller and lighter, which matters in an airplane.
Context Anchor
Seen in instrument-system descriptions, especially older or more complex aircraft systems that use 400-Hz power for heading, navigation, or compass equipment.
Derivation
Hz stands for hertz, the unit for cycles per second, named after the German physicist Heinrich Hertz. So 400 Hz simply means 400 cycles per second. 'Alternating current' means current that reverses direction repeatedly, as opposed to 'direct current' which flows one way only.
Why Pilots Care
Supplies the exact frequency needed for accurate, drift-free operation of the flux gate compass and other sensitive instruments; wrong frequency can cause erratic readings or instrument failure.
Analogy
It is like a tool that must be plugged into the right kind of outlet. If the instrument is designed for 400-Hz A.C., a different kind of electrical power may not run it properly.
Intuition Check
A.C. does not mean air conditioning here. It means alternating current, a type of electrical power. Current does not mean wind or water movement here. It means the flow of electricity.
Example Sentence 1
The flux gate compass system runs on 400-Hz alternating current supplied by the aircraft's inverter.
Example Sentence 2
The aircraft inverter supplies 400-Hz alternating current to the instrument panel.