Definition
A form of alternating electrical current that reverses direction 400 times per second, used as the standard electrical power frequency for many aircraft systems and instruments. It allows the use of smaller, lighter electrical components than the 50 or 60 Hz current used in homes and industry.
Plain English
It's the type of electricity used in aircraft. It switches direction 400 times a second, which is much faster than household electricity. Running at this higher rate lets aircraft use smaller, lighter motors and transformers — a big advantage in flight.
Context Anchor
Seen in discussions of electrically powered compass systems, especially the flux gate compass system used to send heading information to cockpit instruments.
Derivation
Hz stands for hertz, the unit of frequency, named after the German physicist Heinrich Hertz. One hertz means one cycle per second, so 400 Hz means 400 cycles per second. 'Alternating' refers to the current changing direction back and forth, rather than flowing one way only (which would be direct current, or DC).
Why Pilots Care
Higher frequency allows smaller transformers and motors, reducing aircraft weight while maintaining reliable instrument operation.
Intuition Check
“Current” here does not mean wind or water movement. It means the flow of electricity, and “alternating” means that electrical flow repeatedly reverses direction.
Example Sentence 1
The flux gate compass operates on 400 Hz alternating current supplied by the aircraft's electrical system.
Example Sentence 2
Many navigation instruments are designed to operate on 400 Hz alternating current because it permits compact and lightweight construction.