Definition
Alternating current electrical power that cycles 400 times per second, used in aircraft to drive certain gyroscopic instruments and other electrical systems. The higher frequency (compared to the 60-Hz A.C. used in homes) allows transformers, motors, and generators to be smaller and lighter while delivering the same power.
Plain English
A type of aircraft electrical power that switches direction 400 times every second. It's used because the equipment that runs on it can be made smaller and lighter, which matters in an aircraft.
Context Anchor
Seen in instrument flying discussions about electrically powered gyroscopic instruments and the aircraft electrical system that supplies them.
Derivation
Hz' is short for hertz, the unit for cycles per second, named after physicist Heinrich Hertz. 'A.C.' stands for alternating current — electricity that reverses direction many times per second, as opposed to direct current (D.C.) which flows one way. So 400-Hz A.C. simply means alternating current that reverses 400 times each second.
Why Pilots Care
The higher frequency permits smaller and lighter gyro motors while still delivering the high rotor speeds required for stable instrument indications.
Intuition Check
Do not read 400-Hz as a voltage or strength of electricity. It describes how fast the alternating current cycles each second.
Example Sentence 1
The attitude indicator on this aircraft is driven by 400-Hz A.C. supplied through an inverter.
Example Sentence 2
Loss of 400-Hz A.C. power causes the heading indicator to slow down and eventually become unusable.