Definition 1 of 2
Definition
A small, sealed sensing unit, typically mounted in a wingtip or tail where magnetic interference from the airframe is minimized, that detects the direction of the Earth's magnetic field and sends that information electrically to a remote-indicating compass system such as the HSI or RMI. It uses a three-legged spoke-shaped core wrapped with coils; an alternating current excites the core, and the Earth's magnetic field induces voltages in the coils that vary with the valve's orientation relative to magnetic north.
Plain English
A device hidden out in the wingtip or tail that senses which way magnetic north is and sends that information to the compass display in the cockpit.
Context Anchor
Seen in instrument flying when studying HSI systems, remote compass systems, and heading indications.
Derivation
Flux' comes from the Latin fluxus, meaning 'flow' -- here referring to magnetic flux, the flow of a magnetic field through the sensor. 'Valve' is used loosely; it doesn't open or close anything. It comes from older electrical usage where 'valve' meant a device that controlled or detected an electrical signal. So a flux valve is a sensor that 'reads' the flow of the Earth's magnetic field.
Why Pilots Care
It supplies the stable magnetic heading reference that keeps the HSI aligned when the aircraft turns or when other navigation sources are unavailable.
Analogy
Think of it like moving the sensing part of a compass away from the noisy cockpit and letting it send the result back to the panel.
Intuition Check
Do not read “valve” as something that opens or closes a fluid line. A flux valve is a magnetic sensor, not a plumbing valve.
Example Sentence 1
The flux valve is mounted in the wingtip to keep it away from the magnetic interference of the engine and avionics.
Example Sentence 2
During the preflight the mechanic confirmed the flux valve output matched the known runway heading on the HSI.