Plain English Meaning
A piece of iron shaped into a frame that magnetic fields pass through easily. Because magnetism flows through it so well, it can be used to sense the direction of the Earth's magnetism.
Full Definition
A frame made of iron that readily allows magnetic field lines to pass through and concentrate within it. In the flux gate compass, this frame is the sensing element that captures the Earth's magnetic field so the system can determine direction.
Context Anchor
Seen in descriptions of the flux gate compass or flux valve, the sensor that supplies magnetic heading information to cockpit instruments.
Derivation
Permeable' comes from the Latin permeare, meaning 'to pass through.' In magnetism, a material is called permeable when magnetic field lines pass through it easily. Iron is one of the most permeable common materials, which is why it is used here.
Why Pilots Care
The flux gate compass is the behind many cockpit instruments. Knowing it works by sensing the Earth's through a permeable iron frame helps why the system must be shielded from nearby disturbances and why it is mounted in a quiet of the like a or .