Definition
The magnetism that remains in a ferromagnetic material after the external magnetizing force has been removed. In aircraft magnetos and certain instruments, this leftover magnetism in the iron core or pole pieces is what allows the device to begin generating a magnetic field again the next time it is operated, without needing an outside power source to start it.
Plain English
It is the magnetism that stays behind in a piece of iron or steel after the thing that was magnetizing it is taken away. The metal does not go fully back to zero -- a small amount of magnetism is left in it.
Context Anchor
Seen in aircraft electrical system discussions, especially when explaining how a generator starts producing voltage and why it may fail to charge after maintenance or long storage.
Derivation
Residual comes from the Latin residuum, meaning 'that which is left over.' Flux comes from the Latin fluxus, meaning 'a flow.' So residual magnetic flux is literally 'the magnetic flow that is left behind' after the magnetizing force is gone.
Why Pilots Care
Unwanted residual magnetism in airframe parts or instruments can produce compass errors and navigation inaccuracies.
Analogy
A screwdriver can stay slightly magnetic after it has touched a magnet. Residual magnetic flux is similar: a small amount of magnetism remains in the metal even after the original source is gone.
Grounding Statement
Picture a generator sitting still after shutdown: some of its metal parts can still hold a faint magnetic field.
Intuition Check
Residual magnetic flux is not electricity left in the wire. It is leftover magnetism in the metal parts that helps electrical output begin.
Example Sentence 1
The magneto relies on residual magnetic flux in its pole shoes to begin generating voltage as soon as the engine is cranked.
Example Sentence 2
After the lightning strike, the mechanic demagnetized the wing spar to remove residual magnetic flux.