Definition
In an aircraft ignition system, the wire that connects one side of the magneto's primary circuit to the ignition switch, allowing the switch to short the magneto to ground and stop it from producing sparks. When the switch is OFF, the ground wire is connected to the airframe (ground), routing the primary current to ground and disabling the magneto. When the switch is ON, the ground wire is open (not connected to ground), allowing the magneto to operate normally.
Plain English
The wire that lets the ignition switch turn a magneto off. With the switch OFF, the wire is connected to the metal airframe, which kills the magneto's spark. With the switch ON, that connection is broken, so the magneto can fire the spark plugs.
Context Anchor
Seen in magneto ignition system descriptions, ignition switch operation, and troubleshooting an engine that may still fire when the switch is OFF.
Derivation
Called a 'ground' wire because, when active, it carries current to the airframe, which serves as the electrical 'ground' (the common return path). The term 'ground' in electrical work comes from early systems that used the actual earth as the return conductor.
Why Pilots Care
A missing or broken ground wire can allow an engine to continue running after the ignition switch is turned off, creating a serious safety hazard during shutdown.
Intuition Check
Do not picture a wire connected to the physical ground outside the airplane. In this context, “ground” means an electrical path that shuts the magneto off.
Example Sentence 1
If the ground wire to the left magneto breaks, turning the ignition switch to OFF will not stop that magneto from firing.
Example Sentence 2
During preflight, a loose ground wire was found that could have allowed the engine to keep running after shutdown.