What is the function of the P-lead in a magneto system?

The P-lead serves a critical function within the magneto ignition system of an aircraft engine. Its primary role is to connect the ignition switch to the primary circuit of the magneto. When the ignition switch is turned off, the circuit is opened, which effectively grounds the P-lead. This grounding action disables the magneto's ability to generate a spark at the spark plug, preventing the engine from firing. When the switch is in the "on" position, the P-lead is no longer grounded, allowing the magneto to operate normally and generate high voltage sparks for combustion.

The other options do not relate to the function of the P-lead. The battery charging is not a task performed by the P-lead; that responsibility lies with the alternator or battery system of the aircraft. Measuring engine RPM is typically performed by other systems, such as a tachometer, while fuel injection control is managed by the engine's fuel delivery system and not influenced by the P-lead. This clarity on the P-lead's role helps in understanding its importance in the ignition system of an aircraft engine.