What function does the P-lead serve in the Cessna 172?

The P-lead serves an important safety function in the Cessna 172 by grounding the magnetos. When the switch is in the OFF position, the P-lead prevents the magnetos from sparking, thereby ensuring that the engine cannot start. This is a critical feature because it helps to prevent inadvertent engine starts while maintenance is being performed or when the aircraft is not intended to be operational.

In a normal operational scenario, when the key is turned to the START position, the P-lead is disconnected, allowing the magnetos to generate spark, enabling engine start. This grounding mechanism is essential for ensuring safety on the ground and during maintenance, effectively preventing unwanted engine ignition without pilot intervention.