Definition
The primary wire connecting a magneto's primary ignition circuit to the ignition switch in the cockpit. When the switch grounds the P-lead, it short-circuits the magneto's primary coil to ground, preventing it from producing a spark and turning the magneto off.
Plain English
The wire that lets the cockpit ignition switch turn a magneto off. Grounding this wire shuts the magneto down; if the wire breaks or comes loose, the switch can no longer shut the magneto off, and the engine can fire even with the key off.
Context Anchor
Encountered during magneto inspection, ignition switch checks, engine run-up checks, and troubleshooting a magneto that does not shut off correctly.
Derivation
The 'P' stands for 'primary,' referring to the magneto's primary (low-voltage) winding that this wire is connected to. Calling it the 'P-lead' is just shorthand for 'primary lead.'
Why Pilots Care
A disconnected or faulty P-lead can allow an engine to start unexpectedly or continue running after the ignition is turned off, creating a serious safety hazard on the ground.
Grounding Statement
The P-lead is the magneto’s shutoff wire: when it is grounded, the magneto stops making spark.
Intuition Check
Do not read “lead” here as the metal or as “to lead the way.” In this term, a lead is a wire, and the P-lead’s job is to help shut the magneto off.
Example Sentence 1
During the post-maintenance run-up, the mechanic confirmed both P-leads were properly secured before signing off the ignition system inspection.
Example Sentence 2
During the magneto check, the pilot confirmed that each P-lead properly grounded its magneto when the switch was moved to the off position.