Definition
A primary lead, also called a P-lead, is the wire connecting a magneto's primary coil to the ignition switch in the cockpit, allowing the pilot to ground the magneto and stop ignition. When the switch is in the OFF position, the P-lead grounds the primary circuit, preventing the magneto from firing the spark plugs. When the switch is in any ON position, the P-lead is open, allowing the magneto to operate normally.
Plain English
It's the wire that lets the cockpit ignition switch turn a magneto off. When the switch is OFF, this wire connects the magneto to ground, which stops it from making sparks. If this wire breaks or comes loose, the magneto cannot be switched off — even with the key in the OFF position.
Context Anchor
Seen in aircraft engine ignition-system maintenance, especially when checking magnetos, ignition switches, and shutdown problems.
Derivation
P stands for primary, referring to the primary winding of the magneto coil — the side of the coil where low-voltage current flows before being stepped up to fire the spark plugs. The P-lead is simply the lead (wire) attached to that primary circuit.
Why Pilots Care
Ensures positive engine shutdown and prevents accidental engine start during maintenance or parking.
Intuition Check
Do not read lead here as the metal. In this term, lead means an electrical wire or connection.
Example Sentence 1
During the post-maintenance runup, the mechanic confirmed that grounding the P-lead stopped the right magneto cleanly.
Example Sentence 2
The mechanic inspected the PP-lead for chafing before reinstalling the magneto.