On any reciprocating aircraft engine, the magneto is a self-contained electrical generator that produces high-voltage spark entirely independent of the aircraft battery or master switch. That independence is what makes the magneto so reliable in flight — but it also means the engine can fire at any moment the magneto is capable of generating a spark and the propeller moves. The only thing that normally prevents this is the grounding circuit, a deceptively simple wire that connects the magneto's primary coil to ground, collapsing the magnetic field before it can produce an ignition pulse. Understanding that circuit, recognizing when it fails, and testing it correctly is one of the most safety-critical skills in powerplant maintenance.
The FAA's Aviation Maintenance Technician Powerplant Handbook (FAA-H-8083-32) treats magneto grounding and the P-lead circuit as a fundamental topic for good reason: a broken ground wire creates a magneto that appears safe but is actually live. Every year, maintenance technicians and pilots are injured or killed by propellers connected to engines with failed grounding circuits. Knowing the theory is not enough — you must also know the test procedure that proves the circuit is actually working.
How the Magneto Grounding Circuit Works
Inside the magneto, a rotating permanent magnet induces current in the primary winding of the ignition coil as the magnetic field builds. A set of breaker points then interrupts that primary current at precisely the right moment, causing a rapid collapse of the magnetic field that induces an extremely high voltage — often cited as up to 20,000 volts or more, depending on the system — in the secondary winding. That voltage fires the spark plug. This cycle happens completely without any external power supply, which is why a magneto can fire even with the aircraft battery disconnected and the master switch off.
The grounding circuit, commonly called the P-lead (primary lead), connects the primary side of the magneto coil directly to aircraft ground (the airframe). In a standard magneto and switch configuration, when the ignition switch is placed in the OFF position, it closes the P-lead circuit, effectively shorting the primary coil to ground. With the primary coil shorted, any voltage induced in it immediately dissipates harmlessly into the airframe rather than building up to interrupt through the breaker points. No primary current interruption means no high-voltage secondary pulse, and the spark plugs stay cold. The engine cannot fire.
When the ignition switch is turned ON (either to LEFT, RIGHT, or BOTH), it opens the P-lead circuit, disconnecting the ground path from the primary coil. Now the magneto is free to operate normally whenever the engine rotates. This is an important conceptual point: turning the ignition switch ON does not supply power to the magneto — it removes the ground that was suppressing it. The magneto is always generating; the P-lead only prevents that energy from reaching the plugs.
P-Lead Failure Modes
Because the P-lead is a single wire routed from the magneto to the ignition switch, it is vulnerable to the kinds of failures common to any wire: corrosion at connectors, chafing through the insulation where the wire passes near structure, loose terminals at the magneto end, or simply a broken conductor. The consequences of failure depend on whether the circuit fails open or closed.
Open P-lead (broken or disconnected wire): This is the most dangerous failure mode. If the P-lead wire breaks or pulls off its terminal at the magneto, the primary coil permanently loses its ground path — even when the ignition switch is in the OFF position. The magneto is now always live. Rotating the propeller by hand during preflight, pulling the prop through during a compression check, or attempting a hand start will fire the engine regardless of where the cockpit switch sits. There is no external indication of this condition without testing.
Closed (shorted) P-lead: If the P-lead wire chafes through its insulation and contacts ground somewhere along its run, the primary coil remains permanently shorted. The engine will not start and will not run, or it may run very roughly if only one magneto is grounded. While this failure is far less dangerous to personnel, it grounds the aircraft and must still be identified and corrected.
Grounding Circuit Test Procedure
The grounding circuit test is typically performed at the end of engine runup, or any time a magneto system is disturbed during maintenance. The procedure verifies that the magneto actually goes dead when the switch is moved to OFF, and that it produces no ignition when it should be grounded.
During a normal runup grounding check, the pilot or technician brings the engine to a specified RPM (refer to the aircraft's Pilot's Operating Handbook or maintenance manual for the exact value, since this varies by aircraft type). With the engine at that RPM, the ignition switch is momentarily moved to OFF, then immediately returned to BOTH. On a properly grounded system, the engine should die completely — stop running — the instant the switch reaches OFF, confirming that the magnetos actually grounded and ignition ceased. The switch is returned to BOTH immediately once this is confirmed, so the engine does not fully wind down. If the engine does not die completely when the switch is placed to OFF, one or both P-leads are open, meaning the magnetos did not ground — a serious airworthiness defect requiring correction before further flight.
For a more precise bench or in-aircraft test using instruments, a technician can use a magneto timing light or an ohmmeter (with the engine shut down and the propeller secured). With the ignition switch OFF, the technician checks continuity between the P-lead terminal on the magneto and ground. Continuity (near-zero resistance) should exist, confirming the ground path is complete. With the switch ON, the circuit should be open (infinite resistance). An open reading with the switch OFF, or a closed (shorted) reading with the switch ON, indicates a defect in the switch or wiring.
After any magneto work — timing checks, point gap adjustment, distributor block inspection, or impulse coupling service — the P-lead must be reconnected and verified before the cowling is closed and before anyone is allowed near the propeller arc. This is not a step that can be deferred.
Why It Matters: Safety and Airworthiness
The immediate safety implication is obvious: a magneto with an open P-lead can fire the engine when the cockpit ignition switch reads OFF. Ground personnel who believe the engine is safe because the switch is off have been struck by propellers connected to live magnetos. Maintenance procedures must treat any propeller as potentially live until the grounding circuit is positively confirmed.
From an airworthiness standpoint, 14 CFR Part 43 requires that maintenance be performed in accordance with the manufacturer's maintenance manual and FAA-approved data. An engine returned to service with a defective P-lead circuit is not airworthy regardless of how well all other systems perform. The FAA considers a magneto that cannot be reliably grounded a safety-of-flight defect requiring correction before further flight.
Key Numbers and Rules
- P-lead function: Grounds the magneto primary coil; closing the circuit (switch OFF) suppresses ignition.
- Open P-lead effect: Magneto permanently live — engine can fire with ignition switch in the OFF position.
- Grounded (shorted) P-lead effect: Magneto permanently suppressed — engine will not run or will run on one magneto only.
- Runup grounding check: The engine should die completely when the switch is briefly moved to OFF, confirming grounding circuit integrity; failure to die completely indicates an open P-lead on at least one magneto.
- Ohmmeter check (engine off): Switch OFF → near-zero resistance (continuity) to ground; Switch ON → infinite resistance (open circuit).
- Regulatory basis: Maintenance must comply with 14 CFR Part 43 and the applicable Type Certificate Data Sheet and maintenance manual requirements.
- Post-maintenance requirement: P-lead must be reconnected and tested before any personnel are allowed in the propeller arc.