Of all the electrical circuits in a reciprocating aircraft engine, few are as misunderstood—or as quietly dangerous—as the P-lead circuit. This small, unassuming wire runs from the primary (low-voltage) side of each magneto to the cockpit ignition switch, and its entire job is to give the pilot a reliable way to stop the magneto from firing. Understanding how the P-lead works, and what happens when it fails, is essential knowledge for every aviation maintenance technician (AMT) working on piston powerplants—and it is a perennial topic on the FAA Powerplant Knowledge Test.
This article covers the circuit's function, why the magneto is "on" when the P-lead is open, how to verify P-lead continuity, and the critical safety implications that make this one of the most important concepts in engine electrical systems.
How the Magneto Ignition System Works
A magneto is a self-contained alternating-current generator that produces high-voltage sparks entirely independently of the aircraft's battery and alternator. A rotating permanent magnet induces voltage in the primary coil of a transformer. At the moment of peak primary current, the breaker points open, collapsing the primary magnetic field rapidly and inducing a very high voltage—commonly cited as up to around 20,000 volts, though the precise figure varies by magneto design—in the secondary winding. This surge travels through the distributor, out to the correct spark plug, and ignites the fuel-air mixture. The magneto requires no external power source; as long as the engine is rotating, the magneto can fire.
This independence is a major safety advantage in flight—you lose the bus bar, you still have ignition—but it creates a critical ground-handling hazard: a magneto connected to a turning engine will produce sparks regardless of what any switch on the panel says, unless the primary circuit is properly grounded.
The P-Lead Circuit Explained
The P-lead (short for primary lead) is a shielded wire that connects the breaker-point assembly (the primary circuit of the magneto transformer) to the ignition switch in the cockpit. When the ignition switch is placed in the OFF position, it completes a path to ground through the P-lead, shorting out the primary coil. With the primary circuit grounded, the breaker points cannot build up the voltage differential needed to fire the coil—the magnetic field collapses harmlessly into the short circuit instead of inducing a high-voltage spike in the secondary. The magneto is effectively silenced.
It is critical to understand the logic is reversed from most electrical devices you encounter. In a typical circuit, completing a path to ground through a switch turns something on. In the P-lead circuit, completing the path to ground turns the magneto off. This means:
- Switch ON (RUN, BOTH, LEFT, RIGHT): The P-lead circuit is open. The primary coil is free to develop its voltage pulse normally. The magneto fires.
- Switch OFF: The P-lead circuit is closed to ground. The primary coil is shorted. The magneto cannot fire.
Most light aircraft with two magnetos have two P-leads—one for each magneto—and the ignition switch grounds them individually (for single-magneto operation) or together (for OFF). The switch positions L, R, BOTH, and OFF correspond directly to which P-leads are grounded at any given moment.
What Happens When the P-Lead Fails
P-lead failures fall into two categories: an open (broken wire, disconnected terminal, corroded connector) or a short to ground (chafed insulation, pinched wire).
Open P-Lead
An open P-lead is the most dangerous failure mode. Because the switch can only ground a magneto through an intact P-lead, a broken wire means the switch has no authority over that magneto. The magneto behaves as if the switch is always in the ON position. If a technician moves the propeller by hand—during a compression check, timing procedure, or simple post-maintenance inspection—that magneto is live and will fire if the piston reaches firing position. The engine can kick or start unexpectedly, with catastrophic potential for the person at the propeller.
From the cockpit, an open P-lead is often detectable during the pre-takeoff magneto check: the engine will not show any RPM drop when the switch is moved to OFF (or to the single-magneto positions), because the affected magneto never actually shut down. However, this test is not always performed with a full OFF check, so the fault can remain hidden until a ground accident occurs.
Short-to-Ground P-Lead
A P-lead shorted permanently to ground means that magneto is always grounded—always off—regardless of switch position. The engine will still run on the other magneto, but the affected magneto contributes nothing. This shows up as an excessive RPM drop during the single-magneto check. While not an immediate safety hazard in the same catastrophic sense, it is still an airworthiness defect requiring correction.
Why This Matters: Safety on the Ramp and in the Shop
The P-lead circuit is at the heart of one of the most important safety rules in propeller-aircraft maintenance: treat any propeller as if the engine could start at any time. Even with the ignition switch in the OFF position, you cannot be certain the P-lead is intact unless you have verified it. The FAA Aviation Maintenance Technician Handbook and industry safety standards emphasize this point. Specific safe practices include:
- Always verbally call out "CLEAR" before moving a propeller for any reason.
- Position yourself and bystanders away from the propeller arc whenever the engine has been recently run or fuel may be present in the cylinders.
- Never use the ignition switch position alone as confirmation that a magneto is grounded; always verify with a P-lead continuity check when in doubt.
- After any magneto removal, installation, or timing work, confirm P-lead connection and shield grounding before returning the aircraft to service.
Key Numbers and Regulatory Points
- Magneto-to-engine timing: Most horizontally-opposed engines are timed between approximately 20° to 25° before top dead center (BTDC) on the compression stroke; the exact value is engine-model specific and found in the Type Certificate Data Sheet (TCDS) or engine manufacturer's manual.
- RPM drop during mag check: A typical single-magneto RPM drop at run-up is generally in the range of roughly 50–125 RPM; the exact maximum allowable drop and the differential between left and right magnetos are specified by the aircraft/engine manufacturer in the POH/AFM rather than a single universal figure.
- P-lead continuity test: Using an ohmmeter or continuity tester with the ignition switch OFF, verify low resistance (near zero ohms) from the magneto primary terminal to ground. With the switch ON, the circuit should be open (infinite resistance). Perform this test with the aircraft grounded and ignition key removed where possible.
- Shield grounding: The outer braid of the P-lead's shielded cable must be grounded at both the magneto case and the ignition switch to prevent radio frequency interference (RFI) from magneto pulses affecting avionics and to ensure the overall circuit integrity.
- 14 CFR Part 43: Magneto maintenance, timing, and P-lead inspection are regulated maintenance tasks; the FAA requires that all work affecting airworthiness be performed, supervised, or signed off by appropriately certificated personnel.
Memory Aid
"Ground to stop, open to go." This simple phrase captures the reversed logic of the P-lead circuit. When the P-lead is grounded (switch OFF), the magneto stops firing. When the P-lead is open (switch ON, or broken wire), the magneto goes right on firing. Remembering this inversion prevents the dangerous assumption that an OFF switch means a dead magneto.
Common Test Traps
- "The switch is OFF, so the magneto is safe." False. If the P-lead is open due to a broken wire or loose terminal, the magneto is still live no matter what the switch says. This is the most-tested concept related to P-leads on the FAA Powerplant exam.
- Confusing open vs. shorted P-lead effects. An open P-lead leaves the magneto always live (dangerous). A shorted P-lead leaves the magneto always grounded (engine won't use it, but no prop-kick hazard). Know which failure mode is which.
- Thinking the magneto needs aircraft power to fire. Magnetos are entirely self-powered by engine rotation. The only connection to the aircraft electrical system is the P-lead ground path. There is no "battery power" to the magneto primary.
- Forgetting the P-lead shield must be grounded. The outer shielding braid is not just for RFI suppression—it is part of the circuit integrity. An ungrounded or broken shield can cause intermittent grounding issues and erratic magneto behavior.
- Misidentifying the RPM drop cause. A zero RPM drop on one magneto during the run-up mag check does not mean that magneto is performing perfectly—it may indicate an open P-lead, meaning the magneto never actually switched off. Conversely, an engine that quits entirely when switched to one magneto position has a much bigger problem than a P-lead issue.
Mastering the P-lead circuit is foundational to safe magneto work. The reversed logic—ground means off, open means on—runs counter to intuition but is easy to internalize with practice. Every time you work near a propeller or perform magneto maintenance, let this knowledge guide your safety habits: verify the P-lead, respect the arc, and never assume an OFF switch guarantees a silent magneto.