What Is the Purpose of the P-Lead, and What Happens If It Is Broken?

What the P-Lead Actually Does

Inside every magneto-equipped aircraft is a small but critical wire called the p-lead — short for primary lead. Its sole job is to connect the magneto to the ignition switch, and that connection serves one essential purpose: grounding the magneto when you turn the key to the OFF position. Grounding the magneto interrupts the primary circuit, stops spark production, and shuts the engine down. Without a functioning p-lead, the magneto has no way to receive that ground signal — and it keeps doing exactly what magnetos are designed to do: generate high-voltage sparks continuously, independent of any external power source.

This is the detail that trips up a lot of students. A magneto is not like a fuel pump or a landing light — it does not need the aircraft electrical system to operate. It is a self-contained generator driven by engine rotation. The ignition switch does not supply power to the magneto; it only grounds it to stop it. The p-lead is the wire that makes that grounding possible. Confusing the p-lead with a coil wire or some other ignition component is a common mistake, but the examiner is looking for you to understand this specific grounding function clearly. Chapter 7 of the Pilot's Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25) covers the ignition system and lays out this relationship directly.

Why a Broken P-Lead Is Extremely Dangerous

When a p-lead breaks or becomes disconnected, the magneto loses its only path to ground. The ignition switch becomes functionally irrelevant to that magneto — it is always hot, regardless of key position. This means the engine could fire and the propeller could rotate even with the key in the OFF position, sitting on the ramp, with the master switch off and nobody in the cockpit.

Think about what that means in practice. A well-meaning student, line technician, or bystander who grabs the propeller to hand-prop the engine through compression or to move the aircraft manually could find themselves facing a spinning prop with no warning. This is not a theoretical risk — propeller strikes are fatal. Whenever a broken p-lead is suspected, the aircraft must be treated as if the engine could start at any moment, and everyone near the aircraft needs to be warned to keep clear of the propeller arc. That safety communication step is something many students forget to mention during their oral exam, and it matters.

An aircraft with a broken p-lead is grounded — it is not airworthy and must not be flown until the problem is repaired. There is no workaround, no acceptable level of risk. The examiner expects you to say this without hesitation.

How to Catch It During the Run-Up

Here is where the dead-cut test becomes critical, and here is where a lot of students misunderstand its purpose. During your run-up, you perform a magneto check by moving the ignition switch from BOTH to the right mag, back to BOTH, then to the left mag. Most students know this checks whether each magneto is functioning and whether the RPM drop stays within limits — typically no more than 125 RPM drop on either mag and no more than 50 RPM difference between them.

But the dead-cut test is a separate check. After the standard mag check, you briefly move the switch to OFF and then immediately back to BOTH. The engine should stumble or nearly quit — you want to see that momentary RPM drop. If the engine keeps running smoothly with no response when you switch to OFF, that magneto is not being grounded. The p-lead is not doing its job. No drop during the dead-cut test means you likely have a hot mag on your hands.

This is a critical distinction: the mag check verifies ignition performance, but the dead-cut test verifies that the ignition can actually be turned off. Both checks serve different purposes, and conflating them is a mistake that could leave you taxiing back with an unsafe aircraft and no idea why.

What to Tell Your Examiner

When your designated pilot examiner asks about the p-lead, they are not just testing vocabulary. They want to know whether you understand the safety implications from ramp to runup to shutdown. A complete answer covers four points: the p-lead grounds the magneto when the key is off; a broken p-lead leaves the magneto permanently hot; the dead-cut test during run-up is the primary way to detect this failure; and a suspected broken p-lead means the flight is cancelled, the aircraft is secured, and anyone near the prop is warned immediately.

Referencing the ignition system section of Chapter 7 in the PHAK demonstrates you have studied the right material. More importantly, showing the examiner that you understand the real-world danger — not just the textbook definition — is what separates a confident pilot from someone who memorized facts without understanding them.

If you want to practice questions like this in a realistic oral exam format, try SimulatedCheckride.com.