How Does a Magneto Ignition System Work, and Why Does an Aircraft Engine Have Two Magnetos?

What Is a Magneto and How Does It Generate Power?

Most student pilots assume that everything electrical in an airplane traces back to the battery. The magneto ignition system is one of the most important exceptions to that assumption — and misunderstanding it is one of the most common mistakes candidates make during the oral exam.

A magneto is a self-contained electrical generator. It produces its own high-voltage current using a rotating magnet and a set of coils, requiring no input from the aircraft battery or alternator whatsoever. Once the engine is turning, the magnetos are generating. This is by design: if your battery dies or your alternator fails in flight, your engine keeps running without interruption. The ignition system is completely isolated from the rest of the electrical system, which is a foundational concept covered in Chapter 7 of the Pilot's Handbook of Aeronautical Knowledge (PHAK), FAA-H-8083-25.

The high-voltage current the magneto produces travels through ignition leads to the spark plugs, where it ignites the fuel-air mixture inside each cylinder. The timing of this spark is precisely calibrated to fire slightly before the piston reaches top dead center, maximizing the energy extracted from combustion.

Why Two Magnetos? Understanding Redundancy and Efficiency

Your aircraft engine does not have two magnetos because one would not be enough to start the engine — it has two because losing one in flight should never mean losing the engine. This is the redundancy argument, and your examiner will want you to articulate it clearly.

Each magneto fires one spark plug per cylinder. Because each cylinder has two spark plugs, each plug is fired by a separate, independent magneto. The left magneto fires the left plug in each cylinder; the right magneto fires the right plug. If one magneto fails completely, the other continues firing its set of plugs, and the engine keeps running. You will likely experience a modest reduction in power and efficiency, but you can continue the flight and land safely. That is the entire point of the dual-ignition design.

The benefits do not stop at redundancy. With two spark plugs firing in each cylinder, the fuel-air mixture ignites from two points simultaneously. This produces a more complete and efficient burn, which translates directly into better engine performance and power output. So the two-magneto system is not just a safety feature — it is also an engineering advantage under normal operating conditions.

The Magneto Check: What You Are Actually Testing

The pre-takeoff magneto check is a procedure most student pilots can perform, but fewer can fully explain — and your examiner knows the difference. During the run-up, you switch from BOTH to one magneto at a time and note the RPM drop. A typical acceptable drop is around 125 RPM on a single mag, with no more than 50 RPM difference between the two. An excessive drop usually indicates a fouled plug or a timing issue.

Here is where many candidates stumble: the dead-cut check. At the end of the mag check, you briefly move the key to OFF and then quickly back to BOTH. The engine should stumble or drop slightly when you hit OFF. If it does not — if there is no change when you select OFF — that is not a good sign. It means the p-lead on at least one magneto may be broken or disconnected.

The p-lead is the wire that grounds the magneto when you turn the ignition key off. Grounding the magneto is what actually shuts it down. A broken p-lead means the magneto remains live even with the key in the OFF position. The engine will shut down normally because you cut fuel, but the magneto is still capable of firing if the propeller is moved by hand — a serious ground safety hazard. This is why the dead-cut check matters, and it is exactly the kind of nuanced follow-up question an examiner may use to separate a prepared candidate from one who just memorized the checklist steps.

Putting It All Together for Your Oral Exam

When your examiner asks about the magneto system, they are not just checking whether you know a definition. They want to see that you understand the system at a functional level: why it is designed the way it is, what happens when part of it fails, and how the pre-flight checks connect to real-world safety.

Be ready to explain that magnetos are self-powered and independent of the battery. Know that each magneto fires only one plug per cylinder — not both. Understand that a single magneto failure allows continued flight, just with reduced performance. And be prepared to explain the dead-cut check and what a failed response tells you about p-lead integrity.

These details are all grounded in the PHAK Chapter 7 ignition system discussion, and mastering them shows your examiner you are thinking like a pilot, not just reciting facts.

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