top of page

How Magnetos Work in Airplane Engines – The Unsung Heroes of Ignition

Updated: Jan 18

When you think about what keeps an airplane engine running smoothly in flight, the usual suspects come to mind: fuel, oil, cylinders, and maybe even spark plugs. But tucked away inside the engine cowling are two small but vital devices that work tirelessly to keep the combustion process alive—magnetos.


Despite their simple appearance, magnetos are a key reason aircraft engines can run even if all electrical power is lost. Let’s break down what they are, how they work, and why they’re so important for safe flight.



Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >


The Role of a Magneto in an Airplane Engine

A magneto is a self-contained ignition system that generates the electrical spark needed to ignite the fuel-air mixture in the cylinders. Unlike automotive ignition systems that rely on the vehicle’s battery, a magneto works entirely independently of the aircraft’s electrical system.


This means even if the alternator or battery fails mid-flight, your engine will keep running—one of the many redundancy features designed into aviation.


The Basic Principle: Electricity from Magnets

At its core, a magneto operates on a simple principle: moving a magnet past a coil of wire induces an electrical current.


Here’s a simplified breakdown:

  1. Permanent Magnet – Inside the magneto, a strong magnet spins (driven by the engine’s crankshaft).

  2. Coil of Wire – A winding of copper wire sits in the magnetic field.

  3. Induced Current – As the magnet spins, the magnetic field changes, producing an electric current in the coil.

  4. High Voltage Boost – This small voltage is stepped up by a transformer-like coil system to several thousand volts.

  5. Spark Delivery – The high voltage travels through the ignition harness to a spark plug, jumping the gap and igniting the mixture.


Why Two Magnetos?

Aircraft piston engines almost always have two magnetos—a left and a right—each driving its own set of spark plugs. For example, a four-cylinder engine will have eight spark plugs, with each cylinder having two plugs fired by separate magnetos.


Benefits of dual magnetos:

  • Redundancy – If one magneto fails, the other keeps the engine running.

  • More Complete Combustion – Two sparks in each cylinder ensure a faster, more even burn of the fuel-air mixture.

  • Safety in All Conditions – Even with partial system failure, power loss is minimized.


Inside the Magneto: Key Components

  1. Rotating Magnet – Creates the changing magnetic field.

  2. Primary Coil – Generates a low-voltage current.

  3. Secondary Coil – Steps up voltage to the level needed for a spark (typically 12,000–20,000 volts).

  4. Breaker Points – Open and close mechanically to control when the spark is generated.

  5. Capacitor (Condenser) – Prevents arcing at the breaker points and helps produce a sharper spark.

  6. Distributor Block – Routes the spark to the correct spark plug in the firing order.


The Firing Process

When the engine crankshaft turns:

  1. The rotating magnet sweeps past the coil, generating current.

  2. At the exact right timing (measured in degrees before top dead center), the breaker points open.

  3. Current stops flowing through the primary coil, collapsing the magnetic field.

  4. The sudden collapse induces a high-voltage surge in the secondary coil.

  5. This surge travels through the distributor to the correct spark plug.

  6. The spark ignites the fuel-air mixture in the cylinder.


Timing is critical—just a few degrees too early or late can cause rough running or engine damage.


Magneto Checks in Flight Operations

During the run-up before takeoff, pilots check each magneto by turning the ignition switch to LEFT and RIGHT individually.

  • A small drop in RPM is normal (usually 50–150 RPM).

  • A large drop or rough running could indicate a faulty plug, ignition lead, or magneto problem.


This simple test ensures both systems are working before committing to takeoff.


Maintenance and Care

While magnetos are rugged, they do require periodic inspection and servicing:

  • Timing Adjustments – Ensuring spark occurs at the correct crankshaft position.

  • Internal Cleaning – Removing carbon tracking or corrosion.

  • Component Replacement – Breaker points, capacitors, and coils wear over time.


The FAA generally recommends inspecting magnetos every 500 hours of operation or as specified by the manufacturer.


Why Magnetos Still Matter in Modern Aviation

Even in an age of electronic ignition and FADEC (Full Authority Digital Engine Control), magnetos remain a tried-and-true system in general aviation. Their mechanical simplicity, independence from the electrical bus, and proven reliability make them a cornerstone of piston engine safety.


In short: without them, that reassuring hum of your Lycoming or Continental engine wouldn’t be so reassuring.



Study Full Aviation Courses:

wifiCFI's full suite of aviation courses has everything you need to go from brand new to flight instructor and airline pilot! Check out any of the courses below for free:


Study Courses:


Checkride Lesson Plans:


Teaching Courses:



 
 
bottom of page