How Do Aircraft Magnetos Work?
Learn how aircraft magnetos create spark for piston engines, why most airplanes use two magnetos, and what students should know during runup.
Magnetos are part of the ignition system on many piston aircraft engines. Their job is to create the high-voltage spark that fires the spark plugs and ignites the fuel-air mixture inside the cylinders.
The student-pilot version is simple: a magneto is a self-contained electrical generator for ignition. It does not need the aircraft battery or alternator to keep the engine running. That independence is one reason magnetos have stayed common in training aircraft.
Why Airplanes Use Magnetos
In a car, the ignition system usually depends on the vehicle’s electrical system. In an airplane, losing electrical power should not automatically mean losing engine power. A magneto solves that problem by generating its own ignition energy as the engine turns.
Most familiar training aircraft engines use two magnetos. Each magneto normally fires one spark plug in each cylinder. Two spark plugs improve combustion and give redundancy. If one magneto fails, the engine can often keep running on the other, though with reduced performance and a reason to land as soon as practical.
This is one part of a bigger piston-engine picture. Fuel grade, mixture, induction icing, and ignition all interact, so it is worth pairing this with types of avgas and carburetor icing during systems study.
The Main Parts
A traditional magneto includes several key pieces:
- Permanent magnets.
- An iron core or armature.
- A primary coil.
- A secondary coil.
- Breaker points.
- A capacitor.
- A distributor or routing system to send the spark to the correct plug.
You do not need to become a mechanic to fly safely, but knowing the flow helps you understand the runup check and engine roughness.
How the Spark Is Created
As the engine turns, magnets move past the magneto’s core and create a magnetic field. Current builds in the primary coil. At the correct moment, the breaker points open, which collapses the magnetic field quickly.
That rapid collapse creates a voltage spike. The secondary coil steps that voltage up dramatically, high enough to jump the spark plug gap. The spark ignites the fuel-air mixture in the cylinder.
The timing matters. The spark must occur at the correct point in the piston’s travel. If it fires too early or too late, the engine can run poorly, lose power, overheat, or suffer damage.
What the Ignition Switch Is Really Doing
The cockpit ignition switch usually has positions such as OFF, R, L, BOTH, and START. During normal operation, you run on BOTH. During runup, you check each magneto individually.
When you select LEFT, you are typically turning off the right magneto and running on the left. When you select RIGHT, you are turning off the left magneto and running on the right. That sounds backward at first, but the practical result is easy: you are checking whether each side can carry the engine by itself.
When the switch is OFF, the magnetos are grounded so they cannot fire. This is why a broken P-lead or ignition grounding problem is serious. A propeller can be dangerous if a magneto is not properly grounded, even when the cockpit switch appears off.
The Magneto Check During Runup
During runup, you set the specified RPM and check the magnetos according to the aircraft checklist. A small RPM drop on each side is normal because the engine is running with fewer spark plugs firing. What you are watching for is whether the drop is within limits and whether the difference between the two sides is acceptable.
A rough mag check may come from fouled spark plugs, mixture issues, ignition trouble, or other engine problems. Sometimes leaning the mixture during ground operations or using an approved clearing procedure can help with lead fouling, but use the aircraft checklist, operating handbook, and instructor guidance. Do not invent your own fix on the ramp.
If the RPM drop is excessive, there is no drop, or the engine runs very rough, stop and troubleshoot before flight. A bad mag check is not something to “watch in the air.”
Advantages
Magnetos are valued because they are independent, compact, and proven. They let the engine keep producing spark even if the battery, alternator, or much of the electrical system fails.
That independence changes how you think about emergencies. An alternator failure may create avionics and battery-management problems, but it should not stop a magneto-equipped piston engine by itself.
If you are also learning constant-speed or high-performance airplane systems, manifold pressure is another engine indication that becomes more important as the aircraft gets more capable.
Limitations and Failure Modes
Magnetos are reliable, but they are not maintenance-free. Oil contamination, worn parts, arcing at the breaker points, internal timing problems, heat, and age can all create issues.
Starting can also be a special case because the engine turns slowly during start. Many systems use an impulse coupling or other starting aid so the spark is strong and timed properly at low cranking speed.
Why Student Pilots Should Care
Understanding magnetos makes the runup more meaningful. You are not just moving a switch because the checklist says so. You are verifying that each independent ignition system can run the engine, that the spark plugs are behaving, and that the engine is ready to produce dependable power.
The magneto system is simple in concept, but it protects you in a big way. Treat the ignition check as a real airworthiness check, not a formality.
Official References
Need help applying this to your training?
Use this guide as a starting point, then bring the confusing parts to a focused ground lesson. Diego works with Louisville-area and remote students on FAA knowledge, oral-prep, and practical training decisions.