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Aircraft Magnetos Explained: How They Work, the Magneto Check, and Why the Prop Is Always Live

Updated: 1 day ago

Most pilots know the magneto check is part of the runup. Fewer can explain exactly what they're checking for and why. Even fewer know that the magneto system is the reason the propeller of any piston aircraft must always be treated as if the engine could fire — even with the master switch off, even with the key out of the ignition.


This post covers how magnetos work, why aircraft use two of them, what the runup check is actually testing, how to interpret abnormal results, and the safety implications that every pilot should understand from the first day of training.



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What a Magneto Does and Why It Matters

An aircraft magneto is a self-contained electrical generator that produces the high-voltage spark needed to ignite the fuel-air mixture in each cylinder. The key word is self-contained — a magneto generates its own electricity from rotating permanent magnets and requires no external power source of any kind. It is completely independent of the aircraft's battery, alternator, and electrical bus.


This independence is intentional and critical. If the alternator fails, if the battery is dead, if the master switch is off — none of it matters. As long as the engine is turning, the magnetos are generating spark and the engine will continue to run. This is fundamentally different from an automotive ignition system, which depends on battery power and stops working if electrical power is interrupted.


The implication for emergency procedures: in the event of a complete electrical failure in flight, the engine keeps running. You lose your radios, your avionics, your electric fuel pump, your flaps if they're electrically operated — but you do not lose the engine.


How a Magneto Generates Spark

The operating principle is electromagnetic induction — the same principle underlying every electrical generator. Moving a magnetic field past a conductor induces an electrical current in that conductor.


Inside the magneto, a set of permanent magnets is mounted on a rotor driven directly by the engine's accessory drive (and therefore by the crankshaft). As the rotor spins, the rotating magnetic field passes through a coil of wire — the primary winding. This induces a low-voltage current in the primary coil.


The key to generating the high voltage needed to jump the spark plug gap is a rapid, controlled collapse of the magnetic field in the primary coil. This is accomplished by breaker points — a mechanical switch that opens at the precise moment in the rotation cycle when maximum current is flowing in the primary coil. When the breaker points open, current flow in the primary stops abruptly. The sudden collapse of the magnetic field in the primary coil induces a very high-voltage surge in the secondary coil through transformer action — typically 12,000 to 20,000 volts.


A capacitor (condenser) connected in parallel with the breaker points prevents arcing across the points when they open and helps produce a sharper, cleaner collapse of the primary current, which improves the quality of the secondary voltage spike.


The high-voltage surge then travels through the ignition harness — the shielded cables running from the magneto to the spark plugs — and through the distributor, which routes the spark to the correct cylinder in the firing order at exactly the right moment.


Ignition Timing

The spark must arrive at the spark plug at a precise moment in the piston's travel — typically 20–30 degrees of crankshaft rotation before the piston reaches Top Dead Center (TDC) on the compression stroke. This advance allows the flame front to develop fully so that peak combustion pressure occurs just after TDC, pushing the piston down at the most mechanically efficient angle.


Timing that is too advanced (spark fires too early) causes the combustion pressure to build while the piston is still rising, fighting against the combustion force — this can cause detonation and engine damage. Timing too retarded (spark fires too late) means the piston has already passed TDC before combustion builds, reducing power and increasing exhaust temperatures.


Magneto timing is set during maintenance and should be verified during annual inspection. Changes in timing can occur due to normal wear of the breaker points or mechanical components, which is why periodic inspection matters.


The Impulse Coupling

Starting a piston engine at low cranking speed presents a problem: magnetos produce their best spark at higher rotational speeds. At the slow speeds of an engine being hand-cranked or starter-turned, the magneto may not generate enough voltage for reliable ignition.


Most aircraft engines use an impulse coupling on one magneto (typically the left) to solve this. The impulse coupling is a spring-loaded mechanical device in the magneto drive that temporarily stores rotational energy during slow cranking, then releases it suddenly — giving the magneto rotor a brief rapid spin at the moment of ignition. This produces a strong spark even at low cranking speeds, and also slightly retards ignition timing at startup to prevent kickback (the engine reversing direction momentarily on a strong compression stroke during start).


Once the engine starts and reaches normal RPM, the impulse coupling disengages and the magneto runs normally. You can often hear the impulse coupling as a distinct clicking sound during engine cranking.


The magneto without the impulse coupling (typically the right) is used to verify both ignition systems are functioning during the runup but relies on the left with impulse coupling for the initial start.


Why Two Magnetos

Standard GA piston engines have two completely separate magnetos — left and right — each with its own set of spark plugs. Each cylinder has two spark plugs, one fired by the left magneto and one by the right. A four-cylinder engine has eight spark plugs. A six-cylinder has twelve.


The dual system provides two independent benefits:

  • Redundancy: If one magneto fails completely in flight, the other continues to fire its set of plugs and the engine keeps running. You'll notice a power reduction and may notice rough running, but you won't lose the engine. The magneto check during runup is specifically designed to verify this redundancy before every flight.

  • Combustion efficiency: Two spark plugs firing simultaneously from opposite sides of each cylinder create two flame fronts that advance toward each other. This burns the fuel-air charge more completely and more quickly than a single plug firing from one side. The result is slightly more power, more consistent combustion, and lower peak cylinder temperatures compared to single-plug ignition.


The Magneto Check: What You're Actually Testing

The magneto check during runup is one of the most important items on the before-takeoff checklist — not because a problem is expected, but because it verifies the redundancy you're depending on for the flight ahead. Here's what's actually happening:


Setting up: Engine at runup RPM (typically 1,700–1,800 RPM per the POH). Mixture rich.

BOTH to LEFT: You've disconnected the right magneto from the circuit by switching to LEFT only. The engine is now running on the left magneto and left spark plugs only. You're testing the left magneto in isolation.


The engine should continue to run but with a slight RPM drop — typically 50–150 RPM depending on the engine. This drop is normal and expected. It occurs because you've gone from two flame fronts per cylinder to one, which is slightly less efficient combustion. The engine runs on one magneto but not quite as well as on both.


LEFT back to BOTH, then BOTH to RIGHT: Same test on the right magneto.

Back to BOTH: Verify RPM returns to normal and the engine is running smoothly.

Interpreting Abnormal Magneto Check Results


Large RPM drop (more than ~150 RPM or your POH limit): Indicates that magneto is producing significantly weaker spark than normal. Could be fouled spark plugs on that side, a failing magneto, a cracked distributor block, or damaged ignition leads. Do not depart — investigate before flight.


Rough engine on one magneto: Indicates uneven combustion when running on that set of plugs alone. Most commonly caused by fouled spark plugs on the rough side. Can often be cleared by running at high RPM with a lean mixture for a minute to burn off lead fouling. If the roughness doesn't clear, investigate before flight.


No RPM drop when switching to a single magneto: This is the most concerning result. If switching to one magneto produces no RPM drop at all, it means that magneto's plugs are not contributing anything — the engine is already running entirely on the other set. The magneto you just selected may be completely dead, or there may be a P-lead grounding issue (explained below) that is keeping the magneto from firing when it should, while also grounding it when it shouldn't.


RPM drop on both magnetos is equal: Normal. The key is that both drops are within limits and neither side runs rough.


The P-Lead and Grounding: The Safety Issue Every Pilot Must Know

The ignition switch does not interrupt power to the magneto the way a light switch turns off a light — there's no power flowing to a magneto from an external source to interrupt. Instead, the ignition switch controls a P-lead (primary lead) that grounds the primary coil of each magneto when the switch is moved to the OFF position.


When the primary coil is grounded, it shorts out the current and prevents the voltage spike from reaching the secondary coil and spark plugs. The magneto's rotor is still spinning, still generating current in the primary — but that current is immediately shunted to ground and never reaches the plugs.


The critical safety implication: If the P-lead is broken, disconnected, or has an open circuit fault, the ignition switch has no effect on that magneto. The magneto will continue to fire regardless of switch position. A magneto in this condition is called a "hot magneto."


A hot magneto means:

  • Turning the ignition switch to OFF does not stop that magneto from firing

  • If someone pulls the propeller through by hand — for a compression check, clearing a flooded engine, or any other reason — the engine could fire and start


This is why the propeller of any piston aircraft must always be treated as capable of starting, regardless of what the ignition switch says, regardless of whether the master is on or off. "All clear" before pulling the prop through, standing clear of the prop arc, and treating every propeller as potentially live are not just good habits — they're the correct response to the real possibility of a P-lead failure.


How to check for a hot magneto: During the runup, after the magneto check, briefly turn the ignition switch to OFF at runup RPM. The engine should immediately stop running (or begin to stop) as both magnetos are grounded. Immediately return to BOTH. If the engine doesn't respond to the OFF position, you have a grounding problem with at least one magneto. Do not fly — this must be investigated by a mechanic before departure.


Magneto Maintenance Intervals

Magnetos are mechanical devices subject to wear and require periodic inspection and overhaul. The FAA and most manufacturers recommend inspection every 500 hours of operation and complete overhaul every 500 hours or at engine TBO, whichever comes first.


During inspection:

  • Breaker points are checked for gap, condition, and correct opening

  • Capacitor is tested

  • Distributor block is inspected for carbon tracking (arcing paths that can cause misfires)

  • Ignition timing is verified

  • Harness continuity and insulation are checked


Between scheduled inspections, the runup magneto check is the pilot's primary tool for monitoring ignition system health.


Electronic Ignition: The Alternative

Some aircraft have replaced one or both magnetos with electronic ignition systems — computer-controlled ignition that continuously optimizes spark timing based on RPM, manifold pressure, and other parameters. Electronic ignition can improve fuel efficiency, reduce engine roughness at idle, and extend spark plug life.


The trade-off is that electronic ignition systems typically require the aircraft's electrical power to operate — they're not independent of the electrical bus the way magnetos are. For this reason, most certified GA aircraft that convert to electronic ignition retain at least one magneto for redundancy. FADEC (Full Authority Digital Engine Control) systems used in some modern certified aircraft integrate electronic ignition with electronic fuel control, but these designs include battery backup specifically to address the electrical dependency.


For the vast majority of GA aircraft currently flying, the dual magneto system remains the standard.



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Author: Nathan Hodell

CFI, CFII, MEI, ATP, Creator and CEO

Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.



 
 
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