Magnetos, Ignition and the Mag Check
Aircraft magnetos are independent ignition generators providing redundant spark; the mag check verifies both function properly by confirming predictable RPM drops within POH limits when operating individually.
Magnetos, Ignition and the Mag Check
Most training aircraft rely on a dual-magneto ignition system that's fundamentally different from the electrical system in your car. Understanding how magnetos work and why we check them before every flight is essential knowledge for safe aircraft operation.
How Aircraft Magnetos Work
A magneto is a self-contained electrical generator that produces high voltage to fire the spark plugs, completely independent of the aircraft's electrical system. Unlike automotive ignition systems that depend on a battery, magnetos generate their own electricity through engine rotation. Inside each magneto, a rotating magnet induces current in coils, which is then amplified to thousands of volts and distributed to the spark plugs.
Most aircraft engines use two separate magnetos—typically called "left" and "right"—each firing its own set of spark plugs in each cylinder. This means each cylinder has two spark plugs, creating redundancy and improved combustion efficiency. If one magneto fails, the engine continues running on the other, though with reduced power and efficiency.
The magneto switch in the cockpit (often labeled "L-R-BOTH-OFF") controls grounding wires. In the "BOTH" position, neither magneto is grounded, so both operate. When you select "L" or "R," you're actually grounding one magneto to stop it from firing. This counterintuitive design is a critical safety feature: if a grounding wire breaks, the magneto defaults to the "on" position rather than failing off.
Why We Perform Mag Checks
The pre-takeoff magneto check serves multiple purposes. First, it verifies both magnetos are functioning properly before committing to flight. Second, it confirms each magneto can independently sustain engine operation. Third, it checks that the ignition switch properly grounds each magneto when selected.
During the mag check, you're looking for a small, predictable RPM drop when switching from "BOTH" to a single magneto. This drop occurs because combustion becomes less efficient with only one spark plug firing per cylinder instead of two. The Pilot's Operating Handbook (POH) specifies acceptable limits, typically around 125-175 RPM drop maximum, with no more than 50 RPM difference between the two magnetos.
An excessive drop indicates problems: fouled spark plugs, incorrect timing, or magneto malfunction. No drop at all is actually worse—it suggests the other magneto isn't firing at all, or the grounding wire is broken. A rough-running engine on one mag might indicate a dead cylinder on that magneto's circuit.
Proper Mag Check Technique
Perform the mag check at the manufacturer's specified RPM, usually between 1700-2000 RPM. This higher setting provides better indication than idle and helps burn off any lead deposits on the spark plugs. Lower RPM can produce misleadingly large drops that don't indicate actual problems.
Move the ignition switch smoothly but deliberately: BOTH → RIGHT → BOTH → LEFT → BOTH. Always return to "BOTH" between checks and before returning to idle. Note the RPM on "BOTH," then the drop on each individual magneto. Some pilots verbalize the numbers: "Both, eighteen hundred... right, seventeen hundred, drop one hundred."
Never linger on a single magneto longer than necessary—you're operating with reduced redundancy and creating more lead deposits on the inactive plugs. If you need to repeat a check due to an excessive drop, try leaning the mixture slightly and running at higher RPM for 10-20 seconds to clear fouled plugs, then recheck.
Common Mistakes and Misconceptions
Many pilots perform mag checks too quickly, not allowing RPM to stabilize on each magneto. Wait two to three seconds after switching before reading the tachometer. Rushing through the check can cause you to miss important indications.
Another common error is checking mags at idle or very low RPM, which exaggerates normal drops and can cause unnecessary concern. Always use the POH-specified RPM.
Some pilots mistakenly believe a larger drop on one mag compared to the other is always acceptable if both are within limits. While often true, a significant split (approaching that 50 RPM differential limit) warrants investigation and trend monitoring. What's acceptable today might indicate developing problems.
Finally, never assume an engine is safe because it runs smoothly on "BOTH." A completely dead magneto won't be noticed until you check each individually. This is why the mag check is mandatory, not optional.
After Landing Checks
Before shutdown, briefly check that the ignition switch properly grounds each magneto. At idle, momentarily select each individual magneto—the engine should continue running. Then, just before mixture cutoff, briefly rotate to "OFF"—the engine should quit immediately. If it continues running (a "hot mag"), you have a dangerous grounding problem requiring immediate maintenance attention.
--- *Reference and study only — not for operational use. Always follow your POH/AFM, current regulations and a qualified instructor.*