Magneto & Ignition System

The ignition system of a piston aircraft engine exists to deliver a precisely timed, high-voltage spark to each cylinder during the compression stroke. Unlike a car, an aircraft engine does not rely on the battery for its spark. Instead it uses magnetos: self-contained, engine-driven electrical generators that produce their own high-voltage current the moment the crankshaft is turning fast enough. As described in the FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), Chapter 7, this independence is a deliberate safety feature. Because the magneto makes its own electricity, the engine keeps running and keeps making spark even if the aircraft's electrical system, battery, or alternator fails completely.

Certificated trainers use a dual-magneto, dual-ignition arrangement. Each engine carries two magnetos, and each cylinder has two spark plugs. One magneto fires the top plug of one set of cylinders and the bottom plug of the others; the second magneto fires the remaining plugs, so both plugs in every cylinder fire on each power stroke. This design serves two purposes. First, redundancy: if one magneto, one lead, or one plug fails, the engine still runs on the other ignition system. Second, performance: firing the charge from two points in the cylinder at once produces a faster, more complete burn, which yields slightly more power and smoother running than a single plug. Losing one magneto in flight is not an emergency in itself, but it does produce a noticeable, expected drop in power and roughness.

The ignition switch has positions for OFF, R (right magneto only), L (left magneto only), BOTH (normal operation), and usually a spring-loaded START position that engages the starter. Turning the switch to a single-magneto position does not power one magneto; it grounds out the other. This grounding is done through a small wire called the P-lead. When the switch is OFF, both P-leads are grounded, which shorts the magnetos to prevent them from firing. If a P-lead breaks or its ground connection fails, the magneto is no longer grounded and remains live, or hot, even with the switch OFF. A hot magneto means a hand-turned propeller can start the engine, which has caused serious injuries. This is why propellers are always treated as live, and why the switch OFF and mixture at idle cutoff are both used to secure the engine.

The magneto check is a standard part of the engine run-up. At the specified run-up RPM, the pilot moves the switch from BOTH to R and back to BOTH, noting the RPM drop, then repeats on L. A small drop on each single magneto is normal and confirms both systems are working; no drop at all can indicate a grounding fault, while an excessive drop or rough running points to a fouled plug, a bad lead, or a failing magneto. The pilot's handbook and the aircraft POH give the maximum allowable drop and the maximum allowable difference between the two magnetos. A rough or out-of-limits mag check is a no-go item that becomes a maintenance squawk.

For a flight school, the magneto check is one of the most frequently performed and most frequently fudged checks on the field. Students and even experienced renters can be tempted to accept a marginal mag drop to avoid canceling, so a school's dispatch and training culture has to reinforce that an out-of-limits or rough mag check grounds the aircraft. Fouled plugs from improper leaning are a recurring cause of unnecessary write-ups and lost revenue hours, and they tie the ignition system directly to how well students are taught to lean the mixture on the ground and in cruise.

The hot-magneto and P-lead hazard is a safety-culture issue as much as a technical one. Every person who handles a propeller — students, instructors, line staff, and mechanics — must treat it as capable of starting regardless of switch position. Recurring mag-check discrepancies across a fleet are also a maintenance-trend signal worth watching, because they often point to a specific engine developing a magneto, harness, or plug problem before it becomes an in-flight power loss.

Aviatize's Maintenance Control module turns a rough or out-of-limits magneto check into a tracked defect with a clear trail from the pilot's report to the plug cleaning, harness repair, or magneto overhaul that clears it, and Safety Management captures the hot-magneto and propeller-safety reporting that keeps the hazard visible across the whole operation rather than buried in one instructor's memory.

Aviatize's Training Management module lets instructors grade the run-up magneto check and the correct securing of the engine as assessed items, so the school can confirm each student actually understands what the mag check proves and why a propeller is always treated as live, rather than simply reciting the checklist.