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PHAK · PHAK Chapter 6

Magnetos and the Ignition System

Master aircraft magnetos and the dual ignition system: how magnetos work, the run-up mag check, hot mags, and what RPM drops really mean.

CFI's Whiteboard Explanation

Your piston engine has its own private electrical system just for ignition—two magnetos, each firing one spark plug per cylinder. They're engine-driven, so the engine keeps running even if the battery and alternator die.

During run-up you switch from BOTH to R, then BOTH to L, watching for a small RPM drop (usually under 150 RPM, with less than ~50 RPM difference between mags). A drop that's too big = fouled plug or bad mag. No drop at all = a hot mag—the P-lead isn't grounding, and the prop could fire even with the key OFF. Always treat the prop like it's live.

Handbook Reference
PHAK Ch 6

6.magnetos-and-ignition. Magnetos and the Ignition System

The ignition system in a reciprocating aircraft engine provides the spark that ignites the fuel/air mixture in each cylinder at the proper moment. Unlike an automobile, which relies on the battery and alternator for ignition, certificated piston aircraft use a dual magneto ignition system that is completely independent of the aircraft electrical system. This independence is a critical safety feature: the engine will continue running even after a complete electrical failure, alternator loss, or dead battery.

Magneto Construction and Operation

A magneto is a self-contained, engine-driven AC generator that produces high-voltage current using a permanent magnet, a primary and secondary coil, breaker points, a condenser, and a distributor. As the engine rotates the magneto's permanent magnet past the coil, a low-voltage current is induced in the primary winding. When the breaker points open at the precise instant of ignition timing, the magnetic field collapses, inducing a very high voltage (roughly 20,000 volts) in the secondary winding. The distributor routes this high-voltage pulse to the correct spark plug at the correct time in the firing order.

Because the magneto's output depends on the speed of the rotating magnet, magneto voltage—and hence spark intensity—is weak at low RPM. To overcome this during engine start, most systems incorporate an impulse coupling (a spring-loaded device that snaps the magnet through the coil rapidly) or a starting vibrator (a buzz coil that supplies a continuous shower of sparks). After start, the magneto operates normally on engine rotation alone.

Dual Ignition

Each cylinder has two spark plugs, one fired by the left magneto and one by the right magneto. Dual ignition provides two important benefits:

  • Redundancy. If one magneto fails, the engine continues to run on the other, although with a slight power loss and rougher operation.
  • More complete combustion. Two flame fronts originate simultaneously from opposite sides of the combustion chamber, producing more uniform burning, slightly higher power output (typically 75-100 RPM), better fuel economy, and lower cylinder head temperatures.

The Ignition Switch

The ignition switch has five positions: OFF, R (right), L (left), BOTH, and START. In flight the switch is normally set to BOTH. The R and L positions are used only during the magneto check on the ground; BOTH is required for takeoff and all normal operations. The START position is spring-loaded and engages the starter motor; on release it returns to BOTH.

The magneto switch is unique in that the OFF position grounds the P-lead (primary lead) of each magneto, short-circuiting the primary winding and preventing spark. If a P-lead becomes disconnected or broken, the magneto is hot—it will fire any time the propeller is turned, even with the master and ignition switches OFF. For this reason, the propeller must always be treated as live, and pilots should never move a propeller by hand without proper precautions.

The Magneto Check (Run-up)

During the before-takeoff check, the pilot performs an ignition check at the manufacturer's specified RPM (commonly 1,700-2,000 RPM in trainers). The procedure:

  1. Set throttle to the prescribed run-up RPM with the switch on BOTH; note RPM.
  2. Move switch from BOTH to R; note the RPM drop, then return to BOTH and allow RPM to stabilize.
  3. Move switch from BOTH to L; note the RPM drop, then return to BOTH.

A small, normal RPM drop (typically 100-175 RPM, manufacturer-specified maximum often 150 RPM, with a maximum differential between mags of about 50 RPM) confirms each magneto and its plugs are operating. Common findings and interpretation:

  • Excessive drop on one magneto: fouled plugs, faulty harness, bad plug, or weak magneto.
  • No drop at all: a grounding (P-lead) problem—the magneto being switched off is not actually being grounded, indicating a hot mag condition. This must be investigated before flight.
  • Engine quits when switched to one mag: that magneto or its harness has failed completely.

A fouled spark plug (typically from lead deposits at idle or excessively rich mixture) can sometimes be cleared by leaning the mixture aggressively at run-up RPM for 15-30 seconds and re-checking.

Operational Considerations

  • Always perform the mag check at the recommended RPM; doing it at idle produces unreliable results.
  • Return the switch to BOTH between L and R selections to avoid extended operation on a single magneto, which causes raw fuel to accumulate.
  • After shutdown using the mixture (idle cutoff), turn the ignition switch to OFF and verify the key is removed; this is the only positive way to verify the P-leads ground properly.
  • An end-of-flight grounding check (briefly going to OFF at low idle and noting the engine begins to die before returning to BOTH) confirms P-lead integrity.

Understanding the magneto system allows the pilot to recognize anomalies during run-up, diagnose in-flight roughness, and appreciate why a propeller is treated as armed any time the aircraft is approached.

Oral Exam Questions a DPE Might Ask
Q1Why does an aircraft engine use magnetos instead of relying on the battery for ignition?
Magnetos are self-contained, engine-driven generators that produce their own high-voltage spark independent of the aircraft's electrical system. This means the engine will keep running even if the alternator fails or the battery is dead.
Q2During your mag check the RPM doesn't drop at all when you select one magneto. What does that tell you?
Zero RPM drop indicates a P-lead grounding problem—the magneto being switched off isn't actually being grounded, meaning it's a 'hot mag.' The propeller could fire even with the ignition OFF, so the aircraft is unsafe to fly until it's corrected.
Q3What is the purpose of dual ignition, and what RPM drop is typically acceptable on the mag check?
Dual ignition provides redundancy if one magneto fails and produces more complete combustion from two flame fronts, gaining roughly 75-100 RPM. A typical acceptable drop is around 100-150 RPM per magneto with no more than about 50 RPM difference between the two, but always reference the POH limits.
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