Magneto Inspection and Troubleshooting

The magneto is one of the most safety-critical components on a reciprocating aircraft engine. Unlike the electrical system in an automobile, aircraft magnetos operate completely independently of the battery and alternator, generating their own high-voltage current to fire the spark plugs at precisely the right moment. This self-contained design means that even a total electrical failure leaves the engine running — but it also means that a magneto problem can silently rob the engine of power, efficiency, or reliability. For Aviation Maintenance Technicians (AMTs) working on powerplant systems, understanding how to inspect, test, and troubleshoot magnetos is not just a knowledge-test requirement — it is a fundamental airworthiness skill.

This article covers the complete inspection and troubleshooting workflow for aircraft magnetos, grounded in FAA guidance from the Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32) and relevant sections of 14 CFR Part 43.

How the Magneto Works

A magneto is a self-contained alternating-current generator that produces a high-voltage spark independent of any external power source. It consists of five main elements: a rotating permanent magnet, a coil assembly (primary and secondary windings), a breaker point assembly (or electronic ignition trigger), a condenser (capacitor), and a distributor. As the engine crankshaft turns, it rotates the magneto's permanent magnet via a gear or coupling. The rotating magnet induces a magnetic field through the coil core.

When the breaker points open at a precise crankshaft angle, the primary circuit collapses suddenly. The condenser prevents arcing across the points by absorbing the sudden surge of current and then discharging it, sharpening the collapse. This rapid field collapse induces a very high voltage — capable of reaching up to 20,000 volts or more depending on magneto design — in the secondary winding. The distributor then routes that high-voltage pulse to the correct spark plug at the correct time in the engine cycle. Most aircraft engines use a dual magneto system: two completely independent magnetos (left and right) that each fire one spark plug per cylinder. This redundancy improves combustion efficiency and provides a safety backup.

Routine Inspection Procedures

FAA guidance emphasizes that magneto inspections follow a structured approach tied to the manufacturer's maintenance manual and the aircraft's inspection program. During routine inspections, the AMT should address the following areas:

External condition: Check the magneto housing for cracks, corrosion, or security of attachment. Inspect all leads and harness grommets for chafing, cracking, or moisture intrusion.
Ignition leads (high-tension leads): Inspect leads for insulation condition, proper routing, and secure connection at both the spark plug and distributor block ends. Cracked or deteriorated insulation allows high-voltage leakage, especially in damp conditions.
Distributor block and rotor: Look for carbon tracking — thin, conductive carbon paths that form on the distributor block from arcing. Carbon tracking is a common cause of misfires and cross-firing between cylinders. The rotor should be free of cracks, and the carbon brush should be checked per the manufacturer's maintenance manual for proper contact and condition.
Breaker points (on point-type magnetos): Inspect the points for pitting, burning, and proper gap. Breaker point gap varies by magneto model — always verify the specific setting with the manufacturer's data rather than assuming a single typical value. Points with excessive pitting or a rounded contact surface must be replaced, not dressed with a file.
Condenser: The condenser should be tested with a condenser tester for capacity (usually in the range of 0.3 to 0.4 microfarads), series resistance, and leakage. A shorted or open condenser causes severe point burning and unreliable ignition.
Timing gears and impulse coupling: Inspect the impulse coupling for worn pawls, weak springs, or damaged flyweights. The impulse coupling retards ignition timing during starting to prevent kickback and produces a hot spark at low RPM. A malfunctioning impulse coupling is a frequent cause of hard starting.

Magneto-to-Engine Timing

Accurate magneto timing is arguably the most critical element of the inspection. Timing defines exactly when the spark plug fires relative to piston position, expressed in degrees of crankshaft rotation before top dead center (BTDC). For most normally aspirated engines, timing is set somewhere between 20° and 30° BTDC, though the exact value is always found in the engine manufacturer's specifications — not assumed.

Timing is verified using a timing light or a timing buzzer/continuity tester in conjunction with the engine's timing marks on the crankshaft flange or prop flange. The procedure involves rotating the engine to the specified BTDC position for the number-one cylinder on its compression stroke, then checking that the magneto's internal timing (the E-gap position) coincides with the breaker points just beginning to open. E-gap is the angular position of the rotating magnet at which the rate of change of magnetic flux through the coil core is greatest — the ideal moment for the points to open to produce maximum spark energy. If timing is off, the spark fires either too early (advanced) or too late (retarded), reducing efficiency and potentially damaging the engine.

The magneto must also be timed internally before it is timed to the engine. Internal timing sets the relationship between the breaker points opening and the magneto's own E-gap position. This is done on the bench using a continuity light and a timing fixture or the magneto's own timing marks.

The Magneto Drop Check

The magneto drop check (also called the mag check) performed during the pre-takeoff runup is the primary in-service operational test of the ignition system. At a specified RPM (typically 1,800 to 2,000 RPM as specified in the POH/AFM), the pilot switches from BOTH to each individual magneto position. A small RPM drop is expected and normal — commonly cited general guidance allows up to about 150 RPM drop on either single magneto, and up to about 50 RPM difference between the two individual magneto readings, though these values vary by aircraft and engine — always defer to the POH limits.

As the technician, understanding what abnormal mag check results indicate is crucial for troubleshooting:

Excessive drop on one magneto: Suggests a problem on that side — fouled or failed spark plugs, broken ignition lead, internal magneto fault, or timing error.
No drop at all: This seemingly good result is actually dangerous. No RPM drop when switching to a single magneto means that magneto is not actually being isolated — the ground wire (P-lead) may be broken or disconnected. The off magneto is still firing, so the engine does not change. A broken P-lead also means the magneto cannot be shut down from the cockpit, creating a serious safety hazard.
Rough running on one magneto without a large RPM drop: Suggests cross-firing or a fouled plug causing poor combustion quality on specific cylinders.
Large drop with roughness: Often points to one or more completely dead cylinders — failed lead, cracked distributor, or defective spark plug.

Why It Matters

Because aircraft magnetos have no backup from the aircraft electrical system, any degradation goes undetected by the ammeter or voltmeter. A partially failed magneto can cause subtle power loss, increased fuel consumption, and elevated cylinder head temperatures — all signs easily attributed to other causes. In the worst case, a magneto failure at a critical flight phase can be catastrophic. The AMT's rigorous inspection and accurate timing work are literally life-safety functions. Additionally, 14 CFR Part 43 requires that maintenance on ignition systems be performed and recorded per approved data, reinforcing the regulatory weight behind these procedures.

Key Numbers and Rules

Breaker point gap: Varies by magneto model — always verify with the manufacturer's manual.
Condenser capacity: Typically 0.3–0.4 microfarads; test for capacity, series resistance, and leakage.
Magneto timing: Usually 20°–30° BTDC depending on engine model — always use the engine manufacturer's specification.
Maximum RPM drop (single mag): Commonly cited guidance allows up to about 150 RPM — verify in the aircraft's POH.
Maximum differential between mags: Commonly cited guidance allows up to about 50 RPM difference — verify in the aircraft's POH.
No RPM drop on mag check: Indicates a broken P-lead — the magneto cannot be grounded and cannot be shut off from the cockpit.
High-tension lead voltage: Up to 20,000 volts or more depending on design; insulation condition and routing are critical safety items.

Common Test Traps

Confusing no-drop with a good result: The FAA knowledge test frequently presents a scenario where zero RPM drop during a mag check sounds ideal. It is not — it almost always indicates a broken P-lead, meaning the magneto cannot be grounded or shut off, which is a serious airworthiness defect.
Assuming timing specs are universal: Students sometimes memorize a single timing value. In reality, timing specifications vary by engine model and must always be taken from the manufacturer's approved data, not from a generic rule of thumb.
Dressing pitted breaker points: Filing or dressing breaker points is not an approved repair. Points with significant pitting or burning must be replaced. Using a file changes the contact geometry and does not restore proper conductivity.
Ignoring internal timing before engine timing: A magneto must be correctly timed internally (E-gap vs. points opening relationship) before it can be accurately timed to the engine. Skipping internal timing and going straight to engine timing produces incorrect results.
Mistaking impulse coupling failure for magneto failure: A worn or broken impulse coupling causes hard starting and may cause a backfire, but once the engine reaches operating RPM the coupling disengages. Students sometimes confuse this symptom pattern with a primary magneto coil failure, which would cause persistent misfires across all RPM ranges.