The aircraft magneto is one of aviation's most elegantly reliable inventions. Unlike automotive ignition systems that depend entirely on the battery and alternator, an aircraft magneto is a self-contained, engine-driven electrical generator and distribution system that produces the high-voltage sparks needed to ignite the fuel-air mixture in each cylinder. It operates independently of the aircraft's main electrical bus, which means that even a complete electrical failure leaves your ignition system fully functional — provided the engine is turning. Understanding how magnetos are built and how they work is essential knowledge for any Aviation Maintenance Technician (AMT) preparing for the FAA Powerplant exam.
Most certificated piston aircraft engines use two independent magneto systems — one firing each cylinder's top spark plug, the other firing the bottom plug. This dual-ignition arrangement is not redundant in the luxury sense; it is a fundamental airworthiness and combustion-efficiency requirement. Each magneto must be capable of sustaining engine operation on its own, which is why the pre-takeoff magneto check (the "mag check") verifies exactly that.
Core Operating Principle: Electromagnetic Induction
A magneto operates on Faraday's Law of electromagnetic induction: when a magnetic field moves relative to a conductor, a voltage is induced in that conductor. In a magneto, a permanent magnet rotor spins inside a stationary coil assembly. As the rotor turns, the magnetic flux through the coil changes direction, inducing an alternating current in the primary winding of the coil. The magneto harvests and transforms this alternating low voltage into the extremely high voltage — often 15,000 to 20,000 volts or more — required to jump the gap of a spark plug under compression.
This transformation happens in two stages through the ignition coil: first, the primary circuit builds magnetic energy; second, a sudden collapse of the primary circuit's magnetic field induces a massive voltage spike in the secondary winding. The ratio of secondary to primary windings determines the step-up ratio, and the timing of the circuit break determines exactly when the spark fires.
Major Components and Construction
Permanent Magnet Rotor
At the heart of every magneto is a rotating permanent magnet assembly, typically made of a high-coercivity alloy such as Alnico (aluminum, nickel, cobalt). The rotor is precision-machined and magnetized to present alternating north and south poles as it spins. Most aircraft magneto rotors have two or four poles. The rotor is driven directly off the engine's accessory drive at a fixed ratio to engine crankshaft speed — commonly one-half engine speed on a four-stroke engine, though this ratio varies with design and number of cylinders. As the poles sweep past the pole shoes of the coil core, they create a rapidly changing magnetic field.
Coil Assembly (Transformer)
The magneto coil is a dual-winding transformer wound around a soft-iron core. The primary winding consists of relatively few turns of heavy-gauge wire, while the secondary winding consists of thousands of turns of extremely fine wire insulated with enamel coating. When the primary circuit is energized by the changing magnetic flux, it carries a modest current. The instant the breaker points open and collapse the primary field, the secondary winding experiences an induced voltage proportional to the turns ratio — generating the high-tension output that fires the spark plug. The coil assembly is typically oil-impregnated or encapsulated to resist moisture and vibration.
Breaker Points and Cam
The breaker points (also called contact points) are the magneto's timing switch. They are driven by a cam that rotates in synchronization with the magnet rotor. While the points are closed, current flows through the primary circuit and builds a magnetic field in the coil core. At the precise moment of maximum rate of flux change — a position called the E-gap position (a few degrees of rotation past the neutral, full-register position, where the rate of change of flux — and thus the induced primary voltage — is greatest) — the cam opens the points. This sudden interruption collapses the primary field and causes the high-voltage spike in the secondary. The points are precision-gapped; typical specifications call for a gap of approximately 0.016 inches, though always verify the manufacturer's data for the specific magneto model. Misadjusted points directly affect ignition timing and engine performance.
Capacitor (Condenser)
In parallel with the breaker points sits a capacitor (often called a condenser in magneto terminology). Its job is to absorb the surge of primary current at the instant the points open, preventing the current from arcing across the point faces. Without the capacitor, the primary energy would dissipate as an arc rather than collapsing cleanly into the secondary, resulting in poor spark energy and rapid erosion of the point surfaces. The capacitor also speeds up the collapse of the primary field, sharpening and intensifying the secondary voltage spike for a hotter, more reliable spark.
Distributor
The high-voltage output from the secondary winding must be routed to each spark plug in the correct firing order. The distributor performs this function. It consists of a rotating finger (the rotor) driven in phase with the magneto internals, and a distributor block with individual high-tension leads for each cylinder. As the rotor sweeps across each terminal, it routes the high-voltage pulse to the appropriate cylinder at exactly the right moment in the engine's firing sequence. The distributor block must be carefully maintained — carbon tracking (conductive deposits bridging between terminals) is a common failure mode that can cause misfires or cross-firing between cylinders.
Impulse Coupling
Starting an engine by hand rotation or starter presents a timing challenge: at low cranking speeds, spark timing that is correct at operating RPM would fire too early (advanced) and cause a kickback hazard. The impulse coupling solves this by temporarily retarding and then snap-releasing the magneto rotor during starting. A spring-loaded mechanism holds the rotor momentarily while the engine continues rotating, then releases it with a snap — producing both a retarded (later) spark for safe starting and a faster snap-through that generates a stronger spark at low RPM. Impulse couplings are typically installed on only one magneto (often the left); the other may use a shower-of-sparks vibrator system or retard breaker points for starting assistance.
Why It Matters: Safety and Airworthiness
The magneto's independence from the aircraft electrical system is its greatest safety feature. Engine-driven, requiring no external power, it eliminates the risk of ignition loss due to a dead battery, failed alternator, or tripped circuit breaker. This is why FAA regulations require that ignition switches allow each magneto to be tested independently — if either fails, the engine must still run on the remaining one with acceptable RPM drop. The dual-magneto design also improves combustion efficiency: two spark plugs per cylinder ignite the mixture from two points simultaneously, producing a faster, more complete burn that yields better power and lower operating temperatures.
For AMTs, magneto maintenance is a recurring airworthiness task. Manufacturers specify inspection intervals (often every 500 hours or at each annual/100-hour inspection) that include checking point condition and gap, capacitor serviceability, distributor block for cracks and tracking, coil insulation resistance, and verification of timing to the engine. Improper timing — either too advanced or too retarded — can cause detonation, loss of power, or engine damage. Always time magnetos to the manufacturer's specified degrees before top dead center (BTDC).
Key Numbers and Rules
- Secondary voltage output: typically 15,000–20,000 volts or higher, depending on magneto design and engine type.
- Breaker point gap: approximately 0.016 inches is a common specification, but always consult the applicable manufacturer's maintenance manual — variation exists between models.
- E-gap angle: the few degrees of rotation past the neutral (full-register, flux-zero) position where the rate of change of flux — and thus induced voltage — is greatest and the points are timed to open; exact value is model-specific but commonly in the range of 5–10 degrees.
- Magneto drive ratio: typically driven at one-half crankshaft speed on a four-stroke engine (varies by design and cylinder count).
- Impulse coupling retard: retards timing approximately 20–30 degrees for start; again, model-specific.
- Dual ignition requirement: required by 14 CFR Part 33 (engine type certification standards) for certificated aircraft engines.
- Acceptable mag drop: typically no more than 125 RPM drop on either single magneto, and no more than 50 RPM difference between the two — but verify against the specific aircraft's POH/AFM.
Common Test Traps
- Grounded vs. open P-lead: The magneto is ON when the P-lead (primary lead) is open (disconnected), and OFF when grounded. This is counterintuitive — a broken P-lead makes the magneto live at all times, a serious safety hazard. Verifying P-lead continuity is critical during inspection.
- E-gap confusion: The E-gap position is NOT neutral (where flux is zero) — it is a few degrees past neutral, where the rate of change of flux, and thus the induced primary voltage, is greatest. The points open at E-gap for maximum spark energy, not at the exact flux-zero point.
- Impulse coupling on only one mag: Examinees sometimes assume both magnetos have impulse couplings. Only one typically does; confirm which magneto is used for starting checks in the specific aircraft documentation.
- Capacitor function: The capacitor does not store energy for the spark. Its role is to prevent arcing across the breaker points and to sharpen the collapse of the primary field — do not confuse it with an energy storage device like the coil.
- High RPM mag drop causes: A rough mag check may point to fouled plugs, worn points, incorrect timing, or a failing coil — not simply a "bad magneto." Systematic diagnosis is required; replacing the magneto without identifying the root cause may leave the underlying problem unsolved.