Engine Ignition Systems: Magnetos, Harnesses, and Spark Plugs

Engine Ignition Systems: Magnetos, Harnesses, and Spark Plugs

Aviation reciprocating engines rely on a self-contained ignition system that operates independently of the aircraft's electrical system. Understanding how magnetos, ignition harnesses, and spark plugs work together—and how to inspect, time, and test each component—is a core competency for the FAA Airframe and Powerplant (A&P) mechanic.

How a Magneto Works

A magneto is a self-generating alternating-current device that produces high-voltage electricity to fire the spark plugs. Inside, a permanent magnet rotates past a set of pole shoes wound with primary and secondary coils. As the magnetic field collapses through the primary coil, a high-voltage pulse is induced in the secondary coil and routed through the distributor to the correct spark plug at the correct instant.

Conventional magnetos use breaker points to interrupt primary current at the right moment. Modern or overhauled units may replace points with an electronic trigger (solid-state) assembly, which eliminates point wear and provides more consistent timing. Either way, the principle is identical: break the primary circuit at the precise crankshaft position to produce maximum secondary voltage.

E-Gap and Internal Timing

The term E-gap (efficiency gap) refers to the small number of degrees of magnet rotation past the neutral position at which the breaker points should open—or the electronic trigger should fire. At E-gap, the rate of magnetic flux change is greatest, so secondary voltage is highest. Setting E-gap correctly is the foundation of internal magneto timing.

To set E-gap:

1. Rotate the magneto until the timing marks align with the E-gap position specified in the overhaul manual.

2. Connect a timing light or continuity device across the breaker points (or use the electronic tool's indicator).

3. Adjust the breaker point gap or trigger position until the light indicates the circuit opens (or the tool signals the trigger fires) exactly at the E-gap mark.

4. Secure the setting and recheck.

After internal timing is set, the magneto is installed and timed to the engine using a degree wheel on the crankshaft and the magneto's timing marks, so the spark fires at the crankshaft angle specified by the engine manufacturer.

Ignition Harness Inspection

The high-tension ignition harness carries voltages high enough to be dangerous and to arc through degraded insulation. Inspect each lead for:

• Chafing or abrasion against airframe structure or exhaust components
• Cracking or brittleness of the outer jacket, especially near terminals
• Terminal condition: corrosion, looseness, or damaged ferrules at the spark plug and distributor ends

After visual inspection, test every lead with a high-tension lead tester. Connect each lead individually and apply the test voltage specified by the tester manufacturer. Acceptable resistance and insulation breakdown values are listed in the engine manufacturer's service manual. A lead that shows excessive resistance or insulation leakage must be replaced—not repaired.

Safety: High-tension testers operate at very high voltages. Keep the lead under test away from personnel and grounded metal. Follow the tester manufacturer's operating instructions exactly.

Spark Plug Removal, Inspection, and Gapping

Aviation engines use dual ignition—two magnetos and two spark plugs per cylinder—for redundancy and combustion efficiency. Most aircraft spark plugs are either fine-wire or massive-electrode designs. Both require periodic cleaning, inspection, and gapping.

Removal: Use the correct spark plug socket and a calibrated torque wrench. Before removal, blow out the well with compressed air to prevent debris from entering the cylinder. Keep plugs organized by cylinder and position (top/bottom) for rotation scheduling.

Cleaning: Use an abrasive blast cleaner approved for aviation spark plugs. Blast time is short—typically just a few seconds per plug per the equipment manufacturer's guidance. Excessive blasting erodes the electrodes. Follow with a vigorous air blast to remove all abrasive material.

Visual inspection: Examine the ceramic insulator for cracks, the shell for thread damage, and the electrodes for excessive wear, erosion, lead fouling, or carbon bridging. A cracked insulator or bridged electrode is cause for rejection.

Gapping: Use a round wire feeler gauge or a purpose-made aviation spark plug gap tool—never a flat feeler gauge, which gives inaccurate readings on worn electrodes. Compare the measured gap to the manufacturer's specified gap. Adjust by carefully bending the ground electrode. Do not strike the center electrode.

Rotation: Manufacturers publish a plug rotation schedule that moves top plugs to bottom positions and swaps cylinders in a defined sequence. Following the schedule equalizes wear across all plugs.

Installation: Apply a thin film of approved anti-seize compound if specified by the manufacturer. Thread in by hand to prevent cross-threading, then torque to the manufacturer's specification with a calibrated torque wrench.

Safety Throughout This Lab

• PPE: Safety glasses are required at all times. Hearing protection when the engine is run.
• Lockout/Tagout equivalent: Before working on the ignition system with the engine stationary, ground the magneto P-leads and use a grounding clip on the spark plug leads to prevent accidental firing if the engine is rotated.
• High-voltage hazard: Treat every ignition lead as energized until confirmed otherwise.
• Hot surfaces: Allow exhaust and engine components to cool before touching.
• Abrasive blast cleaner: Wear eye protection; never direct the blast at skin.
• Torque compliance: Under- or over-torqued spark plugs cause failures and engine damage.

Document every measurement, finding, and corrective action in the lab log. Accurate records support airworthiness and satisfy regulatory requirements.