9.6 Ignition and Starting Troubleshooting

The Magneto and Reciprocating Ignition

A magneto is a self-contained AC generator and ignition unit driven by the engine. A rotating magnet induces a primary current; when the breaker points open, the collapsing primary field induces a high-voltage secondary pulse that the distributor routes to the correct spark plug through the ignition harness. Because the magneto makes its own electricity from engine rotation, it is independent of the aircraft battery once the engine is turning — the reason a certificated piston engine continues running after a complete electrical failure.

Aircraft use dual magnetos (left and right) firing two plugs per cylinder for redundancy and a more complete, faster burn.

Two construction types exist. A high-tension magneto generates the high voltage inside the magneto and distributes it through high-voltage leads — simple but prone to flashover/arcing in the distributor at high altitude where thin air insulates poorly. A low-tension magneto generates and distributes low voltage to individual step-up transformer coils at each cylinder, with only short high-voltage runs, reducing high-altitude flashover. Modern systems often pressurize the magneto to fight altitude arcing instead.

Starting Aids, E-Gap, and Timing

At cranking speed a plain magneto turns too slowly to make a strong spark, and its normal advanced timing would fire too early and cause kickback. Two aids solve this:

• Impulse coupling — a spring-loaded coupling winds up against a stop while the engine cranks, then releases to snap the magnet rapidly through its E-gap position. This both spins the magnet fast (hot spark) and retards the spark to fire near top dead center for easy, kickback-free starting. After start, flyweights disengage the stop and the magneto runs at normal advanced timing.
• Shower of sparks (retard-breaker) — a second "retard" breaker fires a rapid burst of sparks through a vibrator/booster coil during start, both retarding the spark and giving multiple ignition chances across the plug gap to light a hard-starting mixture.

Timing has two parts the ACS separates. Internal timing sets the breaker points to just open when the rotating magnet is at its E-gap (the few degrees past neutral where the magnetic flux change is greatest, producing the strongest spark). Magneto-to-engine timing then installs and times the magneto so the spark fires at the specified crankshaft angle before top dead center (e.g., 25 degrees BTDC), verified with a timing light and the engine set on the timing marks. Late timing causes high CHT and power loss; over-advanced timing invites detonation and kickback.

Spark Plugs, P-Leads, Turbine Ignition, and Starting Faults

Spark plugs foul (lead/carbon bridging the gap), erode, or short; they are rotated and gapped on a schedule, and a fouled plug shows as a single mag-check RPM drop larger than normal with roughness. The harness can short to ground or open, killing one plug. The P-lead grounds the magneto primary to shut it off; if a P-lead is open or broken, the switch cannot ground the magneto and it stays hot even with the switch OFF — every propeller must be treated as live until proven otherwise.

During a mag check, no RPM drop when one magneto is selected means that magneto is not being grounded off (open P-lead / faulty switch) — a hot-mag condition, not a good engine.

Turbine ignition is fundamentally different: a capacitor-discharge ignition exciter stores energy and dumps high-energy sparks across surface-gap igniter plugs in the combustor — but only during start and relight. Once the engine is self-sustaining, the igniters are de-energized; there is no continuous firing like a magneto.

The start sequence is, in order: (1) starter (electric, starter-generator, or air-turbine using APU/cart/cross-bleed air) spins the compressor up to establish airflow; (2) ignition is energized; (3) fuel is introduced and lights off; (4) at self-sustaining speed the starter and ignition cut out.

Faults map to the sequence: a hung start is light-off but failure to accelerate to idle (insufficient starter air/power or low fuel); a hot start is excessive EGT/ITT during start (too much fuel, too little airflow, or a slow starter) and demands an immediate abort to protect the turbine. On recips, a kickback during start indicates the spark fired too early — a failed/incorrectly installed impulse coupling or mistimed magneto.

No-Start Logic and the Mag-Check Procedure

A reciprocating engine starts only with compression, the right fuel-air mixture, a strong spark at the right time, and adequate cranking speed. A structured no-start diagnosis tests these in turn:

Backfire is combustion in the induction system (lean mixture or a sticking intake valve); afterfire is burning in the exhaust (an over-rich mixture). The mag check during run-up grounds one magneto at a time: a normal small RPM drop on each, within limits and roughly equal, confirms both ignition systems. An excessive drop on one mag points to fouled plugs, a bad lead, or a weak magneto on that side; no drop means that magneto is not grounding off (hot-mag/open P-lead).

A recoverable drop that clears after a brief lean-and-burn indicates lead fouling rather than a failed part. Spark-plug servicing — rotating plugs to even out wear, gapping to spec, and torquing with a new gasket — is routine work tied to these symptoms.