A magneto is a generator, not a consumer

Twenty-nine elements. This unit contains the explanation for the hazard that governs the whole of Powerplant, and it is one sentence long.

1. The switch does not power the magneto. It grounds it.

K4 — the three electrical circuits of a magneto system — and R3.

A magneto is self-contained. It has its own magnet, its own coil and its own breaker points, and it generates the high voltage for the spark itself. It needs nothing from the aircraft's electrical system. Turn the engine and it makes sparks.

So how do you stop it? You short its primary circuit to ground, through the P-lead and the ignition switch. The switch at OFF closes a path to ground; the switch at BOTH opens it.

That inverts what everyone assumes. The consequences:

• A broken or disconnected P-lead leaves the magneto live with the switch at OFF.
• A chafed P-lead grounding intermittently produces a magneto that cuts in and out.
• A faulty switch can leave one or both magnetos hot in any position.
• In every case there is no outward sign.

That is why PP-A §1 states the rule as absolute: treat every propeller as live, regardless of switch position. This unit is where the rule stops being a caution and becomes something you understand.

And it gives you the diagnostic: a magneto that will not switch off is a P-lead or a switch fault, and S10's ignition switch circuit troubleshooting is exactly that.

The three circuits worth naming: the magnetic circuit (rotating magnet and pole shoes), the primary electrical circuit (coil primary, points, condenser and the P-lead to the switch), and the secondary circuit (coil secondary, distributor, harness, plugs).

2. Why there are two magnetos, and why the drop check matters

Every cylinder has two spark plugs, fired by two independent magnetos. Redundancy is part of it, and it is not the whole reason.

Two flame fronts burn the charge faster and more completely than one, which is why the engine loses a little power when you run on one magneto — the magneto drop. A drop that is:

• within limits on both — normal;
• too large on one — that magneto, its harness, or those plugs;
• too small or zero — suspicious, and often means a magneto is not actually being switched off;
• rough rather than a clean drop — a plug or a lead rather than the magneto.

The drop check is a diagnostic with a lot of information in it, and it is worth reading rather than merely passing.

3. Timing is two separate operations, and R1 is why it matters

Internal timing (S1) sets the magneto so its points open at the moment its rotating magnet is at the position of maximum flux change — the E-gap — because that is when it makes the strongest spark.

Timing to the engine (S2) sets the magneto on the engine so it fires at the specified number of degrees before top dead center.

Both must be right, and they are independent. A perfectly internally timed magneto fitted at the wrong engine position fires at the wrong moment; a correctly positioned magneto with wrong internal timing fires a weak spark at the right moment.

R1 — advanced and retarded timing — gives the consequence:

• Advanced — the charge burns too early, pressure peaks before the piston is ready for it, and the result is detonation and a destroyed piston (PP-A §3).
• Retarded — the burn is late, power is lost, and the engine runs hot because combustion is still happening into the exhaust stroke.

Neither is visible, and neither shows up on a run-up that passes. This is the invisible-failure family again, and it is why timing is checked at intervals rather than when something seems wrong.

4. Spark plugs have a heat range and it is not a preference

K2, S12, S11.

A spark plug must run hot enough to burn off deposits and cool enough not to become an ignition source. That balance is the heat range, and it is set by the length of the path heat takes from the nose insulator into the cylinder head.

• Too cold — the plug does not reach self-cleaning temperature, deposits build, and it fouls.
• Too hot — the nose glows and ignites the charge before the spark, which is pre-ignition, and it destroys pistons (PP-A §3).

The approved plugs are listed in the engine TCDS. A plug that physically fits and is not on the list is not a cheaper equivalent; it is a different heat range.

Reading plugs is a diagnostic skill. Color and deposit tell you about mixture, oil control and temperature across that cylinder, and a set of plugs laid out in cylinder order is a map of the engine's condition.

5. Turbine igniters hold lethal energy after shutdown

K9, R2, S14–S17.

Turbine ignition is capacitor discharge: energy is stored in a capacitor and released as a very high-energy spark, because a turbine must be lit in a moving airstream.

The capacitor holds that charge after the system is switched off. The stored energy is sufficient to kill.

So the discipline is: switch off, wait the specified discharge period, and follow the manual's discharge procedure before disconnecting anything. Never assume that off means safe — which is the same lesson as §1, arriving from a different direction.

Igniter plugs are also not spark plugs. They erode rapidly by design, they are inspected for erosion against limits, and they are handled carefully because the tips are brittle.

6. Impulse couplings and shower of sparks solve the same problem

K3. At cranking speed a magneto turns too slowly to make a good spark, and the engine needs a retarded spark for starting so it does not kick back.

An impulse coupling is mechanical: a spring winds up and then snaps the magneto rotor through its E-gap quickly, producing a hot spark, and it also delays the spark. You can hear it click when you pull the propeller through — and that click is a useful serviceability check.

Shower of sparks is electrical: a vibrator supplies a train of sparks through a retard breaker during starting.

Both stop working once the engine starts and normal ignition takes over.

7. What consumes this unit

License

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