Most piston airplanes have two magnetos. Each one fires its own set of spark plugs, so the engine keeps running if one quits. Magnetos make their own electricity, which means the engine keeps running even if the battery and alternator both die.
That redundancy is why owners tend to forget about them. A mag can be slowly wearing out for hundreds of hours while the engine runs fine on the other one.
What magneto timing means
There are two kinds of timing, and both matter.
- Internal timing is about the parts inside the magneto working together, so it produces its strongest spark at the right moment.
- Timing to the engine makes the magneto fire at the right crankshaft position, a set number of degrees before top dead center. The number comes from the engine manufacturer and is listed for your engine.
Timing drifts as parts inside the magneto wear. When it's off, you can get rough running, lost power and high cylinder head temperatures. That's why timing gets checked at every annual.
The 500-hour magneto inspection
Both Champion, which makes Slick magnetos, and Continental call for a magneto inspection every 500 hours. It's more than a timing check. The magneto comes off the engine and gets opened up. The mechanic checks the internal parts against the manufacturer's wear limits, replaces what's worn, then reinstalls and times the magneto.
A lot of owners skip it because the magneto seems fine. It usually does seem fine, right up until a run-up far from home.
Signs of magneto trouble
- A big RPM drop or roughness on one magneto during the run-up. Your POH lists the allowed drop.
- No drop at all. That can mean a broken P-lead, so the magneto stays live even with the key off. A "hot mag" can start the engine if someone moves the prop. Treat every propeller as live.
- Hard starting, which often points to the impulse coupling.
- Roughness in flight that clears up when you switch to one magneto.
A failed mag check means the plane stays on the ground. Don't take off and hope it smooths out.
Magnetos in stock
Brooks keeps magnetos on hand, so a bad mag doesn't have to mean days of waiting for a part. If you're stuck away from home, read what to do when you're AOG, then call or text him.