The Basic Principle
An ignition magneto is a self-contained electrical generator that produces the high-voltage pulse needed to fire a spark plug. Unlike an automotive ignition that draws power from a battery, a magneto generates its own electricity from a rotating permanent-magnet rotor driven by the engine. As the magnet sweeps past a coil, it induces a current — and a built-in transformer steps that current up to the tens of thousands of volts needed to jump a spark-plug gap.
This is why piston aircraft engines can lose their entire electrical system and still keep running: the magnetos are independent of the battery, the alternator, and every other piece of avionics. Two magnetos, two independent ignition paths, two chances per cylinder per combustion cycle. That's the redundancy that makes piston aircraft safe.
Why Two Magnetos
Virtually every certified piston aircraft engine runs a dual magneto — a single housing containing two complete magneto systems sharing one drive gear. Each system fires its own spark plug in each cylinder. If one magneto fails mid-flight, the other keeps the engine running. Pilots verify this on every run-up: magneto check, drop ~50 RPM, rise, drop on the other mag, rise. Both must fire independently.
This redundancy is also why a 500-hour inspection matters. A partial failure on one side may not be obvious in normal operation — the engine runs fine on the other mag. But the day you actually need both, you discover the worn points or cracked cap.
500-Hour Inspection: What It Covers
The FAA 500-hour magneto inspection (per the magneto manufacturer's maintenance manual and Type Certificate Data Sheet) typically includes:
- Internal inspection — point pitting, condenser condition, coil insulation resistance
- Cam dwell and advance curve verified against spec
- Bearing endplay and drive gear wear limits
- Cap and distributor — carbon tracking, cracks, carbon brush wear
- Impulse coupling — trip RPM, spring condition (on impulse-equipped mags)
- Calibrated bench test — minimum firing voltage, hot-fire test, drop-off RPM
Pressurized Magnetos
On turbocharged or high-altitude installations, the magneto is mounted inside a pressurized case that scavenges engine sump pressure to keep the inside of the magneto at a higher pressure than ambient. This prevents internal electrical arcing at altitude where the dielectric strength of air drops. Bendix pressurized models (SF-10, S-700) and certain Slick models use this design.
A pressurized magneto must hold pressure on a bench test — leaking seals are an immediate rework item. The 500-hour inspection on a pressurized unit includes a pressure decay test in addition to the standard electrical checks.
Common Failure Modes We See
- Worn breaker points — pitting, transfer, or oil contamination from engine breather
- Weak or shorted condenser — typically a slow-firing, weak-spark condition at low RPM
- Cracked distributor block or cap — carbon tracking, often visible at the inside of the cap
- Failed coil — open primary, shorted secondary, or insulation breakdown under load
- Worn impulse coupling — slips at low RPM, hard starting on the mag
- Damaged drive gear or coupling — usually found during teardown, not external inspection
- Corrosion from water ingestion — common on neglected aircraft stored outside
Service Intervals & Recommendations
Most piston aircraft magnetos are rated for a 500-hour TBO (time between overhauls). Some Slick models carry a longer TBO when used on lower-utilization engines. The 500-hour inspection is separate from TBO — it's the periodic check that catches developing issues between overhauls.
Our recommendation for owners: don't wait for the engine monitor or the mag check at annual to tell you something is off. If an engine is running rough on one mag, or if mag drop exceeds 50 RPM, or if you can hear the impulse coupling slipping on start, send the mag in. The cost of catching a worn condenser at the bench is $40 in parts. The cost of catching it at altitude is the engine.