The 500-Hour Magneto Inspection: What It Is and Whether You Have to Do It

Sooner or later, an A&P looks at your tach and says, “You’re coming up on 500 hours — time to pull the mags.” And a perfectly reasonable question follows: Do I actually have to? Is this the law, or a suggestion? The honest answer is “it depends” — and understanding what it depends on is one of the more useful pieces of knowledge a piston owner can carry.

The 500-hour magneto inspection sits right on the line between regulatory requirements and manufacturer recommendations. Getting that distinction right matters, because it determines whether an overdue item is a genuine grounding issue or simply a maintenance task you should schedule soon.

First, what a magneto actually does

Your piston engine’s ignition doesn’t run off the battery. It runs off magnetos — self-contained generators that produce the high-voltage spark for your plugs entirely on their own, spun by the engine. That’s why your engine keeps running after an alternator failure, and it’s why you can shut the master off in flight and the engine doesn’t quit.

Almost every certificated piston single carries two magnetos feeding two spark plugs per cylinder. That redundancy is a safety feature (it’s what you’re testing during the mag check on runup) and it’s also why a single magneto problem rarely makes the news — the other one keeps you flying. But redundancy is not a reason to ignore a tired magneto. Two marginal mags are a lot less reassuring than two healthy ones.

Inside each magneto is a small mechanical world: a rotating magnet, breaker contact points, a capacitor, bearings, a distributor gear, and — on many installations — an impulse coupling that helps produce a hot spark at low cranking speed for starting. All of it wears. None of it is visible from outside the case.

Where the “500 hours” comes from

The 500-hour interval is not an FAA regulation. It comes from the magneto manufacturers’ service bulletins — Champion Aerospace for Slick magnetos, and Continental (the historical Bendix line) for theirs. These bulletins recommend removing the magnetos and performing an internal inspection at roughly 500 hours of time in service, then repeating it on that cycle.

That’s an important word: recommend. A manufacturer service bulletin (SB) is guidance from the people who built the part. For a Part 91 operator, a service bulletin is generally not legally mandatory on its own. It becomes mandatory only when the FAA adopts it — most commonly by issuing an Airworthiness Directive that references it.

What actually happens at the 500-hour inspection

This is not the same as the magneto check you do on runup, and it’s not the same as the magneto-to-engine timing check your mechanic performs at annual. The 500-hour inspection means the magnetos come off the engine and apart. Depending on the manufacturer’s current bulletin and what the technician finds, the work typically involves:

Opening the case to inspect the contact points for wear and proper gap, checking the bearings for play, inspecting the distributor gear and block for cracks or carbon tracking, examining the impulse coupling for cracked or worn parts, checking the capacitor, and then re-assembling, re-timing the internal magneto, and re-installing and re-timing it to the engine. Some service bulletins call for replacing specific wear components — impulse coupling springs and pins are common examples — rather than just inspecting them.

Whether the outcome is a clean “inspect and return to service” or a more involved overhaul (often called an IRAN — inspect and repair as necessary) depends entirely on internal condition. Two aircraft at the same 500-hour mark can need very different work.

Why the distinction matters. Two owners both hit 500 hours on the tach. Owner A flies a type with no AD on the magnetos — for them, the 500-hour inspection is a strong manufacturer recommendation. Skipping it isn’t an airworthiness violation, but it is a real reliability and safety gamble.

Owner B flies a type where the FAA issued an AD adopting a magneto inspection. For Owner B, that inspection is mandatory, and flying past it without compliance makes the aircraft unairworthy. Same engine family, same interval — completely different legal weight. The only way to know which situation you’re in is to check the ADs for your specific airframe, engine, and magneto.

“Recommended” doesn’t mean “optional”

It’s tempting to read “not legally required” as “safe to skip.” That’s the wrong takeaway. The manufacturers didn’t pick 500 hours arbitrarily — it reflects how these components wear in service. An impulse coupling that fails can leave you with a hard-starting engine at best; internal magneto failures can cause rough running or a partial loss of ignition. The redundancy of two mags buys you margin, not immunity.

Most experienced owners and shops treat the 500-hour inspection as something you do, full stop — the debate is really about scheduling flexibility, not whether to do it at all. The regulatory question simply tells you how much latitude you have: an AD gives you none, while a service bulletin lets you and your A&P plan the work sensibly (for example, aligning it with an annual or a top-end inspection) rather than grounding the airplane the hour it comes due.

The two columns every owner should keep straight

Treating it as a vague “someday”:

• No record of when the mags were last opened
• Can’t tell if a referencing AD applies to your serial number
• Surprise findings — and surprise bills — at annual
• Impulse coupling or points failure with no warning
• No way to prove compliance to a buyer or insurer

Tracking it deliberately:

• Known tach time at last inspection, next interval calculated
• AD status checked separately and flagged if mandatory
• Work planned alongside the annual to save labor
• Wear components addressed before they fail
• Clean, documented history in the maintenance record

Don’t confuse it with magneto timing

Two different things both get called “the mag” work, and owners mix them up constantly. Magneto-to-engine timing is a check (and adjustment) of when the magneto fires relative to crankshaft position. It’s normally verified during your annual or 100-hour inspection and after any work that disturbs the mags, and it doesn’t require opening the magneto. The 500-hour inspection is the internal teardown described above. Good timing on a worn-out magneto is still a worn-out magneto — the two tasks serve different purposes and run on different clocks.

A few numbers worth keeping in mind: a typical certificated piston single carries two magnetos; the manufacturer-recommended inspection interval is 500 hours; and it takes just one applicable AD to turn that recommendation into a mandatory inspection.

How to track it without overthinking it

The magneto inspection is a perfect example of why “is it due?” isn’t the only question that matters — “is it required or recommended?” matters just as much. A good maintenance system should track both: the tach hours since the last inspection and whether the item is regulatory (an AD), manufacturer-recommended (a service bulletin), or your own conservative preference.

That’s exactly the distinction TachMinder is built around. A recurring item tied to an AD is treated as a hard, safety-critical deadline. A manufacturer interval like the 500-hour mag inspection is tracked and surfaced as a recommendation — visible and timely, but flagged for what it is, so you and your mechanic can plan the work intelligently instead of either ignoring it or panicking over it. The point isn’t to replace your A&P’s judgment. It’s to make sure the right items show up at the right time, labeled with the right level of urgency.

Whatever you use to track it — a tracker, a spreadsheet, or notes with your mechanic — the goal is the same: know your hours since last inspection, know whether an AD applies to your specific magnetos, and never let “recommended” quietly turn into “forgotten.”