The runup check most pilots rush and the magneto drop that is trying to tell you something

The magneto check during runup is a diagnostic test, not a pass/fail box to tick. You’re not waiting to see whether the engine quits on a single magneto—you’re reading how much the RPM drops and whether both magnetos drop by a similar amount. A drop within your aircraft’s published limits, with both mags close to each other, confirms both ignition systems are healthy. Anything outside that—zero drop, an excessive drop, or a large difference between the two—is your engine telling you something is wrong.

Why Your Airplane Has Two Magnetos

Most training airplanes—a Cessna 172, a Piper Cherokee—run on magnetos rather than relying on the electrical system for ignition. A magneto is a self-contained generator that produces the spark for your spark plugs. It doesn’t need the battery or the alternator. It spins with the engine and makes its own electricity, which means your engine keeps firing even if your entire electrical system goes dark.

Your engine has two magnetos: a left mag and a right mag. Each fires its own set of spark plugs, with two plugs per cylinder. There are two reasons for this design:

• Redundancy. If one magneto fails, the other keeps the engine running.
• Performance. Firing the fuel-air mixture from two points in the cylinder instead of one produces a more complete, even burn—worth a little extra power and efficiency.

What Is the Magneto Check Actually Testing?

During the mag check, you switch from running on both magnetos down to running on just one, then back to both, then the other one, and back to both again—watching the tachometer the whole time.

The key point: you are not looking for the engine to keep running. You’re measuring how much RPM you lose on each magneto alone, and comparing the two.

Here’s a concrete example. Sitting at 1,700 RPM on both mags, you switch to the right mag alone. Now only one set of plugs is firing per cylinder, the burn is less efficient, and the RPM drops—say to 1,650, a 50 RPM drop. Switch back to both, let it recover, then switch to the left mag. It drops to 1,640, a 60 RPM drop. Both within limits, and close to each other. That’s a healthy result.

What Are the Magneto Drop Limits?

The numbers are airplane-specific, so always check your Pilot’s Operating Handbook. A typical limit might be a maximum drop of around 175 RPM on either magneto and a maximum difference of around 50 RPM between the two. Don’t memorize a number from a different make and model—an examiner expects you to know the limits for the airplane you’re actually flying.

What Does an Abnormal Magneto Drop Mean?

Scenario 1: No drop at all. You switch to a single mag and the RPM doesn’t move. This is not a good sign. A zero drop almost always means a problem with the ignition switch or the grounding circuit—often a magneto that is “hot” (live) when it shouldn’t be, because it isn’t being properly grounded. A magneto that won’t ground can fire when you don’t expect it. This is why every propeller is treated as live, always: a hot mag means the prop can kick over even with the master off and the key in your pocket. No drop is a squawk, not a pass.

Scenario 2: An excessive drop. You switch to one mag and the RPM falls off a cliff—200, 250 RPM—and the engine runs rough. That ignition system isn’t doing its job. The cause could be a fouled spark plug, a bad lead, a bad plug, or a failing magneto. Most often, especially after a long taxi at low power with the mixture full rich, it’s a fouled plug: carbon or lead deposits bridging the gap so the plug won’t fire clean.

Scenario 3: A large difference between the two mags. Say the right mag drops 40 RPM and the left mag drops 140. Both are technically within the maximum, but that big gap tells you the left side isn’t as healthy as the right—maybe a plug starting to foul or a lead going bad. It’s not an automatic no-go, but it’s information. It’s your engine whispering before it has to shout.

How Do I Clear a Rough Magneto or Fouled Plug?

If you get a rough mag or an excessive drop, don’t immediately give up and taxi back. Try leaning the mixture at runup power for 15 to 20 seconds, then run it up again. Leaning raises the plug temperature and burns off the carbon or lead deposits. Very often the rough mag smooths right out, the drop comes back into limits, and you’ve saved yourself a scrubbed flight and a maintenance write-up.

If it doesn’t clear after a reasonable attempt, the airplane needs a mechanic. Don’t fly a sick magneto, and don’t talk yourself into it because you wanted to make that breakfast fly-out.

What Else Should I Check During Runup?

While you’re parked at 1,700 RPM, use the opportunity:

• Carburetor heat: Pull it and watch for a small RPM drop, which confirms warm air is reaching the carburetor. That drop is your proof the system works—and carb ice is a quiet killer on exactly the mild, humid days you’d least expect it.
• Engine instruments: Oil temperature rising, oil pressure in the green, suction/vacuum where it should be, ammeter showing a charge.
• Propeller: If you’re flying a constant-speed prop, cycle it and watch RPM and manifold pressure respond.

The whole runup takes maybe 90 seconds—but 90 seconds of actually paying attention is worth far more than ten minutes of going through the motions.

What If There’s Pressure to Hurry at a Busy Field?

Picture a line of airplanes behind you at the runup area. You feel the pressure to hurry, you run it up, and you get a rough left mag with a bigger drop than you’d like. The right answer: taxi clear. Pull off to the side, out of everyone’s way, and take the time to lean it out and check it properly.

Nobody behind you is in a bigger hurry than you should be to avoid an ignition problem at 400 feet on climbout. That external pressure—get-there-itis—is one of the recognized hazardous attitudes, and the antidote is simple: not so fast. The airplane will fly when it’s ready, and not one second before.

This guidance aligns with the Airman Certification Standards and the engine and ignition guidance in the FAA’s Airplane Flying Handbook, along with the limits printed in your own POH—the document that actually matters for the airplane you fly.

Key Takeaways

• The mag check measures RPM drop, not whether the engine survives on one magneto—it’s a diagnostic test of ignition health.
• Always use your aircraft’s POH limits (often around a 175 RPM max drop and a 50 RPM max difference), not numbers from another model.
• No drop signals a possible hot/ungrounded mag; excessive drop signals a fouled plug or failing ignition; a large difference between mags signals one side is weakening.
• A rough mag or fouled plug can often be cleared by leaning the mixture at runup power for 15–20 seconds.
• Don’t let traffic pressure rush the check—taxi clear and diagnose it properly rather than fly a sick magneto.