The engine runup you rush through and the magneto drop that is trying to tell you something

The engine runup magneto check is one of the most rushed and least understood preflight procedures in flight training. Most student pilots watch the tachometer needle move, confirm “something happened,” and move on without actually reading the numbers or comparing left to right. But the mag check isn’t a pass-fail checkbox — it’s a two-minute diagnostic conversation with your engine, and the numbers it gives you carry real meaning about the health of your ignition system.

Why Does Your Engine Have Two Magnetos?

Your piston engine has two independent ignition systems — two magnetos, two sets of spark plugs, and two completely separate paths to create combustion in each cylinder. This redundancy exists so that if one magneto fails in flight, the other keeps the engine running. You lose some power, but you keep flying.

When you switch from BOTH to a single magneto during the runup, you are shutting off one ignition system entirely and running on just the other side. Half the spark plugs go dark. The engine should still run — just slightly worse. The mag check tells you how much worse, and that number matters.

What Numbers Should You See?

The Airman Certification Standards and your airplane’s Pilot Operating Handbook (POH) specify the acceptable limits. For most training aircraft, you’re looking at:

• Maximum RPM drop per magneto: approximately 175 RPM
• Maximum differential between left and right: no more than 50 RPM

Both numbers matter equally. A lot of students memorize the per-side maximum and forget the differential limit entirely. Examiners check for both.

What Do Different RPM Drops Actually Mean?

A healthy drop (around 50–100 RPM on each side): Both ignition systems are carrying their weight roughly equally. The small, symmetrical drop confirms each magneto is doing its job, and together on BOTH they deliver optimal combustion.

An unusually small drop (under 25 RPM): This might sound like good news, but a near-zero drop can indicate a hot magneto. That means the grounding circuit on the “off” magneto isn’t working properly — both sets of plugs may still be firing even though you only selected one. A hot mag is a serious safety issue because the propeller could fire if someone moves it on the ground, even with the ignition switch in the OFF position. Report this to a mechanic before the next flight.

An excessive drop (over 175 RPM on one side): Something is wrong with that magneto or the spark plugs it feeds. Possible causes include a fouled plug, a magneto timing issue, or a bad ignition lead. You are not taking off with that reading.

Individual drops within limits but differential too large: This is the scenario that catches students off guard. If the right magneto drops 50 RPM and the left drops 160 RPM, both are individually under 175. But the differential is 110 RPM — well over the 50 RPM maximum. One ignition system is doing significantly more work than the other. That is a no-go condition.

How Do You Clear a Fouled Spark Plug on the Runup Pad?

A higher-than-normal drop on one magneto is common on training flights, and the most likely cause is a fouled spark plug. Before calling it a day, try this procedure:

1. Lean the mixture aggressively for 15–20 seconds to raise combustion temperature
2. Return the mixture to full rich
3. Run the mag check again

This often burns off carbon deposits and clears the problem. If the drop doesn’t come back within limits after this procedure, the airplane needs a mechanic. That isn’t a judgment call — it’s what responsible pilots do.

Why You Shouldn’t Talk Yourself Into a Marginal Drop

There is a real temptation, especially when you’ve scheduled a lesson and you’re paying by the hour, to accept a mag drop that sits right at the edge of the limit. The limits exist because engineers determined that’s where the safety line is. You don’t get to renegotiate with engineering because you want to fly today.

Don’t Forget the Carb Heat Check

If your airplane has a carburetor — and most trainers do — the runup includes a carb heat check that deserves the same attention as the mag check.

When you apply carb heat, the RPM should drop because you’re introducing warmer, less dense air. If the RPM drops and then rises slightly, that means carburetor ice was forming and the hot air just melted it. That’s valuable information.

If the RPM doesn’t drop at all when you pull carb heat, the carb heat system may not be functioning. You need that system working before flying into conditions where carb ice could form.

Putting It All Together: A Real-World Scenario

Cool, humid morning at a nontowered field. Dew point and temperature are within a few degrees. You run the mag check and the left magneto drops 190 RPM — over the limit. You lean the mixture, wait, and recheck. Now it drops 160 RPM, within limits. But the right magneto only drops 40 RPM. The differential is 120 RPM.

Both individual drops are technically within the per-side maximum, but the differential is telling you these two ignition systems are not performing equally. On a morning with high carb ice risk, you don’t launch with a question mark on your ignition system. You taxi back.

Common Runup Mistakes to Avoid

Checking at the wrong RPM. Your POH specifies the RPM for the mag check — usually around 1,700–1,800 RPM for most trainers. Checking at the wrong RPM produces misleading results. Look up the number for your specific airplane.

Forgetting to return the switch to BOTH. After completing the mag check, always verify the ignition switch is back on BOTH before calling ready for departure. A student distracted by a radio call can easily taxi to the runway on a single magneto without noticing — the engine runs fine on one, but you’d be taking off with half your ignition system switched off.

Key Takeaways

• The mag check has two limits: maximum drop per magneto (~175 RPM) and maximum differential between sides (~50 RPM). Know both.
• A near-zero RPM drop can indicate a hot magneto, which is a safety hazard — not a sign of a healthy engine.
• Fouled plugs can often be cleared by aggressively leaning the mixture for 15–20 seconds, then retesting.
• The mag check is a diagnostic tool, not a checkbox. Read the actual numbers and compare left to right.
• Always verify the ignition switch is back on BOTH before taxiing to the runway.