175 RPM Magneto Drop vs. 40 RPM Drop During Runup: What It Means and What to Do

Why the Magneto Check Matters More Than You Think

The magneto check is one of those runup items that student pilots can start performing on autopilot — switch to LEFT, glance at the gauge, switch to RIGHT, glance again, switch back to BOTH, and move on. But your designated pilot examiner knows that a rote runup checklist habit masks a deeper gap in understanding. If you sit down for your oral exam and cannot explain what the numbers actually mean, that is a problem. The magneto check is not a formality. It is a direct window into the health of your ignition system, and interpreting it correctly is a genuine airmanship skill.

The Airplane Flying Handbook (AFH), FAA-H-8083-3, addresses abnormal magneto behavior under its emergency procedures guidance, and for good reason — an ignition problem discovered on the ground is an emergency that never happens in the air. Your ability to recognize, interpret, and respond to an abnormal indication is exactly what the checkride is designed to test.

What Normal Looks Like — and Why the Differential Matters

Before you can recognize abnormal, you need a firm grasp of normal. For most training aircraft, an acceptable RPM drop on each individual magneto is typically no more than 125 RPM. Equally important — and frequently overlooked by students — is the differential between the two magnetos. Even if both individual drops fall within limits, the difference between the two readings should not exceed approximately 50 RPM. Always verify the specific limits in your aircraft Pilot Operating Handbook, since values can vary by engine make and model, but these figures are representative of what you will encounter in most training environments.

So when one magneto produces a 175 RPM drop and the other produces only a 40 RPM drop, you have two problems stacked on top of each other. The 175 RPM drop exceeds the individual limit. And the differential between 175 and 40 — which is 135 RPM — blows past the 50 RPM differential limit by a wide margin. Either issue alone would be worth investigating. Together, they are a clear no-go signal.

One more number worth understanding: a drop of zero. Some students assume that no RPM drop at all is ideal — the engine is running perfectly on one magneto, right? Wrong. A zero drop likely means the magneto you switched off is not actually isolating, which means both magnetos are still firing together. That is its own ignition system defect, and it is just as unacceptable as a drop that is too large.

What a Large Drop Is Actually Telling You

A 175 RPM drop on a single magneto is a symptom, not a diagnosis. The most common culprits are fouled spark plugs, a defective magneto, or a problem with an ignition lead. Each of those has different implications, but from a preflight decision-making standpoint, the root cause does not change what you do next: you do not depart.

One of the most dangerous thought patterns a student pilot can have in this situation is rationalizing the discrepancy away. The logic goes something like this: the engine sounds fine, the other magneto is normal, and at cruise power everything will probably smooth out. This reasoning is faulty on multiple levels. First, aircraft engines are certified and operated as dual-ignition systems for a reason — each magneto must function independently and reliably. Second, a problem that shows up at runup RPM does not necessarily resolve itself at higher power settings. It may get worse. Flying with a known ignition defect is not aeronautical decision-making; it is wishful thinking with consequences.

The Right Response: Clear, Then Ground If Necessary

Here is where a lot of students lose points on the oral exam — they jump straight to grounding the aircraft without mentioning the fouled plug clearing procedure. If fouled plugs are the likely culprit, there is a technique worth attempting before you call maintenance. Running the engine at a slightly higher power setting briefly can sometimes burn off lead deposits or oil fouling on the plugs, potentially restoring normal ignition. This lean-out or burnoff procedure is worth trying once under the right conditions.

If you attempt to clear the plugs and re-check the magneto drop and the 175 RPM indication is still there, the aircraft is grounded. Full stop. You write up the discrepancy, you do not fly, and the aircraft goes to a certificated mechanic for inspection. This is not a conservative overreaction — it is exactly what the FAA expects of a certificated pilot exercising sound judgment.

On your checkride, walking your DPE through this sequence — recognizing the values, understanding why both the individual drop and the differential are out of limits, attempting to clear the condition, and then making a firm no-go decision — demonstrates the kind of systematic, safety-focused thinking that earns a certificate. Knowing the numbers without knowing the response is only half the answer. Knowing the response without knowing the numbers is the other incomplete half.

If you want to practice questions like this in a realistic oral exam format, try SimulatedCheckride.com.