During a Magneto Check Before Takeoff, You Notice a 350 RPM Drop on the Left Magneto: What Does This Indicate and What Should You Do?

Why the Magneto Check Exists

Before every takeoff, pilots run through a magneto check at a specified RPM — typically around 1,800 RPM in most training aircraft. This check is not a formality. It is your last reliable opportunity to verify that both ignition systems are functioning independently before you commit to flight. As covered in Chapter 7 of the Pilot's Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25), aircraft piston engines use a dual magneto ignition system specifically for redundancy. Each magneto fires its own set of spark plugs, and when you switch from BOTH to a single magneto, you are deliberately removing half of that redundancy to confirm the remaining half can sustain combustion on its own. A healthy engine will show a modest, predictable RPM drop during this test. What it should never show is a drop of 350 RPM.

What the Numbers Actually Mean

Your DPE will expect you to know the acceptable limits without hesitation. For many common training aircraft like the Cessna 172, the standard thresholds are a maximum drop of roughly 125 RPM on either magneto, and no more than a 50 RPM differential between the two magnetos. These figures are not arbitrary — they reflect how much power loss is acceptable when operating on a single ignition source. A 350 RPM drop blows past that limit by a wide margin and is a clear signal that something is wrong on the left magneto side.

One of the most common mistakes student pilots make is thinking that a large RPM drop is acceptable as long as the engine runs smoothly. Smoothness is not the standard. The RPM numbers are the standard. An engine can feel relatively smooth while still running dangerously lean or misfiring in a way that will not become obvious until you are climbing out over the departure end of the runway. Do not let a smooth-sounding engine talk you out of what the tachometer is telling you.

It is also critical to understand that these specific figures — 125 RPM and 50 RPM differential — are typical for common trainers but are not universal FARs. The actual limits for your specific aircraft are found in the Pilot's Operating Handbook (POH). Always confirm the acceptable limits for the aircraft you are flying, because they vary by make and model.

What Causes an Excessive Magneto Drop?

A 350 RPM drop on one magneto typically points to one of three culprits: a fouled spark plug, a magneto timing problem, or a failed ignition component on that magneto circuit. Understanding the distinction matters because your DPE may press you on it.

Fouled spark plugs are the most common cause in training aircraft. Lead deposits or carbon buildup on a plug can prevent it from firing reliably, starving that cylinder of proper combustion. In some cases — with a minor RPM drop — leaning the mixture aggressively at high power for a short period can burn off light fouling and bring the drop back within limits. However, a 350 RPM drop is far too large for that technique to be appropriate or safe. Attempting it with a discrepancy this significant is not a fix; it is wishful thinking.

Magneto timing issues are more serious and invisible to the pilot during the runup. If a magneto has drifted out of time, the spark is firing at the wrong point in the combustion cycle, reducing power and potentially damaging the engine. A failed P-lead, a cracked distributor block, or worn internal magneto components can also produce large RPM drops. None of these are pilot-fixable on the ramp.

The Only Correct Response: Ground the Aircraft

When you see a 350 RPM drop on one magneto, the correct decision is immediate and non-negotiable: do not take off. Taxi back to the ramp, shut down the engine, and write up the discrepancy for an A&P mechanic to inspect before the aircraft flies again. There is no version of this scenario where pressing forward makes sense.

Some pilots are tempted to take off anyway, rationalizing that they will see how it does in the air or that the engine seemed to recover slightly after the check. This is one of the most dangerous thought patterns in aviation. The dual magneto system exists precisely because engine failures do happen, and launching with a known fault in one of your two ignition systems eliminates the safety margin that system was designed to provide. The FAA takes aircraft airworthiness seriously, and so should you.

According to the PHAK Chapter 7, the ignition system is fundamental to reliable engine operation. Treating an abnormal mag check as a maintenance squawk — not a minor inconvenience — is the mark of a pilot who understands both the system and their responsibility as pilot in command. Your DPE is not just testing whether you know the RPM numbers. They are evaluating whether you have the judgment to act on them correctly under pressure.

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