Magneto Drop After Run-Up
During the run-up of a Cessna 172S with a Lycoming IO-360, at 1,800 RPM, the left magneto drop is 75 RPM and the right magneto drop is 210 RPM. The POH limit is 150 RPM maximum drop and 50 RPM maximum difference between magnetos. The engine was leaned for taxi at a coastal airport with high humidity. After switching back to BOTH, RPM returns to 1,800 but the engine runs slightly rough. The pilot has a 2-hour daylight VFR cross-country planned and is concerned about delaying. Two top spark plugs on cylinder 3 were replaced 5 hours ago; the remaining plugs have 300 hours. Fuel is 100LL, 40 gallons. The pilot considers leaning more aggressively during the run-up to 'burn off' fouling and then departing if the drop improves to 160 RPM.
1. Interpret the magneto check. Which magneto is suspect, and is the drop within limits?
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The right magneto is suspect. Its drop is 210 RPM, above the 150 RPM maximum, and the difference between the two magnetos is 210 - 75 = 135 RPM, above the 50 RPM maximum. The left magneto drop of 75 RPM is within limits. A magneto drop means the engine is running on the other magneto only; an excessive drop means the right magneto or its plugs are not producing normal combustion. Roughness after returning to BOTH supports a fouled plug or ignition fault. Leaning can sometimes clear light fouling, but a drop that settles at 160 RPM is still above the limit and is not acceptable.
2. What should the pilot do before departure, and what is the risk of departing with a 160 RPM drop?
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Before departure, lean at run-up power for 30-60 seconds, then recheck both magnetos. If the drop remains above 150 RPM or the difference above 50 RPM, do not depart; have maintenance inspect the right magneto and spark plugs. Departing with a 160 RPM drop means one magneto is weak. If the healthy magneto then fails, the engine may lose substantial power; unburned fuel can also ignite in the exhaust, and a shorted P-lead can leave a magneto live on the ground. The 2-hour trip and concern about delay are not justification for accepting an out-of-limits check.
3. In flight, if the engine runs rough and the right magneto is suspect, what actions are appropriate?
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Do not deliberately select the suspect magneto in flight. If the engine runs rough, reduce power, lean for smooth operation, check fuel flow and engine gauges, and plan a landing at the nearest suitable airport. If roughness persists on BOTH, suspect the other magneto, fuel, or induction system; avoid repeated mag switching. A known bad magneto is not troubleshooted by cycling it in flight. Declare an emergency if power loss is significant.