Magneto Drop That Won't Lean Away
At Kaduna (elevation 2,100 ft), surface temperature +34 °C, you are running the pre-takeoff check on a PA-28-161 (Lycoming O-320-D3G) with 1,250 hours since overhaul. At 1,800 RPM, after leaning for maximum RPM, the left magneto gives a 60 RPM drop. On the right magneto the engine drops 190 RPM and runs rough; leaning further recovers it to about 140 RPM, but the roughness stays. The plugs were cleaned and gapped 40 hours ago. There is no engine monitor and no EGT gauge, only the tachometer. The aircraft is booked for a two-hour training detail, and the school's maintenance organisation is on the field with a spare set of plugs. A second aircraft on the same line failed the same check last week.
1. Interpret these numbers against the limits and say what the pattern points to.
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Against the limits (Lycoming O-320): maximum permissible drop is 175 RPM and maximum difference between magnetos is 50 RPM; roughness is disqualifying in its own right.
- Left magneto: 60 RPM, smooth - normal.
- Right magneto: 190 RPM, rough - exceeds the 175 RPM limit before you touch anything, and the roughness alone is a fail.
- After leaning: about 140 RPM, which is inside the 175 RPM absolute limit, but the left-to-right difference is now about 80 RPM, over the 50 RPM limit. The roughness persisting matters more than the number.
Interpretation: leaning raises combustion temperature and can burn off lead or carbon fouling, which is why the drop improved. It cannot fix a failed plug, a shorted or chafed ignition lead, a condenser or points problem inside the magneto, or a timing error. A drop that improves but stays rough usually means one cylinder is not firing properly on that magneto - typically a fouled or failed plug on that circuit.
2. Explain why leaning during the check changes the indication, and what it cannot fix.
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On the ground at 1,800 RPM the mixture is rich, and lead/carbon deposits form a conductive path across the plug electrodes. Leaning raises the firing-end temperature and burns deposits away, so the drop improves. That is a legitimate technique during the check; it is not a way to "pass" a bad magneto.
What leaning cannot do:
- Restore a plug with a cracked insulator or a bridged, dead electrode.
- Fix a chafed ignition lead grounding out.
- Fix a magneto with a failing condenser (often a drop that worsens as the magneto heats) or mis-set internal timing.
- Correct a plug of the wrong heat range.
Why the residual 80 RPM differential matters: the differential limit exists because it shows the two magnetos are no longer contributing equally. An engine making most of its power on one magneto has poorer combustion, higher exhaust temperatures on the weak side, a longer take-off roll and degraded climb - exactly the conditions at 2,100 ft with 34 °C.
3. What is your decision, what do you do next, and how does the second aircraft's failure change your thinking?
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Decision: no-go. The run-up is a test, and this one failed. The aircraft is not dispatched until the defect is found.
Steps:
- Shut down normally; do not "try it again after a run" with the same result.
- Write it up: "Right magneto drop 190 RPM, rough; after leaning 140 RPM, still rough. Aircraft not dispatched." Sign, date, and brief the next pilot rather than leaving a note in a seat pocket.
- Ask the maintenance organisation to remove, clean and test the plugs, check the ignition leads and magneto timing, and repeat the check on the ground.
- If the fouling returns quickly, the cause may be plug heat range, mixture setting or oil entering the combustion chamber, not just handling technique.
The second aircraft failing the same check within a week changes the investigation, not the decision. When two aircraft on the same line fail the same check, suspect a common cause - a fuel batch, a plug batch, or the same priming and leaning technique by pilots - and check the fuel and plug stock rather than assuming each aircraft is separately unlucky. Neither aircraft is fit to fly in the meantime.