Single-cylinder roughness and high CHT in a carburetted engine
You are flying a Piper PA-28-181 (Lycoming O-360, carburetted, fixed-pitch propeller) from FIMP on a 2-hour cross-country. Outside air temperature at 3,000 ft is 31°C. Ten minutes after levelling off, the engine runs slightly rough. Engine gauges: CHT #3 245°C (red line 260°C), oil temperature 112°C (red line 118°C), oil pressure 55 psi (normal 55–95 psi), EGT #3 40°C lower than the others, fuel flow normal, tachometer steady. Carburettor heat application causes a 30 rpm drop and no change in roughness. Magnetos: left drop 75 rpm, right drop 80 rpm (max 175, difference 50). You are 45 NM from FIMP and 25 NM from a licensed aerodrome with a 1,000 m paved runway; weather is CAVOK, wind 090/12. MTOW is 1,150 kg; you are 80 kg below MTOW. Fuel endurance 3.5 hours.
1. Interpret the engine indications. What fault is most likely, and what else must you rule out?
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The key combination is roughness plus #3 EGT lower than the others plus #3 CHT high. A low EGT on one cylinder means that cylinder is producing less heat and power, not that the mixture is simply lean. Possible causes are a stuck exhaust valve, ignition failure on that cylinder (plug or lead), an intake leak, or another mechanical fault. The high CHT on the same cylinder raises concern for pre-ignition, detonation or a valve problem. Carburettor ice is less likely because carburettor heat gave no change in roughness. The magneto drops are within limits, so the whole ignition system is not failing, but one plug or lead can still be faulty. Normal fuel flow and a steady tachometer do not rule out a single-cylinder fault.
2. State your immediate actions and the decision you would make about continuing.
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Immediate actions: reduce power, enrich the mixture, open cowl flaps, increase airspeed to improve cooling, and monitor CHT, EGT, oil temperature and oil pressure. Do not lean further. If CHT exceeds the red line, roughness worsens, or oil temperature rises, land at the 25 NM aerodrome. Even if CHT stabilises, a rough engine with one cold cylinder is not a normal condition. In a single-engine aircraft, continuing a 2-hour cross-country with a possible stuck valve or ignition fault is not justified. I would divert to the 25 NM aerodrome and have maintenance inspect the engine.
3. If CHT stabilises at 235°C but the roughness remains, would you continue? Explain.
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No. A stabilised CHT only means cooling has caught up with the heat being produced; it does not fix the mechanical fault. A stuck valve, a failing plug or lead, or an intake leak can progress to engine failure. The nearest aerodrome is 25 NM away and CAVOK, so the safe decision is to land and inspect. I would not continue to destination on the basis that the temperature has stopped rising.