Engine Roughness in Humid Cruise
Pilot in a C172, 6,500 ft, OAT 5°C, relative humidity 80%, cruise 2,300 RPM. After 45 min, RPM falls to 2,150 and engine runs rough. Pilot applies carburettor heat; roughness increases for about 15 seconds and then RPM rises to 2,250, but the engine remains slightly rough. Fuel selector is on BOTH, mixture rich, mags were checked before departure. No carburettor temperature gauge is fitted. The pilot has 2.5 h fuel and is 40 nm from destination, with a suitable aerodrome 12 nm away. The pilot notices the roughness is worse when power is reduced. Outside air temperature is 5°C and the air is humid. The pilot is tempted to lean the mixture and continue because RPM improved.
1. What is the most likely cause of the RPM drop and roughness, and why did carburettor heat initially make it worse?
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Most likely carburettor icing. At 5°C and 80% RH, the carburettor venturi can cool the air enough for ice to form, even though the outside air is above freezing. Applying carburettor heat melts the ice; the meltwater enters the induction system and causes temporary rough running before the ice clears. The subsequent RPM rise to 2,250 supports this. The fact that roughness is worse at reduced power is also consistent with carburettor ice, because the throttle is more closed and the venturi effect is stronger. A magneto or spark-plug problem would not normally improve with carburettor heat. However, the engine is still rough, so the pilot should not assume the problem is fully solved.
2. What actions should the pilot take now, and should the flight continue to destination?
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Keep full carburettor heat on, enrich the mixture if necessary, avoid large power changes, and monitor RPM and engine smoothness. Do not lean the mixture aggressively to mask the roughness. The suitable aerodrome 12 nm away is the better choice: divert and land. Continuing 40 nm with an engine that is still rough, after suspected carburettor ice, leaves no margin if the ice re-forms or if the roughness is caused by something else. If carburettor heat does not restore smooth running, treat it as a serious engine malfunction. The pilot should advise ATC if applicable, plan a precautionary landing, and not attempt to troubleshoot beyond simple, safe actions in flight.
3. If the engine continues to run rough and then loses power despite carburettor heat, what should the pilot do?
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Establish the best glide speed, select a suitable forced-landing area, and declare an emergency. Attempt only quick, memory-item checks: carburettor heat on, fuel selector on a tank with fuel, mixture rich, magnetos on BOTH, primer locked, and auxiliary fuel pump on if fitted. If power returns, land at the nearest suitable aerodrome rather than continuing the original flight. If power does not return, prepare for an off-field landing: secure the cabin, brief passengers, transmit position and intentions, and touch down under control. Do not delay the forced-landing preparation while troubleshooting. The pilot should not attempt to stretch the glide or turn back to a distant aerodrome unless altitude and terrain clearly allow it.