Engine Roughness and Carburetor Ice on a Cross-Country
You are flying a Cessna 152, powered by a Lycoming O-235, at 7,500 ft MSL over central Luzon. Outside air temperature is 8°C and the dew point is 6°C; there is visible haze but no cloud. After 50 minutes in cruise, the engine begins to run rough. The RPM drops from 2,350 to 2,200. You apply full carburetor heat. The RPM drops further to 2,050, the engine stumbles for about 20 seconds, and then the RPM slowly rises to 2,300 and the roughness disappears. Fuel pressure remains normal, oil pressure and temperature are normal, and the magneto check at runup was normal. You have 1.5 hours of fuel remaining and are 25 nm from your destination. The carburetor heat control moves freely and the air filter was clean at the last inspection. You are not instrument rated and the flight is VFR. You must decide whether to continue, return, or divert.
1. What is the most likely cause of the roughness, and what evidence in the scenario supports it?
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The most likely cause is carburetor icing. The outside air temperature is 8°C and the dew point is 6°C, only a 2°C spread, with visible haze indicating moisture. Carburetor icing can occur at temperatures well above freezing because fuel vaporisation and the venturi effect cool the carburetor throat. The gradual onset in cruise, the RPM drop, and the normal fuel pressure, oil pressure, oil temperature, and magneto check all point away from ignition or fuel-delivery failure and toward ice in the carburetor. The decisive evidence is the response to carburetor heat: the engine initially stumbles, then smooths out and RPM recovers, which is typical of ice melting and being ingested.
2. Why did the RPM drop further after carburetor heat was applied, and why did it then rise?
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When carburetor heat is applied, heated air enters the carburetor. Warmer air is less dense, so the mixture momentarily becomes richer and power drops; that explains the initial fall from 2,200 to 2,050 RPM. At the same time, the heat melts the ice, and the engine ingests water, causing the brief stumble. As the ice clears, the carburetor throat returns to normal, the fuel-air mixture stabilises, and the engine smooths out. The RPM then rises to 2,300 because the obstruction is gone, even though carburetor heat remains on and the air is warmer and less dense than before. The important point is that the initial drop is not a reason to remove heat; it is part of the clearing process.
3. What should you do next, and what precautions should you take for the remainder of the flight?
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Continue with carburetor heat on and monitor the engine closely. In cruise, carburetor heat may remain on if icing conditions persist, but expect a slight power loss and lean the mixture as appropriate for the warmer intake air. Avoid abrupt throttle changes, which can trigger further icing, and keep scanning RPM, oil temperature, and fuel pressure. Because the ice cleared but conditions remain favourable for recurrences, plan to divert or land at the nearest suitable airport if roughness returns or if carburetor heat no longer produces improvement. Do not press on simply because the destination is 25 nm away. If the engine runs rough again and carburetor heat is ineffective, treat it as an emergency and land as soon as practicable.