Carburetor Ice on a Cool, Humid Climb
You are cruising a Cessna 172 at 8,500 ft MSL with the engine at 2,350 RPM and 20.5 inHg manifold pressure. The outside air temperature is -5°C, and the air is humid with visible haze below. The engine begins to run rough and RPM slowly drops to 2,150. You apply carburetor heat; the RPM drops further to 2,050 and the engine runs rougher for about 20 seconds, then smooths and RPM rises to 2,250. You remove carburetor heat, and within 2 minutes the roughness and RPM loss return. You have 34 gal of usable fuel, the nearest suitable airport is 12 NM away, and your planned destination is 45 minutes ahead. There is no icing report for your route, and the aircraft has no carburetor temperature gauge.
1. What is the most likely cause of the RPM loss, and why did the RPM initially drop further after carburetor heat was applied?
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The symptoms are classic carburetor ice: rough running and a slow RPM loss in cool, humid air, followed by improvement after carburetor heat. The carburetor venturi can cool the intake air enough for ice to form even at outside temperatures around -5°C, especially with high humidity.
The initial further RPM drop after applying heat is expected. Carburetor heat admits warmer, less dense air, which reduces the mass airflow and enriches the mixture, so power drops briefly. If ice is present, that heat then melts it; the engine runs rough as melted ice passes through, and RPM recovers as the restriction clears. The recovery to 2,250 RPM supports carburetor ice rather than a mechanical failure. The fact that roughness returns after heat is removed shows that icing conditions still exist and the heat was doing useful work.
2. What should you do now? Consider carburetor heat use, mixture, altitude, and whether to continue to the destination.
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Leave carburetor heat on. Since the roughness returns when heat is removed, the ice will likely reform and may progress to partial or total power loss. Carburetor heat causes a power loss and enriches the mixture, so lean as required for smooth operation at 8,500 ft, but do not lean so aggressively that you mask the roughness or raise temperatures excessively. Avoid large, abrupt throttle changes.
The decision should be to divert to the nearest suitable airport, not continue for 45 minutes. You have only one engine, no carburetor temperature gauge, and repeated ice formation. A 12 NM diversion gives a short exposure time. Maintain a safe altitude with terrain clearance, keep carburetor heat on, and plan a normal approach rather than a short-field approach. If power is still rough on arrival, request priority and be prepared for a possible engine failure; do not delay the diversion to “see if it clears.”