Carburettor Icing During Descent
You are flying a normally aspirated, fixed-pitch light aircraft at 6,500 ft AMSL on a cross-country flight. OAT is 8°C; the dew point is 4°C. You have been at cruise power for 40 minutes with carburettor heat cold. While descending to 3,000 ft at 500 ft/min with reduced power, the engine begins to run slightly rough and the RPM falls from 2,300 to 2,150. No manifold-pressure gauge or carburettor-temperature gauge is fitted. Outside, visibility is good with scattered cumulus and a few stratocumulus; there is no airframe icing. Fuel state is 1 h 40 min plus 45 min reserve. The nearest suitable airfield is 12 nm ahead; the destination is 45 nm ahead. The flight manual says: apply full carburettor heat for descent; if icing is suspected, expect an initial RPM drop.
1. Assess the carburettor-icing risk from the temperature, dew point, and phase of flight. Is the absence of airframe icing reassuring?
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OAT 8°C and dew point 4°C give a spread of 4°C, so relative humidity is high. Carburettor icing is a risk roughly between -7°C and +20°C when moisture is present, and the descent at reduced power is the classic high-risk phase: the throttle is partly closed, the venturi pressure drops, fuel evaporation cools the mixture, and ice can build up in the carburettor and throttle body. The absence of airframe icing is not reassuring: carburettor icing can occur without visible airframe ice because the temperature drop inside the carburettor is much greater than the outside air temperature drop. At 8°C/4°C with a 40-minute cruise and a descent, the risk is serious, not marginal. The correct pre-emptive action would have been full carburettor heat before reducing power, not after the RPM drop.
2. Explain the RPM behaviour after carburettor heat is applied and what it tells you. What actions do you take if the RPM continues to fall?
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Applying full carburettor heat normally causes an initial RPM drop because heated air is less dense. If ice is present, the RPM should then recover within roughly 10-30 seconds as the ice melts and the mixture returns toward normal. A recovery confirms carburettor icing. If the RPM continues to fall, or roughness worsens, the heat may be unable to melt the accumulation, or the problem may not be carburettor ice. Keep full heat applied, keep the mixture rich enough for the heated air, avoid large or rapid throttle changes, and prepare to land. Do not use partial heat: partial heat can melt ice and then refreeze it downstream. The nearest airfield is 12 nm ahead, so the practical decision is to divert and land rather than continue 45 nm and hope the RPM recovers.
3. Make the diversion or continuation decision and state the post-landing considerations.
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Divert to the 12 nm airfield and land. The aircraft is running rough, RPM is below the cruise setting, and the descent is exactly when carburettor ice is most likely. Continuing another 45 nm adds exposure with no useful margin; the reserve fuel does not solve an engine-power problem. If the engine smooths out and RPM recovers fully, that supports carburettor ice, but it does not prove the carburettor is clear; ice can recur as soon as heat is removed. Land, keep carburettor heat on during the approach as required by the flight manual, and do not reduce power abruptly. After landing, have the carburettor and induction system checked and do not depart until the cause is understood. Record the temperature, dew point and RPM behaviour for the maintenance investigation.