Carburettor Icing on Descent in a Piston Twin
A piston twin is descending from 9,000 ft to 4,000 ft in visible moisture with an outside air temperature of 4°C at 9,000 ft and 8°C at 4,000 ft. Both engines are carburetted and have carburettor heat available. The left engine begins to run rough, with a 60 rpm drop and a slight rise in manifold pressure. The right engine is smooth. The crew applies carburettor heat to the left engine, and the roughness briefly worsens before improving. The aircraft is in icing conditions and the pitot heat is on. The crew has 30 minutes to destination and the alternate is 15 minutes away.
1. Explain why carburettor icing is likely despite the outside air temperature being above freezing, and why the roughness worsened after heat was applied.
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Carburettor icing occurs because fuel vaporisation and the pressure drop across the venturi cool the mixture well below the outside air temperature, often by 15–20°C. At 4°C outside air, the carburettor throat can be below freezing, so ice forms even though the aircraft is not in freezing conditions. The brief worsening after carburettor heat is applied is a classic sign: the heat melts ice, and the meltwater passes through the engine as liquid, causing temporary roughness before the ice is fully cleared. If the roughness had continued to worsen, the crew should suspect that heat is not reaching the carburettor or that the ice is too thick to clear. The right engine being smooth does not mean it is safe; it may be at an earlier stage of ice accumulation, so the crew should apply heat to both engines as a precaution.
2. Recommend the power and heat settings and the diversion decision.
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The crew should keep carburettor heat fully on for the left engine until it runs smoothly, then apply heat to the right engine as a precaution. They should avoid large power changes, which can shed ice into the engine, and should maintain a moderate power setting. If the left engine does not run smoothly after two minutes of heat, the crew should assume the ice is not clearing and plan to land at the nearest airport. With 30 minutes to destination and 15 to the alternate, the alternate is the safer choice if any roughness persists, because a rough engine may lose power on approach. The crew should also consider that descending into warmer air at 4,000 ft reduces the icing risk, so a descent may help, but the decision should be made before the engine loses significant power. The judgement call is to divert early rather than press on and risk a single-engine approach.