Carburettor Ice at 6,500 ft
An R44 is cruising at 6,500 ft pressure altitude on a cross-country flight. OAT is 8°C and the dew point is 6°C. The engine has been running smoothly at 22 in Hg and 2,100 RPM. Without warning the engine begins to run rough, manifold pressure falls to 19 in Hg, the helicopter yaws slightly, and the low rotor RPM horn chirps. The pilot has carburettor heat selected COLD. There is no engine fire warning, no oil pressure fluctuation, and fuel quantity is normal. The nearest suitable landing site is a cleared field 4 nm ahead; there is rising terrain to the right and a large lake to the left. The pilot's first action is to apply full carburettor heat. The engine roughens further for several seconds, then manifold pressure starts to recover slowly. The pilot must decide whether to continue the flight, land in the field, or turn back.
1. Explain the most likely cause and why applying carburettor heat initially made the engine rougher.
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The most likely cause is carburettor ice. The OAT of 8°C with a dew point of 6°C gives only a 2°C spread, which is a classic high-risk range for carburettor icing. Ice forms in the venturi and around the throttle, restricting the mixture and causing manifold pressure to fall, the engine to run rough and RPM to decay. Applying carburettor heat melts the ice, but the resulting water passes through the engine and temporarily enriches and disrupts combustion, so the engine may run rougher for several seconds before improving. The slow recovery of manifold pressure after heat is applied supports this diagnosis. The pilot should not mistake the initial roughness for a worsening fault and remove carburettor heat; that would allow ice to re-form.
2. State the immediate handling priorities and decide the safest course of action.
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The immediate priority is to protect rotor RPM by lowering collective smoothly, while keeping full carburettor heat applied. The pilot should avoid large throttle movements, monitor manifold pressure and RPM, and select a landing site. Because the engine has already shown it can ice up, and because the aircraft is near rising terrain and a lake, continuing the flight reduces options. The safest course is to land in the cleared field ahead, let the engine stabilise with carburettor heat, and inspect the situation. After landing, the pilot should not take off again until the engine runs smoothly, full manifold pressure is available and carburettor ice has cleared. A return or a lower-level route may then be considered, but only with a positive margin and a watch for recurrence.
3. What conditions would you use to decide whether to continue after landing?
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After landing, the pilot should use the engine response and the weather as the decision criteria. Continued smooth running, normal manifold pressure, normal RPM and no recurrence with carburettor heat selected are the minimum signs that the ice has cleared. The pilot should also reassess temperature and dew point: a small spread, visible moisture, drizzle or cloud at the planned altitude means a high risk of recurrence. If the engine is not completely normal, or if the route remains in the same temperature and humidity band, the flight should be discontinued. A lower altitude with warmer air may reduce icing risk, but only if terrain clearance and VFR minima are assured. The conservative decision is to land and wait, or cancel the leg.