icing, Carburettor Heat and the Go/No-Go
You are cruising at 9,500 ft in a carburettor-equipped piston aeroplane. OAT is −2 °C, visible moisture is present as a thin layer, and the outside air is stable. The engine begins to run slightly rough; RPM has dropped about 80 and the carburettor air temperature gauge reads +3 °C. There is no airframe ice visible yet. Your destination is 40 min ahead, with an en-route minimum safe altitude of 8,200 ft and terrain rising to 10,500 ft to the north. You have oxygen for the pilot only and one passenger on board.
1. What is the most likely cause of the roughness, and what is the immediate action?
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The symptoms point to carburettor ice: falling RPM, roughness, and a carburettor air temperature near the freezing zone, with visible moisture present. Apply full carburettor heat immediately. Expect a further RPM drop as the heat melts ice and rich mixture passes through, then recovery. Do not reduce heat prematurely - leave it fully on until the engine runs smoothly. If heat is applied and the engine does not recover, or worsens, suspect serious ice and plan to descend to warmer air while turning toward the nearest suitable aerodrome.
2. What is the risk of the en-route altitude and the terrain to the north?
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The MSA is 8,200 ft and you are at 9,500 ft, giving 1,300 ft of margin - adequate in normal conditions but not generous if you must descend for engine problems or ice. Terrain to 10,500 ft to the north means any drift or diversion north requires climbing, which a degraded engine may not manage. Plan any diversion south or along the valley where terrain is lower, and keep a continuous escape route in mind. If ice or engine roughness persists, descend only where terrain allows, and treat 8,200 ft as a hard floor, not a target.
3. Why is carburettor heat less effective at high altitude and low power settings?
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Carburettor heat is drawn from exhaust or engine heat, and its effectiveness depends on the temperature rise it can deliver to the incoming air. At high altitude with low power, exhaust heat and mass flow are reduced, so the temperature rise is smaller and the heated air is thinner. That makes prevention more important than cure: use heat proactively in the descent and at low power, and avoid prolonged low-power glides in icing conditions.