Engine roughness after carburettor heat on descent
You are flying a Cessna 172 with a Lycoming O-320 fixed-pitch engine and two persons on board. You are descending from 9,500 ft to 6,000 ft at 500 ft/min over the Zagros foothills with carburettor heat cold. OAT is +6°C and relative humidity is 80%. The engine begins to run rough; RPM falls from 2,400 to 2,250. You enrich the mixture and the roughness increases slightly. You then apply full carburettor heat. For about 20 seconds the engine runs rougher and RPM drops further, then it smooths and RPM recovers to 2,350. Fuel selector is on BOTH and fuel remaining is 1 h 30 min. The nearest airport is 25 NM away; terrain below is rising and mostly unsuitable for a forced landing.
1. What is the most likely cause of the roughness, and why did carburettor heat initially make it worse?
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The most likely cause is carburettor icing. The OAT of +6°C with 80% relative humidity is a classic carburettor-ice range, and the descent at reduced power increases the risk because the carburettor is less heated by the engine. Initially enriching the mixture made the roughness worse because the engine was already running rich and the extra fuel further disturbed combustion. Applying full carburettor heat melts the ice; the first 20 seconds are often rougher because water and ice are passing through the engine, then the RPM recovers as the ice clears. The recovery to 2,350 RPM supports this diagnosis. The correct response is to keep carburettor heat ON and monitor RPM, engine roughness, fuel flow and oil temperature/pressure rather than assuming the problem is only mixture.
2. State your immediate actions and the indications that would make you commit to a precautionary landing rather than continue to the airport.
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Immediate actions: keep carburettor heat fully ON, maintain a safe altitude and best-glide speed if power is uncertain, check fuel selector on BOTH, mixture rich, magnetos on BOTH, and engine gauges. Do not lean aggressively while carburettor heat is applied. Advise ATC of the roughness and your intention. Commit to a precautionary landing if RPM continues to decay, roughness or vibration becomes severe, oil pressure or temperature goes outside limits, or you cannot maintain altitude. The airport is 25 NM away; from 6,000 ft a Cessna 172 has a glide range of roughly 8-10 NM in still air, so the airport is not a reliable forced-landing option. Select a field or road while you still have power and altitude, and fly a stable approach rather than waiting for a total failure over rising terrain.
3. If the engine continues to lose power and you must carry out a forced landing, outline your plan, including radio and passenger briefing.
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Establish best glide speed, about 65-70 kt in a Cessna 172, and turn toward the most suitable landing area. Attempt a controlled restart: fuel selector BOTH, mixture rich, carburettor heat ON, magnetos BOTH, primer locked, and check for fuel starvation or air intake blockage. Make a Mayday call on 121.5 with position, altitude, heading, intention and persons on board; squawk 7700. Brief passengers: seats upright, belts tight, doors unlatched before touchdown, and brace position. Plan the approach into wind, avoid obstacles, use flaps only as needed, and aim for a touchdown point about one-third into the selected field. Secure the aircraft after landing, shut off fuel and electrics, and evacuate. The radio and passenger briefing should be done early, not during the final seconds of the approach.