Engine roughness after a long descent
During a descent from 9,500 ft to 3,000 ft in a carburettor-equipped piston aircraft, you have been at low power (approximately 15 in Hg MAP, 2,100 RPM) for 12 minutes in clear air at −2 °C OAT. The engine begins to run rough, RPM fluctuates ±80 RPM, and the EGT drops. Carburettor heat is currently COLD. The engine has a fixed-pitch propeller and no carburettor air temperature gauge. You are 25 NM from your destination with 45 minutes of fuel.
1. What is the most likely cause, and what is the immediate action?
Read the worked answer
The symptoms (rough running, RPM fluctuation, EGT drop, low power, descent in cold humid air) point strongly to carburettor icing. At 15 in Hg and 2,100 RPM with OAT around −2 °C, the carburettor venturi can drop the local temperature well below freezing, and descent power settings are a classic icing regime.
Immediate action: apply full carburettor heat, then lean as required and expect a temporary further roughness as ice melts and passes through the engine. Do not reduce power or make abrupt changes. If roughness worsens or does not clear within ~30 seconds, treat it as an engine failure in progress and prepare to glide/land.
2. Why might the EGT drop rather than rise?
Read the worked answer
Ice in the venturi restricts the airflow and enriches the mixture. A richer mixture burns cooler, so EGT falls. The RPM fluctuation comes from the unstable mixture and uneven cylinder filling. This is why EGT behaviour alone is not diagnostic - it must be read with the power setting, OAT and phase of flight.
3. What should you do after the engine smooths out?
Read the worked answer
Leave carburettor heat ON until you are sure the ice has cleared, then transition to a power setting that keeps the carburettor warm (avoid prolonged low-power descent). Plan to land at the nearest suitable aerodrome if roughness recurs, and note the event for maintenance. Do not turn heat off immediately after smoothing - residual ice can re-form.