Carburettor icing is one of the most common causes of engine failure in carburetted engines. It arises from three mechanisms simultaneously:
| Mechanism | Effect | Temperature drop |
|---|---|---|
| Evaporation cooling | Fuel vaporises in the venturi | up to ~30 °C |
| Pressure (Venturi) cooling | Pressure drop in the venturi further reduces T | further ~5 °C |
| Impact icing | Ice on the air intake in visible moisture (rain, cloud) | environment-dependent |
When is carburettor icing most likely?
OAT roughly −10 °C to +25 °C with high humidity. Most insidious on warm, humid summer days at low power settings (descent, pattern). The official carb icing probability chart in the AIP / AFM is the authoritative reference.
Symptoms
| Fixed-pitch | Constant-speed |
|---|---|
| Slow RPM decay without lever movement | Slow manifold pressure decay |
| Rough running | Rough running |
| Advanced: engine failure | Advanced: engine failure |
Action — carb heat
On suspicion or preventively (descent):
- Carb heat FULL ON.
- Expect: brief further RPM drop (warm, less-dense air = slightly less power). Then RPM rises as ice melts and water flushes through the venturi.
- Before next take-off or full power: carb heat COLD (warm air is less dense and may bypass the filter).
Prevention
- In risky conditions: apply carb heat regularly during descent and low-power cruise.
- Watch the engine indications (RPM or MP, rough running).
- On fuel-injected engines: usually not relevant, but use alternate air in visible moisture if fitted.
Manual on Low-level Wind Shear and Turbulence* (weather application); BFU aggregated reports on engine failures.*