Learning objective: Separate carb ice (venturi + vaporisation) from impact ice at the intake; read carb heat on a fixed-pitch prop.
60 seconds: Carb ice even without visible moisture, often at low power. Impact ice is separate. Carb heat FULL: fixed-pitch RPM drops then rises. Then COLD before full power. No “official AIP chart”.
Carburettor icing is one of the most common causes of engine failure in carburetted engines. Carb ice comes from two cooling paths (not three simultaneous carb mechanisms):
| Mechanism | Effect | Temperature drop (teaching magnitude, not a POH limit) |
|---|---|---|
| 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 ice at the intake is separate (visible moisture at filter/intake; fuel-injected: Alternate Air) — not a third carb-ice mechanism “at the same time”.
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). Typical teaching/met chart (OAT vs dew point/humidity, power setting) — not an “official AIP/AFM chart”. An AFM may include a chart; then that AFM applies, not “the AIP”. Exam band (typical teaching, not a POH limit): often about +10 to −5 °C; other bands depending on the chart (some teaching to ~+15/+25 °C with moisture).
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.*
Exam knowledge
Carb ice: venturi plus fuel vaporisation, even with OAT well above 0 °C. Impact ice is separate. Teaching chart, not AIP. Carb heat FULL: RPM drop then rise (fixed-pitch).
Check yourself
1. When is carburettor icing most likely?
- A) OAT about −10 °C to +25 °C, high humidity, especially at low power
- B) Only below −20 °C in dry air at full power
- C) Only with visible moisture below −30 °C
- D) Only on injected engines above FL 180
2. First action if you suspect carb icing?
- A) Mixture to idle cut-off
- B) Pitot heat full on
- C) Carb heat blip, then immediately COLD
- D) Carb heat full on; a further RPM/MP drop is expected at first
- A — moist, moderate OAT, often low power.
- D — carb heat FULL. Expect a further drop, then a rise as ice melts.