Carburettor icing
Carburettor icing: ice in the venturi throat and on the throttle valve of a float-type carburettor. Chokes the engine of air.
Why it forms:
- Air speeds up through venturi, pressure drops.
- Fuel vaporises at throat.
- Air cools up to 21 C below intake temperature (TP 1102). Moisture freezes on throat and throttle valve (PHAK ch.7).
Conditions:
- Worst: high humidity at -5 C to 15 C (TC sample exam Q29).
- Needs humid air. Can occur from -13 C to +38 C (TP 1102).
- Likeliest at high RH; can occur at 50% RH at +38 C (TP 1102).
- Can occur in clear air (high humidity, above freezing). No cloud, rain or visible droplets needed (TP 1102: "moist" is the key word; TC sample exam Q30).
- Risk drops below 0 C (less humidity). Near -10 C, ice crystals pass through harmlessly (TP 1102).
- MOGAS ices more readily: severe cases ice at OAT up to 20 C higher than AVGAS (TC AIM AIR 2.3).
Power setting:
- Likeliest at LOW power (descent, approach): near-closed throttle cools air most (PHAK ch.7).
- Can strike at any power. Severity (TC AIM AIR 2.3, Fig 2.2):
| Severity | Power setting |
|---|---|
| Serious | Any power in worst band |
| Serious | Descent power |
| Moderate | Cruise |
| Light | Cruise or descent |
- Most carburettor-icing engine failures happen at CRUISE, possibly from lower pilot awareness (TC AIM AIR 2.3).
Symptoms:
| Propeller | First sign | Note |
|---|---|---|
| Fixed-pitch | RPM drops | Then rough running (PHAK ch.7) |
| Constant-speed | Manifold pressure drops | Governor holds RPM, even at smallest blade angle, so MP drops first (TP 1102; PHAK ch.7) |
- Constant-speed: with altitude and throttle held constant, falling MP may indicate ice. High-to-low pressure flight or engine malfunction can also lower it (TP 1102).
Response:
- Apply FULL carb heat at once, until all ice gone. Partial heat or too short a time can make it worse (PHAK ch.7; TP 1102).
- Expect initial RPM drop and rougher running as ice melts, then recovery (PHAK ch.7).
- Constant-speed: manifold pressure drops momentarily, then rises gradually as ice melts; means ice most likely present (PHAK ch.7; TP 1102).
- Use only to check for or clear ice, never when full power needed (PHAK ch.7; TP 1102 Ex.3: avoid during take-off, detonation risk).
- At 75% cruise power or less, any amount of heat does no engine damage (TP 1102).
Trap: leaning or adding throttle does not clear ice, full carb heat does; first RPM drop after applying heat is normal.
References
- TP 1102 (Flight Training Manual)
- TC AIM AIR 2.3
- PHAK ch.7