Understanding Carburetor Icing
Carburetor icing happens when the air-fuel mixture's temperature drops below freezing inside the carburetor, causing moisture to freeze on the internal surfaces. This cooling is primarily caused by the Venturi effect and fuel vaporization. As ice builds up, it restricts airflow, leading to power loss or engine failure.
Conditions for Icing
Carburetor ice can form in ambient temperatures between approximately -7°C and 21°C (20°F and 70°F) with high humidity, above 60%. Monitor the outside air temperature (OAT) and dewpoint, as a small spread indicates high humidity and increased risk. Clouds, rain, or high moisture in the air can trigger it. Very cold, dry air is less likely to cause icing due to low moisture content.
Phases of Flight Vulnerable to Carb Ice
Carb ice is more likely during operations at reduced power settings, where manifold pressure is low and the temperature drop in the carburetor is most significant.
High-Risk Flight Phases:
- Descents and Glides: Throttle reduction maximizes the venturi effect and lowers engine heat.
- Taxi and Idle: Low power settings and RPM can lead to ice formation, especially in moist conditions.
- Cruise Flight: Sustained flight at low-to-moderate power can also create favorable icing conditions.
Recognizing Carb Ice Symptoms
Symptoms vary with propeller type:
- Fixed-Pitch Propeller: A gradual, unexplained loss of engine RPM is a common sign.
- Constant-Speed Propeller: A decrease in manifold pressure is typically the first symptom.
- All Carbureted Engines: Engine roughness, vibration, or a stuck throttle can also indicate carb ice.
Proper Use of Carb Heat
If carb ice is suspected, apply full carburetor heat immediately.
Applying Carb Heat - Step-by-Step:
- Pull Carb Heat Control: Apply full heat by pulling the control knob out completely. Partial heat is generally not advised unless using a carb air temperature (CAT) gauge.
- Observe Engine Response: Engine roughness may increase initially, and RPM may drop. This is normal and indicates melting ice.
- Wait for Recovery: Keep full heat on until the engine runs smoothly and RPM or manifold pressure stabilizes. This can take up to several minutes.
- Return to Cold Air: Once stable, push the carb heat knob back in.
Carb Heat vs. Alternate Air
Carb heat addresses carburetor icing, while alternate air is for bypassing a blocked air filter in fuel-injected engines or some carbureted aircraft. Carb heat uses unfiltered, heated air from the exhaust manifold, while alternate air uses unfiltered ambient air from the cowling.
| Feature | Carburetor Heat | Alternate Air |
|---|---|---|
| Function | Prevents and removes ice from inside the carburetor. | Bypasses the main air filter if it becomes blocked. |
| Engine Type | Carbureted engines. | Fuel-injected engines and some carbureted aircraft. |
| Air Source | Unfiltered, heated air from a shroud around the exhaust manifold. | Unfiltered ambient air from inside the engine cowling. |
| Timing | Applied based on conditions, engine power, or suspected ice. | Used if a blockage is suspected or confirmed. |
Conclusion
Understanding when and how to use carb heat is vital for pilots of carbureted aircraft. By recognizing favorable conditions, identifying symptoms, and following correct procedures, pilots can prevent engine failure due to carburetor icing. Always refer to your aircraft's Pilot's Operating Handbook (POH). Preventative use during descents and low-power operations is often recommended. Staying vigilant and acting promptly ensures safer flights. For further information, consult resources like the FAA's Pilot's Handbook of Aeronautical Knowledge.
Preventing Carburetor Icing: A Checklist
- Monitor Weather: Check OAT and dewpoint for high humidity conditions.
- Use Preventative Heat: Apply carb heat during prolonged descents and low power settings.
- Check Regularly: For aircraft without a temperature gauge, perform periodic checks of the carb heat system.
- Avoid Partial Application: Use full carb heat unless the POH or CAT gauge dictates otherwise.
- Keep RPM High on Approach: Avoid extended idle on final approach.
- Don't Ignore Roughness: Act immediately upon noticing engine roughness or power loss.
- Follow POH: Always consult your aircraft's specific Pilot's Operating Handbook.
