Learn what carb heat does, when to use it, the signs of carb icing, the expected RPM drop and why full heat matters in carburetted aircraft.
Carburettor heat sends warmed air into a carburetted piston engine’s induction system to prevent or melt ice in the carburettor. Use it exactly as the aircraft POH or checklist directs—typically when carb icing is suspected and before low-power descent or landing—not routinely for take-off or prolonged ground running.
In real-world aviation, carb heat applies only to engines fitted with a carburettor. Fuel-injected piston aircraft generally use alternate air instead, while turbine engines have different anti-icing systems. Our guide to how piston-engine induction and controls work explains those distinctions.
How does carb heat work?
Carburettor ice forms because the pressure drop through the venturi and the vaporisation of fuel can cool the carburettor well below the outside air temperature. Ice then accumulates around the venturi or throttle plate, restricting airflow and progressively reducing power.
Most light-aircraft systems draw air warmed around an exhaust shroud when the CARB HEAT control is selected. Exhaust gas does not enter the engine; only heat is transferred. On many installations, this heated-air route bypasses the normal intake filter.
Hot air is less dense than cold air, so selecting carb heat normally reduces power and makes the mixture richer. A fixed-pitch propeller usually shows an RPM drop. With a constant-speed propeller, the governor may maintain RPM, making a fall in manifold pressure the clearer indication.
Carb ice does not require cloud, rain, visible moisture or a sub-zero outside temperature. Humid air and reduced throttle settings increase the risk, although ice can form at other power settings. See our explanation of the conditions that cause aircraft icing for the wider temperature-and-moisture context.
When should I use carb heat?
Use carb heat during the prescribed pre-flight engine check, whenever carb icing is suspected, and at the descent or landing points specified by the aircraft checklist.
- During the engine run-up: Select carb heat as directed and confirm the specified RPM or manifold-pressure change. Return it to cold before take-off unless the checklist says otherwise.
- After unexplained power loss: Apply heat promptly if RPM or manifold pressure falls without a corresponding control movement. Rough running and decreasing airspeed may follow, but a gradual power reduction is often the first sign.
- Before reducing power: Many carburetted aircraft require heat before closing the throttle for descent or approach. Applying it first uses the stronger exhaust heat available at the higher power setting.
- In sustained icing conditions: Use the POH procedure rather than applying heat periodically from habit. Requirements differ between engine and airframe installations.
The AFM/POH takes precedence over a rule learned on another aircraft. Some types require carb heat below a stated power setting; others call for it only when icing is suspected.
What should happen when carb heat is applied?
A normal system produces a predictable power reduction, while melting ice may initially make the engine run worse before it recovers.
| Situation | Expected indication |
|---|---|
| No ice, fixed-pitch propeller | An immediate RPM drop that then remains stable. |
| No ice, constant-speed propeller | Manifold pressure usually falls; RPM may remain at the selected value. |
| Ice is present | Power may drop further or the engine may run roughly, followed by an improvement as the ice melts. After heat is removed, RPM or manifold pressure should be higher than before. |
| No response during the prescribed check | The heat source, ducting or control may not be working, although some installations show only a small change. Compare it with the POH limits and resolve an abnormal check before flight. |
What should I do if I suspect carb icing?
- Apply full carb heat. Do not ease it in unless the POH specifically instructs you to use partial heat.
- Keep the heat selected. Melting ice and water passing through the engine can cause temporary roughness. Removing heat at the first stumble may leave the blockage in place.
- Watch for recovery. Look for increasing RPM on a fixed-pitch installation or increasing manifold pressure with a constant-speed propeller. Recovery may be incomplete while hot air is still selected because carb heat itself costs power.
- Follow the abnormal checklist if power does not return. Fuel starvation, ignition faults and other engine problems can resemble carb icing; carb heat will not correct them.
Should carb heat be full or partial?
Use full carb heat unless the aircraft handbook specifically authorises partial heat or provides a carburettor-temperature procedure. Partial heat can warm the induction air into an icing-prone range without supplying enough heat to clear existing ice.
Prolonged carb heat also enriches the effective mixture. At altitude, the POH may call for mixture adjustment to restore smooth running, but that is aircraft-specific and should not replace the immediate full-heat response to suspected ice.
When should carb heat remain off?
Carb heat is normally cold for take-off and go-around because heated air reduces maximum engine power. Some landing checklists leave it hot until power is reapplied, then require an immediate return to cold; follow the exact sequence for the aircraft.
Avoid unnecessary ground use because many carb-heat systems supply unfiltered air, increasing the chance of ingesting dust or debris. Ground operation should normally be limited to the functional check required by the checklist.
Is carb heat the same as pitot heat?
No. Carb heat protects the engine’s induction system, while pitot heat warms the air-pressure probe used by the airspeed system. Neither control removes ice from wings, propellers or windscreens; our separate explanation covers when pitot heat should be used.