Carby Heat On: Summarised by Greg Connors from the CASA website
Carburettor ice should be the first consideration in the event of a power loss. With a fixed-pitch propeller a power loss is indicated by a reduction of engine speed. When there is a manifold pressure gauge installed, a reduction in manifold pressure would show up along with the engine speed reduction. If the
relative humidity is above 50 per cent and the ambient temperature is below 27°C, use carburettor heat immediately before take-off to clear the system of ice, which may have formed during taxi. Return the selector to “cold” before take-off.
In general, carburettor heat should not be used during taxi because air is unfiltered in the “alternate” or carburettor heat “on” position. Always take-off without carburettor heat unless extreme carburettor icing conditions are present and the use of carburettor heat is approved in the Pilot’s Operating Handbook.
Remain alert after take-off for indications of carburettor icing, especially when the relative humidity is above 50 per cent, or when visible moisture (cloud, rain, snow or fog) is present. If carburettor ice is suspected immediately apply full heat and keep it on. Do not adjust the throttle initially: throttle movement may kill the engine if heavy icing is present. Watch for a further power loss then an increase in power as the ice melts. Once heat has been applied, be patient. Do not expect immediate results as it will take time for the ice to be cleared from the induction system. In case carburettor ice persists after a period of several minutes of full heat, gradually move the throttle to the full open position and climb the aircraft at the maximum rate of climb to obtain the greatest amount of carburettor heat. Cautiously adjust the mixture to the leanest practicable setting.
Remember, carburettor icing can occur with ambient temperatures as high as 38°C and humidity as low as 50 per cent. Before a reduced-throttle operation, such as descent, apply full heat and leave on throughout the reduced throttle sequence unless the Pilot’s Operating Handbook indicates otherwise. Periodically open the throttle during extended reduced-power operation so that enough engine heat will be produced to prevent icing. Be prepared to remove carburettor heat if a go around is initiated. Return the control to the “cold” position immediately after landing to avoid ingestion of unfiltered air.