Even at 32°C your carburetor could well freeze. Do you understand under what conditions your carburetor can freeze? You should because your carburetor icing could well happen the next time you take off or land! Each year, several pilots have been surprised.
Most carburetor icing accidents are caused by the formation of ice during normal cruising. It is possible that this fact comes from a relaxation of the vigilance of the pilot who thinks less about icing at high engine speeds than during a descent at reduced power.
In most accidents involving carburettor icing, the pilot has not fully understood the mechanism of ice formation and what happens when he turns on the carburetor heat. Moreover, it is difficult for him to understand the corrective measures if he does not know the process of carburettor icing. A description of this process can be found in most good aviation reference books and any mechanic working on the type of aircraft can explain the carburetor heat system. The explanations of the mechanic are particularly useful because of the differences between the various devices.
We know that METARs give you the temperature and the dew point.
MONTREAL/ST-HUBERT/QC
METAR CYHU 281400Z AUTO 22010G17KT 9SM FEW220 26/19 A2294 RMK SLP141 DENSITY ALT 1200FT=
For instance. This METAR for St-Hubert gives a temperature of 26 and a dew point of 19°C. If you refer to the graph, you can see that we are in the zone of "Moderate icing in cruise and intense icing at reduced throttle, therefore in descent". Hot and humid days are coming and we can expect even more intense icing conditions.
The carburettor is also likely to ice on the ground before takeoff. If you are idling on the ground for a long time or are waiting, icing can form in your carburetor. And even light icing could well be fatal on takeoff. Several pilots saw their engine suffocate during takeoff.
The carburetor heater (Carbheat) is an anti-icer, not a de-icer. It must therefore be used before the frost appears or at the very beginning. Its use even also to a loss of power. The carburettor heater should never be used during takeoff. If you are in conditions conducive to icing you can use it in prevention, 30 seconds to one minute before reducing throttle. And do not forget; use the carburettor heater fully or not at all. Using it in between would only make the situation worse. But always follow the directions in your manual and if in doubt, ask an instructor.
Good flight.
Contributor: Gilles Jean, Transport Canada (TP 2228E-38)
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