Beware Carby Ice!
Ice, Ice Baby… no, not the 1989 rap song, ice is a danger for aircraft in general. However, this time we are talking about carb icing.
With winter now well and truely upon us, carby icing is more prevalent. In particular the cold temperatures mean the air can be MUCH more humid than it feels. Put this cold, moist air through a venturi, add some atomised fuel and the temperature easily drops below not only the dew point of the air but zero and ice is formed!
Unlike airframe icing which is much easier to detect (and predict), carby ice forms out of sight. In addition the temperature drops from the effects of the fuel and venturi are not normally displayed to the pilot. Generally carb ice forms relatively slowly, insidiously making you sub-consciously increase throttle to compensate. Until you suddenly realise the throttle is WELL advanced for the power output being delivered (think 1/2 throttle but only 1500RPM being output) or the engine starts to display symptoms (missing, rough running, etc) or worse, shuts down completely!
The solution is simple, but it DOES require time (and thus altitude) to work… FULL CARBURETTOR HEAT IMMEDIATELY! Be warned however, it will appear you’ve just made things worse. Because you have just directed warmer air over ice, which is now becoming water. So the engine is going to sound and act temporarily MUCH worse as it digests all this water and ice you are putting into the induction system. But in short order you should notice a marked power increase.
What makes it worse? Generally a higher humidity (remember, if it’s cold it can easily be 80+% humidity without noticing) and lower power settings. Don’t forget to select the carb heat when recommended by the flight manual and also ANY time you suspect you may have built up ice.
Example 1: Pilot was taxing out at a regional airport very early one morning. After arriving at the airport at first light and spending some (very cold) time removing the frost from the aircraft they completed their runups and entered for the lengthy backtrack to the end of the departure runway. During the taxy up the slight incline of the runway in this direction the pilot noticed they were increasing the throttle slowly to maintain the RPM needed for taxi. Luckily on a hunch, they selected full carb heat and were treated to a slightly surprising rapid increase in RPM as the ice was removed. They then kept the carb heat selected and only closed it when they selected full power and released the brakes for take-off.
Example 2: Pilot was conducting circuits at an airport, conscious of the cold temperature they made judicious use of the carb heat on the taxi out (remember at low power settings the ice is more likely). They selected carb heat on at the normal point in the circuit just prior to reducing power for base, returning to full cold at about 300ft AGL on final for their planned touch and go. The touchdown was normal, however when the power was increased to commence takeoff again the engine failed to respond. Luckily the pilot was able to reject the takeoff and clear the runway. Because the ice had quickly melted in the hot engine environment without the cooling effects of the fuel and venturi, the engine was able to be restarted and the aircraft returned to the apron.
In both these examples the pilots were able to recover from the effects of carb ice they experienced. However others have not been so lucky. So please be aware of when carb ice is likely (using the above chart) and use that heat control when recommended in the aircraft flight manual (never hurts to have a refresher on what the manual actually says!) and whenever you suspect you may have built up ice.
The only way to truely beat carburettor icing is to fly a fuel injected aircraft… but that comes with its own unique problems. Just ask any Cirrus pilot who has tried to start a hot engine!