For many subjected to North American wintery climes, the site of a surface of ice begs for blades, a puck, and a slapshot. And while winter and ice may inevitably be mutually associated, that may be so mostly on a pond. On an aircraft – and, in particular, in the throat of its carburetor – they’re not the regular bedfellows many mistakenly seem to think is the case. Here’s a quick refresher as to why.
Carburetor ice occurs under a range of temperatures not usually associated with winter. Therein lies the befuddlement. When is it most likely to occur? Worry not so much in January, but heighten your concerns in July.
Under moist atmospheric conditions, with air temperatures ranging from lows of -5C to highs of 30C, you’re susceptible to ice build-up in your induction system. This accumulation is caused by two processes: (1) a drop in temperature as heat is taken from the air channelling its way through the carburetor to effect fuel vapourization, and (2) the drop in temperature due to the low pressure area created within the carburetor.
To become combustible, fuel must be vapourized and mixed with air, a process that takes place in the carburetor. Heat is necessary to make this vapourization happen and this latent heat is absorbed from the passing air making its way through the carburetor into the manifold. Taking the heat out of this air results in a temperature drop of that air of upwards of 30C. Imagine, then, incoming air at 15C dropping to -15C as it does its business inside your carburetor.
The burst of speed of air sprinting through your carburetor also induces a static pressure drop prompting a cooling effect on the fuel-air mixture. This cooling process, in and of itself, forces a temperature drop in the area of 3C to 5C, adding to the overall temperature reduction taking place inside the carburetor.
Now, if that incoming air contains moisture – and, by no means, must it be visible – this cooling process will cause its constituent water vapour to condense, possibly freeze, and stick to anything it can get its little particles on. This is the cause of carburetor ice. A relative humidity of 50% is a spawning point for worry. With a higher percentage humidity factor, that icing hazard will be greater still.
Any amount of carburetor ice formation depends on the absolute humidity of the atmosphere. Normally, the higher the temperature, the greater the absolute humidity. However, serious icing generally peaks around the 15C mark with relative humidity exceeding 60%. Therefore, think of the threat of carburetor icing during conditions of poor atmospheric visibility, after a heavy rain, just after a morning fog, or in the vicinity below stratiform clouds.
Since colder air is denser air, at atmospheric temperatures below -5C, the threat of carburetor ice is diminished. Within any given cubic block of dense cold air, space for water vapour is reduced thus lessening any nefarious icing effects that can bring about the strangling of carburetor passageways, or the freezing of a throttle valve until it jams.
The misconception exists that carburetor ice is a wintertime problem yet, in actuality, rarely is it so. Your carb is safer when the air is dry; the wetter the air, use caution as you fly.