Icing in the summer

Mention the word “ice” to experienced aviators, however, and their overactive imaginations conjure up horror scenarios of majestic and catastrophic proportions. Most of their nightmare scenarios, however, happen in the winter. They picture flying through a freezing winter storm, with supercooled droplets clinging to wheel pants, leading edges, and propeller blades, disrupting the airflow and weighing them down.

It may sound counterintuitive but icing hazards—in particular carburetor icing—are quite common even in warm weather. For the unprepared, the consequences can be just as serious as a wintertime encounter with structural ice.

A common misconception among novice pilots is that icing only occurs when ambient temperatures are near or below freezing. That can lead to complacency during the warm summer months, and confusion when ice does form.

According to the FAA’s Pilot’s Handbook of Aeronautical Knowledge (PHAK), carburetor icing may happen during cruise power settings during conditions common on typical summer days across much of the country: at temperatures between 50 degrees Fahrenheit and 70 degrees Fahrenheit (but also as high as 100 degrees F) with a relative humidity as low as 50 percent. That’s because the icing risk in carbureted engines is less about outside air temperature and more about what happens inside the carburetor.

“Carburetor ice occurs due to the effect of fuel vaporization and the decrease in air pressure in the venturi, which causes a sharp temperature drop in the carburetor,” the PHAK tells us.

In humid conditions, this cooling can lead to rapid ice formation inside the carburetor throat, restricting airflow and decreasing engine power.

When the ice builds up, in a fixed-pitch propeller aircraft, the rpm drops and the engine runs rough. A variable pitch aircraft may indicate an issue by a decrease in manifold pressure. Propeller pitch is automatically adjusted to compensate for loss of power, and a constant rpm is maintained.

In extreme cases, carburetor ice may cause the engine to “cease to operate,” the PHAK says soberly.

The most insidious thing about carb ice is how quietly and unobtrusively it sneaks up on us. Unlike structural ice on the airframe, which builds and is visible within minutes, carburetor ice doesn’t offer any visual cues.

And lest we forget, that ice you see—on the tires, wing spars, or other leading edges—can also rear its ugly head in the summertime. When flying at high altitudes, for example in the mountains or during convective weather, the supercooled liquid droplets are hiding out in clouds where the outside air temperature is still well below freezing. They freeze on impact. Thunderstorms, towering cumulus clouds, and remnants of cold fronts can all produce these kinds of icing conditions at altitude.

The aerodynamic effects of structural icing are cumulative: thrust is reduced, drag increases, lift is reduced and weight increases.

It’s important to remember that any encounter with ice, in the summer or the winter, could be deadly.

So how do we mitigate the effects of summer ice? Be prepared and know your airplane.

If you’re flying a carbureted engine, engage the carb heat function every so often, especially when you are descending or in other low-power operations, even if you don’t think icing is an issue. Sometimes you don’t even notice the buildup, and prevention is the best cure.

Don’t ignore the signs of carb ice if they appear. A slow drop in rpm, engine roughness, or unexpected power loss are the first clues. Apply full carb heat immediately and be patient—it may take 30 seconds or more to melt the ice and get your engine back to full power.

Plan your flight carefully, especially your altitude selection. When flying in mountainous areas or convective environments, know the freezing level and where visible moisture might occur. Just because it’s summer on the calendar doesn’t mean there is no risk of structural ice at higher altitudes.

The ice in your engine or on your wings has nothing on the frozen cubes in your drink. Make sure you get back on the ground safely in order to be able to enjoy those, too.

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