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Frost-free refrigerationFrost-free refrigeration

Mention “contaminated surfaces,” and the surface that springs to most pilots’ minds is the runway. There’s good reason for this. Every year, a few dozen aircraft come to grief sliding off slippery pavement—chiefly, of course, during takeoff and landing attempts, though a few hapless pilots discover they can’t stop or turn while they’re still on a ramp or taxiway. Overruns and sideways excursions are about equally common during landings. Takeoff accidents tend to run more to lateral deviations, though overruns can and do proceed from late decisions to abort takeoff runs impeded by unplowed snow.

That said, the surface whose contamination is of greatest concern is that of the aircraft itself. About one-quarter of all accidents ascribed to airframe icing are precipitated by moisture that was already adhering to the skin before the pilot climbed into the cockpit. We refer, of course, to frost or snow that should have been completely removed during preflight.

Complete removal is the key, and stopping short of completion is another of those mistakes that’s not restricted to novices. In 2003, for example, a 1,200-hour CFI reported spending an hour and a half cleaning snow off the leading edges of a Cessna 172’s wings but didn’t clear the entire upper surface, apparently in the hope that the rest would blow off during the takeoff run. It didn’t. A designated pilot examiner saw her taxi past with both wings “nearly covered with four-inch-tall chunks of snow and ice,” lift off late, and struggle to an altitude of maybe 200 feet before coming down in a field while trying to turn back to the airport. To her credit, the pilot, who wasn’t hurt, “voluntarily sought and received training regarding frost, icing, and snow and its effects on aircraft performance” afterward.

That degree of initiative is well advised, since skin contamination has brought down airplanes much more capable than a Skyhawk. In 2007, a 14,000-hour corporate pilot had the company King Air pulled out of its heated hangar before his passengers arrived. It sat outside in light to moderate snow for a good 45 minutes before everyone was ready to go. That was plenty of time for the warm skin to melt the snow and the cold air to freeze it again. Immediately after takeoff, the airplane began a series of uncommanded rolls to either side, accompanied by shuddering each time the pilot tried to raise the nose. He managed to maneuver it onto a low downwind leg in an attempt to return to the runway, but was unable to prevent it from descending into a hangar in the turn to base. Only the two rear-seat passengers survived. Just more than a year later, an almost identical sequence of events led to the destruction of a Pilatus PC-12. Witnesses saw that “probably a good inch of wet slushy snow” had accumulated on its wings, but the pilot declined to have it de-iced.

Snow is fairly easy to see, even on a white airplane, but frost presents a subtler hazard. The same month as the King Air accident, the pilot of a Cessna Caravan freighter tried to clear frost from its wings and tail with a broom. He believed they were entirely clean, but found out the hard way that he was wrong; here, too, uncommanded rolls preceded the eventual off-airport descent. At least he tried. The pilots of other high-wing airplanes, including Cessnas, Maules, and Super Cubs, professed to have no idea that frost had accumulated on their horizontal surfaces until after they found that their machines wouldn’t fly. One could be forgiven for wondering how hard they’d looked.

To be fair, high-wing designs present particular challenges not only for inspection but also for actually removing frost. Much of the wing area can’t be reached without a stepladder. If that’s not available and the sun is obscured (or temperatures are too cold for the frost to melt), de-icing fluid or time inside a heated hangar are about the only options. The former is messy, and both can be expensive. But of course trying to take off with the frost still in place can prove considerably messier and a great deal more expensive.

It’s also worth bearing in mind that water and temperature can interact in unexpected ways. After a TBM 700 took off in a snowstorm only to be forced down a half-mile beyond the threshold, rough granular ice was found over both fuel tanks but nowhere else. The airplane had been kept in a cold hangar, but the truck that had delivered 177 gallons of fuel had been parked in a hangar with heat. The pilot saw snow blowing off the wings and never noticed patches of ice large enough to render it unable to climb.

ASI Staff

David Jack Kenny

David Jack Kenny is a freelance aviation writer.

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