Okay, this is probably not the place to get all poetic about the ways things change with the seasons. In most of the country, the seasonal effect of greatest concern to flight schools and FBOs is probably the drop in traffic once the weather starts getting cold and nasty—not to mention all the teaching time lost whenever icy clouds push the weather below VFR minimums or taxiways are left treacherous by snow or freezing rain. There are limits to how many times students can be persuaded to trek out to the airport just to take a ground lesson.
Of course, the winter slump extends to most light general aviation, not just flight instruction. This shows up clearly in the accident data: There were almost 13,000 fixed-wing accidents on non-commercial flights between 2001 and 2010, and more than one-third of them were in the months of June, July, and August. Less than half as many took place in December, January, and February, which accounted for just one-sixth. The numbers in spring and fall were almost equal, each making up about a quarter of the total, though there were slightly more in spring (an additional 249 over 10 years).
So far, no surprises: more people crash when more people fly. (Helicopter accidents follow almost exactly the same pattern.) Some of the details aren’t surprising, either. Many of the risks you wouldn’t expect to vary do not; fuel mismanagement, for example, causes about 8 percent of all accidents regardless of season. Other hazards show predictable variation. Poor airmanship (or poor planning) during takeoffs and go-arounds cause 10 percent of winter accidents but 17 percent of those in the summer, with spring and summer right between at 14 percent apiece. Density altitude, anyone? While icing can occur any time, it’s more common when the air’s cold right down to the ground; 42 percent of icing accidents were in the winter, just 14 percent in the summer. For thunderstorms, the situation’s almost exactly the reverse: 44 percent in summer, 13 percent in winter.
Other findings are a little less intuitive. The differences are small, but landing accidents—the most prevalent category year-round—actually peak in the spring, where they make up 34 percent of all accidents, decrease over the summer, and bottom out in fall and winter, dropping to 28 percent. You might be tempted to guess that some pilots who sit out the winter months underestimate how rusty they’ve gotten by the time the weather gets warm enough to tempt them back into the cockpit. We’d have a hard time proving you wrong. Gusty spring winds probably don’t help, either.
Cold starts may accelerate engine wear, but there’s not much evidence that they increase risk in the short term. The share of accidents caused by either confirmed mechanical failures or unexplained losses of engine power shows almost no change over the course of the year. Of course, it could be that more burst oil lines in North Dakota in January are balanced out by an increased number of brake fires in Florida in July, but this turns out not to be the case: The specific types of mechanical failures that led to accidents don’t change much, either. Fluctuations in the relative frequencies of powerplant failures, gear and brake malfunctions, and fuel-system discrepancies are small and apparently random.
Weather accidents, on the other hand, are twice as prevalent in the winter as in the summer, and the increased risk of icing is a fairly small component of that. Even without ice, instrument flying seems to be trickier in the winter. Only half as many accidents of all types occurred in winter as in summer, but winters saw two and a half times more accidents blamed on deficient instrument flying by appropriately rated pilots on active IFR flight plans. Moreover, in winter about two-thirds of IFR accidents occurred during approach procedures; in summer, it was just about fifty-fifty.
Accidents in IMC are fatal more often than not, so this is one factor in another notable difference: accidents in the winter are the most likely to end in death. A quarter of all winter accidents were fatal compared to 21 percent of those in the fall and 18 percent of accidents in spring and summer. Beyond seasonal changes in accident causes, darkness and poor visibility also come into play. Accidents in VMC during daylight hours are the most survivable, and 90 percent of summer accidents are in daytime VMC. By winter, it drops to 75 percent. But that doesn’t tell the whole story, either: winter weather reduces the chances that those who survive the initial impact will last long enough to be rescued. As one example, all 30 of the winter accidents blamed on pilot incapacitation were fatal. A quarter of those in the other three seasons were survived.
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