In the movie This Is Spinal Tap, bass player Derek Smalls (portrayed by Harry Shearer) tells the interviewer that his bandmates are “distinct types of visionaries … like fire and ice.” His own place was “in the middle of that, kind of like lukewarm water.”
Why bring this up now (aside from the fact that it’s fun, and we haven’t worked in a Spinal Tap quote for a while)? When pilots’ minds turn to seasonal weather hazards, most go straight to fire—in the form of thunderstorms—and icing. But while both are terrifying and potentially lethal, we’ve learned to do a fairly good job of avoiding them. In recent years, thunderstorms and icing accounted for only one to two dozen accidents combined. Going by the numbers, a far more prevalent threat is posed by, yes, lukewarm water—suspended as droplets in the atmosphere.
Most people who make it as far as the presolo written test know what “clouds” look like, so the continuing toll taken by attempts to fly VFR in IMC is bewildering as well as deadly. It accounts for twice as many accidents as thunderstorms and icing combined, year after year, and very few are survivable: Fatality consistently runs around 90 percent. And few of those pilots were cut off by unforecast weather that encircled them before they could react. More typically, they pressed on rather than turning around in the face of clearly deteriorating conditions—or, worse, they decided to leave the safety of the ground in order to blast off into a low overcast. What might they have been thinking—that it was probably just a thin layer they should have been able to climb through? How hard could that be?
Of course, in most of this country clouds are a year-round phenomenon. Fog, on the other hand, has both distinctly seasonal tendencies and some potential to take pilots by surprise. In the fall, the air cools more rapidly after the sun sets even as the nights get longer, giving fog more time to form. Large bodies of water retain their heat far longer than the air above them, making airports situated next to them susceptible to being fogged in one night after another. In fact, the peak months for accidents involving fog are October and December. (We’re not sure why November’s not as bad, though a let-up in GA traffic over the Thanksgiving holiday might play a part.)
Since autumn fog tends to form on clear, calm nights—which is to say after dark in otherwise perfect flying weather just when many of us start thinking about re-establishing our night currency—it does have a way of sneaking up on pilots who don’t expect it. That’s not to say that they shouldn’t; this time of year, checking the last few hours’ METAR observations before heading out to the hangar is a particularly good idea. If they show a temperature/dew-point spread that’s rapidly converging on zero, fog should be anticipated. Training flights should think about staying local, monitor any nearby AWOS or ASOS frequencies, and keep a close watch on the visibility. Of course, for students and instructors alike, the early sunsets and cool rather than frigid weather also make this a particularly inviting time to do those required night cross-countries. Just make sure your CFIs put some advance thought into where they could reliably divert if either their destination or home base gets socked in—and that they train their students to do the same.
As we’ve noted elsewhere, fog’s treachery proceeds from simple geometry. A layer just a few hundred feet thick is easy to see through from directly above but turns into a mile of milky nothingness on that three-degree descent path. Going around before visual references are lost preserves the chance to look for better conditions elsewhere. Otherwise, instrument pilots have scant seconds to transition to the gauges before they face the same consequences VFR pilots can expect when deprived of visual references: catastrophic loss of control or an equally catastrophic collision with some obstruction.
Thunderstorms create extreme turbulence that can tear an airplane apart in midair. Icing can render an unprotected airplane incapable of maintaining altitude, not to mention reshaping its airfoils in ways that changes stall behavior in unpredictable ways and even choking off an engine’s air supply—all in IMC. But sudden spatial disorientation below 500 feet agl? To quote from Derek’s bandmate Nigel (played by Christopher Guest), “This one goes to 11.”
David Jack Kenny is manager of safety analysis for the AOPA Air Safety Institute.