Every pilot knows—or certainly ought to know—that airframe icing represents one of flying's greatest hazards. Textbooks and the more thorough accident reports address these subjects frequently, but a seasonal reminder is always in order. Besides, researchers and accident reconstructions are always coming up with new insights into the icing problem. Their findings deserve to be widely disseminated, but for one reason or another many pilots aren't exposed to late-breaking information and safety advice. Let's review some icing basics, then look at some icing issues that have been in the news but somehow haven't become—but ought to be—knowledge resident in all pilots.
First, the basics. Airframe icing comes in six main types—clear, rime, mixed, large-droplet, freezing rain, and frost. Here are the characteristics and typical environments identified with each type:
How big are the water droplets that cause icing? This question is answered in terms of diameters measured in microns. A micron is one-thousandth of a millimeter. If you twirled a pencil point on a piece of paper, the resulting dot would have a diameter of about 500 microns. So when we talk about icing droplets, we're talking small.
Rime icing happens with cloud droplets in the 15- to 40-micron droplet size. Clear icing droplets are slightly larger—in the 15- to 50-micron range. Those are the numbers used in icing certification tests, and which define the icing environment in the eyes of the FAA. But the certification envelopes were developed back in the late 1940s. Now, it seems, nature doesn't always agree with the government. Studies have shown that most documented icing encounters—80-plus percent, whether clear or rime—involved droplets in the 10- to 15-micron range.
As for large-droplet icing, droplet sizes are in the area of 1,000 microns or more in diameter—as big as small raindrops. This helps further explain the runback problem.
The horizontal stabilizer is an airfoil, too. It just provides negative lift—lift that balances out the lift produced by the wings and gives the airplane pitch stability. In the past few years, there has been a good deal of study on tailplane icing. What experts have found is that the small radius of a tailplane's leading edge—like any small-radius object on an airplane, such as a temperature probe—is a very efficient ice collector.
When a tailplane ices up and stalls, negative lift is eliminated and the airplane pitches nose-down. This is now suspected as the cause of several previously unexplained airline accidents.
Adding to the tailplane icing problem is the fact that it's most likely to occur on final approach, when the airplane is being slowed and flaps are being extended. When flaps are extended there is a significant increase in the horizontal stabilizer/tailplane angle of attack, and wing downwash can further increase the tail's angle of attack, causing it to stall. Ice accretions on the tail leading edge cause this type of premature stall because they've reduced the tail's lifting force.
Pilots can tell if a tail stall is impending. Here are the warning signs, which may be masked if an autopilot is flying:
To recover from a tail stall, test pilots recommend doing the following:
The tough part about the recovery procedures is that they're the diametric opposite of what our training recommends for a conventional stall—that is, pulling on the control column instead of pushing.
We'll talk in more depth about strategies for dealing with in-flight icing next month, in a second installment of this story.
Links to additional information on icing can be found on AOPA Online ( www.aopa.org/pilot/links/links0010.shtml). E-mail the author at [email protected].