Winter’s coming (or here), and that means ice. If you were merrily motoring along and suddenly noticed ice forming on your wings, windshield, and elsewhere, aside from vacating that altitude pronto, you probably wouldn’t slow down (unless it involved a maximum rate climb to warmer air). Your pulse would quicken, and most likely so would your engine, at your behest. That’s probably a good idea — except for one thing…
SLOW IS BAD, RIGHT?
We all know what happens to a wing’s angle of attack when it moves at a higher speed. As Langewiesche wrote, describing an airplane’s “gaits“, when an airplane slows down, the angle of attack of the wing must become greater to provide the same amount of lift and support the aircraft at the same altitude. You may recall the admonition not to increase your pitch too much when flying through (and hopefully out of) icing conditions, lest you allow increased ice formation on the underside of your wings. The other good reason to maintain airspeed is that whenever an airfoil is contaminated by anything (in this case ice), stall speed will likely rise — maybe by a little, maybe by a lot. I can understand that, and I never had any argument with it, until I thought about another kind of (less visible) icing: tailplane icing.
UH, OKAY, SO … FAST IS BAD?
In the winter, as we know, the traditional remedy of keeping extra airspeed, for the reasons mentioned, sounds like a good idea. But let’s do a little gedanken, or a “thought experiment“. When you look at an aircraft from the side, what’s different when you compare the angle of incidence that a wing’s chord line makes, with that made by the horizontal stabilizer? Ah. The light bulb probably just went on for some of you, if it wasn’t on already. The angles of incidence are not the same, of course. (There’s even a term for it, called decalage.) The horizontal stabilizer, as you doubtless recall, is usually rigged at a negative “AOA” (angle of attack) and provides what we were told is a “tail down” force. Well now, aside from some seeming aerodynamic idiocy (it’s an airplane, why would you intentionally build anti-lifting forces into the design … but I digress), what does that imply?
ICE’S COFFIN CORNER
This: At higher airspeeds (or with your flaps deployed), the wing’s angle of attack is decreased, but the angle of attack of the tail (even though it may be negative) can actually be increased! We tend to think about icing on our pitot tube, propeller, windshield, and of course, wings — things we can often see from a warm cockpit. Of course, ice forms where we can’t see, too. One of those places, is the tail. Think about this: Since the tail generates a “reverse” or downward force, if it stalls because of an ice build-up, what will happen? The tail will rise, the nose will pitch down and, if you try to stop it, it will only get worse. Can you hear the horror movie music now … or are you already screaming too loud?
TALE OF THE TAPE
A number of accidents have been attributed to just this scenario: an airplane is on approach in icing conditions, initiates a missed approach, accelerates, pitches over, and crashes. In other cases, a violent stall is experienced immediately upon extending flaps. For many years, the cause was a mystery. Then at least 16 accidents (which involved turboprops) of mysterious origin resulted in 139 fatalities. The cluster provided an impetus for the first FAA/NASA tailplane icing workshop. You probably recall a term like “supercooled drizzle droplets” in reference to the American Eagle crash near Roselawn, Indiana in 1994. These types of icing encounters are the worst, occurring in cumulus clouds where there is freezing rain. The drops adhere quickly, and freeze as they run back from their initial impact points. As you know, small radius objects get clobbered first … a small radius not unlike the leading edges of most horizontal stabilizers.
TAILPLANE ICING DANGER ZONES
That airliner (an Aérospatiale ATR-72) was holding in icing conditions for well over a half hour, but it can happen in much less time, especially to smaller aircraft. In addition to acceleration and flap deployment, two other catalysts of catastrophic divergence are equally innocuous: frontal gusts and downdrafts. Either one will momentarily increase the tailplane’s angle of attack, bringing it closer to stall. But remember also:
- FLAPS: Most tailplane icing incidents occur during the approach phase of flight. Flaps are usually extended, and workload is already high (especially in IMC). The greater the flap deployment and the better they work, the worse it can be.
- AIRSPEED: As stated already, the higher airspeeds don’t usually help, due to the need for a greater compensating nose-down elevator force (which of course increases the AOA on the tailplane). Not only that, but ice builds faster when you go faster, blasting further past deiced areas.
- LOADING: Both aerodynamic and otherwise. Forward CGs and side-slips also increase the liability.
Important: Low wing airplanes usually pitch downward with flap deployment, increasing the (negative) AOA on the tail. (High-wing airplanes tend to do the opposite, which can lessen the danger, somewhat.) Airplanes with trimmable stabilizers (read that again: trimmable stabilizers, not elevators … Mooneys come to mind) are at an advantage here, as the entire tail can be trimmed to a lower AOA.
WHAT TO LOOK FOR: These symptoms can serve as a warning that tailplane ice exists:
- any pulsing or oscillation in the elevator or stabilizer
- abnormal nose-down pitch trim changes
- other pitch anomalies
- reduction in elevator effectiveness
- sudden elevator force changes
- uncommanded nose-down pitch
Warning: Autopilots can mask any and all of the above!
THE BOTTOM LINE: Adding airspeed may protect you from a stalled wing, but you can almost literally get bitten on the tail, if it gets iced. If you suspect tailplane icing, immediately retract the flaps to their previous setting, apply moderate nose-up force, try adding some power and airspeed to eliminate vibration, make any nose down changes slowly, exercise any active deicing gear frequently, and get out of the ice as soon as possible.