Wx Watch: Icing Escape Clauses

Preparing for the worst

January 1, 2008

Here’s hoping it never happens to you, but let’s look at some of the emergency measures you can take should you ever become caught in icing conditions. We’re assuming that you’ve made all the necessary preflight checks to prevent an icing encounter in the first place, received encouraging updates through Flight Watch or (if so equipped) your satellite weather provider, and made your “go” decision based on a conviction that icing conditions won’t be a factor.

But the weather changes. Forecasts sometimes don’t hold up. Fronts and cloud masses can move faster than predicted and become denser and wetter than expected—and certain regions of the nation are, by nature, more likely to turn icy when the altimeter setting drops and moist air flows over your route. The Pacific Northwest, Great Lakes, and New England states are especially susceptible to the most dangerous form of icing—that caused by supercooled large drop-lets. When these droplets strike an airplane, they flow back from the point of impact (called the impingement point) and refreeze farther aft along the chord of a wing or other surface.

For whatever reason, let’s say you’re picking up ice. It’s time to take immediate action, and here are some strong suggestions for a safe escape. First of all, make sure your pitot heat is on. (You did check it during the preflight, yes?) With carbureted airplanes, full carburetor heat application also is in order; this provides an alternate path for engine intake air, and this heated air will prevent any ice from building in the carburetor venturi. If ice is already in the venturi, expect a rough-running engine during the melting process, and don’t turn off the carburetor heat when the roughness begins. It can be disconcerting, but after a minute or so the roughness should subside. In fuel-injected airplanes, open the alternate engine air doors (your air filter may be covered in ice, which will soon starve the engine of air). Turning your defroster on may help prevent ice from forming on the windshield. Some claim this works, some don’t buy it; either way, it’s worth a try.

If you’re lucky enough to have deicing boots, propeller anti-icing, windshield ice protection, or any other ice protection gear, be prepared to use them. Propeller anti-icing—like other anti-icing equipment—should have been turned on prior to entering icing conditions. Deicing equipment is used to remove ice after it has accumulated.

After taking the above measures, make a call to air traffic control, explain your problem, and declare an emergency. Controllers can feed you helpful weather information and help direct you to the nearest suitable airport for an instrument approach or other method of performing a precautionary landing. This is one very good reason for filing an IFR flight plan—or obtaining VFR advisories (a.k.a. flight following) at the very least. ATC is just a mic click away. But the real work, of course, is up to you.

The main memory items are deceptively easy to call up: You want to maintain positive control of the airplane, get to above-freezing temperatures, avoid terrain, and avoid clouds or visible precipitation if freezing temperatures extend to the surface.

Ice causes thrust to drop off, lift to decrease, and weight to increase. If you’re taking on ice, then all of these factors translate into a loss of airspeed. If you maintain altitude, airspeed can drop to dangerous levels. With nonturbocharged airplanes not approved for flight into known icing conditions, the way to preserve vital airspeed is to descend. Climbing to cloud-free, on-top conditions may sound tempting, but many times this tactic will only compound your troubles if the airplane collects more ice.

Of course, descending can mean terrain issues. Hopefully, you have planned your flight so as to have low minimum en route altitudes (MEAs) and no clouds or freezing temperatures at the MEA. If this is not the case, you are in serious trouble and must land at the nearest airport having the longest and widest runway—preferably one with a tower and radar service. Unfortunately, being over mountainous terrain usually means that airports are few and far between. The airports that do exist are usually small, nontowered, and without instrument approaches. They also tend to be located in steep valleys or in the narrow floodplains of rivers surrounded by high terrain. This situation is the main reason why crossing mountains in wintertime is a really bad idea if you’re flying a low-powered airplane without known-ice certification unless it’s solid VMC.

Reversing course is often the best idea, assuming you can maintain altitude in ice-free air at the MEA. But be prepared to land. A 180-degree turn isn’t a guarantee that danger is past. The system causing the ice may be all around you by the time you make your turn.
Here are some other must-do items:

Power up and stay clean. Descending will keep your airspeed up, but so will adding power and keeping flaps and (in retractables) gear retracted. Don’t be shy about adding power. Firewall the throttle(s) if need be. You may even need to land with full power if you’ve accumulated enough ice.

Cycle the propeller. Running the propeller from low to high rpm can dislodge propeller ice accretions. A caution is in order: The ice may shed unevenly, causing damaging propeller imbalances.

Exercise the flight controls, gently. Ice can freeze ailerons, elevators, and rudders in one position. The antidote is to move the controls occasionally to keep them from locking in place. Don’t make abrupt movements, lest a too-slow airplane encounters gusts and/or shear that could cause a stall or a high sink rate.

Keep the autopilot OFF. This rule is a corollary to the previous one. Autopilots are great workload-saving devices, but they can spell trouble in icing conditions. Any ice-induced handling anomalies, such as unusual pitching and rolling moments, may go unnoticed when using the autopilot. These warnings of an impending stall or upset would normally be detected by a hand-flying pilot. The landmark October 31,1994, crash of an ATR-72 at Roselawn, Indiana, is an example of the dangers of using autoflight in icing conditions (see “Wx Watch: New From the Icing Front,” February 1998 Pilot). The ATR’s fatal roll began when the autopilot automatically kicked off line when it could no longer compensate for ice-induced rolling moments. If the pilots had been hand-flying, they would have noticed the control abnormalities.

Landing with a load of ice brings up some more critical memory items. Your airspeed should be higher than normal. It should be significantly higher if airspeed and sink rate are deteriorating. That means keeping up the power and delaying the deployment of flaps and landing gear. If you are flying in visual meteorological conditions, do everything you can to preserve your altitude, and be prepared for a no-flap landing at a much-higher-than-normal airspeed.

If you’re flying an instrument approach, by all means try to fly the depicted vertical profile as published, all the while using power as needed and—as in the VFR scenario, delaying gear and flap extension. The touchdown should be flat and fast. Save that nose-high, full-stall landing attitude for a sunnier day. This is the main reason for using the longest and widest runway you can find.

All this talk of flying fast approaches raises a complication if the airplane is equipped with a stabilator: ice-contaminated tailplane stalls. The greater your approach speed, the more nose-down control pressure you must exert to lower the wing’s angle of attack. But this nose-down force means the tail-plane’s compensatory angle of attack will be greater. A forward center of gravity, headwind gusts, downdrafts, and large flap extensions (in low-wing airplanes; high-wing airplanes tend to pitch up with flap extension) add to the problem by causing nose-down pitching moments of their own.

The result can be a tail stall. This means a rapid pitch-down as the negative lift exerted by the tailplane suddenly vanishes. At low altitude, this obviously can have fatal results. The problem is that tailplane stall recovery procedures are counter-intuitive—they’re exactly the reverse of those for a wing stall. To recover from a tailplane stall, you should follow these steps:

  • Apply nose-up elevator/stabilator force;
  • Retract flaps;
  • Apply power;
  • Make subsequent nose-down pitch changes slowly.

Tailplane stalls are a complex topic and have been the subject of numerous icing conferences (see “Wx Watch: Tailplane Stalls,” March 1994 Pilot). In the meantime, you can draw at least some reassurance that tailplane stalls are most likely to occur in large-droplet icing conditions. The potential presence of these conditions is identified on the icing page of the Aviation Digital Data Service’s Web site (http://adds.aviationweather.noaa.gov/icing/icing_nav.php). The red stippled areas mark the large-droplet trouble spots.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.