Wx Watch: Escaping the Frozen Zone

January 1, 2005

Let's say the worst has happened: You're picking up ice. In spite of following a careful preflight weather briefing checklist, in spite of checking all those TAFs, pireps, airmets, and everything pertinent on the many aviation weather Web sites, and despite all these sources indicating an ice-free trip, a layer of ice is beginning to adhere to your airplane. If you've been observant you noticed that the ice formed first on the airplane's small-radius objects — things such as rivet heads, or the outside air temperature (OAT) probe, or the pitot tube (assuming you didn't turn the pitot heat on — which you must do before flying in or near potential icing situations). But your attention has been glued to the instruments, and you missed these early clues. And you haven't looked at the OAT gauge — perhaps the most neglected instrument in the cockpit — for indications in the icing range, zero to minus 20 degrees Celsius. Now your airspeed is dropping off, the windshield is opaque with ice, and all the power you've got isn't enough to tempt you to roll in some nose-up trim in hopes of maintaining altitude. What to do?

The answer to that question hinges on several factors. One is the type of airplane you're flying. Got a huge twin with ice-protection equipment and certification for flight into known icing? That's better than a Cessna Skyhawk with a mere heated pitot tube and alternate engine air as weapons in an ice war, although even pilots of the best-equipped airplanes must still give ice the respect it deserves. Flying a turboprop or a jet? You've probably got gobs of extra power — plus ice protection — to help you better cope with ice. That's still no guarantee of a trouble-free exit from icing, but at least you have more options.

For the purposes of this article, we'll assume you're flying a piston single, one with a bare minimum of ice-protection equipment. Here are some strategies to think about should you encounter icing:

Take action. Don't just sit there and fly on as though nothing has happened. Even so-called "light" rates of ice accumulation will turn into severe ice buildups if you mill around in icing conditions long enough. Now is the time to act quickly — though not impulsively — to extract yourself from the situation. But what will your plan of action be?

Climb. Think hard before trying this option. Do you know how high the cloud tops are? If you climb, will temperatures rise above freezing? A good preflight weather briefing can give you a fair idea of the answers to these questions, but like other aviation hazards icing can be highly localized, and micro-scale in coverage. It's one thing if you can see blue sky through breaks in the clouds above. It's another to enter a climb on instruments into unknown conditions, where cloud tops may extend into the flight levels. You often hear the advice to climb into warmer air aloft if freezing rain is encountered. The theory is that you can penetrate a warm frontal surface aloft, where the (non) freezing rain originated. It may be only 2,000 feet above your original altitude, but you'll be picking up loads of clear ice on the way up — and drastically altering your airfoils as ice accretes on the underside of your wings and tail. There's no guarantee that you'll have enough performance to reach those warmer temperatures, so taking the climb-in-freezing-rain advice may not be a good idea. You could end up with a buildup of clear ice, no climb performance at all, airspeeds and angles of attack near the stall (remember, stall speeds rise dramatically as ice builds), and nowhere to go but down — back through the icing layer you tried to leave behind.

Turn around. A 180-degree turn is probably a good rule of thumb — assuming you came from ice-free conditions and made your decision quickly. The longer you fly on in icing conditions, the longer you'll have to fly in them on the return trip to better conditions. This is another reason to quickly develop an icing escape plan, and quickly carry it out. However, there is a problem with any weather avoidance strategy based on the 180-degree turn: It's based on the assumption that the weather hasn't deteriorated behind you. And we all know that weather can go downhill in short order — particularly near fronts and over high terrain.

Descend. Again, a potentially risky step. Are cloud bases higher than the minimum en route or minimum safe altitude? If so, descending below the clouds may work. Do you know that you'll be descending into above-freezing temperatures? Let's hope so, because you want to lose ice as you descend, not collect more. This would leave you with minimal performance and control at dangerously low altitudes.

Land. This marks the end of a successful escape from icing, but complications arise when the airplane still carries ice accretions. Ideally, you have shed ice during the descent for landing. If you haven't, you're faced with maneuvering an airplane with degraded aerodynamic characteristics, an increased — and unknown — stall speed, and reduced lift. In short, you become a test pilot, making your first landing in a dangerous airplane. Here are the ironclad rules for landing an iced-up airplane:

  • Find the nearest airport served by air traffic control radar services. The longer and wider the runway(s), the better. Instrument-rated pilots should seek out runways served by instrument landing system (ILS) approaches if ceilings and visibilities are too low to permit visual approaches. Declare an emergency and ask for vectors to the final approach path or course.
  • Keep your airspeed up. You don't know your stall speed, and as you approach the runway you certainly don't want to find out.
  • Keep your power up. Use it to maintain altitude and airspeed as best you can.
  • Keep your flaps up. Extending the flaps increases the horizontal stabilizer's, or stabilator's, angle of attack. This can set you up for a tailplane stall, which causes the airplane's nose to suddenly pitch down.
  • Fly gently. No more than 15 degrees of bank in turns, and no sudden changes in pitch.
  • Land with power. Don't reduce power until the wheels are on the runway. Any power reductions prior to touchdown can cause the airplane to stall, causing a hard landing — or worse.

Escape strategies can carry big downsides if you make the wrong choices. Depending on the situation, it can look like no options are good ones. That's why it's so important to minimize risk before you even take off. We've said it before and we'll say it again — you want weather conditions that will let you climb ice-free to on-top cruise altitudes, then descend to warmer temperatures as the flight reaches the approach and landing phases. You also want to avoid flying at night or over mountainous regions. That's because cloud avoidance is much more difficult, and airports are less plentiful in high terrain. Those that do exist are often of little use. Most airports in mountainous areas have short, narrow runways, little in the way of radar services, and are tucked near river valleys, where pockets of fog and low-lying clouds like to form.

No doubt about it, when you try to fight ice with a typical piston single, you fight with a small stick. That's why there's so much emphasis on avoidance — even if it means staying on the ground.


E-mail the author at tom.horne@aopa.org.


Wx Briefs

CIP and FIP, revisited

In " Wx Watch: Icing on the Web" (see November 2004 Pilot), we discussed two Web sites dealing with current and forecast icing potentials for various altitudes and time frames. These were the Current Icing Potential (CIP) and Forecast Icing Potential (FIP) sites ( http://adds.aviationweather.noaa.gov/icing/), located within the National Weather Service's Aviation Digital Data Service (ADDS) domain. No sooner had the article hit the streets than officials in the FAA's Flight Standards office scolded us for advising pilot use of the CIP and FIP for operational decisions. The CIP and FIP are operational as they appear on the ADDS site, but intended for use only by dispatchers and meteorologists. They are experimental products that can be useful for pilots on another, experimental ADDS site ( http://weather.aero). Perhaps any cautionary announcements should be displayed more prominently on these Web sites in the future. In the meantime, we should all adhere to FAR Part 91.103, which states that pilots, "before beginning a flight, become familiar with all available information concerning that flight."

Chesapeake Aviation Training weather seminars

Anyone seriously interested in more in-depth knowledge about aviation weather — as well as pure meteorology — should check out the seminars offered by Chesapeake Aviation Training. The firm, headed up by CFII and meteorologist Scott Dennstaedt, emphasizes numerical weather prediction and computer-model interpretation, and offers a seminar each month on several main topics at various locations. Courses address forecasting, icing, thunderstorms, radar, thermodynamics, and decision making. Two-day courses run $395; one-day courses are $180. For more information, and to sign up, visit the Web site ( http://chesavtraining.com).