Features  / 11.12 /

Icy Reception

Don’t crash this party


Is it your imagination, or does this aircraft feel strange?

The queasy thought upends the tranquility of an easy cruise in instrument conditions.

What’s going on? The aircraft feels almost like it does when you practice slow flight—not quite mushy, but a little unresponsive, and perceptibly more draggy than before. If this were a complex aircraft, not a fixed-gear Cessna Skylane, you’d be tempted to look outside and see if you had forgotten to raise the landing gear based on how it’s performing.

So you look outside anyway—and observe a white, crusty froth that is beginning to coat the front of the gear legs and (probably) everything else that forms a leading edge as your aircraft plows ahead through the clouds. It’s rime ice, looking exactly like photos you’ve seen of it. There isn’t much yet, but it is still accreting.

A glance at the instrument panel shows an absurdly low airspeed for your cruise power setting. At this combination of manifold pressure and rpm your cruise speed should be about 130 knots indicated, not slightly less than 120.

iceMemories of hangar-flying sessions invade your head. “Flying an iced-up airplane makes you a test pilot,” someone said. You remember spirited debates about deice devices versus anti-ice devices, and about an aircraft certification standard referred to as approval to fly in known icing conditions that is conferred on some aircraft. (Not your Skylane.) You recall ground school review of the various types of ice: clear ice, rime ice (which you are accumulating in these stratus clouds), mixed icing, and the worst ice of all—freezing rain (see “Icy Options, below”).

What are your escape options? That’s the only question that matters.

If you are flying at the minimum en route altitude, descent isn’t necessarily out of the question. If a lower altitude will get you above-freezing temps, ask ATC for help. The minimum vectoring altitude is the absolute floor, and it may be considerably lower than the MEA. Your situational awareness also plays an important role in finding escape. A short time ago you heard a pilot report cloud tops at 7,500 feet. You could request a climb from 6,000 feet to 8,000. But is a climb a good idea—or even possible—with the airplane already starting to feel like a lead sled? The request could be denied or delayed for traffic. You might have to raise the stakes by announcing your situation as urgent, or worse, although that’s something you may end up doing soon regardless.

What about diverting? That could mean flying numerous vectors in icing conditions, followed by an approach to an unfamiliar airport (assuming you brought the proper charts along and not just the ones you expected to use at the destination—a mistake some pilots make). As with the preceding option, the question of how much time you would spend in the ice is the major factor. But it’s a strong candidate.

You could turn around—but that would mean a new route clearance, perhaps an altitude change, and at least as much time in the clouds as you have already flown.

You could plow ahead and hope.

Those are the choices. Right now you have to do two things: evaluate them on the quick, while immediately doing everything you can to offset the ice accumulation.

For starters, turn on the pitot heat; knowing that you can trust your airspeed indication is essential. Speed up the prop; increasing rpm might shake loose some ice and retard further accumulation. It may also smooth out the ride, relieving more stress. Check for carb ice. Note the time and update your mental math about fuel remaining.

Concerning those escape options, you can count on this much: None of them will be the perfect answer. Only breaking out unexpectedly into clear air would be a jackpot of that sort. Then the next course of action, as the Aeronautical Information Manual recommends, would be to maintain VFR conditions and land as soon as possible. Otherwise, you are going to have to choose between options that all require some continued exposure to the icing.

It’s time to let ATC know what’s going on. They might be able to provide a vector to a nearby approach, or a call-out to nearby aircraft for a pirep on their flight conditions. Wouldn’t it be nice to hear someone say they’re flying in the clear nearby? Be ready to choose your plan, because once ATC provides you with information, they will ask for your intentions. (They will also stay with you until you are safe.)

Avoidance starts at home. What the scenario painted above illustrates is that whenever an instrument flight is likely to put you in the clouds at or near freezing temperatures, an avoidance plan is called for before you leave the ground. (This is assuming you don’t decide to stay on the ground instead, which is always the prudent course with instrument meteorological conditions near or below freezing temperatures.)

For cold-weather IFR flight in an unprotected airplane, the choices boil down to selecting altitudes where you can be assured of well-above-freezing temperatures when flying in IMC, or knowing that you can fly in the clear at levels where icing conditions lurk.

In extremely cold air, the chances of airframe icing diminish. That’s because those supercooled water droplets you had never heard of until you began to study aviation weather—droplets that exist as liquid until they strike a surface such as a moving aircraft and freeze solid—are not a hazard at extremely low temperatures.

One difficulty of assessing the possibility of an ice encounter is that measuring the risk involves more than studying the weather forecast. “Mountains, bodies of water, wind, temperature, moisture, and atmospheric pressure all play ever-changing roles in weather-making,” says the AOPA Air Safety Institute’s Aircraft Icing Safety Advisor. That explains why some regions (above or downwind of the Great Lakes, or above the Appalachian ridges) can be notorious for icing potential, while a colder region lacks the deadly reputation. It may also explain why winter-flying veterans of your area have different expectations about ice potential than those you read in an official forecast.

iceOnce ice begins to accrete, two characteristics should be noted: the type of ice and the rate of formation. For reporting purposes, there are four rates of accumulation. Three of the four are bad news. Only the barely perceptible trace ice implies an accretion rate barely in excess of the rate at which it is being removed (or sublimated) by the airflow. Light accumulation, despite the benign-sounding term, is problematic. Moderate and severe icing—even more so.

Ice, spoiler of lift. As a pilot you learned early in training about the urgent need to remove frost from your airplane before flight, so you know that it is not the weight of airframe ice that robs an airfoil of its ability to fly. What spoils aerodynamic effectiveness is the ice’s rough, uneven character, disrupting adherence of airflow to wing and tail surfaces, swapping lift for drag. Icing on the leading edge of a horizontal stabilizer also presents a new issue for the icing-uninitiated: a possible tail stall, which can pack its worst punch when flaps are deployed. A clue that a tail stall is imminent is buffeting felt through the control column, as opposed to a wing stall buffet that you sense through the airframe itself.

Don’t relax after escaping. You still have to land an iced-up aircraft, so add extra airspeed to the approach.

How much extra? You’re the test pilot, so you decide—but be generous. A February 2011 hard landing of a deice-protected twin-engine Cessna 310Q in Romeoville, Illinois, serves to illustrate the multiple problems of predicting ice, assessing its rate of accumulation, and getting rid of it: “The pilot reported that she checked the weather prior to departing on the cross-country flight and it appeared that an area of icing had cleared out of the area near the destination airport,” said a National Transportation Safety Board accident summary. “While descending through 4,000 feet near the destination, the airplane began to pick up ice quickly. She advised the air traffic controller and cycled the deice boots several times. She reported that she was unable to determine how much ice was accumulating on the airplane and she made the landing approach using normal approach speeds. The airplane touched down hard, resulting in substantial damage to the internal structure of the left wing. She reported that after the landing ice was visible along the leading edge of the wings. She stated that she should have maintained a higher airspeed during the approach and landing to prevent the accident.”

The NTSB assigned the pilot’s failure to maintain adequate airspeed during the approach and landing as a probable accident cause.

One more tip about keeping icing from doing its worst damage to your aircraft is to move your control surfaces often enough to avoid or detect any freeze-ups. Don’t forget the trim tab.

Use your knowledge of aviation weather to avoid ice. Failing that, execute your escape at the first sign, thereby shortening the time you will have to fly an aircraft with unknown—and changing—aerodynamic properties.

Dan Namowitz is an aviation writer and flight instructor. He has been a pilot since 1985 and an instructor since 1990.

Icy options

There are a number of different types of icing. Here are four common varieties:

Clear ice, sometimes called glaze, looks most like the icicles you sucked on as a kid. It has a generally smooth texture with small air pockets that result in a bumpy surface. With high levels of accretion, it can form “horns” that stick up from leading edges.

Rime ice is rough, milky, and forms rapidly from supercooled droplets. This type generally forms to the shape of the wing leading edge, which makes identification easier.

Mixed ice is, as the name implies, a combination of rime and clear ice. It has the color and characteristics of both.

Frost settles on the airplane while it’s on the ground. It’s not unlike what you’ll find on your car or other metal structure. Don’t take off with any of it on the airplane because it can reduce lift by up to 30 percent.

Getting rid of ice

Airplanes that are certified for flight into known icing have certain capabilities to either prevent ice accumulation, called anti-ice, or shed it once it has appeared, called deice. There are three main types of systems that accomplish this.

Pneumatic boots—In this deice system, a rubber chamber is affixed to the leading edges and then pumped up and down repeatedly to shed ice. These frequently are powered by bleed air from a turbine engine.

Weeping wing—Used mostly as an anti-ice device, this type of system pumps a special glycol-based fluid through millions of microscopic holes on the leading edges, making ice formation nearly impossible. Called a weeping wing because of the running fluid from a distance, this system can be hard to distinguish from the hot wing. Look closely at the alcohol system and you’ll see evidence of the small holes, while a hot wing will be perfectly smooth and shiny.

Hot wing—Whether through bleed air from a turbine engine or electrically, a metal surface is heated to prevent ice formation on the wings, tail, and other leading edges.



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