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| You don't need to be flying in clouds to encounter conditions that can cause ice to form on your aircraft. Freezing rain, or light rain and drizzle that is supercooled, can do the job just as well. |
This risk hasn't changed in decades
It's the late 1930s, and Gann is the co-pilot of an American Airlines DC-2 flying from Nashville, Tennessee, to New York City on a February night. Clouds had snuffed out the stars overhead when ice began forming on the windshield and the wings.
Gann looks through the ice that's "easily three inches thick in places" on his side window to see "the engine, now grizzle-bearded in ice. The carburetor air scoop is an oval-shaped metal mouth on top of the engine. Through it must pass the air, which is as important to any combustion engine as fuel. Without air the engine dies as surely as a drowning human being."
As the captain flies the airplane, Gann uses the direction-finding radio to plot lines of position on a chart that show they are approximately 50 miles north of Knoxville, Tennessee, with a 4,150-foot peak somewhere nearby. "We are now at 4,500 feet and still sinking" as the ice strangles the flow of air into the engines. "We have merely nodded to fear," he writes. "Now we must shake its filthy hand. Both engines suddenly begin cutting out."
The experienced captain responds to the immediate threat by leaning the mixture. The increased amount of air mixed with the gasoline causes the engines to backfire. Flames and air shoot back through the carburetors, shattering the ice forming around the air intakes. The DC-2 stays above the surrounding mountains and flies out of the icing conditions.
Today's pilots can learn much from this and other stories by Gann. Even with the great improvements in navigation and aircraft technology since the 1930s, the principles of meteorology that determine when and where ice forms on an aircraft, and the laws of aerodynamics that determine how an aircraft reacts to icing, remain the same.
Understanding what causes ice to form on aircraft begins with forgetting something you probably learned in grammar school: Water freezes when the temperature falls to 32 degrees Fahrenheit (0 degrees Celsius). Strictly speaking, this isn't the freezing point of water, but the melting point of ice. While it's true the water you put in an ice-cube tray and stick in the freezer turns to ice at close to 32 degrees, smaller amounts of water, such as the tiny drops of water in a cloud, can stay liquid until the temperature drops as low as approximately minus 40 degrees.
To describe what causes icing, we need to take a brief look at the science of phase changes-the transformations among the gaseous, liquid, and solid phases of matter, including water.
The molecules of any substance are always moving. When the substance is a gas, such as water vapor in the atmosphere, the average speed of its molecules is very fast, averaging approximately 1,000 miles an hour at temperatures around 60 degrees, with some moving faster, some traveling slower. Such fast-moving mole-cules mostly bounce off each other when they collide, because their speed overcomes the atomic forces that bind molecules together in a liquid or solid.
As air cools, the average speed of the molecules slows enough for water vapor molecules in the air to begin latching onto tiny particles in the air, such as dust, known as condensation nuclei, to form tiny cloud drops. If the cloud drops are colder than 32 degrees F, either because the air is cooling because it's rising or the surrounding air is already colder than 32 degrees F, the water in the drops doesn't immediately begin turning into six-sided ice crystals. To transform into ice, liquid water needs a template, known as a freezing nuclei. As you can imagine, ice crystals are the best templates. Some types of other natural particles in the air act as freezing nuclei and can transform supercooled water into ice at various temperatures, depending on the substance. A few kinds of bacteria, for example, can turn supercooled water drops into ice when the drops are just below 32 degrees F.
With the millions upon millions of water molecules in a "large" container, such as a compartment in a freezer's ice tray, however, a few ice crystals are likely to form spontaneously and hold together when the water falls a little below 32 degrees F. With a few ice crystals in place, slow-moving water molecules begin attaching themselves, and the crystal grows into the ice cube.
This isn't likely to happen, however, in tiny cloud drops. Thus, the drops will cool below 32 degrees F while remaining liquid. We describe them as being made of supercooled liquid water. Supercooled drops will freeze when they hit something, such as a power line during a freezing rain episode-or the wing of an airplane flying through a cloud of supercooled cloud drops. That is, an airplane needs to encounter supercooled, liquid water, which can be in the form of cloud drops, freezing drizzle, or freezing rain. This is why pilots are taught that for aircraft structural icing to occur, visible moisture (such as cloud or rain drops) and temperatures close to freezing are needed.
While pilots flying in clouds on an instrument flight plan are the most likely to run into supercooled liquid drops, a pilot flying under visual meteorological conditions (VMC) who is staying well clear of clouds and who has at least three miles visibility can encounter supercooled liquid water. If light drizzle or rain is falling, the visibility can remain above visual flight rules (VFR) limitations. The rain or drizzle could be supercooled water. Also, freezing rain could surprise a VFR pilot who isn't obtaining updated weather information.
Structural icing refers to ice on the outside of an airplane's structure, such as on the wings, the horizontal stabilizer, or even on antennas. Gann's DC-2 was a classic case of structural icing. In addition to blocking airflow into the engines and reducing lift, it interfered with radio reception, which made it almost impossible for Gann to find the airplane's location over the mountains.
Pilots also need to know about two other kinds of icing: carburetor icing and frost. Carburetor icing is a completely different animal from structural icing because it can happen on a hot summer's day. Air flowing into a carburetor is forced through a smaller opening called the venturi, which speeds up the air and lowers its pressure. At this point, fuel is added to the air and evaporates into it. Since both lowering the air pressure and evaporation cool the air, it's possible for the air's temperature to drop maybe 60 degrees F. If the air is humid enough, the water vapor can instantly turn into ice, blocking the flow of air and fuel into the engine.
Carburetor icing is often classified along with icing that chokes engine air intakes as "induction system icing." While airplanes with fuel injection aren't subject to carburetor icing, their pilots can encounter the same threat that Gann and his captain faced. The correct response for today's aircraft is to open the alternate air induction door (it may happen automatically).
Frost is ice that forms when water vapor in the air deposits directly as ice without first condensing into liquid water. It's most likely to form on a cold night when the sky is clear. While frost on an airplane can appear to be harmless, FAA policy prohibits a pilot from taking off with frost, snow, or ice on an aircraft's wings, propellers, or control surfaces. Wind tunnel and flight tests have shown that even a thin layer of ice, including frost, reduces wing lift by as much as 30 percent and increases drag by as much as 40 percent.
Even a thin layer of ice disrupts the smooth flow of air over the airplane's wings, which reduces lift. This can do more than reduce the rate of climb because the reduction isn't likely to be even along a wing's span. An airplane with frost on the wings could not only take longer to lift off the runway, but could unexpectedly bank to one side because the frost has reduced lift on one wing more than the other.
Gann's story takes us back to a time when airplanes, ice protection systems, and weather observation and forecasting were crude by today's standards. But the lessons he learned have much to teach today's pilots.
Even after new ice stopped forming on the DC-2 after it flew out of the clouds, Gann, the captain (Gann gives us only his last name, Hughen), and the eight passengers aboard weren't safe. "Though we are flying in clear, smooth air, and thus no longer accumulating ice, the load we already have still renders the ship nearly unmanageable."
A major lesson that Gann describes learning during his apprenticeship as an airline pilot still applies to all flying: "Good pilots held an alternate in their minds for every eventuality. Expecting the worst, they skipped one emotion when trouble appeared, and thus moved without pausing past disappointment to decision and action."
Preparing for the eventualities of flying includes learning as much as possible about potential hazards, such as icing, how to use forecasts and reports, and being prepared to take the correct actions if ice begins to form on your airplane.
Jack Williams, a freelance science writer specializing in weather and climate, is the author of six books. He is an instrument-rated private pilot. His latest book, The AMS Weather Book: The Ultimate Guide to America's Weather, is forthcoming from the University of Chicago Press.
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