Learn from some chilling experiences
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While pilots flying under visual flight rules hardly ever have to worry about ice forming on their airplane, as long as they stay out of clouds and precipitation, every pilot should be aware of the danger--including the rare cases when you don't have to be in a cloud to pick up ice.
A Piper Saratoga pilot's story in National Transportation Safety Board files gives an idea of what can happen when ice begins to form on your airplane.
The pilot told an NTSB accident investigator that 30 minutes into a January 2005 flight from Minnesota to Michigan he noticed a little ice was forming on the wings when the indicated airspeed dropped to 130 knots and the Saratoga would no longer climb. When the airspeed dropped to 120 kt with full throttle in level flight, the pilot asked air traffic control to clear him to land at the nearest airport with an instrument approach.
Losing the ability to climb or maintain airspeed, even with full throttle, is often the beginning of a chain of events that can end with complete loss of control. Fortunately, the Saratoga pilot was only about 500 feet above the ground on the approach when the Saratoga began to buffet and the pilot began to lose directional control. Instead of continuing toward the runway, he immediately landed in a field. The stall warning horn sounded just before the Saratoga hit the ground, shearing off the landing gear. The outcome was better than in many icing accidents: The pilot escaped with minor injuries, and the passenger was uninjured. The story would have been much different if the Saratoga had been higher when the pilot felt a prestall buffet and began to lose directional control.
When ice forms on an airplane it randomly changes the carefully engineered shapes of the wings and horizontal stabilizer in ways that reduce lift and increase drag. Ice can also block the view through the windshield. It can form unevenly on the wings, reducing the lift on one wing more than the other or restricting air flow over the ailerons. Imagine your airplane going into a slight left bank even though you haven't turned the yoke. When you turn the yoke to level the airplane, nothing happens.
One such nightmare occurred on October 31, 1994, when icing caused the pilots of an ATR-72 turboprop to lose roll control; the airplane made at least one complete 360-degree roll before crashing into a soybean field near Roselawn, Indiana. Exactly what happened is extremely well documented, because the airplane's flight data recorder enabled the NTSB to describe each control movement and all of the airplane's gyrations in great detail, including how much force the pilots were applying to the yoke as they tried to recover.
Ice that forms on an airplane's horizontal stabilizer is especially dangerous, because the horizontal stabilizer is a wing with the lift force acting downward. Increasing the lift that pulls the tail down brings the nose up since an airplane rotates around its center of gravity. When the nose pitches up, you decrease the tail's downward force, and the nose pitches down as the tail rises. When a horizontal stabilizer or stabilator is iced up, pitch control is lost and the airplane can nose over into an uncontrollable dive.
To understand aircraft icing, you need to know a few basic things about how ice forms. While you often hear that "water freezes at 32 degrees Fahrenheit," that's not true most of the time. The water that you put in an ice-cube tray and stick in the freezer turns to ice at close to 32 degrees, but smaller amounts of water, such as the drops of water in a cloud, can stay liquid until the temperature drops as low as minus 40 degrees.
The molecules of any substance are always moving. When the substance is a gas, such as water vapor, the average speed of its molecules is very fast, in the neighborhood of 1,000 miles an hour at temperatures around 60 degrees. Molecules of water vapor in the air, like those of any gas, are moving so quickly they bounce off each other when they collide. The speed almost completely overcomes the atomic forces that bind molecules of a liquid or solid substance.
As air--including its water vapor--cools, the average speed of the molecules slows enough for atomic attractions to begin holding the slower-moving molecules together, but not firmly. When water vapor molecules cool enough, they begin to latch onto tiny particles in the air, known as condensation nuclei, to form the tiny cloud drops.
As the air and the water drops in the cloud cool further, some of the water molecules begin locking together more firmly to form six-sided ice crystals, but this doesn't happen instantly at 32 degrees. While some of the millions of molecules in a cloud drop lock together as microscopic ice crystals, many surrounding molecules are traveling fast enough to break up the crystals when they collide. Ice crystals are always forming and being broken apart.
To begin really turning into ice when it's warmer than about minus 40 degrees, the moving water molecules need freezing nuclei to latch onto. Ordinary air, even "clean" air, has millions of tiny particles that help water to begin turning into ice. Water vapor first condenses onto these ice-forming nuclei and then freezes. Most such nuclei aren't effective until the air cools to around 15 degrees F. Some particles, which have shapes even closer to those of ice crystals, provide a place for water vapor to latch onto directly as ice crystals instead of first condensing and then freezing. These work best in temperatures around 0 degrees F or colder.
Water in "large" containers, such as the sections of an ice-cube tray, turns to ice not too far below 32 degrees because slight imperfections in the metal or plastic ice tray's surface act as nucleation sites; they act like freezing nuclei. Also, with millions more molecules than a single cloud drop, larger numbers of microscopic ice crystals form, and all aren't as likely to be shattered before they grow larger.
The important point for pilots is that a cloud that's colder than 32 degrees F but warmer than about 5 degrees F has good odds of consisting mostly of drops of supercooled water that's below 32 degrees, but is still liquid. Each one of these drops will freeze instantly when it hits something that provides a freezing nucleus, such as a microscopic imperfection on an airplane wing. Once ice begins to form on an airplane, it acts as a huge condensation nucleus that turns more supercooled drops into ice.
As a cloud grows colder, more and more of its supercooled water drops turn to ice, but low, generally flat stratus clouds can have enough supercooled liquid water to cause serious icing when they are as cold as minus 20 degrees F. Cumulus clouds offer an even greater icing danger because the air is rising so fast that water is still supercooled without turning to ice even as the temperature approaches minus 40 degrees F.
Once almost all of the supercooled water in a cloud turns to ice crystals, you don't have to worry about icing since crystals will bounce off your airplane when they hit it. But don't be complacent if you see snow (which consists of ice crystals) hitting the windshield as you fly through a cloud, because snow crystals can be mixed in with drops of supercooled water.
Even if you've never flown into a cloud of supercooled liquid water drops, you've probably seen "structural icing" (even though it wasn't called that) unless you live in the deep South. Structural icing on an airplane in a cloud is the same kind of thing that people on the ground know as an "ice storm, " which have occurred as far south as northern Florida.
When supercooled drops of rain or even drizzle fall to the ground, the water freezes instantly when it hits trees, power lines, and roads. If freezing rain is heavy enough, the weight of the deposited ice can bring down tree limbs and power lines, causing widespread blackouts. Lighter freezing rain or even freezing drizzle can leave "black ice" on roads. It's called black ice because you see the dark road through the ice, which you don't realize is there until your car begins to skid.
Ice from freezing rain or drizzle, of course, can coat airplanes on the ground. The same black ice that your car almost skidded on as you were driving to the airport could be as hard to see--without a close look--on your airplane. You need to make sure no ice, even the apparently harmless-looking, thin layer of ice known as frost, is on an airplane that you are about to fly.
Researchers have found that as little as 0.8 millimeters of frost--just over a hundredth of an inch--can reduce lift by 25 percent, while increasing drag, because the frost's roughness disrupts the airflow over the wing that creates lift.
While you will expect your airplane to be icy if an ice storm has hit since you last flew it, frost could surprise you since it forms on clear nights. Frost forms when humidity in the air turns directly into ice without condensing into water; and it's most likely to occur on clear nights, since such nights are colder--everything else being equal--than cloudy nights.
The general rule for structural icing is that it occurs only in below-freezing temperatures and in visible moisture. In other words, you don't have to worry about invisible water vapor forming structural ice on your airplane. But, water vapor can become frost.
Icing can affect pilots following the visual flight visibility and cloud height rules. You can fly into very light freezing rain or drizzle, or snow with freezing rain or drizzle mixed in. Dangerous freezing precipitation isn't always heavy enough to reduce visibility below VFR minimums. Icing is possible any time precipitation is falling and the temperature at your altitude is 32 degrees or below.
An NTSB icing accident report from the pilot of a Cirrus SR22 illustrates the worst icing nightmare: "As the airplane reached the cloud tops at 8,000 feet in visual flight conditions, the airplane began to buffet. The pilot looked at his airspeed indicator and it indicated 80 knots. The airplane stalled, the nose pitched down, and the airplane started spinning to the left while reentering instrument flight conditions. The pilot reduced power, neutralized the flight controls, and applied right rudder with negative results."
In most cases this would be the end of the story; the airplane is spinning down into the clouds and the pilot can't recover.
This time, however, the NTSB says: "The pilot activated the Cirrus airframe parachute system, and the parachute system deployed. The pilot informed the controller...'we have uhh experienced icing we have uhh had a stall we're under the parachute we're an emergency situation.'
"The airplane descended to the ground under the parachute canopy, collided with trees, and came to a complete stop about four feet above the ground. All personnel exited the airplane" without serious injury.
The NTSB reports that in both the Cirrus accident and the one involving the Saratoga that landed hard in a field, the National Weather Service had issued icing alerts for the areas where the airplanes iced up.
Icing is difficult to forecast; sometimes meteorologists predict it where none occurs and at other times they miss icing that does occur. But, safe flight in clouds begins with a complete weather briefing that includes asking about pilot reports of icing that has been encountered as well as predictions for future icing.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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