One sometimes hears of "icing" after a crash. How can ice or snow cause a plane to crash?

During flight, ice can accumulate in different areas -- on the leading edges of wings, engine inlets, etc. (It will stick to the thinner, lower-profile areas, but usually not the larger expanses or fuselage -- a function of aerodynamics; let's not go there.) This occurs during visible precipitation, or when suspended moisture sublimates directly to the surface. The monster here isn't the weight of the frozen material, but the way it changes the contouring of the wings. Even a half-inch ridge of ice can wreak havoc with an airfoil. This is especially important during takeoff and landing, when the speed is slowest and the margin of lift is most critical.

Sitting at the terminal, a plane will collect precipitation the same way your car does -- via snowfall, sleet, freezing rain or frost. Thanks to supercooled fuel in the wings, frost can form insidiously even during temperatures above freezing. But not to worry (you were waiting for that), as all of this is scouted out before flight. An airline's preflight de-icing checklist can take up several pages of a pilot's manual.

Whether aloft or gateside, rules and equipment are on hand for the occasion. Bringing us to the next question

What is the pink liquid used to de-ice snow-encrusted planes? And what about in the air? How do planes de-ice themselves during flight?

The delicious-looking (apricot-strawberry) fluid used for ground de-icing is a heated combination of glycol and water. There are different mixtures for different conditions, varying in temperature and viscosity. It helps remove existing material and prevents the buildup of more. How long a plane is good for after application is not just a matter of giving it a look-see, but follows something called "holdover time," accounting for the rate and type of current precipitation and ambient temperature.

The fluid is often collected and recycled, but at $5 per gallon de-icing a plane is extremely expensive. When the costs of handling, storage and disposal are considered, relieving a single jet of unwanted ice or snow can cost tens of thousands of dollars. Making a messy situation worse, glycol is toxic. What does our de-icing future look like? It looks like a hangar: At a few airports, planes are steered through enclosures that use infrared heat instead of fluid. Continental now uses a facility like this at Newark.

All airliners are equipped with onboard gear to deal with the stuff encountered aloft. On smaller planes, pneumatically inflated boots will break ice from the leading edges of wings and stabilizers. On larger planes the wings, engine inlets and a few other spots are heated with air bled from the engine compressors. Windshields and various probes are kept clear electrically. These systems use redundant sources and are separated into independently operating zones to keep a failure from affecting the entire plane.

Offhand, I can't cite a single case of a large plane crashing from ice that accrued during flight, but there have been a handful of takeoff accidents over the years, most notoriously the one involving Air Florida flight 90 in Washington in 1982. In addition to buildup on the wings, iced-over engine probes gave a faulty, less-than-actual thrust reading after the crew had failed to run the engine anti-ice system. The most recent serious crash, a more true-to-form ice-on-the-wings scenario, was that of a USAir Fokker jet at La Guardia in 1992 (24 of the 51 occupants survived). Not bad, considering there have been about 12 million takeoffs in the country since then.

It seems to me that some runways go a little bit uphill. Is this true, or are my eyes failing me?

Stop blinking. Runways often slope slightly, and some even dip toward the center and rise on both ends. The steepness of the grade often appears much greater than it is -- a trick of perspective. A typical grade is maybe 1 percent overall. And this is to accommodate the existing ground angle; runways are not intentionally canted to make takeoff or landing easier. Occasionally slight performance (weight) penalties apply on an uphill strip.

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Do you have questions for Salon's aviation expert? Send them to AskThePilot and look for answers in a future column.

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