Weather

November’s chilly temperatures are a good reminder to review the dangers of ice as it affects our flying. The most obvious danger is a layer of snow or a coating of ice on ramps, taxiways, and runways. A snow-covered surface could keep you from reaching takeoff speed before you run out of runway. Landing and safely stopping without skidding off an icy runway can be a major challenge.

While ice on a runway is a challenge worth a consideration, ice on airplanes is a no-no.

WHAT ICE DOES TO AIRCRAFT. Ice that forms on an airplane undermines the work of the aeronautical engineers who designed the airplane. Its wings are designed to create the right amount of the upward-acting force called lift, which gets an airplane in the air and keeps it there—despite the force of gravity pulling it toward Earth.

Lift depends on the smooth flow of air over the carefully designed wings. Ice disrupts this orderly flow, reducing the amount of lift that can be produced. If a pilot tries to take off in an airplane with ice on it—even the thin, often hard-to-see ice known as frost—the airplane might not leave the ground before running off the runway. What often happens, however, is the airplane staggers into the air, coming down soon after.

WHY FROST IS DANGEROUS. Frost is insidious because it’s hard to see and isn’t as obviously dangerous as ice. Frost 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 harmless, the FAA—and common sense—precludes 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.

Since frost can be hard to see, especially on white airplanes, look closely at the propeller, wings, and the top and bottom of the horizontal stabilizer—or even run your bare hand over them, to feel for roughness during the preflight. Frost or other ice on a propeller is dangerous because a propeller is a twisted wing that produces lift, acting to pull the airplane forward. Frost or ice will reduce thrust.

ICE AND LOSS OF CONTROL. Pilots who survived after flying into clouds that deposited more than a trace of ice on their airplanes reported that the airplane didn’t respond to control inputs.

Ice that forms on an airplane’s horizontal stabilizer is especially dangerous, because it is a wing with the lift force usually acting downward. When a pilot pulls back on the yoke, it increases the lift force that pulls the tail down. Since the airplane rotates around its center of gravity, this brings the nose up. Pushing the yoke forward decreases the stabilizer’s downward lift; the tail goes up as the nose goes down. When ice covers part of an airplane’s horizontal stabilizer or stabilator, the pilot has less pitch control. In the worst cases, the pilot loses pitch control.

Ice near the ends of the wings can degrade the pilot’s control of the airplane’s roll. One of the worst such cases 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, killing all 64 people on board.

Unlike in suspected similar crashes of general aviation airplanes, the National Transportation Safety Board used the airplane’s flight data and cockpit voice recorders to document exactly what happened before and after the pilots lost control.

HOW ICE FORMS ON AIRCRAFT. To understand aircraft icing, you need to toss out something you probably learned in elementary school: Water freezes when the temperature falls to 32 degrees Fahrenheit (0 degrees Celsius). While that’s not a bad description of what happens most of the time in familiar situations, it’s far from always the case.

Here’s a quick look at what happens. 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, approximately 1,000 miles an hour at temperatures around 60 degrees. Some move faster; some travel slower. Such fast-moving molecules 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 slows enough for water vapor molecules to begin latching onto tiny particles in the air, such as dust—known as condensation nuclei—to form tiny cloud drops. Even when the cloud drops are 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. 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 bottle of water in a freezer, a few ice crystals are likely to form spontaneously and hold together when the water falls a little below 32 degrees F. When a few crystals form, slower-moving water molecules begin attaching themselves, forming solid crystals until eventually the container is filled with solid ice.

This isn’t likely to happen, however, in tiny cloud drops, which commonly cool below 32 degrees F while remaining liquid. We call them supercooled liquid water drops. Theses supercooled drops will freeze when they hit something, such as a power line during a freezing rain episode, or an aircraft. Airplanes can run into supercooled liquid water in the form of cloud drops, freezing drizzle, or freezing rain. This is why pilots should be aware that visible moisture, such as cloud or rain drops, and below-freezing temperatures create a danger of icing.

THE KINDS OF ICE. As a practical matter for most pilots, the kind of ice that forms on airplanes is of little more than academic interest. Nevertheless, you may encounter a question on an FAA knowledge test that requires knowing the difference between clear ice (also called glaze) and cloudy (also called rime) ice. A mix of the two is possible.

Clear or glaze ice is formed by larger supercooled water droplets that don’t entirely freeze when they hit an airplane. The unfrozen supercooled water flows back to freeze and deposit ice over more of the wing or horizontal stabilizer. Such ice can be smooth and translucent, which makes it hard to see. Smaller drops that freeze when they hit cause cloudy rime ice, which is easier to see. At times an airplane can pick up a combination of the two kinds of ice.

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 who is staying well clear of clouds and has at least three miles visibility also can encounter supercooled liquid water. If light drizzle or rain is falling, the visibility can remain above VFR minimums. The rain or drizzle could be supercooled water. Also, freezing rain could surprise a VFR pilot who isn’t obtaining updated weather information. Finally, a report of ice pellets, which most people call sleet, is a sign there are drops of supercooled liquid water aloft.

Sleet forms when raindrops—often melted snow from higher clouds—fall through a layer of above-freezing air, then into air that’s below freezing—there, the drops freeze into tiny pieces of ice about the size of a pencil point that bounce when they hit.

If you take off with sleet pellets rattling against your airplane, you’re likely to climb into drops of liquid freezing rain, which could quickly coat your airplane with ice.