Weather: Jack Frost

Frost is ice that forms when water vapor in the air deposits directly as ice without first condensing into liquid water. If you’re a gardener or a farmer, an early frost can kill your plants.

If you don’t thoroughly scrape frost off your car’s windshield, windows, and mirrors before pulling out of your driveway, you could hit or be hit by a vehicle you don’t see.

These dangers are trivial compared to what frost on an airplane’s wings can do.

FROST CAUSES A SMALL JET TO CRASH. On January 4, 2002, a Canadair Challenger—a small jet—was taking off from Birmingham, England, for a transatlantic flight to Bangor, Maine. “Immediately after liftoff, the aircraft started to bank to the left,” the British Air Accidents Investigation Branch said in its report. “The rate of bank increased rapidly and two seconds after liftoff the bank angle had reached 50 degrees. At that point, the aircraft heading had diverged about 10 degrees to the left. Opposite aileron, followed closely by right rudder, was applied as the aircraft started banking; full right aileron and full right rudder had been applied within one second and were maintained. As the bank angle continued to increase, progressively more aircraft nose-up elevator was applied.

“The left winglet contacted the runway shoulder, the outboard part of the left wing detached and the aircraft struck the ground inverted, structurally separating the forward fuselage, which caught fire.”

The crash killed the jet’s two pilots and all three passengers aboard.

The report said the crew had not ensured that the wings were clear of frost before takeoff. The frost’s surface roughness reduced the wings’ stall angle of attack and made it such that the airplane’s stall protection system no longer worked.

The airplane’s quick roll to the left tells us that the right wing was producing much more lift than the left, causing the quick roll that turned the jet upside down before it hit the ground.

The pilots knew of the danger that ice could form on the airplane. The report said the pilots’ pretakeoff conversation “Included confirmation that the control checks had been completed and that anti-ice might be required immediately after takeoff.”

The report said that “possible impairment of crew performance by the combined effects of a non-prescription drug, jet lag, and fatigue also played a role” in the crash. This might explain why the pilots didn’t closely examine the wings and horizontal stabilizer for frost before taking off for a transatlantic flight on a frigid day that they knew could include a danger of ice forming on the airplane at soon after takeoff.

A CLOSER LOOK AT FROST. Frost is most likely to form on a calm, frigid night when the sky is clear because heat radiating away from the ground isn’t absorbed by water drops in clouds, which reradiate some of the heat back toward the ground. When wind is blowing, it stirs the air, mixing cold air near the ground with warmer air aloft.

While frost on an airplane can appear to be harmless, 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 take longer to lift off the runway. Even worse, as the Birmingham, England, crash shows, the airplane could unexpectedly bank to one side too quickly for a pilot to respond because the frost has reduced lift on one wing more than the other.

Since a propeller is, in effect, a wing that produces lift acting in the direction of travel, frost, like heavier ice, reduces thrust.

Obviously the FAA has good reasons for its policy that prohibits a pilot from taking off with frost, snow, or ice on an aircraft’s wings, propellers, or control surfaces

While you often hear that “water freezes at 32 degrees Fahrenheit,” that’s not true in many cases.THE KINDS OF AIRCRAFT ICING. Frost is only one of the kinds of icing that can threaten aircraft and most discussions of aircraft icing focus on structural icing, which forms on the outside of an airplane’s structure, such as the wings, the horizontal stabilizer, or radio antennas.

Another kind is carburetor icing, which is very different from frost and 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 causes the air to speed up, decreasing the air’s pressure. At this point, fuel is added to the air and evaporates into it. Since both lowering the air’s pressure and evaporation both cool the air, it’s possible for the air’s temperature to drop maybe 60 degrees Fahrenheit. If the air is humid enough, the water vapor can instantly turn into ice, blocking the flow of air and fuel through the carburetor into the engine.

The procedures for preventing or reacting to signs of carburetor icing vary among aircraft and you need to learn the procedures for any airplane you fly.

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 have to be aware of the dangers of structural icing that restricts air flowing into the engine.

ICE SCRAMBLES AIRCRAFT DESIGN. 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—as the Birmingham, England, crash shows.

The October 31, 1994, crash of an ATR-72 turboprop near Roselawn, Indiana, is a classic example of icing causing pilots to lose roll control high in the air.

What happened is well documented, because the airplane’s flight data recorder enabled the National Transportation Safety Board 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 yokes as they tried to recover.

TAILPLANE ICING CAN SNATCH CONTROL FROM A PILOT. 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.

When you pull back on the yoke, it increases the lift that pulls the tail down. As the tail goes down the nose rises, since an airplane rotates around its center of gravity. When you push the yoke forward 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 can be lost and the airplane can nose over into an uncontrollable dive.

THE BASIC SCIENCE OF AIRCRAFT ICING. 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 F,” that’s not true in many cases. The water that you put in an ice-cube tray and stick in the freezer turns to ice at close to 32 degrees F, 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 F.

Coincidently, 40 degrees F is also 40 degrees Celsius. It’s the temperature at which Fahrenheit and Celsius scales meet. This is a coincidence; the two scales were designed separately and before anyone knew small amounts of water can remain liquid to 40 degrees F or C.

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 who has at least three-mile 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 and deposit enough ice to cause problems. Also, freezing rain could surprise a VFR pilot who hasn’t obtained updated weather information.

The lesson is: When temperatures are frigid you need to be aware of the danger of ice, beginning with frost that might have formed on your airplane overnight.