The Weather Never Sleeps
Baby, it's cold outside
BY JACK WILLIAMS (From AOPA Flight Training, January 2004.)
By the middle of winter pilots across the northern United States can begin thinking that bears have the right strategy: crawl into a cave and hibernate until spring.
On the other hand, a pilot who's found a bear willing to share his cave for a long winter's sleep will miss flying on those crisp days when the sun makes the snow sparkle, the winds are calm, and you're convinced you can see 100 miles in the clear air.
Winter brings more than its share of serious weather hazards, but some caution and a thorough preflight weather briefing can help you enjoy those great winter flying days.
You would have to be out of your mind to take off into the face of a howling blizzard, and a weather briefing will help ensure that a quick-moving blizzard or even scattered snow squalls doesn't catch you unaware.
Airframe icing is a big winter danger (it can occur in other seasons), but low-time pilots aren't likely to be caught unless they are careless. Airframe icing refers to ice that forms on an airplane's wings, horizontal stabilizer, and other places such as the windshield you need to see through in order to land.
An airplane needs to be flying in "visible moisture" such as a cloud or through falling precipitation for a layer of ice to form on it. Pilots who haven't learned to fly under instrument flight rules (IFR) should be especially obsessive about avoiding clouds and precipitation in the winter.
Even though you avoid winter's big dangers of storms and icing, the season still presents some subtle dangers. One of these subtle dangers is frost. Though it conjures up images of icy patterns on windows on cold mornings or grass sparkling in the winter sun, frost has snagged some very experienced pilots.
Frost is ice that forms when water vapor in the air sublimates — changes directly from water vapor to ice without first condensing into liquid water.
You might go out to fly on a clear, cold winter morning and never see the frost on your airplane's wings unless you are looking for it. You are more likely to notice that the airplane's windows are frosted over — and if there is frost on the windows, the rest of the airplane is surely frosty. How, you might wonder, can something I can hardly see be all that dangerous? This question falls into the category of famous last words.
Back in 1950, the Civil Aeronautics Board-the forerunner of the FAA-established regulations prohibiting a pilot from taking off if "frost, snow, or ice" is "adhering to wings, propellers, or control surfaces of the aircraft." Numerous crashes led to the regulation, which is still in effect (with some minor changes in wording).
The reason, the FAA explains, is: "Wind tunnel and flight tests indicate that frost, ice, or snow formations having a thickness and surface roughness similar to medium or coarse sandpaper on the leading edge and upper surface of a wing can reduce wing lift by as much as 30 percent and increase drag by as much as 40 percent."
The thin layer of ice disrupts the smooth flow of air over the airplane's wings that creates the lifting force. "Reducing lift" is more dangerous than it might sound. Much more than a slower-than-normal rate of climb is involved. The reduction in lift isn't likely to be even, to put it in simple terms.
Several accidents have been reported in which airplanes with frost on the wings went out of control — imagine what it would be like close to the ground in an airplane with the right wing unexpectedly developing more lift than the left. Your efforts to level the wings may be fruitless.
To catch frost that could do this, you might have to look closely, perhaps run your bare hand over the top of a wing, to realize that it's covered by a thin layer of clear ice-frost.
Getting rid of the frost can be a hassle. Airlines spray airplanes with deicing fluid, but that isn't usually an option for general aviation pilots. Unless the temperature is climbing above freezing, you can't count on the sun to melt the frost.
If you can move the airplane into a warm hangar, that could be the best bet. But, you need to make sure water from the melting frost doesn't remain any place on the airplane where it could freeze after you take off and interfere with movement of control surfaces.
Pilots should also beware of apparently light snow showers. Such flying can be safe, but it has its hazards, beginning with the possibility that the snow could begin falling faster, reducing visibility to unsafe levels.
Snow is nothing but falling ice crystals, or ice crystals that are stuck together to make snowflakes. The air can be full of ice crystals that bounce off your airplane because solid ice crystals aren't sticky. But, ice crystals aren't always solid ice. You can't always be sure what looks like "snow" is always nothing but snow. At times freezing rain or drizzle can be mixed with snow.
In the mid-1990s, scientists from the National Center for Atmospheric Research (NCAR) studied 10 airline takeoff crashes caused by ice. They found that snow that falls when the temperature is between about 25 and 31 degrees Fahrenheit is wet; that is, the ice crystals have liquid water sticking to them. This liquid water is likely to be supercooled.
You might have learned in school that water "freezes" at 32 degrees Fahrenheit (0 degrees Celsius). But, that's not true. It's correct that ice begins melting at 32 degrees, but water can be as cold as minus 40 and still liquid — it's supercooled and it freezes on contact with anything, such as your airplane's wings. Supercooled water causes airframe icing and "ice storms" that coat trees and power lines with ice.
In their mid-1990s study, the NCAR scientists discovered that the supercooled water sticking to snowflakes in 25- to 31-degree air freezes when it hits airplanes, creating a layer of ice sticking to the airplane that looks like snow.
Flight and ground crews sometimes assume the light layer of "snow" on the wings will blow off during the takeoff run. Most of the time this is true, but in the cases studied-remember they were all crashes — the "snow" didn't blow off, and the reduced lift brought the airplanes down.
In several cases, the snow had appeared light because the snow crystals were small. The good visibility made the snow seem less dangerous than it really was.
Large snowflakes, which can be dry, block more light than smaller, wet snow crystals, reducing visibility more than the smaller crystals. Poor visibility is more likely to alert pilots to the potential danger of snow than smaller, but wetter, snow crystals.
Imagine that after carefully making sure no frost is adhering to your airplane's wings or horizontal stabilizer, you take off on a bright day-with no snow falling. You fly a couple of hundred miles, land, have a snack, and then fly back home. While you were gone, the temperature rose, some of the snow banked up along the runway at your home airport melted, and the water flowed onto the runway and froze as the temperature fell in the afternoon.
As you are landing, nothing seems amiss. But when you land in a slight crosswind, your airplane begins sliding to one side, and your brakes seem to have failed completely. The runway is a sheet of ice, and you've suddenly learned about black ice — a clear layer that leaves a runway or road surfaces the normal black color. Such ice sends your airplane or car skidding without warning.
If you are flying to an airport with a control tower, the controller should warn you about an icy runway. If the airport has only a unicom operator, ask on your initial call-up about ice.
Even if the runway isn't icy, you should assume that there's ice on the taxiways. Maybe the bears have the right idea about winter after all.
Jack Williams is weather editor of usatoday.com. As 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.