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Lesson 1 - Basic Cold-Weather Meteorology and Terms


With thanks to contributors from the NWA Aviation Weather Committee, USAToday.com, the National Center for Atmospheric Research and the University of Illinois On-line Guide to Meteorology
...and there I was...

Ever wonder what it is like to be flying in REALLY cold weather? Maybe you've already been there. Maybe not. Let's get a look at what its really like...

Cold Weather Flying

by Mr. Jack Williams
USAToday.com and AOPA Flight Training Magazine


As the main skis of the LC-130 touch the snow near a remote science camp in Antarctica, the pilot Colonel Graham Pritchard pulls back on the yoke, keeping the nose in the air.

The plane throws up plumes of snow as it slows and Pritchard allows the nose to slowly come down until the nose ski touches.

If this sounds a lot like the "soft-field" landings instructors and their students practice in small Cessnas and Pipers, it is.

"One thing that’s helped me in my ski flying is my general aviation flying," Pritchard says. "What I learned in doing soft-field landing technique in a Cessna 150 and how I learned to teach that as a civilian instructor, I’ve carried on.
"The technique you use of a power-on approach, setting the mains down very carefully, holding the power on, letting the airplane decelerate, holding the nose off as long as you can, that’s almost the same thing we do on a big C-130 on a ski landing."

Col. Pritchard is the commander of the 109th Airlift Wing, the New York Air National Guard unit that flies the LC-130s (L means ski equipped) to haul people and equipment to National Science Foundation research sites in Greenland and Antarctica. It’s also ready to fly any military missions requiring airplanes that can land on ice and snow using skis and even missions that don’t require skis, such as hauling relief supplies to the Caribbean after Hurricane Georges.
He’s been flying LC-130s since 1969 when he joined a regular Air Force squadron in Alaska that supported the Distant Early Warning (DEW) Line radar stations across the Arctic. During his spare time in Alaska he earned his civilian flight instructor and instrument flight instructor licenses. He has kept his hand in as a small airplane instructor, and looks forward to returning to civilian instruction when he retires.

Many of the lessons he’s learned flying in the world’s coldest weather also apply to general aviation pilots flying in cold parts of the United States.

When you go out on a cold morning to fly, "take the time necessary to get the airplane fully ready to go," Pritchard says. "You could approach it as we do when we approach one of our cold-soaked airplanes that’s been sitting there in minus 40 temperatures. We have a very slow, step-by-step process." To begin with, the batteries are taken out of the airplane, kept in a warm place and reinstalled when it’s time to start.

External heat is used to warm the engines, cargo compartment and cockpit. "We warm them up very slowly, we don’t want to thermally shock the instruments and avionics. Also, if you try to warm a C-130 too quickly you can crack the cockpit windows."

Warming the engines is important because when oil is cold it doesn’t do as good a job of lubricating the engine as when it’s warmed.

While the airplane is warming up is a good time to get a complete weather briefing, including finding out if you’re likely to encounter icing or whether snow or freezing rain are likely.

"Some aspects of winter flying are more challenging in a temperate climate (such as Upstate New York) than in the polar regions," Pritchard says.

For instance, in the Arctic and Antarctic, the is air is so cold and so dry that airframe icing and heavy wet snow are rarely encountered.

Of course, polar fliers have to worry about storms that come up quickly with 100 mph winds and blowing snow that cut visibility to zero.

In Antarctica and Greenland, pilots, other flight crew members and the scientists and others who fly as passengers expect to be grounded by the weather from time to time. There’s no pressure to take off in risky weather, no matter how important the mission.

When the weather is good, just getting off the ground can often be a challenge both in polar regions and northern parts of the U.S.

While taking off on groomed snow runways or open snow presents its own challenges, "slush is about the worst thing in the world to try to operate in," Pritchard says. But it’s not common in polar regions.

"One of the things that’s often misunderstood is how much adverse effect slush has on takeoff performance. You might not think two to three inches of slush is not that big a deal until about half way or two thirds of the way down the runway you realize the airplane is not accelerating."

Performance charts for the LC-130, like those for general aviation airplanes, do not give takeoff distances for slush or snow, just for dry runways, Pritchard notes. Pilots have to figure out whether a takeoff is possible.

When they’re taking off from a snowy landing strip, the pilots in the 109th have flags at the strip’s halfway point. "The pilot knows, I’m half way down the runway, what is my performance? Do I need to think about doing something else.," Pritchard says. "A general aviation pilot can do the same kind of thing. Nothing fancy is needed. You pick an intersecting runway, a taxiway, something that’s about half way down the runway. If you don’t have normal performance by then, abort the takeoff."

The crews and all passengers flying on the 109th’s LC-130s in Antarctica and to Greenland’s Ice Cap all wear clothing intended for extreme cold weather. They also all carry polar sleeping bags. The airplanes carry enough survival gear, including tents, to take care of all aboard for a week. The crews all go through survival school.

In many parts of the U.S. even a short flight can take you to places where roads and houses are far apart, such as New York’s Adirondack Mountains only an hour or so flying from Schenectady, where the 109th is based.

Pritchard says anyone flying even in populated areas or the U.S. in the winter should "always be thinking about survival. You want something to keep yourself warm, blankets, a heavy coat you can grab real quickly. You should have a flash light, a first aid kit, fire starting materials, and a night signaling device, since you’re dealing with longer nights.
"As soon as you find yourself down, you should start preparing to stay by yourself for a long time. If help comes in an hour, or a day you should see it as a bonus."

Pritchard suggests that "it’s a good idea to fly with a partner in the winter. Solo flying is fine, but if you have a problem, having a partner is a good idea. Maybe in the winter it’s a good idea to call one of your buddies to go out with you. "
Antarctica is about one and a half times as large as the area of the contiguous 48 U.S. states, but has only three radio aids to navigation and only a couple of dozen automated weather stations to supply data for forecasts. The pilots of the 109th fly to places such as the South Pole where they land on groomed skiways. But they also fly many missions to remote science camps where they land on unprepared snow and ice.

Pritchard says in many ways it’s like flying in the 1920s and 1930s when pilots didn’t have radio navigation aids, much less GPS navigation and the ability to stay in constant touch with controllers. If a pilot needed to make an unplanned landing or got lost, there was no one who could help.

Even though the 109th’s LC-130s have the latest in navigation and other technology, pilots are expected to be strong in the "basics of flying: time, distance, planning out your mission, keeping a sense of awareness of where you are, using every input you can, knowing the lay of the land you’re flying in," Pritchard says.

"The new technology is great, it helps us do our mission better than ever. But It would be real easy right now to punch in a set of points in a computer, engage the auto pilot and then sit back and let the airplane go there."

But, "this is fraught with danger." he says. "learn your basics first and always work on your backups." Even on a short, winter’s trip from one urban area to another, an electrical failure could instantly put you in the position of a 1930s pilot who needs to rely on a practiced air sense to make it safely home.

Winter Weather

by Jack Williams

Across the northern parts of the United States, winter often seems like a conspiracy aimed at those who want to learn to fly, keep their skills sharp, or take to the air for the fun of flight.

While some pilots push their planes into a hanger until spring, most flight schools and pilots, even in the coldest parts of the country, keep flying during the winter.

In fact, in many ways the fall is a good time to begin to learn to fly. Winter shouldn’t be a reason to stop work on a license or new rating as long as you’re willing to recognize that more often than during other seasons, the weather will tell you to stay indoors and study for your written exam.

Fall is a good time to begin to learn to fly because the atmosphere tends to be more stable than during the spring and summer. Unstable air gives birth to the up and down air motions known as convection. In its mildest form, convection can make a ride bumpy, which can be disconcerting to someone who’s not used to being aloft in a small airplane. Turbulence can leave a new student wondering whether control inputs or the air caused a particular movement. At its worse, unstable air spawns thunderstorms.

When the atmosphere is stable, smooth air makes it easier for a new student to get used to the idea of controlling an airplane. If a wing dips, a student pilot will know it’s because he or she did something with the controls to make it dip.
In general, the atmosphere is unstable when the temperature contrast between warm air near the ground and cold air aloft is the greatest. In the spring, the sun begins heating the ground, warming the lowest layer of air while air aloft retains its winter chill longer. The result is the bumpy air and thunderstorms that are typical of spring.
As nights grow longer in the fall, the ground begins rapidly cooling while the air aloft is slower to give up its summer warmth. The resulting decrease in temperature contrast makes the air more stable, giving us fall’s calmer days.
Winter brings larger and more frequent storms, which move across the country, blowing away fall’s calm.

The worst winter storms can ground flights, sometimes including even airline flights, across the country with a grab bag of nasty weather.

A winter storm is like a huge mixer with winds spiraling in toward its low-pressure center.

A winter storm that moves into California, Oregon or Washington from over the Pacific Ocean brings with it humid ocean air that can condense into heavy rain for lower elevations and deep snow for the mountains. As it moves inland, the storm pulls in cold air from western Canada, creating mixtures of rain, freezing rain, sleet and snow across the West.
When a storm moves east of the Rockies, it often strengthens as it begins pulling in relatively warm, humid air from over the Gulf of Mexico. At the same time, no mountains slow the cold air is flowing southward southward across the Plains from the Canadian Arctic.

At this time, a weather map will show the storm’s low pressure center, maybe somewhere over the Middle West, with a cold front stretching to the south and west to the Gulf of Mexico and a warm front running east, maybe roughly along the Ohio River and across the Appalachians to the Atlantic Ocean.

The cold front is the boundary at the ground where cold air is advancing, pushing warm air upward. The warm front is the boundary at the surface where warm air is replacing cool air. While the word "warm" might sound good in the winter, a warm front often brings some of the season’s most dangerous weather.

A warm front commonly brings a variety of bad weather during the winter. The big danger for pilots is aircraft icing that occurs over large areas where cloud drops are made of water that is "supercooled;" that is, it is colder than 32 degrees Fahrenheit but has not turned to ice.

Supercooled water is likely to be found aloft above places where sleet is hitting the ground and also above places where freezing rain is falling. (Link here to read more about the process of freezing rain.) In addition, various parts of the storm will have clouds made of supercooled water over places where no precipitation is falling. In fact, the warm air flowing into the storm and upward along the slope between cold and warm air normally wraps around the storm’s center, creating another area where icing is possible.

While falling snow, which is made of ice crystals, doesn’t stick to airplanes in the air, you can’t always be sure that "snow" is always nothing but snow. At times freezing rain can be mixed with snow and the rain drops are not always obvious.

Okay, if you really want to be a forecaster of winter precipitation...read this.

At times, especially on the ground, snow that’s not mixed with freezing rain can leave ice on an airplane. In the mid 1990s, scientists from the National Center for Atmospheric Research, who studied 10 takeoff crashes caused by ice, found that snow that falls when the temperature is between about 25 and 31 degrees Fahrenheit is wet. When it falls onto an airplane, the water in the snow can freeze into ice. Dry snow on an airplane’s wings quickly blows off as the airplane picks up speed early in the takeoff run.

But, if the snow is wet, the ice it has left on the wings won’t blow off. In the crashes the scientists studied, ice on the wings kept the airplanes from developing enough lift to take off.

In several of the 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, dry snow flakes that are likely when the air is colder than 25 degrees, block more light than the smaller, wet snow crystals, reducing visibility more than the smaller crystals. Poor visibility can make pilots more alert to the potential danger of snow than they would be when the visibility is better, but the snow is really more dangerous.

The National Center for Atmospheric Research explains why that is in an article (in red) called "How Snow Can Fool a Pilot." Read that material.

As as result of the mid-1990s study, devices are being installed at some large airports to measure the water content of snow. Airlines, and others, can use this data when they decide whether aircraft should be deiced on the ground and if so, how much fluid should be used.

The lesson for pilots who don’t have access to the deicing trucks that spray jets as the await takeoff, should be to never assume that snow will blow off the wings when you start the takeoff roll. If snow has accumulated, you should always make sure that it’s all snow, not ice.

If the snow is falling while the winds are blowing from around 9 to 15 mph, the researchers found there is an added danger. Normally you will taxi downwind to get to the end of the runway that heads into the wind for your takeoff. While you’re moving downwind on the taxiway, a 9 to 15 mph wind can pack snow on the top of the airplane’s wings. If the snow is wet, you can pick up dangerous ice while taxiing.

As a storm moves on toward the east, cold air flows in behind it, usually clearing the sky and creating good flying weather, at least until the next storm moves in. Even after the sky begins clearing, you still might have to wait a while for comfortable flying. Cold air often arrives with strong, gusty winds that can create turbulence equal to that of the worst, unstable spring day. You should check the winds aloft because they can still be strong after surface winds have calmed down.

Even after the winds die down, clear skies can bring another hazard that is easily overlooked.
Temperatures usually drop as the sky clears because the water drops in clouds absorb heat that’s radiating away from the earth. The water drops, in turn, radiate heat, including back toward the ground. The result is that if everything else is equal, a clear night will turn colder than a cloudy night.

As the air cools on a clear night, it can reach a temperature -- called the frost point -- at which water vapor in the air begins to sublimate onto objects, such as the grass or an airplane that’s sitting outdoors. Sublimation refers to water vapor turning directly into ice without first condensing into water.

The ice formed on objects such as airplanes by sublimation is frost. Often you can hardly see frost on an airplane’s wings, especially if they are white. Even if you can see some frost, it seems pretty harmless. But it’s not. If you take off a glove and rub your hand across the frost it can feel like fine-grained sandpaper. This roughness is enough to disturb the smooth flow of air across the wing, which reduces lift. Researchers have found that frost can reduce lift by 5 to 10 percent. This can be enough to make a safe takeoff impossible.

Parking the airplane so the sunlight falls on the wings, which can help melt frost even when the air temperature is a little below freezing. Sometimes you see pilots and would-be passengers rubbing the wings vigorously with rags to melt the frost. The easiest way to get rid of frost is to push the airplane into a heated hanger, if one is available, to melt the frost, or help melt it as you continue rubbing the wings with rags.

If you push the airplane inside to melt frost, you need to make sure the melt water doesn’t end up in places where it could refreeze and interfere with the movement of the flaps and ailerons.

In some parts of the country, fog becomes more of a problem in the winter than during during other seasons.
Valley fog is a common winter hazard in many parts of the West. When the winds are calm, cold air flows down the mountains into a valley where there can be enough moisture to condense into a thick fog. While the air next to the ground is cold, the air above is warm. This is an "inversion," which keeps air near the ground from rising and air aloft from descending. Such "mixing" of the air would clear the fog; without it the fog stays. Winter’s short days with the sun low in the sky means that there is often not enough solar heat to "burn off" the fog. Many times valley fog will hang around for days until the winds of a new storm arrive to scour out the valley.

When the ground is cold, especially when it’s snow covered, warm, humid air moving in from the south will cause widespread "advection" fog.

Meteorologists use the term "advection" to refer to the horizontal movement of air. As warm air is moved, or advected, over cold ground, the ground cools it to the point at which its water vapor begins condensing to form fog.
Despite its hazards and hassles, winter can also bring beautiful flying.
When the cold, clear air has settled in with only light breezes and you’ve cleared the frost off the wings -- or even better kept the airplane in a warm hanger over night so there’s no frost and the seats are warm as you sit down - and pulled onto the runway for takeoff, you’ll find the cold air improves your airplane’s performance. The airplane will seem to leap off the runway.

Once above a snow-covered landscape in the stable air that can follow a storm, the air is likely to be so clear that you see distant mountains you’ve never seen before from above your home airport. With the next storm a day or two or maybe longer away, you can turn to improving your flying skills or just enjoying being in the air on a sparkling day.

Winter Flying Hazards—Icing

by
Terry T. Lankford

Although not restricted to the Winter Season, induction system icing seems a good place to begin a discussion on icing. Induction system icing consists of carburetor ice and any other ice blockage of the induction system.
Normally aspirated engines can develop ice in the carburetor throat. Conditions most favorable for carburetor ice are outside air temperatures between 0? C and 20? C, relative humidity greater than 50%, and low-power settings; although ice can form with outside air temperatures as high as 32? C. As air accelerates through the carburetor and fuel evaporates, temperatures can be lowered as much as 35? C. Whether ice will develop depends on the velocity of the fuel/air mixture, outside air temperature, humidity, and carburetor system.

Carburetor ice is detected in aircraft with fixed pitch propellers by a loss of engine RPM; in aircraft with constant speed propellers by a loss of manifold pressure. At the first indication of carburetor ice, power loss, or engine roughness, apply full carburetor heat. Be prepared; this will result in additional power loss and engine roughness. Leave the heat on until the engine smoothes out, which might take several minutes. Avoid using partial heat unless the aircraft is equipped with a carburetor air temperature gauge. At times it might be necessary to leave heat on for an extended period. If carburetor heat is left on, be sure to relean the mixture.

Carbureted engines are more susceptible to icing during reduced-power operation. Some manufacturers recommend the use of carburetor heat during all power reductions, others only when ice is suspected. If full power is required, such as a go-around, full carburetor heat and full power might cause detonation or engine damage. It will certainly prevent the engine from developing full power, which might be critical in low-power aircraft at high-density altitudes. Know and follow the manufacturer's recommendations.

I had remained over night in Amarillo, TX. The Cessna 150 was parked into the wind when torrential rains moved through the area. The following morning was clear, temperature in the 15º C, and nearly 100% relative humidity.
The first clue of trouble was the increased throttle setting required to obtain idle RPM; engine runup also took more throttle than usual. I suspected carburetor ice because of a water saturated air filter.

At full throttle the engine developed only about 2200 RPM. The increased ground run to rotation speed—about 7000 feet—should have been another clue. I was off the ground, with no runway remaining, and 200 feet of altitude when the engine started losing RPM! I applied carburetor heat and the engine was running very rough producing about 1700 RPM.

There was a tremendous psychological urge to reduce carb heat and get back the RPM. I was preparing to crash straight ahead, but the engine was still producing power. I decided to make a 180? turn and land on a taxiway. Then I informed a surprised tower controller of what happened; remember, a pilot's first job is to fly the airplane. This is a perfect example of having the clues and ignoring them. I was extremely fortunate.

Induction system icing takes place anytime structural icing occurs. Ice forms at temperatures below freezing on elements of the induction system which are at temperatures of 0° C or below. Symptoms are a gradual loss of power.
On one flight we encountered light icing after an ATC clearance to climb. Periodically carburetor heat was applied. With carburetor heat on, the engine ran fine; off, the engine faltered. The air filter had iced over; the carburetor heat in the Cessna 172 was functioning as an alternate air source.

There is a tendency, especially with new or low-time pilots, to overestimate airframe icing intensity. A recently rated instrument pilot, after experiencing his second encounter with icing in a Cessna 172, reported severe icing. The encounter lasted about 30 minutes, the pilot was unable to maintain altitude and forced to descend. The situation was certainly severe; however, the description is only of light to moderate intensity! Icing intensities are contained in the Aeronautical Information Manual and other publications.
Behold...the AIM portion that tells you about airframe icing...



7-1-20. PIREP's Relating to Airframe Icing

a. The effects of ice on aircraft are cumulative-thrust is reduced, drag increases, lift lessens, and weight increases. The results are an increase in stall speed and a deterioration of aircraft performance. In extreme cases, 2 to 3 inches of ice can form on the leading edge of the airfoil in less than 5 minutes. It takes but 1/2 inch of ice to reduce the lifting power of some aircraft by 50 percent and increases the frictional drag by an equal percentage.

b. A pilot can expect icing when flying in visible precipitation, such as rain or cloud droplets, and the temperature is between +02 and -10 degrees Celsius. When icing is detected, a pilot should do one of two things, particularly if the aircraft is not equipped with deicing equipment; get out of the area of precipitation; or go to an altitude where the temperature is above freezing. This "warmer" altitude may not always be a lower altitude. Proper preflight action includes obtaining information on the freezing level and the above freezing levels in precipitation areas. Report icing to ATC, and if operating IFR, request new routing or altitude if icing will be a hazard. Be sure to give the type of aircraft to ATC when reporting icing. The following describes how to report icing conditions.

1. Trace. Ice becomes perceptible. Rate of accumulation slightly greater than sublimation. Deicing/anti-icing equipment is not utilized unless encountered for an extended period of time (over 1 hour).

2. Light. The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the deicing/anti-icing equipment is used.

3. Moderate. The rate of accumulation is such that even short encounters become potentially hazardous and use of deicing/anti-icing equipment or flight diversion is necessary.

4. Severe. The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the hazard. Immediate flight diversion is necessary.

EXAMPLE-

Pilot report: give aircraft identification, location, time (UTC), intensity of type, altitude/FL, aircraft type, indicated air speed (IAS), and outside air temperature (OAT).

NOTE-

1. Rime ice. Rough, milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets.

2. Clear ice. A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets.

3. The OAT should be requested by the AFSS/FSS or ATC if not included in the PIREP.
Study Questions:

1. Why should you warm the engines before you begin to fly?
2. According to Col Prichard, what is the worst surface to takeoff from?
3. What is a "supercooled" water droplet and why is it important to aviators?
4. What is sleet? How is it formed?
5. What is freezing rain? How is it formed?
6. What is snow? How is it formed?
7. Which is more dangerous to pilots, snow above 25 degrees which is smaller and has better visibility, or bigger snow below 25 degrees with reduced visibility?
8. What is the added danger with winds between 9 and 15 mph?
9. How is frost formed? Is it harmless to aviation?
10. What is advection?
11. What temperatures and humidity are right for induction icing?
12. According to the AIM, what is the temperature range for icing?
13. What is the definition of TRACE?
14. What is the definition of LIGHT?
15. What is the definition of MODERATE?
16. What is the definition of SEVERE?
17. Is HEAVY an official term to describe icing?
18. What is rime and clear icing?