Snowhere to Hide

Macro weather and micro weather determine whether a winter trip is cool, or insane


Ill admit right up front that I live for winter. After dealing with the sweltering heat and high density altitudes of the summer, its a relief to have the cooler temperatures.

However, there are the obvious dangers with operating in a snowy climate, to include frost and snow covered aircraft surfaces, snow covered runways, rapidly changing weather, lower visibilities, more hours of darkness and icing, just to name a few.

In snow country, the preparation for a winter flight is more time-consuming than a summer flight because of these conditions, and requires some additional items and procedures.

When planning a winter GA flight, watch The Weather Channel. You can get a feel for the movements and developments of the weather fronts. If you keep track of the movements day to day, you quickly realize whether the fronts are slow, fast or relatively stationary, and how much of a factor the weather might be on your next flight.

Television weather is limited in its ability to give specific aviation forecasts for ceilings or visibility, so make sure you augment it with DUATS briefings or a call to flight service. I tend to call flight service on the night before a long cross-country trip and ask for an outlook briefing. Sometimes its obvious early on that the forecast predicts weather beyond the capability of a piston single or twin. Other times, you get one of those delightful forecasts of crisp clear weather when its an easy lets go fly tomorrow decision.

Unfortunately, there are also plenty of forecasts for marginal weather, which lead to the really tough decisions. Winter flying typically involves more encounters with bad weather.

In a recent study of medical helicopter accidents from 1983 to 2000, I discovered that the most commonly fatal accident involved the inadvertent penetration of IMC. Of these, the vast majority of the accidents involved weather forecasts of chances of occasional marginal VFR weather.

As you weigh the go/no-go decision, the chances of good weather may seem to be in your favor. It is tempting to consider a 90 percent chance of satisfactory weather as an easy go, but that remaining 10 percent can be a tempting trap in the winter time.

There are a number of strategies to combat this. First, consider using the most conservative decision. If there is a chance of bad weather, consider postponing or delaying a cross-country flight.

If the flight is for pleasure, this is an easy call, but those who use their airplanes for serious travel have a fine line to walk. You dont want to cancel a trip unnecessarily, but neither do you want those sweaty minutes worrying if the aircraft can handle the weather.

The Siren Song
If youre not instrument-rated, winter cross-country trips can be real crapshoots. Try to postpone your flight until one of those nice high pressure systems brings widespread VFR conditions to your region for a few days before initiating a cross-country flight.

Even if youre instrument-rated and consider yourself an all-weather warrior, theres no guarantee your airplane will be able to endure the weather demons winter will throw at you.

Always make certain you leave yourself a way out. Dont wait for the weather to get bad before you decide to end the flight and proceed to an alternate. Make the decision early if there is a deteriorating trend.

If you keep your head in the game, you can be pretty sure you can divert to an alternate location that has satisfactory weather. Try to make that decision sooner rather than later.

Of course, the biggest obstacle to flying light planes in winter is icing. Its easy to get into a situation where the ice exceeds the aircrafts ability to fly well, if at all.

A pilot of a light aircraft that has no icing protection should be especially wary of entering visible moisture whenever the outside air temperature drops below 10 degrees C. Despite the claims of some salesmen, even aircraft with boots and heated props have limited capabilities against ice.

The problem with icing forecasts is that the weather service predicts a chance of ice any time the temperature is below 10 degrees C and there is visible moisture. Although airplanes vary in how efficiently they collect ice and how well they fly when encrusted, the bottom line is that any ice is bad ice. The average single-engine aircraft doesnt have the performance capability to deal with anything more than a very brief icing encounter.

Because the weather forecasts almost always call for a chance of icing, the decision is left entirely up to you. You can ask for Pireps to try to determine if anyone has actually encountered icing along that route, but use Pireps with a strong degree of caution.

You can fly two airplanes through the same spot only minutes apart and have one pick up ice and the other come through clean. There has been some excellent atmospheric research performed on icing, but forecasters ability to understand and predict atmospheric icing conditions is in its infancy.

Because of the uncertainties involved – and the high cost of being wrong – consider the lack of a Pirep for icing as a statement that there might not be ice, but there could be.

Words of caution about flying into icing conditions always seem to come off as excessively conservative. There have been many pilots who have encountered icing and come through it with nothing more permanent than a good hangar tale. At the same time, flying in ice without de-ice gear can quickly give you serious doubts about the continued operation of the airplane. In that sense, its like flying in a thunderstorm.

A good preflight weather briefing can give you many of the answers you need to determine where ice is likely. Try to find out where the bases of the clouds are, where the layers are and how high the tops are.

If your airplane can handle it, you might be able to punch through the clouds and cruise on top, but this requires both thin layers of ice-laden clouds and a strong rate of climb. Many airplanes simply dont have the climb capability – particularly as performance degrades with the accumulation of ice.

If you manage to get on top, the ride is usually smooth and sunny and the ice on the airframe sublimates during the cruise portion of the trip. If the airplane runs out of umpf before the cloud does, youll have to drop back down through those clouds, icing up more on the way.

Therefore its helpful to know the height of the cloud bases, particularly in relation to the terrain. If you ice up, you can then drop below the clouds and still stay above the rocks.

However, if the clouds and low temperatures extend all the way to the ground, the door just shut behind you. To avoid getting trapped in the icing layer, sometimes you may have to shoot an instrument approach to descend safely through the clouds, then abandon the approach once you break out underneath and continue on your way.

If you break out into air thats above freezing, youll have time for the ice to melt before you have to slow for landing.

Preflight Actions
One thing thats unique about winter is that just about any form of precipitation is going to cause problems with the aircraft. If the aircrafts skin is cold due to a low ambient temperature, rain that is close to freezing will form a coat of ice all over the aircraft. This is very common when a warm front moves faster than a cold air mass, forming an occluded front.

Occluded fronts can have a severe impact on flight operations. For example, last year a particularly nasty occluded front was forecast to move over the Midwest. Hours before the front moved in, some of the airlines decided to move their aircraft. The aircraft that remained on station were later coated with several inches of solid ice.

In some cases, people were forced to use fire axes to chip away the ice in order to open the aircraft doors. Thousands of gallons of de-icing fluid were required to melt the ice. There is no compromise with airfoil contamination. You must have clean surfaces before you take off, period.

In the winter, a heated hangar is a godsend. It keeps the surfaces clear of snow and keeps the engine from being cold-soaked. An unheated hangar is still good, though the engine will probably require preheat.

If the airplane is outside, it takes longer to get ready for flight.

Brushing the snow from an aircraft is effective only in the case of very cold powder snow that didnt freeze to the aircrafts surface. This type of snow is typical of drier climates where the snow is formed at very high altitudes – the light powdery snow famous for skiing in the Central Rocky Mountains. When both the surface of the aircraft and the snow are cold, the falling snow is less likely to melt and freeze on the surface.

With most snow, however, its a different story. If the aircraft skin is slightly warmer than the snow, some melting will occur when the snow lands. The melted layer then refreezes.

This is very bad news for a speedy departure because a brush isnt going to remove this layer. It is typically a very rough layer with more performance-robbing effects than frost. The rough surface disrupts the smooth flow of air in the boundary layer and hastens boundary layer separation. The net effect is less lift and more drag.

Surface roughness is so destructive to the production of lift that when Dutch engineers roughened the leading edges of an airfoil by some light sanding, the airfoils lost a significant portion of their lift, on the order of 30 percent.

Some pilots try to prevent wing contamination during interim stops by carrying containers of glycol mixture in their aircraft to spray on the wings when the airplane is on the ground. While it may be better than nothing, its unlikely to be sufficient in cases where precipitation is anything other than light snow. To work properly, the glycol solution for de-icing an aircraft should be heated.

Another consideration is that the glycol mixture falls within the definition of hazmat, and should be handled only with proper precautions.

In most airplanes and under most circumstances, if the airplane has the kind of airfoil contamination that it needs to be sprayed with de-ice fluid, you might want to reconsider your decision to fly. A better strategy is to put the aircraft in a heated hangar and let the airfoil contamination drip off. Then wait for better weather.

In any case, the one precaution you need to observe any time your aircraft has been exposed to precipitation is the risk of water running down into the control surfaces and freezing. You usually cant detect the problem when doing the before takeoff check of the flight controls because often it doesnt happen until you have climbed into freezing temperatures.

The same thing also applies to retractable landing gear. Taxiing or taking off through puddles or slush could cause the gear to get frozen in the gear wells.

There are several accident reports in the NTSB database of aircraft that lost control because of frozen flight controls. When you cruise at altitude with an autopilot, the flight control deflections are usually kept small, and thus any water on the flight or engine control cables can freeze.

Its advisable to move the flight and engine control surfaces on a frequent basis to keep any water from freezing the control cables in place.

Sometimes heated hangar space is too expensive to rent, or it simply isnt available. Wing covers can be useful to help prevent airfoil contamination in light precipitation or frost. They can be quite effective in some conditions but are not a guaranteed cure.

I used wing covers to prevent frost on the wings back when I flew gliders frequently and wanted to launch in morning conditions to take advantage of adjacent ridge winds. A friend flying EMS aircraft in Northern Arizona during severe winter weather also used wing covers to protect the wings from surface contamination.

There are many pilots, some even within the FAA, who contend you can polish the ice or frost on a wing to make it safe to depart. However, wind-tunnel tests show that it takes very little surface roughness to degrade the boundary layer. Simply sanding the leading edge of an airfoil can result in a 30 percent loss of lift. Polishing a pair of wings to defeat this kind of performance demon takes more time and effort than most people realize.

You should also realize that the maximum angle of attack decreases substantially if the wing is contaminated. A normal airfoil that stalls at something near 17 degrees angle of attack may stall at 11 degrees or less with airfoil contamination.

The end result is that the wing will stall much earlier and at a much lower angle of attack than you might be used to. There are two ramifications to this. First, that the stall will occur at a higher airspeed, is fairly obvious. Perhaps not as apparent is that the wing will not be producing as much lift – threatening takeoff performance especially.

If you are fortunate enough to keep your aircraft in a hangar, just trying to get the aircraft onto the ramp can be a struggle. Without fail, ice and crusted snow banks always seem to form at the edge of the hangar doors. Its very difficult trying to pull or push an aircraft over these obstacles.

You will first need to clear an area so you can pull your aircraft onto the ramp. Under most circumstances, one person cannot do this without some sort of powered tug. Inevitably the surface is slick and, as you try to exert some force to get the aircraft in motion, you will fall.

Winter flying may have its rewards, but those rewards are sweetened by the fact that it takes a lot more effort to get into the air in the first place.

From preflight planning to airplane preparation, winter operations are both more difficult and time-consuming than similar summer operations.

Also With This Article
Click here to view “Ground De-Ice Like the Big Guys?”
Click here to view “Tricks and Traps.”

-by Pat Veillette

Pat Veillette is a safety researcher who works in the training department of an air carrier when hes not stoking the wood-burning stove.


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