Winter Weather’s Dirty Half-Dozen

Darkness
By last September, the hours of darkness caught and passed the amount of daylight in every 24 hours in the Northern Hemisphere. By the time the cold of winter set in, the number of daylight hours, depending on your latitude, shrank to eight or less in the northern parts of the contiguous United States. In Alaska, the sun barely made an appearance.

With such short days, we find ourselves flying trips in darkness that only a few months ago would have been routinely completed in daylight, or at least with twilight landings. And every study of general aviation accidents shows that a hugely disproportionate number of accidents happen in darkness compared with the total exposure of hours flown at night.

This added risk of night flying is not so apparent in scheduled airline flying, or even in the activities of the major freight lines that operate mostly at night. But if you think for a moment about the difference between how and where the major airlines fly, night or day, that situation makes sense.

The foremost difference in the night safety record is that the big airlines use big airports. And the airlines almost always use runways that have glideslope guidance, electronic or visual or both. Also, the airlines are almost always in radar contact and are being vectored to intercept the approach. The controller not only provides course guidance with the vectors, but also assigns safe altitudes and monitors the crew to be sure it maintains those altitudes.

Compare the pilot workload and attendant risks of being vectored to an ILS approach to a 7,000-foot or longer runway with the typical general aviation airport operation. The airline crew is well-trained and current in night operations and has every bit of help available from the system. The general aviation pilot might be a little rusty on night flight, is typically headed for a runway that is not served by an ILS or other vertical guidance, and often descends below radar coverage to intercept the final course.

Also, the runway lighting at airports used most often by general aviation pilots is minimal with only widely spaced edge lights of marginal brightness, no touchdown or centerline lights, and no approach-light system to help you align with the runway. It's not such a great mystery why the airlines post essentially the same safety record in darkness as they do in daylight, or why GA pilots don't.

Darkness is the one climate-related winter flying risk that is absolutely predictable, and the only way to minimize that risk is to operate differently than you would with longer days. An obvious risk mitigation is to go out and practice night flying as winter approaches. The required three takeoffs and landings to a full stop in order to be passenger-carrying current are a good start, but not really enough. It is much better to fly a few short trips in the system so you remember what it's like to deal with the cockpit lighting and how to scan the instruments when more visual clues are missing.

You should also reconsider airports you will use at night. For example, a nonprecision approach to a short runway surrounded by hilly terrain might make sense in daylight, but not darkness. Giving up some ground transportation convenience can be an excellent safety trade after the sun has gone down.

Wind
Winter weather systems are usually more potent than those in the summer, with higher pressure in the highs and lower in the lows. That means the wind blows harder as air rushes between faster moving weather systems, and surface wind is a significant risk to all pilots.

Crosswinds are the most apparent challenge, and every airplane has a limit. At some point a crosswind can overpower the control authority available to stop the airplane from drifting sideways. The best pilot in the world can move the controls only to the stops, and if that is not enough to keep you over the runway, the airplane has exceeded its crosswind limit.

There are also unpublished, but very real, overall wind limits for most airplanes. Again, a pilot can position the controls to counteract the force of the wind while taxiing, but at some point the wind can overpower those control inputs and flip the airplane. Even if the wind is more or less on the nose, a strong enough gust can knock many general aviation airplanes off of their wheels.

Handling the risk of winter winds is the same as for dealing with darkness: Be aware that it's a risk, and set limits on your flight operations to minimize the risk.

Fog
In most areas of the country, winter weather brings more fog. As you know, fog is usually caused by a contrast in temperatures. Warm air over cold ground can do it, as can cold air and warm surface temperatures. Winter brings plenty of both conditions.

Fog occurs year-round, but winter fog is often much more widespread than the local patches that develop in the warmer months. One of the most common widespread occurrences of fog happens over the Plains and is called upslope fog. The Great Plains gradually gains elevation west of the Mississippi River, reaching more than 1,300 feet in Wichita, Kansas, more than 3,000 feet in western Kansas, and more than 6,000 feet by the foothills of the Rockies. In the winter, warm moist air can be transported westward and, as it climbs with the terrain, the air becomes saturated and fog forms, creating near zero visibility conditions for hundreds of miles.

Forecasting fog — particularly the time fog will dissipate — is notoriously difficult. Very slight changes in air temperature or wind direction can send the fog packing, or cause it to linger.

The only way to minimize the risk of flying on foggy days is to be totally suspicious of the terminal forecast at your destination airport, and alternate airport, and load on lots of extra fuel. Forecasts will be updated frequently on foggy days, so it's vital to monitor those changes and look for trends. Forecasts made before sunup are particularly unreliable, so if you are making an early morning departure, be especially conservative with your flight planning and alternate airport choice.

Flying an instrument approach in foggy conditions is challenging because you never really "break out" of the clouds into decent visibility. An approach-light system with its flashing "rabbit" lead-in strobe is particularly important in the fog because, as you near decision height, the lights sort of ooze into view and you can continue down the ILS seeing the required runway environment lights.

Low Ceilings
Low clouds are fog moved up a few hundred feet into the air, and they are also much more common in winter. The big difference is that there can often be very good visibility below the cloud ceiling.

Low uniform ceilings are most common over the broad middle of the country and are typically the product of a temperature inversion. An inversion is a layer of warmer air that has overrun the cold air below.

The issue for pilots with low ceilings and inversion is that the conditions can be very widespread and persistent, often lasting all day or even for several days. The clouds are frequently lower than the minimum descent altitude on many nonprecision approaches, so airports without an ILS or GPS LPV become unusable.

Low ceilings with good visibility at the surface are in some ways even more hazardous than fog, because we are tempted to cheat on minimums. With foggy conditions, flying lower doesn't make anything better, but with a low ceiling and good visibility, we know that if we just descend another couple hundred feet we will break out into good conditions. It takes strong self-discipline to stay at the MDA when you know the runway is just ahead and the visibility would allow for an easy landing if you could just get down below the clouds.

To avoid the potential trap of low ceilings, the only answers are very generous fuel-reserve planning and excellent IFR flying skills so that you nail all approach altitudes exactly to give yourself a safe chance of making the landing.

Snow
Snow is a serious winter flying hazard because it can be difficult to forecast and because it cuts forward visibility to near zero.

Snow showers are a particularly potent problem for pilots flying VFR, because their formation and location is impossible to predict with accuracy. The best the forecasters can do is indicate that the conditions are present for snow showers to form, but when and where that will happen is nothing more than a guess.

So-called instability snow showers are potentially the most hazardous to VFR pilots because they occur in otherwise excellent flying conditions. After a weather system moves on, the air behind is often cold, clear and crisp — and unstable. In this case, "unstable" means the air is cooling rapidly with altitude. Because the air is unstable, the warmer air near the surface rises rapidly. In the warm months, thunderstorms could form in such conditions, but in the winter, the small amount of moisture available is literally wrung out of the rising air in the form of snow showers.

Instability snow showers are typically scattered to broken in an otherwise clear sky, but if you fly into the snow column, forward visibility will be near zero. You can see straight down, but not the horizon, so VFR conditions are gone.

Snow is not particularly hazardous for airplanes in flight because it will not stick to the airframe. On most airplanes, a small strip of snow will adhere at the stagnation point on the leading edges of the wings and tails, and many pilots believe that is airframe ice. It's not. The air is dividing to flow over and under the surface at that point, so a small amount of the snow just stays there and doesn't build.

Snow can be mixed in with unfrozen supercooled water droplets that do create airframe ice, but that is not the norm. Usually the atmospheric conditions that create snow freeze all moisture available. I don't know how many times I have flown from an area of rain into snow without accumulating airframe ice. I don't know how or why that can happen. It seems like there would need to be a transition zone between rain and snow where the supercooled droplets would be present to make ice, but that's not usually what happens.

An issue when flying through snow, particularly very dry cold snow, is precipitation static. P-static, as it's called, forms when the snow particles rebound after impact with the airframe. This process leaves behind a small electrical charge, and soon the entire airplane has a different electrical potential than the air around it.

The first sign of P-static will be noise in the com radios and loss of VOR reception, or at least an unsteady VOR indication. GPS reception usually hangs on pretty well as the P-static builds, but even it can be lost in a severe encounter.

The windshield is an excellent P-static collector because it is nonconductive, so the charge can't flow toward the trailing edges and static wicks to be dissipated back into the atmosphere. If you want to experiment, hold your hand near the windshield when flying in snow. If the charge is small, you can feel the hairs on your arm stand up. If the charge is significant, your finger will get zapped. A really big charge makes Saint Elmo's fire with the spooky greenish glow, usually forming at the edge of the windshield. I have seen arcs between the windshield and the whiskey compass, particularly at night.

P-static isn't particularly dangerous in itself, but it can become a hazard if it persists, because of lost communication and navigation capability. The only solutions are to change course to exit the snow, or to try a different altitude where the snow is less dense; or the character of the snowflakes may be different enough to not form P-static.

Slippery Runways
Snow and ice are a problem in the air, but when they collect on the runway, the slippery conditions are a hazard for all pilots. Sliding off the runway on landing is an obvious threat, but on takeoff the lack of traction can send you off the runway if you have to abort, or send you sliding off the side of the pavement in a gusting crosswind when airspeed is below control effectiveness.

The worst aspect of slippery runways is that there is no uniform way to measure and report the conditions. Controllers ask pilots for braking action reports, and we respond with the terms "good," "fair" or "nil." But what one pilot experiences as fair could turn out to be nil for another.

A reason for the differences in braking action reports is often the airplane itself. A jet with a good antiskid braking system may find the stopping action to be fair, but the pilot of a piston airplane with basic brakes and no antiskid protection may have a totally different experience. The size and number of main gear tires, and even their pressure, can make a difference in traction. An effective spoiler system that kills most residual lift from the wing also makes a big difference in braking action for airplanes that have the spoilers versus those that do not.

In any case, braking reports are not an issue for most general aviation pilots because many use airports without control towers, so there is nobody to pass on the reports. When you see ice or snow on the runway at an uncontrolled and unbusy airport, there is nobody to ask and it's up to you to decide whether to attempt a landing.

When you believe the runway surface might be slippery, it is prudent to have at least twice the normal required runway length available. Try to touch down firmly in the normal touchdown zone and then immediately test the brakes for traction. If there is any doubt you will have enough runway ahead to stop, immediately go around.

The Bottom Line
The defense against winter's big weather worries always comes down to the same thing, which is to be more conservative and give yourself a lot more margin in every phase of flight. And on some days, particularly at airports with shorter runways or poor night lighting, the real safety solution is to simply wait for better weather. Spring, with its new set of weather challenges, is on the way.