Preflight planning and adhering to your flight department’s safety guidelines make winter operations safer.
By Karsten Shein
Comm-Inst. Climate Scientist
Battling blinding snow and a gusty crosswind, the pilots were doing their best to maintain the glidepath. As the aircraft passed over the snow-covered runway threshold, the pilot in command (PIC) transitioned out of the sizable crab angle he had been holding.
The left main hit hard, then the right. As soon as the nose wheel touched, the copilot deployed the thrust reversers and the PIC stood on the breaks. They had carried too much speed to the short runway, but with the 40-kt wind gusting to 65, the longer runway exceeded the aircraft’s crosswind limits.
As the slowing jet approached midfield, a gust slammed the aircraft that had been rolling over a patchwork of wet concrete, snow, and ice. The combined conditions pushed the aircraft into a skid that took out an intersection sign, collapsed the nose gear, and sent the aircraft sliding to a stop against a pile of plowed snow.
For many pilots operating in the middle and higher latitudes, winter is marked by a variety of aviation weather hazards. From long nights and biting cold capable of coagulating engine oil and hydraulic fluid to icy runways and zero/zero conditions in heavy snow and wind, it takes additional planning and caution to maintain a solid safety margin in winter flying.
In fact, according to the National Transportation Safety Board (NTSB) database, accidents in instrument meteorological conditions (IMC) are over 4 times more likely in winter than summer. While this is partly due to the increased frequency of IMC weather in winter, the weather normally doesn’t cause accidents. Rather, those accidents happen because a pilot didn’t consider the weather appropriately.
As the Earth’s axis tilts away from the sun in winter, it does several things. First, it increases the length of the night. This may not seem like a big deal for pilots, but darkness makes it more difficult to see other objects and judge distance, course, and speed correctly. Many winter accidents involve ground collisions with other aircraft or airport vehicles, and most of those happen at night or in other cases of low visibility.
Second, the increased night means an energy deficit in the atmosphere, so the air at higher latitudes loses more heat than it gains from the shorter length of daylight. This continual loss of heat makes polar air colder and denser, pushing the polar vortex equatorward and strengthening the temperature difference between the polar and subtropical air, which helps to generate strong winter cyclones beneath transient waves in the polar jet.
Winter cyclones are normally well forecast, and most will produce the same set of conditions to different levels of severity. As the cyclone forms, its central surface low deepens and circulates the air around it, drawing warm air poleward ahead of the low, and cold polar air equatorward behind it in a classic warm and cold frontal pattern.
Because even the warm air around these storms is cooler than it would be in summer, it requires far less moisture to saturate. A little lifting or moisturizing can produce low ceilings, widespread fog, and, frequently, freezing rain around and beneath the front.
Preflight awareness of the freezing level and frequent outside air temperature (OAT) checks when flying near a front are essential to minimize the chances for ice accretion. Icing is possible any time there is visible moisture present in sub-freezing temperatures. Liquid precipitation in air between 0° C and –10° C (32° F to 14° F) presents the greatest icing threat.
Also, given the lowered cloud bases and the gradually sloping frontal boundary, pilots approaching a warm front beneath the cloud base, particularly from the warm sector, should rely on their instruments to avoid following a false cloud horizon below the minimum sector altitude (MSA).
Trailing the warm front is the more abrupt cold front displacing warm sector air as the cold air from behind the low is advanced by the system’s circulation. Although the compressed winter troposphere normally limits midlatitude storm tops to under FL400, winter cold fronts can be every bit as active as those in other seasons. Early and late winter fronts are often the most disruptive to aviation, and the rules for thunderstorm flying apply year-round.
The poleward rear quadrant of winter cyclones is blizzard territory. Normally, a midlatitude cyclone will have a trailing high pressure that enhances the pressure gradient, and therefore the wind between them. The strong and gusty winds may be accompanied by low nimbostratus that wrap around the low like a comma head. While limited moisture translates into light snow, when the snow is driven by strong wind, it can create whiteout conditions. The wind will also blow and drift snow that may have fallen beneath the cold front, and the air trailing the low is normally the coldest, which combined with the high wind can rapidly produce frostbite on exposed skin.
Operating in snow and ice
While we can often route around or over much winter weather, and most business aircraft are equipped to prevent or remove inflight icing, the greatest challenges often occur on the ground. Anytime there is falling or blowing snow, pilots should presume the runways, taxiways, and tarmac will be slick. While airports work tirelessly to remove snow, melting agents are generally not used on runways and taxiways.
The first snow falling on a warm runway may melt, only to freeze as temperatures plummet. In heavy, prolonged, or blowing snow, removal is a continuous process that can pack drainage grooves with snow or leave patches of snow to melt and refreeze. Icy spots can be quickly covered. In a blizzard, runways that have been cleared recently can recontaminate rapidly with a loose and very slick coat of blowing snow.
Pilot operating handbooks (POHs) and many flight department manuals provide guidance for takeoff or landing on wet/contaminated runways, but crosswinds should be factored in as well, particularly if you suspect the runway may be slick. At high speeds, even a moderate gust could be sufficient to loosen the grip of the tires and produce a skid. Halving the aircraft’s maximum demonstrated crosswind as a not-to-exceed condition may help to avoid a wind-induced skid on an icy runway.
After landing, rudder steering is recommended until your speed drops, and spoilers and thrust reversers can help maintain sure footing while slowing. Autobrake can avoid putting too much loading on either main, but pilots should switch to manual braking if they notice autobrake is making it difficult to maintain directional control. High-speed turn-offs should be avoided.
On the ground during or after snow or freezing rain, deicing is a must that pilots should plan into their departure times and FBO/FSO requests, as even a thin layer of frost can reduce lift substantially. All snow and ice must be removed before takeoff, even if it is just a light dusting that covers the aircraft during taxi. Barring a long delay, deicing fluids will keep the aircraft free of snow and ice until it is in the sky. Never use thrust reversers to back up in snow or slush, as that will contaminate your aircraft.
Finally, sometimes it is not a blizzard that is the problem, but an intense high pressure that causes headaches for pilots. Pressure altimeters are limited to a maximum of 31.00 inHg (1050 hPa). Winter highs in higher latitudes can sometimes exceed this, rendering pressure altimeters unreliable for flying below the flight levels. Under these conditions, unless a temporary flight restriction (TFR) is issued, pilots may be able to use radar altimeters or GPS to determine their altitude correctly.
Regardless of what weather you encounter, comprehensive preflight planning and sticking to your safety guidelines should help you avoid a weather-related accident.
As always, if you encounter weather that other pilots should be aware of, file a pirep.
Karsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and a NOAA and NASA climatologist. Shein holds a comm-inst pilot license.