Pat could almost taste the apple pie waiting for her at her family’s holiday party. With her airplane configured and the ILS needles perfectly crosshaired, she was peering into the milky white skies ahead for a glimpse of the runway. A few moments later, as the gusty crosswind subsided, Pat saw its dull grey outline and a glimmer of approach lights through the driving snow. Focusing on that small bit of real estate, she notified the tower the runway was in sight and breathed a sigh of relief. During the brief final approach, pulses of snow would momentarily obscure the runway again, but visibility would return a second or two later. Just as she crossed over the runway end lights, however, she felt a buffet and lost all visual reference. Knowing she was just a few feet from landing and anxious to get to the party, Pat continued to flare. Sure enough, it was just a gust and as soon as it passed, visibility returned, but not quickly enough for Pat to see she had been pushed from the centerline and was about to touch down over the runway edge. One main hit the runway while the other buried itself in the soft verge.
Even if runways and taxiways have been cleared of snow by airport crews, pilots should assume there still may be slick spots and braking could be a challenge.
For many, winter means months of cold, dark days and for some pilots, it’s dealing with snow and ice. Flying in the snow requires an additional level of prepara - tion and care. A search of NTSB reports shows hundreds of snow-related avia - tion accidents. The presence of snow can cause anything from a reduction in visibility to loss of control on the ground or in the air. Even a millimeter or less of snow adhering to the wings or horizontal stabilizers can reduce lift by 20% or more. Most accidents in which snow is a contributing factor fall into three categories – loss of spatial awareness, ice accretion and ground excursions.
Nearly all precipitation begins as ice crystals in the clouds. Low winter freezing levels mean that that it often doesn’t melt before reaching the surface and that much of the precipitation we fly through will be snow. As with rain, snow needs a lifting mechanism to form, such as air rising over terrain or rising as it encounters a shoreline (lake effect snow). The atmosphere itself can provide that lifting in the form of fronts, where warmer and humid air is forced to rise over cooler air at the surface. Occluded fronts associated with strong, low pressure systems are often the cause of blizzard conditions.
Occluded fronts occur near the center of the low as the cold front overtakes the warm front. When this happens, warm and humid air is lifted to wrap around the back side of the low, falling as snow into the cold air below. That region behind the low is also where the surface pressure gradient is strongest, meaning strong winds to blow the snow around. A blizzard is defined as having heavy falling or blowing snow, sustained or gusting winds above 30 kt (35 mph or 56 kph), and visibility of less than one-fourth of a statute mile (0.22 nm or 400 m) for 3 hours or more. Blizzards often shut down airports over large areas for a day or more, meaning your alternate may not be in any better shape than your original destination. Fortunately, high pressure and clear skies often set in just after a blizzard, allowing operations to resume quickly. Cutting a trip short and spending the night at an intermediate airport may be the best option.
Snowflakes have very high reflectivity. This is the reason that even light snow can greatly reduce visibility as it reflects and scatters light, including light that was emitted by or reflected from other objects toward your eyes. It also backscatters any light coming from your aircraft, making it even harder to see things in low light conditions. As with haze, snow can diminish depth perception, making it difficult to determine just how far away an object might be. Whenever flying in snow, assume that you are operating in IFR conditions where visibility has the potential to quickly deteriorate or even disappear entirely. Even if you are flying VFR, awareness of obstacles, terrain and altitude minima is critical in snowy conditions.
Snow on runways and taxiways contributes to several aircraft accidents every year. As falling snow makes landfall, it may initially melt, wetting the surface only to eventually refreeze as the surface chills to below 0°C. Snow can also partially melt and refreeze if it is compacted beneath a thick snow layer.
Though airport plow crews work diligently to keep runways and taxiways clear of ice, even their best efforts may leave slick spots. This is often the case in grooved pavement, where plows pack the snow into the grooves and deicer may not be fully effective in very cold temperatures or where drifting snow quickly covers a plowed runway. In snowy conditions, even if the runways and taxiways appear clear, but especially if they are snow covered, pilots should assume there will be slick spots and braking action may be poor.
Importantly, with the reduced visibility of falling or blowing snow, it may be challenging to see other aircraft or ground vehicles. Can you be sure a disoriented pilot isn’t accidentally crossing the active runway at midfield? It’s always a good idea to avoid a takeoff or landing in a snowstorm (or even taxiing) if you can’t see at least as far as it would take to avoid a collision with another vehicle on the ground.
Falling snow can also be an icing hazard for aircraft. Aircraft leaving a warm hangar are particularly prone to snow melting and refreezing on them as the aircraft skin cools down. Similarly, aircraft warmed by the sun before a snowstorm may accrete ice once the flakes start to fall. Just clearing the snow off the aircraft may not be enough, as the time between startup and takeoff may be sufficient to allow the falling snow to coat the taxiing aircraft with enough ice to bring down the departing airplane. If snow is falling, and you suspect it could melt and refreeze on your lifting surfaces in the time it takes to get airborne, a coat of deicing fluid before takeoff is an important safeguard. Pilots should never attempt takeoff if their lifting surfaces are contaminated in any way.