Snow on the Runway

Snow that has persisted on the ground for a long time will form crusted layers from wind packing, melting and refreezing. Often the base layer and areas between the crusted layers are hollow. This happens when lighter, fluffy snow consolidates into more dense grain structures. These larger grains, referred to as depth hoar, result from water vapor depositing or desublimating onto existing snow crystals. Granular depth hoar acts like a layer of ball bearings beneath the sheet of frozen crust. Backcountry skiers know this is the stuff avalanches are made of. For pilots, the crusts have a similar catastrophic effect of catching gear or skis resulting in airplanes getting stuck, or worse, getting flipped.

Mike Hart

Thursday, January 07, 2016 at 12:00 AM ET

Verified

The RNAV (GPS) Y Runway 21 approach into Atlanta's DeKalb-Peachtree airport. Jeppesen

Key Takeaways:

Snow is a diverse and dynamic substance, with its various forms (e.g., light flakes, wet slush, graupel) and on-ground characteristics (like crusts and depth hoar) posing unique and significant risks to aircraft operations.
The nature of snow on the ground evolves with temperature, wind, and freeze-thaw cycles, creating hazards such as compacted crusts or unstable depth hoar that can damage aircraft or lead to loss of control.
Effective airport snow and ice removal is crucial for aviation safety and is often regulated; however, pilots must remain vigilant, check NOTAMs, and verify runway conditions, especially at smaller or non-towered airports.
Operating on snow-contaminated runways, whether for takeoff or landing, is highly perilous due to reduced friction, obscured visibility, and hidden hazards, necessitating extreme caution, thorough assessment, and a readiness to divert to an alternate.

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Winter has arrived, especially here in the northern latitudes where the snow season is upon us. Y’all living in the south may not have a lot of experience with the stuff. And you may not think you need it. But knowing a bit about snow is a good idea for most of us, if only because a few hours in an airplane easily can transport us from sunny and dry 70-degree comfort to the hidden dangers of the Great White North.

Spend a few moments thinking about it, and you might share my view that snow is a fascinating substance. For example, it can come down in super light flakes with little moisture content, or it can splatter down in a wet slush. Once on the ground, it can it can gently curl away from a dry runway in wisps carried by wing vortices or form hardened berms and crusts capable of shearing off main landing gear and flipping planes on their backs. If nothing else, it’s a good idea for pilots to reflect on which kinds of snow we might encounter, and how it can influence aircraft operations and risk management.