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On the Plane, Snow Is Glistening ...

Source: FAA Safety Briefing - Jan/Feb 2021

Among the many challenges that winter weather presents for aviators, snowfall is likely the most multi-faceted foe. It can severely restrict visibility, make airport surfaces slippery, render signage and lights difficult to discern, significantly increase rollout distances and, for those hangar-less aircraft exposed to the elements, wreak havoc on delicate aircraft parts and surfaces, particularly after a heavy snow. But as the National Transportation Safety Board (NTSB) points out in a recent Aviation Safety Alert (SA), there is yet another critical danger to consider with these falling frozen flakes: wet snow.

Yes, those large clumpy flakes might be fan favorites for snowball makers, but for pilots, they pose a serious threat to safety. Issued September 2020, SA-082 (Flight in Snow) advises pilots to carefully assess the risk of flight in wet snow conditions. The SA stresses that while snow is typically thought to consist of all-frozen water, snowfall can also contain liquid particles either on the flakes or in liquid particles falling amongst the snowflakes. This wet snow can freeze on contact with aircraft surfaces and pose a hazard for structural, engine, and windshield ice accumulation.

snow on aircraft

The SA also mentions an investigation that revealed some common incorrect assumptions pilots have about snow. For example, many believe that flight in snow is generally safe as long as minimum ceiling and visibility requirements are met, or that snow conditions are too dry or cold to pose an icing hazard. The NTSB also recorded in its findings that some pilots believe flying in snow is safe as long as you can see through it. The Alert notes that these assumptions can lead to an inadequate review of icing-related forecasts or tools.

SA-082 details several aircraft accident scenarios where wet snow was present, including a VFR helicopter flight returning from a glacier dog camp that encountered IMC, snow, icing, and gusting wind before impacting terrain. This occurred despite the pilot’s encounter with inflight icing in wet snow on an earlier flight that same day and flight manual prohibitions against flight in these conditions.

So what can a pilot do to avoid and/or mitigate potential wet snow conditions? For starters, it’s important to fully comprehend the potential for icing in these conditions, especially when the outside air temperature hovers around freezing and wet snow is possible. Keep in mind that a cloud that produces dry snow could also contain super-cooled liquid.

When you are assessing risk for flight into snow and potential icing, be sure to review more than just visibility and ceiling conditions. Check en route weather conditions along your route of flight, check for pilot reports (PIREPS), and leverage the current and forecast icing products at aviationweather.gov/icing/fip. You can also check out the Graphical Forecasts for Aviation at aviationweather.gov/gfa or the icing overlays available on the Helicopter Emergency Medical Services Weather Tool aviationweather.gov/hemst. A sound investment might be to seek training on using and understanding these and other weather resources. Browse the course library on FAASafety.gov for online courses and webinars you can take to up your game on icing and winter weather flying. It doesn’t hurt to also take some time to learn more about your aircraft’s equipment features.

Finally, know your limitations. If you suspect you’ll be in over your head on a flight weather-wise, cancel, divert, or make alternate plans. It’s just that simple.For more information, and for a list of helpful tips and resources on how to mitigate the dangers of wet snow, see SA-082 (link below).

Learn More

NTSB Safety Alert 082, Flight in Snow - www.ntsb.gov/safety/safety-alerts/Documents/SA-082.pdf (PDF download)

FAA Advisory Circular 91-74B, Flight in Icing Conditions - www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-74B.pdf (PDF download)

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