An especially snowy winter in some parts of the United States highlights a specific hazard for general aviation pilots: Wet snow.
In fact, flight in wet snow and icing conditions can lead to deadly consequences, according to officials with the National Transportation Safety Board.
While snow is typically thought to consist of all-frozen water, snowfall can also contain liquid particles either on the flakes or liquid particles falling along with the snowflakes. This is what is commonly referred to as wet snow.
These liquid particles have the potential to freeze onto surfaces that the snow comes in contact with — for example, aircraft — posing a significant hazard to the safety of flight through structural, engine, and windshield ice accumulation, NTSB officials warn.
Although FAA Advisory Circular (AC) 91-74B states dry snow is unlikely to pose an icing hazard while wet snow could, the FAA does not define either term. That makes it imperative that pilots review all potential snow forecasts for an icing threat in addition to potential instrument meteorological conditions (IMC), NTSB officials added.
A 2020 NTSB investigation revealed that pilots assumed flight in snow was safe as long as minimum ceiling and visibility requirements were met or that snow conditions were too dry or cold to pose an icing hazard.
Pilots also assumed that snow conditions are safe to fly in as long as you can see through it — they did not consider the effects of icing, NTSB officials said.
These assumptions can result in inadequate reviews of icing-related forecasts or tools.
“A comprehensive review of en route weather conditions, including the likelihood of wet snow and icing, can prevent accidents,” officials noted.
Fatal Accidents
NTSB officials note the agency has investigated several fatal accidents in which wet snow was present, including the April 2018 crash of a Cirrus SR22 that was destroyed after the pilot reported ice accumulation and then hit terrain near Williamsburg, Pennsylvania.
While the pilot received a weather briefing the night before the accident and filed a flight plan, at the time of the accident, an active weather advisory was in effect for moderate icing, instrument flight rules, mountain obscuration, and low-level turbulence near the accident site.
The pilot and his passenger died in the crash
In another accident, this time in January 2018 in Perrysburg, Ohio, a helicopter hit terrain after departing in snow, icing, and IMC for a power line aerial observation flight.
Satellite, radar, and weather observations near the accident site reported low clouds and visibility with light to moderate snowfall at the time of the accident.
Weather models and advisories supported the probability of structural icing in the area at the time of the accident.
Despite accessing the NOAA Doppler radar loop weather imagery depicting these conditions before the accident flight, the pilot elected to depart in snow, which illustrates his incomplete understanding of the hazard, NTSB investigators noted. The pilot and a crew member died in the crash.
What Can You Do To Protect Yourself?
- Be aware that wet snow has the potential to freeze onto surfaces, such as the airframe, engine, or the windscreen, potentially compromising flight safety.
- If you find yourself faced with forecast or observed snow conditions, consider the icing threat and follow icing mitigation or avoidance procedures.
- Be aware that freezing drizzle can coexist with snow. If you are flying into or over areas reporting snow, it is important to understand that the presence of snow does not necessarily mean that icing conditions are not present.
- Stay updated with the latest SIGMET, AIRMET, and CWA information and pilot reports (PIREPs) along your flight route. Pilots are encouraged to ask air traffic control about PIREPs, as well as provide reports to apprise others in the National Airspace System of current weather conditions.
- If your airplane is equipped with deice equipment, know and follow the aircraft flight manual or pilot’s operating handbook for proper use.
- Seek training to ensure that you are proficient and fully understand the features and limitations of the equipment in your aircraft, particularly how to use all features of the avionics, autopilot systems, and weather information resources.
- Be honest with yourself about your skill limitations. Plan ahead with cancellation or diversion alternatives.
Additional resources concerning the hazards of icing can be found at AviationWeather.gov/icing/fip or by using the Forecast Icing Potential/Current Icing Potential overlays on the Graphical Forecasts for Aviation or the icing overlays on the HEMS Weather Tool.
