Supercooled Large Droplets During Winter Operations
There are just a few events that strike real fear in an experienced pilot – one is an in-flight smoke or fire event and another is the loud splat, splat, splat of supercooled large droplets (SLD) hitting your windshield and obstructing your view of everything. Fortunately, neither are common, but the SLD event during winter operations requires quick action, just like the fire does.
The Dangers of SLD
Small liquid droplets that contact an airfoil tend to freeze on a wing instantaneously, forming rough, opaque, white ice deposits called rime ice. This ice is relatively easy to remove with deicing systems.
But if the SLD is large enough, it will be too heavy for the pressure wave traveling ahead of the wing to deflect it. In this case, the droplet will encounter the airfoil and only the part immediately in contact with the surface will freeze. The rest of the droplet will be swept back by the airflow and will freeze beyond the capabilities of the ice protection system. This ice formation leaves a smooth, transparent, relatively difficult to remove, and relatively difficult to see “clear” ice.
How SLD Forms
There are two different atmospheric conditions that produce SLD:
Temperature Inversion
Where a layer of cold air lies beneath a layer of warmer air. Most often associated with warm fronts and stationary fronts. o Freezing rain and freezing drizzle form when snow falls into above freezing temperatures, where it melts. These droplets then continue to fall into the colder air below and refreeze. o The potential for encountering SLD is from the bottom of the warmer air all the way down to the surface.
Collision-coalescence Process
This tends to form freezing drizzle. Far more likely to produce SLD. Droplets collide within the cloud and coalesce into larger droplets. These can be found throughout the entire cloud depth. Look for a maximum cloud height of 12,0000 feet with cloud top temperatures warmer than -12 °C.
What To Do When You Encounter SLD
- Identify The SLD Threat. Ice on cockpit side window panels. o Ice aft of ice protected regions. o Ice aft of normal buildup on spinner. Any other unusual or more extensive ice formations than normal.
- Take Action. Immediately change altitude and exit the conditions. 3000 feet is usually sufficient. SLD conditions are typically found below 12,000 feet. Do not assume there is warmer air above.
- Understand the Danger. Certification for flight into known icing does NOT include SLD conditions. Very few aircraft are capable of safe flight through SLD. It is probable that ice will accrete aft of the ice protection region.
Winter operations have the potential to create dangerous conditions for aviators. Check the forecast before a flight and utilize the SLD forecasting tool at https://aviationweather.gov/gfa/#ice. Keep a flashlight handy for night operations and if anything looks unusual, exit the danger area by changing your altitude immediately.
