I’ve studied SLD (Super-cooled Large Droplet) Icing on aircraft, but the study always left me with more questions than answers. I simply understood that SLD meant that there could be “big drops of water” that stick to the airplane and create a more dastardly result. But, exactly how does it work?
I had a breakthrough in my understanding of the phenomenon when I was taught that water does not always freeze at 32F. For it to freeze at the 32F, the pressure must also be correct. So, water can exist in liquid form in an atmosphere when the pressures exerted on the droplet are less. When the pressure changes, the water will instantly freeze. And…it doesn’t have to be a tiny water droplet…it can be a VERY large droplet…and, as Dr. Thomason demonstrates, even a full bottle of water can freeze instantly.
A good friend of mine (and PA46 owner/pilot), Dr. Thomason made a really cool video that shows how water can be super-cooled and remain in liquid form until the pressure changes.
So, the super-cooled liquid water is present in a cloud or as falling precipitation. When the airplane goes through the air and hits the super-cooled water, it freezes instantly because the pressure increases dramatically. The icing in SLD on the airplane will be clear icing (the worst kind) and because of the large size of the droplets (see the Advisory Circular below) the airplane becomes heavily ladened with ice quickly.
The same phenomenon happens in a non-aviation environment with freezing precipitation. A few years ago we had a terrible power outage in the area that I live because of freezing precipitation. The temperature was freezing at the surface, but due to a temperature inversion, the temperatures aloft were above freezing. So, the rain fell from the warm clouds, entered the cold airmass, but didn’t freeze. When the rain hit the local trees and power lines, it froze instantly due to the change in pressure and stuck. When the weight became too great, the power lines and trees fell causing huge power outages.
Bottom line…water doesn’t always freeze at 32F, but it will when it receives a pressure increase. Pilots can apply this knowledge to icing aloft and gain a firmer understanding of how severe icing can occur in aviation.
For more discussion, here’s a good link to FAA AC91-74A that discusses aircraft icing: Link to FAA Icing A/C