Definition
Liquid water droplets larger than about 50 micrometers in diameter that remain in liquid form at temperatures below freezing. When SLD strike an aircraft, they spread back along the wing before freezing, producing ice that forms aft of the leading edge and beyond the reach of most ice-protection systems.
Plain English
Unusually large raindrops or drizzle drops that are still liquid even though the air is below freezing. Because they are big, they hit the wing and run backwards before turning to ice, so the ice ends up in places that wing de-icers cannot reach.
Context Anchor
Seen in icing discussions, weather briefings, and decisions about whether to avoid or leave clouds, rain, or mist in freezing temperatures.
Derivation
Super-cooled' means cooled below freezing while still liquid. Water normally freezes at 0°C, but very pure or undisturbed droplets can stay liquid well below that until something disturbs them — like contact with an aircraft. 'Large drops' distinguishes SLD from the smaller cloud droplets that produce ordinary rime or clear ice on the leading edge.
Why Pilots Care
SLD can create rapid, severe icing that exceeds the capability of many aircraft de-icing systems and may lead to loss of lift or control.
Analogy
It is like very cold water that somehow stays liquid in a freezer, then turns to ice when it touches something. SLD can do that on the aircraft surface.
Grounding Statement
Picture freezing drizzle hitting a windshield: the drops are big enough to splash and run before they freeze, leaving ice spread across the glass rather than just where they landed. The same thing happens to a wing.
Intuition Check
SLD does not mean snow, hail, or ice pellets. It means liquid water that is already below freezing and can become ice when it strikes the aircraft.
Example Sentence 1
The crew reported SLD conditions after observing ice forming well aft of the wing's leading edge despite the de-ice boots cycling normally.
Example Sentence 2
Unexpected SLD during the descent produced rapid ice on the leading edges.