Definition
Water droplets in a cloud or precipitation that remain in liquid form at temperatures below freezing (0°C / 32°F) and are larger than the droplets typically found in icing-certified conditions, generally exceeding 50 micrometers in diameter. Because they are both supercooled and unusually large, they freeze on impact with an aircraft and can strike surfaces well aft of the leading edges, including areas not protected by deicing or anti-icing equipment.
Plain English
Big water drops that are still liquid even though they are colder than freezing. When they hit an aircraft, they instantly turn to ice — and because they are heavy, they can splash and freeze in places further back on the wing where the ice protection systems can't reach.
Context Anchor
Seen in icing forecasts, weather briefings, and discussions of conditions that may be unsafe to fly through.
Derivation
‘Supercooled’ describes water that has stayed liquid below its normal freezing point — a real phenomenon caused by very pure droplets having nothing to freeze onto. ‘Large’ here is a technical threshold: drops bigger than the small cloud droplets most aircraft icing systems are designed to handle. The term is descriptive rather than figurative.
Why Pilots Care
These drops create severe, rapid ice accretion that can reduce lift and control authority, requiring avoidance or certified anti-icing equipment.
Grounding Statement
Picture flying through cold rain where each drop hits the wing as a liquid splash and then freezes in place — including on parts of the airframe that no ice protection system was designed to cover.
Intuition Check
Do not assume “supercooled” means the drops are already ice. They are liquid water below freezing, which is why they can freeze onto the airplane on contact.
Example Sentence 1
After noting ice forming aft of the deicing boots, the crew recognized they had entered supercooled large drops and immediately requested a lower altitude to exit the icing layer.
Example Sentence 2
After encountering supercooled large drops the wing inspection light showed rapid ice formation on the leading edges.