Definition
The condition that exists when an aircraft, or a component of an aircraft, has been exposed to very low temperatures long enough that its entire mass has cooled to the surrounding ambient temperature. A cold-soaked structure can chill any moisture that contacts it, often causing fuel-cooled wing skins to form frost or clear ice on the upper surface even when outside air temperatures are above freezing.
Plain English
The aircraft has been sitting in the cold long enough that it isn't just cold on the outside -- it's cold all the way through. Even after it's brought into warmer air, the metal and fuel stay cold for a long time, and that cold can cause frost or ice to form on the surface.
Context Anchor
Pilots encounter this during preflight after an aircraft has been parked in freezing weather, or after a long flight in very cold air.
Derivation
From the everyday sense of 'soak' -- to be steeped in something for a long time. The aircraft has been 'soaked' in cold until the cold has fully penetrated it, just as a sponge becomes thoroughly saturated when soaked in water.
Why Pilots Care
A cold-soaked wing can produce clear ice or frost from rain, drizzle, or even humid air on a warm day -- conditions where a pilot would not normally expect icing. This ice is often hard to see and must be removed before flight.
Grounding Statement
A cold-soaked aircraft may still be deeply cold inside its fuel, oil, and metal parts even after the outside air begins to warm.
Intuition Check
Cold soak does not mean the airplane was briefly cold or got wet. It means the cold lasted long enough to chill the material, fuel, or oil throughout.
Example Sentence 1
After the overnight stop in Minneapolis, the wings were cold soaked, and a light morning rain froze on contact with the upper surface.
Example Sentence 2
Cold soak conditions required extra time for the fuel to warm before the first engine start of the day.