Cold soaked fuel frost (CSFF) occurs when very cold fuel in the wing tanks cools the wing surface. This cooling can lead to the surface temperature dropping close to that of the fuel. If the surrounding air becomes moist and cools below its dew point, frost can form on the wings. Typically, this happens when the fuel tanks are full.
The term “cold-soaked” refers to the condition of airplane wings containing extremely cold fuel. This situation often arises after a flight at high altitude or when the aircraft is refueled with cold fuel. If precipitation falls onto a cold-soaked airplane sitting on the ground, clear icing may develop. Even with ambient temperatures between -2°C and +15°C, ice or frost can appear if there’s visible moisture or high humidity, especially if the airplane structure remains at 0°C or below. Clear ice is challenging to spot visually and may detach during or after takeoff.
Several factors contribute to cold-soaking, including the temperature and amount of fuel in the tanks, the type and location of those tanks, the duration spent at high altitude, the temperature of the refueled fuel, and the time elapsed since refueling.
In-flight, CSFF or non-environmental icing can happen during a rapid descent from high altitude, particularly if the wing fuel tanks are still full. This scenario may occur when tankering fuel into warm, humid air.
