Non-Environmental Icing (NEI)

Description

Cold soaked fuel frost (CSFF) forms as a result of cold soaked fuel in contact with the top surface of the wings cooling the surface towards the temperature of the fuel. If the surrounding moist air cools below its dew-point, frost forms on the wing surface.

This phenomenon is only normally seen when the wing fuel tanks are full.

Cold-soak effect

The wings of aeroplanes are said to be "cold-soaked" when they contain very cold fuel as a result of having landed after a flight at high altitude or from having been refuelled with very cold fuel. Whenever precipitation falls onto a cold-soaked aeroplane on the ground, clear icing may occur. Even in ambient temperatures between -2C and +15C, ice or frost can form in the presence of visible moisture or high humidity if the aeroplane structure remains at 0C or below. Clear ice is very difficult to detect visually and may break loose during or after take-off.

The following factors contribute to cold-soaking:

• Temperature and quantity of fuel in the fuel tanks;
• Type and location of fuel tanks; 
• Length of time at high altitude; 
• Temperature of refuelled fuel, and 
• Time since refuelling.

Non-environmental icing experienced in flight

Anecdotally, CSFF or NEI can be experienced in flight following rapid descent from high altitude, with wing fuel tanks still full (as could be the case when tankering fuel), into warm humid air.