Aircraft de-icing and anti-icing systems are designed to remove or prevent the formation of ice on a limited area of the wings, tail and engine nacelles and would not normally be effective in removing deposits which have accumulated while the aircraft is stationary.
It is most important that all frozen deposits be removed from an aircraft before take-off otherwise its performance may be seriously affected.
This may be due to reduced lift and increased drag resulting from the disturbed airflow over the wing and tail surfaces, or it may be due to the extra weight of the frozen deposit over the whole aircraft.
In addition there could be freezing of moisture in controls, hinges and microswitches or the entry of ice into the engines.
Any measure taken to remove frozen deposits on the ground must prevent the possible re-freezing of liquid during the initial stages of flight when the flight de-icing or anti-icing systems are either not functioning or not yet fully effective.
There are two basic types of aircraft ground de-icing/anti-icing fluids:
• Type 1 unthickened fluids.
• Type 2 thickened fluids.
Type 1 fluids have a high glycol content (minimum 80%) and a relatively low viscosity, resistance to movement, except at very low temperatures. Their holdover time is relatively short. Holdover time is the length of time that the fluid will protect the wing from ice, frost and snow.
Type 2 fluids have a minimum glycol content of 50% and due to a thickening agent, enable the fluid to remain on the aircraft surfaces until take-off. These fluids have a good de-icing performance and significantly longer holdover times than Type 1 fluids, providing protection against refreezing and/or build up of snow.
