Guidance on the de-icing and anti-icing of aircraft on the ground
Holdover Time is the effective period of protection against precipitation adhering to the aircraft surfaces, starting from the beginning of the application of the anti-icing fluid.
When aeroplane surfaces are contaminated, they must be de-iced prior to dispatch. When there is a risk of contamination of the aeroplane surfaces at the time of dispatch, these surfaces must be anti-iced.
If both de-icing and anti-icing are required, the procedure may be performed in one or two steps. The selection of a one-step or two-step process depends upon weather conditions, available equipment, available de-icing fluids and the holdover time to be achieved.
Holdover time is obtained by anti-icing fluids remaining on the aeroplane surfaces. With a one-step de-icing/anti-icing the holdover time begins at the start of the treatment and with a two-step de-icing/anti-icing at the start of the second step (anti-icing).
Holdover times are published on the AEA website and the AEA reference document is “Recommendations for De-icing/Anti-icing of Aircraft on the Ground”. Holdover times may also be provided by de-icing/anti-icing fluid manufacturers. These times have been derived from standard tests specified in the Society of Automotive Engineers (SAE) documents. As these tests could not fully simulate the number of variables existing, information has been included with the holdover timetables that indicate factors that may cause the fluid to fail earlier than the published holdover time.
Flight crew training should include information on the construction and use of holdover timetables, and on how to recognise the point at which the duration of holdover protection is reduced and a fluid is deemed to have failed. Holdover time will have effectively run out when frozen deposits start to form/accumulate on treated aeroplane surfaces.
Due to their properties, Type I fluids form a thin liquid wetting film, which provides limited holdover time, especially in conditions of freezing precipitation. With this type of fluid no additional holdover time would be provided by increasing the concentration of the fluid in the fluid/water mixture.
Type II, III, and IV fluids contain a pseudo plastic thickening agent, which enables the fluid to form a thicker liquid wetting film on external aeroplane surfaces. This film provides a longer holdover time especially in conditions of freezing precipitation. With this type of fluid additional holdover time will be provided by increasing the concentration of the fluid in the fluid/water mixture, with maximum holdover time available from undiluted fluid.
The time frame of protection that could reasonably be expected under conditions of precipitation due to the many variables that can influence holdover time should not be considered as minima or maxima as the actual time of protection may be extended or reduced, depending upon the particular conditions existing at the time.
Type II, III, or IV fluid may be applied to clean aeroplane surfaces prior to the start of freezing precipitation. This will minimise the possibility of snow and ice bonding or reduce the accumulation of frozen precipitation on aeroplane surfaces and facilitate subsequent de-icing.