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The holdover time associated with anti-icing and de-icing procedures varies, depending on the prevailing weather conditions and temperatures. The time of the protection will be shortened...
  • A
    when the aeroplane skin temperature is higher than the ambient temperature.
  • B
    in heavy weather conditions.
  • C
    in light wind conditions.
  • D
    in light precipitation.

ICAO Doc 9332
Chapter 4. Holdover Time (HOT)

4.2 HOT is the estimated time the anti-icing fluid will prevent the formation of ice and frost and the accumulation of snow on the protected (treated) surfaces of an aeroplane. These HOTs are generated by testing fluids under a variety of temperature and precipitation conditions that simulate the range of weather experienced in winter.

4.3 Numerous factors that can affect the de-icing/anti-icing performance and HOTs of de-icing/anti-icing fluids have been identified. These factors include, but are not limited by the following:

a) type and rate of precipitation;
b) ambient temperature;
c) relative humidity;
d) wind direction and velocity; including jet blast;
e) aeroplane surface (skin) temperature; and

f) de-icing/anti-icing fluid (type, fluid/water ratio, temperature).

CAUTION
Owing to the many variables that can influence HOTs, the time of protection may be reduced or extended, depending on the intensity of the weather conditions. Heavy precipitation, high moisture content, high wind velocity and jet blast can reduce HOT below the lowest time in HOT guidelines. HOT may be reduced when aircraft skin temperature is lower than outside air temperature.
Weather conditions for which no HOT guidelines exist are referenced in the HOT guidelines.

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