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Which of the following will shorten the applicable holdover time (HOT) for de-icing/anti-icing procedures?

1. High wind velocity
2. Light wind velocity
3. Ambient temperatures above 0 degrees Celsius
4. High relative humidity
5. Aeroplane skin temperature below the ambient temperature
6. A low concentration of anti-icing fluid in the anti-icing fluid/water mix

  • A
    1, 2 and 3.
  • B
    1, 2, 3, 4 and 6.
  • C
    1 and 6.
  • D
    1, 4, 5 and 6.
Refer to figure.
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Learning objective Interpret the guidelines for fluid holdover times and list the factors which can reduce the fluid protection time. Source: ICAO Doc 9640 ‘Manual of Aircraft Ground De-icing/Anti-icing Operations’, Chapter 5: 5.1, 5.2 and Attachment ( 5 tables)

ICAO Doc 9640
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).

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.

Some further explanation can be gained from studying the example guideline table for holdover times, which includes differences due to OAT, fluid/water ratio, cold-soaked wings (skin temperature lower than ambient), etc.

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