HOLDOVER TIMES (HOT)


Each aircraft manufacturer has adopted the SAE charts for their specific aircraft. Fluid type & manufacturer, temperature on a/c time or OAT, fluid mix ratio (fluid/water) and prevailing precipitation factors are used to determine an estimated time. It is imperative to use the correct, authorized HOT chart indicating the amount of effective time in keeping surface contamination from forming. These charts do not and cannot take into account all the many factors that influence the HOT. Hold over time starts when you begin the last fluid application.


There are five SAE HOT charts approved for type IV fluids, because there is a difference in the performance from one manufacturer to the other.


When the deicing/anti-icing fluids start to lose their effectiveness they become dull, losing their liquid shine or gloss and solid contamination will start to form or build up. Most fluid manufacturers recommend an inspection that requires touching the surface if in doubt as clear ice is nearly impossible to see with the naked eye.


If the anti-icing fluid becomes contaminated you must start the process over, from the beginning, with the deicing process.


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 to:


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).


The air operator should publish the HOTs in the form of a table or diagram to account for the various types of ground icing conditions that may be encountered and the different types and concentrations of fluids used. A range of HOTs for a particular condition is recommended to account, to some degree, for the variation in the existing local meteorological conditions, particularly the aeroplane skin temperature and the rate of precipitation being encountered.


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