Chapter 2 - Theory and Aircraft Performance

Holdover Time Guidelines - General

22.  Holdover Time tables are referred to as holdover time guidelines because this term more appropriately represents their function in providing guidance to flight crew and the need for the flight crew to use judgment in their interpretation.

23.  Holdover time guidelines provide an estimate of the length of time anti-icing fluids will be effective. Because holdover time is influenced by a number of factors, established times may be adjusted by the pilot-in-command according to the weather or other conditions. Air Operators' manuals must describe the procedures to be followed for using holdover time guidelines. When the guidelines are used as decision-making criteria, the procedures to be followed by the pilot-in-command for varying the established values must also be specified.

24.  The estimated time is expressed as a range in the guidelines and is based upon the type and concentration of the specific fluid, the outside air temperature, and the kind and intensity of precipitation involved. The HOT guidelines are applicable to an aircraft experiencing ground icing conditions and are not applicable to airborne icing conditions.

25.  The time that the fluid remains effective in ensuring a safe take-off is the time from first application of anti-icing fluid on a clean wing until such time as ice crystals form or remain in the fluid creating a surface roughness for take-off that deteriorates the performance or controllability of the aircraft. Holdover time cannot be precisely determined because it depends on many variables. Some of the variables include: prevailing environmental conditions, variation in precipitation intensity, temperature, wind effects and the humidity, aircraft type and its configuration, effectiveness of the treatment on surfaces, taxiing direction relative to the wind and jet blast from other aircraft. The effects of these variables need to be taken into account by the pilot when establishing the HOT value. There is no simple solution to this complex issue.

26.  Transport Canada has, for a number of years, published Holdover Time Guidelines that were the same as those published by the Society of Automotive Engineers (SAE) for generic fluids and were based upon the recommendations of the SAE G-12 Holdover Time Subcommittee. The SAE has chosen to cease publishing generic HOT Guidelines, as of 2002.

27.  The Federal Aviation Administration (FAA) and Transport Canada (TC) jointly support the testing of anti-icing fluids and, with the assistance of the members of the SAE Holdover Time Subcommittee, evaluate the test results and publish the recommended HOT guidelines for the manufacturer specific fluids. The generic table for Type II, III and IV fluids are based on these. This procedure will continue with both the FAA and Transport Canada publishing the HOT Guidelines.