These tests determine performance characteristics for equipment that must operate when exposed to icing conditions that would be encountered under conditions of rapid changes in temperature, altitude and humidity.
Three icing test procedures are specified according to the category for which the equipment is designed to be used and installed in the aircraft (see Subsection 24.3).
Note: The selection of icing category depends on equipment location in (or on) the aircraft and the type of icing conditions expected. These conditions must be considered by the equipment designer in evaluating these requirements, which are determined by the end application and use of the equipment. These tests generally apply to equipment mounted on external surfaces or in non-temperature controlled areas of the aircraft where rapid changes in temperature, altitude and humidity are generally encountered.
These procedures specify test methods for evaluating the effects of various icing conditions on the performance or aircraft equipment, namely:
The effects of external ice or frost adhering to it
The effects of ice caused by freezing of water condensation or by re-freezing of melted ice
The effects of ice build-up caused by direct water exposure
The following categories cover the anticipated ice formation conditions generally encountered in aircraft.
Category A
This test is applicable to equipment mounted externally or in non-temperature-controlled areas of the aircraft, where ice or frost may form due to condensation when the equipment is cold soaked to extremely low temperatures and subsequently encounters humid air at above freezing temperatures.
Category B
This test is applicable to equipment with moving parts where such movement could be prevented or impeded by ice formation, or where forces resulting from the expansion of ice could damage structural or functional components. The ice formed in or on the equipment results from condensation, freezing, melting and/or re-freezing and may progressively accumulate water or ice inside non-sealed enclosures.
Category C
This test is applicable to items mounted externally or in non-temperature-controlled areas where there is risk of accumulating free water, which could subsequently freeze on the cold surfaces of the equipment. The test is intended to examine the effects of a representative thickness of ice on the performance of the equipment or to determine the maximum thickness that can be permitted before de-icing action is necessary. The required thickness and distribution of ice and any requirement for progressive build-up of ice shall be defined by the APPLICABLE EQUIPMENT PERFORMANCE STANDARDS.
