(Also referred to as High Level Ice Water Content)
Previously, the term “icing conditions” has generally referred to conditions where super-cooled liquid drops adhere to cold airframe surfaces. However, ice crystals in significant quantities can be lofted high into the atmosphere by convective activity typical of thunderstorms, squall lines and tropical cyclones.
The crystals do not build up on the airframe (but simply bounce off) and are largely invisible to on-board weather radar and ice detectors. Only engine surfaces and pitot tubes are affected.
Research has found that ice crystal icing can occur deep in the engine where surfaces are warmer than freezing. It has also been noted that ice crystals impacting heated windscreens can result in pilots observing “rain” as the crystals rapidly melt on contact with the heated windscreen.
A number of agencies, such as the FAA, and engine and airframe manufacturers, have conducted various studies into the impact of high altitude ice crystals on aircraft operations. They have identified at least 150 incidents since 1989 where ice crystals caused issues for the flights concerned. High altitude ice crystals in convective weather are now recognized to be the cause of engine damage and engine power loss including engine surge, stall, flameout and rollback. In addition, some examples of engine blade damage have been identified.
An analysis of all reported events concerning ice crystal icing has concluded that the most favourable conditions for high altitude ice crystals are in the vicinity of convective clouds/thunderstorms, with CB anvils and outflow being at high risk of containing ice crystals. Ice particles in anvils have been measured during research flights and found to be very small; about the particle size of baking flour, and can in some cases last for several hours after the CB has decayed.
SIGMETs and SIGWX forecasts do not currently provide specific information about ice crystal icing. However, the inclusion of information concerning thunderstorms, especially in tropical areas, can provide pilots with an indication of the potential risk of encountering high altitude areas of hazardous ice crystals.
See the Australian Bureau of Meteorology’s High Ice Water Content brochure which has further information on high altitude ice crystal icing.