Ice crystals accreting inside a hot aircraft engine. Photograph: Nasa
An uncommon kind of icing threatens aircraft by accumulating on warm metal.
All pilots are familiar with icing caused by supercooled water droplets, which remain liquid even even though the temperature is effectively under freezing. The droplets stick to cold surfaces such as wing edges and turn to ice. This can boost drag and prevent the flaps from working.
By contrast, ice crystal icing happens inside the engine. The convection currents that give rise to a thunderstorm can carry water vapour to higher altitude, where it condenses into tiny ice crystals. These crystals bounce off the wings of an aircraft, but on touching a warm engine surface such as a fan blade they partially melt and stick.
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When there are as well numerous crystals – higher ice water situations (HIWC) – a deposit builds up that can block air intakes or trigger other troubles. There might be no warning until the engine loses energy.
Ice crystal icing was little known until the 1990s, and the situations that cause HIWC are nevertheless not nicely understood.
The clouds of ice crystals can’t be detected with existing climate radar, but Nasa lately explored the phenomenon in test flights with a new mixture of instruments.
Steve Harrah, principal investigator in the Nasa project, does not anticipate to detect HIWC straight, but hopes to determine the signature situations in which it occurs.
This ought to lead to a sensor that can warn pilots they are flying via potentially hazardous icing circumstances.