An unusual form of icing threatens aircraft by accumulating on warm metal.
All pilots are familiar with icing caused by supercooled water droplets, which remain liquid even though the temperature is well below freezing. The droplets stick to cold surfaces such as wing edges and turn to ice. This can increase drag and prevent the flaps from working.
By contrast, ice crystal icing occurs inside the engine. The convection currents that give rise to a thunderstorm can carry water vapour to high 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.
When there are too many crystals – high ice water conditions (HIWC) – a deposit builds up that can block air intakes or cause other problems. There may be no warning until the engine loses power.
Ice crystal icing was little known until the 1990s, and the conditions that cause HIWC are still not well understood.
The clouds of ice crystals cannot be detected with existing weather radar, but Nasa recently explored the phenomenon in test flights with a new combination of instruments.
Steve Harrah, principal investigator in the Nasa project, does not expect to detect HIWC directly, but hopes to identify the signature conditions in which it occurs.
This should lead to a sensor that can warn pilots they are flying through potentially dangerous icing conditions.
