David Hambling reports in NewScientist (2 July 2016, paywall) how “hot ice” or crystsal icing may have taken Air France 447 down – and how it ties into climate change:
… one apparently minor detail is beginning to take on new significance: ice blockage in a sensor called a pitot tube, which measures air speed.
That shouldn’t have happened at 35,000 feet. According to our understanding of weather, it is not possible for ice to form at this altitude. …
Unlike normal icing, crystal icing happens at altitudes where water should not be liquid. The culprits are plumes of crystals around 40 micrometres across, no bigger than grains of flour, which are invisible to weather radar that pick up normal precipitation.
Because they’re solid, these crystals bounce off the wings and other areas equipped with sensors and defences. But when they land on the warmest parts of the plane – such as the engine or pitot tube – they melt. This also happens in contact with the heated windscreen, where they can cause the weird “rain” sometimes reported.
Once a layer has partially melted, it accretes more crystals. When these layers accumulate in the engine, they can stall it, or break off in solid chunks that damage the engine (see diagram).
Ice build-up in the sensors leads to more insidious damage. On flight 447, the ice crystals blocked the pitot tube, which started giving false readings, showing the plane flying too slowly.
[Edit] Here’s that diagram:
[End edit] And the connection to climate change?
There’s even a chance the problem could get worse. “The warmer, moister world predicted by climate change will have more convective instability, ” says Sue Gray, a meteorologist at the University of Reading, UK. “These systems will be more vigorous and more frequent.”
And according to a recent analysis by Rolls Royce engine labs, these increases in extreme weather could make the conditions in which crystal icing flourishes more frequent. In a presentation, Rory Clarkson, an engine specialist with the company, offered an inconvenient but undeniably safe answer: “Restrict operation during severe weather.”
