AEROSPACE
Shaken and stirred,
without ice
Aviation accidents sometimes leave clues about risks to flight safety. To safeguard against icing, it is critical to understand and predict how ice accumulates on aircraft structures in various conditions. Andy Pye looks at recent developments in testing and specifications
I
t is difficult to believe that the engine of an aeroplane flying in the tropics in the summer could fill with ice, freeze, and shut down. Yet the phenomenon, known as engine core ice accretion, has happenedmore than 150 times
since 1988 – frequently enough to attract the attention ofNASA aviation safety experts. And it can happen on aircraft fromlarge commercial airliners down to business-sized jets. No accident has been attributed to the
phenomenon in the 23 years since it was identified, but there have been some harrowingmoments in the air. Inmost of the known cases, pilots havemanaged to restore engine power and reach their destinations without further problems. According to the Federal Aviation Administration there have been two forced landings: for example, in 2005 both engines of a Beechcraft business jet failed at 38,000 feet above Jacksonville, Florida. The pilot glided the aircraft to an airport, dodging thunderstorms and ominous clouds. Engine core ice accretion was to blame. Little is understood about ice crystal properties at
high altitude and how ice accumulates inside engines. The enginesmay be warminside at such heights, but the air outside is frosty cold. The prevailing theory holds the trouble occurs around tropical storms in which strong convection currentsmovemoist air
Icing on control surfaces has long been known to be a factor in aviation accidents
fromlow altitudes to high altitudes where the local temperatures are very cold, creating high concentrations of ice crystals. But the properties of the ice crystals, such as their size and howmany of themare in a given volume of air, are hard tomodel. Following two such incidents, investigations and
studies revealed new ice cloud threats – supercooled large droplets and ice crystal/mixed-phase ice. As a result, in 2015North American and European aviation regulators introduced requirements that aircraft be certified to these atmospheric conditions. In response to the newregulations, Canada’s
NationalResearch Council (NRC) has upgraded its altitude icingwind tunnel – one of the fewin the world that can simulate and test aircraft surfaces, components and probes in icing conditions at altitude. “The expansion of our wind tunnel capabilities
allows us to collaborate with aerospacemanufacturers whomustmeet new and evolving regulations to prove the airworthiness of their products,” says Jerzy Komorowski, generalmanager, aerospace at theNRC. “This new capability will enhance aviation safety and ensure safer air travel.” Able to simulate altitudes up to 40,000ft and
temperatures down to -40°C, the altitude icing wind tunnel can test sections and scaled down versions of aircraft, as well as engine components and engine
August 2017 /// Environmental Engineering /// 49
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