Boeing, NASA and a host of international partners are seeking to develop new cockpit instruments to reveal previously hidden dangers that can rob engines of power at altitude.
According to NASA, there have been nearly 100 documented cases since 1989 of turbofan engines losing power when flying near, but not in, highly convective clouds.
Those cases include a Beechjet 400A that experienced dual flame-out at 38,000ft (11,600m) in November 2005 while flying in clear weather but near cumulonimbus clouds. The pilots later performed a successful dead-stick landing in Jacksonville.
Engineers Tom Ratvasky and Peter Struk at the NASA Glenn Research Centre have been studying the problem for four years, along with Boeing, Canadian and Australian authorities and other NASA centres. They say the most plausible theory is that unseen ice crystals associated with the convective clouds, potentially at densities much higher than water content typically associated with airframe icing, are causing the engines to flame out.
Ratvasky says the ice crystals may "reside momentarily on the stator blades" of the compressor portion of an engine, changing to liquid and thereby lowering the temperature of the blade to the freezing point, after which more ice accumulates and causes the engine to flame out and potentially damaging the blades.
"They're flying in what looks like nominal conditions, with nothing on the radar," says Ratvasky. "There's no reason to think there are any issues, but they have engine power loss events. In most cases the engines relight and the flight continues."
NASA in 2008 had secured and restored two surplus US Navy Lockheed S-3B Vikings for in-flight sampling campaigns to gather evidence about what is happening in the convective areas, data that would allow them to modify existing windtunnel engine tests with ice crystal injection and to develop warning or avoidance sensors for the cockpit.
The S-3B was to fly its first mission in autumn 2008 over Puerto Rico, carrying a suite of sensors mounted on its drop-tank pylons. Upon further analysis, however, NASA determined in 2009 that the S-3B did not have the performance to be able to carry such an external sensor suite to the 36,000ft test altitude.
Instead NASA is running an open competition with several vendors that can supply a research aircraft capable of performing the test, which the researchers hope to perform initially in the three-month Australian monsoon season that starts in January 2012.
Source: Flight International
