Wayne Farley's Aviation Blog - Air Traffic Management

The danger of flying inside highly convective clouds is well documented, but flying outside and near these clouds poses another danger from unseen ice crystals.
Since 1989, NASA has been studying this danger and documented more than 100 cases of turbofan engines losing power when flying near, but not inside, highly convective clouds.
Among the cases was a Beechjet 400A that experienced dual flame-out at 38,000ft in November 2005 while flying in clear weather but near cumulonimbus clouds. The pilots later performed a successful dead-stick landing in Jacksonville.
Engineers studying the problem 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.
The theory is that 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.
Pilots who have experienced this phenomena were flying in what looked like nominal conditions, with nothing on the radar. There was no reason to think there were any issues, but then they experienced the loss of engine power. In most cases the engines restarted and the flight continued.
Together with Boeing and a host of international partners, NASA is seeking to develop new cockpit instruments to reveal this previously hidden danger that can rob engines of power at high altitude.
This phenomena is a reminder that managing the risk of flying is an ongoing battle which pilots and air traffic controllers need to keep in mind when doing flight planning and routing air traffic around convective clouds.