tdracer
Good posts!
A couple of added things:
In the recent past, 60% of engine rollbacks or flameouts at high altitudes occur in the Asia-Pacific region, probably because of the propensity of deep updraft convective buildups on the number of airliner routes flown over tropical water in that part of the world. Most occur at cruise or during initial descent. It is now believed that localized areas of high ice crystal density have up to 8 grams per cubic meter of ice crystals or water content whereas the current engine design standard for super cooled liquid water has only 2 grams per cubic meter.
Originally the problem of high bypass turbofan powered large transports at high altitudes between 35K - 41K feet was thought to be relatively the same as encountered by turbofan powered commuter aircraft that generally flew at lower altitudes between 28K - 31K feet altitudes around major thunderstorms. The commuter aircraft fly through clouds at high continuous thrust settings and noticeable moisture in the form of water or ice. However, convective buildups are different over land where most commuter aircraft fly verses deep updraft convective buildups over tropical waters that are supplied continuous water through the updraft cores. These deep draft convective buildups are dominated by ice particles and have little in the way of supercooled LWC (liquid water content).
For the GEnx engines, the solution, tracer pointed out, to end the rollback or flameouts is by opening the bleed valves, it should work so long as the location of ice accretion is not in the HPC beyond any bleed valve points.
It should be noted, that earlier this year for the first time, NASA Glen researchers were able to re-create the high altitude ice crystal cloud environment in the propulsion testing lab during a full scale engine test that resulted in loss of engine power. Although the shape of the ice crystals can't be varied, this is the first step to give the industry a new tool in design, testing and certification of turbofan engines relative to the ice crystal phenomena.