Hazelnuts basically nailed it. The problem with Ice Crystal Icing (ICI) is that it's a relatively new phenomena and is poorly understood.
The FAA regulations don't even account for high altitude icing - I understand that EASA has an engine level regulation that's applicable, but I don't think anyone really knows how to show if they comply.
I've been "involved" in ICI issues for the last 10 years but I'm far from an expert (I probably know just enough to be dangerous
) but the threat is something like this:
Conventional icing involves super-cooled liquid hitting cold aircraft surfaces and freezing. Commercial jetliners have typically addressed this threat by heating the threatened surfaces. Conventional icing was not much of threat to the core of the engine because the surfaces were already warm.
Ice Crystal Icing is fundamentally different - it occurs where it is far too cold for super cooled liquid to exist. The ice crystals are extremely small and extremely cold. When they hit the typical cold aircraft surfaces they just bounce off (pilot reports often refer to 'rain on the windscreen' despite TAT way below freezing). However ice crystals in the core of the engine sometimes accumulate and freeze in areas of the engine that are normally far too warm for ice to form (as in 80+deg F/25+deg C). As I understand it (again, I'm not the expert
), the ice hits the warm surface, melts, then additional ice hits and cools the liquid water enough for it to refreeze.
When this ice sheds, it can quench the flame and/or cause impact damage to the compressor.
So far, we've seen ICI flameouts on the CF6-80C2, CF6-80E, PW2000, and the GEnx. The circumstances vary between the various engines (CF6 typically ices up during an idle descent, then a shed/flameout results during the accel for a level-off).
The first few GEnx-2B events resulted in short term rollbacks where the ice quenched the flame, but auto-relight got the fire back before the engine went sub-idle. However in the last couple events, there was also compressor damage, which is what got the regulators so excited. None of the affected engines (to date) have gone sub idle before they recovered, so technically they were not in-flight shutdowns, but with compressor damage there is a real threat of a common mode multiple engine power loss.
The current plan is to mitigate the risk with a software change that will open the bleed valves during ICI to dump enough of the ice overboard to prevent a problem. Time will tell if it's enough
. A large part of the problem is that the conditions are nearly impossible to re-create on the ground (among other things they can't generate ice crystals that small). And even in the convective weather systems that can result in ICI issues, the actual conditions that adversely affect the engines are rare.
While it's true most of the events to date have been the GEnx-2B on the 747-8, the -2B engine has way more operational hours than the -1B on the 787. Since the -1B has also had an event, there is no reason to believe it's fundamentally different.
Now, as to why GEnx engines seem to have a problem and not RR Trent? Figure that one out and I'm sure GE has a fat paycheck waiting for you
