(Lengthy explanation of how this was not an uncontained engine failure per section 372, subsection 85, chapter 47, paragraph 31(b)(2) of some engine manufacturer's manual)
I totally get that according to the "official" definition, this was not an uncontained engine failure.
But to me this is a "spirit of the law vs. letter of the law" thing. The whole purpose of rules forbidding uncontained engine failures is to prevent injuries/did_diedness/property damage if something breaks inside an engine.
Again, no. The spirit of the law has always been avoiding high-energy pieces flying out and severing a wing spar, cutting electric wires or hydraulic lines, or taking out the other engine. There have ALWAYS been things flying out of engines when blades fail.
Now, the issue of the "cowling coming apart when a fan blade fails" seems to be something relatively new, associated with some ultra-high bypass turbofans, that is happening with some frequency, and I agree that it needs to be addressed. But I would not put it in the same box together with uncontained failures.
Yet this is just one of a number of incidents where failures have occurred in the fan section of turbofan engines (United 232, anyone?) and engine bits have been discharged at high velocities, causing injuries, did_diedness, and property damage. So (channeling Evanie here) it seems to me that something should be done about it.
But that is something totally different. That IS uncontained failure. But it is absolutely NOT what happened in this United 777 incident.
In United 232, it was not a fan blade that was shed at high speed, but the whole fan disk fractured and was shed at high speed. This is something that the industry has not totally resolved and we are not anywhere closer to resolve it today than we were 50 years ago. It just can't be done.
For that kind of engine failures, the only remedies are prevention (designs and inspections that make that occurrence almost impossible, and that's why there are so few of these) and mitigation (separating electric lines, fluid lines, adding automatic valves to fuel lines to preserve hydraulic fluid and prevent what happened with United 232, adding redundant structure, etc) with the hope that there will be enough of airplane in working order after the event to be able to end this flight with a safe landing.
And I equally get that when a part becomes "suboptimally attached" within an engine, that said part is likely to exit the engine at some point. But a) straight out the back of the engine is already considered a "proper" place for discharging harmful stuff, as high-velocity air and exhaust gases are already discharged there. And b) even if a part is going to be discharged elsewhere, an attempt should be made to ensure it's discharged at a low enough velocity that it's not likely to do things like rip windows out of the aircraft (Southwest 1380)
I agree. The certification requirements should include provisions that make it almost impossible for the cowling to fail in the event of a contained engine failure. The cowling is actually considered part of the airplane (not part of the engine), so it will be somehow a challenge to include the cowling in the engine containment certification (the engine is tested without a cowling and even less with a cowling of one particular plane). But this needs to be resolved. Another challenge is that, in today ultra-high bypass turbofans, almost all of the thrust comes from the fan instead of being distributed between the fan and the low and high pressure compressor stages (it still is, but not as much as t was before). So you have these large but light (and hollow) blades making a lot of force forward, and then try them not to be launched forward when they are released from the hub, plus it is very possible that the incident will cause an immediate engine surge with the stalling of the compressor and fan and burst of airflow being reversed (like in those incidents where "fire" is shot through the engine inlet). I think it will be hard to ensure that no blade will exit through the front of the engine, but it should be possible to ensure that it does so in a safe-ish way (like extending the reinforced containment ring all the way to the inlet lip).
or sever a bunch of hydraulic lines.
Like United 232? Forget it. If a whole fan, compressor or turbine disk breaks, there is no stopping it. Only option is prevent and mitigate the consequence of the damage.
