To be complete, let me add that engines “breaking off cleanly” is not a good thing because “cleanly” is not reliably all that clean. All else equal I’d much rather lug a failed engine along with me rather than having it break off and do indeterminate but hoped for small damage on the way.
There are scenarios where the engine is damaged or unbalanced enough that it’s creating massive buffeting and vibration threatening to damage the rest of the wing structure, or render the plane uninhabitable / uncontrollable. Those are the in extremis situations where the designed-in weak links fail and the engine falls away. You’d much rather have the pylon separate from the wing than have the pylon shake the wing apart at the same place.
This UAL event was an example where the engine staying attached was as designed, and was the better of the two outcomes.
The good news about an engine is it tends to fall as a single fixed unit. Whatever it hits is flattened but good. But measured generously that’s only ballpark 10 x 20 feet. Anything even a few feet away will be undamaged.
Over a town a shotgun of small parts has a much greater likelihood of hurting people than does a single large lump. Anyone it does hurt won’t feel much for long. So there’s that consolation.
777s with the affected engine, Pratt & Whitney PW4000-112, continue to be grounded until strengthening modifications can be made, and those modifications properly certified. They don’t expect to return to service until 2022. United is the only carrier with this combination of plane and engine. They have 52.
United is “really disappointed” that they can’t return the planes to service. This article didn’t have a link to the full quote, so I don’t know if it’s “we’re really disappointed that we can’t use these planes, but safety comes first, and we’re fixing them as fast as possible” or “we’re really disappointed we can’t use these planes right now, because they don’t blow up very often, and really, nobody was even hurt.” Knowing United breaks guitars, and such, I’m pretty sure I know what they’re thinking, I just don’t know what they said.
United Chief Commercial officer Andrew Nocella had said in July the airline was “really disappointed” they were not able to fly 777s with the PW engines.
“The 777 aircrafts that are grounded are large capacity domestic movers. And we used those for Hawaii and hub-to-hub. And so right now, we’re flying well below where we like to be in Hawaii,” he said. “We really want those aircraft back.”
Yeah, that’s the same thing I saw, but it still doesn’t have the full quote. Not that it matters too much. Mostly I was just reacting to the quote seeming to be very much “me, me, me” with no consideration at all about actually fixing a known safety issue.
Here is an NTSB summary, containing a link to the final report, on the cause of the incident.
Like most incidents related to aircraft, it wasn’t just one thing:
inadequate inspection intervals for the fan blades
the most recent inspection found problems, but did not trigger an appropriate re-inspection
inadequate shielding to contain damage from an “in-flight fan blade out event”
damage allowed fuel into the nacelle
which spread the fire into the thrust reverser, where the fire suppression system could not put it out
It seems to me that number 2 is the linchpin. There were design flaws and flaws in the maintenance schedule, but if the inspection that found problems had properly followed up on the problems, then the engine would have been repaired.
As I was able to piece it together, which is NOT definitive …
P&W wrote a procedure that was itself a bit vague, and the inspection interval in that procedure turned out not to be often enough to detect all problems before they became critical.
UAL performed the inspections at the required intervals.
In one case they found something kinda suspicious and the procedure said “more people should look at these sorts of ambiguous suspicious results”. But didn’t exactly specify who would do that to what standards and what outcomes. UAL regarded the kinda suspicious findings as borderline enough to not invoke the vague “double check” language.
So the part was passed on for continued service. And eventually failed before the next inspection was due.
My overall comments:
Under a close-enough magnifying glass, there are always ambiguities and judgment calls in this stuff. If the standard is perfection, no parts would ever fly. Each testing process and testing tool has tolerances and ambiguities.
It sounds to me like this was 90% a case of parts that sometimes wore faster than P&W’s inspection intervals would catch. And a part that by bad luck was a little too under-worn to reject at one inspection, but a little too over-worn to make it to the next inspection intact. Oops.
With maybe a bit of UAL organizational complacency that “light” wear on these parts is totally normal. For maybe ill-chosen definitions of “light”
