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Comment by ohazi | Hacker News Reader

They're demanding inspections to collect more data, and will revise the inspection interval if deemed necessary.

Turbines are designed so that fan/blade separation failures are contained within the engine casing. Uncontained failures are considered a really big deal, because turbine blades carry a shitload of energy, and whether the blade shoots off into the distance or shreds a wing is essentially luck.

The FAA's response to this (uncontained) turbine failure is typical and expected.

Edit: Apparently this may have been a contained failure. See child comments for details. Leaving the text above alone for context.

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Yeah i can’t see how the parent comment is framing this as bureaucracy. This seems like a solid move for relatively little risk to gather much-needed data. I’m glad the FAA is at least trying. This is their purpose.

> whether the blade shoots off into the distance or shreds a wing is essentially luck

The blade shoots off radially. Critical equipment is not located radially to the turbine blades. Critical equipment is also not located outboard of the engines.

(Interestingly, Boeing learned in WW2 not to run electric wires through conduit. A hit on the conduit would take out all the wires in it. But if it was just a bundle of wires, nicking a couple with shrapnel wouldn't affect the others.)

I'm very impressed that modern engines can contain a turbine burst while still being light enough to fly.

If Boeing really learned from that it wouldn't have run redundant wires in the same bundles...

Doesn’t the redundancy show they learned something?

I don't know to what extent redundancy was there in WW2 aircraft.

While there is no component critical to the engine located radially to the blades, there are other things, like wings and passengers.

The wings have redundant structure and will survive. The passengers are likely why there was a push to contain the blades. I don't know when they finally figured out how to do it, but it's an amazing achievement. It's all part of airliners evolving to be safer all the time.

Note that the same problem exists for propeller driven airplanes. I recall one incident where a prop passenger plane was crossing the Pacific when the propeller assembly came clean off. It sliced its way through the fuselage like butter, screwing up the control cables in the process. The pilots got that bird home by the skin of their chinny chin chins.

If you look on the engine, there is a brownish kevlar sheet that is designed to actually contain the blade. The cowling is not part of the testing process for broken fan blades and is not on the test engines.

Ah, kevlar! The same thing put in bulletproof vests. That makes sense. Thanks for the info!

Consider the DC-10 which had the 3rd engine in the tail. One day, the compressor came apart and cut the lines to all three hydraulic systems. The lines drained out, and the pilots had to fly it back using variable engine thrust to control the airplane. They still lost a third of the passengers on landing, but it remains an incredible display of piloting skill that there were any survivors at all.

Slinging the engines under the wings makes it much easier to design the aircraft to be resistant to turbine burst. Modern airliners are now also designed to not lose the entire hydraulic system if a line gets severed (check valves are used).

This looks like it was a contained engine failure, even though the cowling fell off. A portion of a fan blade was embedded in the containment ring and the other damaged blades is missing but probably went out the front, slicing the cowling on its way.

https://www.flickr.com/photos/ntsb/50971339066/in/album-7215... does not look contained.

Oh, so this is what my initial source was talking about. I guess we'll have to wait for the final NTSB report.

I can't quite tell how far back this is, but it's possible that the failure was contained, but the blade still departed out the back at an angle, and then hit the aircraft further back along the fuselage.

You can't tell from that picture, the damage could have been sustained from the cowling. There is speculation that the damage is from a fan blade but that has not been determined yet. The video of the engine doesn't show any corresponding damage on the side where any fan blade could have exited either.

That's fair. I assume the logic here is that a dislodged cowling or other external engine paneling is traveling at a relatively low velocity when it makes contact with another part of the plane, compared to a fan blade which, if successfully ejected from the engine casing, would be traveling at a very high speed?

Cowling is not designed to hold the fan blades. There is a brown kevlar sheet around the fan that keeps the shrapnel in

That's a composite fairing that just smooths out the join between the wing and the fuselage. It wouldn't take a lot to do damage to it, so a chunk of the cowling is pretty plausible here.

All of this is true except the assertion this incident is an uncontained failure. There's no evidence it was uncontained. Parts exiting the rear of the engine is a contained failure.

http://avherald.com/h?article=4e35503b&opt=0

I only took a quick look, and this source doesn't seem fantastic, so perhaps it's wrong, but anyway... here's my source [1]:

> Engine failures are defined as contained or uncontained. When engine debris leaves the engine, it is an uncontained engine failure. There appears to be a tear in the rear of the fuselage possibly from a fan blade strike. If this is the case, UA 328 suffered an uncontained engine failure.

Edit:

Yeah okay, after looking at the video in your link, I agree that it appears contained.

[1] https://www.travelcodex.com/united-flight-328-boeing-777-eng...

It's considered an uncontained failure if debris leaves the engine through a hole that didn't previously exist - if it falls out the back (or manages to bounce off the containment structure and exit the front), that still classifies as contained. Obviously the difference to people on the ground is pretty semantic[1], but the assumption is that a contained failure should have very little chance of putting the aircraft at risk.

[1] Well, as long as the failure doesn't result in an entire plane landing on their neighbourhood instead of just a few parts

Then why dont the test engines for broken fan blades have the cowling installed?

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