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Comment by lisper | Hacker News Reader

The truth is that even the reinforcement they are going to do is more for PR than for actual safety. This test was intended to push the plane to its limit. Not close to, but actually up to the limit, so it was actually expected to fail somewhere around this point. What they call 100% for this test is actually 150% of what the plane is expected to experience in a worst-case real-world situation. The difference between failing at 99% and failing at 101% (which is what happened the last time they ran this test) is negligible from a practical point of view. You'll almost certainly get more variance than that in practice just from manufacturing non-uniformities, random environmental factors, age, etc.

Boeing has committed a lot of sins recently, but this isn't one of them.

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Doesn't the incidental damage like blowing out a cargo door suggest that, while they expected to push the plane to close to it's limit, that prior to the test they assigned very low probability to this kind of failure? Even if this barely-a-failure isn't on its own evidence of a practical safety risk, anytime you're surprised by new data, it does suggest something about your model needs to be updated.

Sure. They'll update their models saying "yeah, this can happen sometimes at 148.5% of max designed wing load. So... don't fly the plane into any >F5 tornadoes."

These max overload wing tests are scary dangerous, but they don't represent real flight situations. 99% of their target safety factor is within the margin of error for manufacturing variance.

While they'll most certainly investigate the causes of the damage down the fuselage from the wing rupture, it's not likely much of it will be unexpected - if a plane experienced that load, the plane is considered a Loss of Hull even if it lands successfully somehow.

I'm immensely paranoid about flying on planes, I have a terrible fear of heights... and nothing I've read in that article bothers me in the slightest.

It is possible to design systems that fail in predictable ways (tear-strips on consumer packaging are an example of this) but airplanes aren't designed that way. It just doesn't matter how the plane fails when it reaches its structural limits, so no design effort is put into this. So no, it is not at all surprising that the energy dissipated in unexpected ways. That's one of the reasons they ran the last test past 100% so they could find out how it would fail because no one knew.

> tear-strips on consumer packaging are an example of this

The ring pull on a can might be a better example. Tear-strips are notoriously unpredictable.

Nice video on the design and manufacturing of aluminum drinks cans here: https://youtu.be/hUhisi2FBuw

The whole thing is pretty ingenious, but the way the internal pressure helps vent the can while the tab is acting as a second class lever, then as soon as it’s vented becomes a first class lever to push the scored section of the top into the can — that’s great engineering.

https://news.ycombinator.com/item?id=8098146

Isn't there an ancient joke about an aircraft manufacturer having problems with the wings breaking off, and some engineer suggesting performating the wing roots, because have you ever seen toilet paper actually tear at the performation?

Not always. The pins that hold the engines on to the wings are "fuses" in that they are designed to break away if the engine is vibrating violently, before it breaks the wing.

That's not true at all. A good example of this would be the requirement that under a blade out condition on an engine it remains contained. They even test this, https://www.youtube.com/results?search_query=ge90+blade+out

That's not true at all. A good example of this would be the requirement that under a blade out condition on an engine it remains contained. They even test this

About that…

https://en.wikipedia.org/wiki/Southwest_Airlines_Flight_1380

> What they call 100% for this test is actually 150% of what the plane is expected to experience in a worst-case real-world situation. [..] The difference between failing at 99% and failing at 101%

So, what is it 100% of?

Not that I even care enough about Boeing clickbait to skimread the article, but this discussion makes it seem like "percent" is a really poor fit for whatever this is actually measuring and trying to communicate.

So, first, airliners aren't really expected to experience more than 1.3G's of loading in normal or even unusual operation.

They are expected to be able to survive 2.5G's (limit load) without structural damage. The 777X satisfied this.

They are expected to be able to survive a few seconds of 1.5x this-- 3.75G-- without outright failure but possibly with (irreparable) structural damage. Instead, it catastrophically failed at a loading corresponding to 3.70G's.

They are going to reinforce and fix the specific failure that happened at 3.7G's, and make an argument based on strain gauges and instrumentation that another, distant failure was not likely before 3.75G's.

So, first, airliners aren't really expected to experience more than 1.3G's of loading in normal or even unusual operation.

This is 100% true. But the unexpected happens. China Airlines 006 saw 5G due to pilot error. Everyone survived and the plane (Boeing 747-SP) saw another 25 years of service.

Additionally planes age. What's your failure point after 10-15 years of service?

Another part that's worrisome is the original angle. Originally the failure was reported as a cargo door blowing out. The pictures released show otherwise. So the question remains: who got reporters to go with a cargo door failure and why?

That depends on how much you trust Boeing's modeling and assembly skills. IMO the pickle forks on the NG cast at least a little bit of shade on Boeing's reputation.

With regards to the cargo door story - a door did blow out, but it was a passenger door, not a cargo door. Passenger doors are plug-fit (they're bigger than the hole, and pressure holds them in place), so it's not really possible for a passenger door to blow out. Cargo doors are not plug fit, so a latching failure could cause a door to blow out. Everyone in the industry remembers the DC10 cargo door failures.

I suspect the early rumours said a door had blown out, and as that's not really a possible failure mode for a passenger door, it was assumed it must have been a cargo door. Now, in this case a passenger door did blow out, but it wasn't a door failure. The fuselage some distance below a door failed, and split right up past the door. The partially unsupported door then blew out. But the door was peripheral to the story. Boeing could definitely had handled this much better, but it's not really surprising that in the absence of full facts the rumour mill got this one wrong.

Cargo doors are not plug fit, so a latching failure could cause a door to blow out.

Right. A cargo door failure could be simply user error.

Everyone in the industry remembers the DC10 cargo door failures.

And the 747 ones too hopefully (user error and poor design).

The fuselage some distance below a door failed, and split right up past the door. The partially unsupported door then blew out.

Right, the door failure was secondary.

Boeing could definitely had handled this much better, but it's not really surprising that in the absence of full facts the rumour mill got this one wrong.

Someone at Boeing obviously leaked part of the story (the door failed) and Gates ran with what information he was provided. The question remains: why only leak part of that story? Either failure will look bad to laypersons and both would be dismissed fairly quickly by people more familiar with airliners.

Is the working theory that Boeing PR is simply that incompetent or short sighted?

> Additionally planes age. What's your failure point after 10-15 years of service?

In practice there is/are none, because would-be points of failure are continuously replaced at a schedule well ahead of the number of cycles at which they are known or expected to fail. This is true for aircraft of all sizes, not just passenger jets.

Sure, but what does this mean for the 777X? With the NG, even if you reduce the inspection interval to a few thousand cycles you're only looking spending an hour on an inspection.

How often will you have to inspect the skin (and/or whatever else failed or saw damage) on the 777X? How involved will the inspection be?

The thing I can't quite get over is that something led Dominic Gates to run with "the door blew out" as the story. Why? If a torn fuselage skin on a destructive test is so minor, why go with misdirection? Think of it in terms of MCAS. Conceptually relying on multiple AoA inputs is simple, MAX specific training is (relatively) simple. Yet the MAX is still grounded and we're seeing leaks that indicate a suitable fix is dramatically more complex than Boeing has been suggesting.

> How often will you have to inspect the skin (and/or whatever else failed or saw damage) on the 777X? How involved will the inspection be?

After every lightning strike, 1.5 strikes / aircraft / year on average.

Note that this test also does not factor in fatigue, maintenance wear and tear, etc. Planes fly for several decades and some will fly with quite a bit of imperfections (i.e. cracks) for a while before it's maintenance day

100% of ultimate load, which is 150% of limit load. The limit load is what goes into the flight manuals and is what the aircraft is certified to withstand. The ultimate load is a safety factor on top of limit load and is what the engineers design for.

The writer has some of the numbers mixed up. Transport category positive load limits are 2.5 to 3.8 (depending on max takeoff weight), not 1.3. I don’t know where the 1.3 number came from, but it might be from the loads required to be demonstrated during flight tests (1.3g pull-up at 1.3x the stalling speed in the landing configuration IIRC).

100% of its expected actual failure point, based on its design.

The plane is rated to 2/3rds of that (I think this is pretty standard in aviation) - the other "100%" - meaning it should never go above that in flight, and if it does it would require a very costly teardown/inspection.

> This test was intended to push the plane to its limit. Not close to, but actually up to the limit, so it was actually expected to fail somewhere around this point.

I would assume that even in a failure scenario the fuselage is expected to fail gracefully instead of catastrophically.

By gracefully I mean having a fuse-like failure mode, where the whe system remains operational while a controlled failure is observed in a specific component. For example, a safety valve door blowout or at best a window/door blowout.

This test shows none of that. This test shows that Boeing's planes fail catastrophically.

there is no way to fail gracefully, because what this test does is to test the breaking point. when there is such a stress, there's no middle ground.

for example, look how the wing was tested: https://www.youtube.com/watch?v=Ai2HmvAXcU0

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