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Comment by mlyle | Hacker News Reader

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.

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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.

> 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.

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?

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

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