Door blows off Boeing 777X during stress test • Hacker News Board

Re: Door blows off Boeing 777X during stress test

#111

Post by i_am_proteus » Sat, Sep 07, 2019, 3:24 PM UTC

inferiorhuman wrote: post #68

i_am_proteus wrote: post #56

Earlier quoted context omitted.

Very importantly, this hasn't been described as a "test failure." No indication of what the test criteria were, if any. Some tests, especially those past design limits, are just for informational data.

Very importantly, this hasn't been described as a "test failure." That was heavily implied: Sources tell KOMO there was a stunned silence after it happened. … an event occurred that forced the test team to halt testing. … The team is currently working to understand what happened and ensure the area is safe for work to continue. Edit: You're not going to silence the crowd with a successful test and you don't temporari…

Yes, but this isn't the language they'd use if one of the test success criteria was "no door blows out" and then a door blows out.

This is evidenced by their plan to "analyze the door and run the test again." If this were actually a test failure, they'd be "analyzing the door, redesigning a bunch of stuff, and running the test again."

Re: Door blows off Boeing 777X during stress test

#112

Post by donarb » Sat, Sep 07, 2019, 4:45 PM UTC

rolph wrote: post #52

rolph wrote: post #25

ok 1: the wings were bent in an extreme manner. so you have a cylinder to a close approximation, with a hole in the side of it. Then you have two levers [wings] affixed to said cylinder, then you apply force to the levers, and distort the cylinder thus distorting the hole in the cylinder.... at this point i would invoke a toplogical parlour trick of making a quarter fall through a dime sized hole as an analogy. https…

If you dont think this is the case then lets hear your critique, please tell me why you think this is not the case. then if there is time left i can change my most recent actions in this thread ..."The test is meant to push the plane beyond its limits. Engineers had the plane pressurized and on the ground. They loaded it up well beyond capacity and BENT ITS WINGS in an extreme manner, in a way almost certain to never…

That was my thought as well, especially since the carbon fiber wings flex so much more. That flex might induce more stress on the wing mounts which in turn deform the cylinder. The good thing is that these test fixtures are fitted with lots of sensors that record all of the stress.

Re: Door blows off Boeing 777X during stress test

#113

Post by rolph » Sat, Sep 07, 2019, 5:05 PM UTC

donarb wrote: post #112

rolph wrote: post #52

Earlier quoted context omitted.

If you dont think this is the case then lets hear your critique, please tell me why you think this is not the case. then if there is time left i can change my most recent actions in this thread ..."The test is meant to push the plane beyond its limits. Engineers had the plane pressurized and on the ground. They loaded it up well beyond capacity and BENT ITS WINGS in an extreme manner, in a way almost certain to never…

That was my thought as well, especially since the carbon fiber wings flex so much more. That flex might induce more stress on the wing mounts which in turn deform the cylinder. The good thing is that these test fixtures are fitted with lots of sensors that record all of the stress.

In my experience there are strain guages installed all over the air frame for testing and for operation, so the strain history of the frame can be reviewed.

The thing to keep in mind, is the different goals in testing engineered products like this.

there is a thing called Mean Time Before Failure [MTBF] that usually involves sampling of a number of units to arrive at a maintenance and replacement interval.

there is also a need to find the maximim service limits of a product, for example the swash plates and rotor linkage of a helicopter has a threshhold requiring at instance replacement. so for example, if you shook the yoke of a heli quickly left and right beyond a point you will probably knock yourself out of the air, but you will also induce damage requireing immediate replacement.

and then, there is what happened with the airframe of topic. immediate catastrophe during testing. I suspect this wasnt the immediate goal, as the engineers projected it would hold up, and were suprised, luckily not injured.

then the last thing that is usually a pre-production test, is an intentional induction of failure. you have a good idea when its going to break, and you take it there and beyond to gawp at the failure mode and the pieces, and re- engineer it until catastrophe is far beyond extreme duty conditions.[ideally]

Re: Door blows off Boeing 777X during stress test

#114

Post by cmurf » Sat, Sep 07, 2019, 5:51 PM UTC

cameldrv wrote: post #80

kbenson wrote: post #19

Earlier quoted context omitted.

> Here is another video of a 777 wing test where it broke at 154% of the normal load I really hope you mean 154% of the max load, and I hope that max load is engineered to be far above what should be seen in actual service. 50% over the normal service load is not what I would consider extreme, and not something I would expect to be acceptable in an aircraft.

Design limit load is for an aircraft at max allowable weight, with the max authorized g loading (often -1 to +2.5g) or the worst anticipated gusts (about 50 ft/sec depending on altitude). You then need to tack on 50% to get the required ultimate load. 50% is indeed not a huge safety margin like you'd get with a bridge or something, which is why you need trained professionals flying it and regular inspections. If you…

This is why there's maneuvering speed (VA), or turbulent air penetration speed (VB). It isn't necessary to over design it, you just slow down and, voila, the airfoil will sooner stall than break - and the stall is brief, feels like sluggishness, like a squishy slow stall, not abrupt, and recovery is similar perhaps a bit more abrupt as the airfoil regains lift.

Re: Door blows off Boeing 777X during stress test

#115

Post by walrus01 » Sat, Sep 07, 2019, 7:46 PM UTC

Aloha wrote: post #88

walrus01 wrote: post #4

Earlier quoted context omitted.

KOMO is owned by Sinclair broadcasting now, which you can Google about and draw your own conclusions.

How do you think Sinclair's purported bias coloring this story?

I don't, I was referencing the tendency of local TV station websites to be absolutely packed with annoying advertising.

Re: Door blows off Boeing 777X during stress test

#116

Post by cameldrv » Sun, Sep 08, 2019, 6:17 AM UTC

cmurf wrote: post #114

cameldrv wrote: post #80

Earlier quoted context omitted.

Design limit load is for an aircraft at max allowable weight, with the max authorized g loading (often -1 to +2.5g) or the worst anticipated gusts (about 50 ft/sec depending on altitude). You then need to tack on 50% to get the required ultimate load. 50% is indeed not a huge safety margin like you'd get with a bridge or something, which is why you need trained professionals flying it and regular inspections. If you…

This is why there's maneuvering speed (VA), or turbulent air penetration speed (VB). It isn't necessary to over design it, you just slow down and, voila, the airfoil will sooner stall than break - and the stall is brief, feels like sluggishness, like a squishy slow stall, not abrupt, and recovery is similar perhaps a bit more abrupt as the airfoil regains lift.

Sure, as long as the gust is mostly vertical (usually true), and you're able to maintain below Va. All of the up and down motion in a serious thunderstorm can easily cause you to exceed Va or even Vne. Many aircraft have broken up in flight because of this. Now it could be that in many accidents, it wouldn't have mattered how strong the wing was -- eventually the speed build build up to the point where a big gust would break it, but I'm sure there are many where say an extra 50% strength would have made the difference.

Now that's also not to say that if you're going to add weight to an airplane in the name of safety that the best place to do it is the wing spar, but there is certainly a tradeoff between safety and weight.

Re: Door blows off Boeing 777X during stress test

#117

Post by cameldrv » Sun, Sep 08, 2019, 6:21 AM UTC

fluffything wrote: post #107

cameldrv wrote: post #80

Earlier quoted context omitted.

Design limit load is for an aircraft at max allowable weight, with the max authorized g loading (often -1 to +2.5g) or the worst anticipated gusts (about 50 ft/sec depending on altitude). You then need to tack on 50% to get the required ultimate load. 50% is indeed not a huge safety margin like you'd get with a bridge or something, which is why you need trained professionals flying it and regular inspections. If you…

> 50% is indeed not a huge safety margin like you'd get with a bridge or something Safety margins for bridges are like 25-50%.

That's interesting. I know that they used to be a lot more. Perhaps we should be glad that most of the bridges we drive over were built before FEM, and they just used a lot more steel. I wonder how well a bridge designed with a 25% safety margin is going to perform when it's poorly maintained for 50 years and rusting like a lot of bridges in the U.S.

Re: Door blows off Boeing 777X during stress test

#118

Post by banannaise » Mon, Sep 09, 2019, 2:46 PM UTC

> Former Boeing Engineer Dr. Todd Curtis runs Airsafe.com and said this doesn't happen often.

"Well the front's not supposed to fall off, for a start."

Re: Door blows off Boeing 777X during stress test

#119

Post by kortilla » Tue, Sep 10, 2019, 1:52 AM UTC

ChuckMcM wrote: post #84

kortilla wrote: post #79

Earlier quoted context omitted.

90% of $100 is not $190

Interestingly the 154% number seems to be gone from the article now The language is tricky, if you interpret it as I did, or 154% "more" than the design limit, then it is the design limit plus 1.54 more design limits. Like saying you salary is $100 and we're paying you 90% beyond your salary. Then you would expect to get $190. If you interpret it to be 154% "of" the design limit, then I would agree it would be 1.54x…

The video was clear and left no room for interpretation.

Planes are safer because “they can withstand twice their design limit” is senseless. The 2x factor you are thinking of is the design limit. The design limit has all of the huge safety margins built in for the normal load limits.

The planes are designed to take at least 2x the max normal loads.

Re: Door blows off Boeing 777X during stress test

#120

Post by fluffything » Wed, Sep 11, 2019, 11:00 AM UTC

cameldrv wrote: post #117

fluffything wrote: post #107

Earlier quoted context omitted.

> 50% is indeed not a huge safety margin like you'd get with a bridge or something Safety margins for bridges are like 25-50%.

That's interesting. I know that they used to be a lot more. Perhaps we should be glad that most of the bridges we drive over were built before FEM, and they just used a lot more steel. I wonder how well a bridge designed with a 25% safety margin is going to perform when it's poorly maintained for 50 years and rusting like a lot of bridges in the U.S.

The reason this works is that the safety margins accumulate. You have a safety margin on:

* the loading on the bridge, which can be higher than the calculated amount.

* the resistance of the materials used, that can be lower than the requested amount.

* the quality of the soil,

* the lifespan of the bridge,

* ...

So the loading can be 25% percent higher, the materials can be 25% worse, the soil can be 25% worse, the lifespan can be 25% higher, ... and at the end you get a bridge that would still work if everything fails with a margin of 25%.

If the loads are 30% higher, the bridge might or might not fail. Who knows. Even though there are many other safety factors, and the bridge probably won't fail, it was not designed for that. There might be a critical part somewhere that has a resistance that's 25% worse than what it should be, and it therefore can only support a 25% overload, and with 30% that part might fail.

Bridges are designed to fail very slowly, loudly, and visibly, to avoid loss of functionality (e.g. when a bridge start to fail, this becomes obvious, and you still have months or years to sanitize the bridge).

I expect airplanes to have much tighter safety margins (Safety margins aren't chosen arbitrarily. The job of a safety margin is to quantify an uncertainty, and with enough money thrown at each one, most of the uncertainties can be quite precisely quantified. Then it's the job of the designer to say "This plane should be in service for 50 years", and from the uncertainties and statistical analysis you can compute the highest load that the plane will receive in those 50 years with a certain quantified margin of error, and continue the design using that.