[quarterdime]

0.005 inch is indeed quite small, and a demanding tolerance indeed for carbon fiber composite construction. I trust that Boeing design engineers would have know full well that this tight tolerance would cost a lot of money, and would therefore specify it only if necessary. When the out of spec assemblies were discovered, Boeing could either use testing and analysis to show these gaps are OK or rework the aircraft to get them in spec. They chose the latter.

In short, just because 0.005 inch is a small number does not mean the article is grasping at straws. I routinely design mechanical assemblies where the difference between 0.005 inch and 0.010 inch is the difference between a comfortable factor of safety and guaranteed failure under design loads.

[stevage]

Are you able to explain how a tiny gap causes a failure under load? I couldn't picture it.

[ew6082]

Because gaps like this multiply out at the end of a beam. If for example the abutting structural member relies on that joint for support and is 12 feet long (144 inches) and lets say the flange is 6" across, .005/6 x 144 = .12" which is about 1/8 of an inch of wiggle at the end. If your gap were, say .010" instead, there is suddenly 1/4" of wiggle and when things can wiggle like that vibration gets much worse.

[munk-a]

There might be someone on HN who can legitimately answer you but I think this question isn't really helpful to discussion. Some experts have said y should be less than x, instead, y is greater than x... this is a problem. Someone may very well chime in with an explanation about how as long as y is less than 1.2 * x it's actually probably fine, but considering this is a highly technical field and considering the expense of making such a small gap I think it's a good idea to just assume there is some really good for y to be less than x.

Edit: Actually there are some highly technical replies and that's awesome! But I still stand by my point - the time to evaluate whether a test is fair or not is generally not when you're failing the test.

[floxy]

>the time to evaluate whether a test is fair or not is generally not when you're failing the test.

...well, this is a technical forum, and mechanical engineers get these types of questions all of the time. "Hey, the machinist accidentally machined this wall 0.010" too thin. Is it OK to use?" Then you run an analysis, and report something like, no, that's too thin, scrap it. Or, yes, because of X, Y, and Z, this one is acceptable. And maybe this means that you can update the drawing to use a looser tolerance. Or maybe it just means that this one time it is OK, because there is another expensive process that you can do to the part to salvage it. Then there is a procedure to track this non-conformance, until it becomes conformant again. And it becomes part of the permanent record. The missing piece of the article's puzzle is whether this was a critical dimension that should have passed some sort of inspection process. Maybe it should have been a critical (inspected) dimension, but wasn't called out as such on the drawing (a documentation error). Or maybe the inspection was called out, but wasn't done. Or the inspection wasn't done correctly (i.e. inspector reported that it met the tolerance). Or the failed inspection reports were ignored. Or the non-conformance was reported, and an analysis was done and was shown to be fine, but that paper-trail has disappeared. Or in fact all the paper-work is in order, but for some reason wasn't available to the person informing the author of the article. Lots of different rabbit holes to go down here, but we don't have much to go off of.

[kortilla]

This response of “trust the experts” is not interesting, intellectual, or appropriate for this particular forum. There are tons of people here both qualified and motivated to answer this question so telling people not to ask is just generating noise.

[avs733]

What matters isn’t the size of the gap, what matters is the size of the gap relative to the size of the gap it was designed for.

If I design a 10” Diam part to be assembled to another with a .001” gap, then a .010 gap is huge. If it’s a 10’ part that has the same tolerance, a 0.01” gap is still huge.

Tolerances aren’t arbitrary, they are analyzed and the issue is you generally don’t k ow what happens accurately if those tolerance limits are violated.

As for the mechanism, you need to worry not just about a single cycle load to failure, you need to also worry about shortened fatigue life (I,e, failure after many cycles - but many less cycles than predicted). Overall, load transfer is highly complicated in thin skin structures and that the gap is small doesn’t mean that a change in that gap crosses a small change in load

[mjevans]

Not a mechanical Engineer, but I can imagine how.

Both for pieces abutted against each other and for E.G. rivet holes, mechanical interfaces have extremely precise tolerances to support a range of possible stresses. Too wide a tolerance in one area can allow deformation and wiggle that applies unexpected forces on other areas. You should also remember that many aircraft are pressure vessels, since they operate at altitudes where the density of our atmosphere is substantially different.

[lazyier]

Why do you suppose there are gaps in the first place?

Why don't they make it one solid piece? You can do that with composite construction. Just overlap layers and glue it all together.

It could have something to do with how the fuselage change shapes and distorts under different conditions. The airplane goes through various different shapes depending on things like pressurization and thermal expansion. The body gets a bit bigger, the wings flap up and down, things get wider and shorter and harder, etc. etc.

With composite construction things are glued into place, but they need to be designed to accommodate this movement. The glues and such things have a particular amount of elasticity and fatigue limits.

Could be that a 0.005 amounts to 10% less gluing surface and thus the projected fatigue life of the glue is now much different because there is much less.

Just speculating.

[bluGill]

Expansion with temperature changes over the length of a plane is surprising large. They put in gaps along the lenght to take this up so the whole is to size

[assttoasstmgr]

> Why do you suppose there are gaps in the first place?

> Why don't they make it one solid piece? You can do that with composite construction. Just overlap layers and glue it all together.

Are you suggesting they build the entire fuselage as one piece and "glue it all together"? It's an airplane fuselage, not a MacBook chassis.

[throwaway0a5e]

>Are you suggesting they build the entire fuselage as one piece and "glue it all together"?

That's basically how they build ships. The glue is just a little hotter.

It seems doable but QC would probably be a nightmare and it wouldn't be very repairable adding up to it not being an economically sane choice.

Edit: Since apparently this has to be said, they weld ships together creating what is in effect one single piece assembly. The point is that while the techniques for joining fibrous materials are very different, large single piece structures that flex and bow are fairly well understood and there's no reason you couldn't create one out of carbon fiber if doing so penciled out.

[hedgehog]

IIRC a major rationale behind the 787 design is to allow sections to be manufactured complete with wiring harness etc in different locations and then shipped for final assembly. You can build really big parts (check out Janicki) but for this application probably not desirable.

[monocasa]

They definitely stich sections of hull together to make all but the smallest ships.

[throwaway09223]

Their point is that the hull sections are welded, becoming one single solid piece. Ship hulls do not have gaps.

[csours]

If you have multiple parts in an assembly, each with small deviations from the specification, those deviations add up. Now add some substantial load (like a whole airplane), and you can have a real bad time.

Edit: in this case it looks like it was airplane skin panels, some (most?) of which may be stressed members - meaning that it's not a cosmetic piece, it's a load bearing piece. If you have multiple panels with tiny deviations, that changes the loading of the whole structure, potentially leading to warping, flexing, and premature failure.

[inkeddeveloper]

It doesn’t matter. They didn’t meet the standard. It was a standard for a reason and it failed.

[iancmceachern]

Exactly. If the spec was wrong, fix the spec. But we must meet the spec, and we must have specs that are meaningful and accurate.

[Dylan16807]

"Exactly"? I'm pretty sure you just contradicted the comment you replied to by partly explaining why it does matter.

It's a failure of something no matter what, but "it doesn't matter how the gap causes problems" ain't true.

[iancmceachern]

It doesn't matter. It just matters that they cause problems, not how. If the gaps cause problems when outside a certain spec, one needs to ensure the gaps are kept within those bounds. Why they fail outside of those bounds matters not.

[Dylan16807]

To maintain your knowledge of whether the gaps cause problems, you need to know why they cause problems.

And for newer rules, you need to know why to double check that the rules have been made well.

[quarterdime]

This is a fair question but the answer can get complex. Honestly the design/manufacturing of this aircraft joint is way above my expertise and pay grade. I would hope that Boeing has some extremely specialized and talented people working on this. In short, the question of how a gap might affect this assembly is far outside my expertise. However if you want a simple example:

Consider a geometrically perfect cylinder resting on a perfect plane. The contact is a line, with zero width. Therefore a contact area of zero. Pressure is force divided by area. So the nonzero weight of the pin, divided by area (zero) is... infinite? You run into the same problem with a pin in a slightly larger hole. How does this seemingly infinite pressure not lead to failures in wheels (think of train wheels on tracks), ball bearings (spherical balls in torroidal raceways with slight clearance), roller bearings, etc? We are surrounded by geometries that have seemingly zero area points of contact, but they support tremendous loads.

Hertz (yeah, the same guy for whom the 1/s unit is named) figured out the math behind these contact stresses. Basically, for round (and round-ish) things in 2d and 3d, the contact stress has a lot to do with the deformation of the materials. To answer the riddle above (of the cylinder on plane infinite contact stress), you have to consider the deformation of the cylinder and the plane. The stiffness of the materials comes into play, as well as the geometry. You can read up on Herz (or Hertzian) contact stresses if you would like to know more. The math is not terribly difficult, especially for 2d geometries. For a 2d case of a pinned joint, you can often find that a change of a couple thousandths of an inch can mean the difference between a comfortable factor of safety and failure.

I have given a hand-waving example of the importance of tight tolerances on clearances for a small class of problems. I hope it is close enough to the subject matter at hand to be of some use. My comment is from memory, so please forgive (and correct!) any mistakes I've made.

edit: I am rereading my comment, and realize that I didn't make explicit the importance of tight fit for Hertzian contact stress. The smaller the gap between a pin and hole, the greater the contact area (with the same amount of deformation). Think of it this way--for a fixed amount of deformation (say strain at failure), you can carry way more load if the contact area is greater. How do you increase this contact area? By a smaller difference in diameters (smaller gap) of pin and hole. So all things equal (material properties, load), a smaller difference between pin and hole diameters will increase load the joint can carry.

Another point: calculating these contact stresses is doable for most metals, but is far more complex for anisotropic materials (mechanical properties vary in different directions) materials like the carbon fiber composites.

[an1sotropy]

Thanks for these details.

I think others might be forgetting (or not know) that the factors of safety* for the parts in airplanes (around 2, or less?) are very different than factors of safety for the structural parts of bridges (around 5?). Compared on those terms, planes are light and fragile, on purpose, so you can't f around with cheating tolerances.

* https://en.wikipedia.org/wiki/Factor_of_safety

[krageon]

It causes a failure because it is not according to the design. Who knows what happens when you start doing things differently? You're not supposed to add or remove wings and the tolerances should be as described. You don't need to understand the math involved to understand that when an engineer tells you something is important, you should probably listen.

[MAGZine]

I'm not sure why you felt the need to explain to someone who was genuinely curious as to how something might fail, that they shouldn't ask questions.

[krageon]

The line of discussion started out with a frankly confrontational tone and had so far not yielded incredibly interesting results. I read this question in the same tone when I probably shouldn't have, that's fair.

[throwaway0a5e]

> I trust that Boeing design engineers would have know full well that this tight tolerance would cost a lot of money, and would therefore specify it only if necessary

Hahahahahahahahahahahahahahahaha

Source: worked in places that manufactured aerospace stuff.

The .005 is likely the output of some other calculation that got copypasta'd because why not.

[orangepurple]

Don't leave us hanging