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.

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.

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

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.

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.

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.

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

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.

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.