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Replacing airframe panels and inspecting hard-to-reach parts would be easier with screws, I presume. So why are rivets used?

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I read somewhere that there are plans for the planes to be glued. I do not have the source anymore but it makes sense: the stress is distributes, no weak points etc. –  WoJ Jun 14 at 15:18
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@WoJ Some aircraft manufacturers are now using Friction Stir Welding techniques, but I haven't heard of glue being used. –  Lnafziger Jun 14 at 18:14
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Airplanes are riveted, not screwed because they are the product of engineers, not lawyers. –  dotancohen Jun 15 at 6:36
    
@Lnafziger glue is very much on the table for fibre reinforced plastic structures. Only it is mostly termed adhesive and way more expensive. –  yankeekilo Jun 15 at 20:20
    
@yankeekilo Are you referring to resin for composite aircraft? –  Lnafziger Jun 15 at 20:27

5 Answers 5

up vote 26 down vote accepted

Rivets need to press the two parts together AND to transmit shear. The pressure results in friction which is responsible for a considerable part of the load transmitted between two riveted panels. In order to hold up to the maximum possible amount of shear, the rivet needs a smooth surface. A thread would make it much more vulnerable due to the notch effect of the thread.

Since skin panels and flanges vary in thickness, a LOT of different screw sizes would need to be kept on stock so that every combination of panel thicknesses is covered with a fitting screw which has just the right amount of smooth shank length. Rivets, on the other hand, will be hammered to the right length during manufacture, so only a few sizes are needed to cover all those panel thicknesses.

Historically, metalworking tools were much less precise than today. For optimum shear transmission, the bolt must sit tightly in its hole. This could in the past not be done with screws, but only with rivets which grow thicker when set in place. And since aviation regulation authorities are VERY conservative, the easiest to certify joints are those proven rivet joints.

Also, repairability is not so bad with rivets, especially if you can reach only one side. The rivets get drilled out (with a slightly larger diameter drill) and after inspection the parts are riveted together again. And if you need the lowest mass for a given strength, again rivets win over screws. The difference might be small for the individual rivet, but it adds up for the whole airframe.

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  • They are cheap and simple, especially in the volumes needed.
  • They are impossible to open, a useful safety consideration. You only have to check that they are properly inserted from the beginning. They won't shake loose.
  • Flush rivets are aerodynamically good since they can be made flush with the fuselage, which is difficult (or possible?) with bolts I believe.
  • (Blind) rivets are great for complex structures, since you can apply them entirely from the one side. You might be able to use screws if you pre-inserted them and held the thread and rotated the nut, but the nuts would still be outside the skin in the end. wing
  • Furthermore, I believe a protective coating is applied between many (commercial) aircraft panels for moisture and corrosion protection, so you wouldn't want to take them off to start with.
  • There's very seldom any reason to start taking apart aircraft. If it's bad, you can rivet over it or drill out the rivet.
  • Rivets can be made a bit smaller than bolts, saving weight. (Not entirely confident on this one)

Replacing airframe panels and inspecting hard-to-reach parts would be easier with screws, I presume.

Every time you make a hatch it introduces a weak spot into the loaded airframe, such as the skin covering the wings and the cabin, it has to be reinforced to bring it up to the required strength, adding weight. That's why these are kept to a bare minimum.


That's not to say aviation doesn't use screws or has its own versions. One screw derivative is called a hi-lok and is a permanent joining like a rivet. It works much like a classic bolt but has a calibrated diameter between the top hex nut and the bottom nut, so it snaps off when it reaches the correct torque. That way it's got a safety guarantee being attached well.

pic

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It's worth noting that rivets can "work loose" over time, though it's not as common as with screws in my experience. When a rivet works loose you frequently see a "smoking rivet" (with a trail of ground aluminum downwind of the loose rivet that looks like "smoke"). The fix is generally simple (re-buck the rivet and it tightens up). –  voretaq7 Jun 14 at 19:09
    
"Cheap" for given values of cheap for aerospace certified components :D –  yankeekilo Jun 15 at 20:36
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@voretaq7: Its just somewhat scary if you find one of them inside a plane; though it is good for a priceless look of a stewardess if you give it to her after touchdown, asking to be delivered to the captain... –  PlasmaHH Jun 15 at 21:23

Vibrating loose is the major concern.

The panels are generally made from two sheets of aluminum skin (.030 - .060 thick) with a paper honeycomb bonded between them and solid aluminum rails from 1" - 3" wide at the edges. This makes the panels incredibly resistant to flex or compression spread over the surface and extremely lightweight. (You could easily, however shove a pencil through the middle of it (not recommended to try in-flight))

Using screws would a) easily distort the surface of the panels (consider the way a screw is applied (torsion, which causes it to pull itself in) vs a rivet (which only applies force against itself until near-full compression is reached) or b) be highly susceptible to vibrating loose ( particularly since the skin is too thin to contain even a single thread )

It is possible to use screws into the solid edges, but you still have the risk of vibrating loose, plus the manufacturing costs of threading holes or using capture-nuts.

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Could you elaborate on what kind of panels you are referring to? Those do not sound like standard paneling for airliners (no sandwich there). –  yankeekilo Jun 15 at 20:46
    
think the BA BAC111 that had a window blow out because the wrong screws were used, causing the captain to be half sucked out of the aircraft, crew pulled at him to prevent him falling to his death while the copilot made an emergency landing... –  jwenting Sep 3 at 9:45

Flush rivets are used on aircraft for aerodynamic reasons.

Rivets stand up to vibration better than normal screws.

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  1. Rivet Joints have more shock absorbing capacity,
  2. They absorb shear stress better,
  3. They are more leak proof and
  4. They are permanent joints.
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This post does not cite any references or sources. Please help improve this post by adding citations to reliable sources. Unsourced material may be challenged and removed.

    
Thanks for your answer! Can you provide citations or links backing up your statements? –  egid Jun 14 at 18:44

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