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I was recently on a RyanAir flight. As far as I know, they use Boeing 737.

When boarding, I noticed a that one of the panels' edge, roughly where the head / chest part of the logo is, was sticking out maybe 4-5 mm compared to the edges of the neighboring panels.

This wasn't something new because the paint above was in perfect shape.

Is it common for panels on a plane's body to not be perfectly aligned like this? I never noticed this on any other plane I boarded.

enter image description here

The best illustration I could find is a bathroom tile picture:

enter image description here

The skin on the right of the door looked exactly this: one panel had one side not flush with the others and intact paint on top of the whole thing.

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    $\begingroup$ You need to provide a bit more detail, like a diagram or sketch. It sounds like you were noticing a skin lap joint (ie, not flush, with a step of perhaps .063" from one panel to the other). Lengthwise skin joints are sometimes done that way because it's a lot simpler and there is minimal drag penalty. Or it's some kind of reinforcing doubler plate or patch that steps down to adjacent skin. $\endgroup$ – John K Feb 8 at 13:50
  • $\begingroup$ I‘ve seen brand new aircraft with joints across the flow direction (i.e. barrel to barrel) with steps of a couple of millimeters between some panels. $\endgroup$ – Cpt Reynolds Feb 8 at 14:10
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    $\begingroup$ Yeah once you are away from the nose, the turbulent boundary layer is deep enough that lap seams don't have a major drag impact. However you will almost always see a sealant fillet to make the joint kind of wedge shaped instead of straight step. $\endgroup$ – John K Feb 8 at 15:55
  • $\begingroup$ I added a picture to better illustrate $\endgroup$ – Thomas Feb 8 at 16:22
  • $\begingroup$ @JohnK No, 4-5mm is about 0.16-0.2in, which is about 3x bigger than you suggest. (Also, it's kinda silly to say "perhaps" and then give a measurement to the nearest thousandth of an inch, as if people can eyeball measurements to about the precision of the thickness of a piece of copier paper.) $\endgroup$ – David Richerby Feb 9 at 10:41
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It looks like you were seeing simple skin lap joints. Flush skin butt joints are generally used on high speed aircraft, but these joints are more complex to make (you have to use a joining splice strip under the butt joint).

A lot of the time the manufacturer determines that the local airflow isn't smooth right at the surface (there is a turbulent boundary layer that is maybe 1/8" deep) and a small step in the surface has a negligible effect on drag. If this is the case, a simple lap joint, that doesn't require the joining splice strip underneath, is used.

There usually isn't a hard 90 degree step on lap joints because it's normal practice to put a fillet of sealant along the edge to make the higher skin taper toward the lower one and this mitigates any drag effects further.

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Yes, different manufactures have different tolerances. When I worked for Continental Airlines ~1998-2004 as a ramp supervisor, I often commented on the difference between Aerobus and Boeing. I preferred to work on Boeing aircraft but Aerobus aircraft were much smoother and prettier.

A good example is the flap track covers. Boeing flap tracks have large gaps and generally sharper corners, while Aerobus has very little if any gaps and smooth fillets. Boeing engine cowls where sometimes so mismatched that it would take two or three of us pushing and twisting to get the latches locked - Aerobus cowlings are a work of art!

Most of my work was on Boeing-757 and you could tell Boeing had started to take closer attention to detail - for example the gears doors and wing/fuselage streamlining was much neater than previous Boeing aircraft. Boeing-737 are very crudely made. It did seem like Boeing was improving the quality on the 777 and even more on the 787. FYI - McDonnell Douglas probably had the worst gaps, seals, and panel mismatch I observed.

enter image description here Aerobus 380 flap track

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  • $\begingroup$ Aerobus? Do you mean Airbus? $\endgroup$ – mins Feb 10 at 16:02

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