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The top section of this green-painted Boeing 737's fuselage was slightly faded and I noticed a few dark-green, rectangular patches on its top. They look like patch kits on a bicycle tyre's inner tube. What are they? My understanding is that hairline cracks/weaker spots in the fuselage can't simply be patched like this?

Boeing 373 fuselage with odd-coloured, rectangular patches on the top

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  • $\begingroup$ Cracks and weak spots are simply patched (after you cut out the cracked/weakened area to keep it from spawning fatigue cracks). $\endgroup$
    – Vikki
    Mar 22, 2021 at 0:19

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They aren't "repair patches" themselves, they are repainted areas where some sort inspection and/or repair was done to the underlying structure, that required removal of the paint in the area. What the inspection or repairs were done? Well, you'd have to have been there to know, but the 737 does have a lot of structural inspection requirements related to fatigue cracking in fuselage frames and skins.

There may be actually be skin outside doubler patches riveted on in those dark spots to reinforce areas where crack repairs were done, but the borders of the dark spots are just paint masking lines and it's hard to tell in the photo what was done.

Another possibility is that surface corrosion was found at those spots and the paint in the area was stripped, the corrosion was blended out and the zone repainted.

They should probably have their paint guy do some mixing to match the fresh paint to the faded paint, and apply the paint with an edge feathering technique to avoid the sharp masking lines (you have the edges of the mask supported above the surface to allow some paint to spread below, to create a soft edge), so that folks in gate areas looking down at their incoming flights would be less alarmed by the Quilting Bee look.

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    $\begingroup$ I think it is better to paint with the fresh color of the same area and wait it to fade and blend in instead of matching the faded color and the matched color fading in its own fashion. $\endgroup$
    – fraxinus
    Mar 22, 2021 at 7:51
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    $\begingroup$ This is getting more into the realm of DIY.SE, but perfectly matching a patch of paint, particularly on an exterior surface that gets long exposure to the elements, can be next to impossible. Even if you get it looking right immediately after painting, the fresh paint and old paint may fade at different rates and the difference can actually increase over time. It'd almost be easier to repaint the whole plane. $\endgroup$
    – Darrel Hoffman
    Mar 22, 2021 at 14:38
  • $\begingroup$ Oh, one other point. This color discrepancy is much more obvious when using bright, vibrant colors like the green in this example. This might be why you don't see this very often, because most planes are painted predominantly white (possibly partly for this reason). The paint can't noticeably fade as much when it was already white to begin with. $\endgroup$
    – Darrel Hoffman
    Mar 24, 2021 at 14:05

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