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Why do certain areas on passenger jet wings go dark during a flight? This effect is observed on high altitude where temperature is low. When airplane descends, wings return to their initial color.

Wing during a flight on high altitude. Wing during a flight on high altitude.

The same wing before landing. The same wing before landing.

I've noticed this effect in several flights on different aircrafts. It is clear that these areas are covered with other coating, but what is the purpose of this?

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    $\begingroup$ Not sure but I think the original colour is as you see it at high altitude. When descending moisture condenses on the cold wing and changes the colour. $\endgroup$
    – DeltaLima ♦
    Oct 17, 2014 at 16:24
  • $\begingroup$ @DeltaLima I think you're right that it's about changes in the reflected light, the leading edge gets lighter too. $\endgroup$
    – fooot
    Oct 17, 2014 at 16:26
  • $\begingroup$ I see you have some dust in your camera's lens assembly. You weren't using a Samsung S4 Zoom were you? $\endgroup$
    – Grimm The Opiner
    Apr 1, 2019 at 14:42
  • $\begingroup$ I was using Sony compact camera. So, not only Samsung cameras are apt to colect dust on its sensors. I've taken the camera to service shop a couple of times to remove dust, but it has restored quickly after that. $\endgroup$
    – ayampolsky
    Apr 2, 2019 at 17:31

1 Answer 1

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As DeltaLima said, the change in brightness is probably due to the difference in lighting. The first picture is above the clouds, so the air is clear and the light is primarily from the sun. The second picture is in the clouds, so the air is moist and the light is much more diffuse, coming from all directions. This diffuse lighting and the moisture on the wing surface help to reflect more light. You can see this in the already reflective leading edge as well. In the first picture, mostly the darker sky is reflected, but in the second picture, the brighter clouds are reflected. The reflected light will be more evident in the darker areas than lighter ones.

Regarding the materials, they represent different parts of the wing. The darker section represents the main structure of the wing, so the surface is made of metal for strength. The sections around it do not carry as much load so they are made of composites to save weight. The metal conducts heat better, so it will tend to condense more moisture than the composite section, as the aircraft is descending from cold, dry air into warmer, moist air.

The surfaces do seem to have different finishes. This is probably related to the different types of material there.

Those pictures illustrate your question very well. I don't specifically remember noticing this sort of thing myself, but I will see if I have any myself showing the same thing.

Also... anyone know what plane this is? The pointed ends of the shock bodies, the VG's, and the static wicks outboard of the aileron make me think a 737 classic with winglet retrofit.

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    $\begingroup$ Seems to be a Boeing 757 to me. $\endgroup$
    – DeltaLima ♦
    Oct 21, 2014 at 17:40
  • $\begingroup$ @fooot, thank you for the explanation. It seems very plausible, if skin on different parts of the wing is made of different materials. I'll try to make more pictures in a couple of weeks and post them if they bring new information to this question. $\endgroup$
    – ayampolsky
    Oct 26, 2014 at 7:22
  • $\begingroup$ The aircraft is probably Boeing 737-500. imgur.com/WEuHEcF $\endgroup$
    – ayampolsky
    Oct 26, 2014 at 7:42
  • $\begingroup$ Also, note that there's not only less moisture around to scatter the light - there's also less atmosphere! Compare with being on the ground (blue sky) with being in outer space (pitch-black sky, since there's no Rayleigh scattering). In fact, 80% of the earth's atmosphere mass is in the lowest layer (up to 12km). So by approximation, the sky above you while flying (which you can't see from the cramped windows) is about 20% as blue - i.e., rather dark, giving reflective surfaces a dark look. $\endgroup$
    – Sanchises
    Jan 21, 2015 at 20:57

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