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Many aircraft have this line above the door as seen in the photos below. What is it, and what is its purpose?
Source
777
Source 777

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up vote 36 down vote accepted

These are rain gutters.

They are designed to catch rain that runs off the upper surface of the aircraft fuselage and channel it away from the open aircraft door so that the water does not enter the cabin.

A jet-bridge may partially deflect rainwater in some cases, but the gutter helps mitigate water exposure for the aircraft interior and passengers. Even gutters cannot completely eliminate such exposure since their size is necessarily limited by aerodynamic constraints

"Rain Gutters" is the technical name, at least for Boeing aircraft, such as the B777 depicted in the photos in the question.

Here is a Boeing reference to such gutters on the 787:

Rain gutter: When customers found that water was not being properly deflected over a passenger entry door, they requested a change. Boeing relocated the gutter to function more efficiently. The solution means happy customers, who had fewer water and maintenance issues, and happier and drier passengers. This was a significant enough change to process to require certification work with the FAA.

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12  
This was a significant enough change to process to require certification work with the FAA. You know I'm not usually one to gripe about onerous regulatory requirements, but sometimes you just have to facepalm. – voretaq7 Feb 10 at 17:51
    
Yeah, I thought that was noteworthy. I suppose there might have been legitimate reasons if moving or reorienting the gutter affected structure somehow. I don't know how Part 25 certification processes go. – Jonathan Walters Feb 10 at 17:53
4  
"Rain gutter" is also the name for the analogous structure on your house that stops water falling off the root onto you. – David Richerby Feb 11 at 4:45
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I like it how the description focuses on the large amount of happiness produced – Hagen von Eitzen Feb 11 at 15:58
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I assume their alignment approximates that of the airflow during cruise. – sdenham Feb 11 at 18:26

It's for the passengers on a rainy day. If this strip would not be diverting the rainwater flowing from the upper fuselage, a curtain of water would soak the passengers upon entering or leaving the aircraft, and the cabin floor.

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