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I recently traveled on the lower deck of an A380 and the section where the side walls of the fuselage and its roof top meet is curved just like any single deck airplane.

enter image description here
Credits: Wikimedia Commons

If there is a deck above me (which there definitely is) then why does the lower deck have to be curved near the top? Shouldn't it be straight on the sides?

Shouldn't it be straight when there is one more deck above it? I'm sure that it's just the interior that is curved and the external structure is indeed straight but does it serve any important purpose?

Details:

Airline: Emirates
Aircraft: A380-800
Seat: Row 58

Here is what i saw:

enter image description here

Notice that even the roof is curved, is there a structural reason for that as well? On the side wall one of the rows has an extra covering on it, I'm almost sure that is to cover one of the brackets as mentioned in the answer below but even without that the whole cabin is curved.

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    $\begingroup$ Not sure I understand the question - are you asking why is it narrower at the top of the cabin than at the floor? If yes, it’s because the deck is required to fit into essentially a long tube with a blunt but definitely rounded nose... google for an outside view, maybe? Does that help? $\endgroup$ – Cpt Reynolds Jun 18 '18 at 19:43
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    $\begingroup$ What I think he's saying is the vertical curvature of the interior panels doesn't make sense knowing what is above. I think that is just the way they integrated the side panels into the overhead bin structure. It is likely that the resulting space that is in the gap between the curved in panel and the gentler curvature of the pressure hull contains ducting or harness runs. $\endgroup$ – John K Jun 18 '18 at 20:44
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    $\begingroup$ One comparing with the photo in the answer might feel like the curvature in the question photo is not totally explained by the answer photo, but it's clear that the photo in the question is also showing some distortion from the lens and focal length used when taking the picture, so the curvature in the question photo is likely exaggerated. $\endgroup$ – Todd Wilcox Jun 19 '18 at 2:56
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    $\begingroup$ Maybe instead of distortion from the camera, the picture is near the front of the cabin, where it is still expanding out from the nose cone. In this part of the aircraft, there may not be a full deck above you. I've ridden in the upper cabin in front of an A380 (a one-time thing, alas), and can tell you that is narrows considerably an the front, and the curvature of the walls becomes pronounced. The mid deck extends further forward. $\endgroup$ – Paul Sinclair Jun 19 '18 at 3:03
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    $\begingroup$ This is a question, and a misconception I also had. The misconception comes from the fact that people forget that there is a cargo hold, and think fuselage should be evenly shared between upper and lower decks. When you unconsciously think that way, you are surprised when you see lower deck walls curve in. $\endgroup$ – sampathsris Jun 19 '18 at 4:48
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The cabin interior is curved to accommodate, among other things, the brackets holding the upper deck floor beams.

These brackets are a structural feature that transfers the weight of the upper deck to the fuselage with considerably less reinforcement that a straight T-joint. The bracket acts as a small truss element, creating a triangle with the deck and the fuselage that partially turns bending loads into axial ones.

Here is an actual production cutaway, noticeably different from wiki's simplified representation of the A380's cross-section:

A380 cutaway

It would be possible to only wrap the interior lining around the brackets themselves, not along the whole length. But they're not the only thing there (although others could be rerouted; structure requires testing), and this curve saves a bit of weight, so Airbus curves the whole interior by default. An airline could request something different if they wanted to.

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    $\begingroup$ This seems to me to be a much better explanation. Except for those curved brackets, the upper portion of the middle deck walls appear to be decidedly more vertical than the lower portion of the wall which curves in and cut into the leg-room of the victim, er, passenger in the window seat. $\endgroup$ – FreeMan Jun 19 '18 at 19:27
  • $\begingroup$ Having flown economy on a Lufthansa A380, in a window seat, I do not recall the bottom curvature cutting into my leg room. Rather, I recall a few inches of extra room at tray-table level, compared to a 747, making the window seat more comfortable than is normally the case. $\endgroup$ – njuffa Jun 21 '18 at 18:00
  • $\begingroup$ Then I wonder why the bottom floor has none of those brackets. Given that it is designed to hold the full pressure difference between a pressurized cabin and a ventilated cargo bay, the loads on it are substantial. $\endgroup$ – Peter Kämpf Jun 22 '18 at 12:41
  • $\begingroup$ Are you sure? The cargo hold of the A380 is fully pressurized. It receives air from the main deck, there's no pressure difference. Also, the tubular pillars in the cargo hold serve a similar role, and do it even better than brackets (but taking more space). These are almost universal, though not always tubular. $\endgroup$ – Therac Jun 22 '18 at 16:55
  • $\begingroup$ Since THY981 the floor must be able to withstand the full pressure difference, so there is a pressure difference in a mishap. But you are right about the tubular pillars (made from carbon fiber) - they are essential in strengthening the floor structure. $\endgroup$ – Peter Kämpf Jun 23 '18 at 5:05
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Consider this photo of the cross-section of the A380 and the illustration below. You can see the cargo deck on the bottom, the lower deck (where you sat) in the middle, and the upper deck on the top. All have curved walls because the entire fuselage is curved. Straight walls would waste the space between the walls and the curved sides of the aircraft, and no airline wants unnecessarily wasted space.

A380 cross section

A380 cross section, Clem Tillier/Wikimedia Commons

We have a previous question, Why is the fuselage on an airliner circular-shaped?, that addresses why elliptical designs are generally used for modern pressurized jet airliners.

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    $\begingroup$ I didn't embed the image myself because it's not freely licensed (and I'd include a photo credit one way or another). Is that a requirement here? $\endgroup$ – Zach Lipton Jun 18 '18 at 22:24
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    $\begingroup$ I swapped in the public domain illustration (as a sidenote, Clem sure gets around! I usually know him from local rail politics) and kept a link to the photo. $\endgroup$ – Zach Lipton Jun 19 '18 at 6:47
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    $\begingroup$ Whilst I believe this answer is correct to a degree, when considering the size of the A380 the curve visible in the cabin is too severe. I always thought the curve was there to give the illusion that the cabin is wider than it is. It's a feature I hate personally as I like to rest my head against the wall. $\endgroup$ – Ben Jun 19 '18 at 12:16
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    $\begingroup$ I agree with @Ben . The photo appears to show much more curvature than this diagram. I would have guessed the photo was from the top deck, not the middle. $\endgroup$ – zundi Jun 19 '18 at 18:12
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    $\begingroup$ @zundi it's from the front cabin on the lower deck, which is narrower at the point you can see it as it's starting to curve into the nose. $\endgroup$ – Moo Jun 20 '18 at 6:02
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enter image description here
(imgur.com)

Apart from the triangular braces whose curved covers are more prominent than the rest of the wall in the new image added in the question, the cross section does not fully explain the rest of the curvature. The new photo is from row 58, which is away from the nose section and exhibits minimal barrel distortion.

enter image description here
(emirates.com)

This photo of an Emirates A380 undergoing a C-Check shows that the insulation -- with some of the panels removed -- is less curved where it meets the ceiling (upper deck), and that the curved panels are likely there to accommodate the plumbing, such as the air conditioning plumbing (note the pipes in the image).

enter image description here
(wikimedia.org)

This photo of the main passenger deck of an A380 with the interior panels not installed shows the walls curvature and the plumbing. Compare with the upper deck.

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    $\begingroup$ Nice! That adds a new twist to the mystery. $\endgroup$ – Hanky Panky Jun 21 '18 at 12:36
  • $\begingroup$ That's certainly a contributing reason. But the systems can also cut into the overhead bins as well. This can be a chicken vs egg question. $\endgroup$ – Therac Jun 22 '18 at 17:01
  • $\begingroup$ Now I'm split between this answer and the answer by @Therac :-/ $\endgroup$ – Hanky Panky Jun 22 '18 at 18:12
  • $\begingroup$ TBH it's probably both plus someone else. Sometimes you fit something in the space available, other times you make space for it. I'm not privy to the A380's decision-making, so no idea what's the reason and what the consequence. My own bias is structure first, someone else's can be the opposite. $\endgroup$ – Therac Jun 22 '18 at 18:16

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