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Passenger aircraft have fuel tanks in the wings. Why?
What are the advantages and disadvantages of this location ?

examples of disadvantages I would suspect:

  • added weight increases the structural load applied to the wings
  • different gravitational forces and wing-bending between full and empty tanks result in repeating stresses shortening the aircraft life-span
  • higher risk of catastrophic damage to wings in case of in-flight fuel ignition
  • higher risk of fire when lightning strikes a wing

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    $\begingroup$ Keeping fuel in the wings actually helps to maintain their structural integrity, see aviation.stackexchange.com/questions/42613/… $\endgroup$ – DeepSpace Dec 10 '18 at 11:46
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    $\begingroup$ On the ground, the aircraft rests on the wheels. In flight, it rests on the wings - so weight in the wings reduces structural loading, not increases it. $\endgroup$ – Therac Dec 10 '18 at 18:53
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    $\begingroup$ @summerrain it doesn’t reduce it per se, it reduces the need for an even high structural (bending) load that would come from storing it in the main body. $\endgroup$ – Notts90 Dec 11 '18 at 8:01
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    $\begingroup$ Where else would you put the fuel??? There's literally nowhere else. I mean, you could turn the passenger compartment in to a fuel tank I guess? $\endgroup$ – Fattie Dec 11 '18 at 12:23
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    $\begingroup$ @Fattie I'm pretty sure there are some people who would pay to sit on the wings. Wonder if this is a commercially viable concept... $\endgroup$ – Doktor J Dec 13 '18 at 4:20
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Several advantages:

  1. Wing structures are hollow and voluminous in order to provide structural rigidity against flutter and carry flight loads. This provides the space needed to store fuel.
  2. On a conventional aircraft, placing fuel tanks in the wings places the fuel mass very close to, or on, the center of lift. This dramatically reduces Cg shift during flight and reduces the size and weight of the tailplane to maintain stable flight. It also reduces Cg shifts due to sloshing of the fuel inside the tanks, due to the limited constraints of longitudinal travel for the fuel in the tanks.
  3. In the event of a crash landing, having the fuel in the wings keeps it away from the cabin and the occupants, reducing risks of cabin fires.
  4. The weight of the fuel reduces the loading moment on the wing roots, reducing the weight of the structure needed to support the aircraft during flight.

Disadvantages:

1) Fuel sloshing with in the tanks laterally as a result of turbulence, or uncoordinated flight, can lead to lateral weight shift and potential lateral instability. At low quantities of fuel, and in prolonged uncoordinated flight, there is a chance that the engine can suffer fuel starvation simply because the fuel has flowed out of the subs in the tanks These problems can be alleviated with proper fuel tank baffling and the use of feeder hoopers fed by the main tanks which the engine drinks from.

2) On aircraft which utilize a siphon feed fuel system such as low wing aircraft, the fuel cannot be evenly siphoned from both tanks at once. This is a particular problem in single engine aircraft, where separate fuel systems are not dedicated to one engine in particular. In such cases the engine will feed off either the left wing tank with a right wing tank and this is controlled by means of a fuel selector valve in the cockpit. On aircraft that do not have automatic fuel management systems, engine fuel feed must be manually selected. Care must be taken to alternate feed from both tanks periodically to prevent a lateral in balance and fuel quantity. Additionally this fuel tank switching schedule, if ignored long enough, can potentially lead to fuel starvation of the engine and a forced landing. This is particularly problematic in the low wing light single engine aircraft such as the Piper PA-28 or Cirrus SR-2X, especially if the pilot has recently transitioned into this airplane after flying high wing aircraft, which use gravity feed fuel systems, and allow the engine to feed from both tanks at once. Larger single engine aircraft like the TBM have automatic fuel tank switching systems to address this problem. Large multi engine jet aircraft have dedicated fuel management systems which address these problems.

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    $\begingroup$ Possible additional: adding weight to the wings increases their inertia, reducing the influence that turbulent airflow has on them. In simple terms, if the air/wind spends all its time trying to wag the wings up and down and snap them off the plane, making the wings heavier makes them harder to wag, reducing the amount of repeated bending stress the mount points are subject to, and providing for a plane that flies more stably $\endgroup$ – Caius Jard Dec 12 '18 at 10:29
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    $\begingroup$ Also: increasing fuel capacity in the fuselage would reduce available cargo space; the wings can't be practically used for this, but they can be used very practically for fuel because it's not constrained to a particular container height/size. It's liquid, so they can fit the cell around and between structural parts of the wing and make use of a great deal of it. Conversely, relatively few suitcases could be fit practically along the wing, probably primarily near the root, and trying to design around the necessary volume would be an engineering challenge to keep the wings structurally sound. $\endgroup$ – GalacticCowboy Dec 12 '18 at 14:43
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    $\begingroup$ @GalacticCowboy not to mention, trying to balance the weight, since same-sized pieces of luggage may weigh differently. $\endgroup$ – Doktor J Dec 13 '18 at 4:22
  • $\begingroup$ You list advantages only. Are there also any disadvantages ? $\endgroup$ – summerrain Jan 2 at 3:48
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    $\begingroup$ Added more on that subject. $\endgroup$ – Carlo Felicione Jan 2 at 6:05
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I see what you're saying, but there's something you're overlooking in your logic. You're looking at an airplane sitting on the ground, where the wheels are near the fuselage and most of the wings are dead weight that creates strain on the structure.

Think about one in flight. Now all the lift is coming from the wings, imagine the airplane suspended by a couple dozen (billion) cables spread around the wing surfaces. Now the fuselage is dead weight and the strain in the structure is from carrying the fuselage.

So when you add weight to the wings evenly, it adds practically zero structural load for the wings. What's being lifted is inside the source of the lift. So from a structural load perspective, it's a wash: it doesn't matter.

Whereas if you add more tanks in the fuselage, that's fine on the ground, but it adds huge stresses to the wings in flight, effectively reducing practical cargo capacity.

The strain on wings from sitting on the ground is much less worrisome to designers than the strains in flight.

See also "Zero Fuel Weight".

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    $\begingroup$ Exactly what I was thinking, but you said it much better! $\endgroup$ – Michael Hall Dec 10 '18 at 22:44
  • $\begingroup$ +1, for a great explanation. $\endgroup$ – Wossname Dec 12 '18 at 11:56
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    $\begingroup$ "where the wheels are under the fuselage and the wings are dead weight that creates strain on the structure." One important note here: Most airplanes with tricycle landing gear actually have the main gear mounted to the wings, not the fuselage. Some very large aircraft do have an additional inboard set of main gear under the fuselage, but they usually still have outboard main gear under the wings. $\endgroup$ – reirab Dec 14 '18 at 17:47
  • $\begingroup$ For example Boeing 757, PA-28-140, Airbus A320, Boeing 747 $\endgroup$ – reirab Dec 14 '18 at 17:53
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added weight increases the structural load applied to the wings different gravitational forces and wing-bending between full and empty tanks result in repeating stresses shortening the aircraft life-span

As a result of the effects of lift (and the deceasing need for it as the plane lightens) the reverse is actually true see here

higher risk of catastrophic damage to wings in case of in-flight fuel ignition

As opposed to higher risk of catastrophic damage to the cabin in the case of in-flight fuel ignition?

Assuming a non-explosive ignition having the fuel in the wings means you can take action to dump the fuel. If you have a fire begin in the main fuselage however you've got a higher chance of the fire incapacitating the crew before they can take steps. Or damage occuring to the avionics, the pressure cabin etc.

maybe higher risk of fire when lightning strikes a wing ?

Wing tips are one of the locations on a plane that is more prone to lightning strikes - and the potential for fuel fires is there but steps are taken to counter this and in the vast majority of cases lightning does very little damage

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  • $\begingroup$ which are the steps taken? $\endgroup$ – summerrain Jan 2 at 3:45
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Quite simply: there's a lot of empty space in those wings, and there's a lot of empty space needed for fuel.

Creating space elsewhere for fuel would make the entire aircraft larger and heavier, so makes little sense.

And it's not just the wings, many aircraft carry fuel in the vertical stabiliser as well.

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    $\begingroup$ In other words: Why are fuel tanks located in the wings? Because the passengers wouldn't fit in there. :) $\endgroup$ – Terran Swett Dec 10 '18 at 13:14
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    $\begingroup$ @TannerSwett Don't give them any ideas... $\endgroup$ – pipe Dec 10 '18 at 15:33
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    $\begingroup$ @pipe en.wikipedia.org/wiki/Junkers_G.38 ? $\endgroup$ – DeepSpace Dec 10 '18 at 17:24
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    $\begingroup$ @TannerSwett hmm, Ryanair wants to talk with you about that idea. $\endgroup$ – jwenting Dec 11 '18 at 5:05
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Along with the other answers, I'll point out most of the recent the cases where an aircraft fuel tank exploded, the center tank, which is in the fuselage, was implicated. There are two reasons:

First, a fuselage tank is located lower than the engines and requires pumps to raise the fuel. Electrical pump failures have caused explosions. This also means that a pump failure results in unusable fuel, whereas wing tanks can naturally feed the engines via gravity.

Second, fuselage tanks are closer to sources of heat. This was a cause of the TWA flight 800 accident, where heat from nearby air conditioning equipment lead to a flammable vapor in the fuel tanks. In contrast, wing tanks are naturally cooled by airflow and are less susceptible to forming such explosive vapors.

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    $\begingroup$ Not sure if you meant "that in all the cases ... it was the center tank which was implicated" (which is apparently true), but if so, you could probably make it more explicit. $\endgroup$ – jcaron Dec 11 '18 at 13:27
  • $\begingroup$ @jcaron Reworded. I'm sure a wing tank has exploded at some time in the history of aviation, but certainly the center tank has been the main cause of accidents recently. $\endgroup$ – user71659 Dec 12 '18 at 16:31
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  • added weight increases the structural load applied to the wings

Only when the plane's on the ground. When it's in the air, it decreases the load on the wings because their lift balances the weight.

  • different gravitational forces and wing-bending between full and empty tanks result in repeating stresses shortening the aircraft life-span

At the rate of one cycle per flight. And the wings already go through a stress cycle once per flight (flexed down when the plane's on the ground and up when it's in the air).

  • higher risk of catastrophic damage to wings in case of in-flight fuel ignition

The fuel tanks catching fire in flight is catastrophic wherever you put them.

  • higher risk of fire when lightning strikes a wing

When did that last happen? Wikipedia's list of plane crashes suggests LANSA flight 508 in 1971. Such incidents are now even rarer, because fuel tanks are fitted with inerting systems. This was originally recommended after the crash of Pan Am flight 214 in 1963 but it took a long time for it to actually happen.

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    $\begingroup$ This answer seems to imply that inerting systems have been used since sometime shortly after 1963, or at least that inerting systems are responsible for the lack of lightning-induced fire since 1971. The link you provided, however, says inerting systems were not installed for 40 years after the crash of Pan Am 214. Here's an article describing the state of the art in 2004: airspacemag.com/how-things-work/safer-fuel-tanks-5883916 $\endgroup$ – David K Dec 11 '18 at 13:15
  • $\begingroup$ @DavidK Yes, it looks like I've exaggerated what actually happened. I'll come back and edit when I have time, or you're welcome to propose an edit yourself if you have the time. Thanks for letting me know. $\endgroup$ – David Richerby Dec 11 '18 at 14:50
  • $\begingroup$ @DavidK Edited. $\endgroup$ – David Richerby Dec 13 '18 at 23:35
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    $\begingroup$ @summerrain Do you have any examples of planes that have made successful emergency landings after fuel tank fires? It feels like your argument is completely hypothetical. $\endgroup$ – David Richerby Dec 14 '18 at 16:50
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    $\begingroup$ @summerrain I already explained why there aren't fuel tanks under the cockpit. Your question amounts to "In an alternate universe, would..." and we're not in an alternate universe so there's no value in addressing your question. As for an example, you are claiming that putting fuel tanks in different places would give more time for emergency landings. I suggest to you that fuel tank fires are so catastrophic that no emergency landing is going to be possible. A great way for you to argue against that claim would be to say "But it happened on flight XYZ." $\endgroup$ – David Richerby Dec 14 '18 at 17:19
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More weight on the wings is actually good as it makes the aircraft more balance and more resistant to unnecessary swerving during turbulence or wind current, like a person walking on tight rope carrying a horizontal rod(bar) for balancing. Check out radius of Gyration in mechanics.

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There is no other reason. The engine is attached to the wing and They are trying to design a fuel tank to fuel the engine, There is no space on the torso. So they make a hole in the wings.

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    $\begingroup$ welcome to aviation.SE. Please be aware that this is an english-only website. as such, we will have to remove the part of your answer written in Korean. Moreover, please be aware that we like answers that contain a bit of detail, and possibly references. $\endgroup$ – Federico Dec 16 '18 at 8:24

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