Cfrtjryk

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

Several advantages:

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.

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.

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.

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.

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

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.

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 under the fuselage and the wings are deadweight 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 deadweight 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 inflight, effectively reducing practical cargo capacity.

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

See also "Zero Fuel Weight".

• 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 so rare because fuel tanks have been fitted with inerting systems as recommended after the crash of Pan Am flight 214 in 1963.

Along with the other answers, I'll point out 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.