11
$\begingroup$

This is a safety question about fuel and fires. This all regards commercial aviation for large passenger planes.

Most fuel is stored in the wings, but in the largest planes, there's also a center tank for the longest flights. It's this center tank I'm concerned with. In the event of a fire, this seems to be the most dangerous tank.

I'm obviously concerned with fires in the fuselage, or very close to it. Obviously this is where all the people are and we don't want smoke or flame there, but remember also there are overhead oxygen canisters that if heated or fueled, will explode.

Jet fuel is flammable by its nature. This cannot be changed because its purpose is to combust in an internal combustion engine. So it seems the best we can do is to keep the fuel as far from the fuselage as possible, to make it unlikely that fuel will cause a fire there (initially).

Isn't there someplace we can move this center tank? Some ideas I thought of: store fuel in anti-shock bodies on the wings (or they could be just small sears-haack bodies on the wings); store fuel in the empannage; store fuel in nacelles that have a larger diameter than the engine itself, to make room for fuel.

Another exotic idea: Is it possible to engineer the wing such that the most likely structural failure will happen first near the wingtip rather than the wingroot? This would spill the fuel farther from the fuselage. (I got this idea from a really wierd Boeing 747 crash in Netherlands where one engine jumped off then fell back into the other engine, causing 2 engine losses. They said some pins were designed to fail first for a more "benign" failure, but apparently those pylon pins were engineered wrong.)

There's one more tricky caveat, though: the cross feed line to balance fuel loads in the wings. If we eliminate this line, we increase the dangerous consequences of a fuel imbalance. But, it's interesting to think of what might cause a major fuel imbalance other than a leak, in which case the cross feed valve should not be opened. I can't think of anything so I'm leaning toward the opinion that this line can be eliminated too.

EDIT: Thanks to bodargpd for pointing this out. Losing an engine in flight means the consumption rates will be hugely different. Without a cross-feed line, all that fuel in the other wing is unavailable and we may have to ditch early. So the cross-feed line is still necessary. I still say this is much less dangerous than a center fuel tank, however. It would also be nice if we engineered the cross-feed line to be weakest at the sides, so any leak will most likely be as far to the outside of the fuselage as possible.

To sum up, the basic question is, can we viably move the center tank volume of fuel somewhere outside the fuselage?

EDIT: I'm not primarily worried about fuel explosions. I'm concerned mostly about fuel leaks during hard emergency landings. I'd rather have a fuel leak out on the wing than in the fuselage.

$\endgroup$
  • 10
    $\begingroup$ If it explodes in the wing or the fuselage the effect will probably be the same, and considering how rare center tank explosions are (I know of two examples, only one, TWA 800, was fatal), not sure this is really a major issue. The bigger issue is empty tanks, not full ones. $\endgroup$ – Ron Beyer Feb 23 '16 at 4:06
  • 14
    $\begingroup$ Let's say you lose the left engine on a two-engine aircraft. You'll drift down to a lower altitude and be at a lower speed. If you eliminate the cross feed line, you have lost access to all the fuel in the left wing, and being now low and slow, you may need that fuel. If you're in the middle of an ocean, being unable to use that fuel may mean you'll have to ditch. Just as a matter of an old man's opinion, I spent 10 years on 747-100/200 aircraft with center tanks holding, as I remember, around 340,000 lbs, and I never felt uneasy about that, even after TWA 800. $\endgroup$ – Terry Feb 23 '16 at 8:13
  • 2
    $\begingroup$ I believe most commercial pax flights use chemical oxygen generators and do not have compressed O2 stored in bottles above the seats. I would imagine that those are far less dangerous in a fire than the O2 cylinders. $\endgroup$ – FreeMan Feb 23 '16 at 14:14
  • 4
    $\begingroup$ @FreeMan Not necessarily; chemical oxygen generators are still providing oxygen. Chemical oxygen generators feeding a cargo hold fire were the problem in the ValuJet Flight 592 crash, for example. $\endgroup$ – bogardpd Feb 23 '16 at 16:35
  • 1
    $\begingroup$ How do you propose to engineer a wing to fail at the tip rather than the root? There's very little load at the tip so there's nothing to make it fail there. Indeed, whenever something strikes the wing, the wing almost has to break somewhere between the impact point and the root, because that's where the torque is being applied. $\endgroup$ – David Richerby Feb 24 '16 at 2:23
15
$\begingroup$

I'm assuming you're talking passenger safety in the event of a ground impact - if anything happened midair, it wouldn't really matter if it were in the fuselage or one of the wings.

Moving the fuel somewhere else is going to have center of gravity implications. Moving it to wing pods or lager nacelles in particular would increase drag, which would require the plane to have even more fuel on board, which seems counterproductive to decreasing the fuel risk to passengers (and, of course, would substantially increase the cost of operating the flight).

Some military aircraft do carry fuel in removable underwing fuel pods (particularly fighters being ferried somewhere, where increased endurance/time between refuelings is more important than performance for that particular mission). However, that's less a safety design and more the lack of anywhere else to put the extra fuel on a small aircraft.

If an engine fails, the rate of fuel consumption is going to be different on each side of the plane. Although both are rare, you likely have far more situations where different wings use fuel at different rates than situations where having fuel feed under the body would be dangerous. If that's true, then removing the cross-feed line would be more dangerous than leaving it in.

$\endgroup$
  • 1
    $\begingroup$ Yeah I'm worried about ground impact (hard landings in emergencies), not fuel explosions mid-air. I edited the OP. $\endgroup$ – DrZ214 Feb 24 '16 at 0:41
  • 1
    $\begingroup$ Removing the centre fuel tank doesn't mean you have to have no feed line between the two wing tanks. If we accept the hypothesis of the question, it's surely safer to have a fuel pipe passing through the fuselage than to have a tank containing thousands of litres of fuel there. That pipe would be protected by closed valves at each wing tank so would have a relatively small amount of fuel in it, compared to a tank. $\endgroup$ – David Richerby Feb 24 '16 at 2:26
  • $\begingroup$ @DavidRicherby: You mean like the tank arrangement on the 777-200? $\endgroup$ – Sean Mar 27 at 21:08
5
$\begingroup$

Extending bogardpd's answer: moving the fuel from the center to the wings will also upset the weight and balance. When the plane rolls, since the mass is further away from the rotation center, the moment of inertia is increased. Which means slower response and a need for stronger wings, which means more weight, which means even stronger wings...... You get the idea.

$\endgroup$
  • $\begingroup$ But weight in the wingtips enhances roll stability-- or so we can read on Aviation Stack Exchange $\endgroup$ – quiet flyer Aug 11 at 0:43
1
$\begingroup$

Yes the fuel from the centre tank can be moved to another place in the aircraft. It's a lot of fuel though.

Some ideas I thought of:

  • Store fuel in anti-shock bodies on the wings (or they could be just small sears-haack bodies on the wings). Technically possible, but there will be a large amount of pods.
  • Store fuel in the empannage. This is already done for cruise trim drag elimination.
  • Store fuel in nacelles that have a larger diameter than the engine itself, to make room for fuel. These would be huge and the fuel would be directly around the heat source of the combustion chamber.

All of this is technically possible, but is there really a problem to solve? The centre tank is the first one to be emptied, since the fuel in the wing provides bending relief.

All solutions would introduce economical issues: the extra drag makes our tickets more expensive.

Is it possible to engineer the wing such that the most likely structural failure will happen first near the wingtip rather than the wingroot?

The root of the wing has the largest stresses due to bending moment, and will unfortunately fail first.

$\endgroup$
1
$\begingroup$

Probably safer in the belly than anywhere else due to stress, bending, and weight balance issues. The wings would be closest to pitch CG, but loading them with fuel would affect the roll rate as well as a possibility of creating a sickening uneven "sloshing" motion that could affect passenger comfort.

The solution is called "inerting" or eliminating one of the spark, oxygen, fuel components that a fire needs. Nitrogen will not support combustion of jet fuel, so filling the fuel tank with nitrogen greatly improves safety. Nitrogen is cheap, and a gentle flow into the tank will keep even volatile liquids from igniting.

$\endgroup$
  • $\begingroup$ Bending is not a good argument since bending relief is the reason wing tanks are emptied last. The underside is also the most exposed area of the airframe in the event of a crash or even a gear-up landing, so I don't buy the "safer" argument. Regarding inerting, nitrogen is cheap, but the tanks are not, weight-wise, see this report: fire.tc.faa.gov/pdf/TG3.pdf $\endgroup$ – AEhere Aug 13 at 9:39
  • $\begingroup$ @AEhere I understand spanwise loading provides bending relief for wings, but what about fuel lines? Wings do bend. Maybe I'm old fashioned, but putting the fuel tank where the bomb bay was seems good! I would avoid crashes too, but one Russian CNG design did put the tank ABOVE the passenger compartment. Final note about "sloshing" or "Dutch Roll", getting that weight in the center and building good strong wings would be my approach. $\endgroup$ – Robert DiGiovanni Aug 13 at 9:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.