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The answers to this this question make it clear how modern airliners dump fuel in the case of an emergency. As far as I understand, fuel dumping might be done to lower an aircraft's weight to the maximum landing weight, particularly soon after takeoff. As far as I understand, jet fuel is typically not safe. When I, as a complete outsider, hear of fuel dumping, I imagine a whole bunch of dangerous corrosive fuel being dropped on whatever happens to be below, and it sounds like a frankly awful idea. What makes fuel dumping safe, or if it is dangerous what makes it worth it?

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Jet fuel isn't corrosive and the dumped fuel evaporates before it reaches the ground anyway. –  David Richerby Aug 14 at 23:56
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Lol at "dangerous corrosive" fuel. It's kerosene, sprayed over a very big area. –  Martin Argerami Aug 15 at 10:13
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@DavidRicherby: That depends on many conditions like rain, temperature and altitude of dumping. I can remember a situation where some test airbus dumped fuel over our living area and you could smell it. –  PlasmaHH Aug 16 at 17:03
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Here's some aircraft maintenance trivia: we called the valve that controls the dump a Jesus Valve. I'm not sure what the pilots call it, but its a bad day if they have to use it. –  jww Aug 17 at 5:50

4 Answers 4

up vote 32 down vote accepted

Fuel dumping is almost always an emergency maneuver, and it is never good for the environment. So the alleged danger from the dumping must be balanced with the imminent danger to the 200–400 passengers involved. It is never an easy decision but that's why the Captain gets paid the big bucks...

Dumping fuel sounds dangerous but it is not. It is not going to explode. Jet-A can be compared to kerosene and I don't have that statistic so let's compare it to Diesel Fuel which is not dissimilar to Jet-A. Diesel Fuel at 400°F(204°C) is as volatile as gasoline is at 70°F(21°C). Obviously that is a big difference. Jet-A also evaporates into the air, so little if any hits the ground.

The biggest danger I see is to the atmosphere.

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misted fuel can still ignite but if you are going at mach 0.7 it won't catch up to you –  ratchet freak Aug 15 at 0:19
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Since when do airline pilots get paid the big bucks? ...or is that a joke? :) –  egid Aug 15 at 5:55
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@egid - The starting salary for a captain with a medium-sized airline may range from £57,000 to £78,000, while those with the major operators could earn from £97,000 to more than £140,000. A pilot's salary is often incremental, rising with each year of service with the company. Sure, it's not millions, but that is pretty big amount of money. –  Davor Aug 15 at 12:19
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@egid: Yes, meant to be a joke. –  Skip Miller Aug 15 at 13:09
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@Davor those numbers seem pretty high for US carriers. Maybe it's different overseas. –  egid Aug 15 at 15:33

It's not really safe in an environmental sense, but it's not that bad. Jet fuel is essentially kerosene, which is harmful, but as far as engine/industrial fluids go, it's pretty benign. Additionally, when an airplane dumps it, it's diluted so much by the time it makes it anywhere that it's not a significant concern anymore.

On the flip side, if an airplane didn't dump fuel and landed overweight, the potential side effects could be disastrous and could result in many injuries and damages.

Basically, it's choosing between two bad options, landing overweight or dumping fuel, and dumping fuel is the least bad of the two of them.

In any case, it's quite a rare occurrence. You are exposed to far more toxins just walking down the street of a city than from airplanes dumping fuel.

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Up-voting @skipmiller, I offer this addendum

if it is dangerous what makes it worth it?

Emergency Landing After Takeoff

Large aircraft typcially cannot land as heavily as they can take off. It's a structural issue. So fuel dumping would be done if an emergency did not allow the time to burn down to allowable landing weight.

Maintain flight with engine/power loss

If available power did not allow sustained flight one might dump fuel. Worse if you were over mountains and needed to stay high.

Ditching

There was a case of a C-130 - a high wing plane - ditching in the Atlantic. It floated for a couple days with empty gas tanks and had to be shelled to force it to sink.

Fuel Leak

If a wing tank fuel leak was causing aircraft balance problems one might dump from the opposite wing to keep the aircraft controllable.

Life Imitating Fiction

15 minutes after takeoff we lost all thrust on 3 engines. Our gross weight forced us into a controlled descent. Dumping most of our fuel might very well have kept us airborne on 1 engine, if not I think empty tanks would have limited possible fuel fire in the controlled crash. I was about to dump 45,000 lbs of JP-4 all over west Texas when we solved our engine problem.

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Very cool points, but I don't understand the ditching example. Was fuel dumped to make the plane lighter so it would float better? –  raptortech97 Aug 14 at 23:32
    
Navigation error caused them to run out of fuel. –  radarbob Aug 14 at 23:34
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It's a consideration. And I just illustrated the point where empty tanks helped keep the aircraft afloat. But I may concede a point - I don't think Sullenburger dumped fuel when he went into the Hudson. I don't think his airplane had a dump system. –  radarbob Aug 15 at 0:14
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Just to point out that while an empty tank will certainly float better than a tank full of Jet-A, the full tank will also float because Jet-A is around 6.7 lbs/gal and sea water is greater than 8 lbs/gal. –  Terry Aug 15 at 1:59
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@Terry only if the Jet-A + the steel/aluminum container and whatever is attached to it is lighter than water, it's not enough that the Jet-A is :) –  falstro Aug 15 at 12:59

Fuel dumping is done by many types of Aircraft(fighter etc.) it is not limited to the role(cargo, passenger etc.). Although the purpose may be same.(Sometimes its done to reduce the load, to avoid any accident).

In a fighter plane fuel is dumped to reduce the weight and increase the distance to cover(the weapons it carries already has tons of weight on the plane). This could be the same in passenger but situations may vary.

Fuel dumping is really a serious threat to the environment or the biological impact especially if it's dumped in the water(causes damage to maritime species and irreversible damage to the coral reefs if present).

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