Posted on Nov 11, 2011 in Travel
I’ve heard of planes having to dump or drop fuel before landing. I know when I spill a bunch of gas on the ground it makes a mess and smells bad, and there isn’t even that much of it. So when an airplane does this, where does all that fuel go?
I’m going to try to not be twelve years old here and keep the number of “dump” jokes to a minimum. I’m a guy, it will be difficult — but we’ll get through it together.
No doubt you’ve seen planes on TV dump fuel, or you’ve heard about a plane having to do so on the news — or maybe you’ve even been on a plane that’s had to jettison excess fuel. So… where does all that stuff go? Well, the answer is pretty simple, but let’s take a minute to go over why planes need to — er — take a dump, sometimes.
Airplanes have two major types of weight limits that have to be considered in their day to day operation — maximum takeoff weight and maximum structural landing weight. The maximum structural landing weight is always the lower of the two weights, meaning a plane can be heavier when it takes off than when it lands. In the case of many widebody airliners (think Boeing 747 and the like) and most military planes, the difference between the two maximum permissible weights can be in the tens of thousands to even over a hundred thousand pounds for very large planes.
So what this means is that if you’re on a 747 headed from JFK to London. and 30 minutes into the flight you have to turn around and go back (mechanical failure, passenger illness, or any of the various reasons a flight would be turned back), you have a problem: Your plane is tens of thousands of pounds overweight to land safely. The landing gear could collapse, the plane could overrun the runway since the brakes can’t handle the extra mass — or both.
It doesn’t even take a case this extreme. If an airliner had a light passenger load and got a big fuel efficiency boost from riding the jet stream for a few hours, even a routine flight could leave a plane slightly overweight for landing. In all of these cases, the crew of the aircraft will, if the plane has a dump system (not all do — some planes can meet FAA landing weight requirements without one), send the excess overboard.
There are, as one would expect, guidelines in place as for when a plane is allowed to jettison fuel. You can read the specifics for yourself here, in FAA Order 7110.65R, Chapter 9, Section 4 — but in short, it reads like this: Fuel must be dumped at a minimum altitude of 2000 feet above any object within a 5 mile radius (to facilitate evaporation), and the dumping aircraft must be separated from other aircraft by at least 5 lateral miles. Air traffic control will also instruct planes to dump fuel away from large bodies of water and populated areas as much as reasonably possible.
So I mentioned evaporation above as one of the requirements for altitude, and that brings us to the meat of the answer. Jet fuel comes in many different varieties these days, but all of them are a form of kerosene. Kerosene evaporates extremely rapidly in the atmosphere, and little to none of it will survive in liquid form to fall on the earth, especially at high altitudes on a warm day.
In short, it doesn’t exactly disappear — but it doesn’t rain jet fuel on your head when a nearby 747 has to drop 10000 pounds of kerosene in order to land safely, either.
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