Not normally. That stuff costs money. Planes normally take off carrying sufficient fuel for the planned flight, plus some reserve, so that they will expect to land with nearly empty tanks. A fully fueled plane is quite heavy for landing safely, so if some emergency condition requires a premature but not immediate landing, fuel might be dumped to lighten it for an easier landing, or to reduce the amount of fuel available in case of fire.

As it was mentioned, dumping fuel is NOT a normal proceedure but it is occasionally warranted during emergency situations. (Providing they are equipped to do so. As illustrated in the recent Jet Blue incident, the A320 does not have this capacity.) Jet fuel is not intentionally dumped unless there is an abnormal situation. Why? The most common scenario for a fuel dump would be an emergency that occured after lift off that necessitated an immediate (or ASAP) return to the airport. If the aircraft was very heavy on departure (with fuel, pasengers, cargo ect) they will exceed the aircrafts structural "MLW"/Maximum Landing Weight limit if they turn right around and come back for a landing. Most jet aircraft have quite a differance between the "MTOW"/Maximum Take Off Weight and MLW. For example the wingleted version of the Boeing 737-700 has a MTOW of 153,000lbs* and a MLW of 128,000lbs*. Jet A weighs 6.8 lbs per gallon, so you can see that a great volume of fuel would need to be dumped in such emergency situations! (3,676 gal in this example.) If the aircraft had full fuel, but the passenger & baggage weights onboard did not cause the aircraft to exceed MLW, an immediate return for landing could be anticipated (depending on the nature of the emergency of course) and no dumping would be necessary. Since tossing off passengers and cargo to lighten the load on heavier flights is obviously not an option, weight needs to be reduced for a safe landing somehow: It must either be dumped or burned off. In the case where the a/c is equipped to do so, dumping is the most expeditious manner to reduce weight. If an a/c touched down at a weight beyond its MLW airframe stress, wrinkling of the aluminum skin, bending of structures, rupture of the fuel cells (in the wings) or even a failure of the landing gear could occur. If an aircraft does return for landing and for whatever reason is overweight upon touchdown, it must be throughly inspected by mechnics for damage before it is returned to service. Some definitions: Maximum Design Taxi Weight (MTW). Maximum weight for ground maneuver as limited by aircraft strength and airworthiness requirements. (It includes weight of taxi and run-up fuel.) Maximum Design Takeoff Weight (MTOW). Maximum weight for takeoff as limited by aircraft strength and airworthiness requirements. (This is the maximum weight at start of the takeoff run.) Maximum Design Landing Weight (MLW). Maximum weight for landing as limited by aircraft strength and airworthiness requirements. Maximum Design Zero Fuel Weight (MZFW). Maximum weight allowed before usable fuel and other specified usable agents must be loaded in defined sections of the aircraft as limited by strength and airworthiness requirements. Operating Empty Weight (OEW). Weight of structure, powerplant, furnishing systems, unusable fuel and other unusable propulsion agents, and other items of equipment that are considered an integral part of a particular airplane configuration. Also included are certain standard items, personnel, equipment, and supplies necessary for full operations, excluding usable fuel and payload. *From the Boeing company technical specifications- http://www.boeing.com/assocproducts/aircompat/acaps/737sec2.pdf

Well, then I am totally confused. On Sunday, the sky above our valley was like a tic-tac-toe game for some eight to twelve very high altitude jets. All day long, there were at least five overhead leaving horizon to horizon trails. These trails did not go away. They stayed clear and visible for twenty minutes to three hours each. There were about twenty or so countable trails at any given time. Today there are none, but you can see far off to the south of the valley, it is happening there. Then, it moves over the mountains to the west of us. Then a day later it starts north of us and works it's way overhead. All the craft seem to be identical. The trail is two streams, one from each side if the tail. Once in a while, one goes over whose trail is evident, but fades right away. That seems to be about the same altitude, judging from the size the jet appearance. Again , these trails hang for at least twenty minutes, sometimes, for hours. As I understand it, a vapor trail would not last anywhere near that long. And, if they are dumping fuel, as I was told yesterday, then I am amazed the whole place don't blow up, as much as we see. Anybody care to straighten me out before I get too paranoid?

I work close to DFW airport and I see passanger jets dump fuel befor every landing. I can get video of this. I think its a wast of fuel and money. I do beleave it is harming the air we breath . I do love to fly. but when I have to get inspection for my car , and be careful of not spilling a drop of gas. I think I can sew the air lines for X amount of money.Its not fare for them to get away with it. and the cost of jet fuel is much higher than cars.I can guess that one jet alone will dump at least 1000 gallons of jet fuel. I had have enough . I beleave the EPA is getting payed off......