Fuel dumping

Fuel dumping (or a fuel jettison) is a procedure used by aircraft in certain emergency situations before a return to the airport shortly after takeoff, or before landing short of its intended destination (emergency landing) to reduce the aircraft's weight.

Aircraft have two major types of weight limits: the maximum takeoff weight and the maximum structural landing weight, with the maximum structural landing weight almost always being the lower of the two. This allows an aircraft on a normal, routine flight to take off at the higher weight, consume fuel en route, and arrive at a lower weight. There are other variables involving takeoff and landing weights, but they are omitted from this article for the sake of simplicity.

It is the abnormal, non-routine flight where landing weight can be a problem. If a flight takes off at the maximum takeoff weight and then faces a situation where it must return to the departure airport (due to certain mechanical problems, or a passenger medical problem for instance), there will not be time to consume the fuel meant for getting to the original destination, and the aircraft may exceed the maximum landing weight to land at the departure point. If an aircraft lands at more than its maximum allowable weight it might suffer structural damage, or even break apart on landing. At the very least, an overweight landing would require a thorough inspection for damage.

As jets began flying with US airlines in the late 1950s and early 1960s, the FAA rule in effect at the time mandated that if the ratio between an aircraft's maximum structural takeoff weight and its maximum structural landing weight was greater than 105%, the aircraft had to have a fuel dump system installed. Aircraft such as the Boeing 707 and 727 and the Douglas DC-8 had fuel dump systems. Any of those aircraft needing to return to a takeoff airport above the maximum landing weight would jettison an amount of fuel sufficient to reduce the aircraft's weight below that maximum landing weight limit, and then land.

During the 1960s, Boeing introduced the 737, and Douglas introduced the DC-9, the original models of each being for shorter routes; the 105% figure was not an issue, thus they had no fuel dump systems installed. During the 1960s and 1970s, both Boeing and Douglas "grew" their respective aircraft as far as operational capabilities were concerned via Pratt & Whitney's development of increasingly powerful variants of the JT8D engines that powered both aircraft series. Both aircraft were now capable of longer duration flights, with increased weight limits, and complying with the existing 105% rule became problematic due to the costs associated with adding a fuel dump system to aircraft in production. Considering the more powerful engines that had been developed, the FAA changed the rules to delete the 105% requirement, and FAR 25.1001 was enacted stating a jettison system was not required if the climb requirements of FAR 25.119 (Landing Climb) and FAR 25.121 (Approach Climb) could be met, assuming a 15-minute flight. In other words, for a go-around with full landing flaps and all engines operating, and at approach flap setting and one engine inoperative, respectively.

...
Wikipedia