Fuel dumping

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.

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 must land well before its destination, even returning immediately after takeoff to the departure airport (for example, because of mechanical problems or a passenger medical problem), it will contain more fuel than was intended for landing. If an aircraft lands at more than its maximum allowable weight, it might suffer structural damage or even break apart on landing.

As jets began flying in the US in 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 1.05, 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.

A large-scale fuel dumping occurred on September 11, 2001, when U.S. airspace was closed due to the September 11 attacks. International flights en route to the U.S. were either turned back to their point of origin or diverted to land in Canada and other countries. Many of these flights were fuelled for travel well into the American interior; for those mid-flight aircraft unable to land due to excessive fuel weight, dumping was necessary.

Since most twin jet airliners can meet these requirements, most aircraft of this type such as the Boeing 737 (all models), the DC-9/MD80 and Boeing 717, the A320 family and various regional jet ("RJ") aircraft do not have fuel dump systems installed. In the event of an emergency requiring a return to the departure airport, the aircraft circles nearby in order to consume fuel to get down to within the maximum structural landing weight limit, or, if the situation demands, simply lands overweight without delay.[1] Modern aircraft are designed with possible overweight landings in mind, but this is not done except in cases of emergency, and various maintenance inspections are required afterwards.

Long-range twin jets such as the Boeing 767 and the Airbus A300, A310, and A330 may or may not have fuel dump systems, depending upon how the aircraft was ordered, since on some aircraft they are a customer option. Three- and four-engine jets like the Lockheed L-1011, McDonnell Douglas DC-10 / MD-11, Boeing 747 and Airbus A340 usually have difficulty meeting the requirements of FAR 25.119 near maximum structural takeoff weight, thus most of those have jettison systems. A Boeing 757 has no fuel-dump capability as its maximum landing weight is similar to the maximum take-off weight.

Fuel-dumping operations are coordinated with air traffic control, and precautions are taken to keep other aircraft clear of such areas. Fuel dumping is usually accomplished at a high enough altitude (minimum 6,000 feet, AGL), where the fuel will dissipate before reaching the ground. Fuel leaves the aircraft through a specific point on each wing, usually closer to the wingtips and farther away from engines, and initially appears as more liquid than vapor. Specific areas have been designated where fuel dumping is allowed to avoid damage or harm where the fuel may drop; generally speaking, this is above seas or unpopulated areas above land. Delta Air Lines Flight 89 is an example of fuel dumping that violated established regulations: on 14 January 2020, it dumped more than 10,000 gallons of fuel at a low altitude over a populated area in Los Angeles, causing injuries to 56 people including school children below.[2]

It is difficult to quote specific dump rates even for specific types of aircraft since the dumped fuel is not pumped but delivered by gravity feed so as to be more independent of electrical systems, which might be unavailable in a fuel-dump scenario. This means the actual rate depends on the pressure exerted by the fuel head: the more fuel on board, the higher the rate at which it flows out. This also means that the dump rate is not constant, but decreases while dumping because the fuel head and its pressure decrease.

As a rule of thumb for the Boeing 747, pilots quote dump rates ranging from a ton per minute, to two tons per minute, to a thumb formula of dump time = (dump weight / 2) + 5 in minutes.[3] In 2009,[4] an Airbus A340-300 returning to its airport of departure shortly after takeoff dumped 53 tons of fuel in 11 minutes.[5]

The average rate of descent of the dumped fuel is approximately 500 feet per minute. Air traffic control, after receiving information by the pilots executing the dumping procedure, usually separates other traffic by 2,000 feet vertically and 5 nautical miles (9.3 kilometres) laterally, since the dumped vapours if ingested by a jet engine might cause serious problems to the engine's normal operation.