Speaking for myself, the only time I ever had to jettison fuel was when, on a direct flight from Taiwan to Mauritius, we arrived overhead the island slightly overweight for the landing. Some crew members have flown their entire professional lives without ever having to do so. Such is the reliability and dependability of jet airliners, that there is seldom a need for pilots to jettison fuel. The procedure must take place away from areas that are environmentally sensitive and at a height of 4000 feet above ground level so as to ensure complete evaporation of the fuel. Obviously, adequate weather minimums have to exist at the airport of intended landing. Whenever fuel jettisoning is to be accomplished there are certain aspects that need to be considered. The amount of fuel that needs to be jettisoned and then also the amount of fuel that is to be left in the tanks is pre-programmed so that the procedure terminates before all of the fuel is pumped overboard and there is nothing left for the engines to run on! The fuel jettison system allows for fuel to be pumped overboard through two jettison nozzles, one on each side of the aircraft, located close to the aft of the wing tips. In this regard, the effect of any technical abnormalities on the aircraft’s operational capability as well as the reported braking action on the runway must also be taken into account. The aircraft should ultimately be light enough for it to be able to be brought to a stop within the available runway distance. The solution, time permitting, is to jettison enough fuel to bring the aircraft’s weight down, certainly to below its maximum landing weight and then, if necessary, even further. Also, given the chosen airport’s runway length and the higher than normal approach airspeed because of its weight, the aircraft could touch down too fast for the wheel-brakes and reverse thrust to bring it to a stop within the available length of the runway. However, touching down too heavily could lead to damage of the aircraft’s structure. In an emergency, a successful overweight landing could in fact be achieved. There are many varied examples of this having happened in the past, such as technical abnormalities, medical emergencies, freak weather changes, threats to safety and security, or a sudden closure of the destination airport due to an accident. This could either be back at their point of departure or along the route, at the nearest suitable airport. However, within the first few hours of having got airborne, the occurrence or presence of certain unusual circumstances could lead to situations where the crew would be forced to execute a premature and over-weight landing. By then the approach for landing would be underway, with the aircraft being lighter than 285 000 kilograms. If an uneventful and uninterrupted flight exceeding this amount of time is completed, the landing weight limitation is an easy one to meet. With an average consumption rate of about 10 000 to 11 000 kg per hour, this takes some ten to eleven hours of flying. This means that if a 747-400 in fact takes off at a weight of 395 000 kilograms, it would have had to burn off 110 000 kilograms of fuel, before it got to a weight below the landing weight limit. The Boeing 747-400 aircraft has a maximum take-off weight of 395 000 kilograms and a maximum landing weight of 285 000 kilograms.
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