The Boeing 747-400 aircraft has a maximum take-off weight of 395 000 kilograms and a maximum landing weight of 285 000 kilograms.
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. With an average consumption rate of about 10 000 to 11 000 kg per hour, this takes some ten to eleven hours of flying. If an uneventful and uninterrupted flight exceeding this amount of time is completed, the landing weight limitation is an easy one to meet. By then the approach for landing would be underway, with the aircraft being lighter than 285 000 kilograms.
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. This could either be back at their point of departure or along the route, at the nearest suitable airport. 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.
In an emergency, a successful overweight landing could in fact be achieved. However, touching down too heavily could lead to damage of the aircraft’s structure. 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.
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. The aircraft should ultimately be light enough for it to be able to be brought to a stop within the available runway distance. 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 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. 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!
Whenever fuel jettisoning is to be accomplished there are certain aspects that need to be considered. Obviously, adequate weather minimums have to exist at the airport of intended landing. 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.
Such is the reliability and dependability of jet airliners, that there is seldom a need for pilots to jettison fuel. Some crew members have flown their entire professional lives without ever having to do so. 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. This was because we had made up time as a result of obtaining shorter en-route clearances from ATC and also due to us picking up some up some incredible tailwinds that had not been planned for. We established ourselves in a holding pattern that was aligned with the runway centreline and about ten miles offshore and commenced with the procedure. The wind was calm and so accurate was the way that the aircraft’s navigation system flew the holding pattern, that on turning so as to remain in the hold for a second race-course pattern, we flew back into the vapour that we had pumped into the air earlier on. Within seconds, coming from the air-conditioning system, was a strong smell of fuel throughout the aircraft. We sure were lucky that the “No Smoking” ban was already in force!