Aircraft have the ability to dump the fuel which they do not need, the procedure is called the Fuel Jettison, also known as Fuel Dumping. But why do pilots drop the expensive fuel that we as passengers paid for when purchasing a ticket? Have they miscalculated how much fuel they need for a particular flight or is there another reason? The answer is in the text that follows.
The aircraft itself with the crew has a certain weight called DOW - Dry Operating Weight, to this weight we add Payload (weight of passengers and cargo), which in total gives ZFW - Zero Fuel Weight. Fuel is added to Zero Fuel Weight (fuel for the entire flight from A to B, fuel for taxiing at the airport, fuel needed for the flight to the alternation, etc.). Zero Fuel Weight + fuel will give TOW or Take-off Weight. But in the case of fuel dump, MLW - Maximum Landing Weight plays a major role. Namely, each type of aircraft has a prescribed maximum weight during landing, and any landing above the maximum permissible weight can be dangerous and can cause certain damage to the aircraft.
To simplify, let's say that during a certain flight, the plane declared an "emergency" and pilots are searching for the nearest airport where they can land (the reasons for such a thing are numerous, from medical emergency to various technical problems). The aircraft will probably be heavier than the allowed maximum landing weight at that time, and to avoid any risk, the cockpit will begin the fuel dumping procedure, thereby reducing the weight of the aircraft.
Through the pipes at the ends of the wings, away from the engine, fuel will be dropped from the aircraft. Fuel dumping is coordinated with ATC to prevent other aircraft from flying through the area. Once the aircraft reaches the maximum allowable landing weight, it can proceed to the final approach and landing.
It should be noted that regional aircraft, such as the B737 and A320 (which are used on almost all flights within Europe) do not have this option, so if they declare an emergency, these aircraft will generally remain circling until they consume fuel above the maximum landing weight.
The following video can be good example. El Al’s B767 after taking off from Toronto International Airport lost. one of its engines and requested to return to the airport, but before that it must drop a certain amount of fuel for the reasons mentioned earlier. The flight controller gave to the cockpit vectors and the altitude at which the procedure will take place, and upon completion the aircraft was guided back to Toronto International Airport where it landed without a problem.
Some of the basic rules when fuel dumping is in the place: 1.) Fuel dumping must take place outside populated areas, preferably above water, and in areas of stable atmosphere,
2.) The fuel should not be discharged at an altitude of less than 6000 ft, 3.) Pilots will give the flight control an estimated duration of the procedure so that the ATC can notifies all aircraft in the area to avoid flying through the area where the dumping procedure is in the place.
ATC must announce to all aircraft located in the area that the fuel dumping is in progress, location, altitude, the type of aircraft that is performing it and the direction of flight. The information is repeated every 3 minutes to all aircraft on the frequency, as well as the special info about the completion of the procedure.
Many studies have proven that fuel evaporates at certain altitudes and should not come to the ground. There were cases when this happened, but such situations are rare and happen in exceptional occasions. The procedure is normal and well-established, takes place with the coordination of ATC and cockpit and is actually, given the number of daily flights in the world, quite rare (which is not surprising given that technical failures are rare nowadays) .