Who often drives long distances by car, you normally fully refueling the car before the trip. By contrast, aircraft are almost never fully fulled. Only in very rare cases is the tank full. Why is that?
Aircraft have the tank in the wings and some additionally installed in the fuselage. If it is a larger aircraft, the wings and thus the tank is larger.
The larger an aircraft is, the more passengers will fit into it. Usually, fewer passengers are transported on shorter flights. But the planes fly more frequently. Therefore 150-200 passengers fit in an Airbus A320 or a Boeing 737. As a result, both aircraft are used on short to medium flights.
Whereas a Boeing 747 with approximately 470 passenger seats and an Airbus A380 with up to 580 seats are built for long distance flights. Both aircraft therefore have a larger tank so they can fly the long distances in one go.
On the one hand the kerosene consumption on a flight depends on the flight distance. On the other hand, it depends on the total weight of the aircraft.
Normally, the pilots already know during the flight planning how many passengers are to be expected on a flight. Based on this, the kerosene consumption for the flight is calculated.
Additionally to the fuel for the flight distance, come even more legal minimum quantities, which must be taken. On top of that, the pilots can increase the minimum amount of kerosene. Especially in bad weather, rather more fuel is taken.
But then the tank is still not full in most cases. With this amount of kerosene, a plane now safely arrives from A to B.
If an aircraft is now fully refueled, the total weight of the aircraft increases, which also increases consumption.
If you carry less fuel in a tank and do not fill the tank to the top, on the one hand you will save fuel costs and on the other you will use less kerosene, which is also better for the environment.
It is best to take less fuel and take as many passengers as possible on board. Then we come to another argument, not fully fuel the plane.
If a plane for long distances (an A380 or the 747) is fully booked to the last seat, the plane can no longer be fully fueled. The tanks are so big that very long distances can be flown. But if the plane would be fully occupied and fully fueled, the maximum take-off weight would be exceeded.
In such a case, you have to weigh how much fuel is needed and how many passengers can be transported. However, this phenomenon does not only apply to large aircraft for long flights. Even on short-haul aircrafts, it may be the case when the weather conditions are poor. For example, if you can only start with a tailwind.
This is how an airplane with free seats and not fully refueled can take off, and yet this flight is at the limit of utilization.
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