What kind of fuel do airplanes use? And where are the fuel tanks located? And how is the right amount of fuel required for a flight figured out?
Let's proceed in order.
The fuel used by jet airplanes is kerosene, and it has specific names (JP-8, Jet-A1 etc.) depending on its characteristics. If you have never seen it, just to give you an idea, consider that it's very similar to the liquid used in "zippo" lighters: clear, a bit oily.
It has the advantage of resisting very well to low temperatures without freezing (during a flight, ambient temperature can easily reach -60 degrees centigrade), as well as high temperatures, without catching fire.
Yes, it doesn't catch fire very easily. In order to burn, kerosene needs to be in the form of small droplets, like a spray, and this is the way it is actually transformed and used inside jet engines.
Well, let's say that in an airplane, every place not used for something else is a great place to put some fuel!
The biggest amount is usually stored inside the wings, in several tanks located along the whole wing structure.
Having fuel inside the wings has some advantages, like keeping the airplane more "balanced" as tanks deplete, and also making it easier for the airframe to withstand the stress caused by the aerodynamic forces.
Another advantage is that, if the engines are located under the wings, in case of a catastrophic failure involving all the fuel pumps (extremely unlikely scenario!), fuel can still feed the engines by "gravity drop".
Other fuel tanks are usually located in an area under the passenger cabin floor, and sometimes also inside the vertical tail plane.
Something you probably don't know is that airliners hardly ever get a "full tank fill up" before the flight.
For an airplane, it is very important to keep the weight as low as possible; carrying more fuel than the required amount would mean carrying unnecessary weight, with a subsequent increase in fuel consumption, and sometimes a reduction in paying load (passengers and baggage) capacity.
So... how can they decide what is the right amount to carry without the risk of running out of fuel?
For every single flight, a computerized system calculates the expected fuel consumption with high accuracy, keeping into account the airplane's weight, the route and altitude of flight, the wind forecasts, even the age of the engines of the specific airplane flying the sector.
On top of this calculated amount, there is the legal requirement to add a "reserve fuel", specifically:
- a spare quantity that allows to reach a close by airport (the so called "alternate") in case the landing cannot be completed, for whatever reason, at destination. The alternate airport is decided during the planning phase, based on the weather forecast and the airport usability at the time of arrival;
- a spare quantity that allows to reach an alternate airport from any position along the route, in case of need;
- a spare quantity that allows to deviate from the planned path, in case there is bad weather expected along the route;
- a spare quantity that allows to "wait" in flight for a certain amount of time in the vicinity of the destination airport, in case there are delays expected due to air traffic congestion, fog, bad weather and so on;
- a spare quantity, called "final reserve", that is always required inside the tanks at landing (it depends on the aircraft's type).
Before each flight pilots evaluate, using all the information available to them, whether the assigned fuel calculated by the computerized system is enough or not, and they can add an extra amount if they consider it appropriate.
There are also times when it is convenient to board the maximum amount of fuel possible. This happens when the cost of fuel at the departure airport is a lot cheaper than the one at destination, enough to justify a higher weight, thus a bigger fuel consumption for the outbound flight.
