Within most large commercial airlines you will find a department that deals solely with flight planning, and the people within this department are responsible for mapping out the route each flight will take and also the fuel that will be required for those flights.
Once the route has been determined, the fuel required to get from origin to destination is calculated, taking into account wind direction and strength along the route. This is defined as the trip fuel and constitutes the majority of the fuel that will actually be used.
Now in case there is a problem at the destination airport such as bad weather or other factors, airlines also need to carry fuel to get from that airport to an alternate diversion airport. This fuel is called diversion fuel and is calculated from carrying out a missed approach at the destination airport to landing at the diversion airport. Usually the diversion airport will be another suitable airport not too far from the original destination airport.
On top of this, what is called contingency fuel is also carried. This can vary in amount but usually equates to approximately 3% of the trip fuel and is there in case of situations such as stronger headwinds or ATC re-routes.
Then there is taxi fuel which is enough fuel for the aircraft to taxi between the gate and the runway and vice-versa. This is usually not too high a figure as the engines pretty much run at idle power while taxiing.
And finally, there is a 30 minute reserve of fuel added. This is called final reserve and should only ever be used in extreme circumstances. It’s very rare that an aircraft will have to land with less than their final reserve fuel in the tanks.
So the total fuel required for a commercial flight is:
Trip fuel + diversion fuel + contingency fuel + taxi fuel + final reserve.
The final decision on how much fuel to take lies with the captain of the aircraft and often he or she will elect to take some extra fuel in case of the possibility of delays along the route or at the destination.
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