there is a fuel calculation method used by airlines which is called reclearence or decision point procedure (or even other names for the same procedure).
Normally you have to have certain amounts of fuel on board to be legal:
1. Trip Fuel: A to B
2. Contingency Fuel (5% of trip fuel, or 20min fuel calculated with trip fuel flow, wichever is lower)
3. Alternate Fuel: B to C
4. Final Reserve Fuel: 30min fuel calculated with trip fuel flow
That is the fuel amount which is required by the authorities, otherwise you would fly illegal.
If you have the capacity (limited either by takeoff weight, landing weight, or tank capacity), you can also take some extra fuel. The extra fuel calculation is done by the pilots. They decide how much extra they would like to have based on weather forecasts (fog forecasted, they expect holding) or their experience (30min extra in LHR
, due to holding) or many other factors....
That is the planning method which is used for most of the flights. But sometimes there are reasons leading to a reclearence flight plan:
You are planning a flight from A to B. Unfortunately you cannot fuel the amount from above to fly this sector (to much load, to much headwind.....). Then you can plan a flight from A to D. D is an airport a few hundred miles short of your proposed destination B and is called Reclearence Airport. So you make a legal flight plan from A to D with an alternate E.
Now you start your flight and you try to safe fuel where ever you can. There is a preplanned point ~100nm-200nm away from D which is the Reclearence Point X. There you have to look at you remaining fuel. If you saved enough fuel to fly from X to your proposed destination B legally, you can continue (legally: Trip fuel from X-B plus the rest mentioned above). Otherwise you have to land in D.
Sounds complicated in the beginning. But it works in 99% of all reclearence flights that you land in B. That makes it interesting for the airlines, because they save fuel or can carry more load. Even with 1% of the flights landing in between it is still profitable for the operator. If you are on board on one of those flights it is bad indeed. You miss your connections...... and so on.
Any questions? Don't hesitate to ask.