Put simply, you MUST arrive at your destination with enough fuel for:
A. An approach, go-around, cruise and descent to the alternate airport; and
B. Arrive at the alternate airport with 30 minutes holding fuel.
In addition, you SHOULD arrive at your destination with enough fuel (extra to A and B) to enable some holding at destination before having to decide whether or not to divert. That extra fuel will depend on weather, NOTAMS, pilot experience of the destination, company statistics, etc.
To estimate some actual figures:
A. Obviously will vary with the distance from the destination to the alternate, but for a “Large Twin”, in a “non-remote” location, assume about 6 tonnes.
B. Depends on type and weight at the alternate but assume 3 tonnes.
C. Most crew would want 15-20 minutes extra time, especially after a very long flight, so allow an extra 2 tonnes for that.
D. Then there is the fuel consumed during start-up and taxi (0.5 tonnes).
E. In addition, there is the contingency fuel, the amount of which depends on the approvals and SOPs held by the operator, and the extra fuel which may be required due to ETOPS. Let’s assume 3 tonnes for this.
F. Finally, all of this “extra” fuel will increase the weight (mass) of the aircraft, which will further increase the fuel burn. On a super-long flight, we might burn an extra 1.5 tonnes, just to carry our additional “extras”.
So, in summation, the fuel we must deduct from the total carrying capacity, in order to calculate a figure that we can use just for the flight from departure to destination is:
A. 6.0
B. 3.0
C. 2.0
D. 0.5
E. 3.0
F. 1.5
Total 16 tonnes.
So for a total capacity of 145.5 tonnes, you could use 129.5 from departure to destination. Assume an average burn of 7.5 tonnes per hour and an average groundspeed of 480 kts gives a flight time of 17hrs 15mins and a range of 8,288 nms.
The actual fuel remaining in tanks on arrival at the destination will depend on how much (if any) contingency fuel has been used. If you’ve used the ETOPS fuel, it means you’ve diverted en-route and so this fuel should still be available to you at the end of a normal flight. The only fuel that you will definitely have used will be D and F so you might arrive at destination with as much as 14 tonnes.