My current work involves a mission management system, which includes fuel calculations.
To keep things simple, the minimum fuel state seen on commencing a go-around is the sum of the fuel required to reach the alternate, 30 min of holding at the alternate and the desired touchdown fuel state. Default values are currently 2000 kg for 30 min holding, 360 kg for the approach and 0 kg on landing - it is assumed that the aircraft will land within those 30 minutes. We also assume that the 5% contingency will have been used before reaching the go-around at destination.
However, the estimation of fuel required to reach the alternate and carry out an IFR approach is far from simple. Aircraft designers will pretend that low values are sufficient, so that their customers can dispatch with the absolute minimum of fuel to maximise payload within RTOW constraints (or whatever JAR calls it).
Currently we propose a linear relationship of fuel vs distance for the diversion trip to the alternate. For the aircraft in question, this is 800 + (12 x great circle distance in nm from dest to altn). However, the end-user will be able to amend the distance as necessary. Thus he will see:
Trip distance (dest to altn): nm (edit if required)
thus Trip fuel (800 + 12d) = kg
+ approach (default 360kg) = kg (edit if required)
+ 30 min hold (default 2000kg) = kg (edit if required)
+ min landing fuel (default 0 kg)= kg (edit if required)
= min fuel at go-around from dest.
So all elements of the diversion process are clearly defined and may be edited by the end-user. On a day-to-day basis, all the user will need to do is to type in the ICAO of the dest and altn and review the results.
This defines the minimum landing fuel weight at the dest, although it is assumed for fuel burn calculations that the contingency is carried right up to the start of the go-around at destination. The system can then calculate the fuel required for the flight given this weight, the ZFW and the route, including taxi allowance.