Calculation
The weight of fuel forms a significant part of the total weight of an aircraft, so any fuel
calculation must take into account the weight of any fuel not yet burned. Instead of trying to
predict the fuel load not yet burned, a flight planning system can handle this situation by
working backward along the route, starting at the alternate, going back to the destination,
and then going back waypoint by waypoint to the origin.
A more detailed outline of the calculation follows. Several (possibly many) iterations are
usually required, either to calculate interdependent values such as reserve fuel and trip fuel,
or to cope with situations where some physical constraint has been exceeded. In the latter
case it is usually necessary to reduce the payload (less cargo or fewer passengers). Some
flight planning systems use elaborate systems of approximate equations to simultaneously
estimate all the changes required; this can greatly reduce the number of iterations needed.
If an aircraft lands at the alternate, in the worst case it can be assumed to have no fuel left (in
practice there will be enough reserve fuel left to at least taxi off the runway). Hence a flight
planning system can calculate alternate holding fuel on the basis that the final aircraft weight
is the zero fuel weight. Since the aircraft is circling while holding, there is no need to take wind
into account for this or any other holding calculation.
For the flight from destination to alternate, a flight planning system can calculate alternate trip
fuel and alternate reserve fuel on the basis that the aircraft weight on reaching the alternate is
zero fuel weight plus alternate holding.
A flight planning system can then calculate any destination holding on the basis that the final
aircraft weight is zero fuel weight plus alternate holding plus alternate fuel plus alternate
reserve.
For the flight from origin to destination, the weight on arrival at the destination can be taken
as zero fuel weight plus alternate holding plus alternate fuel plus alternate reserve plus
destination holding. A flight planning system can then work back along the route, calculating
the trip fuel and reserve fuel one waypoint at a time, with the fuel required for each inter-
waypoint segment forming part of the aircraft weight for the next segment to be calculated.
At each stage and/or at the end of the calculation, a flight planning system must carry out
checks to ensure that physical constraints (e.g., maximum tank capacity) have not been
exceede d. Problems mean that either the aircraft weight must be reduced in some way or the
calculation must be abandoned.
A - Z of Flight
July 2017
www.alliance-airways.net
1 ! 7