Commercial airlines do not want an aircraft to change altitude too often (among other things,
it may make it more difficult for the cabin crew to serve meals), so they often specify some
minimum time between optimisation-related flight level changes. To cope with such
requirements, a flight planning system must be capable of non-local altitude optimisation by
simultaneously taking a number of waypoints into account, along with the fuel costs for any
short climbs that may be required.
When there is more than one possible route between the origin and destination airports, the
task facing a flight planning system becomes more complicated, since it must now consider
many routes in order to find the best available route. Many situations have tens or even
hundreds of possible routes, and there are some situations with over 25,000 possible routes
(e.g., London to New York with free-flight below the track system). The amount of calculation
required to produce an accurate flight plan is so substantial that it is not feasible to examine
every possible route in detail. A flight planning system must have some fast way of cutting the
number of possibilities down to a manageable number before undertaking a detailed
analysis.
Reserve reduction
From an accountant's viewpoint, the provision of reserve fuel costs money (the fuel needed to
carry the hopefully unused reserve fuel). Techniques known variously as reclear, redispatch, or
decision point procedure have been developed, which can greatly reduce the amount of
reserve fuel needed while still maintaining all required safety standards. These techniques are
based on having some specified intermediate airport to which the flight can divert if
necessary; in practice such diversions are rare. The use of such techniques can save several
tons of fuel on long flights, or it can increase the payload carried by a similar amount.
A - Z of Flight
July 2017
www.alliance-airways.net
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