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
exceeded. Problems mean that either the aircraft weight must be reduced in some way or the
calculation must be abandoned.
An alternative approach to fuel calculation is to calculate alternate and holding fuel as above
and obtain some estimate of the total trip fuel requirement, either based on previous
experience with that route and aircraft type, or by using some approximate formula; neither
method can take much account of weather. Calculation can then proceed forward along the
route, waypoint by waypoint. On reaching the destination, the actual trip fuel can be
compared with the estimated trip fuel, a better estimate made, and the calculation repeated
as required.
Cost reduction
Commercial airlines generally wish to keep the cost of a flight as low as possible. There are
three main factors that contribute to the cost:
•
the amount of fuel needed (to complicate matters, fuel may cost different amounts at
different airports),
•
actual flying time affects depreciation charges, maintenance schedules, and the like,
•
overflight charges are levied by each country the aircraft flies over (notionally to cover
air traffic control costs).
Different airlines have different views as to what constitutes a least-cost flight:
•
least cost based only on time
•
least cost based only on fuel
•
least cost based on a balance between fuel and time
•
least cost based on fuel costs and time costs and overflight charges
Basic improvements
For any given route, a flight planning system can reduce cost by finding the most economical
speed at any given altitude and by finding the best altitude(s) to use based on the predicted
weather. Such local optimisation can be done on a waypoint-by-waypoint basis
A - Z of Flight
July 2017
www.alliance-airways.net
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