•
Routes and flight levels
The particular route to be flown determines the ground distance to cover, while winds on that
route determine the air distance to be flown. Each inter-waypoint portion of an airway may
have different rules as to which flight levels may be used. Total aircraft weight at any point
determines the highest flight level which can be used. Cruising at a higher flight level
generally requires less fuel than at a lower flight level, but extra climb fuel may be needed to
get up to the higher flight level (it is this extra climb fuel and the different fuel consumption
rate that cause discontinuities).
•
Physical constraints
Almost all the weights mentioned above in "Overview and basic terminology" may be subject
to minimum and/or maximum values. Due to stress on the wheels and undercarriage when
landing, the maximum safe landing weight may be considerably less than the maximum safe
brake-release weight. In such cases, an aircraft that encounters some emergency and has to
land immediately after taking off may have to circle for a while to use up fuel, or else jettison
some fuel, or else land immediately and risk having the undercarriage collapse.
Further, the fuel tanks have a maximum capacity. On some occasions, commercial flight
planning systems find that an impossible flight plan has been requested. The aircraft cannot
possibly reach the intended destination, even with no cargo or passengers, since the fuel
tanks are not big enough to hold the amount of fuel needed; it would appear that some
airlines are over-optimistic at times, perhaps hoping for a (very) strong tailwind.
•
Fuel consumption rate
The rate of fuel consumption for aircraft engines depends on the air temperature, height as
measured by air pressure, aircraft weight, aircraft speed relative to the air, and any increased
consumption as compared with brand-new engines due to engine age and/or poor
maintenance (an airline can estimate this degradation by comparing actual with predicted
fuel burn). Note that a large aircraft, such as a jumbo jet, may burn up to 80 tons of fuel on a
10-hour flight, so there is a substantial weight change during the flight.
A - Z of Flight
July 2017
www.alliance-airways.net
1 ! 6