User manual

PFPX - Professional Flight Planner X
Aerosoft GmbH 2013
80 81
Cruise Speed
Speed control during flight is both a requirement of air traffic control in
parts of a flight, and of airlines in controlling fuel burn rates.
Constant speed
In some segments of flights, aircraft are required to fly at specific speeds.
Particularly over oceans and remote territories, where radar coverage is
ab-sent, air traffic control may only be able to ensure sufficient aircraft
separa-tion if all aircraft in the sector are flying at the same speeds.
This speed is usually defined as a constant mach number (representing
the ratio of aircraft speed and the local speed of sound). Mach 0.78
would mean an aircraft is travelling with 78% of the local speed of
sound.
Cost index
A flight incurs time-related direct operating costs as well as fuel costs.
Crew wages, aircraft, engines and auxiliary power units may have direct
hourly costs or fixed costs over a period little related to flying time. Fuel
costs, on the other hand, are very much related to flying time.
As the ratio of the costs of these two elements change, the airline can
use the cost index feature of the aircraft‘s flight management computer
to sig-nificantly reduce operating costs.
An airline will normally treat the calculation of the value of the elements
as a commercial secret. The outcome of their calculations can, though,
be ex-pressed as a set of cost index values used for flight planning.
A flight planning system will use the value of the cost index to deter-
mine economy climb (ECON), cruise and descent speeds.
Cost index usually ranges from 0 to 99 or 999. A low cost index is used
when fuel is expensive compared to operating costs. The aircraft would
fly slower, benefiting from a lower fuel burn. This would of course incre-
ase flight time.