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of course the number one priority in any airline operation.
The purpose of this document is to examine the influence of flight operations
on fuel conservation with a view towards providing recommendations to enhance
fuel economy.
It is very rare that the reduction of fuel used is the sole priority of an airline.
Such instances are to maximize range for a given payload, or to decrease fuel
uplift from a high fuel cost airport. Generally fuel is considered one of the direct
operating costs and an airline tries to minimize total direct operating costs. This
introduces the concept of Cost Index and is the scope of another brochure
(Getting to Grips with the Cost Index). However it is sometimes necessary to
consider the cost implication of a fuel economy, and this is done where necessary
in this brochure.
This brochure systematically reviews fuel conservation aspects relative to
ground and flight performance. Whilst the former considers center of gravity
position, excess weight, flight planning, auxiliary power unit (A.P.U.) operations
and taxiing, the latter details climb, step climb, cruise, descent, holding and
approach.
None of the information contained herein is intended to replace procedures
or recommendations contained in the Flight Crew Operating Manuals (FCOM), but
rather to highlight the areas where maintenance, operations and flight crews can
contribute significantly to fuel savings.
3 - INTRODUCTION Getting to grips with Fuel Economy
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3. INTRODUCTION
This brochure considers the two flight management modes: “managed”
mode and “selected” mode.
The managed mode corresponds to flight management by means of a
dedicated tool, the flight management system (FMS). Crews interface through the
multipurpose control and display unit (MCDU) introducing basic flight variables
such as weight, temperature, altitude, winds, and the cost index. From these
data, the FMS computes the various flight control parameters such as the climb
law, step climbs, economic Mach number, optimum altitude, descent law. Hence,
when activated, this mode enables almost automatic flight management.
When in managed mode, aircraft performance data is extracted from the
FMS database. This database is simplified to alleviate computation density and
calculation operations in the FMS, but individual aircraft performance factors can
produce good correlation with actual aircraft fuel burns.
When in selected mode, crews conduct the flight and flight parameters
such as speed, altitude, and heading have to be manually introduced on the flight
control unit (FCU).
The cost index (CI) used in the managed mode provides a flexible tool to
control fuel burn and trip time to get the best overall economics. A technique that
reduces fuel burn often requires more trip time. Hence fuel savings are offset by
time related costs (hourly maintenance costs, flight and cabin crew costs and
marginal depreciation or leasing costs). The cost index is the cost of time ($/min)
compared with the cost of fuel ($/kg) and is used to obtain the best economics.
If fuel costs were the overriding priority, because fuel costs were much
more significant than the cost of time, then the cost index would be low. With
zero cost of time it would be zero and the FMS would fly the aircraft at Mach for
max range (MMR).
However if the cost of fuel was very cheap compared to the cost of time,
then speed would be important and the CI would be high. For zero cost of fuel,
the Cost Index would be 999 and the FMS would fly the aircraft just below MMO.
Best economics would be between these two speeds and would depend on
the operator’s cost structure and operating priorities. For more information on
Cost Index see “Getting to Grips with the Cost Index”
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
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4. PRE-FLIGHT PROCEDURES
Operation of the aircraft starts with the aircraft on the ground by aircraft
maintenance, preparation and loading.
This part intends to highlight the impact of some ground operations on fuel
consumption. Even if these operations enable only little savings in comparison
with savings made during the cruise phase, ground staff has to be sensitive to
them and should get into good habits.
This part is divided into seven different sections:
• Center of gravity position
• Excess Takeoff weight
• Flight Planning
• Ways of taxiing to save fuel
• Auxiliary Power Unit
• Fuel Tankering
• Aerodynamic Deterioration
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
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4.1 CENTER OF GRAVITY POSITION
　
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