getting to grips with fuel economy(3)

 
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”

 
 
 
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
　
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