(2)  
Several options are available to the pilot through entries in the REF NAV DATA page.

 (a)  
Entries in the NAV INHIBIT field inhibit both DME/DME and DME/VOR VHF NAV radio updating through autoselection. It will not inhibit the navaid to be used in manual, procedure or route tuned modes.

 (b)  
Entries in the VOR INHIBIT field will inhibit VOR/DME VHF NAV radio updating in autoselection. It will not inhibit DME/DME VHF NAV radio updating in either autoselection or manual, procedure or route tuned modes.

 (c)  
Entries in the VOR/DME INHIBIT field result in all VOR/DME VHF NAV radio updating modes being inhibited.

 (3)  
Navaids blackballed through the navigation data base are inhibited for use in VHF NAV radio updating. If the VOR only portion is specified as blackballed in the data base, only the VOR is inhibited for use. If the DME portion is specified as blackballed in the data base, the DME is inhibited for use and the VOR will not be used. If both the VOR and DME portions of the navaid facilities are specified as blackballed, the whole navaid is inhibited. Whenever the DME portion of a navaid is inhibited for use through the navigation data base, the navaid will not be used, regardless of the tuning mode including manual, procedure or route tuned stations.

 7. Performance
___________
 A. General (Fig. 32)
 (1)  The performance function manages the economy of the airplane with respect to fuel and time. Data is received from the Performance Data Base (PDB), guidance function and sensor data as well as pilot input data through the CDU. FMC software uses this data to assemble an economically feasible flight path profile. In addition the FMC provides optimal values for speed and thrust while the autothrottle is engaged.
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 34-61-00
 ALL  ú ú 22 Page 72 ú Jun 18/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (2)  
Cost index is a value which relates direct hourly flying cost to the cost of fuel. Due to its day to day changing characteristics, this value can be manually entered by the pilot through the CDU or transmitted by ACARS prior to takeoff.

 (3)  
The performance function performs various subfunctions so that the values provided can be realized. These subfunctions are described in the following sections.

 B. Speed target generation
 (1)  
This function uses a variable mach number to calculate the various speed targets. This number combines altitude, pressure, cost index, gross weight, wind, PDB data and temperature in order to minimize airplane operating cost per mile traveled. The speed targets generated are:

 Minimum transition climb
 Economy climb
 Economy cruise
 Economy descent
 Maximum angle climb
 Long-range cruise
 Engine-out long-range cruise
 Engine-out minimum drag
 Best hold

 (2)  
The FMC, makes invalid, all speed targets and blanks any FMC computed speeds from all CDU pages in the event any minimum required performance targets become invalid during flight.

 C. Atmospheric model
 (1)  
The atmospheric model allows for wind and temperature factors to be included in the prediction of speed targets.

 (2)  
Winds can be entered through the CDU by the pilot. These values are entered on the WINDS page. The WINDS page provides for entry and display of forecast winds and temperatures at specific altitudes for up to 120 waypoints.

 (3)  
These winds are predicted winds through the linear interpolation of an entered wind at a waypoint and the present position wind. These wind predictions allow for planning of the fight path.

 (4)  
Temperature predictions are calculated in the same manner as wind predictions, through linear interpolation of an entered value at a waypoint and the current temperature.

 D. Speed and Thrust Envelope Protection
 (1)  This function provides a protection to ensure that the calculated speed and thrust targets stay within airplane and engine limits. If a calculated speed or thrust target goes beyond the envelope value, that target is flagged invalid and is not used.
 E. Fuel and Weight Management
 (1)  This function continuously monitors the airplanes gross weight. The instantaneous fuel remaining and the initial zero fuel weight or gross weight which are entered into the CDU by the flight crew are used for the gross weight calculations. These values will be displayed in pounds or kilograms depending upon weather the kilogram option is selected.
　
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