Then it computes predictions along the flight plan based on these speeds,
 weather parameters, and weight. It performs vertical guidance referenced to
 these predictions.
 Other miscellaneous performance computations are also made.
 (Ref. para. 3.C.).

 The crew can insert various data or select function modes through two MCDUs
 (Multipurpose Control and Display Unit) linked to the FMGCs (Flight
 Management and Guidance Computer).
 The two MCDUs, both NDs (Navigation Display), and, for some parameters, the
 PFDs (Primary Flight Display) are also used by the system to display

R information related to the above-mentioned functions (Ref. para. 3.D.).

 2. System Description
__________________
 A. General The FMS general architecture which shows the two FM portions incorporated R in the FMGCs, with the MCDUs and the DMCs (Display Management Computer)
R  for display is given in figure. (Ref. Fig. 001) It has to be noted that the navigation data base must be loaded once
 every 28 days by the uploading sytem in the cockpit.

 The FMS navigation, radio navigation and lateral guidance architectures
 are shown in the following figures.
 (Ref. Fig. 002, 003, 004)
 The architecture for weight and speed calculation and for vertical

 guidance/predictions is given in the following figures. (Ref. Fig. 005, 006)

 FMS - General Architecture
 Figure 001

 FMS - NAV Architecture
 Figure 002

 FMS - RAD NAV Architecture
 Figure 003

 FMS - Lateral Guidance Architecture
 Figure 004

 Weight - Architecture
 Figure 005

 FMS - Prediction/Vertical Guidance Architecture
 Figure 006

 B. Multifunction Usage
 (1) General The MCDU is a color Multipurpose Control and Display Unit capable of communicating with two FMGCs and up to other subsystems.
 The MCDU design also includes BITE functions to continuously ascertain the integrity of the MCDU and that disable the MCDU for serious malfunctions.
 (a) Operation (Ref. Fig. 007) The MCDU display operates in one of several contexts or "states", which control the MCDU display at any given moment. Additionally, there are two transitory states that exist when switching between these states. The primary and transitory states are:
 -Primary states
 . Active subsystem page displayed
 . MCDU MENU page displayed

 -Transitory states
 . Long-term power up
 . Subsystem selected

 These states and the transitions between them are described as follows:
 1 Initial subsystem selection
_ The initial subsystem selection transitory state occurs after a normal long- term power up with the BRT knob in ON position or in transition from OFF to ON. After successful completion of the long-term power up as described in para.(2) MCDU power up/resets, the MCDU monitors its input ports for subsystem identifiers, requests menu text data from any subsystem for which an identifier has been received and:
 -either identifies the highest priority available subsystem and selects it as the subsystem to be given control of the display (A),
 - or determines that no subsystems are available to receive control and displays the MCDU MENU page (B).

 MCDU Page Display Control
 Figure 007

 2
_
 3
_
 a Highest priority available subsystem identified (A)
_ If after a long-term power up, valid subsystem identifiers and menu text data have been successfully received on the port associated with the selected FM, MCDU1 and MCDU2 initiate normal communications with that port immediately. If neither the subsystem identifier nor menu text has been received from the selected FM, and a minimum of 13 seconds has elapsed since power up, MCDU1 and MCDU2 commence normal communication with the next highest priority subsystem available. Each successive subsystem is selected and evaluated until valid subsystem identifiers and menu text data are established with the highest priority subsystem available or all subsystem ports have been examined and no subsystems have been found to be available.
 Once a subsystem is found that has sent both valid identifiers and valid menu text, it is selected as the active subsystem and is given control of the display via an Enquiry for a page update (ENQ). A third MCDU may be installed as an option for maintenance purpose (CFDS and AIDS) and ACARS system (ATSU). The third MCDU is not connected to the FMGC and can not be used as backup MCDU for FMS purpose in case of MCDU1 or MCDU2 failure.
　
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