.
Chapter 10, Flight Instruments, Displays.

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Flight Management, Navigation -Flight Management System Description 737 Flight Crew Operations Manual

The FMC and CDU are used for enroute and terminal area navigation, RNAV
approaches and to supplement primary navigation means when conducting all
types of instrument approaches.

YA686, YA687, YC153 - YK104
The dual FMC installation is certified as a “sole source” navigation system.
Airplanes equipped with two FMCs are certified to operate outside radio navaid
coverage. The second FMC serves as a backup, providing complete navigational
functions if the other FMC fails.

YA686, YA687, YC153 - YK104
With a dual FMC installation, one FMC is always designated as primary. This is
controlled by the position of the FMC Source Select switch. Refer to Chapter 11,
FMC Source Select Switch.

YA686, YA687, YC153 - YK104
The primary FMC:

.  
allocates navaid tuning and updating functions between FMCs

.  
insures synchronization between FMCs

.  
controls CDU displays

.  
provides input to the autopilot

.  
provides input to the autothrottle system
YC411 - YD655

.  
processes ACARS (data link) messages.

YA686, YA687, YC153 - YK104 Positioning the FMC Source Select Switch to BOTH ON L or BOTH ON R isolates FMC operation to use only the left or right FMC respectively. In the NORMAL position, the left FMC is primary by default. Although the aircrew can enter information into either CDU, the primary FMC is responsible for synchronizing this information with the secondary FMC and updating both CDU displays.
When external position updating is not available, the FMC uses the IRS position as reference. When the IRS is the only position reference, the FMC applies an automatic correction to the IRS position to determine the most probable FMC position. This correction factor is developed by the FMC’s monitoring IRS performance during periods of normal position updating to determine the typical IRS error value. It is important to note that, when external position updating is not available, navigation accuracy may be less than required. Flight crews should closely monitor FMC navigation, especially when approaching the destination. The accuracy of the FMC navigation should be determined during descent phase by using radio navaids and radar information if available.
Note:  Inaccurate position updating may cause the airplane to deviate from the desired track.
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Flight Management, Navigation -Flight Management System Description
737 Flight Crew Operations Manual
Control Display Units (CDUs)
Two identical, independent CDUs provide the means for the flight crew to communicate with the FMC. The crew may enter data into the FMC using either CDU, although simultaneous entries should be avoided. The same FMC data and computations are available on both CDUs; however, each pilot has control over what is displayed on an individual CDU.
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Flight Management, Navigation -Flight Management System Description 737 Flight Crew Operations Manual

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737 Flight Crew Operations Manual

Introduction
When first powered, the FMS is in the preflight phase. As a phase is completed, the FMS automatically transitions to the next phase in this order:
.
preflight .  descent

.
takeoff .  approach

.
climb .  flight complete.

.
cruise

Preflight
During preflight, flight plan and load sheet information are entered into the CDU. The flight plan defines the route of flight from the origin to the destination and initializes LNAV. Flight plan and load sheet information provide performance information to initialize VNAV.
Required preflight information consists of:
.
initial position .  performance data

.
route of flight .  takeoff data.

Optional preflight data includes:
.
navigation database .RTA data

.
SID .  cruise wind

.
STAR  . reduced takeoff and climb
thrust limits.

Each required or optional data item is entered on specific preflight pages.
Preflight begins with the IDENT page. If the IDENT page is not displayed, it can be selected from the IDENT prompt on the INIT/REF INDEX page. Visual prompts provide assistance in selecting the appropriate CDU pages. Preflight pages can be manually selected in any order.
　
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