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Lateral Navigation
All computations and commands from the ANCDU are based on a direct great–circle course between waypoints. Complete departure or arrival/approach procedures cannot be manually entered or crossloaded from the FMC. This is because the ANCDU does not accept “undefined” waypoints/legs, that is, no fixed heading or course legs, and no conditional waypoints. The ANCDU automatically constructs a great–circle course to bypass such legs. However, individual legs of a procedure, if they are great–circle courses, can be manually entered or crossloaded.
Computed courses are referenced to magnetic north for the active leg. Downpath legs are displayed as true courses, because the IRS can provide magnetic variation only for present position.
Vertical Navigation
The ANCDU does not perform computations related to vertical navigation and does not provide VNAV or autothrottle commands. However, an operable FMC can provide thrust limits to the autothrottle during ANCDU LNAV operation. Autothrottle commands are provided by MCP speed and mode selection.
Radio Tuning
The ANCDU cannot tune the VHF navigation radios and does not use radio information for position updating.
Electrical Power
Both ANCDUs are powered via transfer bus No. 1. Response to power loss is the same as for the FMC/CDU, except that the MOD IRS LEGS page is displayed in flight following a power interruption of ten seconds or more.
Flight Management, Navigation -Alternate Navigation System (ANS) 737 Operations Manual

System Downmoding
The ANS may fail due to either the loss of IRS navigation mode inputs or ANCDU malfunction. If an IRS fails, the IRS transfer switch cannot transfer navigation inputs from the other IRS. If continual self–tests detect a failure, the CDU display is blanked, and LNAV disengages
CDU Messages
There are only four messages associated specifically with ANS operation, all related to CDU entries. See section 11.60.
Waypoints
All waypoints in the ANCDU flight plan must be defined in terms of latitude and longitude.
Waypoint Identifiers
When an FMC flight plan is crossloaded, the ANCDU flight plan maintains the same waypoint identifiers used by the FMC. The associated latitude/longitude of those waypoints is also automatically crossloaded from the FMC.
If manually entering waypoints on the IRS LEGS or IRS WPT DATA pages, the waypoint identifier is limited to a maximum of 6 alpha/numeric characters. Manually entered waypoints are keyed into the scratch pad as any of the following:
.  
an identifier which already exists in the ANCDU flight plan (e.g., SEA,
KBFI, ELN01, or WPT03).

.  
a crew–assigned identifier and associated latitude/longitude (e.g.,
SIMONE/N4802.2W12241.3).

.  
latitude/longitude (e.g., S3618.5E14136.9). Upon entry, the ANCDU
assigns a sequential identifier number such as WPT04, etc.

 

Preflight Introduction
Preflight entry of an ANCDU flight plan is accomplished after entry of the FMC/CDU flight plan is complete. The following diagram depicts crossloading of the FMC flight plan into the ANCDU.
Copyright . The Boeing Company. See title page for details.
11.50.4   June 8, 2001

Flight Management, Navigation -Alternate Navigation System (ANS)
Preflight Page Sequence

Page Displays Introduction
This section provides detailed descriptions and examples of the three page displays which are unique to the ANCDU mode of operation. These page displays are only intended to provide a general description of overall system capability; they are not intended to reflect valid navigational data.
IRS Progress
The IRS PROGRESS page is an advisory page which displays current dynamic data concerning progress along the active ANCDU flight plan route. No entries are permitted.
Page access is as shown. The page is also automatically displayed upon FMC failure when IRS inputs are valid and an active ANCDU flight plan exists.
Flight Management, Navigation -Alternate Navigation System (ANS)

IRS Progress Page

Active (TO) Waypoint Line
Displays:
.
identifier of the active waypoint (highlighted in reverse video)

.
distance to go (DTG) from present position to the active waypoint

.
estimated time en route (ETE) to the active waypoint.

 

Track/Desired Track (TK/DSRTK)
Displays present IRS magnetic track/desired magnetic track (i.e., the flight plan great–circle course).
Copyright . The Boeing Company. See title page for details.
11.50.6  December 1, 2000

Flight Management, Navigation -Alternate Navigation System (ANS)
737 Operations Manual

Groundspeed (GS)
Displays current IRS ground speed in knots.
　
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