737 Operations Manual

Introduction
This section provides a detailed description of the alternate navigation system (ANS). Information in this section is limited to that which is unique to the ANS; information which is common with the flight management computer system (FMCS) is not repeated.
General
The ANS is an IRS–based system which provides lateral navigation capability independent of the FMCS. The ANS with the control display units (ANCDU) can be operated in parallel with the FMCS, providing an independent crosscheck of FMC/CDU operation. In the event of FMC failure, the ANS provides a backup mode of operation which is immediately available for use.
As does the FMCS, the ANS performs computations related to lateral navigation; it also provides LNAV commands which can be coupled to the AFDS for integrated operation with the flight management system.
System Components
The ANS consists of two separate systems; ANS–Left and ANS–Right. Each system is independent of the other, consisting of its own ANCDU and “on–side” IRS.
During FMC/CDU operation, all memory and computing capability are contained within the FMC. The same information is available for display on either CDU.
During ANCDU operation, each CDU uses its own internal memory and computing capability. Since each ANCDU performs its own computations based on inputs from its own IRS, the information available for display on each CDU is independent of that displayed on the other CDU.
Each pilot has a navigation mode selector, which is used to specify the source (FMC, ANS–L, or ANS–R) of the navigation information being provided to the related EFIS symbol generator and flight director.
If the associated flight control computer (FCC A or FCC B) is the master, then the related navigation mode selector is also the master and determines the source of LNAV commands provided to the autopilot.
Flight Management, Navigation -Alternate Navigation System (ANS) 737 Operations Manual

Memory
In contrast to the FMC, the ANCDU has neither a performance database nor a permanent nav database. Thus, there is no stored information from which the desired waypoints can be extracted by use of their identifiers. All waypoints in the ANCDU flight plan must be defined in terms of latitude and longitude. The ANCDU has a limited memory which can store up to 20 waypoints. The ANCDU flight plan is then assembled from this memory. Waypoints can be entered into the ANCDU flight plan either during preflight or while in flight. Navaids and airports can also be entered as “waypoints,” as well as certain procedural waypoints (only those defined by latitude and longitude).
If the crew has not entered an ANCDU flight plan, the ANCDU memory automatically stores the active FMC flight plan. This makes a flight plan available for the ANCDU in the event of FMC failure. All waypoints in the ANCDU memory and flight plan are automatically erased at flight completion.
Operation Overview
When the FMC is operable, the CDUs are used in the normal manner during preflight to activate an FMC flight plan. With the FMC/CDU preflight complete, the IRS LEGS page may be used to crossload (copy) the FMC flight plan into the ANCDU. If executed, the ANCDU flight plan becomes identical to the FMC flight plan (except that the ANCDU bypasses any undefined procedural legs contained in the FMC flight plan). If desired, the flight plan may be modified prior to execution. Additionally, the entire ANCDU flight plan may be entered manually. When entering flight plans into both ANCDUs, note that each flight plan (whether identical or not), must be entered and executed individually.
ANCDU operation requires valid inputs from the related IRS navigation mode for present position, ground speed, track, and wind. Based on the flight plan and IRS inputs, the ANCDU computes and displays courses to fly, leg lengths, estimated time en route, and crosstrack deviation. Associated roll commands are available to the AFDS.
With the FMC operable, the ANCDU can be used to monitor FMCS operation. Note that a CDU can be switched between FMC/CDU and ANCDU page displays as desired, since source selection of control and guidance commands is determined separately by the navigation mode selector.
Copyright . The Boeing Company. See title page for details.
11.50.2  June 8, 2001

Flight Management, Navigation -Alternate Navigation System (ANS)
737 Operations Manual
If the FMC flight plan was crossloaded into the ANCDU, only the first 20 waypoints appear in the initial ANCDU flight plan. However, as the flight progresses, downpath FMC waypoints are automatically added to the ANCDU memory, becoming available for subsequent crossloading into the ANCDU flight plan. Anytime the FMC flight plan is modified, the ANCDU memory is automatically updated with applicable changes.
If the FMC should fail, the IRS PROGRESS page is automatically displayed, and ANCDU LNAV operation is immediately available via the navigation mode selector. If an ANCDU flight plan had not been previously executed, the IRS LEGS page is automatically displayed. The most recent FMC flight plan is stored in the ANCDU memory and can be activated if desired, or a new flight plan may be manually entered. Additional crew entries may be required as the flight progresses, due to the ANCDU limit of 20 waypoints.
　
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