Flight Management Navigation -
Alternate Navigation System


CDU Instrument Source Select Panel.

The CDUs can be used as an alternate navigation system if both FMCs fail. The CDUs perform lateral navigation computations. LNAV and VNAV are not available. The route can be flown using HDG SEL.

During normal FMC operation, all system capabilities are contained within the FMCs. During alternate navigation operation, the CDUs use their own internal memory and computing capability.

Each CDU performs its calculations based on inputs from its own IRS and provides information for display independent of the other CDU. Each CDU can display its route on its respective HSI in the map mode without database symbology.


AMM Aircraft equipped with a CDU with an Alternate Navigation System capability have a 3-position, rotary, Navigation Source Select Switch instead of a push-button ALTN switch.

Radio Tuning

The radios must be manually tuned. Navigation computations use the IRS present position. No radio updating occurs.

Alternate Navigation Waypoints

The CDUs do not have a performance or navigation database. The CDUs continuously load the active route from the FMC. If both FMCs fail, the CDUs keep flight plan waypoints except for conditional waypoints, offsets, and holding patterns. Waypoints which are part of the route when the FMCs fail can be referenced by either their identifier, or latitude and longitude.

New waypoints can only be entered as latitude and longitude. This includes waypoints the flight crew has deleted from the CDU. Complete departure or arrival/approach procedures cannot be manually entered.

The CDU does not accept "undefined" waypoints/legs; that is, no fixed heading or course legs, and no conditional waypoints. The CDU automatically replaces an undefined leg with a discontinuity. However, individual legs of a procedure that are a great-circle course can be entered.

Waypoint Operations

Waypoint operations include:

Alternate Lateral Navigation

All CDU calculations are based on a great-circle course between waypoints.


Route Changes

Route changes are made on IRS LEGS page in almost the same manner as normal FMC operations. All courses between waypoints are direct routes. When the active waypoint is modified, the only navigational choice is present position direct to the modified active waypoint.

The two CDUs operate independently. A route change to one CDU does not change the other one. The route entered in the left CDU can be displayed on the captain's HSI using his NAV source select switch. The route entered in the right CDU can be displayed on the first officer's HSI using his NAV source select switch.

Course Reference

Each CDU uses its associated IRS for navigation data. The IRS supplies magnetic variation only for the present position. Therefore only the active waypoint course can be referenced to magnetic north. All subsequent courses are referenced to true north.

Alternate Navigation CDU Pages

The alternate navigation system provides two CDU pages:

Failure of a single FMC causes the related CDU to display the MENU page. Selecting the operable FMC on the NAV selector switch restores the CDU displays to normal.

If the other FMC fails, the IRS LEGS and IRS PROGRESS pages are available on either CDU via the LEGS and PROG mode select keys. Selecting CDU on the NAV source selectors provides a CDU generated map on the HSI.

IRS Legs