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October 1, 2000
747 Operations Manual
Flight Management, Navigation Chapter 11
FMS Alternate Navigation System Description Section 50
Copyright © The Boeing Company. See title page for details.
D6-30151-425 11.50.1
Introduction
The CDUs can be used as an alternate navigation system if both FMCs fail. The
CDUs perform lateral navigation computations; however,LNAV, VNAV, and
autothrottle are not available. The CDU can be used to tune navigation radios.
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 IRU. Usually,
the left CDU provides information to the left ND map and the right CDU provides
information to the right ND map. The center CDU can be selected as an alternate
to the left or right CDU.
Alternate Navigation Waypoints
The CDUs do not have a performance or navigation database. The CDUs
continuously copy the active route from the FMC. If both FMCs fail, the CDUs
retain flight plan waypoints except for conditional waypoints, offsets, and holding
patterns.Waypoints on the copied route can be referenced by either their identifier,
or latitude and longitude.
New waypoints can only be entered in latitude and longitude. This includes
waypoints the flight crew has deleted from the copied route.
Waypoint Operations
Waypoint operations include:
• add new waypoints (latitude/longitude entry only)
• remove existing waypoints
• change the sequence of existing waypoints
• connect discontinuities.
Alternate Lateral Navigation
All CDU calculations are based on a great–circle course between waypoints.
Route Changes
Route changes are made on ALTERNATE NAVIGATION 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.
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
11.50.2 D6-30151-425
Flight Management, Navigation -
FMSAlternateNavigationSystem
Description
A route change to any one CDU does not change the other two. The Captain may
view the route entered in either the left or center CDU on the ND using the NAV
source select switch. The First Officer may view the route entered in either the
right or center CDU on the ND using the NAV source select switch.
When displaying center CDU data on the First Officer’s ND, the ND range
selector must agree with the range selected on the Captain’s selector. If different,
the message MAP RANGE DISAGREE displays on the First Officer’s ND.
Course Reference
The IRU supplies magnetic variation for present position. Only the active
waypoint course can be referenced to magnetic north. All subsequent waypoint
courses are true courses.
Alternate Navigation Radio Tuning
The radios must be manually tuned on each CDU in alternate navigation. The left
CDU tunes the left VOR, DME, ADF, and left ILS. The right CDU tunes the right
VOR, DME, ADF, and right ILS. The center CDU tunes the center ILS. Manual
tuning is accomplished on the ALTERNATE NAVIGATION RADIO page.
Alternate Navigation CDU Pages
Alternate navigation is accomplished from three CDU pages:
• ALTERNATE NAVIGATION LEGS
• ALTERNATE NAVIGATION PROGRESS
• ALTERNATE NAVIGATION RADIO.
Failure of an FMC displays the related CDU MENU page and the scratchpad
message TIMEOUT-RESELECT. Rotating the related Navigation Source Selector
to the operable FMC restores the CDU display and NDs to normal.
If both FMCs fail, the IRS LEGS, IRS PROGRESS, and ALTN NAV RADIO
pages are available on all CDUs. Rotating Navigation Souce Selectors to a CDU
position restores NDs and the alternate navigation pages.
December 01, 1999
747 Operations Manual
Flight Management, Navigation -
FMS Alternate Navigation System
Description
Copyright © The Boeing Company. See title page for details.
D6-30151-425 11.50.3
Alternate Navigation Legs Page
This page displays data about each leg of the route. The route can be modified.
Waypoint speed and altitude restrictions are not displayed because performance
data is not available.
1 Leg Direction
Displays course to the waypoint.
Course reference is M for magnetic, T for true.
Active waypoint leg direction can be magnetic or true. Subsequent waypoint leg
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