Joined: 11 Dec 2005
|Posted: Thu Aug 09, 2012 1:34 pm Post subject: Flight Management, Navigation (Technical Notes)
In rare cases, a new airplane present position update of the ADIRU may cause the EICAS message NAV UNABLE RNP to momentarily display. This occurs if the entered ADIRU position is far different than the GPS position. The message displays until the FMC reconciles the difference between the new ADIRU position and the GPS position.
A position update is recommended if the total time in the navigation mode is expected to exceed 24 hours.
ADIRS Schematic, Normal Mode
ADIRS Schematic, With NAV ADIRU INERTIAL Message
FMC Position Update
• The FMC calculates present position based on ADIRU and/or the GPS data.
• If GPS updating is not available due to loss of signals, pushing a TO/GA switch updates the FMC position to the runway threshold or to the position shift position, when entered.
• When making an intersection takeoff, the intersection displacement distance from the runway threshold must be entered on the TAKEOFF REF page.
• With GPS NAV ON, the TO/GA update is inhibited.
• The FMC position is the ADIRU inertial position updated by GPS source data to compensate for inertial reference errors. Radio navaid updating can be activated by selecting the RAD NAV INHIBIT toggle to OFF on the REF NAV DATA page. Updating priority is based on the availability of valid data from these sources.
• The FMC automatically tunes VOR, DME, and ILS radios for position updating. Selection is related to the active route and any procedure (SID, STAR, etc.) in the active route.
• Manually selecting VOR frequencies or identifiers precludes the FMC from autotuning other VOR/DME frequencies for position updating; however, the FMC continues to tune DME-DME pairs for position updating.
• ADIRU inertial position updating occurs in the following priority order:
- LOC and DME-DME (NAV RAD INHIBIT - OFF)
- LOC and collocated VOR/DME (NAV RAD INHIBIT - OFF)
- collocated VOR/DME
• With all ADIRUs failed and GPS operative and valid, the FMC position is the GPS position.
• Localizer updating is inhibited when GPS/INERTIAL is the navigation update mode.
• The FMC changes to LOC updating when:
- the tuned localizer is associated with the destination runway
- the airplane is less than 6,000 feet above localizer navaid elevation
- the airplane is less than 20 nm from the localizer navaid for a front course approach or less than 12 nm for a back course approach
- the airplane is within a 25° sector of the inbound localizer course
- the difference between airplane track and the localizer course is less than a 45° intercept angle
Single FMC Failure
• LNAV and VNAV, if active, remain active and all flight plan and performance data is retained.
• A software reset may occur while in single FMC operation. The active route becomes inactive, the performance data is erased, and LNAV and VNAV (if engaged) modes fail.
• To regain FMC operation, activate and execute the flight plan, enter the necessary performance data, and engage LNAV and VNAV.
• If the MENU page and the scratchpad message TIMEOUT RESELECT display, the FMC is no longer connected to the CDU. Selecting the <FMC prompt connects the CDU to the FMC.
Dual FMC Failure
• The CDUs supply route data to the NDs, and one of the CDUs supplies LNAV guidance to the autopilot.
• LNAV can be reselected on the mode control panel.
• Dual FMC failure may inhibit the autothrottle system. If it is available, it may be used in conjunction with any valid autopilot roll and/or pitch mode.
• If the MENU page displays and the <FMC prompt is not displayed in line 1, pushing the LEGS function key displays the ALTN NAV LEGS page, the PROG key displays the ALTN NAV PROGRESS page, and the NAV RAD key displays the ALTN NAV RADIO page