ADIRU Alignment

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

On Ground

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.

In Flight

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:

- GPS

- LOC and DME-DME (NAV RAD INHIBIT - OFF)

- LOC and collocated VOR/DME (NAV RAD INHIBIT - OFF)

- LOC

- DME-DME

- 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

FMC Failure

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