787 Flight Crew Operations Manual
DO NOT USE FOR FLIGHT
Flight Management, Navigation -
Navigation Systems Description
Copyright © The Boeing Company. See title page for details.
D615Z003-TBC 11.20.5
The IRU has the additional capability to complete an alignment while in flight.
This capability allows the IRU to completely recover from an in flight loss of
alignment due to a power interruption or inadvertent shutdown. When power is
restored to the IRU while in flight, attitude information becomes available within
a few seconds and full navigation capability is restored within about 10 minutes.
This capability relies on GPS data and is therefore not available if the GPS
receivers are not functioning.
Note: A position update is recommended if the total time in the navigation mode
is expected to exceed 24 hours.
IRU Failure
Should both IRUs fail, the EICAS advisory message NAV IRU is displayed. The
FMC will continue to receive a hybrid GPS-inertial position solution using the
AHRUs inertial sensors combined with GPS position.
Attitude and Heading Reference Unit (AHRU)
Like the IRUs, the AHRUs supply a stable source of attitude, heading and rate
information used in voted solutions for flight controls, displays, and other using
systems. Although the AHRUs do not have the ability to provide an independent
inertial position, they do contribute to a hybrid GPS-inertial position that
combines the accuracy of the GPS position with the stability of the AHRUs
inertial sensors.
IRS Failure
If the IRS fails, meaning both AHRUs and both IRUs fail, the EICAS caution
message NAV INERTIAL SYS is displayed. GPS output from the integrated
navigation radios (INRs) provides the necessary position and track data for the
FMC to continue to operate without the IRS. In this mode, the FMC continues to
provide flight planning, navigation, and performance information to the crew. ND
map center and map expanded modes are still available. LNAV and VNAV
guidance coupled to the autopilot however are not available when the IRS is
inoperative.
The CDU POS INIT page displays the SET HDG prompt immediately after any
IRU or AHRU failure. However, voted heading information is still being sent to
the IRS from the working IRUs and AHRUs and no aircrew action is required.
Should the IRS fail however, the aircrew must use the SET HDG prompt to
periodically sync the display heading to the standby compass magnetic heading.
The following functions are inoperative after failure of the IRS:
AFDS Modes:
February 15, 2010
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