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80
RANGE
846.0z
12.7
GPS
40
MAP PLAN
MAG
287
294o / 3 TRK
GS 287 TAS MUMPP
>
POS REF 3 / 4
RECEIVERS
GPS
GPS-INERTIAL
HYBRID
POSITION TO FMC
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COMMON COMPUTING RESOURCE (CCR)
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IRU
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POSITION
787 Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
Boeing Proprietary 11.20.3
Inertial System
Inertial Reference System (IRS)
The IRS calculates airplane altitude, airspeed, attitude, heading, and position data
for the displays, flight management system, flight controls, engine controls, and
other systems. The major components of IRS are the inertial reference units
(IRUs), and attitude and heading reference units (AHRUs).
Inertial Reference Unit (IRU)
The IRU combines inertial reference functions with GPS position information to
provide hybrid GPS-inertial position outputs. The IRU is fault–tolerant and fully
redundant.
IRS Power
Initial power–up requires battery bus power and the IRS switches to be ON. If the
IRSs are switched off, they must complete a full realignment cycle before the
airplane can be moved.
If electrical power is subsequently removed from the airplane and the Battery
switch is switched OFF, the hot battery bus continues to supply electrical power
to the IRS. The ON BAT light illuminates, and the horn in the landing gear wheel
well sounds to alert maintenance personnel the IRS is on battery power.
IRU Alignment
On initial power–up, the IRUs enters the align mode. The EICAS memo message
INERTIAL ALIGN MODE displays. Attitude or heading/track data is removed
from the PFDs. When the GPS system is operating normally there is no
requirement for the flight crew to enter present position on the CDU POS INIT
page. The POS INIT page displays dashes in the ENTER PRESENT POSITION
line to allow the option of manually entering present position. If the GPS position
is not available during the alignment, the dashes are replaced with boxes and
present position must be entered to complete the alignment. Alignment time is
approximately 7 to 10 minutes at mid latitudes and up to 17 minutes at high
latitudes. The airplane should not move until alignment is complete.
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.
787 Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
11.20.4 Boeing Proprietary
If the latitude/longitude position is not close to the position of the origin airport,
the EICAS advisory message FMC MESSAGE is displayed along with the CDU
help window message INERTIAL/ORIGIN DISAGREE. If the crew–entered
latitude/longitude position does not pass internal IRU comparison tests, the
EICAS advisory message FMC MESSAGE is displayed along with the CDU help
window message ENTER INERTIAL POSITION.
If a new airplane present position entry fails the internal check twice, the EICAS
advisory message FMC MESSAGE is displayed along with the CDU help
window message ALIGNMENT REINITIATED. The system automatically starts
a new alignment cycle.
Attitude and Heading Reference Unit (AHRU)
Like the IRUs, the AHRUs supply a stable source of attitude, heading and rate
information to the FCE. The FCE uses all those inputs to provide voted solutions
for flight controls and output to displays and other using systems.Although the
AHRUs do not have the ability to provide an independent inertial position, they
do provide a hybrid GPS-inertial position that combines the accuracy of the GPS
position with the stability of the AHRUs inertial sensors.
If the IRS fails, meaning both AHRUs and both IRUs fail, the EICAS caution
message NAV INERTIAL SYS is displayed. The 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
　
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