737 MAX Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
11.20.4
MN-FLT-OH-201
Inertial System
The Air Data Inertial Reference Unit (ADIRU) combines the Air Data Reference
System (ADR) and the Inertial Reference System (IRS) into a single Line
Replicable Unit (LRU). For information about the air data system, see chapter
10.20.
The IRS partition of the ADIRU computes and provides airplane position, ground
speed, heading, and attitude data to the DUs, flight management system, autoflight
system, and other systems.
The major components of the inertial system (IRS) are:
• two Air Data Inertial Reference Units (ADIRUs)
• one Inertial System Display Unit (ISDU)
• one IRS Mode Select Unit (MSU)
• one IRS transfer switch.
Inertial Reference System (IRS)
1B402 - 1B786, 1B807 - 1J594
The hybrid GPS-inertial reference function (as installed) is activated when the IRS
is connected and has received GPS data and time mark inputs from at least one of
the two Multi-Mode Receivers (MMRs). The hybrid GPS-inertial reference
function combines IR functions with GPS position information to provide high
integrity, high availability data outputs to be used by the flight management
system and other airplane systems. When GPS data is not available, the IRS is able
to work independently to provide navigation data to other airplane systems like the
ADIRU without hybrid GPS-inertial.
Two independent IRSs are installed. Each IRS uses three sets of laser ring gyros
and accelerometers to sense the airplane's attitude, rate changes, and accelerations.
The IRSs are the airplane's sole source of attitude and heading information, except
for the Integrated Standby Flight Display (ISFD) and standby magnetic compass.
In normal navigation mode, the IRSs provide attitude, true and magnetic heading,
acceleration, vertical speed, ground speed, track, present position, and wind data
to airplane systems. IRS outputs are independent of external navigation aids.
IRS Alignment for the ADIRU with Hybrid GPS-Inertial
1B402 - 1B786, 1B807 - 1J594
An IRS must be aligned before it can enter the navigation mode. When hybrid
GPS-inertial is enabled, the ADIRU uses GPS position from the MMRs as present
position to initialize the IRS by default and can complete the alignment without
flight crew's manual inputs of present position. If GPS position is not available
from any MMR, the present position can be entered through the FMC CDU. If the
present position cannot be entered through the FMC CDU, it may be entered
through the ISDU keyboard. The airplane must remain stationary during
alignment.
March 1, 2021
