737 MAX Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
11.20.10
MN-FLT-OH-201
If alignment is lost in-flight, the navigation mode (including attitude, present
position and ground speed outputs) is inoperative for the remainder of the flight.
However, rotating the MSU switch from NAV to ATT allows the IRS to erect
attitude mode. The primary function of the Attitude Mode is to rapidly establish
basic pitch and roll attitudes, body rotational rate, and linear accelerations. The
attitude mode requires approximately thirty seconds of straight, level, and
unaccelerated flight to allow the platform to align itself to the airplane vertical axis
for subsequent pitch and roll sensor outputs. The ALIGN light on the MSU is
turned on during the 30 seconds attitude mode erection, and extinguish when the
IRS enters attitude mode. Some attitude errors may occur during acceleration, but
will be slowly removed after acceleration stops.
The FMC/CDU scratchpad message ENTER HEADING is displayed when IRS
enters attitude mode. The attitude mode can provide heading information, but to
establish compass synchronization the crew must manually enter the initial
magnetic heading via the FMC/CDU or ISDU. Drift of up to 15 degrees per hour
can occur in the IRS heading. Therefore, when in attitude mode, an operating
compass system must be periodically cross-checked and an updated magnetic
heading entered in the IRS, as required.
IRS Entries
During IRS alignment, manual IRS entries of present position is normally
accomplished on the POS INIT page of the FMC/CDU. The ISDU may also be
used for entry of present position.
In attitude mode, manual IRS entries of magnetic heading are required
periodically to correct sensor drift overtime. The magnetic heading entries are
accomplished on the FMC/CDU or the ISDU.
IRS Power
The IRS can operate on either AC or DC power. The left IRS is normally powered
from the AC standby bus, and the right IRS from the AC transfer bus 2. If AC
power is not normal or removed, either or both systems automatically switch to
backup DC power from the switched hot battery bus and the ON DC light
illuminates. Backup DC power to the right IRS is automatically terminated if AC
power is not restored within 5 minutes. If either IRS is on backup DC power on
the ground, the ground-call horn in the nose wheel sounds to alert maintenance
personnel that the IRS is on battery power.
March 1, 2021
