737 Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
11.20.6 D6-27370-866-EGP
IRS Power
The IRSs 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, either or both systems automatically switch to backup DC
power from the switched hot battery bus. Backup DC power to the right IRS is
automatically terminated if AC power is not restored within five minutes.
Initial power–up requires battery bus power available and the IRS mode selector
to be in ALIGN, NAV, or ATT. If the IRS is turned off, it must complete a full
realignment cycle before the airplane can be moved.
If AC electrical power is subsequently removed from the airplane, the switched
hot battery bus continues to supply electrical power to the IRS. The ON DC light
illuminates, and the ground-call horn in the nose wheel well sounds to alert
maintenance personnel that the IRS is on battery power.
When the IRS mode selector is turned OFF, the IRS remains powered for
approximately 30 seconds. The ALIGN light illuminates until the system is
completely shut down.
Inertial System Display Unit (ISDU)
The ISDU is located on the aft overhead panel and displays data according to the
position of the display selector and system selector. The ISDU also contains a
keyboard for entry of present position and heading.
Mode Select Unit (MSU)
The MSU is located on the aft overhead panel and is used to select the operating
mode for each IRS. Indicator lights on the MSU show status of each IRS.
IRS Transfer Switch
Should either IRS fail, the IRS transfer switch is used to switch all associated
systems to the functioning IRU.
September 28, 2006
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