3.  IRS Mode Select Unit (MSU) (Fig. 8)__________________________
 A.  The mode select unit (MSU) provides mode selection for two IRU's through two mode select switches and monitors the operation of the IRU's through two sets of alert lights mounted on the unit.
 B.  The MSU is mounted on the aft overhead panel P5 in the flight control cabin. The MSU is mounted with four DZUS fasteners and is interfaced with airplane wiring through two rear mounted connectors.
 C.  The MSU contains two rotary mode select switches, one for the left (L) IRU and one for the right (R) IRU. The positions are OFF, ALIGN, NAV and ATT.
 D.  When the mode selector switches are in OFF, power is removed from the IRUs.
 E.  When ALIGN is selected, power is applied to the IRU's and the IRU's normally progress through an alignment period of approximately 10 minutes before the navigational mode is armed. When the switches are maintained in ALIGN, however, the IRU's remain in the align mode. Normal alignment requires the entry of present position into the IRU's.
 (1)  The NAV position of the switches enables the navigational mode, provided the alignment period is completed in the IRU's. The NAV position may be selected directly from OFF. The NAV mode is the normal operating mode for the IRS. In this mode, the IRS performs inertial navigation functions and outputs normal IRS data to be displayed or used by other systems.
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IRU 1
ALIGN
BRT/DIM GND
TEST DC 10
IRU 1
FAULT
PRESS TO TEST DC
IRU 1 DC FAIL
5V AC

IRU 1
ON DC
GROUND

A

737-300/400/500
MAINTENANCE MANUAL

IRU 2
ON DC

18  BRT/DIM (GRD)
10 TEST DC
15 IRU 2 DC FAIL
IRU 2
FAULT

14  IRU 2 ALIGN

IRU 2 DISCRETES
IRU 1 DISCRETES

MSU Schematic
Figure 8

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (2)  The ATT position of the switches places the IRU's into a reversionary mode of operation. The ATT (attitude) mode is used when failure or total power loss (AC and DC power) is detected in the NAV mode. In this mode, only attitude and heading data is output to the user systems.
 F.  Each mode select switch has detented positions. The detented positions prevent accidental movement of the switch. In a detented position, the switch must first be pulled out of the detent before selecting another position to prevent damage to the switch. In the NAV position, the switch must first be pulled out of detent before selecting the ATT position. In the ATT position, the switch must first be pulled out of detent before selecting the NAV position. In the ALIGN position, the switch must first be pulled out of detent before selecting the OFF position.
 NOTE: On IRU P/N S242T101-105 and on (user friendly features) there is a
____ 2 second time delay before the IRU recognizes attitude mode after the mode select switch is placed in the ATT position. This is to prevent inadvertent selection of ATT mode.
 G.  Two sets of four annunciators are installed on the MSU, one set for each IRU. Each set of annunciators consists of an ALIGN annunciator (white when lit) and three warning annunciators (amber when lit). The three warning annunciators are ON DC, DC FAIL, and FAULT. DC power for lamp illumination is derived from the IRU or from the master dim test circuit.
 (1)  
The ALIGN light illuminates steady during alignment or flashes when an abnormal alignment is sensed.

 (2)  
The ON DC lights illuminate when 115 volt ac power is removed from the IRU's and the IRU's operate on 28 volt dc. The 28 volt dc continues to power the left IRU but after 5 minutes, power is removed from the right IRU.

 (3)  
The DC FAIL lights illuminate when the airplane battery power is insufficient to maintain IRU operation.

 (4)  
　
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