3. IRS Mode Select Unit (MSU) (Fig. 8)__________________________
A. The mode select unit (MSU) provides mode selection for two IRUs through two mode select switches and monitors the operation of the IRUs 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 IRUs and the IRUs 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 IRUs remain in the align mode. Normal alignment requires the entry of present position into the IRUs.
(1)
The NAV position of the switches enables the navigational mode, provided the alignment period is completed in the IRUs. 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.
(2)
The ATT position of the switches places the IRUs 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.
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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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A
737-300/400/500MAINTENANCE MANUAL
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.
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 IRUs and the IRUs 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)
The FAULT Lights illuminate when an abnormal condition exists in the IRUs.
H. The lights may be tested by pressing each light switch assembly, by activating the bright-dim-test circuit or by initiating an IRU test.
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