B.  Each mode select switch is detented in the NAV position which prevents accidental movement of the switch. When the switch is in the NAV position, it must first be pulled out of detent before selecting a new position to prevent damage to the switch.
 4.  IRS Switch on the Source Select Module
_______________________________________
 A.  The switches are located on the captain's and F/O's source select modules.
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Not Used
Figure 6

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 B.  The IRS switches allow the captain and first officer to switch between the left, right and center IRUs.
 C.  For normal operation the captain's switch is set to the L position and the first officer's switch is set to the R position.
 D.  The IRS source select switches contain switch contacts which are used by the IRS and other systems.
 (1)  
One set of contacts provides three analog discretes to each of the three EIU's. These discretes select the IRU data bus to provide data to the EIU's.

 (2)  
Another set of contacts provides discretes which control source select relays. These relays channel data from the selected IRU to the brake system and to the stall warning system in the MAWEA.

 (3)  
The left and right weather radar transceivers use different sets of switch contacts. The IRS switches change the IRU source as follows: ~................................................................. | | Source Select Module | | ....................................................... | Interface | Captain's | First Officer's | | System | IRS Switch | IRS Switch | | ....................................................... | | L | C | R | L | C | R | ................................................................... | Left WXR | L IRU | C IRU | C IRU | N A | N A | N A | ................................................................... | Right WXR | N/A | N/A | N/A | C IRU | C IRU | R IRU | 2...........…........…........…........…........…........…........1

 (4)  
The Center IRU receives discretes from another set of contacts on the first officer's IRS source select switch. These contacts, in addition to the contacts on the ADC source select switch determine which air data output is used by the center IRU.

 E.  When the captain's and first officer's Source Select switches are set to select the same IRU, the EICAS message SOURCE SEL IRS will be displayed.
 5.  Heading Reference Switch
________________________
 A.  The HDG REF switch is used to manually select either magnetic north (NORM position) or true north (TRUE position) reference on the PFDs and NDs. It is located on the EICAS control panel / FMC switch module, P2-4.
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 6.  Control Display Units (CDU) (Fig. 7)
___________________________
 A.  The CDU is used to enter present position data during alignment, to initialize heading data during ATT mode, to initiate the IRU self test through the CMC, to check IRU rejection criteria, to view ground speed, position, and to view status messages, (Ref 34-61-00/1).
 B.  IRS - related CDU pages
 (1)  
The POS INIT page is used to enter present position or heading into the IRS.

 (2)  
The MAINTENANCE INDEX page provides access to the IRS MONITOR page.
　
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