(a)  
OFF to ALIGN. Switching from OFF to ALIGN requires that the initial position (latitude and longitude) be entered in order to complete the alignment satisfactorily. This requires that initialization of the IRU be performed and that the airplane is not moved during the alignment cycle. If left in the ALIGN position, the system will continue to perform the alignment process.

 (b)  
OFF to ALIGN to NAV. Switching from OFF to ALIGN to NAV requires that the initial position be entered in order to complete the alignment and enter the navigate mode and that the airplane is not moving. Once the alignment process is complete, the align annunciator will go out and the IRU will enter the navigate mode.

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A
737-300/400/500MAINTENANCE MANUAL
 (c)  
NAV to ALIGN. Switching from NAV to ALIGN will zero the system residual velocity errors and level the system. If the ground speed is greater than 20 knots, the IRU will remain in the NAV mode of operation. A maximum of 30 seconds is required to complete the process. It is permissible to enter a second, more accurate latitude and longitude to improve the system alignment. Updates to latitude and longitude are allowed only during the ALIGN mode. The system will enter the navigation mode when selected manually.

 (d)  
NAV to ALIGN to NAV. This sequence, when completed in less than 30 seconds, will zero the system residual velocity errors and level the system. The system will automatically revert to the NAV mode after the 30 second releveling process is complete. The above sequence should only be performed when the airplane is not moving.

 B. Alignment Conditions and Indications (Fig. 18)
 (1)  
Fig. 18 lists annunciation for both good alignment and alignment problems.

 (2)  
The IRU compares entered longitude with the last known longitude stored in non-volatile memory. If the difference is greater than one degree, the ALIGN annunciator flashes. This could legitimately occur (i.e. the entry is correct) if the IRU was newly installed. In this case, a re-entry of the same (correct) position will be accepted.

 (3)  
The IRU compares entered latitude with the last known latitude stored in non-volatile memory. If the difference is greater than one degree, the ALIGN annunciator flashes. A re-entry of the same (correct) position will be accepted, causing the align annunciator to return to its constant-on condition. After the alignment period, the entered latitude is compared with the IRU computed latitude during alignment. A difference in latitude sine > 0.15 or a difference in latitude cosine > 0.01234 will cause the ALIGN annunciator to flash after the ten-minute alignment period has elapsed.

 NOTE: For IRU P/N S242T101-105 and on (user friendly features),
____ the position test is inhibited after the IRU is returned from a shop visit, after an abrupt power down, or after selection of ATT mode, This is to eliminate nuisance flashing of the ALIGN lights.
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A 737-300/400/500MAINTENANCE MANUAL
ALIGNMENT CONDITIONS TABLE
ACTION  RESULTS 
ALIGN ANNUNCIATOR  FAULT ANNUNCIATOR  OPERATIONAL MODE 
ELAPSED TIME < 10 MINUTES. 
LATITUDE DATA ENTERED IS NOT WITHIN ONE DEGREE OF STORED VALUE OR MISCOMPARES WITH PREVIOUS ENTRY.  FLASHING  OFF  ALIGN (DATA STORED) 
SAME LATITUDE DATA ENTERED SECOND CONSECUTIVE TIME (IDENTICAL DATA).  ON  OFF  ALIGN (DATA ACCEPTED) 
ACCEPTABLE LATITUDE DATA ENTERED (WITHIN ONE DEGREE OF STORED VALUE).  ON  OFF  ALIGN (DATA ACCEPTED) 
LONGITUDE DATA ENTERED IS NOT WITHIN ONE DEGREE OF STORED VALUE OR MISCOMPARES WITH PREVIOUS ENTRY.  FLASHING  OFF  ALIGN (DATA STORED) 
SAME LONGITUDE DATA ENTERED SECOND CONSECUTIVE TIME (IDENTICAL DATA).  ON  OFF  ALIGN (DATA ACCEPTED) 
ACCEPTABLE LONGITUDE DATA ENTERED (WITHIN ONE DEGREE OF STORED VALUE).  ON  OFF  ALIGN (DATA ACCEPTED) 
　
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