(b)  
Set DSPL SEL in the PPOS position.

 (c)  
Enter the present position (latitude and longitude) by pressing in proper sequence, two of the following keys; N(2), W(4), E(6), or S(8) and the input position.

 (d)
Press ENT key to transmit the data displayed to both IRU's.The alignment mode can be entered by turning the mode select switch in any of four sequences: OFF to ALIGN, OFF to ALIGN to NAV, NAV to ALIGN, or NAV to ALIGN to NAV.

 (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.

 (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.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 B. Alignment Conditions and Indications (Fig. 22)
 (1)  
Fig. 22 lists annunciation for both good alignment and alignment problems.

 (2)  
The IRU compares entered longitude with the last known longitude stored in nonvolatile 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 nonvolatile 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: Prior to opening the IRS circuit breakers or removing power
____ from the airplane, turn the mode selector switch on the MSU to OFF and wait until the ALIGN annunciator extinguishes. Failure to do so will prevent the IRU's from storing the current present position in nonvolatile memory, and at the next start up the ALIGN annunciator will flash when present position is entered. Present position must then be re-entered to extinguish the ALIGN annunciator.
 NOTE: For IRU P/Ns 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.
 If the airplane is moved with the IRU's not aligned in NAV mode, the change in present position may be sufficient to cause the ALIGN annunciator to flash during initialization. Present position must be re-entered to extinguish the ALIGN annunciator.
 NOTE: For IRU P/Ns S242T101-105 and on (user friendly features),
____ the IRUs will automatically restart the alignment process if airplane motion is detected. The ALIGN light will flash for 30 seconds when excess motion is detected.
　
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