(8)  
IRS Alignment

 (a)  In the alignment mode, the IRU reference axis is aligned to the local vertical, the IRU calculates latitude by estimating the horizontal earth rate components, and velocities are set to zero. To complete the alignment process, the IRU needs present position data. This data can be entered using the CDU.

 (9)  
Alignment criterion

 (a)  A successful alignment requires the following: 1) Present position (latitude and longitude) has been entered
 from the CDU.
 2)  The total time in the ALIGN mode is 10 minutes or greater.
 3)  The quality-of-alignment index is less than 1. (This is a
 measurement of how fast the computed earth rate is reaching a steady state value). 4) For latitude, the cosine of the entered amount equals the computed cosine amount within 0.01234. 5) For latitude, the sine of the entered amount equals the computed sine amount within 0.15. 6) The latitude and longitude in memory equals the input latitude and longitude within one degree.
 (b)  If during the process of full alignment, the criteria outlined above is not met, several indications occur. If the quality-of-align index is greater than one at ten minutes, the IRS LEFT, CENTER and/or RIGHT messages appear on the main EICAS display. The fault ball is set on the IRU and this fault is also stored in BITE memory.
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747-400 MAINTENANCE MANUAL
 (c)  If the entered latitude fails the sin/cos test, an error message will be displayed on the CDU. If the same latitude is entered a second time, the IRS LEFT, CENTER and/or RIGHT, messages will appear on the main EICAS display. If the second latitude entry does not agree with the first entry, either the error message will continue to display (entry does not agree with computed latitude), or the message will be removed (entry agrees with computed latitude). An IRU fault is displayed only after the same latitude is entered twice and fails the sin/cos test after the second entry. A latitude may be entered at anytime during the alignment process. If it passes the tests, the system will enter the NAV mode providing that the MSU switch is in the NAV position.
 NOTE: The latitude sin/cos test will be performed only once
____ per latitude entry and once prior to entering NAV mode.
 (d)  
The entered latitude and longitude are compared to the last computed latitude and longitude stored in the IRU memory. If the difference is greater than one degree, an ENTER IRS POSITION message appears on the CDU scratch pad. If the second entry is identical to the first it is then taken as correct. If the remaining tests pass, the IRU enters the NAV mode providing that the MSU switch is in the NAV position. Once in the NAV mode, no further latitude or longitude updates are accepted by the IRU.

 (e)  
If excess motion is detected, alignment is automatically restarted and the IRS MOTION message appears on the main EICAS display.

 (10) BITE Interface Test
 (a)  Pushing the interface test switch causes the following to occur: 1) For 0 - 2 seconds - BCD and BNR output data with SSM set to
 functional test and data set at the pre-determined values.
 IRS valid output discrete is set to invalid.
 2)  For 2 - 10 seconds - No BCD output data. BNR output data with SSM set to failure warning and data set to the pre-determined values. IRS valid output discrete is set to actual status.
 3)  After 10 seconds - BCD and BNR output data with SSM set to functional test and data set to pre-determined values.
 (b)  
The interface test mode is inhibited whenever the indicated ground speed exceeds 20 knots or the IRU is in the attitude mode.

 (c)  
The interface test can also be initiated by the CMC through the CDU ground tests page.

 B.  Control
 (1)  To activate the IRS, align the IRS (AMM 34-21-00/201).
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