34-28-00

 ú ú 08 Page 46 ú Mar 15/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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A
737-300/400/500MAINTENANCE MANUAL
When the mode select switch on the MSU is placed in the ALIGN position, the respective IRU enters the alignment mode. The IRU reference axis is aligned to the local vertical. It computes latitude by estimating the horizontal earth rate components. True north is computed from the vector sum of these earth rate components. While in the alignment mode, the IRU will output a signal to light the ALIGN display on the MSU.Minimum of 10 minutes are required to complete the alignment. The system can engage the NAV mode at any time after a satisfactory alignment as determined by the following: 1) The total time in the ALIGN mode is 10 minutes or greater. 2) The quality-of-alignment index is less than 1. (This is a
 measurement of excessive IRU sensor noise or excessive airplane movement during alignment. 3) For latitude: a) The cosine of the entered amount equals the computed cosine amount within 0.01234. b) The sine of the entered amount equals the computed sine amount within 0.15. c) The latitude in memory equals the input latitude within one degree. 4) The longitude in memory equals the input longitude within
 one degree. 5) The Mode Select Switch is set to NAV. 6) There has been no detected motion during alignment.If during the alignment process the criteria outlined is not met, several indications occur. If the quality-of-align index is greater than one at ten minutes, the IRU transmits a signal to the MSU. The signal lights the FAULT annunciator and sets the fault ball on the IRU. This fault is also stored in BITE memory.If the entered latitude fails the sin/cos test after the 10 minute alignment period, the ALIGN annunciator will flash. If the same latitude is entered a second time the flashing ALIGN annunciator will go steady and the FAULT annunciator will light. If the second latitude entry does not agree with the first entry, the ALIGN annunciator will continue to flash. A FAULT is annunciated only after the same latitude is entered a second time (after a 10 minute alignment period) and fails the sin/cos test after the second entry. However, a latitude may be entered at anytime during the alignment process. If it passes the tests, the system will enter the NAV mode.The entered longitude is compared to the last computed longitude stored in the IRU memory. If the difference is greater than one degree, the ALIGN annunciator will flash. If the second longitude entry is then taken as correct, the ALIGN annunciator goes out and the IRU enters the NAV mode if the remaining alignment completion conditions are satisfied. Once in the NAV mode, no further updates are accepted to the latitude or longitude by the IRU.
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 34-28-00
 ú ú 08 Page 47 ú Mar 15/90

BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
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When the mode select switch is in NAV or ALIGN position (ALIGN mode only), updating of the present position latitude and longitude is allowed from the FMCS CDU or from the ISDU. This, however cannot occur once the IRS has entered the NAV mode. In the NAV mode, the IRS performs inertial navigation. It provides data on airplane attitude. It outputs body rates and accelerations. It also provides true and magnetic heading along with velocity vectors. Latitude and longitude are also output. The IRS accepts the latitude and longitude entered during the alignment mode as the starting point for the dead-reckoning computation.

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The attitude mode provides a rapid attitude/heading restart capability. The ATT mode is used if the IRS has had a total power shutdown or BITE has detected a failure in the NAV mode. ATT mode outputs are attitude/heading, rate, acceleration, vertical velocity, and altitude.

 (aa) The attitude mode is selected by switching the mode select switch to the ATT position. Magnetic heading inputs from the FMC are only accepted by the IRU while the mode select switch is in the ATT position.
 (ab) If the attitude mode is selected, the magnetic heading changes will track the platform heading changes after heading is entered.
　
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