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 (a)  In the rapid re-alignment mode, the IRS ALIGN MODE message(s) appears on the main EICAS display, the IRU reference axis is aligned to the local vertical and residual velocities are set to zero. The rapid re-alignment mode can be entered either by turning the mode select switch from NAV to ALIGN or from NAV to ALIGN to NAV. 1) NAV to ALIGN. If left in the ALIGN position the system
 will continue to zero the residual velocities and after several minutes begin to update the heading and latitude calculations. The IRS ALIGN MODE message(s) will be removed and the IRU will enter the navigation mode when NAV is selected.
 2)  NAV to ALIGN to NAV. The IRU will perform a rapid re-alignment and after 30 seconds the IRS ALIGN MODE message(s) will be removed and the IRU will reenter the NAV mode.
 NOTE: During rapid re-alignment mode, sensor biases are
____ not recalculated and using method 2) above heading is not re-estimated. Heading errors and sensor bias drift from the previous flight leg will remain for the next flight leg. For this reason, rapid re-alignments should not be used prior to flights where the IRS will be used as the sole means of navigation if the combined operating time from the last full alignment to expected destination arrival time exceeds 18 hours.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
 (b)

(c)

(d)

(e)

A
747-400
MAINTENANCE MANUAL

 NOTE: During rapid re-alignment mode, sensor biases are
____ not recalculated. Sensor bias drift will therefore accumulate with time and IRS navigation performance will degrade slowly. It is recommended that a complete alignment be performed at least once per day to prevent this.
The IRU will not enter rapid re-alignment mode if IRU computed ground speed is greater than 20 knots.As an option, latitude and/or longitude may be entered while the IRS ALIGN MODE message(s) appears on the main EICAS display. Entering a more accurate position will eliminate initial position error.If position is entered, the IRU compares entered latitude and longitude with the last computed latitude and longitude. If the difference in longitude is greater than one degree or, if the difference in latitude is greater than 0.5 degrees, the ENTER IRS POSITION message will appear on the CDU. 1) If a second position entry is made that is not identical to
 the first entry and, if the position comparison test fails, the ENTER IRS POSITION message will continue to be displayed.
 2)  If a second position entry is made that is not identical to the first entry and, if the position comparison test passes the ENTER IRS POSITION message will be removed.
 3)  If a second position entry is made that is identical to the first entry, the IRU assumes its navigational accuracy was extremely poor during the last flight leg and the message CYCLE IRS OFF-NAV appears on the CDU. If a subsequent position entry is made which passes the position comparison test, the message will be removed.
The airplane must not be moved during the alignment period. If excess motion is detected by the IRU, the IRS MOTION message appears on the main EICAS display for a minimum of 30 seconds and until the motion stops. A complete 10 minute alignment automatically restarts after the motion ceases. It is not necessary to enter position should this occur.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 34-21-00
 ALL  ú ú 16 Page 25 ú Feb 18/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (3)  
IRS navigation mode

 (a)  
In the NAV mode, the IRS performs inertial navigation, calculating position using the position entered during the alignment mode as the starting point for a dead reckoning computation. It also provides outputs of airplane pitch and roll, true and magnetic heading and track, rotation rates, accelerations, velocities, windspeed and direction.

 (b)  
IRU navigational accuracy is statistical. That is, the position will drift in a random direction from actual position. The rate of this drift is also statistical but is certified with the FAA to be less than two nautical miles per hour 95% of the time. This means that for 5% of the flights, the computed position of a normally operating IRU will drift at greater than two nautical miles per hour. The higher the drift rate the less likely it is to occcur.
　
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