If the airplane takes off or you do a new alignment, the FAULT light goes off. If the LIM senses the voltage again in the air, you will see the LIM fault conditions on the next landing.
A34-21-00-049.fm
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737-700 MAINTENANCE TRAINING MANUAL

A34-21-00-049.fm
ADIRS - ADIRS MAINTENANCE CODES

January 2005
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SOUTHWEST AIRLINES PROPRIETARY(c) All rights reserved
SOUTHWEST AIRLINES.
737-700 MAINTENANCE TRAINING MANUAL

ADIRS - TRAINING INFORMATION POINT - FMCS BITE PAGES - IRS MONITOR

A34-61-300-062.fm
General
To do a check of the IRS position error rate, select IRS MONITOR on line select key (LSK) 5R, from the FMCS BITE page.
IRS MONITOR Page
The position error rate (nautical miles per hour) for each IRS shows on this page.
The FMCS calculates the position error rate for each flight. It divides the difference between the FMCS position and IRS position by the total flight time. The FMCS calculates the error rate at the end of each flight. The error rate does not show in-flight.
The error rate does not show for an IRS that is not aligned in the NAV mode or is in the attitude mode.
Using IRS Monitor to Calculate Radial Position Error
You can use the per hour drift rate calculated by the FMC to determine if the total radial position error for the IRU exceed limitations. The radial position error chart is found in the fault isolation section of the maintenance manual, part II, section 34-21-00-2, figure 201.
Example:
The crew writes up that the #2 IRU position is off compared to the #1 IRU and the FMC. You pull up the IRS Monitor page in FMCS BITE, and it shows the #2 IRU with a drift rate of 4.0NM per hour.
The crew states the IRU’s have been in NAV for approximately 4 hours.
First you need to determine the total position error for the #2 IRU.Do the following:
-Multiple the #2 IRU per hour drift rate by the time-in-nav (4.0 x 4=16 nm total error).

Next, go to the radial position error chart. Read the total error of 16 nm across the bottom line on the chart.
Draw a line vertically from 16 nm until you intersect the horizontal line for 4 hours in nav.
If the intersect point is to the left of the heavy diagonal line on the chart, the total IRU position error is “excessive” and the IRU should be replaced.
Another way to determine if the total error is excessive without using the chart, is to use the formula: 3 + 3T, where T = Time in Nav. This will give you the maximum total error possible.
In our example, if the IRU’s had been in NAV for 4 hours and applying that to the formula would give you the following:
-3 + (3X4) = 15 nm

15 nm is the maximum total error you can have with the IRU’s in NAV for four hours.
As the #2 IRU had a total position error of 16 nm, you have exceeded the maximum total error limit.
January 2005
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737-700 MAINTENANCE TRAINING MANUAL

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ADIRS - TRAINING INFORMATION POINT - FMCS BITE PAGES - IRS MONITOR

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737-700 MAINTENANCE TRAINING MANUAL

IRS - TRAINING INFORMATION POINT - IRU DRIFT CRITERIA

General
You use the position error chart to find the IRU drift.
Time In Navigation Mode
To use the position error chart, you must know the time that the IRU was in the navigation mode. The time is on the vertical axis of the position error chart. If the flight crew did not do any new alignments, the time can be for multiple flight legs.
　
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