Boeing has received several questions regarding the criteria for making IRS maintenance log book entries. The following information is being supplied to help each airline establish IRS monitoring guidelines within their own operating specifications and maintenance programs.
Boeing recommends two IRU performance removal criteria. First, if the position error at the end of two consecutive flights is more than 3 + 3T nm (where T is the time since alignment in hours). Second, if the residual ground speed at the end of the flight is greater than 20 kt for two consecutive flights. These criteria are in the Fault Reporting Manual (FRM).
Trying to apply the criteria stated above can be a problems for pilots because the ADI ground speed is an average of the ground speeds the FMC receives from the IRSs. Also, the airplane position as displayed on the HSI map, is normally an average of the three IRS positions with radio update corrections added by the FMC.
If the ADI ground speed or HSI map display contains an obvious error, the cause may be an IRS problem. Another indication of an IRS problem is displayed on the CDU PROGRESS page where the number of IRSs being used by the FMC is displayed. If an IRS ground speed differs from the average by more than 20 kt, or the position differs from the average by more than 30 miles, the associated IRS will not be used by the FMC for the remainder of the flight. When an IRS position is not being used by a FMC the associated position is not displayed on the CDU POS REF page.
When the accuracy of an IRS is a suspected problem, the IRS performance can be checked after arriving at the destination gate. If the airplane is equipped with an IRS control panel with a display, it may be used for determining residual ground speed. An estimate of the accumulated position error can be seen by comparing individual raw (not radio updated) IRS positions from the IRS control panel display with the FMC position on the CDU POS REF page. Valid raw IRS positions are also displayed on the CDU POS REF page. The actual position error can be measured by entering the present position (actual gate position) and raw IRS position as waypoints on a legs page and reading the distance between the entered positions (accumulated position error).
Boeing Flight Operations Technical Bulletin 757-09, 19th April 1983