Originally Posted by
Pepperseed
Would you then agree that the only ways for a 73 CL IRS to be removed from the NAV loop would be either by lack of elec power or by the system itself if the FMC Kalman filter detects an unacceptable drift from said IRS ?
Probably, yes. However, the FMC should disregard the IRS which is drifting away automatically. In any case, you should get the VERIFY POS: IRS-FMC, and then during the FMC Nav Check supplementary procedure, you can force the FMC to take one of the available sources as the current position (IRS L/R, RADIO), when you have confirmed that that is the most accurate source.
Note that as in the Honeywell document, for this chart to be used accurately, full IRS align is required before each flight.