As others have said, once the IRSs have aligned, that’s it. I think even the runway update only affects the FMC position, not the IRS. The FMC position will be based on:
DME/DME
LOC/DME (during approach)
VOR/DME
IRS
in that order, as far as I can remember.
During my FO initial line training on the 737-300 in 1987 (pre-GPS), we were on a LGW-TFS sector. Some time after coasting out near Faro, we got the “IRS NAV ONLY” message. My trainer asked me, “so, what’s navigating the aircraft now?” “IRS”, I replied confidently. “Actually, it’s still the FMC, but the FMC position will gradually drift towards the IRS position, until we get radio updates again” came the wise response.
Fast forward a few years and this time I was in the LHS, again approaching Faro, our destination. Suddenly the aircraft turned left. Inspection of the EHSI showed that the pink string was a few miles to our left and as we were in LNAV, the autopilot was doing its best to keep up. “Ah, we must have lost radio updating and the FMC is using the IRSs now”, I informed the co-pilot. Imagine my surprise when he said, “actually, we’re still using DME/DME!”
POS INIT page showed that the radio (and FMC) position and the IRS position differed by about 5nm, which was a bit excessive after only 2.40 flight time. We reverted to “raw data” and confirmed that in fact the DME/DME position was in error. Staying with old-fashioned methods for the reminder of the descent, approach and landing, proved uneventful (day, VMC).
On the northbound home sector, we were prepared and when the same thing happened, we inhibited the DMEs that the FMC was agility-tuning until we were well clear of the area. It turned out that the FMC was using some TACAN equipment which had been moved temporarily on the back of a lorry for a military exercise!
Great photo, by the way! Brings back happy memories…..