737 FMC position
Hi all,
Question: consider a 737 classic with one FMC, two IRUs, two GPS receivers, and a single frequency-scanning DME radio. Should it be possible for a single malfunctioning IRU to cause a noticeable shift in the FMC position (thus a mapshift on the EHSI) if all other sensors are working fine?
I know it is possible, since this scenario happened during an actual flight, but I wonder if this is normal system behaviour. Based on the information from the Boeing FCOM, I'd say the FMC position is derived from GPS, DME/DME, VOR/DME or Localizer/DME in that order of priority, while IRS position is used as a reference only if external position updating is not available (of course, that raises the question: which IRS? Left or right?). Did the impossible occur?
Any ideas would be appreciated!