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IRS drift (GPS Fail) causes ND intermittently fail

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I did a little experiment with GPS failures (AFAIK some 747-400s did not have GPS, but it's not an airline option in the QOTSII) and the IRS drift was significantly large over a 2 hour time-span that the ND failed during the approach, and the map started to shift. No EICAS messages however, apart from the original "GPS" failure message.


The QOTSII doesn't seem to do any position updating through navaids when GPS fails, but ANP remained at 0.05nm. The ND map shift (when it didn't intermittently fail) is ~1nm. I never used PURGE or GPS NAV INHIBIT in the POS REF page.


The GPS failures were triggered mid-flight between Hong Kong and Guam. A FMC hold was manually entered at OBALE (IAF at Guam) and outbound was flown using heading select. During the first turn inbound to join the hold (which I immediately exited for the ILS 06L approach) the ND glitched with rapid map shifts before completely failing. The ND came back occasionally for ~5 seconds during the approach but by then the map shift became very large.



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