B737-800
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Situational Awareness; Troubleshooting
Aircraft Equipment Problem Critical
Flight Crew Overcame Equipment Problem; General Declared Emergency
Human Factors; Aircraft
During cruise flight; we received a warning on the FMC; 'VERIFY POS: IRS-IRS.' We noted that the left IRS was showing a very large error; over 25 miles at the time and growing. We consulted the QRC; FOM; Flight Handbook and Systems Manual for guidance as to specifically what the error meant and how to handle it. We cross checked our remaining navigation information and determined the right IRS was working properly. We notified Dispatch via ACARS; and contacted Commercial Radio to let them know of our navigation failure as per guidance in our manuals. We continued to carefully monitor and cross check our position and performed several extra plots on our plotting chart to ensure our position was correct. Our position was indeed correct operating on the good IRS. We also began monitoring a coastline VOR as soon as it was in range and used it as an additional check to continuously verify our position. After radar contact with Center; and after beginning our descent the left IRS generated a FAULT annunciation and apparently failed as the Captain's flight instruments failed. The First Officer then became the pilot flying. We then declared an emergency and got radar vectors to the airport. The checklist was consulted and that restored adequate instrumentation for the Captain to continue flying. We then landed uneventfully and wrote the discrepancy in the aircraft logbook.
An air carrier flight; eastbound over the Pacific Ocean; suffered a single IRS failure and eventual loss of the Captain's flight instruments. The flight continued utilizing the First Officer's instruments and IRS data.