B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 217; Flight Crew Type 217
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Aircraft; Human Factors
[While] under Center control; an IRS-Right message; followed by an unable RNP message appeared on the FMC. Although LNAV was still active; the aircraft appeared to be right of the magenta navigation line. I believe we were about two miles right of course. I was the pilot monitoring. I notified ATC of the navigation error and he cleared us direct [to a new fix]. I asked ATC to ensure we appeared to be headed in the right direction; as I was not confident in the accuracy of the navigation at this point. ATC indicated it looked good to him.Subsequent discussions with ATC resulted in our vacating RVSM airspace. At one point I switched the IRS switch to both on left. This disconnected the autopilot [and] it would not re-engage. I put the IRS switch back to both and reconnected the autopilot. The RNP value remained in acceptable limits for the remainder of the flight; although the right IRS remained inoperative.We landed uneventfully at our destination; logged the discrepancy in the logbook; and notified Dispatch and Maintenance.
While at cruise; we got a navigation error message unable required Navigation Performance RNP and verify position IRS. We noticed the position star on the HSI showed one of the positions way off to our right. On the position reference page; the right IRS showed a position way different than any of the other sources. I think the Captain in trouble shooting switched to left IRS and at some point the aircraft began a left turn of about 20-30 degrees.
B737-700 flight crew track deviation resulted when the right IRS failed in flight.