Narrative:

Enroute at FL380; we experienced a gradual failure of the right IRS/IRU while the first officer was the pilot flying. Initially; we observed the aircraft begin to deviate to left heading with rapidly increasing left crosswind at 180/200+ knots and increasing. Next; we observed autopilot trying to correct and aircraft was deviating south of course. At this point; we began crosscheck with other instruments and communicated to each other what we observed. As we were trying to determine who had good data; the right IRS fault light illuminated; and I took command of the aircraft; as the first officer's CDU lost attitude and altitude data. I was suddenly hand flying at FL380 as first officer pulled QRH to begin checklist. We were in clear air and I stabilized heading and altitude while we communicated with ATC that we needed a heading. I was hand flying at FL380 for first time; probably ever; in my career. It's a real different experience than hand flying below 10;000 feet.as I tried to follow with back up the first officer as he read and executed the QRH; I unintentionally allowed the aircraft to climb to 38;320 feet; momentarily; before correcting back to FL380. Cle center asked us our altitude and we acknowledged that we had briefly been at 38;300 feet; but had corrected to FL380. We told them we were having to hand fly the aircraft and had a systems malfunction we were trying to diagnose and to stand by. Once we completed QRH; we told them we could not use autopilot and would have to leave rvsm airspace; which we did. Later; after communication with dispatch; we diverted flight for maintenance and fuel.

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Original NASA ASRS Text

Title: A B737-700 flight crew; level at FL380; experienced a malfunctioning IRS causing the First Officer to lose altitude and attitude data. When the Captain assumed control with the autopilot off a modest altitude deviation occurred in part due to lack of experience with hand flying at altitude.

Narrative: Enroute at FL380; we experienced a gradual failure of the right IRS/IRU while the First Officer was the pilot flying. Initially; we observed the aircraft begin to deviate to left heading with rapidly increasing left crosswind at 180/200+ knots and increasing. Next; we observed autopilot trying to correct and aircraft was deviating south of course. At this point; we began crosscheck with other instruments and communicated to each other what we observed. As we were trying to determine who had good data; the right IRS Fault light illuminated; and I took command of the aircraft; as the First Officer's CDU lost attitude and altitude data. I was suddenly hand flying at FL380 as First Officer pulled QRH to begin checklist. We were in clear air and I stabilized heading and altitude while we communicated with ATC that we needed a heading. I was hand flying at FL380 for first time; probably ever; in my career. It's a real different experience than hand flying below 10;000 feet.As I tried to follow with back up the First Officer as he read and executed the QRH; I unintentionally allowed the aircraft to climb to 38;320 feet; momentarily; before correcting back to FL380. CLE Center asked us our altitude and we acknowledged that we had briefly been at 38;300 feet; but had corrected to FL380. We told them we were having to hand fly the aircraft and had a systems malfunction we were trying to diagnose and to stand by. Once we completed QRH; we told them we could not use autopilot and would have to leave RVSM airspace; which we did. Later; after communication with Dispatch; we diverted flight for maintenance and fuel.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.