Narrative:

I was the pilot flying with autopilot on while in navigation mode with the ILS frequency for the runway set in both the captain's and first officer's navigation windows. The intent was to select ILS once the navigation function had completed the turn; allowing the final approach segment to be flown in ILS mode. We were just above 6;500 ft MSL descending to 5;200 MSL and cleared for the approach. As we proceeded inbound the aircraft started the turn toward the FAF. About halfway through the turn; a boxed; amber rnp indication flashed on the pfd and the aircraft started to roll out. I disconnected the autopilot and intervened but we still overshot the localizer.approach was informed of our degraded navigation and we requested a vector back to final. Rnp stayed illuminated for about 20-40 seconds then went out. We were back on course prior to the final approach fix and before intercepting the glidepath from below. I estimate we were back on course by 8 DME. Approach asked us about the overshoot as it occurred.it is my understanding that an FMS aircraft should have performed this maneuver correctly and legally. We were outside the final approach segment at the time of the overshoot. It was about a 45 degree right turn to final. I thought a re-vector would be smoother than intercepting the localizer at that angle. We were in VMC conditions with the visibility about 9 miles - the runway was not yet in sight. The rest of the approach was uneventful. A logbook write-up was made after the flight.

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Original NASA ASRS Text

Title: While maneuvering to intercept the ILS in the NAV mode; an MD-80 flight crew received an RNP warning; disconnected the auto pilot and requested additional vectors to final. The RNP warning extinguished well before intercepting final and the approach was completed successfully.

Narrative: I was the pilot flying with autopilot on while in NAV mode with the ILS frequency for the runway set in both the Captain's and First Officer's navigation windows. The intent was to select ILS once the NAV function had completed the turn; allowing the final approach segment to be flown in ILS mode. We were just above 6;500 FT MSL descending to 5;200 MSL and cleared for the approach. As we proceeded inbound the aircraft started the turn toward the FAF. About halfway through the turn; a boxed; amber RNP indication flashed on the PFD and the aircraft started to roll out. I disconnected the autopilot and intervened but we still overshot the localizer.Approach was informed of our degraded navigation and we requested a vector back to final. RNP stayed illuminated for about 20-40 seconds then went out. We were back on course prior to the final approach fix and before intercepting the glidepath from below. I estimate we were back on course by 8 DME. Approach asked us about the overshoot as it occurred.It is my understanding that an FMS aircraft should have performed this maneuver correctly and legally. We were outside the final approach segment at the time of the overshoot. It was about a 45 degree right turn to final. I thought a re-vector would be smoother than intercepting the localizer at that angle. We were in VMC conditions with the visibility about 9 miles - the runway was not yet in sight. The rest of the approach was uneventful. A logbook write-up was made after the flight.

Data retrieved from NASA's ASRS site as of April 2012 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.