Air carrier flight crew reported approximately 10 NM from the fix WYLER in the vicinity of PIT; the aircraft began to experience navigational errors. The pilots received RNP; ANP; LOC and GPS flags.
Synopsis
Air carrier flight crew reported approximately 10 NM from the fix WYLER in the vicinity of PIT; the aircraft began to experience navigational errors. The pilots received RNP; ANP; LOC and GPS flags.
Narrative
PIT approach control cleared us to WYLER in order to intercept the localizer in expectation for the ILS 32 at PIT. The weather was a broken ceiling so the ability to visually navigate to the field visually was impaired. We were proceeding direct to WYLER; descending from approx 10;000 to 6;000. Somewhere outside of WYLER; best guess is within 10 NM; the aircraft symbol began to drift off course and the autopilot began to attempt to correct back to course (LNAV active and center autopilot active). The RNP was 1.0 at the time and the ANP started to vary; turning amber as the number went from approx 2.0 to 4.2.We ensured that the aircraft leveled at 6;000 and the Controller queried our ground track. We promptly notified them that our navigation system was no longer reliable at that point and required vectors to localizer final for the ILS 32. Approach control gave us vectors back around to localizer final inside of WYLER; where the localizer was successfully intercepted; cleared the ILS 32 and performed an uneventful autoland. During the vectors the ANP began to return green and displayed a value of 0.01-0.03 through landing.Upon shutdown and the aircraft secured; a mechanic came on board asked us if we experienced a map shift. He informed us that he has seen this similar write up from other crews; both aircraft type X and aircraft type Y; with approximately 4-5 occurrences from his memory in the last month coming in for the ILS 32 via proceeding through WYLER. This is concerning because it is a repeating event that both aircraft type X and aircraft type Y are experiencing that prevents the crew from navigating accurately when transiting via the narrative above.
Second reporter narrative
On an intercept heading for the ILS 32 into PIT outside of WYLER four amber flags (RNP; ANP; LOC and GPS) appeared and we started to veer off course. The flags went back and forth between green and amber several times while the aircraft chased the proper course and went off course. ATC finally questioned us and we gave a quick response while taking vectors back to try again. They then vectored us inside of WYLER where we intercepted final and made an uneventful auto landing. My real concern was when we asked to talk to maintenance after shutdown he immediately asked if we had a map shift at WYLER. According to him this was the fourth or fifth such occurrence in about the last month.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.