Date: 201809
Local time of day: 0601-1200
Location: OUN.Airport, OK
Flight conditions: Mixed
Weather elements / visibility: Fog; 2
Light: Daylight
Aircraft: PC-12
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Initial Approach
Airspace: Class E OKC
ATC / advisory: TRACON OKC
Aircraft component: GPS & Other Satellite Navigation
Component problem: Malfunctioning
Reporter organization: Personal
Reporter function: Pilot Flying; Single Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Human factors: Human-Machine Interface; Troubleshooting; Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification; Flight Crew FLC complied w / Automation / Advisory; Flight Crew FLC Overrode Automation; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Aircraft; Airport
Primary problem: Ambiguous

Narrative: During arrival into Oklahoma City terminal area; I was routed via the GULLI 2 Arrival for the OUN airport. Navigation systems and all aircraft systems appeared to be functioning normally. The aircraft was on autopilot with NAV mode engaged. Upon reaching the GULLI intersection the aircraft failed to turn toward the next fix. The advisory caution concerning anticipated turn was indicated as normal. During the period after GULLI while the aircraft was being hand steered to the next segment; I noted the GPS signal was not in error mode. I requested and received a heading from the Approach Controller to assist with troubleshooting the issue. I was then able to reset the avionics as required. All normal functions were confirmed to 'restored/normal' and the autopilot was re-engaged to track the heading assigned. I concurrently prepared for the RNAV 18 Approach to OUN. During the approach sequence all was normal with glide slope arming and subsequent 'capture' of glide slope. At approximately 2.5 - 3.5 mile final the LPV annunciation was displayed; but the EFIS red X appeared on the vertical flight director segment. The #1 GPS then changed to LNAV instead of LPV. In accordance with established self-regulating 'policy'; I executed a missed approach and requested a vector for an ILS approach to Runway 18. During the vectors for ILS; I set up number 2 NAV for the RNAV 18 Approach as a backup and to assist with an anticipated PIREP. The ILS approach was completed normally; but the #2 GPS had the same failure as during the first approach; with a loss of GPS accuracy. Weather conditions were 200 OVC 2SM on the approach with tops of ground fog/clouds at 3;000. When reported to ATC; I received information that at least one other aircraft has a similar experience. There were NOTAMS out for high level GPS abnormalities but I believe this incident was related to ground based GPS augmentation equipment. (GBAS).




Synopsis: PC-12 pilot reported a failed LPV approach due to a loss of GPS accuracy.