Date: 201909
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: Q400
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Landing
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Situational Awareness
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Crew initially planned for and briefed the CAT 3 approach to ZZZ Runway XXR. While on arrival; ATC informed crew of a runway swap to XYL. Crew reloaded FMS for ILS CAT 2; (using CAT 3 procedures) to Runway XYL and reviewed the approach review items. While on final vector to join the localizer; the HSI showed red X on CDI signifying lack of localizer signal. Altitude was 2;400 ft. MSL approaching the final approach fix. We were the first aircraft to make an approach to Runway XYL. After querying them; ATC informed us that they had not yet switched the localizer antenna to Runway XYL (major contributing factor). Hence the lack of reception. Aircraft was VMC at this point and continued visually. Pilot monitoring changed the approach in the FMS to a GPS approach for vertical guidance and cross-filled for Pilot Flying. Just prior to reaching 1;000 feet. AFE; Pilot Flying called for flaps 35 and Pilot Monitoring selected them. Final approach was flown at Flaps 15 open bug speed (120 kts). While crossing the runway threshold; crew received aural GPWS 'too low; Flaps' warning. GPWS warning resulted from mis-compare between GPWS ground proximity flap setting (15 degrees) and actual flap setting (35 degrees). At this point; the aircraft was on glide path (PAPI) and stable on-speed. Given the prevailing conditions; stable aircraft condition; and suitable runway remaining; Pilot Flying elected to not to execute a missed approach (normal GPWS response to warning issued). A safe and uneventful landing was made within the touchdown zone.[Company] crews are very standardized in procedural practice. Hence when the approach became a GPS approach; crew automatic response was to fly a flaps 35 approach. Given the phase of flight and close proximity to the runway; sufficient time was not available to re-calculate landing speeds. Crew would have been best to fly the approach at flaps 15 and appropriate calculated speed. In this case; crew situational awareness broke down to some extent; resulting in the GPWS warning. Flight crews must remain mentally flexible during situations like ours and be able to think beyond our routine procedures and check lists in order to compensate for non-standard variables in the flying environment. Had the prevailing conditions (ceiling/visibility) been worse or runway was shorter crew would have executed a missed approach procedure.




Synopsis: Q400 Captain reported that loss of situational awareness and a last-minute runway change resulted in an unstabilized approach and an erroneous GPWS warning.