Date: 202001
Local time of day: 1201-1800
Location: JAX.TRACON, FL
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb; Final Approach
Aircraft component: Autoflight System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Aircraft operated by First Officer with A/P in 'B' mode; vectoring from left downwind approach to Runway XXC. FOQA (Flight Operational Quality Assurance) hit graphic indicates whilst on a 30-45 degree intercept to the final approach course to Runway XXC and whilst at altitude descending from altitude 2;500 ft. to 2;000 ft. as directed by ATC. No aural alert received in the cockpit however; was notified next day by FOQA gate keeper of the GPWS FOQA advisory. Aircraft did not experience this. Altitude was correct per altimeter setting and following ATC instructions. No aural warning received in cockpit nor from ATC. Aircraft was fully within radar environment and control of ATC. Suspect an altitude calibration issue or other equipment anomaly with Aircraft X. Preceding leg while PIC and acting as Flying Pilot; Autopilot 'A' mode would not engage. Made note verbally that we would check and verify this when level off occurred. Due to short duration of the flight; we neglected to check this and continued the flight and subsequent approach to Runway XX at ZZZ. at approximate 300 feet; in Clear VMC conditions; a GPWS alert did occur advising 'Glide Slope.' Aircraft was on the ILS Glide slope and visual glide path indicator with a sink rate of 500-700 ft. per minute. All approach parameters were within limited and cross checked. Approach continued to landing well within touch down zone with no other abnormalities existing. The alert was dismissed as an erroneous message. Again the altimeter setting was correct and verified; no advisories were provided by ATC. Conversation on [date] with FOQA gate keeper regarding a false FOQA advisory about GPWS while on approach the following leg to ZZZ1 while in autopilot 'B' mode caused me to compare the two events and postulate that the autopilot 'B' computer may have a technical issue. Autopilot 'A' mode would not engage after takeoff during climb. Utilized 'B' mode and all operations normal for entire flight. Forgotten that 'B' mode was engaged and mistakenly did not write up anomaly of 'A' mode autopilot. Could not identify why the GPWS alert occurred and did not know what action would be needed as a normal approach to landing was possible with correct glide slope and visual path indications. Any erroneous indication or failure of equipment should be immediately reported and addressed by Maintenance. Flight Mechanic should have been consulted along with Maintenance control for appropriate action.




Synopsis: B737 Captain reported that an autopilot malfunction resulted in an erroneous GPWS warning during approach.