Date: 202201
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Dusk
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Autopilot
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Human-Machine Interface; Time Pressure; Troubleshooting; Distraction; Workload; Confusion
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning; Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Airspace Structure; Software and Automation; Aircraft
Primary problem: Aircraft

Narrative: FMS/FMC/Autopilot Failure. I looked at the instrumentation to confirm glide slope intercept and immediately recognized that we were descending early. I went to announce it to the CA (Captain) (Pilot Flying) who also recognized the situation at the same time. The aircraft was on autopilot; flying level; with VOR/LOC activated after being cleared for the approach into ZZZ. The aircraft; without being prompted; began to descend before intercepting glide slope at some point. The Captain instantly disconnected the autopilot and auto throttles; initiating a full power climb back to the intercept altitude. We received a call from ATC and received a minor GPWS warning as we were climbing. I feel that crew coordination; confirmation of what the aircraft was supposed to be doing and the reaction by the Pilot Flying (CA) were proper and allowed us to detect the situation early on. Because it was an automation failure; the aircraft will be written up for maintenance.


Second narrative: FMS/FMC/Autopilot Failure. It was a dark night; but severe clear. Being intimately familiar with the ZZZ topography; I am keenly aware of the high; rising terrain between the Final Approach Fix and Runway XXR. The airplane started descending without pilot input. I immediately noticed this and announced out loud: 'Something is wrong!'. The aircraft was on autopilot; flying level; with VOR/LOC activated after being cleared for the ZZZ ILS XXR approach. Before intercepting the glide slope; the aircraft began to descend on its own without being prompted. I disconnected the Autopilot and Auto-throttles and initiated an immediate climb. Within seconds after proactively initiating the climb; we received a single GPWS: 'Terrain'; and seconds later; ATC called and said: 'Check Altitude' and gave us the Altimeter Setting (which was set correctly.) I continued and landed safely. I believe that this was an automation failure of some kind and that the immediate action taken was just. Emphasis should be made; and recognition of; the excellent crew coordination by both myself and a superior FO (First Officer) ameliorated what could have been a potential negative outcome.


Synopsis: An air carrier flight crew reported the autopilot descended the aircraft below glide slope causing a Ground Proximity Alert and Low Altitude Warning from the Tower.