Date: 202101
Local time of day: 0601-1200
Location: SEA.Airport, WA
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B SEA
ATC / advisory: Tower SEA
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Human-Machine Interface
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control; Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors; Aircraft
Primary problem: Aircraft

Narrative: While on final approach into Seattle on a visual approach to [Runway] 16R; after Capturing glide path with the auto pilot; we were configuring the last of our flaps through 1;000 ft. and the Captain called for below the line; while running the checklist the Captain made the remark we were looking low. As I came up the plane was in the flight director; however for some reason the auto pilot had gotten knocked out of GS mode and defaulted to pitch hold. As the Captain started to correct back to the PAPIs we received a low alt alert from Tower; along with a glide slope warning from EGPWS. The Tower saw us correcting during the call however and noted disregard. As we were able to re established on glide path and were stable before 500 ft. we continued the approach to land. Not sure why the flight director disconnected from the glide slope and defaulted to pitch hold; and if was something we did while configuring; or if it was a fault of the plane or ground equipment.I was distracted by the running of the checklist and did not see the entities deviation from glide path; more why the autopilot changed modes. After returning to the displays I was getting conflicting information from the flight director and the glide slope indication. It would have been easier to just look outside and get the indications from the PAPI as to our current airplane state.


Second narrative: ILS [Runway] 16R into Seattle; visual conditions; LOC and GS captured with autopilot engaged. After aircraft configuration complete I felt like we were low. I saw that we were now in pitch hold and below glide path. LOC was still captured. I disconnected autopilot and leveled off; heard 'Glide-slope' as well. ATC was informing us about low altitude alert; then immediately said disregard. I regained glide-slope utilizing PAPI.Not sure why the GS disconnected; if it was something we did wrong or if it was a fluke or something to do with the system. I talked to the Captain taking the airplane about the event and suggested to use the ILS. Lesson learned; at all times monitor glide path and lat/vertical modes. The transition from GS to pitch hold was very subtle.


Synopsis: Air carrier flight crew reported that during final approach to SEA airport; the autopilot changed from glide slope mode to pitch hold causing the aircraft to descend below glide path. The crew regained control following a ground proximity warning and low altitude alert from air traffic control.