Date: 202105
Local time of day: 1201-1800
Location: RST.Airport, MN
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach; Final Approach
Airspace: Class D RST
ATC / advisory: TRACON RST; Tower RST
Second aircraft: Commercial Fixed Wing
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Confusion; Distraction
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Detector: Automation Aircraft Terrain Warning; Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Human Factors; ATC Equipment / Nav Facility / Buildings
Primary problem: Human Factors

Narrative: Glide slope alert flying into RST. During approach phase of landing into RST. I was the Pilot Monitoring. We were cleared for the visual to runway 13. I recall we were descending and the Pilot Flying had the FAF altitude of 2;600 set; and the banana bar was indicating that we should level off at that altitude at approximately 0.25-0.75 prior to Ellie (FAF). I believe the approach mode was also activated; and we were fully configured. At the same time we were setting up for the approach there was a GA aircraft taking off; and a [air carrier] taxiing and holding short of Runway 13. I was focused outside and listening to ATC. We were both surprised when we received a glide slope alert. I believe that we were approximately 200 feet below the FAF when we received the alert. Around about this time I believe the 757 was cleared for takeoff. When we received the alert the Pilot Flying kicked off the autopilot; increased power and worked to re-establish a stabilized approach. We were still operating in visual conditions and had the runway in sight. From my perspective it seemed he had re-established a stabilized approach. Around 200-300 feet; or so; we received another glide slope alert. I do not recall us being excessively low to have activated the glide slope aural alert. The aircraft landed normally and safely. The Pilot Flying was frustrated that we had by all indications an unstable approach. In rethinking the events I am confident that we had the aircraft correctly configured and ready to fly the ILS with the aid of the autopilot. While we could not confirm this; our best guess was that the taxiing air carrier had somehow interfered with the glide slope and we began descending prior to the FAF. I felt that he had established a stabilized approach prior to 1;000 AGL; so the second aural alert was also a surprise. I believe it was the PF's first time flying into RST. After the first aural glide slope alert we probably should have called for a go-around. The alert caused a couple moments of confusion. We probably should have just gone around and removed the distraction.




Synopsis: Air carrier Captain reported GPWS Alerts on approach that was potentially due to an interfering aircraft taxiing.