Date: 201712
Local time of day: 1801-2400
Location: RAP.Airport, SD
Flight conditions: VMC
Light: Night
Aircraft: Medium Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach; Final Approach
Airspace: Class D RAP
ATC / advisory: Tower RAP
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Communication Breakdown; Situational Awareness
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Contributing factors: Human Factors; Environment - Non Weather Related; Airport
Primary problem: Environment - Non Weather Related

Narrative: We became low too far out because we were cutting corners instead of flying to the FAF and following the vertical guidance down. Even though it was severe clear; it was dark and this was only our 4th time collectively we had flown into the airport.It was day three of four for me. We had picked up the street Captain associated with the incident halfway through day two; so this was day two of three for him.We climbed out on the Departure; no issues. It was fully dark at this point; but an almost full moon was shining on this clear night. Cruise was uneventful. The Captain and I briefed ILS 32 and started our descent into RAP; asking Center for lower so we did not have to descend at a breakneck pace.We were handed off to approach and were told to descend to 7000. The landscape around RAP is pretty desolate; so there weren't many lights and buildings to judge distance to the ground. I knew elevation around there didn't drastically change as much as it did west of the airport; but I knew there were rolling hills of decent size.As we approached from the south; setting up to land Runway 32; approach asked us if we had the airport in sight about 19nm away almost directly south of the airport. We both did; and we were cleared Visual approach Runway 32; as expected; then to switch to tower. I 'cleaned up the FMS' for the Captain (CA); meaning I had extended the centerline out past the final approach fix of YAVUC (5000'; 7.3 DME) and HELTA (5500'; 13.3 DME). The Captain elected to turn in toward the airport and start his descent. He had also spun in an altitude of 400'. (I'm not sure the exact altitude; I just knew it began with a 4). I had noticed that he was going to join the approach inside the FAF; so from what I could gather; he didn't want to ALTS CAP too high and have to chase the glideslope all the way down.I had questioned him about setting an altitude below the FAF; and although I believe he had an idea of what was going on in his head; I don't recall if he verbalized his intentions; because as we were descending; the gear horn began to go off and the green radio altimeter was showing us progressing through 1000; 900; and then maybe 800' above the ground. I recognized the sound; he was confused on what was happening; and I noticed what was happening; so I said; 'we need to climb' and I pushed the autopilot disconnect while pitching the plane up. He began to understand what was happening; so he moved the thrust levers - that were currently at idle; which I realize why the horn was going off - up to CLB or TOGA; I don't fully recall. We climbed about 400 feet or so and were about 13nm away from the airport now. I asked him if he wanted flaps; he said; 'Yeah; 1; 8; and 20.' I checked the speed (saw and confirmed about 220 kts) and added flaps.I think he was still in blue needles and I had switched over to green at this point; and he followed shortly thereafter. We dropped the gear about 5nm out from the runway; and fully configured right away. The airport and PAPIs were in sight the entire time during this approach; and I never saw four red - always at least one white. According to Flightaware.com; we joined the localizer about 4 or 5 nm away from the runway; inside the FAF. Although it was flown visually; he was about 1 dot below the GS almost all the way down; though it was stable and he was attempting to make corrections to his airspeed and altitude. By the time 500' rolled around; he was stable; just a couple degrees left of centerline; and we elected to continue and land.I did not feel like we were unstable during that segment of the approach; or I would have called for a go-around. He was shaken; and pretty behind the plane; but he seemed to have caught up in time to make a successful landing. We rolled out and went to the gate with no issues.We should have joined the approach much further out; and configured much earlier. There was no one behind us or in front of us; so speed was not an issue. I should have been more commanding as a First Officer (FO) with a brand new street Captain with 50 hours in the plane; although I think I handled the situation to the best of my abilities. If I were to do it all over again; I would have tried suggesting to fly the published approach and I would have slowed and configured much earlier.The main problem I saw was the lack of confidence the CA had in himself as a pilot. He's a solid pilot; with thousands of hours of experience; but he was behind the plane a lot this trip; and this was just one notable example.




Synopsis: First Officer of a turbojet reported experiencing a marginally stable approach at night into an airport with mountainous terrain.