Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Inflight Event / Encounter CFTT / CFIT
Flight Crew Took Evasive Action
Human Factors; Procedure
While being vectored on a base leg to final; approach requests us to call the field in sight. It was a night approach with good weather and we had the field at 10 miles but I initially declined to call the field because of our angle and unfamiliarity with the area. After the second (or third?) request from ATC we accepted the visual at around 6 mile dog leg. They had directed us to keep speed up so now I was gauging the energy state while working toward a straight in final. As we descended I scanned the radar altimeter and noticed it fluctuating between 1;300-900 feet. We were configured at flaps 25 and I verbalized this to my FO (First Officer) as I temporarily leveled our descent and checked our distance from the runway. DME was NOTAMed out of service so I put [the] runway on fix page during brief and it showed around 4-5 miles. Bar altimeter was 2;300 with 800 ft elevation so we determined that hilly terrain was causing the spike. By this time radar altimeter normalized above 1000 ft and we had selected flaps 30 so we continued approach to landing.Approach brief covered obstacles but I did not take into account hilly terrain in the area. Also; our desire to facilitate ATC requests to keep speed up and accept visual approach.At unfamiliar airports during night operations it is advisable to let ATC vector to the initial or final approach fix.
Air carrier Captain reported noting radar altimeter readings as low as 900 feet AGL on a night visual approach to SAT.