Medium Transport; Low Wing; 2 Turbojet Eng
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 82; Flight Crew Type 5364
Situational Awareness
Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Human Factors
While on visual approach to SAT we were given a runway change. We executed the runway change in the FMC and verified the changes.  As the Captain intercepted the localizer we began our approach. As the glide slope started to come in we started our descent. I could start to see that we were visually low on approach and stated that a correction was necessary. He also verified this and began to correct the altitude. The GPWS warning was activated around the time of the correction. We had both the runway and Tower in sight as we corrected. The Tower Controller noticed our altitude and informed us that the 13R ILS was OTS. We landed without further incident.
Air carrier First Officer reported receiving a GPWS terrain warning as well as a low altitude alert from ATC on a visual approach to SAT.