MD-82
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Other / Unknown
Flight Crew Became Reoriented
Human Factors
First Officer flying modified left downwind Runway 12R San Antonio in VMC; after minor weather deviations on arrival. About 5 north of Alamo OM; First Officer pointed out visually SAT; and I acknowledged. I am 100% certain that we were looking at SAT during this exchange. After a minute or so; Approach turned us towards the south on a left base leg and asked if we had the airport; to which the First Officer nodded/gave thumbs up; and I answered yes to ATC; and we were cleared for the visual approach 12R. The First Officer started a slow turn to intercept final. I did a quick mental altitude check using 3 for 1 from Alamo; which showed us in the slot; and I then noticed that the ILS Glideslope for 12R showed capture on the FMAs; and with the heading we were on localizer capture was sure to soon follow. I then went heads down to get Tower frequency from the iPad; and put it in the Number 1 radio in anticipation of being handed off. As soon as I finished this; Approach handed us off; so I flipped the switch and checked in with Tower; and were cleared to land; after which I put Ground frequency in the unused side of Number 1 radio. During this period I was heads down for maybe 20 seconds or so; and when I looked up we were on a 45 degree intercept angle for a runway about 8 miles ahead; and all seemed OK for an instant; but our jumpseat rider then shouted out that we were headed for Lackland Air Force Base; not SAT.  I then realized that when we turned base leg; the First Officer had visually locked onto the wrong airport; and we had shot through the final for 12R while I was involved with the radio frequencies. At this same time; Tower advised us that we appeared to be heading too far south; and I asked permission to come left and enter a right base for 12R; which we did and landed uneventfully. I thanked the jumpseat rider; and telephoned the Tower. They did not seem too concerned; and said this happens rather frequently there; although I have never noticed that airport to the south in all the many times I have arrived to SAT in this exact scenario.Obviously I failed to adequately monitor the localizer capture; allowing all my attention to be dedicated to the radio frequency juggling. I do not know why the autopilot did not capture the 12R localizer; but I assume that while I was heads down; the First Officer used a different roll/pitch mode to take a 'better' angle towards what he thought was the right airport; thus dropping the ILS out of the arm mode that it was in. I have flown with this First Officer before; and know him to be a good pilot; so I probably let my guard down a little too much. I also feel that the fact that I had not flown for over 5 weeks because of vacation time was a factor; in that this was my second leg back; and I had that feeling of being just a little behind things compared to normal.
MD-82 Captain reported First Officer was flying a visual to SAT and locked in on Lackland; overshooting the localizer.