CREW IN AN ACR B727 MAKING A RADAR SURVEILLANCE APCH TO RWY 21 AT SAT WERE ISSUED A DSCNT TO THE 1280 FT MDA 10 NM FROM THE RWY. THE FLT NEARLY COLLIDED WITH A 1448 FT RADIO TWR APPROX 5 MI FROM THE RWY.

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APCHING SAN ANTONIO FROM THE N AND JUST SWITCHED OVER TO APCH CTL WE WERE INFORMED THAT RWY 12R; THE ATIS LNDG RWY; WAS CLOSED AND THAT WE WOULD BE USING AN ASR APCH TO RWY 21. THERE IS NO OTHER APCH TO RWY 21. THE WX WAS 1000 FT OVERCAST WITH 10 MI VISIBILITY AND WIND AT 160 DEGS; 7 TO 10 KTS. WE BRIEFED THE APCH AND WERE VECTORED FOR IT. AT 10 NM FROM THE THRESHOLD; WE WERE CLRED TO THE MDA -- 1280 FT; ON AN ASSIGNED HDG. NO OTHER XMISSIONS FROM APCH CTL WERE RECEIVED UNTIL 5 NM FROM THE RWY. WE WERE THEN TOLD THAT WE WERE 'ON COURSE 6; NO 5 MI FROM THE RWY.' WE BROKE OUT OF THE CLOUDS AT ABOUT 1700 FT MSL AND OBSERVED A TWR (TV; RADIO?) AT 12 O'CLOCK AND CLOSE. WE WERE AT 1500 FT. THE TWR IS CHARTED AT 1448 FT. THE FO WAS THE PLT FLYING AND HE TURNED IMMEDIATELY ABOUT 20 DEGS TO THE R. WE SOON SAW THE RWY; STILL QUITE A BIT TO THE R AND CONTINUED TO LAND WITHOUT INCIDENT. DURING THE APCH IT WAS OBVIOUS THAT THE CTLR WAS DOING IT ALL. IT WAS NIGHT AND HE WAS APCH/DEP/TWR AND GND. IT ALSO SEEMED TO ME THAT THE CTLR WAS ONLY USING THE ASR APCH TO GET US BELOW THE CLOUDS SO WE COULD CONDUCT A VISUAL APCH FROM THAT POINT. SOMEHOW THE TV TWR JUST TO THE L OF THE APCH COURSE AND HIGHER THAN THE MDA; WAS LEFT OUT OF THE EQUATION. TALKING TO AN FAA SUPVR LATER; I WAS TOLD THAT THE ASR TO RWY 21 WAS SUPPOSED TO BE A STEP DOWN APCH. THIS WAS ONLY THE SECOND ASR APCH I HAVE DONE IN OVER 30 YEARS OF AIRLINE EXPERIENCE. THE OTHER WAS NO BETTER. I WONDER IF AN ASR APCH IS REALISTIC THESE DAYS. THE CTLR DID NOT FOLLOW THE COM PROCS AS I REMEMBER THEM. WE WERE L OF COURSE AND DRIFTING FURTHER L AND WE WERE TOLD THAT WE WERE ON COURSE. WE WERE DESCENDED TO AN ALT THAT WAS LOWER THAN AN OBSTACLE JUST L OF THE APCH COURSE. NO ALT ADVISORIES WERE ISSUED AND NO LOST COM PROCS WERE DISCUSSED. THE CTLR WAS APPARENTLY OVERLOADED. EVEN THOUGH IT WAS NIGHT; THERE WERE SEVERAL ACFT IN HIS CTL. HOW COULD HE CONDUCT A PROPER SURVEILLANCE APCH UNDER THOSE CONDITIONS? WE WERE AWARE OF THE TWR AND IN FACT LEVELED OFF ABOVE ITS CHARTED ALT. WE ASSUMED THAT THE ASSIGNED HDGS WOULD KEEP US CLR OF THE OBSTACLE; BUT WE WERE CAUTIOUS NONETHELESS.