MLG READ BACK WRONG ALT AND DESCENDED BELOW MINIMUM SAFE ALT TOWARDS TERRAIN. APCH CTLR AND GPWS CAUGHT ERROR IN TIME TO PREVENT SERIOUS INCIDENT.

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ON 10/SUN/88; DURING THE TIME XA16Z; MLG CHKED IN APPROX 10 MI E OF COREL INTXN WBOUND DSNDING TO 8000'. I DSNDED HIM TO 7000' (THAT IS THE MVA IN THE AREA). I WAS THEN SCANNING MY RADAR SCOPE FOR TFC THAT SAN DIEGO TRACON HAD PREVIOUSLY TRIED TO POINT OUT WHEN I NOTICED MLG DSNDING THROUGH 6400'. I IMMEDIATELY INSTRUCTED MLG TO CLB TO AT LEAST 6500'. THE PLT RESPONDED WITH 'I WAS CLRED TO 5000'.' I SAID; 'NEGATIVE; CLB TO AT LEAST 6500'.' THE PLT RESPONDED AND CLBED TO 6900'. I LATER HEARD A RECORDING OF THE INCIDENT AND I HAD TOLD THE PLT TO DSND TO 7000' BUT HE READ BACK THAT HE WAS DSNDING TO 5000'. THE ALT READOUT WAS 6400' WHEN I CLBED HIM; BUT PRIOR TO CLBING THE READOUT GOT AS LOW AS 5700'. SUPPLEMENTAL INFO FROM ACN 96167: UNDER THE CTL OF COAST APCH ON V372 JUST W OF HOMELAND (HDF 13.4) WE WERE AT 8000' MSL. WE WERE DIRECTED TO THE S OF V372 ON HDG 240 DEGS; VECTOR FOR LNDG AT LGB. WE 'HEARD' A CLRNC TO 5000' AND RESPONDED MLG TO 5000'. PASSING THROUGH 6000' THE CTLR COMMENTED THAT WE WERE AT 6000' AND CLRED TO 7000'. WITH BACKLIGHTING THE SHAPE OF MOUNTAINOUS TERRAIN CAME INTO VIEW. I SAID WE WERE CLRED TO 5000' IN MY REPLY TO THE CTLR AND IN THE SAME BREATH ORDERED 'CLB' TO THE F/O WHO WAS AT THE CONTROLS. SECONDS LATER 'TERRAIN' WAS ANNOUNCED BY GND PROX WARNING. BOTH F/O AND I ABRUPTLY PULLED BACK ON THE CONTROL COLUMN AND WERE PASSING 6500' ABOUT THE TIME THE CTLR SAID SOMETHING LIKE 'YOU CAN LEVEL AT 6500';' THEN SEEING OUR RATE OF CLB SAID 7000' WOULD BE OK. WE LEVELED AT 7000' THEN WERE IMMEDIATELY CLRED TO 5000'. I SAW THE HAZARD DUE TO BACKLIGHTING BUT DON'T KNOW HOW MUCH CLRNC FROM THE HILLS THAT WE HAD. LATER I SPOKE WITH APCH SUPVR WHO REVIEWED THE TAPE. THE CTLR HAD CLRED US TO 7000' FROM 8000' NOT 5000'. WHY I HEARD 5000' IS A MYSTERY TO ME AS I WAS CERTAIN IN MIND OF THE CLRNC TO 5000'. THE CTLR MISSED MY READBACK OF 5000'. 7 DOES NOT SOUND LIKE 5; SO I CAN'T EXPLAIN THAT. I HAD REVIEWED THE LA AREA CHART WHICH IS BETTER THAN THE LOW ALT CHART AND I KNEW THAT V372 MSA W OF HDF WAS 5000'. IN EVERY PREVIOUS APCH TO LGB THAT I HAVE MADE I CONTINUED TO SLI ON V372 THEN WAS VECTORED OVER THE PACIFIC FOR A LEFT 270+ DEG TURN BACK TO INTERCEPT THE LOC FOR RWY 30. THIS TIME THE VECTOR OF 240 DEGS FOR A NIGHT TURN IN TOOK US OVER HIGHER TERRAIN. I'M NOT CONSCIOUS OF THOUGHTS ABOUT THE 5000' MSA AS THE CTLR GAVE CLRNC TO 7000'; BUT I 'HEARD' 5000' AND READ BACK 5000'. FORTUNATELY THE CTLR CALLED US AT 6000'; AS WE DSNDED TO PERHAPS 5700' BEFORE REVERSING TO 7000'. HAD HE NOT NOTICED; THE BACKLIGHTING OF THE MOUNTAIN WOULD HAVE MADE THE NEED FOR A CLB APPARENT (I THINK); BUT THE TERRAIN WARNING WAS THE ICING ON THE CAKE. IT'S SIGNAL WAS FOLLOWED BY IMMEDIATE CREW RESPONSE AND A HAPPY ENDING. I DON'T KNOW THE ALT OR TOPS OF THE MOUNTAINS ON OUR ASSIGNED HDG. I DON'T KNOW HOW MUCH CLRNC FROM THE MOUNTAINS WE HAD; BUT IT CERTAINLY MAKES CLEAR THE IMPORTANCE OF GOOD COM BTWN THE CTLR THE PLT. SUPPLEMENTAL INFO FROM ACN 96578: FORTUNATELY THE CTLR NOTICED OUR DSCNT BELOW 7000' AND STATED SUCH.