APCH CTLR GIVES MD80 CLRNC TO 2000 FT VICE REQUIRED 3000 FT FOR ILS RWY 28L INTERCEPT AT PIT. PF RECOGNIZES LOW ALT AND INITIATES CLB. GPWS ACTIVATES.

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WE WERE FLYING THE PIT ILS RWY 28L IN VFR CONDITIONS ON RADAR VECTORS WITH APCH CTL AT THE TIME OF THE INCIDENT. I; THE FO; WAS FLYING THE APCH WITH THE USE OF THE AUTOPLT AND AUTOTHROTTLES. BOTH PLTS SAW THE LNDG RWY AT APPROX 10 MI OUT. APCH ISSUED THE FOLLOWING CLRNC: 'ACR X; FLY HDG 320 DEGS; DSND AND MAINTAIN 2000 FT. CLRED THE ILS RWY 28L. SPD 180 KTS TO THE MARKER. CONTACT TWR AT THE MARKER.' THE PNF READ BACK THE CLRNC. THE PF THEN SET THE ALT WINDOW TO 2000 FT. BOTH PLTS POINTED AND VERBALLY VERIFIED THE ALT SET IN THE WINDOW. THE PF SIMULTANEOUSLY INITIATED AN IAS DSCNT AT THE ASSIGNED AIRSPD; A HDG CHANGE USING THE HDG SELECT MODE; AND ARMED THE ILS. DSNDING THROUGH 3000 FT MSL; I VISUALLY DETECTED A CLOSURE RATE WITH THE GND. I BROUGHT THE GND INTO MY XCHK WHILE SIMULTANEOUSLY MONITORING THAT THE AUTOPLT WAS GOING TO CAPTURE THE ALT SET IN THE WINDOW AND LOC. AT THIS TIME I STILL HAD THE LNDG RWY IN SIGHT AS DID THE PNF. PASSING THROUGH 2500 FT MSL; I SAW THAT THE GND APPEARED TO BE MUCH CLOSER THAN IT SHOULD BE AT THIS POINT DURING AN ILS APCH. I ELECTED TO DISENGAGE THE AUTOPLT AND START A CLB WHILE TURNING TO INTERCEPT THE LOC. I DID SO WITH THE RWY IN SIGHT. ALMOST SIMULTANEOUSLY AS I WAS DOING THIS; APCH CTL ISSUED A LOW ALT WARNING AND A GPWS ALERT SOON FOLLOWED. BECAUSE WE WERE IN VMC WITH THE LNDG RWY IN SIGHT AND HAD A POSITIVE VECTOR AWAY FROM THE GND WITH NO OBSTACLES AHEAD; WITH THE VERBALLY STATED CONCURRENCE OF THE CAPT I ELECTED NOT TO GO FIREWALL THRUST; PITCH TO 20 DEGS NOSE HIGH; AND FLY THE GPWS RECOVERY PROC. AS WE CLBED TO 3000 FT; WE WERE WITHIN 1 DOT OF THE LOC COURSE AND ON AN INTERCEPT HDG. APCH THEN CALLED AND VERIFIED THAT WE WERE IN A SAFE POS; APOLOGIZED FOR ISSUING THE WRONG ALT; AND ASKED IF WE WOULD LIKE TO CONTINUE WITH THE APCH. THE CAPT ACCEPTED THE APCH CLRNC AND I THEN RECAPTURED THE ILS AND FLEW AN UNEVENTFUL APCH AND LNDG. ALMOST INSTANTLY AFTER MAKING THE DECISION TO DISENGAGE THE AUTOPLT BECAUSE I SAW THE CLOSURE RATE WITH THE GND; I QUICKLY REFED MY PLATE AND SAW THAT THE FAF ALT WAS 3000 FT MSL; NOT THE 2000 FT MSL WE WERE ISSUED. I BELIEVE; ALTHOUGH I AM NOT ABSOLUTELY CERTAIN; THAT THE LOWEST ALT WE GOT TO WAS 2300 FT MSL. IT WAS THEN THAT THE GPWS WARNING CAME ON. I DID NOT SEE THE ALT DISPLAYED BY THE RADAR ALTIMETER. AFTER BLOCKING IN; THE CAPT AND I DISCUSSED THE EVENT. HE INTRODUCED THE TECHNIQUE OF SELECTING THE 'DATA' DISPLAY WHICH WILL GIVE HARD APCH ALTS. IF THIS WAS INTRODUCED IN MY TRAINING; AND I DO NOT BELIEVE IT WAS; THIS TECHNIQUE WAS NOT EMPHASIZED. NEEDLESS TO SAY; I WILL ALWAYS BE FLYING WITH 'DATA' SELECTED.