AN AIRBUS 319 CREW HAS LAST MIN RWY CHANGE AT PIT; PA. APCH COORD AND REPROGRAMMING OF AUTOMATION LEAD TO DISORIENTATION AND DSCNT BELOW MSA UNTIL ALERTED BY ATC TRACON CTLR.

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THIS PROB TURNED OUT TO BE A MIXTURE OF LATE ATC INSTRUCTIONS; FALSE GS; AND EQUIP FAILURE. WE WERE ON A VECTOR FROM PIT APCH. WE HAD INITIALLY SET UP AND BRIEFED FOR RWY 32 IN PIT; THEN THEY CHANGED OUR LNDG RWY TO RWY 28R. THEY THEN WANTED TO CHANGE IT BACK TO RWY 32 AFTER WE HAD ALREADY SET UP AND BRIEFED RWY 28R. THE CAPT ASKED THE CTLR IF WE COULD KEEP RWY 28R; AND THEY BOTH AGREED. WE RECEIVED A CLRNC TO CROSS COFEE AT 4000 FT AND ON THE VECTOR WE WERE CLRED FOR THE ILS RWY 28R IN PIT. THE APCH PUSH BUTTON WAS SELECTED; SECOND AUTOPLT ENGAGED; AND THEN WE REALIZED THAT THE TURN WAS GIVEN LATE AND WE WERE ALREADY PAST THE LOC. THE CAPT DISCONNECTED THE AUTOPLT; AND STARTED A R TURN BACK TO THE LOC. SOMEHOW HE THOUGHT WE WERE HIGH; THOUGHT THE GS DEV WAS SHOWING HIGH; AND HE INITIATED A DSCNT. I WAS BUSY RELOADING THE LNDG DATA IN THE APCH PHASE WHICH HAD SOMEHOW DUMPED. THE APCH CTLR GAVE US A 300 DEG HDG TO REINTERCEPT THE LOC; AND QUERIED US AS TO IF WE HAD RWY 28R TUNED IN FOR THE APCH; BECAUSE TO HIM IT LOOKED AS IF WE WERE LINED UP FOR RWY 28L. (THIS WAS DUE TO THE WINDS; AND THE LATE TURN-ON.) IN THE MEANTIME; WE HAD DSNDED TO 2800 FT AND BROKEN OUT OF THE CLOUDS. THERE WERE 2 DISTINCT RADIO TWRS DIRECTLY AHEAD; AND ATC SAID MAINTAIN 3000 FT. SINCE WE HAD THE ARPT IN SIGHT; WE ASKED FOR A VISUAL APCH. THE CTLR SAID NO; NOT YET. BUT AS WE CLRED THE TWRS; HE CLRED US FOR THE VISUAL. EVEN THOUGH THE APCH MODE WAS SELECTED; IT IS MY OPINION THAT THE ALT ALERTER SHOULD NOT BE INHIBITED UNTIL GS CAPTURE. I HAD NO IDEA WE HAD DSNDED THAT LOW UNTIL THE CTLR TOLD US TO CLB TO 3000 FT. I DO NOT KNOW IF THE FALSE GS HAD ANYTHING TO DO WITH THE INHIBIT FUNCTION OF THE ALT ALERTER. BUT; IF THE CEILING WAS ANY LOWER; THE EVENT COULD HAVE BEEN MUCH WORSE. DUE TO TASK LOADING; AIRBUS ANOMALIES -- SUCH AS KNOWN FALSE GS CAPTURES; AND POOR SITUATIONAL AWARENESS -- ALL ADDED TO THE SERIOUSNESS OF THIS EVENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THE CHANGE IN RWY ASSIGNMENT REQUIRED THE REPROGRAMMING OF THE APCH DATA. THIS CAUSED HIM TO BE HEADS DOWN AND LOSE POSITIONAL AWARENESS. THE CAPT APPARENTLY REACTED TO A FALSE GS AND WITHOUT CHKING POS; DSNDED BELOW THE NORMAL APCH ALT FOR THE POS OF THE ACFT. THE RPTR SAYS THAT THERE IS ANECDOTAL EVIDENCE OF FALSE GS CAPTURE AT CLT; NC; SPECIFICALLY AND OTHER ARPTS USED BY HIS CARRIER AS WELL. THIS ANOMALY OCCURS PRIMARILY IN THE A319 VERSION OF THE AIRBUS. HIS COMPANY IS AWARE OF IT AND IS INVESTIGATING. THE AURAL ALT ALERTER IS INHIBITED ANY TIME THE AUTOPLT IS ENGAGED. THERE SHOULD HAVE BEEN AN AURAL ALT ALERT SINCE THE CAPT WAS HAND FLYING AT THE TIME OF THE ALT EXCURSION AND THERE WAS NO GS CAPTURE.