ACFT ON FINAL APCH HAD ANOTHER ACFT PASS DIRECTLY BELOW WHILE IN CONTACT WITH THE TWR.

Narrative

WHILE ON FINAL APCH TO ONE OF THE PARALLEL RWYS; I OBSERVED AN ACFT PASS UNDER MY ACFT WITH APPROX 200 FT OF CLRNC. BOTH ACFT WERE IN CLASS C AIRSPACE UNDER CTL AND CONTACT WITH THE TWR. MY APCH WAS STABILIZED AND PROPERLY POSITIONED FOR THE RWY I WAS USING. IT TURNED OUT THAT THE OTHER ACFT'S PLT GOT CONFUSED; AND BLEW THROUGH THE APCH LEG OF THE RWY SHE WAS CLOSED FOR; AND THEN PASSED UNDER OUR ACFT. UNFORTUNATELY; I NOT ONLY HAD AN ADDITIONAL PLT IN THE ACFT; BUT NEITHER OF US SPOTTED THE ACFT UNTIL AFTER IT HAD PASSED UNDER OUR ACFT AND OUT THE OTHER SIDE. FROM A CRM STANDPOINT; THE PLT FLYING WITH ME WAS DOING AN EXCELLENT JOB OF HELPING KEEP THE COCKPIT STERILE AND PROPERLY DIVERTED. IN OTHER WORDS; WE WERE CONCENTRATING ON FLYING THE ACFT AND WE WERE DOING A GOOD JOB OF MAINTAINING SITUATIONAL AWARENESS AT ALL TIMES. THE SIT COULD HAVE BEEN AVOIDED IF THE OTHER PLT HAD PROPERLY STUDIED THE ARPT LAYOUT AND QUERIED THE TWR ABOUT ANY CONFUSION SHE HAD. ALTHOUGH WE TOOK IMMEDIATE EVASIVE MANEUVERS AFTER SEEING THE OTHER ACFT; HAD THE OTHER ACFT BEEN CLOSER VERTLY; THERE MIGHT HAVE BEEN A DIFFERENT OUTCOME. IN ADDITION TO THE OTHER PLT'S IMPROPER APCH; THE TWR CTLRS WERE NOT MAINTAINING VIGILANCE WITH MONITORING THE APCHING ACFT. AFTER THE INCIDENT; I QUERIED ATC ABOUT THE OTHER ACFT; AND THEY WERE UNAWARE OF ITS POS I BELIEVE THE CONTRIBUTING FACTORS TO THIS SIT WERE: 1) THE OTHER PLT'S IMPROPER DECISION MAKING DURING APCH; 2) THE OTHER PLT'S LACK OF PREFLT PLANNING WITH RESPECT TO ARPT LAYOUT; 3) OUT LACK OF VISUAL SCANNING VIGILANCE; 4) THE LACK OF THE CTLRS TO MAINTAIN CONTACT AND MONITORING OF THE ACFT APCH. AS FAR AS OUR LACK OF SCANNING VIGILANCE; WE WERE WORKING HARD ON WATCHING FOR OTHER ACFT; BUT AT THE TIME OF THIS SIT; WE WERE CONCENTRATING ON WATCHING THE SPACING WITH ACFT IN FRONT AND TO THE SIDE OPPOSITE FROM WHICH THE OTHER ACFT APCHED. IN OTHER WORDS; WE WERE CONCENTRATING MAINLY ON THE ACFT IN OUR PATTERN; AND NOT THE ACFT IN THE PATTERN OF THE OTHER PARALLEL RWY.