RPT OF A NEAR MISS AND LOSS OF SEPARATION BTWN AN ACR AND A SINGLE ENG CESSNA WHILE ON A 3 MI FINAL APCH FOR LNDG RWY 33L.

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BWI APCH CTL WAS VECTORING OUR FLT FOR RWY 33L AT BWI. WE WERE ASKED IF WE HAD THE ARPT IN SIGHT (WE REPLIED YES). WHEN ON A R BASE FOR LNDG; AND SUBSEQUENTLY CLRED FOR A VISUAL APCH TO RWY 33L; I WAS DSNDING AND IN A SHALLOW R TURN TO FINAL WHEN I SPOTTED A SINGLE ENG CESSNA JUST BELOW US AND FILLING UP MY GLARESHIELD. OUR PRESENT DSCNT RATE HAD US POINTED RIGHT AT HIM! I ADDED PWR (THRUST) AND LEVELED OR SLIGHTLY CLBED THE ACFT TO AVOID HIM AND CALLED THE TFC TO THE FO AND SO WHO REPLIED THAT THEY SAW HIM PASS UNDERNEATH US AND TO THE R. I INSTRUCTED THE FO TO ASK THE TWR (WHO HAD ALREADY CLRED US TO LAND); IF THEY KNEW A CESSNA WAS OUT HERE CLOSE IN. THE CESSNA CALLED THE TWR FOR LNDG CLRNC ABOUT 10 SECONDS LATER ON SHORT FINAL TO RWY 33R; AND RECEIVED LNDG CLRNC. AFTER DISCUSSING THE INCIDENT ON THE GND; NONE OF MY CREW KNEW THE CESSNA WAS IN THE AREA; WE JUST HAPPENED TO SEE THE ACFT. I DID RECALL HEARING EARLIER THAT SOME ACFT ON ONE OF THE APCH FREQS WAS HAVING TROUBLE WITH HIS XPONDER. THE OTHER 2 CREW MEMBERS ALSO RECALLED HEARING THAT XMISSION. I BELIEVE THIS TO BE THE SAME CESSNA; BECAUSE HE DID NOT SHOW UP AT ANY TIME ON OUR TCASII DISPLAY. OUR CONCERN IS THAT THIS ACFT HAD NO XPONDER; AND THAT WE HAD NO TCASII INFO AS WELL AS HAVING NOT BEEN TOLD ABOUT THE TFC. I WOULD ESTIMATE WE MISSED HIM BY 100-300 FT. APPARENTLY OUR WAKE TURB CAUSED HIM NO PROBS. THE OTHER 2 CREW MEMBERS (FO AND SO) SAID THEY WERE GLAD I SAW HIM. THE BWI TWR SAID THE CTLR WAS UNAVAILABLE (DOWNSTAIRS) BUT HE FELT THAT THEY MAY HAVE WANTED THE TWR TO SEPARATE US; AND THAT HE WOULD CHK INTO THE INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR DID NOT MAKE A FOLLOW UP CALL TO THE FACILITY AFTER THE INITIAL CALL. THE RPTR STATED AN AIR SAFETY RPT WAS FILED WITH THE COMPANY. RPTR'S ACFT WAS A B727. RPTR GUESSED THAT THE OTHER ACFT WAS A C172 WHICH WAS ON A L TO R CONVERGING COURSE FOR RWY 33R. RPTR CLAIMED THE CESSNA PASSED UNDERNEATH THEIR ACFT. RPTR SPECULATES THERE WAS A BREAKDOWN OF COORD BTWN APCH CTL AND THE TWR.