B737 ON VISUAL TO DCA RWY 19 EXPERIENCED TCAS TA WITH ISSUED VFR HELI TFC.

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THIS IS MY SECOND ROTATION OFF OE; SO I AM STILL GETTING USED TO THIS ACFT. I AM VERY FAMILIAR WITH THE LOCAL PROCS IN DCA; INCLUDING THE OP OF MIL HELIS ALONG THE RIVER. NON STANDARD SEPARATION OVER THE RIVER SEEMS TO BE THE NORM FOR DCA FOR SOME REASON. I HAVE NOT ENCOUNTERED THIS TYPE OF OP AT ANY OTHER US AIRPORT. WE WERE APPROACHING DCA FROM OVER IAD ON THE FINKS ARRIVAL. I WAS THE PF. WE HAD A MODERATE TAILWIND COMPONENT IN THE DSCNT WHICH INCREASED THE WORKLOAD TO PREPARE FOR THE RIVER VISUAL 19 APCH. AS ANYONE WHO HAS FLOWN THIS APCH BEFORE CAN ATTEST; IT IS ONE OF THE MOST CHALLENGING APPROACHES IN THE COUNTRY. IT TAKES A HIGH LEVEL OF CONCENTRATION. THAT SAID; WE WERE FULLY CONFIGURED; STABILIZED AND ON SPEED AT FLAPS 40 BEFORE 1000 FT AGL (VREF=127K). AS WE ROUNDED THE CORNER OVER ROOSEVELT ISLAND (TO THE L; APPROX 500-700 FT); WE RECEIVED A TCAS TA. TWR REPORTED 2 HELIS ALONG THE RIVER AS WE RECEIVED 2 MORE TA'S. WE HAD THE HELIS' LIGHTS IN SIGHT AT ALL TIMES; AND BECAUSE OF MY FAMILIARITY WITH DCA; I ELECTED TO CONTINUE TO LAND. I FELT CONFIDENT THAT WE WOULD STAY CLEAR OF THE TFC; ALTHOUGH IT SEEMED CLOSER THAN NORMAL. THE REST OF THE LNDG WENT NORMALLY. ONLY AFTER I HAD A WHILE TO LET WHAT HAPPENED SINK IN DID I REALIZE HOW CLOSE THE HELIS HAD BEEN. I WAS LOOKING AT POINTS OF LIGHT; AND MANEUVERING TO AVOID THEM; BUT I COULDN'T SEE THE ACTUAL ACFT. IN DISCUSSING IT WITH THE FO LATER; HE REPORTED THAT WE CAME ABOUT 200 FT AND 0.2 MILE AT THE CLOSEST POINT. I WAS LOOKING THROUGH THE HUD AND NOT AT THE TCAS DISPLAY SINCE I WAS FOCUSED ON MAKING THE TURN TO FINAL. I FAILED TO REALIZE AT THE TIME THAT THE TCAS RA FEATURE WAS INHIBITED DUE TO OUR LOW ALT. IN RETROSPECT; I BELIEVE THAT WE WOULD HAVE CERTAINLY RECEIVED AN RA. IF I WERE FACED WITH THIS SITUATION AGAIN; I WOULD PERFORM AN GAR. BUT I HAVE TO QUESTION THE WISDOM OF DCA TWR LETTING HELIS FLY IN THE APCH PATH OF ONE OF THE COUNTRY'S HIGHEST PROFILE ARPTS; PARTICULARLY ON AN APCH AS CHALLENGING AS THIS ONE. NOTE: IT IS COMMON PRACTICE TO HUG THE LEFT BANK OF THE RIVER FOR THIS APCH; BECAUSE YOU CAN MAKE THE TURN TO FINAL AT A SLIGHTLY HIGHER ALT WHILE REMAINING ON PROFILE FOR TOUCHDOWN IN THE TDZ (IT'S NOT A LONG RUNWAY).