FLC OF AN MDT ACR ACFT TOOK EVASIVE ACTION TO AVOID A HELI ACFT CLBING OUT FROM THE ARPT.

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I WAS TURNING FINAL FOR STOL LNDG RWY 21 DCA. I SAW A HELI APCHING ON CONFLICTING FLT PATH. IT WAS APPROX 400 FT BELOW US; BUT WE WERE DSNDING FOR LNDG. I ADVISED DCA TWR OF TFC. HE SAID 'TFC LOW LEVEL; 200 FT.' JUST AS I WAS GOING TO MANEUVER TO AVOID; THE CHOPPER MADE A STEEP TURN AWAY FROM US. AT THIS POINT WE WERE WITHIN 100 FT VERTICALLY AND 1000 FT HORIZLY. BECAUSE OF THE LIMITATIONS ON PERFORMING A STOL LNDG AT DCA; MY TURN TO AVOID HIM WOULD ALSO CAUSE A GAR. TWR HAD PUT US ON RWY 21 WHILE THE ARPT WAS IN A 'N' OP (ALL OTHER TFC LNDG ON RWY 36). I RECEIVED NO WARNING FROM TWR OF HELI TFC AND I HAD NO WAY OF KNOWING IF CHOPPER PLT HAD US IN SIGHT (UNTIL HE MADE THE EVASIVE TURN). THIS IS BY NO MEANS THE FIRST TIME OF SUCH AN OCCURRENCE AT DCA. I HAVE HAD MANY CONFLICTS WITH CHOPPERS; BOTH ON LNDG AND TKOF AT DCA. I HAVE TALKED TO TWR SEVERAL TIMES (INCLUDING THIS TIME) AND HAVE RECEIVED APOLOGIES AND ADMITTING OF ERRORS BUT THE PROB CONTINUES. THE HELI IS A VERY MANEUVERABLE ACFT. IF THE CHOPPER PLTS WOULD JUST GIVE US MORE SEPARATION; THE PROB WOULD BE SOLVED. WHEN THEY SEE US THEY COULD SLOW OR HOVER; TURN; ETC. CONFLICTS OCCUR WHEN TWR FORGETS TO ADVISE ME THAT CHOPPER HAS ME IN SIGHT AND THE CHOPPER MAINTAINS HIS 'TRAJECTORY' WHICH CONFLICTS WITH MY LIMITED ARR OR DEP AIRSPACE.