ACR X HAD NMAC LTSS FROM MIL HELI IN ATA. EVASIVE ACTION TAKEN GAR. SYS ERROR.

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WASHINGTON APCH CTL CLRED ACR X FOR THE MT. VERNON 36 APCH. AT THE WILSON BRIDGE THE TWR CLRED US TO LAND STOL ON RWY 33. I TURNED OFF THE MT. VERNON 36 APCH ONTO A L BASE LEG FOR RWY 33. THE PATTERN FLOWN WAS SQUARE. I TURNED FINAL ABOUT 1/2 MI S OF THE E BANK OF THE POTOMAC RIVER. AT NO TIME WERE WE ADVISED OF A HELI IN OUR AREA AND WE DID NOT HEAR THE TWR TALKING TO A HELI ON 119.1 (1-WAY OR 2-WAY COM). WE DID NOT SEE A HELI DURING OUR APCH. THE SKY WAS HAZY AND THE SUN WAS OFF THE L SIDE. IT WAS DAYLIGHT. LINED UP ON FINAL AND STABILIZED FOR A STOL LNDG; I SUDDENLY CAUGHT SIGHT OF A PRESIDENTIAL TYPE HELI XING OUR FLT PATH; APPROX 50-100 FT BELOW AND 100 FT IN FRONT OF US AT OUR 11:30 POS; AS WE PASSED THROUGH 600 FT MSL. THE HELI WAS MOVING L TO R AT A GOOD SPD; BUT I DETERMINED A COLLISION WAS POSSIBLE AND IMMEDIATELY APPLIED PWR; PULLED UP AND CALLED 'GAR' TO THE FO. AS WE PULLED UP I CALLED OUT THE HELI TO THE FO WHO THEN SAW IT CLOSE AND BELOW AS IT PASSED OFF THE R SIDE. THE GAR WAS UNEVENTFUL AND COMPLIED WITH TWR INSTRUCTIONS WHICH WERE GIVEN AFTER WE CALLED WITH GAR AND NEAR MISS. THE TWR'S RESPONSE WAS 'ROGER AND UNDERSTAND' ONLY. THE HELI IN QUESTION DID NOT APPEAR TO ALTER COURSE OR FLT PATH UNTIL DSNDING AND LNDG AT ANACOSTRA. ON THE GND I ATTEMPTED TO CALL THE TWR ON LANDLINE BUT KEPT GETTING A BUSY SIGNAL (3 ATTEMPTS). SINCE I WAS DUE BACK OUT IN 10 MINS FOR MY NEXT FLT; I RETURNED TO MY ACFT AND CONTINUED ON MY SCHEDULE. I WAS NEVER ABLE TO DETERMINE IF THE TWR KNEW OF OR HAD TALKED TO THE HELI IN QUESTION. MANY TIMES THE TWR TALKS TO HELIS ON A DIFFERENT FREQ; BUT YOU CAN HEAR THE TWR'S COM TO THEM ON 119.1. THEY WILL USUALLY POINT OUT HELI TFC TO US AND COMMENT THAT THE HELI IS AT SUCH AND SUCH AN ALT; DIRECTION AND THAT THEY (THE HELI) HAVE US IN SIGHT. THIS INCIDENT WAS 'VERY' CLOSE AND FAR FROM THE NORM I'VE EXPERIENCED IN 15 YRS OF FLYING INTO AND OUT OF DCA.