Narrative:

Washington approach control cleared air carrier X for the mt. Vernon 36 approach. At the wilson bridge the tower cleared us to land STOL on runway 33. I turned off the mt. Vernon 36 approach onto a left base leg for runway 33. The pattern flown was square. I turned final about 1/2 mi south of the east bank of the potomac river. At no time were we advised of a helicopter in our area and we did not hear the tower talking to a helicopter on 119.1 (1-WAY or 2-WAY communication). We did not see a helicopter during our approach. The sky was hazy and the sun was off the left side. It was daylight. Lined up on final and stabilized for a STOL landing, I suddenly caught sight of a presidential type helicopter crossing our flight path, approximately 50-100 ft below and 100 ft in front of us at our 11:30 position, as we passed through 600 ft MSL. The helicopter was moving left to right at a good speed, but I determined a collision was possible and immediately applied power, pulled up and called 'go around' to the first officer. As we pulled up I called out the helicopter to the first officer who then saw it close and below as it passed off the right side. The go around was uneventful and complied with tower instructions which were given after we called with go around and near miss. The tower's response was 'roger and understand' only. The helicopter in question did not appear to alter course or flight path until descending and landing at anacostra. On the ground I attempted to call the tower on landline but kept getting a busy signal (3 attempts). Since I was due back out in 10 mins for my next flight, I returned to my aircraft and continued on my schedule. I was never able to determine if the tower knew of or had talked to the helicopter in question. Many times the tower talks to helicopters on a different frequency, but you can hear the tower's communication to them on 119.1. They will usually point out helicopter traffic to us and comment that the helicopter is at such and such an altitude, direction and that they (the helicopter) 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.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.