ACR DH8 TAKES PRECAUTIONARY AVOIDANCE MANEUVER FOR MIL HELI ON APCH PATH TO DCA.

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ON APCH INTO DCA; WE WERE CLRED BY WASHINGTON APCH FOR A VISUAL APCH TO RWY 15 AT 2000 FT. WE WERE CLRED TO TWR AND UPON CHKING; TWR ADVISED OF A HELI IN THE PENTAGON AREA BUT DID NOT CLR US TO LAND OR RESTRICT OUR APCH. I PICKED UP THE HELI VISUALLY AT 1500 FT ON APCH. HE WAS LOW; JUST N OF THE PENTAGON MOVING E TOWARD THE RIVER. IT APPEARED TO BE A MIL UH-1 TYPE HELI. SHORT OF THE RIVER; THE HELI TURNED S TOWARD THE ARPT. AS WE APCHED THE PENTAGON AT 700-800 FT ALT; THE HELI TURNED W AND CROSSED THE FINAL APCH COURSE AT GLIDE PATH ALT IN FRONT OF US. I DEVIATED SLIGHTLY L OF THE RWY CTRLINE TO GET MORE DISTANCE BEHIND THE HELI AND CONSIDERED A GAR. WE WERE ABLE TO STAY AT A SAFE DISTANCE AND COMPLETE THE APCH. ON SHORT FINAL WE REQUESTED A LNDG CLRNC AND RECEIVED A CLRNC TO LAND AT APPROX 200 FT AGL. I ESTIMATE THAT WE PASSED ABOUT 1/2 MI BEHIND THE HELI. OUR ACFT WAS EQUIPPED WITH TCASII WHICH THE RA FUNCTION IS INHIBITED AT LOW ALTS. WE DID NOT GET ANY TCASII ADVISORY. THE TWR WAS VERY BUSY AT THE TIME AND I WAS NOT ABLE TO MAKE CONTACT TO DISCUSS THE SIT WITH THEM DUE TO FREQ CONGESTION. OUR OPS AREA AT DCA IS NOT EQUIPPED WITH ANY GND COM CAPABILITY.