NMAC BETWEEN DEPARTING COPTER AND LANDING COMMUTER.

Narrative

PRIOR TO DEP WE WERE FACING TO THE E TWR POINTED OUT COMMUTER Y TWO MI TO THE E WE COULD SEE. TWR SAID Y WAS LANDING ON RWY 21. TWR CLRED US FOR DEP TO THE E TO JOIN ROUTE ONE AND FLY NE UP THE ANACOSTIA RIVER. JUST AFTER TKOF AND PASSING E OF RWY 18-36 TWR TOLD US TO PASS BEHIND ACFT Y. SINCE Y WAS HEADING W ON A BASE TO 21 AND WE WERE HEADING E; THIS WAS CONFUSING BECAUSE HOW COULD WE POSITION OURSELVES BEHIND Y. I STARTED A LEFT TURN FIGURING Y WAS GOING TO TURN LEFT ON FINAL; BUT IT APPEARED HE WAS TURNING RIGHT TO GET A BETTER LINE UP FOR 21. SO I STARTED TO TURN RIGHT TO GO BEHIND HIM TO THE S. ACFT Y THEN TURNED LEFT ON FINAL; AND TIME HAD RUN OUT. HE PASSED OVER THE TOP OF US OR JUST BEHIND US. I HAD NOT CLBED BECAUSE OF THE CONVERGENCE. AS A HELI PLT; I HAD COMPLIED WITH THE FAR'S AND STAYED CLEAR OF FIXED WING TFC; BUT NOT BY MUCH. TWR IN RETROSPECT SHOULD HAVE DEPARTED US TO THE N TOWARD MEMORIAL BRIDGE OR HELD US UNTIL ACFT Y WAS ON THE GND OR DEP US HEADING 120 DEG TO CLEAR RWY 21 AS SOON AS POSSIBLE. IN THE FUTURE; I WILL NOT ACCEPT A TKOF CLRNC AT DCA TO THE E IF THERE IS TFC COMING IN FROM THE E AND SO CLOSE TO THE ARPT. HELIS NEED A HELIPORT AT RFK STADIUM TO KEEP A CLEAR OF FIXED WING TFC AT DCA.