OPERROR LTSS BTWN A BE90 OVERFLT ON RPTR'S FREQ AT 5000 FT AND A B757 ARR ON APCH CTL'S FREQ DSNDING TO 5000 FT. BOTH ACFT WERE IN THE ARR SECTOR'S AIRSPACE.

Narrative

IAD HAD A KING AIR APPROX 10 NM S OF IAD AT 5000 FT MSL; HDG NEBOUND. THE ARTS AUTOMATED HDOF WAS FLASHING TO DEP W SCOPE; WHICH I WAS WORKING. I ASKED IAD WHERE HE WAS LNDG. IAD STATED BWI. I TOLD IAD WHEN WE TAKE THE HDOF THE FREQ WILL BE 119.85; WHICH IS W ARR. I REDIRECTED THE HDOF TO W ARR. W ARR REDIRECTED THE HDOF BACK TO ME; AT SAME TIME IAD QUESTIONED THE FREQ. I TOLD IAD 118.95 RADAR CONTACT. AT THIS TIME THE ACFT WAS ENTERING MY AIRSPACE NEBOUND. I ROGERED THE ACFT AND GAVE HIM THE ALTIMETER. AT SAME TIME; A VFR ACFT CALLED ME DIVERTING MY ATTN TO HIM; WHICH WAS ASKING FOR CLASS B ENTRY. WHEN I SCANNED BACK TO ACFT; THE ACFT WAS ENTERING W ARR'S AIRSPACE. THE ACFT HAD TFC APPROX 11 O'CLOCK AND 3-4 MI; A B757 SEBOUND DSNDING TO 5000 FT MSL. I ISSUED TFC AND TURNED SEBOUND. THE W ARR CTLR ISSUED TFC; TURNED B757 L BACK TO THE NW; AND DSNDED THE ACFT TO 4000 FT. THE ACFT HAD AN RA. I'M A DEVELOPMENTAL ON THE APCH SIDE OF THE TRACON. MY PROFICIENCY ON DEP COULD BE LACKING. I WAS ONLY ON THE POS 5 MINS PRIOR TO THE ERROR. THERE IS A TOTAL OF 15 POS AT DCA TO CERTIFY ON AND TO STAY CURRENT ON. THE HIGH AND LOW ON THE DEP POS I WAS WORKING WAS COMBINED. THE VIDEO MAPS DO NOT HAVE SECTOR BOUNDARIES SHOWN ON THE MAPS. ALL OF THESE COULD HAVE CONTRIBUTED TO THE ERROR.