ZDC CTLR RECOGNIZES THAT HGR DEPARTING PA31 CLBS THROUGH ASSIGNED ALT OF 5000 FT AND CONFLICTS WITH AN ENRTE C182 AT 6000 FT.

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AFTER GAINING RADAR CONTACT ON ACFT X (A DEP OFF HGR ARPT); I CONFIRMED THE MODE C AND VERIFIED THE ASSIGNED ALT AS 5000 FT. ACFT X SAID HIS ALT LEAVING AND ANSWERED IN THE AFFIRMATIVE FOR HIS ASSIGNED ALT AS 5000 FT. ACFT Y; A C182; LEVEL AT 6000 FT; WAS ISSUED TFC ON ACFT X CLBING TO 5000 FT. ACFT X WAS ISSUED TFC ON ACFT Y AT 6000 FT. ACFT X'S MODE C INDICATED 5300 FT. I REISSUED HGR ALTIMETER AND ASKED ACFT X TO CONFIRM LEVEL AT 5000 FT; ACFT X DID NOT RESPOND. I ISSUED AN IMMEDIATE DSCNT TO 5000 FT FOR ACFT X THEN TURNED ACFT Y L HDG 270 DEGS (TFC WAS ACFT Y NBOUND; AND ACFT X SBOUND) AND CLBED TO 7000 FT. ABOUT 7 SECONDS LATER; ACFT X RPTED LEVEL AT 5000 FT. SUPPLEMENTAL INFO FROM ACN 506956: I DEVIATED FROM THE ASSIGNED ALT BY INADVERTENTLY CLBING TO 5600 FT ON DEP FROM HAGERSTOWN; MD. WHILE LOOKING FOR THE MARTINSBURG APCH PLATE AND NOT SCANNING THE INSTS; I WAS THROUGH 5000 FT BEFORE I REALIZED IT. THE CTLR CALLED; I IMMEDIATELY DSNDED. HE ALSO CALLED THE CESSNA AND ASSIGNED 7000 FT. THE CTLR DID EVERYTHING HE COULD. VERIFIED THE ASSIGNED ALT; WHICH I SHOULD HAVE GIVEN HIM ON INITIAL CONTACT; VERIFIED THE MODE C RPT AND CALLED THE TFC. FAILURE TO SCAN; THINKING ABOUT THE NEXT APCH AND NOT FOLLOWING THE 'AVIATE; NAV; COMMUNICATE' PRIORITY RULE CONTRIBUTED TO THE DEV. IN ADDITION; I SHOULD HAVE BEEN MORE AWARE OF THE 1000 FT SEPARATION WITH THE TFC. I SHOULD HAVE RETRIEVED THE APCH PLATE BEFORE DEPARTING HGR.