CL65 FLT CREW WITH ZDC DSNDED BELOW ASSIGNED ALT DURING ARR TO BWI; CONFLICTING WITH A PAY3 AT 17000 FT.

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THE FO WAS PF AND WAS CLRED TO DSND TO FL180. THE FO INITIATED THE DSCNT; THE FO LEFT THE COCKPIT TO USE THE LAVATORY. THE CAPT ASSUMED CTL OF THE ACFT. WHILE THE FO WAS ABSENT; ATC ADVISED THE FLT TO EXPECT TO CROSS MUMSY AT 15000 FT. THE CAPT READ BACK THE CLRNC AND ENTERED 15000 FT AT MUMSY IN THE FMS. HE ALSO ENTERED 15000 FT IN THE ALT SELECTOR; THINKING THIS WAS THE CLRNC. WHEN THE FO RETURNED; THE FLT WAS AT APPROX FL200. THE CAPT XFERRED CTL OF THE ACFT TO THE FO; AND PROCEEDED TO OBTAIN ATIS. JUST BELOW FL180; ATC ASKED THE FLT TO VERIFY LEVELING AT FL180. THE FO TOLD ATC WE WERE GIVEN 15000 FT AT MUMSY. ATC INSTRUCTED THE FLT TO HOLD ALT. THE FO SELECTED ALT HOLD; AND ASKED THE CAPT IF WE HAD BEEN CLRED TO 15000 FT. THE CAPT TOLD THE FO WE HAD BEEN TOLD TO EXPECT MUMSY AT 15000 FT. SHORTLY AFTER; ATC CLRED THE FLT TO 15000 FT AND ADVISED THE FLT THAT WE HAD BEEN TOLD TO EXPECT 15000 FT. THE CAPT CONTACTED ATC UPON ARR. THE CTLR LATER CONTACTED THE CAPT AND ADVISED HIM THAT THE FLT HAD A LOSS OF SEPARATION WITH ANOTHER ACFT. SEVERAL FACTORS CONTRIBUTED TO THIS EVENT. THE FO WAS ABSENT DURING A BUSY TIME. HIS ABSENCE WAS UNAVOIDABLE; BUT A MAJOR FACTOR. THE ABSENCE OF THE FO DID NOT ALLOW FOR THE STANDARD PROTOCOL REGARDING ALT READBACK BTWN CREW MEMBERS. THE CAPT SHOULD NOT HAVE SET THE EXPECTED ALT IN THE ALT SELECTOR. THE CAPT DID NOT BRIEF THE FO WHEN THE FO RETURNED. THE FO DID NOT IMMEDIATELY RESPOND WHEN ATC ASKED IF HE WAS LEVELING AT FL180. INSTEAD HE TOLD ATC SOMETHING HE WAS NOT SURE OF; THAT WE WERE CLRED TO 15000 FT. AFTER SAYING THIS; HE ASKED THE CAPT. FINALLY; ALTHOUGH COMMON AND UNDERSTOOD; THE USE OF AN EXPECTED CLRNC BY ATC CONTRIBUTED TO THE WORKLOAD BY REQUIRING A READBACK; BUT DID NOT CONTRIBUTE USEFUL INFO. WE DO NOT NEED TO KNOW WHAT WE CAN EXPECT TO RECEIVE LATER AS A CLRNC. SUPPLEMENTAL INFO FROM ACN 615952: WHEN I RE-ENTERED THE FLT DECK AND BUCKLED IN; THE PLANE WAS IN A DSCNT. I STATED THAT I SAW 15000 FT IN THE ALT PRESELECT AND THE XING RESTR OF 15000 FT AT MUMSY IN THE FMS. THE CAPT AGREED WITH MY READBACK AND XFERRED CTLS TO MY SIDE AGAIN. THROUGH FL180; ZDC ASKED IF WE WERE LEVELING AT FL180. ASKED THE CAPT IF WE WERE GIVEN EXPECT MUMSY AT 15000 FT. AT THAT POINT HE REALIZED HIS MISTAKE. SUPPLEMENTAL INFO FROM ACN 615412: ACFT #2 WAS IN LEVEL FLT AT 17000 FT. ACFT #1 WAS AN ARR CLRED TO FL180 TO CROSS OVER THE ACFT #1. ACFT #1 WAS TOLD TO EXPECT MUMSY INTXN AT 15000 FT. PLT READ BACK FL180 AS THE CLRNC; BUT QUESTIONED TO EXPECT CLRNC. THE FULL CLRNC WAS RESTATED AS 'DSND TO AND MAINTAIN FL180; EXPECT MUMSY INTXN AT 15000 FT.' ACFT #1 MAY HAVE GOTTEN CONFUSED ON THE ASSIGNED ALT BECAUSE THEY DSNDED TO 15000 FT. SEPARATION WAS LOST BTWN THE 2 ACFT.