B737-700 FO MISTAKES BWI HDG CHANGE WITH ALT DSCNT CLRNC FOR SPD CLRNC; NO HDG CHANGE.

Narrative

DSCNT TO BWI AT 11000 FT; FO FLYING; HAD JUST MADE PA SEATING THE FLT ATTENDANTS. I WAS MONITORING COM #1 DURING MY PA AND RETURNED HALFWAY THROUGH THEIR INSTRUCTIONS. I UNDERSTOOD BWI TO SAY SLOW TO 270 KTS; THEN DSND TO 6000 FT. I SLOWED; THEN BEGAN TO DSND ON COURSE. THE CAPT SWITCHED OFF TO COORDINATE WITH OPS AND MAINT. DSNDING THROUGH 10000 FT; BWI APCH ASKED ME IF I WAS HDG 270 DEGS YET. I ASKED IF THEY WANTED ME FLYING 270 DEG HDG; AND THEY SAID YES. I BEGAN THE TURN; THEN THEY SAID FORGET THE TURN; DSND TO 5000 FT AND EXPEDITE THROUGH 6000 FT; TURN L TO 180 DEGS. IN THE TURNING; DSNDING; AND EXPEDITING THE DSCNT; I WENT THROUGH 10 KTS STILL AT 270 DEG HDG. AT ABOUT 8 KTS; CAPT RETURNED TO FREQ; AND I TOLD HIM WHAT HE MISSED. BALTIMORE DIDN'T SAY ANY MORE. SUPPLEMENTAL INFO FROM ACN 573563: WE WERE FLYING THE FMS ON THE PROGRAMMED FLT PLAN (AT 11000 FT) INTO BWI; AND AS WE MADE THE LAST TURN DIRECT TO THE VOR; ATC GAVE US A 270 DEG HDG AND A DSCNT TO 6000 FT. WHEN I CAME BACK ON FREQ; I LOOKED UP AND WE WERE DSNDING THROUGH +/-7000 AND WERE INDICATING 275 KTS. I QUESTIONED THE FO; AND HE TOLD ME THAT HE DID NOT UNDERSTAND THE CLRNC TO 270 DEGS. MY MISTAKE WAS NOT TO MONITOR THE PF CLOSER IN COMPLIANCE WITH AN ASSIGNED CLRNC. I MISINTERPED THAT FMS TURN AS COMPLIANCE WITH THE ASSIGNED HDG.