A BA41 FLC WAS ISSUED CTL INSTRUCTIONS BY BWI APCH CTL THAT; ALTHOUGH READ BACK; WERE EXECUTED DIFFERENTLY THAN EXPECTED. RPTR STATES CTLR SAID 'ONE SEVENTY' AND THE PF READ BACK 'ONE SEVEN ZERO.' THE CTLR APPARENTLY MEANT SPD; BUT THE PLT EXECUTED HDG. FLC CITES CONTRIBUTING FACTORS WORKLOAD; COM; DEFERRED TCASII EQUIP AND WX.

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AFTER BEING HANDED OFF FROM ZDC TO BWI APCH; APCH GAVE US THE APCH IN USE AND RADAR VECTORS. BWI ATC APPEARED TO BE BUSY AND THE WX WAS AT MINIMUMS. WE APPEARED TO BE ON A DOWNWIND AND HAD PASSED THE FIELD. I WAS EXPECTING A BASE TURN AT ANY TIME AND CAPT WAS USING THE #2 RADIO CALLING BWI OPS AND BRIEFING THE PAX. I WAS FLYING THE ACFT AND COMMUNICATING WITH ATC. THE ERROR IN COM OCCURRED WHEN BWI GAVE THE FOLLOWING CLRNC: TURN TO 170 DEGS AND DSND TO 3000 FT. IN RESPONSE; I READ BACK 1-7-0 AND DOWN TO 3000 FT. NOTHING MORE WAS SAID; NO CORRECTION GIVEN BY ATC. WHAT ATC WANTED WAS A SPD OF 170 KTS AND 3000 FT; NOT A TURN! THE RESULT WAS A SPACING/SEPARATION ERROR WITH A B747. VECTORS WERE GIVEN TO US TO CLR THE TFC AND THE FLT ENDED WITHOUT INCIDENT. A COMBINATION OF ERRORS LED TO THE INFRACTION WHICH MAY HAVE BEEN AVOIDED. CTLR AND PLT WORKLOADS; WX; MISCOM; AND A TCASII UNIT BEING DEFERRED EARLIER THAT MORNING WERE ALL CONTRIBUTING FACTORS; WHICH MAY HAVE RESULTED IN THE INCIDENT IN QUESTION. IN CONCLUSION; I HAVE LEARNED FROM THIS INCIDENT AND KNOW TO REQUEST CLARIFICATION OR QUESTION ATC IN THE FUTURE; ESPECIALLY IN A BUSY ATC ENVIRONMENT. SUPPLEMENTAL INFO FROM ACN 392630: ANY OTHER ACFT ON FINAL APCH WERE ALREADY SWITCHED TO A FINAL APCH FREQ. THE CLRNC WHICH WAS HEARD WAS APPROPRIATE FOR THE DISTANCE FROM THE ARPT FOR A TURN TOWARD FINAL APCH COURSE. NO CONFLICTS OBVIOUS. RESULT WAS LOSS OF REQUIRED SEPARATION FROM OTHER ACFT.