Narrative:

After being handed off from ZDC to bwi approach, approach gave us the approach 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 captain was using the #2 radio calling bwi operations and briefing the passenger. I was flying the aircraft and communicating with ATC. The error in communication occurred when bwi gave the following clearance: turn to 170 degrees and descend 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 speed 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 clear the traffic and the flight ended without incident. A combination of errors led to the infraction which may have been avoided. Controller and pilot 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 information from acn 392630: any other aircraft on final approach were already switched to a final approach frequency. The clearance which was heard was appropriate for the distance from the airport for a turn toward final approach course. No conflicts obvious. Result was loss of required separation from other aircraft.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.