Narrative:

At approximately 20 NM from bwi, approach controller instructed, 'proceed direct bwi, maintain 3000' and advise airport in sight.' at least twice first officer advised controller that airport was in sight, but no response from controller. I was hand-flying the aircraft and making a slow descent toward the final approach. Just prior to reaching 3000' controller advised traffic, '12 O'clock, 3 NM at 2500',' VFR, followed by another aircraft at 2500'. Shortly after this advisory, TCAS issued an alert on the first aircraft and shortly thereafter an alert on the second. During these alerts, controller gave a second advisory on these aircraft. Both aircraft was sighted and I began a turn away from the aircraft and more toward the OM for runway 33L. During the rapid callouts from both TCAS and the approach controller, I was confused and under the impression we were cleared for a visibility approach. As the G/south became alive, I started descending, having both VFR aircraft in sight off my left side. At an altitude of 2700', controller advised, 'company identify, you were assigned 3000'.' at that point I leveled off and controller asked, 'do you have the traffic and airport in sight?' first officer responded, 'affirmative, traffic and airport in sight.' controller: 'company identify, you are cleared for the visibility approach.' I feel the primary factor contributing to the misinterpreted clearance was the fact that it was a partial clearance; I.east, direct to the airport but maintain 3000', or cleared to descend at your discretion, but maintain a heading. These clearance types are all in anticipation of a visibility clearance. Also contributing was the rapid succession of advisories from the controller and TCAS. While I feel TCAS will become a very valuable tool, it is presently distracting in a very busy terminal environment.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION EXCURSION FROM CLRNC ALT IN ANTICIPATION OF A VISUAL APCH.

Narrative: AT APPROX 20 NM FROM BWI, APCH CTLR INSTRUCTED, 'PROCEED DIRECT BWI, MAINTAIN 3000' AND ADVISE ARPT IN SIGHT.' AT LEAST TWICE F/O ADVISED CTLR THAT ARPT WAS IN SIGHT, BUT NO RESPONSE FROM CTLR. I WAS HAND-FLYING THE ACFT AND MAKING A SLOW DSNT TOWARD THE FINAL APCH. JUST PRIOR TO REACHING 3000' CTLR ADVISED TFC, '12 O'CLOCK, 3 NM AT 2500',' VFR, FOLLOWED BY ANOTHER ACFT AT 2500'. SHORTLY AFTER THIS ADVISORY, TCAS ISSUED AN ALERT ON THE FIRST ACFT AND SHORTLY THEREAFTER AN ALERT ON THE SECOND. DURING THESE ALERTS, CTLR GAVE A SECOND ADVISORY ON THESE ACFT. BOTH ACFT WAS SIGHTED AND I BEGAN A TURN AWAY FROM THE ACFT AND MORE TOWARD THE OM FOR RWY 33L. DURING THE RAPID CALLOUTS FROM BOTH TCAS AND THE APCH CTLR, I WAS CONFUSED AND UNDER THE IMPRESSION WE WERE CLRED FOR A VIS APCH. AS THE G/S BECAME ALIVE, I STARTED DSNDING, HAVING BOTH VFR ACFT IN SIGHT OFF MY LEFT SIDE. AT AN ALT OF 2700', CTLR ADVISED, 'COMPANY IDENT, YOU WERE ASSIGNED 3000'.' AT THAT POINT I LEVELED OFF AND CTLR ASKED, 'DO YOU HAVE THE TFC AND ARPT IN SIGHT?' F/O RESPONDED, 'AFFIRMATIVE, TFC AND ARPT IN SIGHT.' CTLR: 'COMPANY IDENT, YOU ARE CLRED FOR THE VIS APCH.' I FEEL THE PRIMARY FACTOR CONTRIBUTING TO THE MISINTERPRETED CLRNC WAS THE FACT THAT IT WAS A PARTIAL CLRNC; I.E, DIRECT TO THE ARPT BUT MAINTAIN 3000', OR CLRED TO DSND AT YOUR DISCRETION, BUT MAINTAIN A HDG. THESE CLRNC TYPES ARE ALL IN ANTICIPATION OF A VIS CLRNC. ALSO CONTRIBUTING WAS THE RAPID SUCCESSION OF ADVISORIES FROM THE CTLR AND TCAS. WHILE I FEEL TCAS WILL BECOME A VERY VALUABLE TOOL, IT IS PRESENTLY DISTRACTING IN A VERY BUSY TERMINAL ENVIRONMENT.

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.