Narrative:

While transitioning from the descent phase to approach phase of flight, the approach controller advised us to descend to 3200 ft MSL. While descending we got a TCASII TA, we leveled off but the traffic was climbing. Therefore, we got an RA. After acting immediately we cleared traffic. At this point as we saw the airport and judged our relative ht, we realized we needed to descend rapidly. The PNF called the visual and the controller cleared us. We descended to 1500 ft MSL and requested frequency change to tower. At that point the approach controller advised us that he never cleared us to descend below 3200 ft MSL and we were at 1500 ft MSL at this point. He requested us to call ATC once we landed and we proceeded with tower. It is our understanding that once you have been cleared for the approach, altitude is at pilot's discretion considering ht above obstacle regulations.

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Original NASA ASRS Text

Title: ATC QUESTIONED ATR42 FLT CREW FOR LEAVING ASSIGNED ALT AFTER ISSUING CLRNC FOR VISUAL APCH.

Narrative: WHILE TRANSITIONING FROM THE DSCNT PHASE TO APCH PHASE OF FLT, THE APCH CTLR ADVISED US TO DSND TO 3200 FT MSL. WHILE DSNDING WE GOT A TCASII TA, WE LEVELED OFF BUT THE TFC WAS CLBING. THEREFORE, WE GOT AN RA. AFTER ACTING IMMEDIATELY WE CLRED TFC. AT THIS POINT AS WE SAW THE ARPT AND JUDGED OUR RELATIVE HT, WE REALIZED WE NEEDED TO DSND RAPIDLY. THE PNF CALLED THE VISUAL AND THE CTLR CLRED US. WE DSNDED TO 1500 FT MSL AND REQUESTED FREQ CHANGE TO TWR. AT THAT POINT THE APCH CTLR ADVISED US THAT HE NEVER CLRED US TO DSND BELOW 3200 FT MSL AND WE WERE AT 1500 FT MSL AT THIS POINT. HE REQUESTED US TO CALL ATC ONCE WE LANDED AND WE PROCEEDED WITH TWR. IT IS OUR UNDERSTANDING THAT ONCE YOU HAVE BEEN CLRED FOR THE APCH, ALT IS AT PLT'S DISCRETION CONSIDERING HT ABOVE OBSTACLE REGS.

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.