Narrative:

We were approaching dfw at a busy time. Our controller was talking almost non stop on 2 separate frequencys. Out flight number was xyy and there was another company flight number xyz on the other frequency. Although not directly cautioned on the similar call signs, we were aware of it. I was flying the aircraft and monitoring the TCASII as there were numerous targets near us. My first officer took a call for a descent to 5000 ft and set that in the altitude preselector. I acknowledged 5000 ft and initiated a descent. Being busy as the PF, and with the high volume of communications occurring, I did not personally hear the controller but accepted my first officer's altitude selection. He was matter-of-fact about it and, after flying most of the month together, I had confidence in him. I do not know whether the controller erred or we took a call for xyz. We would not have heard xyz respond and the controller did not correct us if we had responded to another aircraft's instructions. Regardless of how it happened, we were on our way down from 6000 ft to 5000 ft. I had been watching what the controller later told us was a cessna caravan below us flying parallel to us at about 3 NM away. I did not think this was too unusual, as it occurs often around the dfw terminal area, but I did monitor his proximity to us. In retrospect, the controller usually advises you of traffic that close and tries to verify that you have a visual on it. I got 1 TA from TCASII as we passed the aircraft, but had visual contact with it. At about the same time, ATC advised us of the traffic and that our assigned altitude was 6000 ft. I immediately stopped the descent and we reported the traffic insight, now passing behind us. The controller then cleared us to maintain 5000 ft. We were at 5200 ft and 2 1/2 mi from the caravan at our closest point. He was level at 5000 ft. We only received the 1 traffic alert from TCASII and no RA's to take evasive action. We all know and are taught the importance of crew coordination and the responsibility of both pilots being in the loop regarding all aspects of the flight. My not personally verifying the clearance to 5000 ft and accepting my first officer's understanding of that clearance was a factor in this situation. The practice of a controller working 2 frequencys at once can be difficult because you can't hear other aircraft and can transmit simultaneously with another aircraft on the other frequency and not realize it.

Google
 

Original NASA ASRS Text

Title: AN SF340 CREW DSNDS OUT OF THEIR ASSIGNED ALT UPON ACCEPTING ANOTHER COMPANY'S CALL SIGN AND CLRNC, INTO DFW, TX.

Narrative: WE WERE APCHING DFW AT A BUSY TIME. OUR CTLR WAS TALKING ALMOST NON STOP ON 2 SEPARATE FREQS. OUT FLT NUMBER WAS XYY AND THERE WAS ANOTHER COMPANY FLT NUMBER XYZ ON THE OTHER FREQ. ALTHOUGH NOT DIRECTLY CAUTIONED ON THE SIMILAR CALL SIGNS, WE WERE AWARE OF IT. I WAS FLYING THE ACFT AND MONITORING THE TCASII AS THERE WERE NUMEROUS TARGETS NEAR US. MY FO TOOK A CALL FOR A DSCNT TO 5000 FT AND SET THAT IN THE ALT PRESELECTOR. I ACKNOWLEDGED 5000 FT AND INITIATED A DSCNT. BEING BUSY AS THE PF, AND WITH THE HIGH VOLUME OF COMS OCCURRING, I DID NOT PERSONALLY HEAR THE CTLR BUT ACCEPTED MY FO'S ALT SELECTION. HE WAS MATTER-OF-FACT ABOUT IT AND, AFTER FLYING MOST OF THE MONTH TOGETHER, I HAD CONFIDENCE IN HIM. I DO NOT KNOW WHETHER THE CTLR ERRED OR WE TOOK A CALL FOR XYZ. WE WOULD NOT HAVE HEARD XYZ RESPOND AND THE CTLR DID NOT CORRECT US IF WE HAD RESPONDED TO ANOTHER ACFT'S INSTRUCTIONS. REGARDLESS OF HOW IT HAPPENED, WE WERE ON OUR WAY DOWN FROM 6000 FT TO 5000 FT. I HAD BEEN WATCHING WHAT THE CTLR LATER TOLD US WAS A CESSNA CARAVAN BELOW US FLYING PARALLEL TO US AT ABOUT 3 NM AWAY. I DID NOT THINK THIS WAS TOO UNUSUAL, AS IT OCCURS OFTEN AROUND THE DFW TERMINAL AREA, BUT I DID MONITOR HIS PROX TO US. IN RETROSPECT, THE CTLR USUALLY ADVISES YOU OF TFC THAT CLOSE AND TRIES TO VERIFY THAT YOU HAVE A VISUAL ON IT. I GOT 1 TA FROM TCASII AS WE PASSED THE ACFT, BUT HAD VISUAL CONTACT WITH IT. AT ABOUT THE SAME TIME, ATC ADVISED US OF THE TFC AND THAT OUR ASSIGNED ALT WAS 6000 FT. I IMMEDIATELY STOPPED THE DSCNT AND WE RPTED THE TFC INSIGHT, NOW PASSING BEHIND US. THE CTLR THEN CLRED US TO MAINTAIN 5000 FT. WE WERE AT 5200 FT AND 2 1/2 MI FROM THE CARAVAN AT OUR CLOSEST POINT. HE WAS LEVEL AT 5000 FT. WE ONLY RECEIVED THE 1 TFC ALERT FROM TCASII AND NO RA'S TO TAKE EVASIVE ACTION. WE ALL KNOW AND ARE TAUGHT THE IMPORTANCE OF CREW COORD AND THE RESPONSIBILITY OF BOTH PLTS BEING IN THE LOOP REGARDING ALL ASPECTS OF THE FLT. MY NOT PERSONALLY VERIFYING THE CLRNC TO 5000 FT AND ACCEPTING MY FO'S UNDERSTANDING OF THAT CLRNC WAS A FACTOR IN THIS SIT. THE PRACTICE OF A CTLR WORKING 2 FREQS AT ONCE CAN BE DIFFICULT BECAUSE YOU CAN'T HEAR OTHER ACFT AND CAN XMIT SIMULTANEOUSLY WITH ANOTHER ACFT ON THE OTHER FREQ AND NOT REALIZE IT.

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.