Narrative:

We were air carrier X flying from lax to las on jan/xx/97. On arrival into las vegas we were cleared for the creso 3 arrival and given clearance to cross whigg intersection at 12000 ft MSL. At or about the time we reached whigg intersection, ZLA passed us to las vegas approach. As first officer, I was flying the aircraft. I was also responding to ATC radio calls since the captain had gone off the ATC frequency to call company and get a gate assignment prior to our descent below 10000 ft MSL. I checked in with approach control and they assigned us a heading of 050 degrees and cleared us to descend and maintain 10000 ft MSL. Although the captain had just returned to the ATC frequency, I read back the heading and altitude clearance since I didn't know if he heard it. I set 10000 ft in the autoplt and began the descent out of 12000 ft MSL. Flight conditions were night and VMC. Approach was also talking to a brasilia aircraft. As we descended, we were monitoring what I believe was the brasilia, on the TCASII. As we closed on the aircraft we had visual contact and used the TCASII for range and altitude information. During our descent, as we closed inside of 5 mi on the aircraft, TCASII indicated it was approximately 800-1000 ft below us. We were monitoring it and assumed that aircraft was descending to 9000 ft MSL. I believe our altitude was approximately 10500 ft MSL when approach called and told us our assigned altitude was 11000 ft MSL and to turn right to 070 degrees. We turned, climbed back to 11000 ft MSL and notified the controller we had the aircraft in sight. We also advised we had been cleared to 10000 ft MSL and that is what we had read back. (As we discussed this in the cockpit, the captain stated that although I read back the altitude assignment because I wasn't sure he had heard it, he had, and also would have read back 10000 ft MSL if I had him respond to the approach control check in.) from watching TCASII, I believe we closed to about 2 to 2 1/2 mi on the other aircraft with about 500-800 ft vertical separation. We did not close to the degree that a TCASII RA was activated. After separation was obtained, approach control turned us back towards the airport and repeated that 11000 ft MSL had been our assigned altitude. We both believe the controller assigned us 10000 ft MSL and are very sure that is the altitude we read back. If the approach controller had said 11000 ft MSL, he did not correct our readback. The TCASII display is an excellent tool in helping maintain situational awareness. It can, and should be used to xchk the appropriateness of ATC instructions. We monitored this aircraft, and since it was a nice VMC night (he might have been a VFR aircraft) did not query approach control as to his assigned altitude. In retrospect, the frequency was not congested, and a quick call could have prevented the conflict.

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Original NASA ASRS Text

Title: AN ARRIVING B737 IS ISSUED A DSCNT TO 11000 FT. THE RPTR COPIES DSCNT TO 10000 FT AND READS BACK 10000 FT. THE CTLR DOES NOT CATCH THE ERROR UNTIL LTSS OCCURS WITH A COMMUTER ACFT AT 10000 FT. WHEN THE ERROR WAS DETECTED, THE CTLR ISSUED A CLB TO 11000 FT AND A TURN AWAY FROM THE COMMUTER ACFT.

Narrative: WE WERE ACR X FLYING FROM LAX TO LAS ON JAN/XX/97. ON ARR INTO LAS VEGAS WE WERE CLRED FOR THE CRESO 3 ARR AND GIVEN CLRNC TO CROSS WHIGG INTXN AT 12000 FT MSL. AT OR ABOUT THE TIME WE REACHED WHIGG INTXN, ZLA PASSED US TO LAS VEGAS APCH. AS FO, I WAS FLYING THE ACFT. I WAS ALSO RESPONDING TO ATC RADIO CALLS SINCE THE CAPT HAD GONE OFF THE ATC FREQ TO CALL COMPANY AND GET A GATE ASSIGNMENT PRIOR TO OUR DSCNT BELOW 10000 FT MSL. I CHKED IN WITH APCH CTL AND THEY ASSIGNED US A HDG OF 050 DEGS AND CLRED US TO DSND AND MAINTAIN 10000 FT MSL. ALTHOUGH THE CAPT HAD JUST RETURNED TO THE ATC FREQ, I READ BACK THE HDG AND ALT CLRNC SINCE I DIDN'T KNOW IF HE HEARD IT. I SET 10000 FT IN THE AUTOPLT AND BEGAN THE DSCNT OUT OF 12000 FT MSL. FLT CONDITIONS WERE NIGHT AND VMC. APCH WAS ALSO TALKING TO A BRASILIA ACFT. AS WE DSNDED, WE WERE MONITORING WHAT I BELIEVE WAS THE BRASILIA, ON THE TCASII. AS WE CLOSED ON THE ACFT WE HAD VISUAL CONTACT AND USED THE TCASII FOR RANGE AND ALT INFO. DURING OUR DSCNT, AS WE CLOSED INSIDE OF 5 MI ON THE ACFT, TCASII INDICATED IT WAS APPROX 800-1000 FT BELOW US. WE WERE MONITORING IT AND ASSUMED THAT ACFT WAS DSNDING TO 9000 FT MSL. I BELIEVE OUR ALT WAS APPROX 10500 FT MSL WHEN APCH CALLED AND TOLD US OUR ASSIGNED ALT WAS 11000 FT MSL AND TO TURN R TO 070 DEGS. WE TURNED, CLBED BACK TO 11000 FT MSL AND NOTIFIED THE CTLR WE HAD THE ACFT IN SIGHT. WE ALSO ADVISED WE HAD BEEN CLRED TO 10000 FT MSL AND THAT IS WHAT WE HAD READ BACK. (AS WE DISCUSSED THIS IN THE COCKPIT, THE CAPT STATED THAT ALTHOUGH I READ BACK THE ALT ASSIGNMENT BECAUSE I WASN'T SURE HE HAD HEARD IT, HE HAD, AND ALSO WOULD HAVE READ BACK 10000 FT MSL IF I HAD HIM RESPOND TO THE APCH CTL CHK IN.) FROM WATCHING TCASII, I BELIEVE WE CLOSED TO ABOUT 2 TO 2 1/2 MI ON THE OTHER ACFT WITH ABOUT 500-800 FT VERT SEPARATION. WE DID NOT CLOSE TO THE DEG THAT A TCASII RA WAS ACTIVATED. AFTER SEPARATION WAS OBTAINED, APCH CTL TURNED US BACK TOWARDS THE ARPT AND REPEATED THAT 11000 FT MSL HAD BEEN OUR ASSIGNED ALT. WE BOTH BELIEVE THE CTLR ASSIGNED US 10000 FT MSL AND ARE VERY SURE THAT IS THE ALT WE READ BACK. IF THE APCH CTLR HAD SAID 11000 FT MSL, HE DID NOT CORRECT OUR READBACK. THE TCASII DISPLAY IS AN EXCELLENT TOOL IN HELPING MAINTAIN SITUATIONAL AWARENESS. IT CAN, AND SHOULD BE USED TO XCHK THE APPROPRIATENESS OF ATC INSTRUCTIONS. WE MONITORED THIS ACFT, AND SINCE IT WAS A NICE VMC NIGHT (HE MIGHT HAVE BEEN A VFR ACFT) DID NOT QUERY APCH CTL AS TO HIS ASSIGNED ALT. IN RETROSPECT, THE FREQ WAS NOT CONGESTED, AND A QUICK CALL COULD HAVE PREVENTED THE CONFLICT.

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.