Air carrier B737-300 with descent clearance from 12000 ft to 10000 ft. Controller questioned flight crew as to what their assigned altitude was and sounded confused when issuing a radar vector away from a commuter aircraft the air carrier was overtaking and implied to be at 10000 ft or 11000 ft. No less than standard separation.

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Attributes
ACN	358100
Time
Date	199701
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : bld
State Reference	NV
Altitude	msl bound lower : 11000 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : las
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	arrival other enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 250 flight time total : 9000 flight time type : 5000
ASRS Report	358100
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Miss Distance	horizontal : 18000 vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other

Narrative:

During the descent into las, approach controller cleared us to 10000 ft from 12000 ft. A commuter aircraft was in front of us. We were overtaking the commuter. I went off frequency to get a gate assignment and was coming back on frequency when the clearance was issued. The first officer read back 10000 ft and confirmed it with me. I had the commuter airplane in sight and on TCASII. As we descended through 11000 ft, the controller asked what our assigned altitude was, and we confirmed 10000 ft. He sounded confused as he gave us a vector away from the commuter aircraft. The confusion could have been avoided if we would have been aware of the commuter's assigned altitude and our assigned altitude as being the same as we were overtaking the commuter. We could have then questioned the clearance and removed the confusion. I was aware that the commuter was there but I thought he had been cleared to, and was descending to a lower altitude.

Original NASA ASRS Text

Title: ACR B737-300 WITH DSCNT CLRNC FROM 12000 FT TO 10000 FT. CTLR QUESTIONED FLC AS TO WHAT THEIR ASSIGNED ALT WAS AND SOUNDED CONFUSED WHEN ISSUING A RADAR VECTOR AWAY FROM A COMMUTER ACFT THE ACR WAS OVERTAKING AND IMPLIED TO BE AT 10000 FT OR 11000 FT. NO LTSS.

Narrative: DURING THE DSCNT INTO LAS, APCH CTLR CLRED US TO 10000 FT FROM 12000 FT. A COMMUTER ACFT WAS IN FRONT OF US. WE WERE OVERTAKING THE COMMUTER. I WENT OFF FREQ TO GET A GATE ASSIGNMENT AND WAS COMING BACK ON FREQ WHEN THE CLRNC WAS ISSUED. THE FO READ BACK 10000 FT AND CONFIRMED IT WITH ME. I HAD THE COMMUTER AIRPLANE IN SIGHT AND ON TCASII. AS WE DSNDED THROUGH 11000 FT, THE CTLR ASKED WHAT OUR ASSIGNED ALT WAS, AND WE CONFIRMED 10000 FT. HE SOUNDED CONFUSED AS HE GAVE US A VECTOR AWAY FROM THE COMMUTER ACFT. THE CONFUSION COULD HAVE BEEN AVOIDED IF WE WOULD HAVE BEEN AWARE OF THE COMMUTER'S ASSIGNED ALT AND OUR ASSIGNED ALT AS BEING THE SAME AS WE WERE OVERTAKING THE COMMUTER. WE COULD HAVE THEN QUESTIONED THE CLRNC AND REMOVED THE CONFUSION. I WAS AWARE THAT THE COMMUTER WAS THERE BUT I THOUGHT HE HAD BEEN CLRED TO, AND WAS DSNDING TO A LOWER ALT.

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.