Air carrier flight crew return to previous altitude when another aircraft is observed below them. ZAB supervisor advises PIC, when asked to call, that clearance was for company aircraft, who advised that another aircraft acknowledged clearance.

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Attributes
ACN	453131
Time
Date	199910
Day	Sun
Local Time Of Day	1801 To 2400
Place
State Reference	AZ
Altitude	msl single value : 31000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zab.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zab.artcc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 270 flight time total : 18000 flight time type : 15000
ASRS Report	453131
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	atc equipment : conflict alert other controllera other flight crewa other other : 5
Resolutory Action	controller : issued new clearance flight crew : returned to original clearance flight crew : took precautionary avoidance action
Consequence	faa : investigated faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

While cruising at FL310, received clearance to 'descend to FL280, discretion to FL240.' I read back clearance and first officer began descent. First officer was distraction talking to flight attendant and did not hear clearance. 500 ft into descent, first officer noticed aircraft below us on TCASII about 1500 ft. We stopped descent about the time center called to ask our altitude. We stated we were climbing back to our cruise altitude and referenced the previous clearance. Center then gave me a phone number to call on my arrival. After listening to the tapes, center supervisor said the controller had been relieved from duty and that no action would be taken against me. The original clearance had been for company Y, not my airplane. Upon receiving the clearance, company Y told the center someone else had taken their clearance.

Original NASA ASRS Text

Title: ACR FLC RETURN TO PREVIOUS ALT WHEN ANOTHER ACFT IS OBSERVED BELOW THEM. ZAB SUPVR ADVISES PIC, WHEN ASKED TO CALL, THAT CLRNC WAS FOR COMPANY ACFT, WHO ADVISED THAT ANOTHER ACFT ACKNOWLEDGED CLRNC.

Narrative: WHILE CRUISING AT FL310, RECEIVED CLRNC TO 'DSND TO FL280, DISCRETION TO FL240.' I READ BACK CLRNC AND FO BEGAN DSCNT. FO WAS DISTR TALKING TO FLT ATTENDANT AND DID NOT HEAR CLRNC. 500 FT INTO DSCNT, FO NOTICED ACFT BELOW US ON TCASII ABOUT 1500 FT. WE STOPPED DSCNT ABOUT THE TIME CTR CALLED TO ASK OUR ALT. WE STATED WE WERE CLBING BACK TO OUR CRUISE ALT AND REFED THE PREVIOUS CLRNC. CTR THEN GAVE ME A PHONE NUMBER TO CALL ON MY ARR. AFTER LISTENING TO THE TAPES, CTR SUPVR SAID THE CTLR HAD BEEN RELIEVED FROM DUTY AND THAT NO ACTION WOULD BE TAKEN AGAINST ME. THE ORIGINAL CLRNC HAD BEEN FOR COMPANY Y, NOT MY AIRPLANE. UPON RECEIVING THE CLRNC, COMPANY Y TOLD THE CTR SOMEONE ELSE HAD TAKEN THEIR CLRNC.

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.