Flight crew departs assigned altitude in response to another flight's clearance.

Attributes
ACN	318918
Time
Date	199510
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : dln
State Reference	MT
Altitude	msl bound lower : 35000 msl bound upper : 39000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zlc
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 15600
ASRS Report	318918
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 75 flight time total : 18200 flight time type : 300
ASRS Report	318822
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course other
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Cruising at FL390 ZLC issued the clearance 'air carrier X descend to FL310.' I repeated the clearance and reported 'out of FL390 for FL310,' clearly using our full call sign 'air carrier X.' passing FL350 ATC called and said, air carrier X climb to FL390. I asked him to repeat because he had just cleared us to FL310. He said that was for an air carrier Y and that I took his clearance. We climbed back to FL390. ATC did not report traffic in the area nor did TCASII show any. I don't believe there was a conflict. We had been working with this one controller and he had given us 2 subsequent frequency changes. I believe he was working at least 3 different frequencys at the time, and this was without a doubt the reason he did not hear my readback. I suspect both air carrier Y and I read back the clearance simultaneously and ATC heard only 1 readback. I find it very disconcerting when ATC issues a clearance and a readback is not heard on that same frequency. This leads one to believe that the clearance is for him. I'm still not convinced whether ATC misspoke or we misheard the issued clearance.

Original NASA ASRS Text

Title: FLC DEPARTS ASSIGNED ALT IN RESPONSE TO ANOTHER FLT'S CLRNC.

Narrative: CRUISING AT FL390 ZLC ISSUED THE CLRNC 'ACR X DSND TO FL310.' I REPEATED THE CLRNC AND RPTED 'OUT OF FL390 FOR FL310,' CLRLY USING OUR FULL CALL SIGN 'ACR X.' PASSING FL350 ATC CALLED AND SAID, ACR X CLB TO FL390. I ASKED HIM TO REPEAT BECAUSE HE HAD JUST CLRED US TO FL310. HE SAID THAT WAS FOR AN ACR Y AND THAT I TOOK HIS CLRNC. WE CLBED BACK TO FL390. ATC DID NOT RPT TFC IN THE AREA NOR DID TCASII SHOW ANY. I DON'T BELIEVE THERE WAS A CONFLICT. WE HAD BEEN WORKING WITH THIS ONE CTLR AND HE HAD GIVEN US 2 SUBSEQUENT FREQ CHANGES. I BELIEVE HE WAS WORKING AT LEAST 3 DIFFERENT FREQS AT THE TIME, AND THIS WAS WITHOUT A DOUBT THE REASON HE DID NOT HEAR MY READBACK. I SUSPECT BOTH ACR Y AND I READ BACK THE CLRNC SIMULTANEOUSLY AND ATC HEARD ONLY 1 READBACK. I FIND IT VERY DISCONCERTING WHEN ATC ISSUES A CLRNC AND A READBACK IS NOT HEARD ON THAT SAME FREQ. THIS LEADS ONE TO BELIEVE THAT THE CLRNC IS FOR HIM. I'M STILL NOT CONVINCED WHETHER ATC MISSPOKE OR WE MISHEARD THE ISSUED 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.