Flight crew accepted a clearance for another aircraft and departed assigned altitude.

Attributes
ACN	432803
Time
Date	199904
Day	Sat
Local Time Of Day	0601 To 1200
Place
State Reference	NV
Altitude	msl bound lower : 38000 msl bound upper : 39000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa.artcc
Make Model Name	B767 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : enroute altitude change
Route In Use	enroute other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp pilot : commercial pilot : multi engine pilot : instrument pilot : cfi
Experience	flight time last 90 days : 150 flight time total : 20000 flight time type : 1500
ASRS Report	432803
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 non adherence : clearance
Independent Detector	atc equipment other atc equipment : radar other controllera other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned airspace
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

First officer (aircraft X) received clearance to descend to FL310 from FL390 and read back clearance. At FL380 controller stated we should have remained at FL390, climb back to FL390. I was involved with a medical emergency with first flight attendant and never heard the communications. Notably, aircraft Y was on the frequency. I spoke to the controller's supervisor by telephone 1 hour later, and she did not hear the conversation(south) between the first officer and controller. We agreed that aircraft Y may have caused the confusion. No action is planned legally, tapes are being pulled and supervisor and myself will discuss later. Apparently, the controller had 2 sits while we had the medical emergency, all with similar flight numbers X and Y. No evasive action was required and no conflicts existed per the supervisor.

Original NASA ASRS Text

Title: FLC ACCEPTED A CLRNC FOR ANOTHER ACFT AND DEPARTED ASSIGNED ALT.

Narrative: FO (ACFT X) RECEIVED CLRNC TO DSND TO FL310 FROM FL390 AND READ BACK CLRNC. AT FL380 CTLR STATED WE SHOULD HAVE REMAINED AT FL390, CLB BACK TO FL390. I WAS INVOLVED WITH A MEDICAL EMER WITH FIRST FLT ATTENDANT AND NEVER HEARD THE COMS. NOTABLY, ACFT Y WAS ON THE FREQ. I SPOKE TO THE CTLR'S SUPVR BY TELEPHONE 1 HR LATER, AND SHE DID NOT HEAR THE CONVERSATION(S) BTWN THE FO AND CTLR. WE AGREED THAT ACFT Y MAY HAVE CAUSED THE CONFUSION. NO ACTION IS PLANNED LEGALLY, TAPES ARE BEING PULLED AND SUPVR AND MYSELF WILL DISCUSS LATER. APPARENTLY, THE CTLR HAD 2 SITS WHILE WE HAD THE MEDICAL EMER, ALL WITH SIMILAR FLT NUMBERS X AND Y. NO EVASIVE ACTION WAS REQUIRED AND NO CONFLICTS EXISTED PER THE SUPVR.

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.