Narrative:

We were cruising at FL310, direct to appleton VOR. We were told to descend to FL290, and we complied. A few mins later, while we were cruising at FL290, ZID apparently directed 'air carrier X abcd, descend to FL270' (from FL290). I misunderstood the call sign and acknowledged 'air carrier X aecf out of FL290 for FL270.' the controller did not catch the error, and both of the pilots in my cockpit thought we heard our call sign. When we were descending through FL284, ZID asked us what our altitude was. We responded and were told that the clearance to descend was not for our call sign. ZID subsequently cleared us to continue down to FL270. Air carrier X flight abcd was not up on the frequency, and never did respond to further repeated calls from ZID. 1) both of us in the cockpit of 'air carrier X aecf' mistook the call directed to air carrier X abcd. The call signs are similar in that they begin with the same numbers and have the word 'X' in the middle. If not enunciated clearly, and if radio reception is scratchy, they may be mistaken. 2) air carrier X abcd was not up on the frequency, as ZID was expecting. Air carrier X abcd could have assisted in the error if they had been on frequency. 3) ZID did not catch our readback for the wrong flight number. We deviated 600 ft from our assigned altitude before the error was detected by ZID. No traffic conflicts were apparent and no TCASII warnings were given. We made a mistake and fortunately did not come close to another aircraft.

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Original NASA ASRS Text

Title: AN ACR LEAVES ITS ASSIGNED ALT WHEN IT TAKES A CLRNC MEANT FOR ANOTHER COMPANY ACFT WITH A SIMILAR CALL SIGN WHO WAS NOT ON THE SAME FREQ.

Narrative: WE WERE CRUISING AT FL310, DIRECT TO APPLETON VOR. WE WERE TOLD TO DSND TO FL290, AND WE COMPLIED. A FEW MINS LATER, WHILE WE WERE CRUISING AT FL290, ZID APPARENTLY DIRECTED 'ACR X ABCD, DSND TO FL270' (FROM FL290). I MISUNDERSTOOD THE CALL SIGN AND ACKNOWLEDGED 'ACR X AECF OUT OF FL290 FOR FL270.' THE CTLR DID NOT CATCH THE ERROR, AND BOTH OF THE PLTS IN MY COCKPIT THOUGHT WE HEARD OUR CALL SIGN. WHEN WE WERE DSNDING THROUGH FL284, ZID ASKED US WHAT OUR ALT WAS. WE RESPONDED AND WERE TOLD THAT THE CLRNC TO DSND WAS NOT FOR OUR CALL SIGN. ZID SUBSEQUENTLY CLRED US TO CONTINUE DOWN TO FL270. ACR X FLT ABCD WAS NOT UP ON THE FREQ, AND NEVER DID RESPOND TO FURTHER REPEATED CALLS FROM ZID. 1) BOTH OF US IN THE COCKPIT OF 'ACR X AECF' MISTOOK THE CALL DIRECTED TO ACR X ABCD. THE CALL SIGNS ARE SIMILAR IN THAT THEY BEGIN WITH THE SAME NUMBERS AND HAVE THE WORD 'X' IN THE MIDDLE. IF NOT ENUNCIATED CLRLY, AND IF RADIO RECEPTION IS SCRATCHY, THEY MAY BE MISTAKEN. 2) ACR X ABCD WAS NOT UP ON THE FREQ, AS ZID WAS EXPECTING. ACR X ABCD COULD HAVE ASSISTED IN THE ERROR IF THEY HAD BEEN ON FREQ. 3) ZID DID NOT CATCH OUR READBACK FOR THE WRONG FLT NUMBER. WE DEVIATED 600 FT FROM OUR ASSIGNED ALT BEFORE THE ERROR WAS DETECTED BY ZID. NO TFC CONFLICTS WERE APPARENT AND NO TCASII WARNINGS WERE GIVEN. WE MADE A MISTAKE AND FORTUNATELY DID NOT COME CLOSE TO ANOTHER ACFT.

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.