Narrative:

We were at cruise at FL230. ZOB center cleared us to FL220. In the descent; we thought ZOB issued a descent to 11;000 ft. The captain read back the instruction to descend to 11;000 ft. I thought ZOB had said air carrier XXXY instead of our flight number of air carrier XXXX; but I had not heard any other company planes on the frequency and when the captain read back the new altitude; there was no correction from ATC. Also the descent clearance seemed to be correct for our location and phase of flight. At about 21;500 ft; in the descent; ZOB asked our assigned altitude; the captain told him 11;000 ft. ZOB said okay; and gave the current altimeter setting. The captain asked if there was another company plane on frequency and we were told yes. A couple of minutes later; air carrier XXXY checked on and ZOB said they had been looking for them and they must have had a late hand off. The captain and I decided that he had meant to give clearance to air carrier XXXY and we answered for them. Suggestions; I heard air carrier XXXY and not air carrier XXXX when the clearance to 11;000 was given. I should have asked the captain to verify the call sign. It was late and the frequency was quiet so I just assumed that it was for us because often ATC will call and the last number in the call sign will be incorrect. I should always get clarification when unsure. Also we should probably not be sending flight with such similar sounding call signs into the same airport at the same time; it creates potential for confusion which it did this night.

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Original NASA ASRS Text

Title: Air carrier enroute with ZOB initiated a descent reportedly due to four digit same company call sign confusion; reporter suggested company refrain from operating similar sounding call sign flights in the same area at the same time.

Narrative: We were at cruise at FL230. ZOB Center cleared us to FL220. In the descent; we thought ZOB issued a descent to 11;000 FT. The Captain read back the instruction to descend to 11;000 FT. I thought ZOB had said Air Carrier XXXY instead of our flight number of Air Carrier XXXX; but I had not heard any other company planes on the frequency and when the Captain read back the new altitude; there was no correction from ATC. Also the descent clearance seemed to be correct for our location and phase of flight. At about 21;500 FT; in the descent; ZOB asked our assigned altitude; the Captain told him 11;000 FT. ZOB said okay; and gave the current altimeter setting. The Captain asked if there was another company plane on frequency and we were told yes. A couple of minutes later; Air Carrier XXXY checked on and ZOB said they had been looking for them and they must have had a late hand off. The Captain and I decided that he had meant to give clearance to Air Carrier XXXY and we answered for them. Suggestions; I heard Air Carrier XXXY and not Air Carrier XXXX when the clearance to 11;000 was given. I should have asked the Captain to verify the call sign. It was late and the frequency was quiet so I just assumed that it was for us because often ATC will call and the last number in the call sign will be incorrect. I should always get clarification when unsure. Also we should probably not be sending flight with such similar sounding call signs into the same airport at the same time; It creates potential for confusion which it did this night.

Data retrieved from NASA's ASRS site as of April 2012 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.