Narrative:

At approximately 10 miles northwest of ZZZ1 receiving vectors for final approach to ZZZ ILS runway xxl; we were cleared to 5;000 feet. The first officer; as the pilot monitoring; selected 5;000 in the altitude window and pointed at it. I; as the pilot flying; pointed at it and said '5;000'. I selected flch and the aircraft began descending to 5;000 from 6;000 [feet] note; the pilot monitoring was using the radio call 'xxxx; heavy'; while ATC was repeatedly using only our call sign without the qualifier 'heavy'. Also note that there was a flight another company aircraft immediately in front of us getting vectored by same controller to the same runway. As such; just before the FMA annunciated altitude capture (altitude cap) at 5000; ATC evidently cleared the company aircraft to descend to 3000 and cleared that aircraft for approach. Since ATC was not using the word 'heavy' for us; and because of the similar sounding call signs so close together; the pilot monitoring selected 3000 in the altitude window and responded to the ATC clearance. I was not clear which call sign ATC said. I had noticed the pilot monitoring selected 3000 and was waiting for the response from ATC. I became fixated on listening for ATC; and then realized that under these conditions; being at 5000 and still about 10 miles from ZZZ1; it would be highly unusual for ATC to clear us to descend to 3000; and that we must have responded to the other aircraft's clearance. Now; since the pilot monitoring had selected a new altitude in the window before altitude cap was annunciated in the FMA; and flch was engaged; and since the pilot flying was distracted and did not notice this; the aircraft descended below 5000; which was our actual clearance. By the time the pilot flying reacted and descent was arrested; ATC realized that we were descending erroneously and re-emphasized our clearance to only 5000. We noticed that we had descended to 4600 before descent was arrested and climb initiated back to 5;000 [feet]. Flight continued without incident to landing. No conflicts were noted.

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Original NASA ASRS Text

Title: B-767 flight crew reported complying with an ATC descent clearance intended for a company aircraft.

Narrative: At approximately 10 miles NW of ZZZ1 receiving vectors for final approach to ZZZ ILS RWY XXL; we were cleared to 5;000 feet. The First Officer; as the Pilot Monitoring; selected 5;000 in the altitude window and pointed at it. I; as the Pilot Flying; pointed at it and said '5;000'. I selected FLCH and the aircraft began descending to 5;000 from 6;000 [feet] Note; the Pilot Monitoring was using the radio call 'xxxx; Heavy'; while ATC was repeatedly using only our call sign without the qualifier 'Heavy'. Also note that there was a flight another company aircraft immediately in front of us getting vectored by same controller to the same runway. As such; just before the FMA annunciated Altitude Capture (ALT CAP) at 5000; ATC evidently cleared the company aircraft to descend to 3000 and cleared THAT AIRCRAFT for approach. Since ATC was not using the word 'Heavy' for us; and because of the similar sounding call signs so close together; the Pilot Monitoring selected 3000 in the altitude window and responded to the ATC clearance. I was not clear which call sign ATC said. I had noticed the Pilot Monitoring selected 3000 and was waiting for the response from ATC. I became fixated on listening for ATC; and then realized that under these conditions; being at 5000 and still about 10 miles from ZZZ1; it would be highly unusual for ATC to clear us to descend to 3000; and that we must have responded to the other aircraft's clearance. Now; since the Pilot Monitoring had selected a new altitude in the window before ALT CAP was annunciated in the FMA; and FLCH was engaged; and since the Pilot Flying was distracted and did not notice this; the aircraft descended below 5000; which was our actual clearance. By the time the Pilot Flying reacted and descent was arrested; ATC realized that we were descending erroneously and re-emphasized our clearance to only 5000. We noticed that we had descended to 4600 before descent was arrested and climb initiated back to 5;000 [feet]. Flight continued without incident to landing. No conflicts were noted.

Data retrieved from NASA's ASRS site 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.