Date: 201606
Local time of day: 0601-1200
Location: ATL.Airport, GA
Flight conditions: VMC
Light: Daylight
Aircraft: Light Transport
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Approach
Airspace: Class E A80
ATC / advisory: TRACON A80
Reporter organization: Air Taxi
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Communication Breakdown; Fatigue
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: On initial approach vectors to land with Atlanta Approach. The radio traffic was very congested. Approach was very busy. I was given a descent to 12;000; I copied the instruction. We descended to 12;000. Around the time we leveled at 12;000; I was given a speed reduction assignment. I acknowledged and we complied. Shortly thereafter; I thought that I was given another descent assignment to 10;000. I copied the instruction. My copilot was busy with some flying stuff and didn't hear the instruction. He then saw that 10;000 was in the altitude alerter; and asked; 'when were we told to go to 10?' I said; 'just now.' So we started down. At 11;000; ATC came on and told us to stop; and asked why we were descending. I said; 'We were instructed to descend to 10;000'. ATC said we were not instructed. I said that I read back the assignment; and if it wasn't for me; why didn't you correct me. The remainder of the flight was normal. Just before going to tower frequency; approach gave me the TRACON phone number to call. I called. The lady on the phone said that she had pulled the tape; and there was no recording of an instruction for me to descend to 10;000 and furthermore no reply from me to that instruction. I told her that I didn't know why my response was not there; but the radio was so busy; that another plane may have keyed up at the same time and cut me out. But I know that I acknowledged an instruction that I believed was for me even though it was now apparent that it was for someone else. And that since I wasn't corrected; I felt I was properly complying with said instruction. She then said the instruction was for a [commercial] jet with a somewhat similar callsign. She said that paperwork was going to be filed with the FAA and to be waiting for a call.I realize now that I should've clarified the instruction with ATC; when my copilot asked about 10;000 being in the alerter. I was confident I heard correctly; even though it is good CRM to clarify an instruction when a crew member questions the instruction. One of the controllers had told a couple of other planes that they needed to pay better attention to hearing their callsigns because they were calling these planes multiple times with no response. One controller told one plane to 'listen up'. And he said it quite sternly. So when my copilot questioned the assignment; I felt that I had it right and I also didn't want to anger the controller by trying to cut in. Not confirming was my biggest mistake. Fatigue may have had something to do with it also. We were arriving in ATL after a four hour plus flight. We left my home base [late at night]. So; I was tired as well.




Synopsis: Air taxi Captain reported an altitude deviation resulted after responding to a descent clearance that was intended for an aircraft with a similar call sign.