Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Flight Instructor
Flight Crew Last 90 Days 75; Flight Crew Total 3900; Flight Crew Type 100
Communication Breakdown; Fatigue
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Human Factors
We thought we were cleared to descend out of FL430 after responding to and reading back a clearance to descend. As we passed through FL420 ATC cleared us to FL400 with the statement that the previous descent clearance had not been for us. No known loss of separation. We subsequently determined that clearance had been for a similar call sign. My guess is that our readback was either blocked by the other aircraft or ATC did not realize it was us acknowledging the clearance. I believe contributing factors include: We had not been made aware of a similar call sign upon checking in on the frequency. Fatigue we had already completed nearly 9 hours of flying including an ocean crossing in what was a back side of the clock operation for our circadian rhythm. Our failure to adequately monitor the frequency.
A small transport crew acknowledged a descent clearance and began a descent that was intended for another aircraft with a nearly identical 'N' number. No warning of a similar call sign had been given.