Date: 201605
Location: ZZZ.ARTCC, US
Aircraft: Medium Large Transport
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
Second aircraft: Cessna Citation Undifferentiated or Other Model
Reporter organization: Government
Reporter function: Enroute
Human factors: Confusion; Communication Breakdown
Second reporter function: Pilot Flying; First Officer; Check Pilot
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft TA; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Human Factors; Company Policy
Primary problem: Human Factors

Narrative: Citation was cleared to FL180. [Air Carrier X Flight AAB] was on course to [destination]. [Air Carrier X Flight AAB] was NORDO for some period of time where sector 14 did not establish communications. I requested sector 14 start [Air Carrier X Flight AAB] down to get under Citation. They informed me he was NORDO and could not comply. [Air Carrier X Flight ACB] checked in also descending for [he destination]. I asked [Air Carrier X Flight ACB] to go through company and have [Air Carrier X Flight AAB] come to sector 15 frequency. Right after doing so; Approach called and said [Air Carrier X Flight AAB] was on their frequency and I requested communications. [Air Carrier X Flight AAB] checked in and asked if he missed a handoff somewhere. I informed him he did but it wasn't a problem and we would start him down soon. I called traffic to [Air Carrier X Flight AAB] and Citation. My very next clearance was for [Air Carrier X Flight ACB] to cross 5 north of ZZZ VOR at 110 then reduce to 250k. [Air Carrier X Flight AAB] read back [Air Carrier X Flight ACB] clearance and I missed that [Air Carrier X Flight AAB] read it back. [Thirty seconds later] [Air Carrier X Flight AAB] asked if I had traffic for him. I called the traffic again and issued an immediate 30 degree turn to the left. He never advised he received an RA. I asked if [Air Carrier X Flight ACB] got the clearance meant for him and he said affirmative.[Suggest] have [Air Carrier company] eliminate similar sounding call signs aircraft landing/departing same airports.I did not tell [Air Carrier X Flight AAB] about [Air Carrier X Flight ACB]; but [Air Carrier X Flight ACB] was aware of [Air Carrier X Flight AAB] because I asked him to go through company to [Air Carrier X Flight AAB]on my frequency. I should have told [Air Carrier X Flight AAB] about [Air Carrier X Flight ACB] but due to him already coming on frequency late; calling merging target procedures to [Air Carrier X Flight AAB] and Citation; and evaluating getting [Air Carrier X Flight ACB] down through overflight traffic at 110; I did not. I called traffic and a contributing factor to me missing the call sign readback was me assuming [Air Carrier X Flight AAB] wouldn't take a descent clearance immediately after I called traffic.


Second narrative: We took another flight's altitude clearance and it may have resulted in an air traffic control loss of separation. I was the Pilot in Command and was Pilot Flying (PF) from the right seat for this flight. I was also acting as Check Airman and conducting upgrade [check]. We were cruising at FL190 approaching ZZZ VOR from the northwest. Shortly after our frequency change to center on 133.325; the Pilot Monitoring (PM) responded as '[Air Carrier X Flight AAB]' to an ATC clearance to descend to FL180 and to cross 5 miles north of ZZZ VOR at 11;000 feet. The PM set 11;000 feet in the altitude alert window and created a crossing point with the altitude restriction in the FMS. I set FL180 as the new cruise altitude in the FMS and began the descent while FMS started calculating a new top of descent point for the new cruise altitude. We expected a descent clearance as we were a little high for our anticipated crossing restriction at [the VOR].We were not aware that '[Air Carrier X Flight ACB]' was also on the frequency and approaching ZZZ VOR north of our position. In hindsight; it seems that both flights acknowledged the new clearance at the same time and transmitted over one another. The new clearance was intended for '[Air Carrier X Flight ACB]'; not '[Air Carrier X Flight AAB]'. Unfortunately; we understood the transmission as ours and it seems that the Controller didn't realize that we had either both responded to the new clearance simultaneously or that we had inadvertently taken a clearance meant for '[Air Carrier X Flight ACB]'.As we descended through 18;700 feet; proximate TCAS traffic appeared at our 2 o'clock position at the edge of the TCAS display range (RNG 6.5) at FL180 (I believe it was a Cessna Citation). I slowed our rate of descent while the PM queried the controller. The controller advised us that the previously issued descent clearance was for another aircraft ([Air Carrier X Flight ACB]). He instructed us to turn left immediately and we returned to FL190 while completing the turn. I don't recall ever descending below 18;500 feet as we complied with the instructions to turn for separation. We never saw the traffic penetrate the inner TCAS range ring on our TCAS display and we received one Traffic Advisory (TA) as we initiated the turn and the climb. Weather conditions were VMC at the time of the event; but we never visually acquired the aircraft.After the conflict was resolved; we were given further descent and a frequency change. The controller also gave us a telephone number to call once we were on the ground in order to discuss the event with Center. The rest of the flight was uneventful.Once on the ground; I spoke with the ARTCC Operations Manager (TMO) over the telephone. He said that we had taken another flight's clearance and that it had resulted in a loss of separation. He explained that he would submit a Mandatory Occurrence Report (MOR) that would be reviewed by the Quality Control (QC) office. [The controller] said that initial indications showed that the separation had decreased to 2.67 miles and 200 feet; but advised that initial readings were not necessarily as accurate as those that result from the QC review.We were both personally and professionally frustrated that we didn't catch the mistake until the loss of separation had occurred. There were a couple of contributing factors that probably didn't make things any easier for us leading up to the event.First; we discovered an unusual Passenger Address system issue with [this aircraft] during our return flight. When making a passenger address from any flight deck microphone; headset; or the flight deck handset; the PA was always hot on the First Officers (FO) headset and overhead speaker; independent of switch position. One could turn the FO overhead speaker off during a flight deck initiated PA; but the PA was always hot and at full volume on the FO headset. The anomaly may have resulted in a brief period of no radio contact that I believe occurred while the PM was making our arrival PA. When the PM returned from his PA and I felt that I hadn't heard a radio transmission for a while (Center frequency 127.8); I initiated contact and received no reply. I subsequently contacted approach on 118.6 who sent us to Center frequency 133.325.Additionally; it likely would have helped us prevent the error or catch our mistake sooner had we known that there was another [Company] flight on frequency 133.325 with a similar sounding call sign. We did not know that in addition to our '[Air Carrier X Flight AAB]' flight; there was also an '[Air Carrier X Flight ACB]' flight on frequency that was also nearing the ZZZ VOR until after the loss of separation event. That flight eventually ended up landing right behind us.Finally; though challenging from an airline's scheduling perspective; it would help to try to prevent scheduling similar sounding call signs from arriving at destinations at the same time.


Synopsis: A Center Controller and the crew of one of the aircraft involved; described the events leading up to the acceptance of a descent clearance for another company flight with a similar call sign. This resulted in a loss of separation with an opposite direction Citation.