Date: 201309
Local time of day: 0001-0600
Location: ZTL.ARTCC, GA
Aircraft: MD-88
Operator: Air Carrier
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZTL
ATC / advisory: Center ZTL
Second aircraft: A320
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Developmental
Human factors: Communication Breakdown
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Air Carrier X was initially climbed to FL330 on DAWGS departure on J14. Air Carrier Y was stopped at FL310 on the Debie..Bgred departure off CLT. Air Carrier Y was stopped under slow Citation on same route at FL320. with trainer and D-Side; collectively agreed to stop Air Carrier X at FL300 and turned Air Carrier Y to allow to get to requested altitude quicker. Clearance was given to stop Air Carrier X at FL300 and an Air Carrier Z; 50 miles ahead on J14; read the clearance back and neither of us caught the readback error nor did the Air Carrier X. The HALO was on Air Carrier Y at FL310 with Air Carrier X passing underneath when saw the Air Carrier X go through FL304 and immediately questioned his clearance and the Air Carrier Z said they thought that was them and they were the ones who read it back. Air Carrier X said they were still climbing to FL330. Therefore a loss of vertical separation occurred between Air Carrier X and Air Carrier Y. This all occurred after a very busy time when all CLT arrivals in the stram had to be vectored with S-turns to bleed off high TMA numbers.


Second narrative: Air Carrier Z checked in climbing off ATL and was issued a climb to FL330 on J14. A couple minutes later Air Carrier X checked in climbing off ATL and was issued FL310 on J14. At the time we had an Air Carrier Y climbing on the departure off of CLT to FL300. The Air Carrier Y was stuck underneath a slow moving Citation. I prompted my Trainee to stop Air Carrier X off so we could provide Air Carrier Y with a shortcut in order to be able to climb them sooner. The Developmental issued 'Air Carrier X amend altitude to maintain FL300.' At the time we didn't realize that Air Carrier Z was the aircraft that read back the stop off at FL300. The Developmental then issued the shortcut to JAMMR to Air Carrier Y turning him to the right and toward Air Carrier X. Because of the wrong aircraft taking the stop off clearance and neither the Developmental; our D-side; or I noticing the incorrect aircraft took the clearance there was a loss of separation when Air Carrier X and Air Carrier Y pulled along side each other about 2.8 miles apart. It wasn't until they were side by side that Air Carrier X climbed through FL300; this happens too often with [Company XZ] pilots taking each other's clearances just because the last digit of their callsigns is the same. For some reason we have this problem with [Company XZ] pilots more than any others. It seems they are only listening for [Company XZ] and the last digit. As far as we were concerned it wasn't even an issue of similar callsigns with the zero being so apparent in the Air Carrier X callsign that it would keep people from being confused. That being said we somehow missed Air Carrier Z taking the clearance. I'm honestly still trying to figure out how I missed it.


Synopsis: ZTL Controller described a loss of separation when two company aircraft accepted the wrong ATC clearance causing the separation loss; one reporter claiming the subject company pilots frequently confuse their call signs.