Date: 201305
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Aircraft: HS 125 Series
Flight plan: IFR
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Aircraft X checked on frequency from TRACON. TRACON owns airspace up to and including FL190. I was working R-Side ZZZ Center Sector 29. Aircraft in this sector are climbed to FL190 off the area East Bound by the TRACON and are given to ZZZ Center Sector 29 for additional climb. My position at Sector 29 has 'control on contact' from the TRACON per LOA. TRACON must give Center '3 miles separation and increasing.' Aircraft X checked on climbing to FL180; not FL190. I missed that and heard what I had expected to hear him check on climbing to as FL190. I immediately climbed the aircraft to FL290. A few moments later; the TRACON called me on the landline and said he had 'prematurely' shipped Aircraft X to me and said that he had another aircraft at FL190. I said I had already climbed Aircraft X not seeing the FL190 limited underneath. Once he said that; I observed their limited at FL190 diverging from Aircraft X but still very close. I told the TRACON I would quickly stop Aircraft X at FL180. When I issued the clearance to stop Aircraft X at FL180; the observed altitude was FL177. Pilot read it back and said he might 'go through the altitude' but would go back down. I observed Aircraft X (with a 5 mile J-ring I put up) climb to FL181. Aircraft appeared to be at 3 miles diverging at that point. Next update Aircraft X was at FL184; again with under 5 miles separation but I believe was greater than 3 miles. Next update indicated the descent of Aircraft X. Loss of separation never happened in the Center stratum; but I was unsure if we had actually breached the 3 mile separation rule for the TRACON while the vertical loss of separation was occurring; I called over my Supervisor to turn the event in. After the event occurred with no further issues; the TRACON called me back to thank me for helping out. He followed up saying that he thought he had stopped Aircraft X at FL180 (he did) and then made the mistake of changing frequencies prematurely to me. I missed the lower check on altitude of FL180 (expecting to hear FL190) which caused the event. If I had caught the initial check on correctly; I would have immediately called the TRACON to request control for further climb and been referenced traffic. I am not sure if a separation error actually occurred in the TRACON. Both aircraft were below my stratum of FL200 and above. Hearback/readback error on my part. If I had caught the initial check on as being different that what I was expecting; I would have never climbed Aircraft X without getting control. This is a very uncommon incident in this sector. I need to focus more intently on the check on altitude and be diligent not to process what I 'expect' to hear.




Synopsis: Enroute Controller described a near loss of separation event when assuming climbing traffic had reported an 'expected' altitude but in fact stated a different altitude.