Date: 201704
Local time of day: 1201-1800
Location: ZFW.ARTCC, TX
Flight conditions: VMC
Aircraft: Beechcraft Single Piston Undifferentiated or Other Model
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Flight phase: Cruise
Airspace: Class E ACT
ATC / advisory: TRACON ACT
Second aircraft: Cessna Citation Undifferentiated or Other Model
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Confusion
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Contributing factors: Procedure; Airspace Structure; Human Factors; Aircraft
Primary problem: Human Factors

Narrative: My Radar Assist received a call from an adjacent Center sector with a question about a crossing restriction and then a point out for aircraft Y to cross DODJE at 4;000 feet. We did not have any traffic. I noticed a limited data block in TRACON airspace at 9;000 feet opposite direction from aircraft Y. About 3-4 minutes out; I pulled the 9;000 foot data tag and watched it for a minute assuming TRACON would reference the traffic. After not noticing a temporary 10;000 feet assigned to aircraft Y; I checked to see if TRACON had a data block. They did not; so I sent them a Plan View Display (PVD) and made a call to see if they knew about the traffic. They did not and then took action to make sure there was separation.I called back a minute later to let him know the crossing restriction would be different. In the process I interrupted a call between the TRACON controller and the Center Low sector controller. The response from the TRACON controller was ridiculous; saying there was no factor between the two and it was safe to descend. Luckily the other sector controller did not descend aircraft Y and they passed separated vertically; but roughly 1.7 miles laterally while crossing opposite direction.With the required filters for Center Low sector generally set to 10;800-23;200 feet; they would have never seen the target to know there was traffic. I'm not sure TRACON knew or understood that there are limitations to what we see inside their airspace; although I'm not sure it would have made much of a difference based on the initial response.


Second narrative: Working center low radar sector. I coordinated aircraft Y with the high sector direct DODJE intersection approved. Took the handoff from the high sector and coordinated with Sector 29 about what altitude to issue aircraft crossing DODJE and issued a point out for aircraft Y crossing DODJE at 4;000 feet. Sector 29 said point out approved. I then call Waco Approach and offer a point out for code ZZZZ about 5 miles east of Waco VOR descending to cross DODJE intersection at 4;000 feet. The Waco Approach Controller said code ZZZZ point out approved. I issue a crossing restriction for aircraft Y to cross DODJE at and maintain 4;000 feet and issue the Waco altimeter. Several minutes pass then I receive a call from Sector 29 and a display pops on my scope of aircraft X who is level at 9;000 feet and they ask if I am watching this aircraft for my traffic. I advise Approach had taken a point out on aircraft Y; but I knew nothing of the traffic. I thanked him and moved on. I then called aircraft Y and amended his altitude to maintain 10;000 for traffic. I called Approach and inquired about aircraft X; asking what he was planning to use for separation and he advises 'it's no factor'. I told him it absolutely was a factor. After further review; the separation was estimated at 1.4 miles and 500 feet had the descending aircraft not been stopped at 10;000 feet.I suggest retraining for the approach controller that was on the radar position during this event and go over standard separation minimums for IFR aircraft. A huge thank you to the sector which called me and advising me of an aircraft I had no knowledge of. The aircraft was below my altitude filter limits and was never referenced. By the time I knew about aircraft X on my own it would have been too late.


Synopsis: Center Controllers reported receiving approval from TRACON to descend an aircraft into their airspace even though the TRACON had conflicting traffic.