B747-400
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 2.2
Situational Awareness; Human-Machine Interface
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew FLC complied w / Automation / Advisory
Airspace Structure; Procedure; ATC Equipment / Nav Facility / Buildings
Aircraft X was handed off from sector 41 (lower sector working 23;000 feet and below) and we accepted the handoff. Aircraft X checked on with me climbing through 11;500 feet to 23;000 feet. I acknowledged; 'Aircraft X; Oakland Center; Climb and maintain FL 300'. Immediately after that I saw conflict alert activate with a target at 15;500 feet roughly 2-3 miles directly in front of Aircraft X. Subsequently; before I could say anything; the Aircraft X pilot informed me that they had traffic in front at 15;500 feet. I turned on my primary radar and asked if they wanted a vector. I did not see any target on primary at this time. This was about the same time the Aircraft X advised that they had passed the traffic. We began some dialogue around the situation and he said they never visually saw an aircraft BUT they had passed one on TCAS. The pilot said they had to arrest the climb rate in order to avoid an RA. The pilot said they did not get an RA. Thereafter the Aircraft X resumed their normal rate to 30;000 feet. I had my Radar Assist call down to sector 41 and ask about that traffic. My Radar Assist informed me that sector 41 said; 'I didn't see any traffic there; but I do now.' I informed the Aircraft X that my sector was 24;000 feet and above so we didn't use primary radar that high but that we were investigating it. Even with my primary radar on I couldn't see a target in their area. I asked the Aircraft X if he was planning on filing a NMAC report and he said he wasn't; then asked if they needed to. I told them that I was reporting the incident and that they probably didn't need to.I would recommend that we have no communication changes below 18;000 feet. It seems to be a little bit of a gray area. Some feel the higher sectors shouldn't take handoffs below F180 and others say the lower sectors shouldn't issue frequency changes to aircraft below 18;000 feet. It just needs to be re-addressed.
A controller working airspace above 24;000 feet reported taking the handoff of a departure about 11;000 feet. The pilot advised the controller of TCAS traffic in front of them at 15;500 feet and that they were responding by reducing their climb rate. At the same time; the Conflict Alert appeared on the controller's radar display.