Date: 201909
Location: NCT.TRACON, CA
Aircraft: A319
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C SJC
ATC / advisory: TRACON NCT
Second aircraft: B737 Next Generation Undifferentiated
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Troubleshooting; Communication Breakdown; Situational Awareness
Anomaly: ATC Issue All Types; Conflict NMAC
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: ATC Equipment / Nav Facility / Buildings
Primary problem: ATC Equipment / Nav Facility / Buildings

Narrative: I had just taken over the position within 15-20 minutes of the incident. The previous controller had an issue with one aircraft not answering him. The controller got a hold of the aircraft on the standby transmitter and everything went as planned. I switched back to the normal transmitter and had no issue talking to anybody. I had turned; slowed; and based numerous aircraft with no issue. I had Aircraft X on the downwind at 5;100 ft. and issued a base turn; and got no response. I switched headset jacks and tried again; tried the standby transmitter; used a handset; and had another sector try my frequency at their position; and nobody was able to get any response from the aircraft. I broad-casted on guard for Aircraft X to turn and switch to a different frequency; and I believe I transmitted on guard for Aircraft Y who was descending via and cleared for an approach out of 9;000 ft. The heading that Aircraft X was on put the aircraft into conflict with Aircraft Y descending via RNAV Arrival. Initially there were 20 miles of space between the aircraft but because the frequency didn't work the aircraft were converging; and I projected Aircraft Y would descend into Aircraft X if something could not be done.I continuously tried to broadcast on guard and on the frequency to get a hold of either aircraft. Finally; at the last minute I got a response from Aircraft Y and stopped him at 6;100 ft. The aircraft passed directly over each other 1;000 ft. separated. I turned Aircraft X to follow Aircraft Y inbound and then continued to readjust the sequence until everything was worked out. I slowed the final down to recover and everyone landed without incident. It was the scariest moment I've ever encountered in my entire career; and I had no control over the situation. Aircraft Y called in later and said that he heard the broadcast on guard frequency and came over to my secondary frequency which enabled the situation to be handled safely. We also found out later that technicians had been working on the primary frequency and the area was not aware of it; and supposedly a line failed that shouldn't have.I don't know what can be done to resolve this as it was clearly a technical issue. Under normal circumstances the operation was completely routine; done every day; and completely in control. With the loss of the frequency for over 15 to 20 flying miles in a congested airport environment; things quickly escalated to unsafe. The only thing I can say is that based on my personal encounter with this issue; if the frequency is being serviced for any reason whatsoever it would be safer to use a backup frequency until it is complete so that unexpected outages would be less likely to occur. In addition; the saving grace on this issue was possibly the fact that the airline pilots were monitoring guard. At the very least Aircraft Y heard that there was something abnormal and managed to find their way to a working frequency in time to avoid a very unsafe situation. The fact that guard was an available option is a lifesaver; and if anything it should be stressed to pilots that monitoring guard often can be the difference in getting a hold of pilots and keeping the National Airspace System safe.




Synopsis: TRACON Controller reported that their frequency stopped working for several minutes; resulting in a NMAC.