Date: 201603
Local time of day: 1801-2400
Location: SAN.Airport, CA
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B SAN
ATC / advisory: Tower SAN
Aircraft component: Traffic Collision Avoidance System (TCAS)
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Training / Qualification; Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft RA; Automation Aircraft TA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: We were on a visual approach to Runway 27 backed up with the RNAV (GPS) 27; using LNAV and VNAV. With our landing clearance received; we suddenly had a TCAS advisory at about 1300 ft which was immediately followed by an RA commanding a descent. The F/O; being the PF; was already hand flying the aircraft at the time and promptly responded to the RA with a steep descent. In order to respond; we had to go below the [glide] path. At about 700 ft; I asked him to get back on the [glide] path because of the proximity to buildings; even though the RA never advised us that we were 'Clear of Conflict'. At 500 ft we were stabilized. At that stage the target on the screen showed 300 ft right above us; but we never had visual contact. When we asked the Tower about the traffic they stated that they did not see any traffic on short final.San Diego Tower claimed that there was absolutely no traffic seen within two and a half miles of our position. They offered the explanation that there could be a TCAS malfunction called TCAS MIRRORING that happened to us. Since I have never heard of that term; nor was I aware of the possibility that the TCAS could malfunction to indicate a traffic conflict at 1300 ft; with none being there; this incident needs to be researched and the pilot group needs to be educated about this issue. The margins on an approach this low to the ground are tight with a DESCEND RA; especially in SAN. While I understand that there is a lower limit in which the RA function is turned off; this situation would have been bad in IMC conditions; since we are trained to respond to an RA.


Second narrative: The Crew received an ACARS message on the subsequent flight from the Company stating that it could have been TCAS Mirroring that caused the event. This is something that I am not familiar with in 30 years of 121 experience. I feel the Crew handled the situation properly and by Company standards and policy. If this were an actual aircraft conflict we performed to avert such an event. If this is 'TCAS Mirroring' as it was referred to; I would like to see information on its causes; frequency of taking place; and notification to pilots to make them aware of this anomaly. Had we been in IMC conditions on approach to SAN I feel there would have been a greater sense of uncertainty and a higher level of threat with most likely a go-around concern with unknown traffic less than 300 ft above us.


Synopsis: B737 flight crew reported responding to an RA on approach to SAN that took them below desired vertical path. Tower reported no traffic in the vicinity.