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 first officer; 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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.