Air carrier flight crew reported following a TCAS RA command that descended them well below glide slope on approach to SFO. Reporter executed a go-around and expressed concern about the frequent airborne conflicts on parallel approaches to SFO Runways 28L and 28R.
Synopsis
Air carrier flight crew reported following a TCAS RA command that descended them well below glide slope on approach to SFO. Reporter executed a go-around and expressed concern about the frequent airborne conflicts on parallel approaches to SFO Runways 28L and 28R.
Narrative
While on left base in VMC conditions for 28L at SFO; NorCal pointed out Aircraft Y traffic on right base for 28R. They were also on the Bodega Arrival; but were given 28R with the high right downwind down the bay. We were given 310 heading to join and cleared for a visual approach to 28L. I pointed out the traffic and they were in sight by both pilots. We were told to maintain visual separation. I told the FO that there might be a TCAS problem based on their position and relative bearing making it appear they were going end up right off our wing. But; it depended entirely on how Aircraft Y flew their visual ground track to 28R. We still had some time to mitigate the TCAS threat so I disconnected the autopilot and mentioned to the FO that I was going [to] ignore ATC's last instruction to fly a heading of 310 to join final and instead stay south of the 28L extended centerline to keep as much lateral separation as possible. It wasn't enough as Aircraft Y joined the 28R centerline vs. flying a dogleg to join final inside of the bridge. We received a descend RA which I complied with. The FO advised SFO Tower of the RA. The RA continued with the descend command for what felt like an eternity and had us well below glideslope which was full scale. We were now approaching 1000 feet and a high workload and very low on approach. I was uncomfortable with the state of the approach; position; and altitude. I elected to go around and turn left away from the traffic. We returned for an uneventful approach to 28L. My personal take on this event - foundation from which I have the following opinions. I've been based in SFO since a very long time with different airlines and likely have thousands of operations in and out of the airport. This event was probably the most UNCOMFORTABLE that I have ever been in any operation in or out of SFO. The workload was high enough with a descending RA while relatively low then into a go-around. If there was any combination of events that could result in an undesirable aircraft state this was it. The sad part is it was completely preventable and unnecessary. A threat into SFO is and always has been being closely paired up with other traffic and the logic and programming of the TCAS. ATC is of no help because their vectors; speed assignments to pair traffic wing on wing off the high right downwind for 28R with traffic that they've given a 310 vector to join the 28L centerline. My only mitigation strategy available if ATC is the threat and the pilots on 28R have poor Situational Awareness (SA) and decide to join the 28R localizer outside of the bridge is to completely disregard ATC heading and speed assignments to keep the TCAS happy. Our system and industry are not based upon each pilot fending for themselves on a regular basis because they know better." This is not the first time I have been on 28L and received a RA due to aircraft on a right downwind visual for 28R. A Company jump seater; who hasn't been at Company for a long period of time; commented that was the 4th time he's had aircraft cause an RA by how they fly their ground track while on a visual for 28R. A problem exists in SFO. ATC doesn't mitigate the threat; they add to it. The threat of RAs has been mitigated by the redesigned FMS Visual for 28R. It does absolutely nothing to mitigate the threat for right downwind visuals for 28R."
Second reporter narrative
We were cleared for a visual approach for 28L and were paired with traffic making a visual to 28R. The traffic was behind us and above our altitude. We were then given a speed of 170 which caused the traffic to overtake us. Their vertical and lateral track were such that we received an RA that commanded a descent and it was persistent. I did have the traffic in sight and as we descended they only got closer to us laterally. This situation was not comfortable and it appeared that continuing our approach would likely lead to an Undesired Aircraft State (UAS) and a degradation in safety so we executed a go-around with a left turn. The go-around and subsequent approach and landing were uneventful.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.