Air carrier flight crew reported receiving a low altitude alert from SFO Tower while responding to a TCAS RA; and; after executing a go-around; received another low altitude alert when they used FLCH mode that generated a high rate of descent.
Synopsis
Air carrier flight crew reported receiving a low altitude alert from SFO Tower while responding to a TCAS RA; and; after executing a go-around; received another low altitude alert when they used FLCH mode that generated a high rate of descent.
Narrative
After being cleared for the Tipp Toe Visual approach we received an RA as we joined the final approach course. We had the pairing traffic in sight just prior to receiving the RA however it appeared on the Multi-function Flight Display (MFD) that the traffic on our right was possibly drifting through the 28R final approach course and I could not confidently feel that there was no threat of conflict. The RA voice guidance advised a descent. I instructed the pilot flying (PF) to disconnect the Autopilot and auto-throttle and fly the box which he performed correctly. I advised SFO Tower that we were responding to an RA. I estimate we descended approximately 500ft until we received level off instructions and the clear of conflict announcement. About this time SFO Tower advised us of a low altitude alert. I responded that we were responding to RA guidance and that we were going around. SFO Tower gave us instructions and the go around was performed. During our second approach after receiving visual approach clearance the PF set the appropriate altitude of 1800ft for the FAF and began the descent. I soon realized he was in green flight level change and that he was descending much quicker than he needed to. I intervened and selected FPA to shallow the descent. As we began to level at 1800ft SFO Tower advised us of another low altitude alert. I responded that we had the correct altimeter setting and were at 1800ft. I suspect that low altitude alert was triggered by our rate of descent as we never went below 1800ft to my knowledge. The approach continued; was stable and resulted in a normal landing. During post flight debrief we discussed the use of flight level change and the threats of using it in certain situations.Cause: SFO approach congestion as well as the PF's use of flight level change that may have triggered a low altitude alert from ATC.Suggestions: As a crew; briefing potential threats of SFO close parallel operations and the appropriate use of Flight level change.
Second reporter narrative
I was Pilot Flying; we had been cleared for the TIPPTOE Visual 28L. When we began our turn to intercept the localizer; Approach gave us a traffic advisory for traffic on the parallel runway. I could see them initially; Captain reported traffic in sight and we continued the turn to intercept. Shortly after the turn; we lost sight of the traffic; and the aircraft gave us an RA with a descend" command. Since we were not absolutely certain of the position of the conflicting traffic; I complied with the RA and followed the procedure outlined in the SOP. Captain advised Approach about us complying with the RA. Approach gave us a low altitude alert. Once we were clear of conflict; we initiated a go around; and got vectored back on and cleared for a visual approach. When turning from base to final; I was in FLCH with a steep angle of descent. Captain intervened and went to FPA to shallow out the descent; and that is when Approach gave us another low altitude alert; even though we had leveled off at 1800'; on localizer; as outlined by the charted visual. We advised approach of the same; and landed uneventfully. In the debrief; we discussed use of FLCH vs FPA and what to use when appropriate.Cause: Close parallel ops into SFO; steep descent using FLCHSuggestions: Visual approaches are never the same; and will not have the exact same set up every time. Understanding the use of different configurations to descend; as well as different AP modes to descend will lead to better descent profiles and smoother operations overall"
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.