Air carrier flight crew reported receiving a low altitude alert from ATC on descent into SFO; citing lack of autoflight mode awareness as contributing.
Synopsis
Air carrier flight crew reported receiving a low altitude alert from ATC on descent into SFO; citing lack of autoflight mode awareness as contributing.
Narrative
Going into SFO we were assigned the Tip Toe. It was loaded and briefed including the threats of SFO ATC Controllers leaving us high and clearing us late. I have not been into SFO in a little while so that was another threat briefed. They didn't clear us and ended up keeping us on the high side stepping us down gradually as they kept asking for us to identify traffic out in front of us. I was concerned about being high and made the mistake of going into FLCH to descend quickly when we finally got cleared down to 4000'; we finally got cleared for the approach and I armed the approach mode while we were in LNAV and waited for the Flight Computer/GP (Guidance Panel which we discussed and I briefed what we should expect on this approach since we are well trained on this particular approach. I got consumed with the traffic; and the late clearance and did not put the airplane back into VNAV. The Flight Computer became the active lateral mode after SAMML as expected but there wasn't a glide path mode activated and we realized I was below the glide path that was built at that point. I quickly turned off the autopilot and we worked as a crew to regain SA (Situational Awareness) on our flight path. I leveled off momentarily and got on the glide path [and was] was stable well before our 1000' and 500' gates and landed on 28R.
Second reporter narrative
We were cleared to join the lateral portion of the Bridge Visual Runway 28L into SFO. They refused to clear us for the visual because we couldn't verify other traffic in the area; therefore keeping us high on the approach. Finally they cleared us for the visual and we needed to descend. We set the TDZE in the MCP (Mode Control Panel) and did a check to verify everything was where it needed to be. It was very busy with ATC frequency changes; call outs; configuration changes. Next thing you know; SFO Tower calls and tells us to check our altitude. Apparently; we were in FLCH instead of VNAV. We stopped our descent and rejoined our vertical profile. We made all the stabilized gates and were stabilized at 1500' with the correct configuration all the way to landing with no further issues.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.