Air carrier Flight Crew reported receiving a low altitude alert from ATC during the approach to DCA. Crew corrected back to vertical path and continued the approach.
Synopsis
Air carrier Flight Crew reported receiving a low altitude alert from ATC during the approach to DCA. Crew corrected back to vertical path and continued the approach.
Narrative
On RNAV-Z Rwy 19 approach into DCA. First Officer flying the approach. Fully briefed the approach and the demands prior to commencing. Reviewed the approach together the day prior to this flight as well. Cleared for the approach by ATC. At the (IF) set touchdown elevation. LNAV and VNAV verified. We were closely monitoring our lateral path both by instruments as well as visually. I was pointing out some key landmarks to avoid entering P-56A during the approach. While we were both focusing on lateral navigation; we both missed the fact that VNAV had at some point disengaged. Upon selecting Flaps 40 I believe our decent rate increased because we were no longer in VNAV. While initiating Landing Checklist we got a call from ATC (Low Altitude Alert; check altitude immediately.) This brought our attention back to our vertical path. Determined that we were indeed couple hundred feet low. F/O immediately disconnected the autopilot and corrected back to correct vertical path while we assessed what had happened. We still had lateral guidance and vertical indications on PDI but VNAV was no longer engaged. Confirmed stabilized on path and continued the approach and landing visually.Long debrief followed and lessons learned.
Second reporter narrative
This was my first time flying into DCA. I was unfamiliar with the approaches and the airport. We were doing the RNAV Z Rep 19 and were all stable through most of the approach. We were being slowed about 25 miles from the airport due to traffic ahead of us. we were in VNAV with SPD INTV at 170kts with the flaps at 15 and gear down. The autopilot was on and as we were beginning our final decent from SUNEY waypoint we were on VNAV PATH and instructed 170kts to FONVI about a 4 mile final to the runway. As we were descending; I was monitoring the MFD to notice the waypoints and the Prohibited airspace beside us while also looking outside for the airport and river. I should have been more focused on my FMAs and making sure I was monitoring my speed for the final configuration point. In the distraction of looking outside; I came back inside and noticed we were ready to configure and slow. I think at this time I accidentally hit the VNAV MCP instead of SPD INTV and the plane went in to Vertical speed mode. I slowed to final speed manually with the speed knob; this should have alerted me that it was not in VNAV. As were slowing; it was about 1100ft MSL that we got the low altitude verbal alert from ATC. I looked down from looking for the runway; I was about 600 feet low of the approach; but on course at GREYZ to FONVI. I turned off the autopilot; leveled off and flew the approach course till I intercepted the glideslope again. during this time we reached our final speed; configured fully to flaps 40. I hand flew the rest of the approach and were fully stable and configured by 1000 feet and our 500 foot windows.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.