B737 Undifferentiated or Other Model
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 3200
Distraction
Inflight Event / Encounter CFTT / CFIT
Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Human Factors
We were on final approach flying the RNAV 19 when we received the 'CAUTION' 'OBSTACLE' alert. We had the TRUPS Arrival and RNAV 19 approach set in the FMS. The Captain and I verified all the points in the FMS matched the JEPPs arrival and approach. We also read and discussed all of the company pages regarding flying into DCA. This was a night approach. All appeared to be going well during the approach; we had LNAV/VNAV engaged and the Captain was completely configured (Flaps 40) and on speed prior to the recommended point of GREYS. The current altimeter setting was set. As the plane was descending over the river I heard a call from Tower but did not understand what he said because we got the 'CAUTION' 'OBSTACLE' call at the same time. I attempted to respond to the controller to repeat; but I inadvertently pressed the Auto Pilot Disconnect button instead of the Radio button on the yoke. I said to the Captain 'That was me; I disconnected your Auto Pilot; Your Airplane'. He began flying manually. We had the river in sight and then Tower repeated the call I missed earlier saying 'Low Altitude Alert'. The Captain made an immediate correction to begin a slight climb. We were visual of the river and surrounding buildings the entire time. I noticed the airspeed decelerating and I said 'Airspeed.'. The Captain immediately added power and corrected the airspeed. He then intercepted a visual descent and glide path and landed on Runway 19 at DCA. After completing the parking checklist the Captain called the Tower Controller on speaker phone and asked him the details of the 'low altitude' call. The DCA Tower Controller was not concerned and said when his screen flashes he has to make the call. He stated it was not anything 'crazy' and sounded if this was not uncommon. He said it was not a reportable event. In retrospect; I believe we failed to trap an error and descended too low on the RNAV approach. I am not exactly sure how or why considering we were LNAV/VNAV.
B737 flight crew reported receiving a low altitude alert from the tower on final approach.