Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Human Factors
Potomac Approach cleared us for the visual approach to 19C and gave us instructions to descend to 7;000 feet. The Autopilot was in use and the aircraft descended to 7;000 feet. Alts Capped message appeared. I started to sequence the approach in the FMS. After 2 seconds I looked up and I saw the preselect altitude indicator flashing amber and we were at 6;700 feet. I put the airplane in VSI climb mode. A second later ATC transmitted a low altitude alert call and gave us the altimeter setting. I replied we are correcting. In a few seconds the aircraft leveled at 7;000 feet again.After talking it over with the First Officer we came to the conclusion he may have accidentally pressed the VSI mode instead of the NAV mode. When I saw that we were 300 feet below the preselected altitude I did not turn off the Autopilot and used the VSI mode to climb back up to 7;000 feet. The whole event must have taken about 5 seconds. The other option would have been to turn off the Autopilot and maybe the correction could have been quicker.
Air carrier Captain reported a drop in altitude on approach; catching it and getting an alert from ATC.