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Air carrier captain reported a drop in altitude on approach; catching it and getting an alert from ATC.
| Attributes | |
| ACN | 1652797 |
| Time | |
| Date | 201905 |
| Local Time Of Day | 1201-1800 |
| Place | |
| Locale Reference | IAD.Airport |
| State Reference | DC |
| Environment | |
| Flight Conditions | VMC |
| Light | Daylight |
| Aircraft 1 | |
| Make Model Name | Medium Large Transport |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Initial Approach |
| Flight Plan | IFR |
| Person 1 | |
| Function | Captain Pilot Not Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine |
| Events | |
| Anomaly | Deviation - Altitude Excursion From Assigned Altitude Inflight Event / Encounter CFTT / CFIT |
Narrative:
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 navigation 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.
Original NASA ASRS Text
Title: Air carrier Captain reported a drop in altitude on approach; catching it and getting an alert from ATC.
Narrative: 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.
Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.