CRJ900 first officer reports descent below the MCP selected altitude with the autopilot engaged.

Attributes
ACN	912382
Time
Date	201010
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	Regional Jet 900 (CRJ900)
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Commercial
Experience	Flight Crew Last 90 Days 80 Flight Crew Total 3200 Flight Crew Type 80
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Clearance

Narrative:

We were on radar vectors for approach and the captain was flying. We were descending from either 7;000 or 6;000 ft down to to 5;000 ft which was assigned by ATC. All of the sudden I noticed out altitude indicator was blinking and as soon as I realized we had gone lower to about 4;700 ft and started telling the captain; ATC asked us what altitude we were descending to! I said '5;000 ft' and the captain immediately started a climb to 5;000 ft but I believe we went as low as 4;600 ft before correcting the error. Neither one of us was able to figure out why the airplane with auto pilot engaged during this deviation did not capture the pre-selected altitude.

Original NASA ASRS Text

Title: CRJ900 First Officer reports descent below the MCP selected altitude with the autopilot engaged.

Narrative: We were on radar vectors for approach and the Captain was flying. We were descending from either 7;000 or 6;000 FT down to to 5;000 FT which was assigned by ATC. All of the sudden I noticed out altitude indicator was blinking and as soon as I realized we had gone lower to about 4;700 FT and started telling the Captain; ATC asked us what altitude we were descending to! I said '5;000 FT' and the Captain immediately started a climb to 5;000 FT but I believe we went as low as 4;600 FT before correcting the error. Neither one of us was able to figure out why the airplane with auto pilot engaged during this deviation did not capture the pre-selected altitude.

Data retrieved from NASA's ASRS site as of April 2012 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.