Medium large transport flight crew overshot descent altitude and then immediately returned.

Attributes
ACN	503140
Time
Date	200103
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : clt.vortac
State Reference	NC
Altitude	msl bound lower : 8700 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : clt.tracon tower : cyyz.tower
Operator	common carrier : air carrier
Make Model Name	B737-600, -700, -800
Operating Under FAR Part	Part 121
Navigation In Use	other other vortac
Flight Phase	descent : approach
Route In Use	departure : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 10000 flight time type : 6000
ASRS Report	503140
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude
Consequence	Other
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were flying the unarm 1 arrival into clt. Clt approach had given us a turn to 360 degrees and a descent to 9000 ft. The autoplt was engaged and the first officer was flying the aircraft. Approach control had called out traffic below us. Both pilots were occupied searching for the traffic. When I looked back at the instruments, I noticed the aircraft descending below 9000 ft. The first officer immediately disconnected the autoplt and initiated a climb back to 9000 ft. The aircraft had descended to approximately 8700 ft before returning back to assigned altitude. Approach control meanwhile verified our assigned altitude and I acknowledged. I feel that crew fatigue and both pilots being preoccupied looking for traffic were the major factors for deviating below altitude.

Original NASA ASRS Text

Title: MLG FLC OVERSHOT DSCNT ALT AND THEN IMMEDIATELY RETURNED.

Narrative: WE WERE FLYING THE UNARM 1 ARR INTO CLT. CLT APCH HAD GIVEN US A TURN TO 360 DEGS AND A DSCNT TO 9000 FT. THE AUTOPLT WAS ENGAGED AND THE FO WAS FLYING THE ACFT. APCH CTL HAD CALLED OUT TFC BELOW US. BOTH PLTS WERE OCCUPIED SEARCHING FOR THE TFC. WHEN I LOOKED BACK AT THE INSTS, I NOTICED THE ACFT DSNDING BELOW 9000 FT. THE FO IMMEDIATELY DISCONNECTED THE AUTOPLT AND INITIATED A CLB BACK TO 9000 FT. THE ACFT HAD DSNDED TO APPROX 8700 FT BEFORE RETURNING BACK TO ASSIGNED ALT. APCH CTL MEANWHILE VERIFIED OUR ASSIGNED ALT AND I ACKNOWLEDGED. I FEEL THAT CREW FATIGUE AND BOTH PLTS BEING PREOCCUPIED LOOKING FOR TFC WERE THE MAJOR FACTORS FOR DEVIATING BELOW ALT.

Data retrieved from NASA's ASRS site as of July 2007 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.