Captain of an airbus A320 overshot assigned altitude during climb due to the aircraft's FMC did not level the aircraft as programmed.

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Attributes
ACN	446010
Time
Date	199908
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	intersection : fortt
State Reference	PA
Altitude	msl bound lower : 31000 msl bound upper : 31700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zny.artcc
Operator	common carrier : air carrier
Make Model Name	A320
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	climbout : vacating altitude
Route In Use	enroute airway : j60.airway
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer pilot : cfi
Experience	flight time last 90 days : 200 flight time total : 10000
ASRS Report	446010
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance non adherence : far
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued alert 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:

While nearing FL310, in a climb to FL350, center reclred us to maintain FL310. I set FL310 in the FCU. The lead flight attendant entered the flight deck and while I was distraction talking to her, the aircraft continued climbing. The first officer noticed the climb at FL317. I then descended back to FL310. ATC queried us regarding our altitude as I began the descent. ATC indicated there was no traffic conflict as a result. I believe this occurrence was a result of the 'altitude trap' that is inherent in the A320 software. The aircraft vertical mode reverts to vertical speed when the aircraft is acquiring an altitude and a new altitude is entered in the FCU window.

Original NASA ASRS Text

Title: CAPT OF AN AIRBUS A320 OVERSHOT ASSIGNED ALT DURING CLB DUE TO THE ACFT'S FMC DID NOT LEVEL THE ACFT AS PROGRAMMED.

Narrative: WHILE NEARING FL310, IN A CLB TO FL350, CTR RECLRED US TO MAINTAIN FL310. I SET FL310 IN THE FCU. THE LEAD FLT ATTENDANT ENTERED THE FLT DECK AND WHILE I WAS DISTR TALKING TO HER, THE ACFT CONTINUED CLBING. THE FO NOTICED THE CLB AT FL317. I THEN DSNDED BACK TO FL310. ATC QUERIED US REGARDING OUR ALT AS I BEGAN THE DSCNT. ATC INDICATED THERE WAS NO TFC CONFLICT AS A RESULT. I BELIEVE THIS OCCURRENCE WAS A RESULT OF THE 'ALT TRAP' THAT IS INHERENT IN THE A320 SOFTWARE. THE ACFT VERT MODE REVERTS TO VERT SPD WHEN THE ACFT IS ACQUIRING AN ALT AND A NEW ALT IS ENTERED IN THE FCU WINDOW.

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.