B737 crew had altitude deviation.

Attributes
ACN	469390
Time
Date	200004
Day	Mon
Local Time Of Day	1201 To 1800
Place
State Reference	GA
Altitude	msl bound lower : 29000 msl bound upper : 29400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl.artcc tower : phl.tower
Operator	common carrier : air carrier
Make Model Name	B737-200
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Route In Use	enroute airway : ns.airway
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 5200 flight time type : 1200
ASRS Report	469390
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment : altitude alert other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude none taken : detected after the fact
Consequence	other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

We were at cruise flight with altitude hold on autoplt. Captain and I were searching for traffic when we noticed the aircraft started to climb. I wasn't able to correct situation until about FL294. I quickly returned aircraft to FL290. The autoplt altitude hold came off without warning. This is what caused our altitude deviation.

Original NASA ASRS Text

Title: B737 CREW HAD ALTDEV.

Narrative: WE WERE AT CRUISE FLT WITH ALT HOLD ON AUTOPLT. CAPT AND I WERE SEARCHING FOR TFC WHEN WE NOTICED THE ACFT STARTED TO CLB. I WASN'T ABLE TO CORRECT SIT UNTIL ABOUT FL294. I QUICKLY RETURNED ACFT TO FL290. THE AUTOPLT ALT HOLD CAME OFF WITHOUT WARNING. THIS IS WHAT CAUSED OUR ALTDEV.

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.