Autoplt anomaly reported as cause of altitude deviation.

Attributes
ACN	482370
Time
Date	200008
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : bos.vortac
State Reference	MA
Altitude	msl bound lower : 23250 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zbw.artcc
Operator	common carrier : air carrier
Make Model Name	DC-10 30F
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	descent : vacating altitude
Route In Use	arrival : profile descent
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : second officer
Qualification	pilot : atp pilot : commercial pilot : multi engine pilot : instrument pilot : flight engineer
Experience	flight time last 90 days : 100 flight time total : 6000 flight time type : 400
ASRS Report	482370
Person 2
Affiliation	company : air carrier
Function	flight crew : captain
Qualification	pilot : multi engine pilot : commercial pilot : atp pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	aircraft : equipment problem dissipated flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

On descent to assigned altitude of FL240, aircraft was on autoplt and initially leveled off at FL240. ATC assigned new routing, and while setting up navaids, noticed aircraft was at FL235 and descending. While correcting, the aircraft descended to as low as FL232.5. Reached assigned altitude of FL240 within 20 seconds from initial correction. Upon re-engaging autoplt at FL240, autoplt commenced a climb and was immediately disengaged. On subsequent engagement autoplt worked well.

Original NASA ASRS Text

Title: AUTOPLT ANOMALY RPTED AS CAUSE OF ALTDEV.

Narrative: ON DSCNT TO ASSIGNED ALT OF FL240, ACFT WAS ON AUTOPLT AND INITIALLY LEVELED OFF AT FL240. ATC ASSIGNED NEW ROUTING, AND WHILE SETTING UP NAVAIDS, NOTICED ACFT WAS AT FL235 AND DSNDING. WHILE CORRECTING, THE ACFT DSNDED TO AS LOW AS FL232.5. REACHED ASSIGNED ALT OF FL240 WITHIN 20 SECONDS FROM INITIAL CORRECTION. UPON RE-ENGAGING AUTOPLT AT FL240, AUTOPLT COMMENCED A CLB AND WAS IMMEDIATELY DISENGAGED. ON SUBSEQUENT ENGAGEMENT AUTOPLT WORKED WELL.

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.