A MD83 autopilot trim (ap) put the aircraft in a slight descent several times during flight with AP1...

Narrative:

At FL330 approximately 90 minutes into flight with the autopilot on for less than 1 hour; I noticed a slight descent of approximately 100-150 ft. Almost simultaneously the autopilot trim light illuminated; I disconnected the autopilot. It disconnected with a noticeable bump which descended the aircraft another 50 or so feet; for a total of approximately 200-225 ft. I re-trimmed the aircraft; and returned the aircraft to FL330. I checked the autopilot trim to insure it was still operational. Approximately 3 minutes later we were given a vector; as I turned the aircraft with the autopilot; it began a descent of approximately 500 FPM. I again disconnected the autopilot; and switched to autopilot #2. We were not in icing conditions; nor were we at the maximum altitude for our gross weight. We checked the QRH. The aircraft seemed to operate normally with autopilot #2 until at lower altitudes when the autopilot trim light again illuminated while maneuvering. We landed uneventfully; called maintenance control and wrote up the discrepancy in the logbook. There were no ATC conflicts noted. Sometimes things happen. Better maintenance schedules unlike the new schedules which would detect problems before they occur. No fly until failure policy; preventive maintenance!

Original NASA ASRS Text

Title: A MD83 AUTOPILOT TRIM (AP) put the aircraft in a slight descent several times during flight with AP1 selected. At lower altitudes with AP2 the AP TRIM likewise allowed a descent while turning.

Narrative: At FL330 approximately 90 minutes into flight with the autopilot on for less than 1 hour; I noticed a slight descent of approximately 100-150 FT. Almost simultaneously the autopilot trim light illuminated; I disconnected the autopilot. It disconnected with a noticeable bump which descended the aircraft another 50 or so feet; for a total of approximately 200-225 FT. I re-trimmed the aircraft; and returned the aircraft to FL330. I checked the autopilot trim to insure it was still operational. Approximately 3 minutes later we were given a vector; as I turned the aircraft with the autopilot; it began a descent of approximately 500 FPM. I again disconnected the autopilot; and switched to autopilot #2. We were not in icing conditions; nor were we at the maximum altitude for our gross weight. We checked the QRH. The aircraft seemed to operate normally with autopilot #2 until at lower altitudes when the autopilot trim light again illuminated while maneuvering. We landed uneventfully; called Maintenance Control and wrote up the discrepancy in the logbook. There were no ATC conflicts noted. Sometimes things happen. Better maintenance schedules unlike the new schedules which would detect problems before they occur. No fly until failure policy; preventive maintenance!

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	989741
Time
Date	201201
Local Time Of Day	1801-2400
Place
Locale Reference	ZME.ARTCC
State Reference	TN
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	MD-83
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

A MD83 autopilot trim (ap) put the aircraft in a slight descent several times during flight with AP1 selected. At lower altitudes with AP2 the ap trim likewise allowed a descent while turning.