MD-83
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Situational Awareness; Troubleshooting; Distraction
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem; General Maintenance Action
Procedure; Aircraft
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!
[Narrative #2 had no additional information.]
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.