MD-83
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness; Training / Qualification; Troubleshooting
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Other / Unknown
Flight Crew FLC Overrode Automation
Aircraft; Company Policy
I had an aircraft with MEL 22-2 Autopilot Pitch Trim. Under (O) Procedures; step E; it allows use of autopilot for landing with provisions. This MEL does not require a downgrade of the CAT II/III status; however; in Volume I under APPR/LND/GA; the Autopilot Pitch Trim is required for both Cat II and III. Maybe this could be changed; many other autopilot MEL's also require a downgrade under 22-89; one other note about MEL 22-2. Up in RVSM airspace the possibility of losing 200 feet in a turn on the autopilot is high; especially if distracted by ATC communications; etc. It might be worthwhile to consider an altitude restriction under this MEL. We were being vectored and rerouted; (distraction) during this flight and during a 90 degree heading change at FL320; the autopilot could not maintain altitude. Manual intervention prevented altitude deviation; however not by much. I feel changing the MEL as outlined above would help prevent this malfunction from causing problems for other pilots.
A MD83 Captain questioned whether the aircraft should be permitted into RVSM airspace with the autopilot trim MEL'ed because in a turn the aircraft can potentially not hold altitude; and additionally a discrepancy exists between manuals about CAT II/III operations without the trim functioning.