A MD83 captain questioned whether the aircraft should be permitted into rvsm airspace with the autop...

Narrative:

I had an aircraft with MEL 22-2 autopilot pitch trim. Under (O) procedures; step east; 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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	991944
Time
Date	201201
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	MD-83
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Horizontal Stabilizer Trim
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Other / Unknown

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.