A B737 flight crew requested a clearance below rvsm airspace when their autopilot began an uncommand...

Narrative:

During the climbout we noticed that the stab out of trim light had illuminated momentarily a couple of times with the 'a' autopilot engaged. We discussed that this was normal for short periods of time and thought no more about it. After leveling off for cruise at FL330; we diverted our attention away from the instruments to attend to other duties (putting away charts; logbook; getting weather; etc.). Our jumpseater said; 'hey guys...you're descending.' we both looked up to see our altitude at about FL327 and descending. The 'a' autopilot was still engaged and in altitude hold on the scoreboard. The captain immediately corrected back to FL330 and I was about to inform center when he asked about our altitude. I said that we were correcting and leveling at FL330. After level for just a few seconds; it was obvious that the 'a' autopilot was not going to hold altitude so the captain disengaged it and the engaged the 'B' autopilot. The results were the same. The aircraft began a shallow climb and at about 100 ft; the captain disengaged the autopilot and manually flew the aircraft. I informed ATC that we were unable rvsm and asked for a descent below FL290. During the descent; I pulled out the QRH and we ran the stab out of trim checklist. Although the light was not illuminated at the time; this was the best option. The aircraft responded to manual and electrical trim; so the captain elected to not re-engage the autopilot for the duration of the flight. The rest of the flight and landing was uneventful.

Original NASA ASRS Text

Title: A B737 flight crew requested a clearance below RVSM airspace when their autopilot began an uncommanded descent and a short while later an uncommanded climb. An elevator servo was found to have malfunctioned.

Narrative: During the climbout we noticed that the Stab Out of Trim light had illuminated momentarily a couple of times with the 'A' autopilot engaged. We discussed that this was normal for short periods of time and thought no more about it. After leveling off for cruise at FL330; we diverted our attention away from the instruments to attend to other duties (putting away charts; logbook; getting weather; etc.). Our jumpseater said; 'Hey guys...you're descending.' We both looked up to see our altitude at about FL327 and descending. The 'A' autopilot was still engaged and in altitude hold on the scoreboard. The Captain immediately corrected back to FL330 and I was about to inform Center when he asked about our altitude. I said that we were correcting and leveling at FL330. After level for just a few seconds; it was obvious that the 'A' autopilot was not going to hold altitude so the Captain disengaged it and the engaged the 'B' autopilot. The results were the same. The aircraft began a shallow climb and at about 100 FT; the Captain disengaged the autopilot and manually flew the aircraft. I informed ATC that we were unable RVSM and asked for a descent below FL290. During the descent; I pulled out the QRH and we ran the Stab Out of Trim checklist. Although the light was not illuminated at the time; this was the best option. The aircraft responded to manual and electrical trim; so the Captain elected to not re-engage the autopilot for the duration of the flight. The rest of the flight and landing was uneventful.

Data retrieved from NASA's ASRS site as of April 2012 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	879956
Time
Date	201003
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Elevator ControlSystem
Person 1
Function	First Officer Pilot Not Flying
Experience	Flight Crew Last 90 Days 203 Flight Crew Type 5200
Person 2
Function	Captain Pilot Flying
Experience	Flight Crew Last 90 Days 192 Flight Crew Type 18000
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude

A B737 flight crew requested a clearance below rvsm airspace when their autopilot began an uncommanded descent and a short while later an uncommanded climb. An elevator servo was found to have malfunctioned.