B727 crew has trim malfunction which causes aircraft to lose altitude in RVSM airspace.

Narrative

Rewritten in plain English from NASA’s original all-caps telegraphic report — facts, figures, and terminology preserved.

While in cruise flight in RVSM airspace, autopilot began trimming nose down and started slow descent. When autopilot was disconnected, aircraft pitched down and approximately 300 feet altitude loss occurred. Notified ATC of autopilot problem and requested departure from RVSM airspace. ATC cleared us to FL270. Remainder of flight uneventful.

Supplemental info from reporter ACN 719658: Autopilot was disconnected and manually returned to FL310. Heavy control forces were necessary to overcome during maneuver.

Callback conversation with reporter ACN 719659 revealed the following info: The reporter stated the cruise trim motor was not disengaging at the moment trim inputs ceased, causing the trim to coast past the desired set point. This characteristic became rather cumbersome during approach. The crew decided to hand fly the aircraft and make manual trim inputs via the trim wheel. This still did not work well but after a little practice, they were able to fly an approach to a normal landing. The cruise trim motor problem was discovered by maintenance during troubleshooting.