Narrative:

About 1 hour into the flight; aircraft began altitude oscillations similar to mountain wave; one oscillation exceeded 200ft and was reported to center. Center queried other aircraft in our area with no mountain wave reported by other aircraft. I closely monitored the controls and held the yoke; approximately 5 minutes later; oscillations began; and I could feel ratcheting and jerking in the yoke; and disengaged the autopilot when the altitude deviation reached 200. Leveled the plane. There were no warning annunciators or failure flags in the cockpit. I switched the plane from the right afcs to the left. We advised center that our auto pilot had malfunctioned and that we had switched to the second system. Center told us there were no aircraft above or below us and we were good to continue. The airplane operated correctly for about 15 minutes before the oscillations began again. We told center our autopilot was malfunctioning and we were unable rvsm. We were descended to fl 280 and we continued the flight to [our destination]. While hand flying and hand trimming the plane I could feel binding and release in the elevator trim wheel. I wrote this up with maintenance.

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Original NASA ASRS Text

Title: BAE-125-850XP flight crew reported experiencing oscillations in the aircraft pitch mode while in cruise flight. These oscillations were determined to be caused by a malfunctioning autopilot system. The autopilot was disengaged; air traffic control notified; and the flight continued safely to its destination.

Narrative: About 1 hour into the flight; aircraft began altitude oscillations similar to mountain wave; one oscillation exceeded 200ft and was reported to center. Center queried other aircraft in our area with no mountain wave reported by other aircraft. I closely monitored the controls and held the yoke; approximately 5 minutes later; oscillations began; and I could feel ratcheting and jerking in the yoke; and disengaged the autopilot when the altitude deviation reached 200. Leveled the plane. There were no warning annunciators or failure flags in the cockpit. I switched the plane from the right AFCS to the left. We Advised Center that our auto pilot had malfunctioned and that we had switched to the second system. Center told us there were no aircraft above or below us and we were good to continue. The airplane operated correctly for about 15 minutes before the oscillations began again. We told Center our autopilot was malfunctioning and we were unable RVSM. We were descended to FL 280 and we continued the flight to [our destination]. While hand flying and hand trimming the plane I could feel binding and release in the elevator trim wheel. I wrote this up with maintenance.

Data retrieved from NASA's ASRS site 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.