Date: 201602
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Dusk
Aircraft: BAe 125 Series 800
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Troubleshooting; Workload
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

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.


Second narrative: While cruising at FL400; Our Hawker 800XP started altitude/pitch oscillations. Since we were in Center's airspace; we thought it to be mountain wave action. Moments later a similar event occurred. No warnings or Flags were in view. AFCS were switched to no avail; as the same altitude/pitch oscillations occurred. The Captain reported to me that while hand-flying the airplane; he felt the elevator control to be stiff and notching/ratcheting where elevator control should be fluid and smooth. We immediately notified ATC and were told no other aircraft in the vicinity had been experiencing any mountain waves. We requested and were granted lower altitude as we were unable to maintain RVSM Airspace. We continued the trip to an uneventful and safe landing at [our destination].


Synopsis: 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.