Narrative:

During cruise flight at FL400; along [in oceanic airspace]; aircraft experienced a failure of the yaw damp system. Associated with this were the yaw damp 1-2 fail; rudder fault and ap disconnect cas messages. Crew took control manually as aircraft became unstable along the longitudinal (roll) axis. The crew was aware that the majority of the [oceanic] routes were to the south; and that the aircraft would need to descend into thicker air to regain control. There was traffic (a falcon) below; also on same airway. The PF executed the oceanic contingency maneuver; starting with a 45 degree turn to the right (taking the plane north) and establishing a 15 nm offset; paralleling filed airway. While the PF was doing this; the PNF was in contact with arinc on HF to advise them of our situation and request clearance to FL280. The PF took the VHF radio and made broadcasts on 121.5 and 123.45 and was in contact with the falcon traffic; which had visual contact. ATC cleared us to FL380; but the plane was still not controllable at that high altitude. At this point; our position was not quite past the etp (equal time point); so the crew elected to reverse course back to ZZZ; and initiate a descent to FL280. ATC was advised and requested we fly a 60 nm offset from airway. Crew was unable to establish this; and elected to return to the 15 nm standard offset; still to the north of airway. Once reaching FL280; with IAS reduced to 250 kts; the plane was controllable on all axis and crew elected to remain at that altitude. All exterior lights were turned on and advisory calls were made on HF; VHF 121.5 and 123.45. Communication was established with other aircraft and maintained. Crew remained at FL280; 15 nm offset north of airway until back in radar contact with center. Flight landed at ZZZ with partial flap setting (in accordance with operator checklist) on the longest runway. No other issues occurred.

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Original NASA ASRS Text

Title: A Hawker 4000 Captain reported a yaw damper system failure and a rudder fault with associated autopilot disconnect and instability inflight while overwater at FL400. The crew regained control; reversed course; and returned to the departure airport.

Narrative: During cruise flight at FL400; along [in Oceanic airspace]; aircraft experienced a failure of the yaw damp system. Associated with this were the Yaw Damp 1-2 Fail; Rudder Fault and AP Disconnect CAS messages. Crew took control manually as aircraft became unstable along the longitudinal (roll) axis. The crew was aware that the majority of the [Oceanic] routes were to the south; and that the aircraft would need to descend into thicker air to regain control. There was traffic (a Falcon) below; also on same airway. The PF executed the Oceanic Contingency maneuver; starting with a 45 degree turn to the right (taking the plane north) and establishing a 15 nm offset; paralleling filed airway. While the PF was doing this; the PNF was in contact with ARINC on HF to advise them of our situation and request clearance to FL280. The PF took the VHF radio and made broadcasts on 121.5 and 123.45 and was in contact with the Falcon traffic; which had visual contact. ATC cleared us to FL380; but the plane was still not controllable at that high altitude. At this point; our position was not quite past the ETP (Equal Time Point); so the crew elected to reverse course back to ZZZ; and initiate a descent to FL280. ATC was advised and requested we fly a 60 nm offset from airway. Crew was unable to establish this; and elected to return to the 15 nm standard offset; still to the north of airway. Once reaching FL280; with IAS reduced to 250 kts; the plane was controllable on all axis and crew elected to remain at that altitude. All exterior lights were turned on and advisory calls were made on HF; VHF 121.5 and 123.45. Communication was established with other aircraft and maintained. Crew remained at FL280; 15 nm offset north of airway until back in radar contact with Center. Flight landed at ZZZ with partial flap setting (in accordance with operator checklist) on the longest runway. No other issues occurred.

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.