Date: 201710
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Hawker 4000
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Autoflight Yaw Damper
Component problem: Failed
Reporter organization: Corporate
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control; Flight Crew Landed As Precaution; Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

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.




Synopsis: 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.