Date: 201204
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Weather elements / visibility: Turbulence
Light: Daylight
Aircraft: BAe 125 Series 800
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Aeroplane Flight Control
Component problem: Malfunctioning
Reporter organization: Corporate
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Other / Unknown
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Diverted; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft; Company Policy
Primary problem: Aircraft

Narrative: We experienced 'coupled' wing oscillations after experiencing light chop/turbulence at FL400. Oscillations occurred at a rate of approximately 2-3 cycles/second with a travel of approximately 12 inches from neutral both up and down; causing extreme airframe shaking. We declared an emergency; were given a descent and vectors to a nearby airport. On descent at approximately FL320 the oscillations stopped. We elected to continue our diversion as a precautionary measure where a normal approach and landing were accomplished. At no time was positive control of the aircraft lost. An examination of the aircraft was made; directed by our company Director of Maintenance via phone. No airframe damage was noted. Crew and passengers elected to continue the flight to the intended destination and experienced no further incident.

Callback: The reporter advised that; in addition to descending; they had reduced their Mach from about 0.76 to about 0.70 and extended the speedbrakes in an attempt to stop the oscillations. Subsequent to submitting the report he experienced another episode of the same type. After the second event he searched out and spoke with other company pilots who had experienced similar episodes. All the aircraft involved in these events were equipped with after market winglets. The reporter had personally contacted them and discussed the events and was advised that the winglet manufacturer was aware of incidents of this type; but ascribed the problem to incorrect aileron rigging rather than an issue with the winglets themselves. The reporter was advised both ailerons are to be rigged with 1/4 inch droop below neutral. He assumed the different rigging to be necessary due to airflow changes produced by the presence of the winglets.After learning of the earlier events the reporter was concerned that there had been no attempt to determine the cause and the aircraft involved had merely been inspected for damage and; when no obvious faults were found; returned to service. He believes the recurring nature of the incidents and the severity of the oscillations demand investigation and resolution of their underlying cause.



Synopsis: An HS125-800XP flight crew experienced the sudden onset of wing oscillaton/flutter at FL400 which resulted in extreme 'aircraft shaking.' They diverted; declared an emergency; descended; and reduced airspeed; and the oscillations ceased as they passed through FL320. A maintenance inspection was unable to find a cause and the flight crew and their passenger continued to their destination without further incident.