Flight Crew Air Transport Pilot (ATP)
Confusion; Troubleshooting
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Aircraft Aircraft Damaged; Flight Crew Diverted
Aircraft; Human Factors
Aircraft experienced gradual; uncommanded left roll/yaw at 16;000 ft and 300 KIAS; level flight; while vectored direct for the Arrival. Continuous light vibration felt throughout airframe. Slip/skid (ball) indicator displaced completely to the left of center (unusual for a left yaw situation). No unusual engine or flight control malfunctions or abnormalities indicated on the MFD/PFD/MWS.Slowed to 250 KIAS; disengaged Auto Pilot (AP)/ Yaw Damper (YD); and centered the ball by displacing rudder trim 2 indications to the left. Coordinated diversion with ATC. Aircraft remained stable with rudder trim 2 indications to the left; rudder pedals difficult to move as if Yaw Damp still engaged; and vibrations still noticeable. Flew stable visual approach. ATC arranged for emergency vehicles to be on standby. Landed with no complications.On post flight; discovered a fastener on the leading edge of an accessory panel; on the underside of the left engine pylon; became loose and allowed for the leading edge of the panel to be forced aft. Although it was understandable that the panel could cause the aircraft to yaw left; it was confusing as to why the slip/skid (ball) indication was indicating to step on the left pedal and add left trim. Notified ACP; maintenance and steward on duty.
[Report narrative contained no additional information.]
HS125 flight crew on a ferry flight reported experiencing flight anomalies created by an unfastened panel on the engine pylon.