Date: 201602
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: BAe 125 Series 800
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Confusion; Troubleshooting; Workload
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Landed As Precaution; Flight Crew Diverted; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I was serving as First Officer and acting as Pilot Flying. The assigned Captain was acting as Pilot Monitoring. We had accepted the aircraft out of maintenance in the morning and flown two legs. We departed with four passengers on board.  On departure; ATC issued us a heading and instructed us to climb to and maintain 7;000 ft MSL. The departure was uneventful and we experienced occasional light chop through approximately 5;000 ft MSL. Upon exiting the clouds and the chop at approximately 6;000 ft MSL; the Captain and I both noticed a pronounced; abnormal high frequency vibration present in the aircraft. We leveled off at 7;000 ft MSL as instructed and engaged the aircraft's autopilot and began to investigate in an attempt to ascertain the source of the vibration.  The vibration was present in the control column; the seats; the floor; and every place else we could feel on the aircraft's structure. I experimented with changes in speed between approximately 200 KIAS and 250 KIAS and discovered no change in the vibration. I conducted further tests by decreasing engine power to idle and increasing power up to the climb setting; swapping between AFCS 2 and AFCS 1; and turning off the engine sync; none of which resulted in any change in the abnormal vibration. All engine indications were in the normal operating ranges. As we discussed the possible causes of the vibration; ATC issued instructions to climb to and maintain 16;000 ft MSL. I set the autopilot to vertical speed and began climbing. As we passed approximately 11;000 ft MSL; and at 250KIAS; the vibration suddenly became quite a bit more pronounced. We leveled at 16;000 ft MSL and I again attempted to change the vibration by changing power settings and speeds to no avail. At one point; the control column was visibly shaking. The Captain and I briefly discussed our best course of action and both determined that the safest; most conservative response was to divert. We simply did not know what might be causing the vibration and were concerned that continued high altitude; high speed flight might exacerbate the condition to a point where it may cause damage to or failure of a control surface. We elected to [advise ATC] and divert to ZZZ; which we found especially suitable due to its proximity and long runway. Because we had fueled for an extended cross country flight; the aircraft was over its maximum landing weight. We felt that the risk of landing overweight was far lower than the risk in possibly having a flight control failure and losing control of the aircraft while circling to burn off fuel so we computed speeds and briefed for an overweight landing. The approach and landing was uneventful; with a very soft touchdown and regular landing rollout. Upon taxiing to the FBO; we both conducted a thorough external post flight check and noted no abnormalities.


Second narrative: We tried to switch between autopilots and also disconnected the autopilot completely and noticed the vibrations were the same. Finally we tried to increase and decrease airspeed still no change. At that point I decided to contact the company on the flight phone and ask for maintenance assessment of the situation. While I had maintenance on the phone we started to be uncomfortable with the situation as the vibrations were becoming stronger. I elected to divert to the nearest suitable airfield.


Synopsis: BAE-125-850XP flight crew reported developing a pronounced inflight vibration which the crew could not isolate to engines or other components. They elected to divert to a nearby airport.