An hs-125 flight crew encountered moderate to severe airframe vibration while in cruise at FL400. Th...

Narrative:

Cruising in occasional light chop at FL400 at mach .74 = 220 KIAS. The airframe started to vibrate at a rate similar to moderate to severe chop. We thought we had an airframe flutter or automated flight control system malfunction. We requested a lower altitude and were cleared to FL350. In the descent we declared an emergency. The captain used the autopilot to descend and level off at FL350 where he turned off the autopilot. The aircraft stopped vibrating. The captain flew the rest of the flight by hand. We asked center for a clearance to a nearby airport. They cleared us to the airport. In the descent we experienced some turbulence; but no more airframe vibration. We told center fuel and souls on board; and that the aircraft was under control and we no longer considered it to be an emergency. We landed uneventfully. The passengers said the wing tips fluctuated up and down about 3 ft at a rate of about 3 oscillations per second. This lasted for about 3 to 4 minutes.

Original NASA ASRS Text

Title: An HS-125 flight crew encountered moderate to severe airframe vibration while in cruise at FL400. The vibration ceased following a descent and disconnecting the autopilot; subsequently diverting to a nearby airport.

Narrative: Cruising in occasional light chop at FL400 at Mach .74 = 220 KIAS. The airframe started to vibrate at a rate similar to moderate to severe chop. We thought we had an airframe flutter or automated flight control system malfunction. We requested a lower altitude and were cleared to FL350. In the descent we declared an emergency. The Captain used the autopilot to descend and level off at FL350 where he turned off the autopilot. The aircraft stopped vibrating. The Captain flew the rest of the flight by hand. We asked Center for a clearance to a nearby airport. They cleared us to the airport. In the descent we experienced some turbulence; but no more airframe vibration. We told Center fuel and souls on board; and that the aircraft was under control and we no longer considered it to be an emergency. We landed uneventfully. The passengers said the wing tips fluctuated up and down about 3 FT at a rate of about 3 oscillations per second. This lasted for about 3 to 4 minutes.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	988543
Time
Date	201201
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Make Model Name	BAe 125 Series 800
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Person 1
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 20 Flight Crew Total 11876 Flight Crew Type 35
Events
Anomaly	Inflight Event / Encounter Other / Unknown

An hs-125 flight crew encountered moderate to severe airframe vibration while in cruise at FL400. The vibration ceased following a descent and disconnecting the autopilot; subsequently diverting to a nearby airport.