Narrative:

Routine; uneventful climb up to FL380 on way to FL400 in no turbulence when several waves of rather severe vibration occurred thought airframe. Not able to [isolate] source; but reduction of power and airspeed improved situation; yet phenomenon reoccurred. Requested an emergency descent to lower flight level; finding things 'smoothed out' at FL350. Cleared to FL300 where we reported critical situation had resolved and emergency did not currently exist; yet passengers understandably did not what to continue flight in that aircraft. So; we requested 'normal' handling for a landing at [a nearby airport]. Uneventful descent and landing there.

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Original NASA ASRS Text

Title: FA10 flight crew climbing through FL380 reported severe airframe vibration of unknown origin; so they declared an emergency and diverted to a nearby airport.

Narrative: Routine; uneventful climb up to FL380 on way to FL400 in no turbulence when several waves of rather severe vibration occurred thought airframe. Not able to [isolate] source; but reduction of power and airspeed improved situation; yet phenomenon reoccurred. Requested an emergency descent to lower flight level; finding things 'smoothed out' at FL350. Cleared to FL300 where we reported critical situation had resolved and emergency did not currently exist; yet passengers understandably did not what to continue flight in that aircraft. So; we requested 'normal' handling for a landing at [a nearby airport]. Uneventful descent and landing there.

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.