Incident Overview
A pilot reported that while conducting a maintenance test flight, a significant vibration occurred. The airplane was flying straight and level at 160 mph in smooth air with a broken cloud layer at 6,500 feet. The vibration began as a minor shake and intensified rapidly, described by the pilot as "like an earthquake." The pilot pulled the throttle back in less than three seconds, and the vibration stopped immediately. An uneventful landing was subsequently performed.
Post-Flight Inspection
A thorough inspection of the airplane revealed no visible damage. All systems and components appeared normal. The ruddervators were checked and found to be properly balanced. No anomalies were detected in the airplane's configuration or operation.
Previous Occurrence
The pilot noted that this vibration was identical to one experienced in the same airplane on August 1, 1996. Since that earlier event, two modifications had been made: the installation of a B35 model bulkhead and the installation of F35 model equivalent counterweights. Despite these changes, the airplane exhibited no faults during the flight or subsequent inspection.