Incident Description
During a descent at approximately 150 knots, the pilot encountered turbulence. The rudder pedals began shaking uncontrollably, and the entire aircraft, particularly the empennage, vibrated. Pulling back on the control column and pushing on the rudder pedals caused the shaking to subside. The flight landed without further incident.
Inspection Findings
After the flight, the aircraft continued to be operated for several months, despite wrinkled skin on the aft fuselage. A subsequent inspection revealed cracks in all four corners of the aft fuselage bulkhead. The static balance of each ruddervator was measured and found to be excessively tail heavy. Both ruddervator trim tabs had been installed improperly, resulting in free play at the hinge line. No lead balancing washers were found in either ruddervator counterweight housing. Additionally, the thickness of paint on the ruddervators exceeded the manufacturer's factory acceptance criteria.
Maintenance Observations
The combination of tail-heavy ruddervators, improperly installed trim tabs, missing balancing washers, and excessive paint thickness likely contributed to the vibration experienced during the descent. The cracked aft fuselage bulkhead and wrinkled skin suggest significant structural stress. The aircraft was subsequently grounded for repairs.