Incident Overview
A pilot reported experiencing light turbulence while flying at 140 mph, during which the airplane began to shake. The shaking lasted approximately three seconds before the pilot reduced power and pulled back on the yoke to slow the aircraft. The pilot continued to the destination and performed a normal landing. After landing, an inspection revealed extensive damage in the area of the aft fuselage.
Investigation Findings
An investigation determined that the airplane did not have a current annual inspection. The pitot tube had pinholes where it had been welded. The airspeed indicator was tested and found to not meet specifications.
The ruddervators were examined for proper balance and rigging. The service balance range for the ruddervators is 16.8 to 19.8 inch-pounds trailing edge heavy (underbalance). Using the counterbalance method, the underbalance of the left ruddervator was determined to be 22.53 inch-pounds, and the right ruddervator 22.68 inch-pounds. Both values exceeded the specified range.
Additionally, the left and right ruddervator trim tab hinge pins had been installed incorrectly. The incorrectly installed trim tabs exhibited significant vertical free play near the inboard end, and the tabs had restricted up and down angular travel.
The ruddervator balance and the trim tab installation had been performed by two different aircraft maintenance facilities.