Incident Summary
The pilot reported that while cruising at 4,000 feet msl in clear sky with smooth air, he initiated a descent. During the descent at about 3,200 feet msl, with an airspeed of 185 mph, a nose-down attitude of 5 degrees, and a left turn of 15 degrees, the control yoke began to oscillate fore and aft about half an inch. The oscillations increased to about one inch, and the airplane experienced three or four 'solid jolts'. After approximately five seconds, the pilot reduced power, and the vibrations ceased. The pilot continued to the destination and landed without further incident.
The airplane's never-exceed airspeed (Vne) was 202 mph, and the maximum cruise airspeed (Vno) was 175 mph. The descent airspeed of 185 mph was between Vno and Vne.
Inspection Findings
Inspection of the airplane revealed extensive damage to the aft fuselage between fuselage station (FS) 256.9 and the 233.5 bulkhead. The longerons were bent downward with a permanent bend of 2.5 to 3 inches. The left and right side fuselage panels below the stabilizers exhibited 45-degree buckling from the bottom of the FS 256.9 bulkhead to forward of the stabilizer cuffs. The bottom fuselage panel forward of the 256.9 bulkhead had popped rivets and torn skin. There were three creases in the right stabilizer. No pre-existing cracks were found on the longerons. The ruddervators were properly balanced.