CHI98LA125 · Beech B35

The pilot reported that the airplane was just leveling off after a descent of 700 feet. The wind was light in smooth air. The airplane was in a 15 degree angle of bank right turn, 1 to 2 degrees nose down attitude, and at 180 mph. The airspeed was in the middle of the yellow arc. The pilot reported the airplane began to shake violently like a rivet gun hitting against the fuselage, or like a machine gun. He said the vibrations reverberated throughout his body. He did not feel any vibrations in the controls. He pushed over on the control column after about three seconds of vibration. The vibration stopped. An uneventful landing was made. The aircraft inspection revealed cracks in the upper two corners of the FS 256.9 bulkhead. The forward spar of the right ruddervator was broken. A crack had proceeded through the top spar flange and had worked its way down to the bottom flange but stopped about 1/4 inch prior to reaching a rivet hole. The center hinge had broken and a 8.5 inch skin tear was evident on the top surface of the ruddervator. The ruddervators were properly balanced. The inspection revealed no maintenance anomalies. The FAA issued an Airworthiness Directive 98-13-02 on July 7, 1998, restricting the airspeed to 144 mph on Beech 35, A35,and B35 airplanes. The aircraft manufacturer issued a Service Bulletin 27-3358 in February 2000, which required an extensive inspection of empennage components, and it changed the ruddervator underbalance from 16.8 to 19.8 inch pounds to 16.8 to 18.0 inch pounds in order to lift the speed restriction.

Verbatim NTSB analysis and probable cause from the NTSB dataset