Incident Description
A pilot reported that while leveling off after a 700-foot descent in light wind and smooth air, the airplane experienced violent shaking. The aircraft was in a 15-degree bank, 1 to 2 degrees nose down, at 180 mph (within the yellow arc). The pilot described the vibration as similar to a rivet gun or machine gun striking the fuselage, reverberating throughout his body. No vibrations were felt in the controls. After approximately three seconds, the pilot pushed forward on the control column, and the vibration ceased. An uneventful landing followed.
Inspection Findings
A post-flight inspection found cracks in the upper two corners of the FS 256.9 bulkhead. The forward spar of the right ruddervator was broken. A crack had propagated through the top spar flange and continued toward the bottom flange, stopping about 1/4 inch before a rivet hole. The center hinge was broken, and an 8.5-inch skin tear was observed on the top surface of the ruddervator. The ruddervators were confirmed to be properly balanced. No maintenance anomalies were identified.
Related Directives
The Federal Aviation Administration had issued Airworthiness Directive 98-13-02 on July 7, 1998, restricting airspeed to 144 mph on Beech 35, A35, and B35 airplanes. Subsequently, the aircraft manufacturer released Service Bulletin 27-3358 in February 2000, requiring a comprehensive inspection of empennage components and revising the ruddervator underbalance limits from 16.8 to 19.8 inch-pounds to 16.8 to 18.0 inch-pounds, with the intent of lifting the speed restriction.