Incident
During cruise flight at 12,000 feet mean sea level, the airplane began shuddering with a severe high-frequency vibration that affected the entire airframe. The pilot reported no specific source related to the flight controls. Reducing power on the left engine did not stop the vibration, but reducing power on the right engine caused it to cease. The pilot then secured the right engine. Within a couple of minutes, the vibration returned with equal intensity. The pilot diverted to a nearby airport and lowered the landing gear during approach, at which point the vibration stopped again. The airplane landed without further incident.
Post-Flight Inspection
A post-flight inspection revealed that the right elevator trim tab push rod was broken. The clevis connecting to the trim tab surface was pinched due to an overtorqued bolt and nut. Metallurgical examination of the broken rod identified a fatigue crack that had propagated through at least 95 percent of the thread cross section.
Maintenance History
Maintenance records showed that the right elevator trim tab push-pull tube bolts and bushings had been removed and replaced. At the time of the incident, the airframe had accumulated 101.5 flight hours since that inspection, and no additional maintenance on the right elevator trim system had been documented.