NTSB Factual Report

On September 22, 2007, approximately 0015 central daylight time, a Beech C90A twin-turboprop airplane, N871KS, experienced a failure of the right elevator trim-tab pushrod during cruise flight and the flight crew executed an emergency landing to Emporia, Kansas. The pilot, co-pilot, and 6 passengers were not injured. The airplane was registered to and operated by the Kansas State University, Salina, Kansas. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed for the Title 14 Code of Federal Regulations Part 91 business flight. The flight departed Tulsa, Oklahoma, and was destined for Manhattan, Kansas.

According to the pilot, while in cruise flight at 12,000 feet mean sea level, the airplane suddenly began "shuddering with a severe high frequency vibration. The vibration was in the entire airframe, not specifically the flight controls, so I had no clue where it was coming from." The pilot reduced the power on the left engine, however, the vibration continued; the pilot reduced the power on the right engine, and the vibration stopped. The pilot then shutdown and secured the right engine. Within a couple minutes of securing the right engine, the vibration "returned just as bad as before." The pilot then elected to divert to Emporia. The pilot lowered the landing gear during the approach, and the vibration stopped again. Subsequently, the airplane landed uneventfully at Emporia. During a post flight inspection, the pilot observed the right elevator trim tab push rod was broken.

According to a mechanic who removed the broken push rod, the clevis which connected to the trim tab surface was "pinched due to an overtorqued bolt and nut." Breakaway torque was not measured during the removal of the rod. The trim tab surface horn inner bushing was free to rotate within the outer bushing.

The pilot, who regularly piloted the incident airplane, stated that he noted no abnormal operations with the trim system prior to the failure.

On November 19, 2007, the NTSB materials laboratory in Washington, DC, examined the broken push rod. According to the metallurgist, the torque tube/linkage contained a rod end on each side of the fracture. One rod end fractured at the thread portion. Detailed bench binocular and scanning electron microscope examination of the fracture face from the rod end revealed a fatigue crack that propagated through at least 95 percent of the thread cross section.

A review of the airframe maintenance records revealed that the airplane was maintained in accordance with a manufacturer's approved inspection program. On February 14, 2007, during a phase 3 and 4 inspection at a Raytheon service center, the right elevator trim tab push-pull tube bolts and bushings were removed and replaced. At the time of the incident, the airframe had accumulated 101.5 flight hours since the inspection, and no additional maintenance to the right elevator trim system was noted in the maintenance records.

The following is an excerpt from the airframe maintenance manual elevator trim tab installation procedures: "Insure proper lubrication and install the bolt and nut, torque to 25-30 inch-pounds and install the cotter pin. The bolt, inner bushing, and clevis must be held tightly so they will rotate together when the trim tab is operated. The bolt must not turn in the inner bushing. Movement should be between the bushings."

NTSB Probable Cause

The improper installation of the right elevator trim tab attachment hardware by maintenance personnel which resulted in excessive wear, fatigue, and the failure of the push-pull tube.