Accident Overview
On November 17, 2023, about 2030 mountain daylight time, a Beech 95, N2024C, was substantially damaged during a Title 14 Code of Federal Regulations Part 91 instructional flight near Yuma, Arizona. The flight instructor and pilot receiving instruction were not injured.
Flight Events
After departing from the airport, the pilot receiving instruction noticed the rudder pedals moving back and forth. The instructor assumed control and also felt the rudder pedals oscillating. The instructor elected to return to the airport. The oscillation intensified, causing the airplane to shake violently, which resulted in the forward windscreen separating from the airframe. The pilot receiving instruction declared an emergency while the instructor landed the airplane without further incident.
Postaccident Examination
A Federal Aviation Administration inspector examined the airplane and found that the rudder trim control rod end had fracture-separated near the trim tab attachment point. The forward windscreen had separated, leading to substantial damage.
Laboratory Examination
The rudder trim control rod end and jam nut were sent to the National Transportation Safety Board Materials Laboratory in Washington, DC. Examination revealed that the rod end had fractured through the threads approximately 3 to 4 threads away from the clevis end. The upper ear of the clevis was bent and twisted, with contact damage on the left side of the clevis and on the threads between the clevis and the fracture surface. The forward side of the clevis showed a fractured surface with a curving arrest line, features consistent with fatigue.
A scanning electron microscope (SEM) was used for further examination of the fatigue region. The SEM images revealed that fatigue features emanated from a broad origin area at the root of the thread and propagated preferentially along the thread root. Coarse striations and secondary cracking were identified at higher magnifications, showing features consistent with a relatively high-stress, low-cycle fatigue fracture mechanism. The fracture surface appeared to be free of any substantial oxidation from a preexisting crack. The overall fracture appearance indicated that the fracture likely occurred due to repeated high compressive loads.
Maintenance Records
Review of maintenance records revealed that about a month prior to the accident flight, maintenance had been performed on the rudder trim tab hardware.
Flutter Explanation
Flutter is an aeroelastic phenomenon that can occur when an airplane’s natural mode of structural vibration couples with aerodynamic forces to produce a rapid periodic motion, oscillation, or vibration. Flutter can be somewhat stable if the natural damping of the structure prevents an increase in forces and motions. It can become dynamically unstable if damping is not adequate or speed is increased, resulting in increasing self-excited destructive forces. Flutter can range from an annoying buzz to a violent destructive failure in a very short time. Due to the high frequency of oscillation, even flutter on the verge of becoming catastrophic can be hard to detect. Aircraft speed, structural stiffness, and mass distribution are three inputs that govern flutter. An increase in airspeed, a reduction in structural stiffness, or a change in mass distribution can increase susceptibility to flutter.