What happened
On July 27, 1996, a Zlin 126 Trener, registration OH-TZA, was performing a private flight at Hangon lentopaikka when a serious flight control malfunction occurred. The aircraft, carrying a pilot and a five-year-old passenger, had just completed a landing roll when the pilot noticed an unusually strong pitch-up tendency. Upon attempting to adjust the elevator trim, the pilot discovered the trim was stuck in an abnormal position.
Because the remaining runway was insufficient for a second landing, the pilot initiated a go-around. During the climb, the aircraft experienced a heavy left yaw, and the pilot noted that the elevator control effectiveness was nearly non-existent. Visual inspection during the level flight revealed that the left elevator was stuck in a full pitch-up position, while the right elevator was no longer visible, indicating it had moved out of its normal position. To maintain control, the pilot performed the landing at a high airspeed. Upon returning to the apron, it was discovered that the right elevator had detached from the stabilizer and was hanging only by two thin trim cables. The detached elevator had struck the rudder, causing structural deformations.
ed
The investigation
The investigation focused on the mechanical failure of the elevator attachment mechanism. The assembly utilized a bearing system where a bearing pin was secured to the elevator via a nut and a washer. Investigators determined that a specific bearing had detached from its mounting on the control arm and slid out of position. This movement caused the elevator pins to dislodge from their bearings, allowing the elevator to detach from the stabilizer.
The investigation examined the dimensions of the washer used in the assembly. It was found that the washer installed was only 12.5 mm in diameter, which was smaller than the 13 mm outer diameter of the bearing. This undersized washer failed to prevent the bearing from migrating. Furthermore, the investigation looked into manufacturer and maintenance records, noting that while a service bulletin for the Z 226 model recommended a larger 16 mm washer, this information had not been effectively communicated or implemented for the Z 126 in Finland.
Findings
- The primary cause of the failure was the use of an undersized washer that failed to retain the bearing in its mounting.
- Longitudinal forces, likely generated during ground operations or tailwheel flutter, provided the force necessary to displace the bearing.
- The detachment of the bearing allowed the elevator pins to slip out of the bearings, leading to the complete separation of the right elevator from the stabilizer.
- The detached elevator struck the rudder, causing secondary damage to the vertical stabilizer.