What happened
On July 27, 1999, a Cessna 340, registration HB-LGR, was performing a landing at Lugano-Agno Airport in Switzerland. The flight had originated from Calvi, Corsica, and was being operated under VFR conditions by two private pilots.
During the landing roll, the aircraft touched down on the centerline of the runway. Although the touchdown was described as somewhat abrupt, the pilots initially maintained control. However, after traveling approximately 150 to 200 meters down the runway, the aircraft began to vibrate violently and veer toward the left edge of the runway. This vibration was caused by the failure of the left main landing gear torque link assembly. The failure caused the left wheel to rotate 180 degrees, resulting in the tire scrubbing against the runway and the landing gear door being wrenched from the fuselage. The occupants of the aircraft were uninjured.
The investigation
Investigators examined the technical records of HB-LGR and focused on a 200-hour inspection performed on June 18, 1999. During this maintenance, a mechanic had removed the bolt from the left main landing gear torque link assembly to perform other tasks and subsequently reinstalled it.
Technical analysis of the hardware revealed that the washer located under the bolt head was significantly non-compliant with the manufacturer's specifications. The investigated washer had an external diameter of 14.28 mm, whereas the original part required a diameter of 19.13 mm. Additionally, the washer was much thinner (0.78 mm compared to the required 1.6 mm) and possessed only about half the required hardness (189 HV compared to 395 HV).
Findings
- The primary cause of the incident was the displacement of the bolt from the left main landing gear torque link assembly due to vibrations generated during braking.
- The failure was driven by the installation of non-compliant washers that did not meet the manufacturer's specifications.
- Under the heavy loads and rotational forces experienced during landing, the undersized and weakened washer deformed plastically. This allowed the bolt head and the bushing to be pulled through the upper fork hole, leading to the structural separation of the torque links.