What happened
On May 16, 1967, a Piper PA-28-180 Cherokee, registration HB-OWM, was conducting a navigation training flight with a pilot and an instructor. After landing at Courtelary, the crew prepared for a subsequent departure. During the takeoff roll, the aircraft experienced a sudden shock and began to sink. The crew noted heavy vibrations in the flap lever and heard dull thuds emanating from beneath the fuselage.
The instructor took control and observed that the left flap was deformed and the left main landing gear was rotating backward behind the wing. Because no emergency services were available at Courtel and the aircraft was unstable, the crew decided to divert to the military airfield at Payerne to seek assistance. During the flight, the pilot attempted to contact Lausanne tower but was eventually directed to Payerne by Geneva Information.
Upon landing at Payerne, the left main gear—which was held only by the brake oil line—completely detached from the aircraft. The aircraft initially rolled on the remaining gear, but as it slowed, the left wing dropped into the tall grass, causing a 180-degree ground loop. There were no injuries to the occupants.
The investigation
An investigator examined the aircraft and the takeoff site at Courtelary. The runway at Courtelary was found to be in excellent condition with short grass and no obstructions. The investigation focused on the mechanical failure of the landing gear assembly, specifically the torque link (referred to in factory manuals as a 'compass') that connects the cylinder to the piston.
Findings
- The primary cause of the accident was the fatigue failure of the torque link responsible for aligning the wheel and maintaining the gear in the flight position.
- The middle articulation of the link was found to be structurally weaker than the other two joints.
- A radial grease fitting had been drilled into the rings of the articulation, creating an additional point of weakness and a stress concentration site that likely initiated the fracture.