What happened
On October 17, 1968, a Cessna 150 (registration HB-CSL) was conducting a student pilot training flight at the La Côte aerodrome in Switzerland. During the third circuit of the pattern, the student pilot performed a landing on runway 04. Shortly after the aircraft made contact with the grass runway, the right main landing gear spring steel blade snapped near the attachment point.
The structural failure caused the aircraft to collapse toward the right side. The instructor attempted to maintain directional control, but as the aircraft veered toward parked aircraft near a hangar, the instructor released the controls to allow the plane to enter a ground loop. The aircraft pivoted approximately 120 degrees clockwise before coming to a halt at the edge of the runway. There were no injuries to the instructor or the student, though the aircraft sustained damage to the fuselage, tail, and landing gear.
The investigation
Investigators examined the broken landing gear component in collaboration with the Federal Laboratory for Materials Testing. The investigation focused on the physical properties of the steel blade and the surface condition of the fracture site. The aircraft had a relatively high number of landings (6,899) despite its relatively recent manufacture.
Metallurgical analysis identified the material as a chrome-vanadium steel. While chemical composition and hardness tests met standard specifications, specialized testing revealed that the material possessed low toughness. The fracture surface showed the presence of two fatigue cracks originating from the lower surface of the blade, specifically at a location characterized by deep surface ridges left over from the manufacturing process.
Findings
- The primary cause of the accident was a fatigue failure of the main landing gear spring steel blade.
- The failure was driven by the combined effects of the material's low resistance to impact (low toughness) and its vulnerability to cracks caused by manufacturing irregularities (surface ridges).
- The manufacturer's maintenance manual did not require periodic dye penetrant inspections of the landing gear blades, nor did it specify a service life for these components.