What happened
On June 24, 2024, a Piper L-4J Grasshopper, registration LN-SAI, was performing landing practice at Maarud gård in Sør-Odal, Norway. The flight, conducted under VFR conditions, included an instructor and a student. While executing a planned three-point landing on a grass runway, the aircraft experienced an intense braking force immediately upon touchdown. The aircraft subsequently pivoted onto its nose and came to a halt.
Upon inspection, the crew discovered that the left main landing gear was missing. The left gear leg had plowed two furrows into the grass, and the propeller had struck the ground. While the occupants escaped without 0 injuries, the aircraft sustained significant damage, including a broken wheel axle, a bent propeller, and damage to the engine mounts, wingtips, and tailwheel.
The investigation
The Norwegian Safety Investigation Authority (NSIA) conducted a focused investigation, including a non-destructive fracture analysis of the left landing gear leg performed by the Norwegian Defence Research Establishment (FOLAT). The analysis revealed that the wheel axle had snapped inside the bushing of the main gear leg.
Investigators noted that the aircraft is a historical model dating back to 1944. While the specific age of the axle could not be determined, the aircraft has a history of operating on skis and floats, which may subject the landing gear to higher mechanical loads. The investigation also noted that the aircraft has recorded over 6,500 landings.
Findings
Technical analysis of the broken axle identified fatigue cracks originating at the spot welds used to attach the axle to the outer bushing. The NSIA concluded that the failure was likely caused by fatigue originating from these weld points, potentially due to material weakening during the welding process.
Contributing factors may include the increased structural stresses associated with floatplane operations. The investigation also noted that the pilot's focus on maintaining proper airspeed and pitch during the landing prevented a more severe outcome, such as a complete aircraft inversion.
Safety action
While no formal safety recommendations are being issued due to the lack of specific inspection requirements for this vintage component, the NSIA encourages owners of Piper aircraft with similar landing gear configurations to perform periodic internal inspections of the axle using light or a borescope to detect potential issues.