Accident Overview
On the afternoon of 24 June 2024, a Piper L-4J Grasshopper, registration LN-SAI, departed Kjeller (ENKJ) with an instructor and a student on board for landing practice at Maarud farm, which has a grass strip approximately 600 m long. The aircraft is owned by a partnership and based at Kjeller. Both occupants were co-owners.
Flight and Landing
The final landing circuit was reported as normal. The student flew the approach with planned speed and descent rate. A three-point landing had been briefed. Just before touchdown, the aircraft experienced sudden braking, nosed over, and came to a stop. It then fell back onto the landing gear. Both pilots were uninjured and exited the aircraft.
Damage and Findings
Examination revealed that the left main wheel was missing. It was later found months afterward during potato harvesting north of the strip. The aircraft sustained a broken wheel axle, a bent propeller, and damage to the left landing gear, tailwheel, wingtips, and engine mount.
The left landing gear leg was sent to the Norwegian Defence Laboratory (FOLAT) for non-destructive fracture analysis. The axle had broken inside the bushing of the main gear leg. Two of four spot welds attaching the axle to the outer bushing showed fatigue cracks.
Analysis Conclusion
FOLAT concluded that the fracture appeared planar with possible crack arrest lines, and crack growth likely started at the spot welds. The damage most likely resulted from fatigue originating from one or both spot welds, propagating around the axle to a small final fracture area. The small final fracture indicates high fracture resistance.
The aircraft, built in 1944, is 80 years old. No maintenance history for the axle was available. The aircraft had been operated on skis and floats, which may have imposed additional loads, particularly during water landings. Corrosion was not identified as a contributing factor.
The investigation noted that the pilots' focus on proper speed and energy management may have prevented a more severe outcome. A higher-speed landing with a lower nose attitude could have caused the gear leg to dig in deeper, potentially flipping the aircraft onto its back.
The Accident Investigation Board (SHK) did not issue a safety recommendation due to the aircraft's age and lack of inspection requirements for the axle. However, it encouraged owners of Piper aircraft with similar landing gear to periodically perform internal inspections of the axle using light or a borescope.