What happened
On 6 January 2020, a Cessna 172S, registration EI-DDX, was conducting commercial flight training at Cork Airport. During a routine local circuit, the aircraft performed a touch-and-go landing on Runway 25. While climbing back out to approximately 300 feet, the student pilot noticed that the left main undercarriage wheel had detached from the strut.
The instructor immediately took control of the aircraft and declared an emergency. After coordinating with Air Traffic Control to ensure the runway was clear of debris, the instructor executed a controlled two-wheel landing. The aircraft subsequently veered onto the grass verge adjacent to the runway. There were no injuries to the instructor or student pilot, though the aircraft sustained substantial damage, including impact to the nose gear, propeller, and wingtip.
The investigation
Investigators from the AAIU conducted a detailed metallurgical analysis of the recovered left undercarriage leg. The examination focused on the fracture surface and the structural integrity of the component. The investigation also reviewed the aircraft's maintenance history and the practices of the maintenance organization.
Technicians identified several anomalies on the undercarriage leg, including uneven paint and a bulge caused by a bead of weld. The analysis revealed that the leg had been subjected to unauthorized repairs. Furthermore, the investigation looked into the use of clear polyurethane tape, which is often applied to prevent chafing but can potentially trap moisture against the metal surface.
Findings
The investigation established that the primary cause of the failure was unapproved and improper weld repairs performed on the outer diameter of the undercarriage leg. These welds were of poor quality and likely introduced hydrogen embrittlement into the heat-affected zone during the welding process. This structural compromise allowed a fatigue crack to initiate and propagate through the alloy during subsequent landing cycles, eventually leading to the complete separation of the gear during the touch-and-go maneuver.
Additional contributing factors included:
- The presence of severe pitting corrosion on the base metal, which had not been fully removed before the unauthorized welding and subsequent painting.
- The application of polyurethane tape, which may have exacerbated corrosion by trapping moisture against the leg.
- The difficulty in detecting underlying surface anomalies or corrosion through the transparent tape during routine inspections.