What happened
On March 17, 2002, a Robin R2100 A, registration F-GBAJ, was performing a local flight instruction mission at Montélimar aerodrome. During the landing sequence on runway 20, the aircraft touched down normally. However, as the pilot applied braking to decelerate, the aircraft began to veer to the right, accompanied by a gradual sinking of the right wing. Once the aircraft came to a complete stop, the pilot discovered that the right main landing gear had completely collapsed and folded toward the wingtip.
The investigation
The investigation focused on the structural integrity of the main landing gear housing and its attachment points to the wing spar. Investigators examined the gear housing, the attachment hardware, and the assembly of the components.
Physical examination of the gear housing revealed that three of the four attachments securing the housing to the spar had failed. The fourth attachment remained intact but showed signs of cracking. The investigation also scrutinized the welding of the spacers (bushings) to the housing. It was noted that the geometry of the housing prevented complete welding on the rear face, leaving unsealed sections susceptible to corrosion. Furthermore, the investigation reviewed the dimensions of the spacers and the specific bolts used in the assembly to determine if they met manufacturer specifications.
Findings
- The primary cause of the gear collapse was insufficient clamping force within the assembly, which allowed for movement between components.
- Three of the four bolts used in the assembly had a smooth portion under the head that was too long, preventing effective tightening of the assembly.
- Several spacers on the front face were out of tolerance, with thicknesses as low as 1.9 mm, whereas the required specification was 3.0 mm ± 0.5 mm.
- These dimensional discrepancies and improper bolt lengths allowed for bilateral forces (lateral movement) to act on the assembly.
- These forces led to the development of fatigue cracks originating from surface irregularities in the welds, such as weld beads and thickness variations.
- Significant corrosion was present within the fatigue cracks, indicating that the cracks had been propagating for a considerable amount of time prior to the accident.