What happened
On 3 May 2013, a Short SC7 Skyvan 3, registration G-BEOL, experienced a failure of its nose landing gear during landing at Weston-on-the-Green, Oxfordshire. The incident involved the detachment of components from the nose landing gear assembly.
The investigation
Following the occurrence, the manufacturer conducted a forensic examination of the nose landing gear components. The investigation focused on the oleo's sliding tube, which is a hollow component featuring an internal screw thread at its base to secure the nose wheel fork assembly. The examination revealed a fracture located near the top of the internal thread, specifically around the undercut area.
Investigators determined that the failure was likely caused by a fatigue crack propagating from a machining feature within the thread undercut surface. This feature may have been created during the manufacturing process due to the specific dimensions and tolerances defined in the engineering drawings, which allowed the thread-cutting tool to leave a mark on the surface. Additionally, the presence of cracks in certain thread roots suggests that fatigue cracks initiating from those features could also be a contributing factor.
Findings
- The primary cause of the component detachment was a fracture in the sliding tube of the oleo.
- The propagation of a fatigue crack from a machining feature in the thread undercut is the most probable mechanism of failure.
- Manufacturing tolerances for the undercut diameter may have permitted the thread-cutting tool to leave a stress-raising mark on the surface.