What happened
On 28 November 2007, a Britten-Norman Islander BN-2A-26, registration EI-BCE, was being moved across the ramp at Connemara Airport in Co. Galway. The aircraft was being towed by a tug when the pilot attempted to release the brakes. During this process, the right-hand brake pedal disconnected from the assembly. There were no passengers on board at the time, and the incident resulted in no injuries and only minor damage to the aircraft.
The investigation
The investigation focused on the structural integrity of the brake pedal pillar. Technical records showed the aircraft had a very high usage history, having completed over 81,000 landings and 17,155 flight hours since its manufacture in 1976. Because the pedal mounting block obscured the pillar from view, a physical disassembly was required to examine the component.
A metallurgical analysis was performed on both the failed right-hand pillar and the intact left-hand pillar. The laboratory examination identified that the failure originated at a bolt hole within the pillar. Notably, the researchers found machining burrs—rough edges left from the manufacturing process—around the bolt holes of the failed component.
Findings
- The primary cause of the failure was fatigue cracking within the brake pedal pillar.
- The high number of landings and subsequent brake applications on this specific airframe created an extreme number of stress cycles.
- The presence of manufacturing burrs likely acted as a stress concentrator, helping to initiate the cracks.
- The manufacturer's review of the incident led to the issuance of a Service Bulletin (SB BN-2/SB 314), which mandates borescope inspections of the shaft assemblies for all operators.
- To prevent future occurrences, the manufacturer has increased the thickness of the tube for all new and replacement shafts to improve fatigue life and static strength.