What happened
On 4 December 2008, an EC135 T1 helicopter, registration G-CCAU, was performing a routine aerial photography mission near Hindlip Hall, Worcestershire. While flying toward the West Mercia Police Headquarters, the crew experienced a significant increase in vibration levels throughout the aircraft's speed range. As the helicopter approached its landing site, a loud impact occurred, followed by heavy, repetitive vibrations at the same frequency as the main rotor RPM. The pilot performed an emergency shutdown and landed the aircraft safely at Hindlip Hall. Upon inspection, the crew discovered that one of the rotating swash plate scissor link assemblies had become detached from the swash plate, causing the component to strike the engine and gearbox cowlings.
The investigation
The AAIB investigation focused on recent maintenance performed on the aircraft. It was established that the aircraft had undergone repairs to a different scissor link assembly (Scissor link B) just days prior. During this process, the first assembly (Scissor link A) had been removed to serve as a comparison for the replacement bushings. Due to a delay in receiving parts, Scissor link A was partially reinstalled but not fully secured.
Investigators found that the maintenance staff relied on a verbal handover rather than a documented one when transitioning responsibility between engineers. Furthermore, the work pack did not clearly identify which specific assembly had been disturbed. The investigation also noted that the nut used to secure the link was a previously used component, which lacked the visual cues of a new nut that might have alerted staff to an incomplete installation. The inspection process, including duplicate inspections and 'Check A' inspections, failed to detect that the split pin had not been installed and the nut had not been properly torqued.
Findings
- The primary cause of the incident was the incomplete reattachment of the scissor link assembly to the rotating swash plate.
- A lack of documented handover between engineers led to a misunderising of the aircraft's maintenance status.
- The maintenance work pack failed to clearly distinguish between the two different scissor link assemblies involved in the repair.
- The use of a previously installed nut made it difficult to identify that the installation was incomplete.
- High workloads and the use of supervisors to perform certifying duties introduced distractions during the inspection process.