What happened
On 9 October 2006, an Agusta A109A helicopter, registration G-DNHI, was conducting a commercial passenger flight from Redhill Aerodrome to Biggin Hill Airport. During the cruise phase of the flight, a component of the left engine's outboard exhaust duct separated from the aircraft. The detached duct struck the tail rotor assembly, causing the tail rotor gearbox to detach from the helicopter.
Following the initial impact, the pilot experienced a significant yaw to the right. While the pilot managed to regain some directional control by reducing torque, a subsequent structural failure of the upper vertical stabiliser caused a violent pitching and yawing motion. The pilot was forced to enter an autorotative descent. Despite the challenging terrain, the pilot executed a successful forced landing in a field approximately 2 nm west of Biggin Hill. There were no injuries to the crew.
The investigation
Investigators examined the wreckage and found that the tail rotor assembly and gearbox were missing from the aircraft. The upper vertical stabiliser was severely displaced, and the lower vertical stabiliser had sustained damage consistent with being struck by the tail rotor blades.
Upon inspecting the left engine cowl, investigators located a failed 'Mormon' clamp. This two-piece clamp, used to secure the exhaust duct, had a crack in one of its strap sections. The investigation determined that the crack was caused by stress corrosion cracking. Because the crack was located beneath an overlapping section of the clamp, it was not detectable through standard visual or non-destructive testing while the clamp was installed.
Findings
- The primary cause of the accident was the failure of a clamp used to attach the left engine's outboard exhaust duct.
- The failure was due to stress corrosion cracking within the clamp's strap.
- The separation of the duct allowed it to strike the tail rotor, leading to the loss of the tail rotor gearbox and partial failure of the tailboom structure.
- The location of the crack prevented detection during routine maintenance inspections.