What happened
On 7 May 2003, a Schweizer 269C-1 helicopter, registration G-ODNH, was conducting a training flight near Pateley Bridge, Yorkshire. While climbing through approximately 1,500 feet, the pilot experienced sudden and extreme vibrations that were felt through both the structure and the collective lever. Fearing a component separation and a total loss of control, the pilot immediately initiated an autorotative landing.
The aircraft performed a gentle engine-off landing in a field of long grass. While the landing itself was successful and resulted in no injuries to the two crew members, the aircraft sustained significant damage. The vibration caused the failure of the attachment lugs at the forward end of the tail-boom, causing the boom to droop and the tail-rotor drive shaft to disengage. Additionally, the horizontal stabiliser's leading-edge skin was fractured.
The investigation
Investigators examined the fracture of the tail rotor fork bolt, which is responsible for the teetering motion of the rotor. Scanning Electron Microscope (SEM) analysis revealed that the bolt had failed due to fatigue cracking. Evidence suggested the bolt had been rotating within the hub assembly, as indicated by fretting marks and a wear band corresponding to approximately 60 degrees of rotation.
Further examination of the hub assembly revealed that the bolt was extremely tight within the threaded insert. Investigators noted that the contact surfaces were coated in a reddish-brown powder, which was identified as wear debris from the teflon-lined bearings. The investigation also looked into the maintenance history, noting that the bolt had been installed using a torque-tightening method. While this was an approved method at the time, the investigation found that the bolt may not have been fully seated due to high friction during installation, which could have prevented the correct tension from being established.
Findings
- The primary cause of the incident was the fatigue failure of the tail rotor fork bolt.
- The failure occurred because the bolt was operating without the necessary pre-load, likely because it was not properly seated during the last maintenance installation.
- High friction within the assembly, possibly caused by residual teflon dust in the threaded insert, likely masked the fact that the bolt head had not reached its proper position during torque-tightening.
- A required 25-hour torque check was not performed because the instruction for this check was located in a separate section of the maintenance manual rather than the primary re-assembly instructions, leading to an oversight by the technician.