What happened
On 17 June 2000, an Agusta A109E Power, registration G-TVAA, was conducting an air ambulance mission near Arborfield Cross, Berkshire. While performing a gentle right turn at approximately 300 feet to locate a patient, the crew heard a loud bang followed by a sudden sensation of the aircraft sinking. The pilot attempted to arrest the descent by increasing collective control, but the aircraft failed to respond normally, and the main rotor speed increased to 105% NR.
Believing the aircraft had suffered a double engine failure, the pilot initiated an autorotation procedure. During the rapid 10-second descent, the pilot applied full collective control just before touchdown. The helicopter struck the ground heavily, causing the landing gear to collapse. The aircraft came to rest upright against a fence, with 3 minor injuries sustained by the crew and passengers.
The investigation
Investigators examined the engine data and found that the engines had shut down normally without any recorded faults. However, a detailed inspection of the main rotor head revealed that the rotating scissors linkage had detached due to a fractured bolt.
Technical analysis showed that the lower scissors link had been installed in the incorrect orientation during maintenance performed the previous day. This incorrect assembly caused the spherical bearing to reach the limit of its travel and caused the cup washer to contact the face of the link. This mechanical interference created a bending load on the swashplate bolt, leading to a fatigue failure. Furthermore, the investigation found that the bevelled washer was missing from its correct position, and the retaining nut was nearly thread-bound, which prevented the assembly from being tightened to the required torque.
Findings
- The primary cause of the loss of control was the fatigue failure of the swashplate attachment bolt.
- The failure was triggered by the incorrect installation of the lower scissors link, which was fitted back-to-front during recent maintenance.
- The incorrect orientation of the link caused mechanical interference and bending stresses on the bolt.
- The maintenance manual lacked sufficient detail to prevent incorrect orientation, and the design allowed for such an error to occur.
- Evidence suggested that several other helicopters had been manufactured with an incorrect component sequence, specifically regarding the placement of the bevelled washer.
Safety action
Following the investigation, several safety recommendations were issued to the manufacturer and airworthiness authorities. These included mandates to inspect the swashplate scissors linkage for correct assembly, to modify the attachment design to increase the thread margin and prevent thread-binding, and to amend the Maintenance Manual to provide unambiguous instructions regarding the orientation of the lower link.