Accident Details
On 17 June 2000 at 0833 UTC, an Agusta A109E Power helicopter, registration G-TVAA, was operating an air ambulance flight from White Waltham Airfield to Arborfield Cross, Berkshire. The helicopter was carrying one pilot and two passengers (crew and passengers all sustained minor injuries). The helicopter had departed at 0829 UTC and was approaching the pickup point at about 300 feet above ground level, speed between 60 and 80 knots, in a gentle right turn. Occupants heard a loud bang from the upper rear of the cabin and felt a sudden sink. The commander applied collective control but the helicopter did not respond; main rotor speed increased to 105% NR. The commander entered autorotation, lowered the landing gear, and issued a Mayday call. The helicopter touched down heavily with low forward speed; the landing gear collapsed and the helicopter came to rest upright with its nose against a fence. The three occupants evacuated unaided.
Investigation Findings
Examination of the main rotor head revealed that the rotating scissors linkage had become detached due to a failure of the bolt attaching the lower link to the rotating swashplate. This loss of connection caused the swashplate to lag behind the rotor head, reducing rotor blade pitch angles and resulting in sudden loss of lift. The lower scissors link had been installed back to front, restricting the spherical bearing's range of movement. The incorrect installation led to contact and distortion of components, culminating in bolt fracture.
Maintenance Background
The helicopter was manufactured in 1999 and had accumulated 271 flying hours. On 16 June 2000, the day before the accident and 3 hours 10 minutes flying time earlier, the lower scissors link was replaced during an 'A' check due to excessive play in the hinge bearing. The removal was difficult, causing damage to the link bearing, and a new lower link was installed. This was the first disturbance of the scissors linkage since manufacture. The Maintenance Manual diagram lacked sufficient detail to identify correct orientation, and the written instructions contained an error referencing the wrong part number for a washer. The design of the 109E Power scissors linkage is unique; earlier versions have an asymmetric lower link that cannot be installed incorrectly.
Conclusions
The accident resulted from incorrect assembly of the swash plate scissors link, leading to bolt fracture and loss of control. The maintenance manual deficiencies contributed to the incorrect installation.