History of Flight
At 1600 hours on 14 March 2004, a private Agusta A109E helicopter, registration G-HIMJ, departed from Colney Park Heliport at Leeds Bradford Airport. Shortly after takeoff, at approximately 300 feet, the pilot heard a whining noise followed by a bang. All indications were normal and control was maintained, so the pilot elected to land immediately. During the flare and collective application, the fuselage began to spin about the rotor axis. Upon landing, the helicopter remained upright but rotated two or three times before stopping. The tail rotor and its control system were intact but the drive system was non-functional.
Post-Accident Examination
Examination revealed that the forward section of the tail rotor drive shaft had failed about 6 inches aft of the rotor brake. The No. 1 engine output drive shaft had contacted the adjacent bulkhead. Severe heat damage and cracking were found on the titanium alloy tail rotor shaft tunnel at the junction of its tapered front section and parallel-sided mid section. The lower attachment of the rear left brace assembly securing the main rotor gearbox to the structure had failed due to fracture of all four bolts connecting the steel attachment bracket to the aircraft. Fatigue was present in all four bolts, with two having fatigue across at least 95% of their cross-sections and two across 65%. The corresponding bolts on the rear right brace assembly showed low torque values: one required 51 lbf·in to initiate rotation, while the others required less than 40 lbf·in.
Airworthiness Follow-Up
After the accident, the manufacturer issued a Bolletino requiring immediate inspection of the brace assembly attachment bracket bolts. Within the first five operating hours, no failed bolts were reported, but within 25 hours, 12 loose bolts were identified on five aircraft. No pattern in build sequences or service use was determined. The manufacturer had previously been informed of a similar bolt failure on another A109E, but that was assumed an isolated event.
Design History
The 109 Series originally used aluminium alloy lower attachment brackets. Several years prior, a fatigue failure of such a bracket occurred in flight, but the aircraft remained controllable and tail rotor drive was unaffected. As a result, the brackets were replaced with steel alloy components of similar geometry, which are less susceptible to fatigue. The rear left bracket normally carries the highest peak load among the four lower brackets.
Detailed Findings
The tail rotor drive shaft failure exhibited overheating and rotational scoring. The fracture face had a fibrous appearance typical of light alloy components that failed under load after exposure to fire or severe overheating. Internal paint showed heat discoloration, and torsional deformation occurred while the shaft was hot. The bolts from the rear left brace showed fatigue striations indicating fewer cycles than the total flight cycles of the aircraft. All bolts were within dimensional tolerances.