What happened
Following a routine landing on the vessel Star Coral, the AgustaWestland A109E, registration VH-XUM, remained stationary on the deck for several minutes without any initial signs of instability. During the subsequent takeoff attempt, the aircraft encountered sudden and violent vibrations. While the pilots' accounts of the specific sequence varied, the aircraft experienced significant vertical oscillations through the airframe. During this period of instability, the tail rotor separated after the helicopter made contact with the deck. The impact of the main rotor against the tail boom was a consequence of the instability rather than the initial cause. The aircraft was not airborne prior to the onset of the vibrations, though it may have briefly rebounded on the helidecent as the oscillations intensified.
The investigation
Investigators examined the wreckage and the site, finding no evidence that the main or tail rotors struck the ship's structure prior to the incident. The damage patterns observed on VH-XUM were consistent with previous A109E incidents involving ground resonance. The investigation focused on the transition from a stable state to the onset of vibration. It was noted that the helicopter was likely light on its oleos when the vibration began. A critical factor identified was the sudden lowering of the collective, which caused the helicopter to land firmly on the helideck. According to the FAA Helicopter Handbook, such an impact during an unbalanced rotor state can cause blades to move out of phase, leading to uncontrollable oscillations.
While the operator's inspection of the rotor and damping systems showed no obvious defects, technical or environmental factors could not be entirely dismissed. Furthermore, the investigation noted that the pilots' responses were uncoordinated; while one pilot attempted to lift the aircraft, the supervising pilot lowered the collective. Because the rotor speed was within the normal operating flight range, continuing the takeoff might have allowed the vibrations to dissipate.
Findings
- The helicopter likely experienced ground resonance during the takeoff phase.
- The sudden lowering of the collective intensified the instability by forcing the aircraft firmly onto the deck.
- The rapid escalation of the event left insufficient time for the crew to implement established emergency procedures or perform a formal transfer of control.