No obvious preimpact mechanical anomalies were observed on any of the hydraulic system components. All main transmission gear/component damage was consistent with impact and/or overload. The main transmission anti-resonance isolation system mounts were examined on site, with no evidence of preimpact damage. The damage to the main transmission torque strut was consistent with impact and overload.
The engines were removed for detailed study. No evidence of preimpact failure was observed. In addition, the helicopter's gross weight fell within the flight envelope and below the helicopter's maximum certified gross weight at the time of the accident.
In the end, it seemed clear that the main rotor disc had moved from its normal plane, allowing the yellow blade to strike the tail.
Main Rotor Disc Divergence From Normal Plane of Rotation
Eurocopter conducted a study to determine what condition or event could have caused the main rotor disc to divert from its normal plane of rotation and strike the endplates and the tail-rotor driveshaft. The study started with an assumption that contact between the main rotor blades and the endplates or tail boom could only be possible when a pilot carries out an extreme aft cyclic input.
Eurocopter initially studied the possibility of an occurrence termed as an “Aggressive Pull Aft” maneuver during fast forward flight, where the pilot aggressively pulls the cyclic pitch from a forward position fully to the aft stop. In a simulation model, it was demonstrated that during an “Aggressive Pull Aft” maneuver alone, sufficient clearance to the endplates and tail boom existed. As a result, Eurocopter concluded that another flight maneuver must precede the “Aggressive Pull Aft” to explain the excessive flapping required that would result in the main rotor blade collision with the endplates and the tail boom.
To address this issue and through approved engineering and simulation models, Eurocopter provided the investigation team with two accident flight maneuver sequence scenarios. In each case, the maneuver sequence starts at the same cruise flight condition (fast, forward flight, high collective pitch and forward longitudinal cyclic).
Scenario No. 1: Push cyclic full remaining range forward and then pull full range of longitudinal cyclic control backward (cyclic push/pull maneuver). Eurocopter determined that the control sequence in scenario No. 1 resulted in an increased blade deflection when compared to the “Aggressive Pull Aft” maneuver but still showed enough clearance between the main rotor disc and the tail section of the helicopter.
Scenario No. 2: Sudden lowering of the collective to near the lower stop, followed by a simultaneous reaction of nearly full up collective and nearly full-aft cyclic (longitudinal).
“Eurocopter determined that the control sequence in scenario No. 2, based on known conditions at the time of the accident and the laws of physics and aerodynamic principles, was the only maneuver that would result in main rotor blade contact with the endplates and tail boom,” said investigators.