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Analysis

Summary

The circumstances of the accident are consistent with a loss of control due to insufficient main rotor RPM being maintained, and incompatible control inputs from the instructor and the student following the initiation of the simulated engine failure by the instructor. The reported actions by the instructor indicate that he was attempting to recover the situation and allowing the student to follow him through on the controls. The student also recalled attempting to manipulate the helicopter's controls during the descent. It was unlikely that the instructor could have maintained effective control of the helicopter with both pilots manipulating the controls. Procedures for clarifying who is in control at all times, should be established and followed.

The helicopter manufacturer warned that to recover lost main rotor RPM, the pilot must immediately roll on throttle and lower the collective simultaneously. Both pilots reported that they could not lower the collective to the full down position. The activation of the low rotor RPM warning horn during most of the descent confirms that the collective was seldom in the full down position. The instructor reported attempting to increase the throttle position, but it felt like the student had frozen on the throttle. There were no defects found in the examination of the helicopter that would have explained why the collective was not able to be lowered to the full down position or the throttle increased. The manufacturer cautions that once the main rotor RPM decreases below 80%, pilots may not be able to recover control even if the flight controls are correctly positioned. Both pilots recalled seeing the rotor RPM needle in the vicinity of 80% during the descent. The student's recollection suggested that the rotor RPM may have reduced to below 80%.

The investigation was unable to resolve the differences between the statements by the instructor and the student with reference to the way in which the throttle was reduced.

The seat structures are designed to deform during a high G vertical impact, reducing the load transmitted to the seat occupant and increasing survivability. However, in deforming, the seat structure loses significant strength. In this case, the seat structure lost sufficient strength to allow the left anchor point of the left seat lap belt to tear free, increasing the risk of injury to the seat occupant.

 
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