What happened
On August 18, 2014, at approximately 08:50 local time, an MD 500E helicopter, registration HB-ZKD, was conducting a private flight training session near Maschwanden/Rain, Switzerland. The flight was performing practice autorotations and power recovery maneuvers. During the seventh autorotation attempt, at an altitude of roughly 900 feet, the flight instructor attempted to reduce the engine speed to idle using the collective twist grip. This action resulted in the unintentional shutdown of the turbine.
Upon noticing the drop in turbine speed and the activation of the ENG-OUT1 warning, the instructor realized the engine had ceased operation. At approximately 500 feet above the ground, the instructor decided to continue the autorotation due to the low altitude. The aircraft landed without further incident, though the crew sustained significant injuries.
The investigation
SUST examined the mechanical components of the aircraft's collective pitch control. The investigation focused on the design of the twist grip on the MD 500E collective lever, which includes a mechanical lock intended to prevent the transition from the idle position to the cut-off position. This locking mechanism is located on the left side of the collective and is designed to be operated only by the pilot in the left seat.
Following the event, a visual inspection of the aircraft was conducted. The investigation established that the safety pin of the unlocking system on the collective had become dislodged.
Findings
- The primary cause of the engine shutdown was the dislodging of the safety pin within the collective's unlocking mechanism.
- This mechanical failure allowed the twist grip to move past the idle position into the cut-off position, which is normally prevented by the safety lock.
- The flight instructor's attempt to reduce engine speed directly led to the engine flameout because the mechanical protection was compromised.