What happened
On January 22, 2016, an Enstrom 280C helicopter, registration HB-XLS, was undergoing pre-flight preparations for a training flight at Buttwil Airport. The aircraft was positioned on a snow-covered startup pad near a fuel station. During the execution of the pre-flight checklist, the student pilot noted that the Emergency Locator Transmitter (ELT) was missing from its usual location. The instructor left the aircraft to investigate the missing component, leaving the student to continue the checks.
As the student pilot increased the engine RPM to 2900 for a magneto check, the helicopter began to rotate left due to the lack of traction on the icy surface. In an attempt to stabilize the aircraft using the pedals, the tail rotor blades made contact with the left control cable. This contact caused the cable to snap, leading to a sudden and uncontrolled rotation of the aircraft. The student pilot managed to stop the rotation by reducing engine power after the helicopter had completed approximately four rotations and drifted toward a nearby meadow. No injuries were reported, though the aircraft sustained light damage.
The investigation
SUST examined the mechanical condition of the tail rotor assembly and the maintenance records of the aircraft. Investigators discovered that the left tail rotor control cable had severed at two points near a pulley. The investigation also scrutinized the maintenance history of HB-XLS, noting that while a recent 200-hour inspection had been performed, the specific tension measurements for the control cables were not recorded in the logs. Furthermore, the investigation found that the rubber bumpers on the static stop, which are intended to dampen movement, were in poor condition. The maintenance records for the aircraft were also found to be incomplete, making the traceability of certain components difficult.
Findings
- The primary cause of the accident was the loss of tail rotor control resulting from the tail rotor blades striking and severing the left control cable.
- Improper maintenance of the helicopter contributed to the failure, specifically through the poor condition of the static stop rubber bumpers and a pre-existing damage to the left control cable.
- It is highly probable that the control cables were under insufficient tension.
- The use of a snow-covered startup pad with reduced ground friction contributed to the initial rotation of the aircraft.