What happened
On March 19, 2003, an Enstrom 480 helicopter, registration HB-XJQ, was conducting a VFR training flight near Buttwil, Switzerland. The flight, involving a student pilot and an instructor, included various maneuvers such as pedal turns, hover exercises, and several autorotations. During a climb to 3,0-foot altitude, the instructor ordered a sudden autorotation maneuver.
Shortly after initiating the climb, the aircraft experienced intense vibrations and a loud noise, followed by a complete loss of power and a significant drop in rotor RPM. The aircraft began to yaw to the left. Despite the instructor taking control and attempting to manage the descent, the rotor RPM continued to decay. To avoid trees, the pilot had to execute a 90-degree course change. The helicopter eventually struck uneven, rising terrain, causing the main rotor to strike the tail boom. Both occupants survived the impact without injury.
The investigation
SUST investigators examined the wreckage and the drive transmission system. They discovered that the drive belt, which transfers power from the engine shaft to the main gearbox, had jumped off its pulleys and was damaged.
Detailed disassembly revealed that the H-strut, which maintains the correct distance and alignment between the upper and lower pulleys, had fractured. Additionally, the drag links responsible for the alignment of the lower pulley had torn away. Metallographic analysis of the broken H-strut showed that cracks had originated at the weld seam. The investigation found that the welding process had created a heat-affected zone (HAZ) with a martensitic structure, making the material brittle and prone to cracking.
Findings
- The primary cause of the accident was the inability to perform a normal autorotation because the main rotor was being braked by the damaged drive system.
- The failure of the H-strut caused the pulleys to become misaligned, forcing the drive belt to deflect forward where it struck the gearbox housing, creating significant drag.
- The design of the rotor brake/freewheel mechanism on this turbine-powered model was a contributing factor; because the freewheel is located at the engine output, it could not decouple the rotor from the engine when the damage occurred downstream in the transmission.
- The fracture in the H-strut was likely due to improper material treatment during manufacturing, specifically hardening within the heat-affected zone of the weld.
Safety action
Following the investigation, the manufacturer issued a service directive (Bulletin No. T-018) establishing a detailed procedure for inspecting the H-strut welds every 100 flight hours. SUST also recommended that the Swiss Federal Office of Civil Aviation (FOCA) engage with the type certificate holder to review the arrangement of the rotor freewheel to ensure that a functional autorotation is possible even in the event of drive system damage.