What happened
On July 13, 2013, a Cabri G2 helicopter, registration HB-ZLJ, was conducting a training flight near Neuhaus, Switzerland. The flight, involving a flight instructor and a student, initially proceeded without issue through the Bernese Oberland. However, during a series of landing exercises, the crew heard a loud bang followed by a distinct smell. The instructor performed an immediate landing to inspect the engine compartment.
During the inspection, a mechanic joined the crew and identified a loose strobe light component and some singed material on the air filter. After a brief test flight to return to the student, the crew experienced a recurrence of the smell, followed by visible grey smoke emerging from the engine cowling. As the pilot attempted a precautionary landing, a rattling noise and heavy vibrations occurred, leading to a total engine failure. The pilot successfully executed an autorotation, landing in a grain field after striking a fence line. The occupants escaped the aircraft without injury, though the helicopter sustained heavy damage.
The investigation
SUST examined the engine compartment and the components of the cooling system. The investigation focused on the sequence of events following the initial mechanical noise and the subsequent smoke. Investigators analyzed the physical state of the cooling fan and the material properties of its components to determine why the engine failed during the second flight segment.
Findings
- The primary cause of the engine failure was the disintegration of the cooling system fan.
- The fan's front disc failed due to material fatigue.
- The design of the cooling fan was inadequate for the operational stresses encountered.
- The front disc exhibited insufficient lateral stiffness and material inconsistencies, including brittleness and a wall thickness that was up to 25% below specifications in certain areas.
Safety action
Following the investigation, safety recommendations were issued to the European Union Aviation Safety Agency (EASA) to ensure the manufacturer, Hélicoptères Guimbal, implemented a fleet-wide inspection program and took necessary measures to prevent future fan disintegrations.