What happened
On April 3, 2008, a Schweizer Aircraft Corp. 333 helicopter, registration HB-XBQ, was conducting a flight training mission near Thalheim an der Thur. The flight, involving a flight instructor and a student pilot, was intended to include training for a potential change in aircraft ownership. The day's program included several successful autorotation and tail rotor failure simulations.
During the training, the instructor selected a field for a demonstration of a landing with a simulated tail rotor failure. The crew identified the area as a meadow; however, the site was actually a field of winter wheat. As the instructor performed the maneuver, the helicopter's nose drifted left due to the lack of tail rotor assistance. While the instructor managed to realign the aircraft using collective and cyclic controls, the landing gear struck the ground at a point where the left skid made contact first. The soft, dough-like surface of the wet field caused the skid to sink and break, leading to a forward pitch that caused the main rotor blades to strike the ground.
The investigation
Investigators examined the wreckage and the site conditions. A material analysis of the broken left skid revealed a ductile fracture, indicating the failure was caused by a sudden, intense load. The investigation confirmed that the aircraft's weight and center of gravity were within limits and that there were no pre-existing mechanical defects. The weather at the time was characterized by light rain and low visibility due to clouds, with a damp environment contributing to the soft state of the soil.
Findings
- The primary cause of the accident was the loss of control during an emergency landing exercise due to landing on unsuitable terrain.
- The landing site, which appeared to be a meadow from a distance, was actually a wet, soft agricultural field.
- The accident was contributed to by the fact that the crew did not perform a prior reconnaissance of the landing area's ground conditions.
- No technical malfunctions or health issues involving the crew were identified as contributing factors.