What happened
On May 19, 2011, an Eurocopter AS 350 B2, registration HB-ZAM, was conducting a training flight near Niederösch, Switzerland. The crew, consisting of a pilot, an instructor, and an observer, was performing a series of maneuvers to prepare for an upcoming annual check flight. The specific objective of the flight phase was to practice procedures for a flight without hydraulic assistance.
After selecting a landing site in a mown field, the instructor initiated the hydraulic-off exercise by activating the hydraulic test switch and then deactivating the hydraulic system via the main cut-off switch on the collective. During the approach, the aircraft entered a short, turning final approach. As the aircraft's forward airspeed dropped significantly at an altitude of approximately 10 to 15 meters, the aircraft began moving backward and rotating on its vertical axis. Although the instructor attempted to take control, the aircraft could not be stabilized. The helicopter eventually touched down on its right skid with a lateral sliding motion, causing the aircraft to tip over. There were no injuries to the three occupants.
The investigation
SUST examined the flight path using GPS data and analyzed the crew's statements and the aircraft's technical records. The investigation focused on the execution of the hydraulic-off procedure and the flight dynamics during the final approach. Investigators reviewed the aircraft's maintenance history and the flight manual (FLM) requirements for such maneuvers. The investigation also looked into the sequence of switch manipulations, specifically whether the hydraulic test switch was correctly handled according to the manual, as initial crew statements regarding this sequence were contradictory.
Findings
- The primary cause of the accident was a loss of control during a training approach without hydraulic assistance, resulting in a lateral touchdown and subsequent tipping.
- The final approach was characterized by a short, turning path with insufficient forward airspeed.
- The instructor's takeover of the controls occurred too late to prevent the unstable flight condition.
- The crew failed to discuss a coordinated emergency plan for a potential loss of control during the pre-flight briefing.
- The possibility of re-engaging the hydraulic system to prevent a loss of control was not utilized.
- There were no evidence of pre-existing technical defects or medical issues affecting the crew.