What happened
On February 26, 2015, an AgustaWestland AW109SP, registration HB-ZRV, was performing a landing approach at the Rega base in Erstfeld, Switzerland. During the final approach, the helicopter was flying at a high rate of descent with minimal horizontal speed. While at a low altitude, the aircraft entered a vortex state, leading to a loss of control over the descent rate. This resulted in the helicopter colliding with the terrain.
The investigation
Investigators examined the flight data, including the Electronic Flight Instrument System (EFIS) and Data Acquisition Unit (DAU), to reconstruct the flight path. The analysis focused on the aircraft's attitude, vertical speed, and the environmental conditions during the approach. The investigation also reviewed the performance of the aircraft' and the effectiveness of the onboard safety systems, including the crash-attenuating seats and the emergency locator transmitter (ELT).
Findings
The pilot failed to realize that the descent rate was too high relative to the low horizontal speed during the final approach. Additionally, the pilot detected the development of the vortex state too late to prevent the collision. Other contributing factors included the pilot's limited experience with this specific aircraft type and an inappropriate approach tactic given the prevailing wind conditions.
While the impact forces were within a range that should have been survivable, two crew members and one passenger sustained severe spinal injuries. The investigation found that while the crash-attenuating seats met certification standards, the actual dynamics of this specific impact differed significantly from the scenarios used during testing. Furthermore, the aircraft's emergency locator transmitter failed to function because all six of its acceleration sensors were defective.
Safety action
Following the investigation, several safety recommendations were identified:
- Authorities should implement acoustic warnings to alert helicopter crews to an impending or developing vortex state near the ground.
- Certification testing procedures for crash-attenuating seats should be reviewed to ensure they reflect real-world impact conditions.
- Measures should be taken to ensure the continued functionality of the ARTEX C406-N HM emergency locator transmitter following an accident.