Introduction
On 26 February 2015, an AgustaWestland AW109SP helicopter, registration HB-ZRV, operated by Rega, sustained an accident at the Rega base in Erstfeld, canton Uri, Switzerland. The helicopter was conducting an approach when it entered a vortex ring state at low altitude, leading to loss of control and collision with terrain.
Accident Sequence
The helicopter was on a final approach with a steep descent angle, high vertical speed, and low horizontal speed. During this approach, the helicopter entered a vortex ring state. The pilot did not recognize that the descent rate was too high relative to the low horizontal speed, and the pilot detected the developing vortex situation too late. Consequently, control of the descent rate was lost, and the helicopter collided with the ground.
Causes
The accident was caused by an entry into vortex ring state at low height above ground during a steep approach with high vertical speed and minimal horizontal speed. This resulted in loss of control of the descent rate and subsequent collision with terrain.
Causal Factors
The following factors played a causal role: The pilot did not realize that the descent rate was too high compared to the low horizontal speed during final approach; the pilot detected the development of a vortex situation too late.
Contributing Factors
Contributing factors included: low aeronautical experience of the pilot on the specific helicopter type; an inappropriate approach tactic relative to the wind conditions; and insufficient knowledge of a critical essential value.
Risk Factors
The investigation identified two risk factors that did not contribute to the accident but were noted: test procedures for the crash seats did not correspond to reality; and the functioning capacity of components used for the type of emergency locator transmitter was unsatisfactory.