What happened
On the afternoon of January 16, 2024, a Bell 429 helicopter, registered as HB-ZAP, was performing a commercial flight returning to Sion Airport (LSGS). The aircraft, carrying a pilot and one passenger, entered the Sion control zone and followed a flight path toward the southeast sector of the airfield. The pilot executed a dynamic approach, passing over the Final Approach and Take-off Area (FATO) at approximately 25 knots before transitioning into a right turn toward Helipad 3.
At the same time, a Guimbal Cabri G2 was positioned on the adjacent Helipad 4. The pilot of the Cabri G2 was preparing for departure, with the main rotor having just begun to rotate and the anti-collision lights activated. As the HB-ZAP was completing its landing at Helipad 3, the intense downwash generated by the descending aircraft impacted the stationary helicopter. This aerodynamic force caused one of the Cabri G2's rotor blades to strike its own tail boom, resulting in structural damage to the tail boom and a rotor blade. There were no injuries to the crew or passengers, and no third-party injuries were reported.
The investigation
The investigation examined the flight paths, the proximity of the two helipads, and the operational procedures in place at the time. Investigators noted that the helipads in the southeast sector of Sion Airport are positioned relatively close to one another. The inquiry also reviewed the lack of specific Standard Operating Procedures (SOPs) from the operators regarding approaches to these specific helipads, as well as the airport's existing regulations, which prohibit simultaneous hovering on the same helipad but do not specifically regulate the approach via the FATO to adjacent pads.
Findings
- The primary cause of the incident was the increased downwash produced by the Bell 429 during its transition from forward flight to a hover.
- Because the main rotor of the Guimbal Cabri G2 was at a low rotational speed during its startup phase, it was highly susceptible to the aerodynamic disturbance caused by the arriving aircraft.
- The proximity of the helipads at Sion Airport created a high-risk environment for downwash-related incidents.
- The pilot of the arriving aircraft did not sufficiently monitor the stationary helicopter to adjust the approach, such as by utilizing a slower hover taxi instead of a dynamic approach.
- There was a lack of coordination via a common radio frequency between the two aircraft operations.
Safety action
To mitigate the latent danger of downwash in crowded helipad areas, the following measures are recommended:
- Pilots performing approaches should actively monitor stationary aircraft on the ground and adjust their flight path—for example, by terminating the approach over the FATO and transitioning to a slow hover taxi—if a conflict is detected.
- Improved coordination between aircraft using the same area should be maintained through communication on a shared radio frequency.
- Operators should implement Standard Operating Procedures (SOPs) to manage arrivals and departures in close proximity to avoid similar occurrences.