What happened
On September 29, 2018, at approximately 12:23 PM, an Agusta AW10CO9SP helicopter, registration HB-ZRT, was performing a medical evacuation flight toward Jungfraujoch. The helicopter had recently departed from Interlaten Heliport and was climbing at an altitude of roughly 30 meters above the ground. Simultaneously, a Cessna 172P, registration HB-CIE, was conducting a flight training maneuver near the former military airfield at Interlaken. The training flight involved practicing an engine-failure glide towards Ringgenberg.
As the helicopter climbed northeast, the pilot suddenly observed the high-wing aircraft approaching in the opposite direction in a landing configuration. The pilot immediately executed a right-hand evasive maneuver to avoid a collision. At the point of closest approach, the two aircraft were separated by a horizontal distance of approximately 100 meters at nearly the same altitude.
The investigation
The investigation by the SUST focused on why collision avoidance systems failed to alert the crews. The investigation was delayed, as the incident was not reported to the authorities until six days later, resulting in the loss of critical data from the onboard collision warning systems.
Investigators examined the operational status of the electronic safety equipment on both aircraft. On the HB-ZRT, the Traffic Advisory System (TAS) and Floice collision warning system did not provide an alert, and the aircraft's navigation display did not show the approaching plane. On the HB-CIE, while the pilot and instructor saw the helicopter, the onboard Flarm system failed to issue a warning. Furthermore, the investigation found that the training aircraft was monitoring a different radio frequency (Meiringen CTR) rather than the Interlaken frequency, which prevented the crew from hearing the helicopter's initial departure announcement.
Findings
Several contributing factors led to the near-collision:
- Inoperative safety equipment: It is highly probable that the Flarm collision warning system on the HB-CIE was not switched on during the flight.
- Lack of situational awareness: The training crew was monitoring a frequency for a different controlled airspace, meaning they did not receive the helicopter's departure notification.
- Technical limitations of TAS: The helicopter's traffic advisory system failed to detect the HB-CIE because the encounter occurred below radar coverage and the training aircraft lacked ADS-B out capability, which is required for TAS to identify traffic in non-radar environments.
- Delayed reporting: The failure to report the incident immediately prevented investigators from reviewing the recorded data from the collision warning systems.
Safety action
Following the incident, the flight group operating the HB-CIE updated its pre-flight checklists to explicitly include the activation of the Flarm system and landing lights. Additionally, these items were integrated into the group's annual training courses.