What happened
On May 4, 2016, a serious near-collision occurred within the Dübendorf (LSMD) control zone involving two helicopters. The first aircraft, a Swiss Air Force AS332 Super Puma, registration T-320, was conducting a military instrument flight (IFR) approach to runway 29. Simultaneously, a civilian EC 120 B helicopter, registration HB-ZHD, was operating under visual flight rules (VFR) for a training flight, transiting the control zone.
As the T-320 descended to its assigned altitude of 4,000 feet, it encountered the HB-ZHD on a collision course. Although air traffic controllers issued traffic advisories to the military crew and instructed the civilian helicopter to descend to 3,000 feet, the aircraft still experienced a dangerous close proximity. At the point of closest approach, the horizontal separation was only 0.2 NM, with a vertical separation of just 125 ft.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined radar data, radio communications, and flight recorder information from the T-320, as well as Flarm data from the HB-ZHD. The investigation focused on the coordination between the approach control and the aerodrome traffic control, as well as the visibility of the aircraft to one another.
Investigators found that while the HB-ZHD was detectable on the T-320's traffic advisory system via its transponder, the T-320 was not visible to the HB-ZHD's Flarm device because the military aircraft was not equipped with such technology. Furthermore, the civilian crew struggled to identify the military helicopter due to its camouflage paint scheme.
Findings
- The primary cause of the incident was that the required separation between the military IFR helicopter and the civilian VFR helicopter was implemented too late due to a false assumption regarding the flight path of the T-320.
- Air traffic controllers incorrectly assumed the T-320 would continue flying toward a specific waypoint rather than initiating the curved approach procedure.
- A contributing factor was the high workload of the aerodrome traffic controller, who was managing the position alone while simultaneously handling tasks related to an event at the airfield and managing traffic changes from Zurich Airport.
- The military crew's reaction time was impacted by the difficulty of estimating the trajectory of an aircraft approaching at a steep angle.