What happened
On August 11, 2011, a private Mooney M20J, registration HB-DHI, was conducting a VFR flight near Arni at approximately 6,500 feet. The pilot intended to transit through the Emmen TMA Sector 5. Simultaneously, a Swiss Air Force Eurocopter AS53 and2 (registration T332) was performing a radar-monitored ILS approach to runway 22 at Emmen airfield.
Due to ongoing instrument approaches, the air traffic controller denied the HB-DHI entry into the Emmen TMA. The pilot of the Mooney failed to understand the instruction to remain outside controlled airspace and descend. Consequently, the aircraft continued on its original course and altitude. This led to an initial close approach between the two aircraft. Shortly after, the aircraft entered the Emmen local control zone. Despite a second instruction to descend, the pilot did not comply. During this time, the T332 maneuvered toward the glidepath, resulting in a second encounter. The aircraft crossed paths with a lateral separation of only 0.5 NM and a vertical separation of just 100 ft, qualifying the event as an ICAO Category A airprox.
The investigation
The SUST investigation examined the actions of the flight crews and air traffic controllers, as well as the technical capabilities of the aircraft and the ground-based surveillance systems. The inquiry focused on why the unauthorized entry occurred and why the controllers were unable to prevent the near-collision.
Findings
- The primary cause was the uncoordinated entry of a civil aircraft into controlled airspace without authorization, driven by the pilot's misunderstanding of the procedures for obtaining entry clearance.
- The military air traffic control center lacked a Short Term Conflict Alert (STCA) system, which would have provided a critical safety net.
- The Eurocopter AS532 was not equipped with a traffic warning or collision avoidance system, a gap permitted by specific exemptions for state aircraft.
- Contributing factors included poor coordination between air traffic control sectors, inadequate monitoring of the aircraft, and the use of complex or difficult-to-understand instructions by the controller.
Safety action
- Safety Recommendation 473: The Federal Office of Civil Aviation (FOCA), in coordination with skyguide, should ensure that military airfields are equipped with ground-based conflict warning systems.
- Safety Recommendation 474: FOCA and the Air Force should ensure that military aircraft operating primarily in civilian airspace are equipped with collision warning systems compatible with civilian standards.