What happened
On June 19, 2019, a Dassault Falcon 7X, registration HB-JST, was conducting a descent toward Buochs Airport (LSZC) under Instrument Flight Rules (IFR). The aircraft was cleared to an altitude of 6,000 ft AMSL. While operating in Class E airspace, the aircraft entered an area where uncontrolled Visual Flight Rules (VFR) traffic was expected.
Simultaneously, a formation of three Pilatus PC-7 military training aircraft was conducting an air policing mission near Langenthal. The mission, operating under 'Loose Advisory Control,' involved simulating the interception of an unidentified target. One PC-7 acted as the target, while the other two functioned as the intercepting 'fighters.' The formation was flying at approximately 7,000 ft AMSL.
As the aircraft paths approached one another, the civil air traffic controller identified the military formation via their transponders and provided traffic information to the crew of HB-JST. This allowed the crew to visually acquire the training aircraft. Shortly thereafter, the Traffic Alert and Collision Avoidance System (TCAS) on the Falcon 7X issued a warning and a resolution advisory, commanding the pilot to descend. The aircrafts eventually crossed paths with a vertical separation of approximately 1,100 ft.
The investigation
The investigation focused on the interaction between IFR and VFR traffic within Class E airspace. Investigators examined the roles of the tactical fighter controller (TFC) and the responsibilities of the flight crews under the 'Loose Advisory Control' level of control. The investigation also reviewed the effectiveness of the transponders and the TCAS 4 system in providing situational awareness to both the civil and military operators.