What this relates to:
Near-misses or collisions of aircraft in the air. The Airborne Collision Avoidance System (ACAS) is intended to reduce this risk by supplying pilots with evasion instructions. The pilots are obliged to observe all messages immediately, even if the messages contradict the clearances or instructions from air traffic control.
Example:
On approach to the airport, a commercial aircraft receives an abort message with the order to climb immediately. The collision warning device has located a motorised aircraft performing aerobatics in the airspace beneath it.
Remarks on the 2024 figures:
The frequency of evasion instructions (TCAS RA) reported in Switzerland since 2022 has been higher than in previous years but remains stable from 2022 to 2024. There could be several, partially desirable, reasons for this increase. Two possible explanations:
- More transponders in use. The increased number of transponders in operation enables more precise detection and identification of other aircraft in the vicinity. Consequently, the system is able to respond with greater sensitivity to potential conflicts and therefore issues evasion instructions more frequently.
- Transponder Mandatory Zone (TMZ) north-eastern Switzerland. The introduction of the TMZ has led to more aircraft operating in the defined airspace with switched-on transponders. This has increased the visibility of aircraft and the likelihood of evasion instructions being issued.
