What happened
On June 3, 2015, a serious airprox incident occurred within the Zurich Terminal Control Area (TMA). A Cameron A-160 hot air balloon, registration HB-BYI, entered Class C airspace without obtaining prior clearance from air traffic control. During this period, an Avro RJ100, registration HB-IYW, operated by Swiss Global Air Lines AG, was conducting a radar-guided approach to Zurich Airport. The unauthorized presence of the balloon in the controlled zone resulted in a dangerous proximity between the two aircraft.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) established that the balloon pilot operated within the TMA without authorization, although the transponder remained active. The investigation examined the technical performance of ground-based warning systems, the visibility of VFR traffic on controller radar screens, and the communication protocols between the balloon pilot and air traffic services. The inquiry also reviewed the programming of the Short Term Conflict Alert (STCA) and the training standards for balloon pilots regarding airspace boundaries and altitude instrumentation.
Findings
Findings indicate that the primary cause of the incident was the unauthorized entry of the hot air balloon into Class C airspace without air traffic control authorization. This was compounded by the balloon pilot's insufficient awareness of the required separation minima within the TMA and a reluctance to contact air traffic services.
Several systemic contributing factors were identified:
- The ground-based STCA system was programmed to ignore targets with ground speeds below 30 knots, meaning it failed to trigger an alert for the slow-moving balloon.
- VFR traffic, including the balloon, appeared in a subtle light brown color on controller radar screens, making it difficult to detect unauthorized incursions.
- There was a lack of automated warning systems to alert controllers of unauthorized VFR entries into the TMA.
- A general deficit in knowledge regarding the proper use of altimeters and transponders was noted among some pilots.
Safety action
Following the incident, several safety recommendations were issued to the Federal Office of Civil Aviation (FOCA) and Skyguide. These include improving the STCA programming to include slow-moving aircraft, developing systems to better alert controllers to unauthorized VFR airspace incursions, and establishing periodic competency checks and further training for balloon pilots. Additionally, recommendations were made to simplify the airspace structure around major Swiss airports to reduce the risk of accidental incursions by light aircraft.