Factual Information
On 3 June 2015, a serious incident (airprox) occurred between an Avro RJ100, registration HB-IYW, operated by Swiss Global Air Lines AG as flight SWR1193, and a Cameron A-160 hot air balloon, registration HB-BYI, in the Zurich Terminal Control Area (TMA LSZH 11) near Wigoltingen, Canton Thurgau, Switzerland. The airliner was under radar guidance during its approach to Zurich Airport when the balloon entered Class C airspace without authorization from air traffic control, resulting in a dangerous proximity.
Causes
The investigation by the Swiss Transportation Safety Investigation Board (STSB) determined that the serious incident was caused by the hot air balloon entering Class C airspace without ATC authorization, leading to a dangerous proximity with the controlled airliner. A causal factor was the balloon pilot's insufficient awareness of the minimum separation required from aircraft operating in the TMA.
Contributing Factors
Several factors played a role in the incident:
- The balloon pilot's reluctance to contact air safety services.
- Discrete presentation of VFR traffic on air traffic controllers' radar when aircraft crossed TMA boundaries without authorization.
- Inadequate programming of the Short Term Conflict Alert (STCA) system.
Systemic Issues (Not Causal but Identified as Safety Risks)
The investigation also identified systemic factors that did not play a role in this incident but posed safety risks:
- General knowledge deficits among balloon pilots regarding the use of altimeters and transponders.
- Radio communication system configuration prevented approach controllers from using the 122.250 MHz frequency dedicated to balloons.
- Restrictive transponder requirements for aircraft operating below the TMA.
- Airspace organization and vertical limits around Zurich Airport could allow dangerous situations to develop from relatively minor errors.
Conclusion
The STSB issued safety recommendations based on these findings. The investigation report number is 2294.