What happened
On July 16, 2016, at approximately 17:15 UTC, a Bombardier CL30 business jet, registered as OE-HOO, was on an instrument flight rules (IFR) approach to Zurich Airport (LSZH) following a ferry flight from Krakow. While descending through a cloud layer at roughly 8,000 ft, the flight crew spotted a hot air balloon, registered as HB-QTH, approximately 2 NM ahead and to the left.
The crew maintained their descent as cleared by approach control but reported the encounter to the East Approach control unit. The aircraft eventually passed within approximately 0.8 NM horizontally and 200 ft vertically of the balloon.
The hot air balloon had departed from near Dörflingen earlier that afternoon. Seeking specific wind conditions, the pilot ascended to approximately 6,398 ft AMSL. Upon noticing the approaching business jet, the balloon pilot initiated a descent to avoid a collision. The balloon was operating under visual flight rules (VFR) and was not equipped with a transponder.
The investigation
The investigation focused on why the two aircraft came within such close proximity despite being in controlled airspace. Investigators established that the balloon pilot did not carry a transponder because they did not intend to enter the controlled airspace and found portable units difficult to operate.
Furthermore, the investigation noted a second, separate incident involving a different hot air balloon (HB-QRO) occurring on the same day, which also lacked a transponder while operating in the Zurich control zone. This highlighted a recurring pattern of non-transponder equipped aircraft operating in sensitive airspace.
Findings
- The primary safety issue was the absence of a transponder on the hot air balloon.
- Because the balloon was invisible to radar, air traffic controllers were unable to provide traffic information or receive short-term conflict alerts.
- The lack of a transponder signal meant the aircraft's Traffic Alert and Collision Avoidance System (TCAS) could not generate a resolution advisory, leaving the crew of OE-HOO without a critical safety net.
- The encounter was classified as an ICAO Category B airprox due to the lack of automated collision avoidance capabilities.