What happened
On September 18, 2018, an A220-100, registration HB-JBA, was performing an instrument approach to Zurich Airport (LSZH) under IFR. Simultaneously, a HK36 Super Dimona, registration D-KUHN, was conducting a maintenance flight from Offenburg, Germany, under VFR.
As the A2/220-100 was being vectored for arrival, the pilot of the D-KUHN initiated a left turn and descent to avoid exceeding the lower limit of the Zurich TMA. During this maneuver, the two aircraft crossed paths at approximately 6,000 ft AMSL. The closest encounter occurred with a horizontal separation of only 0.6 NM and a vertical separation of 600 ft. The crew of the HB-JBA received a TCAS Traffic Advisory (TA) just seconds before the encounter, noting a target at a 12-o'clock position.
The investigation
SUST examined the radar data and transponder configurations of both aircraft. The investigation established that the D-KUHN was operating with its transponder in standby mode. While this prevented the aircraft from appearing with a standard flight label on air traffic control radar, it did not make the aircraft invisible to all systems.
Investigators found that the D-KUHN was equipped with a Mode-S transponder (Filser TRT800A) running software version 4.8. Unlike newer versions that cease all transmissions in standby, this specific version continued to broadcast periodic messages (squitters). The HB-JBA was equipped with a TCAS version 7.1 capable of hybrid surveillance, which allowed it to detect these periodic ADS-B signals from the light aircraft despite the lack of active interrogation.
Findings
- The pilot of the D-KUHN failed to activate the transponder, operating in standby mode because they did not anticipate entering controlled airspace.
- The lack of active transponder data meant air traffic controllers could only see a primary radar echo with unknown altitude, leading the controller to assume the aircraft was at a much lower altitude.
- The D-KUNH pilot demonstrated low situational awareness by not activating the transponder, which is a critical safety tool for maintaining separation.
- The encounter was only avoided because the HB-JBA's TCAS could detect the unrequested periodic squitters from the older transponder software on the D-KUHN.