Introduction
On 5 September 2007, at approximately 04:11 UTC, a loss of separation minimum occurred between two aircraft near the boundary of FIR Praha and FIR Warsaw. The incident involved flight FAH6984, an ATR 72 operated by FARNAIR Switzerland AG (registration HB-AFJ), and flight WZZ135K, an Airbus A320 operated by WIZZ Air Hungary LTD (registration HA-LPH). FAH6984 was en route from Cologne/Bonn Airport (EDDK) to Katowice Airport (EPKT), while WZZ135K was flying from Katowice to LIME.
Incident Sequence
At 03:14:24, FAH6984 reported to ACC Praha sector LM EC at position ABERU and flight level 210. The controller did not have radar contact with the aircraft and instructed it to revert to ACC Munich frequency. After coordination, FAH6984 was confirmed to be entering FIR Praha. At 03:22, the controller instructed the crew to check their transponder and switch to the second SSR unit. Radar contact was established at 03:22:40. At 03:46, the crew switched back to the first transponder unit without losing radar identity.
At 03:59:54, ACC Praha sent a coordination message to EPWW (ACC Warsaw) regarding FAH6984's planned descent from FL210 to FL150. At 04:07:27, EPWW sent a coordination message for WZZ135K climbing to FL220. At 04:08:14, FAH6984 was cleared to descend to FL150. At 04:11:18, WZZ135K reported climbing to FL200 towards BAVOK at a vertical speed of 2000 ft/min. At 04:13:30, the separation minimum between the two aircraft was reduced. The crew of FAH6984 reported a Traffic Advisory (TA) from TCAS at 04:14, near BAVOK at FL150, with conflicting traffic directly ahead.
Air Traffic Control Actions
At the time, ACC Praha was undergoing a sectorization change from configuration B2 to C3, which began at 04:03. Frequency congestion occurred between 04:11:39 and 04:11:44 due to simultaneous transmissions from two radio stations. The STCA (Short Term Conflict Alert) activated at 04:11:59 on the former LM sector between WZZ135K and FAH6984. Coordination between controllers was hampered by the sectorization change and frequency issues. At 04:14:19, the sectorization change was completed. ACC Praha later contacted EPWW to inquire about WZZ135K's climb, but no problems were reported.
Transponder Malfunction
Investigation revealed that FAH6984's transponder had been malfunctioning. While in EDMM airspace, the aircraft's transponder responded in Mode S only to the Auersberg radar, which locked it for roll-call interrogations from other radars, and did not respond in Mode A/C at all. After switching to the second SSR unit at 03:22, normal operation was restored. However, after switching back to the first unit at 03:46, the transponder again operated only in Mode S, locked to the Pisek radar, and failed to respond to other radars. This situation caused a lack of radar information on the Polish side prior to the incident. The operator later identified the cause as a faulty weight-on-wheels switch (WOW 1) on the main landing gear, which affected transponder performance. After replacement, the transponder functioned correctly.
Meteorological Conditions
According to an expert estimate from the Czech Hydrometeorological Institute, cloud cover from a low-pressure area moved over the Ostrava and Opava regions. At FL160–FL170, there was a significant amount of cloud. The wind at FL160 was variable at 5 knots. The pilot of FAH6984 reported that the descent towards BAVOK occurred in instrument meteorological conditions (IMC).
Conclusion
The incident involved a loss of separation between two aircraft due to a combination of transponder malfunction and air traffic control coordination challenges during a sectorization change. No injuries or damage were reported.