What happened
On June 19, 2017, at 07:03 UTC, an incident occurred affecting air traffic management (ATM) services north of the Terminal Control Area (TCA) east of Pardubice (LKPD). An air traffic controller (ATC) cleared flight PNY501 to descend from FL100 to FL70. However, surveillance data on the controller workstations (CWS) indicated that the aircraft had only descended to FL95 and was failing to maintain the assigned altitude.
Due to the potential for conflict with other traffic climbing to FL110, the controller queried the crew regarding their altitude. The crew confirmed they had reached and were maintaining FL70. Upon switching the radar data source from the ARTAS tracker to a local tracker, the surveillance target split into two distinct tracks: one showing the aircraft at FL95 and another at FL69/FL70. This discrepancy was classified as a serious incident regarding the disruption of safe ATM service.
The investigation
The investigation examined the performance of the P3D-WS and P3D-LKMT Wide Area Multilateration (WAM) systems and the ARTAS surveillance tracker. The investigation established that while conventional radars correctly identified the aircraft at FL70 using Mode C, the WAM systems provided erroneous altitude data.
It was determined that the ARTAS tracker was ignoring Mode C updates from conventional radars because those radars updated every five seconds, whereas the WAM systems updated Mode C every second. Consequently, the ARTAS output relied solely on the incorrect Mode C data provided by the WAM systems. Furthermore, the investigation found that the WAM systems had misidentified military Mode 2 information as Mode C information.
Findings
- The primary cause of the disruption was a combination of incorrect detections by the WAM systems, triggered by a non-standard response pattern from a military transponder.
- The aircraft's transponder responded to Modes A and 2, but the Mode 2 response was not preceded by a Mode 1 response.
- Since 2014, the WAM systems had been configured to query in an A/A/C sequence, assuming that Modes 1 and 2 would always be used in succession. Because the Mode 1 interrogation was missing, the system failed to correctly distinguish Mode 2 from Mode C, leading to the altitude error.
- The ARTAS tracker's reliance on high-frequency WAM updates caused it to prioritize the erroneous WAM Mode C data over the accurate, albeit slower, updates from conventional radars.