Facts
On June 19, 2017, at 07:03 UTC, the Air Navigation Services of the Czech Republic reported a serious incident involving the erroneous display of flight PNY501 on the IDP TR7 system at Ostrava Mošnov Airport. The flight was cleared by an area control controller to descend from flight level (FL) 100 to FL70. According to the display on all controller workstations using the ARTAS tracker, the aircraft descended to FL95 and stopped. Concerned about a potential conflict with opposite traffic climbing to FL110, the controller asked the crew if they were descending to FL70. The crew responded that they had already reached and were maintaining FL70. When the controller switched the workstation to the local PATRON tracker, the target split into two: one at FL95 and one at FL70.
The event was assessed using the risk analysis tool (RAT) as a serious incident – A4 (serious incident).
Analysis
The aircraft was equipped with a Mode 1/2/A/C transponder; Mode S was not available. The military radar at Nepolisy interrogated in Modes 1/2/A/C, while the WAM systems of ŘLP ČR interrogated in Mode A/A/C. The target was detected by radars at BUKOP, PRAHA, PÍSEK, JAVOR, VIENNA, and by the P3D-WS/LKMT/LKTB WAM systems. Conventional radars indicated the correct Mode C value, showing the aircraft at FL70. Erroneous detection was exhibited by the P3D-WS and P3D-LKMT WAM systems.
In the first phase, the controller workstation used the ARTAS tracker, which ignored Mode C values from conventional radars because their update rate is 5 seconds, while WAM systems update Mode C every second. Thus, the output from ARTAS for flight PNY501 showed an incorrect Mode C value from the WAM systems.
In the second phase, the controller switched to the local PATRON tracker. This display showed two targets due to differing Mode C indications from different radar sources.
The root cause of the erroneous WAM detection was a misinterpretation of military Mode 2 as Mode C. WAM systems can distinguish Modes 1 and 2 from Mode A/C only when responses to Modes 1 and 2 are continuous, as typically occurs with military transponders. The original mitigation against misinterpretation was a fixed code set for Modes 1 and 2 used by the military, which the civil WAM systems filtered. Since early 2014, the military began using a full spectrum of codes in Modes 1 and 2. In May 2014, all WAM systems were updated to interrogate in sequence A/A/C to differentiate Modes 1 and 2 from Modes A and C. This update assumed standard use of both Modes 1 and 2 together. However, the transponder on this aircraft responded only to Mode A and Mode 2 without a preceding Mode 1 response. Consequently, the WAM systems failed to correctly identify Mode 2 and misinterpreted it as Mode C.
Conclusion
The cause of the serious incident was a combination of incorrect detections by the WAM systems, which manifested in a non-standard response mode of the military transponder.
Corrective measures were implemented in September 2017: a parameter in the ARTAS system was adjusted to delay WAM data updates, preventing Mode C updates solely from WAM. Additionally, ERA a.s. updated the WAM system software to eliminate misinterpretation even when only one military mode is used. The interrogation sequence was changed to A/A/C, then to C/C/A, and finally to C/C/A/1/2. Flight tests with military aircraft did not reveal any anomalies.