What happened
On 21 July 2023, a Boeing 737-809, registered EC-NGC and operated by Albastar, was cruising at FL 350 near the Limoges VOR/DML near the boundary of the ACC/west and ACC/south sectors. During the flight, the aircraft's transponder ceased transmitting information in response to ground radar interrogations and nearby Airborne Collision Avoidance Systems (ACAS). This malfunction resulted in a total loss of radar contact for air traffic controllers.
Approximately 20 minutes after the failure, the Boeing 737-809 crossed the flight path of an Embraer 190, registered F-HBLD, operated by HOP!, at the same flight level. The horizontal separation between the two aircraft dropped to 2.6 NM, significantly below the required 5 NM minimum for RVSM airspace. While the Boeing 737-809's TCAS did not issue any alerts because it did not perceive a threat, the crew of the Embraer 190 was surprised by the sudden appearance of the other aircraft. Radar contact was eventually restored after a military controller detected the aircraft on primary radar.
The investigation
The BEA examined the sequence of events following the transponder failure, specifically focusing on why the failure went unnoticed by both the flight crew and air traffic controllers. The investigation analyzed the cockpit alert systems, the air traffic controller interface (ERATO environment), and the effectiveness of compulsory reporting points. The investigation also reviewed the safety management systems (SMS) of the French air navigation service provider (DSNA) and the ergonomics of the man-machine interfaces used by controllers.
Findings
- The flight crew did not notice the amber ATC FAIL light on the control panel, which likely illuminated during the flight.
- Air traffic controllers failed to perceive visual warnings on their screens and inadvertently acknowledged the loss of radar contact by deleting the marker indicating the issue.
- The crew did not contact the controller at the compulsory reporting point BALAN, which could have served as a mechanism to identify the loss of radar contact.
- The aircraft's navigation displays lacked specific symbology to help crews differentiate compulsory reporting points from other waypoints.
- The transponder failure prevented the Short Term Conflict Alert (STCA) systems from monitoring the aircraft's position.