Serious incident to the Boeing 737 registered EC-NGC operated by Albastar and the Embraer 190 registered F-HBLD operated by Hop! on 21/07/2023 en route

 In cruise at FL 350, on flight LAV4651 carried out by the Boeing 737-809 operated by Albastar between London-Stansted airport (United Kingdom) and Tarbes-Lourdes-Pyrenees airport, the transponder, switched on by the crew, ceased transmitting any information in reply to interrogations from the ground radar stations and Airborne Collision Avoidance Systems (ACAS) from aircraft nearby. This malfunction meant that radar contact with flight LAV4651 was lost and that the ACAS systems on the aeroplanes flying nearby and the Short Term Conflict Alert (STCA) systems available to controllers were not able to detect or monitor it.

Neither the crew of flight LAV4651 nor the air traffic controllers of the control sector in which the aeroplane was flying perceived or took into account the systems designed to alert them in the event of a transponder failure:
• the crew did not see the amber ATC FAIL light on the ATC Control Panel situated between the two crew seats which probably lit up during the flight according to the examinations carried out during the investigation;
• the air traffic controllers did not perceive the various visual warnings which were displayed on their screens and acknowledged them by deleting the marker which indicated a loss of radar contact.

Radio contact in the vicinity of the compulsory reporting point BALAN, situated at the boundary between the ACC/west and ACC/south sectors, constituted a possibility of recovering the situation. The PCR asked the crew to head directly to this point, defined as a compulsory reporting point. The crew, in the absence of an explicit request from the controllers to contact them on approaching BALAN, continued their route as set out in the flight plan.

 
Around 20 min after the transponder failure, the Boeing 737 crossed the route of an Embraer 190 operated by HOP! (Flight AFR21YB) at the same flight level, with a minimum horizontal separation of 2.6 NM for a required minimum distance of 5 NM. Despite the failure of the transponder, the Boeing 737’s TCAS functioned. Based on the information received from the transponder and from the TCAS of the Embraer 190, the algorithm of the 737 TCAS determined that there was no threat of collision with the Embraer. As a consequence, no Traffic Advisory (TA) or Resolution Advisory (RA) was transmitted on board the Boeing 737. However, the crew of the Embraer 190 had had no information about the position and path of the Boeing 737 and were therefore surprised to see it cross their route ahead of them at the same flight level.

At the same time, a military air traffic controller in the Cinq-Mars-La pile military detection and control centre detected the flight on the primary radar and after several coordination actions between ACC/south-west and ACC/west, and after the loss of separation between the two aeroplanes, radar contact with flight LAV4651 was recovered and the flight was able to continue to destination without any other anomalies.

The BEA has issued four safety recommendations relating to the following topics:
• Certification Specifications for Airborne Communications, Navigation and Surveillance;
• systems for attracting the air traffic controllers’ attention;
• overall management of safety risks by the air navigation service provider in France;
• compulsory reporting points.