What happened
On November 20, 2011, two Boeing 757 aircraft were operating in the Canary Islands TMA. The first, a Finnair BOEING 757-2Q8 with registration OH-LBR, was descending from flight level 390 toward Tenerife South/Reina Sofia Airport. Simultaneously, a Thomas Cook Airlines BOEING 757-28A, registration G-TCBA, was maintaining flight level 370 on a similar arrival route.
Following a shift change in the NWW sector, the air traffic controller received a descent request from the crew of G-TCBA. Due to a radar garbling issue, the radar label for G-TCBA incorrectly displayed an altitude of flight level 405. Believing the aircraft was above the Finnair flight, the controller cleared OH-LBR to descend directly to flight level 250. This instruction caused the two aircraft to violate the minimum required radar separation of 5 NM and 1,000 ft.
As the aircraft approached, the Traffic Collision Avoidance System (TCAS) activated. The crew of OH-LBR reported receiving a resolution advisory (RA) and requested confirmation of their descent clearance. The crew of G-TCBA also experienced TCAS alerts, including a traffic alert and a resolution advisory to descend.
The investigation
The CIAIAC investigation focused on the radar malfunction and the subsequent crew responses. Technical analysis of the radar data revealed a garbling phenomenon, likely caused by similar oblique distances between the aircraft and the radar, which caused the transponder signal for G-TCBA to split into two incorrect labels.
Simulations using the Eurocontrol InCAS tool determined that while the crew of G-TCBA responded appropriately to their TCAS advisories, the crew of OH-LBR likely failed to follow the "climb" resolution advisory, continuing their descent instead. This improper response contributed to a secondary conflict where both aircraft received reversed advisories (climb for one, descend for the other) before the conflict eventually cleared.
Findings
- A radar garbling error caused the controller to see an incorrect altitude for G-TC0BA.
- The controller failed to detect the discrepancy between the radar display and the flight progress strips.
- The clearance given to aircraft OH-LBR to descend through the altitude of G-TCBA caused the separation violation.
- The crew of OH-LBR did not adequately respond to the TCAS climb advisory, continuing their descent despite the instruction.