What happened
On May 26, 2013, a complex airprox incident occurred near the MOLUS waypoint in Swiss airspace involving three separate aircraft. The sequence began when an Airbus A319, registration G-EZAU, operating as Easyjet flight EZY 899B, was climbing through the flight level range of FL 360 to FL 380.
An air traffic controller in sector L5 issued a clearance for the A319 to climb to FL 380. However, this instruction followed a previous error where the aircraft had been intended to climb only to FL 360 due to traffic density. This erroneous clearance caused the G-EZAU to lose separation with a Boeing 737-800, registration D-ABKB, operating as Air Berlin flight BER 17Z, which was crossing perpendicularly at FL 370.
As the onboard collision avoidance systems (TCAS) triggered resolution advisories, the D-ABKB performed an upward avoidance maneuver. This climb subsequently caused a second loss of separation with a third aircraft, a Boeing 737-800, registration G-TAWF, operating as Thomson Airways flight TOM 857, which was traveling on an opposing course at FL 380.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined radar data, radio communications, Mode S downlink transmissions, and pilot reports. The investigation focused on the sequence of clearances provided by the Geneva ACC sectors L5 and L6.
Investigators found that the controller in sector L5 believed they had assigned FL 360 during a second instruction, but the aircraft was actually cleared to FL 380. The investigation also reviewed the performance of the Short Term Conflict Alert (STCA) and the TCAS systems. It was noted that the pilots of all involved aircraft responded to the TCAS alerts within the expected reaction time parameters, effectively following the vertical resolution advisories to avoid a collision.
Findings
- The primary cause was an erroneous clearance to FL 380 issued to the G-EZAU, which violated the required separation from the aircraft at FL 370.
- The upward avoidance maneuver by the D-ABKB directly resulted in the secondary separation loss with the G-TAWF.
- The air traffic controller in sector L5 was managing high traffic complexity and workload at the time of the incident.
- The TCAS and STCA safety nets functioned as intended, providing the necessary warnings to the flight crews to execute avoidance maneuvers.