What happened
On 10 June 2011, a serious airprox incident occurred approximately 18 nautical miles northeast of the Fribourg VOR/DME. An Airbus A319, registration D-AKNK, operated by Germanwings GmbH, was descending through the Swiss airspace when it encountered a significant conflict with a Raytheon 3/90 Premier 1 aircraft, registration D-IAGG, operated by Hahn Air Lines.
The incident began when an air traffic controller in the Geneva Area Control Centre cleared the Airbus A319 to descend to flight level 250. However, the controller simultaneously entered flight level 280 into the air traffic control system, which was the level coordinated for the flight transfer. Because the system reflected a different altitude than what was communicated via radio, the aircraft descended into a flight path that was in direct conflict with the Hahn Air Lines aircraft, which was climbing to flight level 270.
The investigation
The Swiss Accident Investigation Board (SAIB) examined the actions of five different air traffic controllers involved in managing the aircraft. The investigation focused on the discrepancy between the verbal clearance and the data entered into the controller's system. Investigators also reviewed the technical capabilities of the surveillance tools available to Skyguide, the Swiss air navigation service provider, at the time of the event.
Findings revealed that the crew of the Airbus A319 had correctly entered the verbal clearance into their onboard systems. However, the controllers in the Zurich Area Control Centre expected the aircraft to be at flight level 280 and failed to notice that the crew was reporting a descent to flight level 250. The investigation also noted that an ongoing Air Force exercise at the time had increased the workload for civil air traffic controllers, making the oversight more likely.
Findings
- The primary cause was an air traffic controller providing a verbal clearance that did not match the altitude entered into the air traffic control system.
- The flight crew of the Airbus A319 initially followed air traffic control instructions rather than continuing to follow the diverging resolution advisories provided by the Traffic Alert and Collision Avoidance System (TCAS).
- None of the five air traffic controllers involved in the management of the two aircraft identified the discrepancy between the radioed altitude and the system data.
- The incident was exacerbated by increased controller workload due to local military exercises and the use of non-standard phraseology.
- A systemic lack of a technical safety net prevented the immediate detection of the controller's manual entry error.
Safety action
Following the investigation, the SAIB issued safety recommendation no. 462, urging the Federal Office of Civil Aviation to ensure that a system is implemented within Swiss air traffic control capable of triggering an alarm when a discrepancy exists between a cleared flight level and the altitude selected in the aircraft's transponder.