What happened
On April 12, 2013, a serious airprox incident occurred approximately 20 nautical miles southeast of Zurich Airport. Two commercial aircraft, a Boeing 737-800 operated by Ryanair (registration EI-ENK) and an Airbus A319-111 operated by TAP Air Portugal (registration CS-TTD), were cruising in Swiss airspace at different flight levels.
Seeking to avoid expected turbulence, the crew of the Ryanair flight requested a climb from flight level 360 to 380. However, the air traffic controller issued a clearance for flight level 380 using a callsign intended for a different aircraft from the same airline. The crew of EI-ENK mistakenly accepted this clearance and began their ascent. Shortly thereafter, the ground-based conflict warning system triggered an alert, indicating a collision risk between the two aircraft. The separation between the planes dropped to a minimum of 0.8 NM horizontally and 650 ft vertically. Both flight crews immediately executed Resolution Advisories (RA) from their onboard TCAS units to avoid the collision.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the communications between the aircraft and the Zurich Area Control Centre. The investigation focused on the sequence of radio transmissions, the controller's handling of the sector, and the operational environment. Investigators reviewed the radar plots, which showed the activation of the Short Term Conflict Alert (STCA) and the subsequent TCAS maneuvers. The inquiry also looked into the management of the air traffic sectors, noting that several sectors had been merged due to high traffic volume and weather-related flight changes.
Findings
- The primary cause of the incident was that the crew of the Boeint 737-800 initiated an unauthorized climb after mistakenly following a clearance meant for another aircraft.
- The air traffic controller failed to notice that the clearance was not being read back by the intended recipient.
- The crew of the Ryanair flight requested a change in altitude without stating their specific callsign, contributing to the confusion.
- The controller issued a level change without verifying which aircraft had made the initial request.
- The controller's failure to identify that the incorrect aircraft had responded to the instruction was a critical factor.
- Increased traffic complexity and the decision to merge air traffic sectors during periods of heavy weather and turbulence placed additional strain on the controller's workload.