What happened
On December 13, 2001, at approximately 17:12 UTC, two separate separation violations occurred near the RILAX intersection within the Zurich ACC North Sector. The first incident involved a Lufthansa Canadair Regional Jet 100LR (D-ACJE) and a Crossair Embraer145 (HB-JAF). Both aircraft were operating at flight level 180. The pilot of the Embraer145 reported visual contact with the oncoming Canadair Regional Jet 100LR before the two aircraft passed each other with a horizontal distance of 3.7 NM and a vertical separation of only 200 feet.
A second violation occurred shortly after, involving the same Canadair Regional Jet 100LR (D-ACJE) and a Crossair Saab2000 (HB-IZR). The Saab2000 was climbing through flight level 160 toward level 180. As the Canadair Regional Jet 100LR performed a course change to enter the RILAX holding pattern, it crossed the path of the Saab2000, resulting in a horizontal separation of 2.2 NM.
The investigation
The investigation established that the North Sector was managing high traffic density and complex maneuvers due to severe weather at Zurich Airport. Snow and ice required frequent runway closures and clearing operations, leading to significant arrival delays and aircraft accumulation in the RILAX holding pattern.
The air traffic controller (ATC) was operating a single-manned sector, performing both Radar Executive and Radar Planning duties simultaneously. The investigation found that the controller was managing multiple conflicting traffic flows, including a transit aircraft at flight level 190, while also handling several aircraft arriving for the RILAX hold. The controller admitted that proactive planning had become impossible due to the workload.
Findings
- The primary cause was the overload of air traffic controllers managing a single-manned sector during periods of high complexity.
- High traffic density was exacerbated by severe weather, which caused significant delays and forced aircraft into holding patterns.
- The controller failed to recognize a critical pilot report regarding the Canadair Regional Jet 100LR entering the RILAX hold, which was a key factor in the second separation loss.
- A lack of timely support from the Radar Planning function meant the controller was unable to effectively manage the increasing traffic volume.
- While the Saab2000 received an ACAS-TA, the pilots did not perceive the risk as high.
- The controller did not provide traffic information to the involved aircraft regarding the developing conflicts.
Safety action
Following the incidents, supplementary instructions were issued regarding Single Manned Operation Procedures (SMOP). Notably, following the subsequent Überlingen accident in 2002, the use of single-manned operations was prohibited by the Federal Office of Civil Aviation (BAZL) as a precautionary measure.