What happened
On 23 September 1999, a near-collision occurred in the Zurich FIR, approximately 18 NM northeast of ROTIS. The incident involved two aircraft on intersecting paths: a Saab 2000, registration HB-IZY, operated by Crossair, and a Bombardier CL 600, registration F-GRJB, operated by Britair.
At the time, the Saab 2000 was climbing from FL210 toward FL240. To manage traffic, the air traffic controller instructed the aircraft to maintain FL270, with an expectation of FL2 .The Bombardier CL 600 was cruising at FL280. During the sequence, the controller mistakenly cleared the Saab 2000 to climb to FL280, which placed it on a collision course with the oncoming Bombardier CL 600.
As the aircraft approached one another, the pilot of the Bombardier CL 600 initiated a climb due to a TCAS Resolution Advisory. The controller, realizing the error, immediately instructed the Saab 2000 to descend back to FL270 and directed the Bombardier CL 600 to turn right. Despite these rapid interventions, the aircraft passed within 3 NM horizontally and 700 ft vertically of each other.
The investigation
SUST examined the radar communications and the sequence of instructions issued by Zurich ACC Upper 1. The investigation established that the airspace was experiencing moderate to heavy traffic, complicated by the presence of cumulonimbus clouds in the Trasadingen region, which necessitated several course corrections by various pilots.
Investigators reviewed the transcript of the radio communications, noting that the controller's initial error was followed by a period of confusion where an imprecise instruction was issued to the Saab 2000. The investigation also looked at the performance of the Short Term Conflict Alert (STCA) system, finding that the alarm at the air traffic control workstation triggered very late—at the same moment the pilots and controller were already implementing avoidance maneuvers.
Findings
- The primary cause of the incident was a brief lack of attention by the Radar controller, which resulted in the Bombardier CL 600 temporarily falling outside of the controller's active control concept.
- The controller's error was compounded by a moment of shock following the realization of the mistake, leading to an initial imprecise instruction to the Saab 2000 regarding its assigned flight level.
- The Bombardier CL 600 was already performing an automated maneuver based on a TCAS Resolution Advisory (RA) when the conflict reached its closest point.
- The STCA system provided insufficient warning time, as the alert occurred only after the conflict was already being actively resolved by the crew and air traffic control.