What happened
On 18 February 2002, two aircraft flying toward Zurich entered the North sector of ACC Zurich airspace on nearly simultaneous paths. The first aircraft, an Airbus A3/19 with registration HB-IPZ, was cruising at FL 160. Shortly after, a Hawker H25B, registration G-XRMC, was following at FL 130.
As the aircraft progressed, the air traffic controller (ATCO) in the North sector observed the Hawker H25B increasing speed significantly. To prevent the faster aircraft from overtaking the Airbus A319, the controller attempted to reverse the approach sequence by instructing the Airbus A319 to reduce speed. However, as the aircraft were transferred to the East sector approach controller, the separation gap closed rapidly.
The approach controller realized that the Airbus A319's descent clearance would lead to a conflict and attempted to halt its descent. The crew of the Airbus A319 did not initially respond to these calls, partly because the pilot was distracted by a telephone call from the cabin crew regarding cockpit security protocols. During this period, the two aircraft closed to a lateral separation of only 0.6 NM and a vertical separation of 200 FT. The conflict was eventually resolved when the Airbus A319 crew reported an ACAS Resolution Advisory (RA) and initiated a climb, while the controller instructed the Hawker H25B to turn away.
The investigation
The investigation examined the coordination between the North and East sectors and the use of the CALM management tool. It was established that the North sector ATCO was performing single-manned operations, acting as both radar executive and radar planner. The investigation found that the controller had not updated the approach sequence or the increased speed of the Hawler H25B in the management system, likely due to high workload and simultaneous coordination tasks.
Furthermore, the investigation looked into the communication breakdown in the Airbus A319 cockpit. It was noted that post-9/11 security regulations required cockpit doors to be locked, meaning the pilot had to use a telephone to communicate with the cabin. This interaction, accompanied by a loud buzzer, contributed to the crew's failure to monitor ATC instructions during the critical phase of the encounter.
Findings
- The North sector was being operated under Single Manned Operation Procedures (SMOP), placing excessive workload on a single controller.
- The North sector controller failed to update the CALM system with the actual speed of the Hawker H25B or the revised approach sequence.
- The Airbus A319 crew failed to respond to multiple ATC instructions due to a distraction caused by a cabin-to-cockpit telephone call.
- The approach controller's decision to instruct the Hawker H25B to turn left was described as spontaneous and, in retrospect, a right turn might have been more appropriate.