What happened
On 10 October 1999, at approximately 18:15 UTC, a Swissair Airbus A321, registration HB-IOD, was arriving at Zurich Airport from London Heathrow. While transitioning through the Zurich Terminal Control Area, the aircraft was flying at an unusually high speed of approximately 310 KT IAS, significantly exceeding the standard approach speed of 240 KT IAS.
During the descent, the aircraft was cleared by the Zurich Arrival East Sector (APE) controller to descend to FL 70. Shortly after, the flight crew reported that they had leveled off at FL 130 due to a TCAS Resolution Advisory (RA). This maneuver occurred as an Avioimpex MD80, registration Z3-ARB, was departing Zurich for Stuttgart on a nearly reciprocal course at FL 120. The two aircraft eventually crossed paths with a lateral separation of only 1.2 NM and a minimum vertical separation of 800 ft.
The investigation
The investigation examined the coordination between different air traffic control sectors and the technical state of the surveillance equipment. It was established that while the transfer of control from the ACC Sector East to the APE sector was technically clear, a dispute had occurred between controllers. The Radar Executive East (REE) controller had pressured the APE controller to prioritize the Swissair flight in the arrival sequence due to its high speed.
Furthermore, the investigation found that the Short Term Conflict Alert (STCA) system at the approach unit was not operational at the time, as it had been taken out of service for parameter testing due to a high error rate. The investigation also noted that the APE controller lacked a virtual control strip for the departing aircraft, which hindered the ability to monitor the potential conflict.
Findings
- The primary cause was that the APE controller, while authorizing the descent of HB-IOD, failed to maintain awareness of the separation requirements regarding the approaching Z3-ARB.
- The Swissair aircraft was operating at an unusually high speed, which restricted maneuverability and complicated the arrival sequence.
- A lack of coordination regarding the aircraft's speed during the transfer between control sectors contributed to the situation.
- The absence of a functional STCA warning system meant the impending conflict was not automatically flagged to the controller.
- The intense verbal exchange between the REE and APE controllers regarding the arrival sequence distracted from separation management.
Safety action
- The investigation emphasized that the acquisition and installation of a reliable, high-priority STCA warning system that meets all operational requirements must be progressed.