What happened
On 13 July 1999, an Airbus A330, registered HB-IQG, operated by Swissair was on final approach to Runway 14 at Zurich Airport. While flying the ILS approach, the crew received a TCAS warning indicating a potential conflict with another aircraft. The second aircraft, a Hawker Siddeley HS25A with registration I-SDFG, was approaching the same approach axis from the west.
Prior to the encounter, the crew of I-SDFG had been cleared to hold at the EKRON waypoint. However, the aircraft bypassed the holding pattern and instead began executing its own navigation-based approach to the runway. As the two aircraft converged, the vertical separation decreased to approximately 300 feet, and the lateral distance narrowed to 2.6 NM. The Swissair crew eventually established visual contact with the Italian aircraft, and the situation stabilized after the HS2 aircraft performed a sharp turn away from the approach path.
The investigation
The investigation examined the radio communications between the aircraft and the Zurich Arrival control units (FINAL and APW). It established that while the arrival controller (FINAL) acted quickly to instruct the A330 to expedite its descent, there was a delay in the response from the western sector controller (APW).
Investigators analyzed the flight path of I-SDFG, noting that the aircraft did not comply with the initial instruction to hold at EKRON. The investigation also reviewed the weather conditions, which included thunderstorms, rain, and visibility of 6 km, noting that the A330 crew was intermittently flying through cloud layers during the incident.
Findings
- The primary cause of the incident was that I-SDFG failed to respect the EKRON holding clearance and instead initiated a standard ILS approach using its own navigation systems.
- The crew of I-SDFG responded slowly to air traffic control instructions to change heading to avoid the approach track.
- The air traffic controller did not require a read-back of the initial holding instruction, which contributed to the pilot's deviation.
- The use of inexpedient phraseology by both the controller and the crew led to a misunderstanding of the clearance requirements.