Two Swiss Airbus A330s narrowly avoid collision near Zurich

Casualties unknown • 15 NM nordöstlich vom Flughafen Zürich LSZH, ZH, CH

An altitude selection error by a flight crew led to a high-risk airprox between two Airbus A330-223 aircraft 15 NM northeast of Zurich.

What happened

In the early hours of March 20, 2009, two Airbus A330-223 aircraft, both operated by Swiss International Air Lines, experienced a serious near-miss while approaching Zurich. The first aircraft, HB-IQK (flight SWR 155), was established in the AMIKI holding pattern at flight level 120. Simultaneously, the second aircraft, HB-IQC (flight SWR 229), was cleared by Zurich approach control to descend to flight level 130.

However, a manual error during the descent process caused the aircraft to deviate from its assigned altitude. The two heavy jets closed in on each other, reaching a minimum horizontal separation of 1.6 NM and a vertical separation of only 300 ft, well below the required 3 NM horizontal or 1000 ft vertical separation standards. The conflict was only resolved after the Traffic Collision Avoidance System (TCAS) issued resolution advisories to both crews, prompting immediate corrective maneuvers.

The investigation

The Swiss investigation examined radar data, flight parameter recordings, and radio communications. Investigators focused on the Flight Control Unit (FCU) settings of HB-IQC. Data revealed that while the captain had correctly read back the clearance to flight level 130, the altitude selection knob was adjusted to flight level 120.

Investigators also reviewed the actions of the air traffic controller at the Zurich approach east sector. While the controller's Short Term Conflict Alert (STCA) eventually triggered, the investigation noted that the controller initially failed to notice the altitude deviation. Furthermore, a traffic information update provided to SWR 155 contained an incorrect direction of movement, though this did not prevent the crew from identifying the hazard.

Findings

The primary cause was a manual altitude error where the pilot flying on HB-IQC set the flight control unit to flight level 120 instead of the authorized flight level 130.
The crew failed to identify the incorrect setting, meaning the closed-loop principle of verification was not effectively applied.
Crew fatigue, resulting from a long night flight, likely contributed to the error.
The pilot not flying was distracted by using an electronic flight bag to review arrival procedures.
The air traffic controller did not immediately perceive that the aircraft had descended below its assigned altitude.

Probable cause

The flight crew of SWR 229 incorrectly programmed a lower altitude into the flight control unit and failed to detect the error, leading to a violation of separation minimums and a high-risk encounter with SWR 155.

Frequently asked questions

What happened in the 2009-03-19 aircraft accident near 15 NM nordöstlich vom Flughafen Zürich LSZH, ZH, CH?

An altitude selection error by a flight crew led to a high-risk airprox between two Airbus A330-223 aircraft 15 NM northeast of Zurich.

What aircraft was involved and where did it happen?

The accident on 2009-03-19 involved a aircraft, registration HB-IQK, at 15 NM nordöstlich vom Flughafen Zürich LSZH, ZH, CH.

What was the probable cause of the accident?

The flight crew of SWR 229 incorrectly programmed a lower altitude into the flight control unit and failed to detect the error, leading to a violation of separation minimums and a high-risk encounter with SWR 155.

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