TCAS Resolution Advisory Triggered During Near-Collision of Two…

TCAS Resolution Advisory Triggered During Near-Collision of Two Air Liners

Casualties unknown • VANAS 50 NM SSE de Genève, CH

A Boeing 737 and an Airbus A319 experienced a dangerous convergence near Geneva after an incorrect flight level was entered into the flight management system.

What happened

On July 8, 2010, a serious air proximity incident occurred near waypoint VANAS, approximately 50 NM south-southeast of Geneva. The incident involved a Boeing 737-30 (registration D-ABXS) operating as DLH 03K and an Airbus A319-111 (registration G-EZIY) operating as EZY 529Y.

While descending through the controlled airspace, the crew of the Airbus A319 entered an incorrect flight level into their flight management system. As the aircraft descended, it converged with the Boeing 737, which was crossing on a perpendicular route. The proximity became critical enough to trigger a Short-Term Conflict Alert (STCA) in the control center and a "CLIMB, CLIMB" resolution advisory (RA) on the Boeing 737. The aircraft eventually passed each other with a lateral separation of 2.2 NM and a vertical separation of only 125 feet.

The investigation

The Swiss Accident Investigation Board (SAIB) examined radar plots, radio communications, and Mode S transponder downlink data. The investigation focused on the flight management system inputs of the Airbus crew, the air traffic controller's response to initial radio contact, and the technical performance of the onboard collision avoidance systems. The inquiry also reviewed the workload and sectorization of the Geneva Area Control Center (ACC) at the time of the event.

Findings

The primary cause was the dangerous convergence of an aircraft descending below its cleared flight level into the path of a crossing aircraft.
The flight crew of the Airbus A319 entered an incorrect flight level into the flight management system, which was FL 320 instead of the cleared FL 330.
The air traffic controller failed to notice that the crew reported an incorrect flight level during their initial contact on the frequency.
The Airbus crew failed to perform the standard verification procedure to ensure the flight level in the autopilot altitude window matched the ATC clearance, likely due to a mechanical read-back of the instruction.
High workload and complex traffic management in the L14 sector contributed to the delay in controller intervention following the STCA alert.
The Boeing 737's TCAS issued an upward resolution advisory, which caused the aircraft to deviate vertically by approximately 300 feet.

Probable cause

The incident was caused by an aircraft descending below its assigned altitude into the path of another aircraft, driven by an erroneous flight level entry in the flight management system and a failure in the communication verification loop between the crew and air traffic control.

Frequently asked questions

What happened in the 2010-07-07 BOEING B737-300 accident near VANAS 50 NM SSE de Genève, CH?

A Boeing 737 and an Airbus A319 experienced a dangerous convergence near Geneva after an incorrect flight level was entered into the flight management system.

What aircraft was involved and where did it happen?

The accident on 2010-07-07 involved a BOEING B737-300, registration D-ABXS, at VANAS 50 NM SSE de Genève, CH.

What was the probable cause of the accident?

The incident was caused by an aircraft descending below its assigned altitude into the path of another aircraft, driven by an erroneous flight level entry in the flight management system and a failure in the communication verification loop between the crew and air traffic control.

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