What aircraft was involved and where did it happen?

The accident on 2001-03-02 involved a DASSAULT AVIATION F2TH, registration UNKNOWN, at Région du VOR de St-Prex, CH.

What was the probable cause of the accident?

The incident was caused by the approach controller's failure to maintain precise vertical separation between two aircraft on converging tracks, exacerbated by the pressure of a controller handover and heavy traffic density.

TCAS Resolution Advisory Averted Near Collision at Geneva

A mid-air near collision between two commercial aircraft was avoided by TCAS resolution advisories during a period of heavy traffic and controller handover at Geneva.

What happened

On the evening of 2 March 2001, a near-collision incident occurred in the vicinity of the St-Prex VOR during approach operations at Geneva. The incident involved a Falcon 2000 (FPG566) performing a VOR/DME approach to runway 23 and an Air Nostrum CRJ2 (ANS893 34) positioned on the right-hand downwind leg.

Due to a snowstorm, the ILS was out of service, necessitating instrument approaches. During a period of heavy traffic and a simultaneous controller handover, the arrival controller attempted to resolve a conflict between the CRJ2 and a departing aircraft. To achieve separation from the outbound traffic, the controller redirected the CRJ2 onto a new heading that required crossing the final approach path of the Falcon 2000.

As the two aircraft converged, the Short Term Conflict Alert (STCA) was triggered. The controller instructed the Falcon 2000 to descend rapidly to 6000 ft. During the maneuver, the CRJ2 pilot initiated an uninstructed descent to 5400 ft following a TCAS Resolution Advisory (RA). The two aircraft eventually crossed with a minimum separation of 1 NM and 300 ft.

The investigation

The investigation established that the incident occurred at night under conditions of light snowfall and heavy inbound traffic. The investigation examined radar plots, communication transcripts, and the sequence of controller instructions. It was noted that the controller had recently taken over the position during a complex traffic situation. The investigation also reviewed the functionality of the TCAS systems on both aircraft, noting that the CRJ2 pilot followed an RA, while the Falcon 2000 pilot observed a brief RA intruder alert on their display.

Findings

The primary cause was the controller's lack of precision in applying vertical separation between two aircraft on converging tracks.
A controller handover occurred during a period of high traffic density and unresolved conflicts.
The tactical decision to change the approach sequence for the CRJ2 to avoid departing traffic inadvertently created a new conflict with the incoming Falcon 2000.
The controller's instruction to the Falcon 2000 to descend quickly was issued without explaining the underlying reason for the urgency.
The CRJ2 pilot performed an unassigned descent to 5400 ft in response to a TCAS Resolution Advisory.

Safety action

An Air Traffic Incident Report (ATIR) was lodged by the Control Tower operations manager regarding the separation loss.