What happened
On August 13, 2001, two aircraft operated by Crossair experienced a close proximity event near the Zurich airport area. The first aircraft, an Avro146 RJ100 with registration HB-IXV (flight SWR504), departed Zurich on a scheduled flight to Hamburg. Shortly after takeoff, the aircraft was instructed by Departure Control to expedite its climb to flight level 70.
Simultaneously, a Saab2000 (registration HB-IYE, flight CRX57A) was departing Zurich for Nuremberg. The aircraft was instructed to level off at flight level 60 and turn left. During the maneuver, the Avro146 RJ100 received a Traffic Advisory and subsequently a Resolution Advisory from its Airborne Collision Avoidance System (ACAS), commanding a climb.
As the Saab2000 climbed in response to its own ACAS instructions—first receiving a command to climb and later a command to descend—the two aircraft crossed paths. The vertical separation between the two planes dropped to approximately 400ft, while the horizontal distance at the crossing point was only 0.3NM.
The investigation
The investigation examined the sequence of air traffic control (ATC) instructions and the performance of the aircraft involved. It was established that the Departure Controller was managing high traffic density and attempted to use radar separation by maneuvering the Saab2000 to stay below the Avro146 RJ100.
Investigators found that the Avro146 RJ100 was in an acceleration phase following its takeoff, which limited its ability to increase its rate of climb as requested by ATC. Furthermore, the investigation looked into the communication delay from the Saab2000 crew, who had delayed contacting Departure Control to complete post-takeoff cockpit procedures (clean-up).
Findings
- The primary cause of the incident was the simultaneous departure of both aircraft in close succession, combined with ATC-directed deviations from their assigned standard instrument departure routes.
- The controller's decision to only stop the climb of the Saab2000 at flight level 60, rather than at 5000ft, contributed to the loss of separation.
- The Avro146 RJ100 crew did not inform the controller that they were unable to expedite the climb due to the aircraft's performance limitations during the acceleration phase.
- The Saab2000 crew delayed contacting Departure Control to prioritize internal company procedures for gear and flap retraction.
- The ACAS system on the Saab2000 issued conflicting instructions (climb followed by descent) that led the aircraft into the path of the other aircraft.