What happened
On September 19, 2000, at approximately 07:37 UTC, an air traffic incident occurred near the KLO DVOR/DME, roughly 5 nautical miles west/southwest of Zurich. The incident involved two aircraft operating under Instrument Flight Rules (IFR) in Class C airspace: a Swissair Boeing MD-83, registration HB-ISZ, and a Eurowint ATR42-300, registration D-BCRT.
The ATR42-300 departed Zurich for Dortmund, following the B3Y departure route. Shortly after takeoff, the pilot was instructed by Zurich Departure Control to execute a left turn toward the Zurich East VOR after passing 5,000 feet. Approximately one and a half minutes later, the Boeing MD-83 departed Zurich for Basel, following the H3Y departure route.
As the Boeing MD-83 climbed through Flight Level 60 at a rate of approximately 1,000 to 1,200 feet per minute, the pilot reported receiving a Traffic Collision Avoidance System Resolution Advisory (TCAS-RA), prompting an immediate reduction in climb rate. At that moment, the ATR42-300 was climbing through Flight Level 66 at approximately 500 feet per minute. The two aircraft were flying nearly on opposing tracks, resulting in a loss of required separation. The closest approach reached a horizontal distance of 1.7 NM and a vertical distance of only 600 feet.
The investigation
The investigation by the Swiss Transportation Safety Investigation Board (SUST) focused on the separation standards and air traffic control performance. Investigators examined the takeoff interval, noting that while the Tower Approach Communication System (TACO) suggested a 3-minute separation, the actual interval between the two departures was only 91 seconds.
Technical reviews confirmed that the Boeing MD-83 was equipped with TCAS, whereas the ATR42-300 was not. The investigation also reviewed radar data and controller communications to determine why the separation minima were breached.
Findings
- The primary cause of the incident was a misjudgment of the traffic situation by the air traffic controller and their subsequent passive behavior.
- The controller, aware of the approaching traffic via radar, recognized that minimum separation was being compromised but chose not to intervene, assuming the aircraft would maintain separation.
- The incident was further contributed to by an unusually short departure interval between a faster jet aircraft and a slower turboprop.
- Although the Boeing MD-83 pilot responded correctly to the TCAS-RA by reducing the climb rate, this action only prevented further vertical closure and did not address the horizontal separation breach.