Accidents

Uberlingen Mid-Air Collision

Contradictory advice from Traffic Collision Avoidance System (TCAS) and an air traffic controller led to a mid-air collision between two TCAS-equipped aircraft. 71 fatalities.

On 1 July 2002, a passenger jet (Bashkirian Airlines Flight 2937) and a cargo jet (DHL Flight 611) collided  in mid-air. The collision happened over the south German town of Überlingen; it occurred despite both  aircraft being equipped with TCAS.

The two aircraft were in airspace that was controlled from Zürich and were on a collision course. A single  controller was on duty and they were responsible for controlling two workstations. This arrangement was  against regulations, but was tolerated by management and had become accepted practice. Initially, the  controller did not appreciate the dangerous situation that was developing.

Maintenance was being conducted on the main radar system, which meant that the controller was reliant  on a backup system. This delayed the presentation of radar information. In addition, a ground-based  optical system that would have provided warning of the impending collision was turned off, also for  maintenance: the controller was unaware of this.

Less than a minute before the collision the controller became aware of the situation. He instructed Flight  2937 to descend. Seconds after initiating this decent, the TCAS on Flight 2937 requested a climb, with the  corresponding system on Flight 611 requesting a descent. Flight 2937 continued to follow the controller’s  direction, meaning that both aircraft were descending.

Unaware of the TCAS instructions the controller repeated the request for Flight 2937 to descend; he also  provided Flight 2937 with misleading information on the relative location of Flight 611. The planes collided,  resulting in the deaths of all 69 people on Flight 2937 and both people on Flight 611.

One of the actions resulting from the accident was a clarification from International Civil Aviation  Organization (ICAO) of how pilots should respond to contradictory information from a controller and TCAS.

This incident illustrates the importance of the following Data Properties: consistency, with regards to the  instructions provided to Flight 2937; availability, with regards to information from the ground-based optical  system; and timeliness, with regards to information from the radar system.

Links 

• https://en.wikipedia.org/wiki/Überlingen_mid-air_collision
  (accessed 29 November 2017).