On the night of 1 July 2002, Bashkirian Airlines Flight 2937, a Tupolev Tu-154 passenger jet, and DHL Flight 611, a Boeing 757 cargo jet, collided in mid-air over Überlingen, a southern German town on Lake Constance.
All 69 passengers and crew aboard the Tupolev and the two crew members of the Boeing were killed
Here is an Air Crash Investigation video of Bashkirian Airlines Flight 2937 mid-air collision with DHL Flight 611
Bashkirian Airlines Flight 2937 was a chartered flight from Moscow, Russia, to Barcelona, Spain, carrying sixty passengers and nine crew.
The two aircraft were flying at flight level 360 (36,001 feet (10,973 m)) on a collision course. Despite being just inside the German border, the airspace was controlled from Zürich, Switzerland, by the private Swiss airspace control company Skyguide.
The only air traffic controller handling the airspace, Peter Nielsen, was working two workstations at the same time. Partly due to the added workload, and partly due to delayed radar data, he did not realise the problem in time and thus failed to keep the aircraft at a safe distance from each other.
Less than a minute before the accident he realised the danger and contacted Flight 2937, instructing the pilot to descend to flight level 350 to avoid collision with crossing traffic (Flight 611).
Seconds after the Russian crew initiated the descent, their traffic collision avoidance system (TCAS) instructed them to climb, while at about the same time the TCAS on Flight 611 instructed the pilots of that aircraft to descend.
Had both aircraft followed those automated instructions, the collision would not have occurred.
The official investigation by the German Federal Bureau of Aircraft Accident Investigation identified as the main cause of the collision a number of shortcomings on the part of the Swiss air traffic control service in charge of the sector involved, and also ambiguities in the procedures regarding the use of TCAS, the onboard aircraft collision avoidance system.
A year and a half after the crash, on 24 February 2004, Peter Nielsen, the air traffic controller on duty at the time of the collision, was murdered in an apparent act of revenge by Vitaly Kaloyev, a Russian citizen who had lost his wife and two children in the accident.