On 1 July 2002, BAL Bashkirian Airlines Flight 2937, a Tupolev Tu-154 passenger jet, and DHL International Aviation ME Flight 611, a Boeing 757 cargo jet, collided in mid-air over Überlingen, a southern German town on Lake Constance, near the German-Swiss border.

[3] The official investigation by the German Federal Bureau of Aircraft Accident Investigation (German: Bundesstelle für Flugunfalluntersuchung; BFU) identified the main cause of the collision to be a number of shortcomings on the part of the Swiss air traffic control (ATC) service in charge of the sector involved, as well as ambiguities in the procedures regarding the use of the traffic collision avoidance system (TCAS) on board.

[4]: 110 [BFU 1] 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 architect whose wife and two children died in the accident.

[8] Forty-six of the passengers were Russian schoolchildren from the city of Ufa, in Bashkortostan, on a school trip organised by the local UNESCO committee to the Costa Daurada beach area of Catalonia.

[13] They travelled on an overnight train to Moscow and arrived on 29 June, then, as their driver accidentally took them to the wrong airport, they missed their original flight.

[15] DHL International Aviation ME flight 611, was a 1990-built Boeing 757-23APF and was first delivered to Zambia Airways as 9J-AFO before being sold to Gulf Air as VH-AWE[16] in late 1993.

The flight was being flown by two Bahrain-based pilots,[17][18] 47-year-old British Captain Paul Phillips and 34-year-old Canadian first officer Brant Campioni, who had to fly the second leg of the journey.

At 23:21 CEST (21:21 UTC),[21] DHL Flight 611 reported to the area control center responsible for southern German airspace.

[4]: 76 [BFU 5] About eight seconds before the collision, Flight 611's descent rate was about 12 m/s (2,400 ft/min), not quite as rapid as the 13 to 15 m/s (2,500 to 3,000 ft/min) range advised by the TCAS.

Flight 611, now with 80% of its vertical stabilizer lost, continued for a further 7 kilometres (4.3 mi; 3.8 nmi) for two minutes, before crashing into a wooded area close to the village of Taisersdorf at a 70° downward angle.

Each engine ended up several hundred meters away from the main wreckage, and the tail section was torn from the fuselage by trees just before impact.

Bahrain's statement also mentions the lack of crew resource management in the Tupolev's cockpit as a factor in the crash.

[22] Switzerland notes that the Tupolev was about 33 m (108 ft) below the flight level ordered by the Swiss controller, and still descending at 580 m/min (1,900 ft/min).

[25] Skyguide, after initially having blamed the Russian pilot for the accident, accepted full responsibility and asked relatives of the victims for forgiveness.

In another case before the court in Konstanz, Skyguide's liability insurance is suing Bashkirian Airlines for 2.5 million euro in damages.

The case was opened in March 2008; the legal questions are expected to be difficult, as the airline has filed for bankruptcy under Russian law.

[25][32] The Swiss police arrested Kaloyev at a local motel shortly afterward, and in 2005, he was sentenced to eight years for manslaughter.

[33] He was released in November 2007, having spent three and a half years in prison, because his mental condition was not sufficiently considered in the initial sentence.

Kaloyev was treated as a hero back home at Vladikavkaz, and expressed no regret for his actions, instead blaming the victim for his own death.

[23] The medal is awarded for the highest achievements, improving the living conditions of the inhabitants of the region, educating the younger generation, and maintaining law and order.

[4]: 103  The BFU recommended that this ambiguity should be resolved in favor of obeying TCAS advisories even when these were in conflict with ATC instructions.

[36]: 2, 176, 134, 22  Japan published its report 11 days after the Überlingen accident, in it calling on the International Civil Aviation Organization (ICAO) to make it clear that TCAS advisories should always take precedence over ATC instructions.

This proposal would have created a "reversal" of the original warning – asking the DHL plane to climb and the Tupolev crew to descend.

[4]: 35  According to an analysis by Eurocontrol, this would have avoided the collision if the DHL crew had received and followed the new instructions and the Tupolev had continued to descend.

[4]: 50 The investigation report contains a number of recommendations concerning TCAS, calling for upgrades, better training and clearer instructions to the pilots.

[4]: 111–113 [BFU 16] The TCAS II system was redesigned, with its ambiguous "Adjust Vertical Speed" RA voice command changed to "Level-Off", to increase proper responses from pilots.