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. All of the passengers and crew aboard both planes were killed, resulting in a total death toll of 71 including 52 children.

The official investigation by the German Federal Bureau of Aircraft Accident Investigation (; 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.Section 3.2 "Causes", page 110

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.

Background

Tupolev aircraft and crew BAL Bashkirian Airlines flight BTC2937 was a chartered flight from Moscow, Russia, to Barcelona, Spain, carrying 60 passengers and 9 crew. 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. Most of the parents of the children were high-ranking officials in Bashkortostan. One of the fathers was the head of the local UNESCO committee. 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. They remained there for two days until 1 July in order to find the arranged chartered flight. Flight 2937 departed from Moscow Domodedovo Airport at 22:48 Moscow Time (18:48 UTC) bound for Barcelona International Airport (now Josep Tarradellas Barcelona–El Prat Airport).

The aircraft was a 1995-built Tupolev Tu-154M, with aircraft registration .

The flight was piloted by an experienced Russian crew: 52-year-old captain Alexander Mikhailovich Gross () and 40-year-old first officer Oleg Pavlovich Grigoriev (). The captain had more than 12,000 flight hours (including 4,918 hours on the Tu-154) to his credit. Grigoriev, the chief pilot of Bashkirian Airlines, had 8,500 hours of flying experience (with 4,317 hours on the Tu-154) and his task was to evaluate Captain Gross's performance throughout the flight.

Murat Akhatovich Itkulov (), a 41-year-old pilot with close to 7,900 flight hours (4,181 of them on the Tu-154), who was normally the first officer, did not officially serve on duty because this was the captain's assessment flight. Sergei Gennadievich Kharlov (), a 50-year-old flight navigator with approximately 13,000 flight hours (including 6,421 hours on the Tu-154), and Oleg Irikovich Valeyev (), a 37-year-old flight engineer who had almost 4,200 flight hours (all of which were on the Tu-154), joined the three pilots in the cockpit.

Boeing aircraft and crew DHL International Aviation ME flight 611 was a 1990-built Boeing 757-23APF in service with DHL International and registered in Bahrain as . The flight was being flown by two Bahrain-based pilots, 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. Both pilots were very experienced — Phillips had logged close to 12,000 flight hours (including 4,145 hours on the 757) and Campioni had accumulated more than 6,600 flight hours, with 176 of them on the Boeing 757. Campioni had also previously worked as an air cadet and corporal in the Canadian Military Engineers. At the time of the accident, the aircraft was en route from Bahrain International Airport in Manama, Bahrain, to Brussels Airport in Brussels, Belgium, with a stop at Orio al Serio Airport in Bergamo, Italy, and departed Bergamo at 23:06 CEST (21:06 UTC).

Accident

The airspace was controlled from Zürich, Switzerland, by the Swiss federal airspace control Skyguide. Air traffic controller Peter Nielsen, the only controller handling the airspace, was burdened with working two workstations at the same time.

At 23:21 CEST (21:21 UTC), DHL Flight 611 reported to the area control center responsible for southern German airspace. Nielsen then instructed Flight 611 to climb from flight level 260 () to flight level 320 (). Flight 611 requested permission to continue the climb to flight level 360 () to save fuel. Nielsen permitted Flight 611 to climb after about four to five minutes, after which Flight 611 reached the desired altitude at 23:29. Meanwhile, Flight 2937 contacted Nielsen at 23:30, which also reported to be at flight level 360. Nielsen acknowledged the flight, but did not assign a different altitude to either aircraft. That meant that both were now at the same altitude and on a collision course.

At 23:34:49 CEST (21:34:49 UTC), about a minute before the collision, Flight 2937's TCAS instructed them to climb; seconds later, Nielsen, realizing the oncoming danger, contacted Flight 2937 and instructed the pilot to descend to flight level 350 (1000 ft lower) to avoid collision with crossing traffic (Flight 611). The crew of Flight 2937 disengaged the autopilot and initiated their descent at 23:35, obeying the controller instead of the TCAS. At about the same time, the TCAS on Flight 611 instructed the crew of that aircraft to descend.Section 4 "Safety Recommendations", pages 111–113 The collision would thus not have occurred if both the aircraft's crews had obeyed the TCAS's instructions.Section 1.16.2 "ACAS/TCAS II analysis", page 34

Flight 611's pilots followed their TCAS instructions and initiated a descent, but could not immediately inform Nielsen as he was dealing with Flight 2937. The crew of Flight 2937, however, descended as instructed by Nielsen,Section 2.7 "Analysis summary", pages 104–106 contrary to the TCAS's instructions which advised the crew to climb. Both planes were now descending.

Unaware of the TCAS-issued alerts, Nielsen repeated his instruction to Flight 2937 to descend, giving the crew incorrect information as to the position of the DHL plane, telling them that the plane was to the right (2 o'clock position) when it was in fact to the left (10 o'clock position).Section 2.4.1 "ATC Zurich", page 76 About eight seconds before the collision, Flight 611's descent rate was about , not quite as rapid as the range advised by the TCAS.

Flight 611, responding to the developing situation, increased its descent rate.Section 1.1.1 "Boeing B757-200", pages 6–7Section 1.1.2 "Tupolev TU154M", pages 7–9 Eight seconds before the collision, Flight 2937's crew became aware of the situation when they gained visual sight of Flight 611 incoming from the left, and, two seconds before the collision, obeyed their TCAS instruction and attempted to put the aircraft into a climb. Unfortunately, the collision had by then become inevitable. The two aircraft collided at 23:35:32 CEST (21:35:32 UTC), at almost a right angle, at an altitude of , with Flight 611's vertical stabilizer slicing completely through Flight 2937's fuselage just ahead of the wings. Flight 2937 broke into several pieces, scattering wreckage into the Brachenreute neighborhood over a wide area. The nose section of the aircraft fell vertically, while the tail section with the engines continued, stalled, and fell. Many of the children's bodies fell in the field just southeast of the memorial.

Flight 611, now with 80% of its vertical stabilizer lost, entered a flat spin dive, and continued for a further 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 after being torn from the plane due to airframe stress, causing the black boxes to stop before the plane hit the ground, and the tail section was torn from the fuselage by trees just before impact.Section 1.12 "Wreckages and impacts information", pages 19–33 All 69 people on board Flight 2937 and the two crew members on board Flight 611 died.Section 1.2 "Injuries to persons", page 9

Other factors in the crash Only one air traffic controller (ATC), Peter Nielsen of ACC Zurich, was controlling the airspace through which the aircraft were flying. The other controller on duty was resting in another room for the night. This was against Skyguide's regulations, but had been a common practice for years and was known and tolerated by management. Maintenance work was being carried out on the main radar image processing system, which meant that the controllers were forced to use a fallback system. Telephone systems were also under maintenance, preventing contact between other air traffic controllers. A delayed Aero Lloyd flight approached Friedrichshafen at a time when no additional landings were expected that night, and with the telephone system under maintenance, Nielsen was forced to remain in contact with the pilots until they came within radio range of the Friedrichshafen tower, while the other two planes neared collision.Section 1.17.1 "ATC Zurich", pages 35–42

The ground-based optical collision warning system, which would have alerted the controller to the pending collision about two minutes before it happened,Section 2.6.1.4 "Warning systems", page 88 had been switched off for maintenance. Nielsen was unaware of this. An aural short-term conflict alert warning system released a warning addressed to workstation RE SUED at 23:35:00 (32 seconds before the collision). This warning was not heard by anyone present at that time, although no error in this system could be found in a subsequent technical audit;  however, whether or not this audible warning is functional is not something that is technically logged. Even if Nielsen had heard this warning, at that time finding a useful resolution order by the ATC was impossible.

Deviating statements in the official report

All countries involved could add additional "deviating" statements to the official report. Bahrain, Switzerland, and Russia did submit positions that were published with the official report. The deviating statements were published verbatim as an appendix to the report by the BFU investigators.

The statement by Bahrain, the home country of the DHL plane, mostly agrees with the findings of the report. It says that the report should have put less emphasis on the actions of individuals and more on the faults within Skyguide's organization and management. Bahrain's statement also mentions the lack of crew resource management in the Tupolev's cockpit as a factor in the crash.

Russia states that the Russian pilots were unable to obey the TCAS advisory to climb; the advisory was given when they were already at , while the controller wrongly stated conflicting traffic was above them at . Also, the ATC gave the wrong position of the DHL plane (2 o'clock instead of the actual 10 o'clock). Russia asserts that the DHL crew had a "real possibility" to avoid a collision, since they were able to hear the conversation between the Russian crew and the controller.

Switzerland notes that the Tupolev was about below the flight level ordered by the Swiss controller, and still descending at . The Swiss say that this was also a cause of the accident. Switzerland also requested that the BFU make a formal finding that the TCAS advisories would have been useful if obeyed immediately; the BFU declined to do so.

Aftermath

Nielsen needed medical attention due to traumatic stress caused by the accident. At Skyguide, his former colleagues maintained a vase with a white rose over Nielsen's former workstation. Skyguide, after initially having blamed the Russian pilot for the accident, accepted full responsibility and asked relatives of the victims for forgiveness.