2002 Überlingen mid-air collision  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, near the Swiss border. All 69 passengers and crew aboard the Tupolev and both crew members of the Boeing were killed. The official investigation by the German Federal Bureau of Aircraft Accident Investigation (, (BFU)) 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 as well as ambiguities in the procedures regarding the use of TCAS, the on-board 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. Bashkirian Airlines Flight 2937 was a chartered flight from Moscow, Russia, to Barcelona, Spain, carrying 60 passengers and nine crew. Forty-five 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 Dorada area of Spain. 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. The aircraft, a Tupolev Tu-154M registered as was piloted by an experienced Russian crew: 52-year-old Captain Alexander Mihailovich Gross (Александр Михайлович Гросс) and 40-year-old First Officer Oleg Pavlovich Grigoriev (Олег Павлович Григорьев). 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.  41-year-old Murat Ahatovich Itkulov (Мурат Ахатович Иткулов), a seasoned pilot with close to 7,900 flight hours (with 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. 50-year-old Sergei Gennadyevich Kharlov (Сергей Геннадьевич Харлов), a flight navigator with approximately 13,000 flight hours (including 6,421 hours on the Tu-154), and 37-year-old Flight Engineer Oleg Irikovich Valeev (Олег Ирикович Валеев), who had almost 4,200 flight hours (all of which were on the Tu-154), joined the three pilots in the cockpit.