Earlier this year the US witnessed its worst mid air collison in 25 years when an collided with an army helicopter near Reagan Washington National Airport .

The crash killed all 67 aboard the DC flight and all three soldiers on the - with wreckage from both aircrafts falling into the The demand for answers from the victims's grieving families is fierce and grows louder as the US National Transportation Safety Board investigation continues.

Above the small town of Überlingen in Germany, 22 years ago - another shocking mid-airoccurred - killing all 71 people on boards - leaving all relatives desperate for answers. The majority of those lost their lives were children.

On July 1 2002, passenger jet Bashkirian Airlines Flight 2937, and DHL International Aviation ME Flight 611, a Boeing 757 cargo jet, collided in mid-air over Uberluigen, a southern German town on Lake Constance near the German-Swiss border. The accident is the subject of a new episode of premiering tonight at 9pm on the

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The passenger jet was flying from Moscow to Barcelona and had a total of 69 occupants including the crew. There were 45 Russian schoolchildren on the flight on a school trip. The DHL cargo flight was flying from Bergamo to Brussels and had two pilots on board.

Relatives of the deceased were horrified to discover that Peter Nielsen, the controller at Zurich who was solely controlling the air space, did not know that the ground-based optical collision warning system at the control tower had been switched off for maintenance that fateful night. Nielsen worked for the Swiss federal airspace control . The system would have alerted him to the impending about two minutes before it happened - enough time to prevent it. Instead he only noticed and advised the crew 43 seconds before the collision.

Both planes had (traffic alert and collision avoidance systems) and both systems were working correctly. However while the pilots of the DHL plane obeyed TCAS's instruction to descend, the Russian crews were told by TCAS to climb. Instead, they followed Neilson's instruction to descend - putting them right back in the path of the DRL flight. This grave error doomed all on board the two planes.

Russian architect Vitaly Kaloyev lost three people he loved in the crash - his wife Svetlana and two children, 10-year-old Konstantin and 4-year-old Diana. Vitaly was working in Spain at the time and it was a much longed for family reunion. In 2004, while investigators were writing up the final report, Vitaly flew into Zurich, crazed with grief. He went to Nielsen's house and stabbed him to death. He found Nielsen in his garden and plunged a 14cm knife into his stomach before fleeing the scene.

He was arrested at Zurich airport and charged with murder. The dad was given an eight-year prison term for killing Neilsen. However, his sentence was later reduced after a Swiss judge ruled that he had acted with diminished responsibility.

After completing two-thirds of his term and receiving parole, Kaloyev was freed from prison on November 8th, 2007. Speaking on National Geographic Channel's Vitaly says of the killing: "This was not revenge. This was punishment. I wanted to hear them say we are responsible but forgive us if possible. They had killed so many children but nothing had happened. No apologies. No expressions of sorry."

The final report into the said that a chain of errors and technical problems led to a mid-air collision. Nielsen made mistakes but was also overworked. The Swiss air traffic control system Skyguide suffered serious technical problems that night and had failed properly to organize the night shift.

Skyguide paid out compensation to the families which was equivalent of $22,300 – 26,700 for each victim. In September 2007, three Skyguide managers were given suspended prison terms and a fourth was ordered to pay a fine. The 2017 Aftermath, a film starring Arnold Schwarzenegger, is partly based on the Überlingen midair crash and Kaloyev's story.

Air Crash Investigation 'Deadly Climb' premieres tonight (Monday March 10) on National Geographic Channel at 9pm

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