What happened
On 1 July 2002, at approximately 21:35:32 hrs, a Tupolev TU-154M and a Boeagling B757-200 collided in the night sky near Ueberlingen, Germany. The Tupolev aircraft was operating a charter flight from Moscow-Domodedovo to Barcelona, carrying 60 passengers and 9 crew members. The Boeing aircraft was performing a cargo flight from Bergamo to Brussels, operated by a two-pilot crew.
Prior to the impact, the crew of the Tupolev TU-154M had been instructed by air traffic control to descend to FL 350 to avoid conflicting traffic. However, the aircraft's TCAS issued a "climb, climb" Resolution Advisory (RA). The crew responded to this RA by pulling back on the control column, which conflicted with the air traffic controller's descent instruction.
Simultaneously, the Boeing B757-200 was executing a descent following a TCAS Resolution Advisory to "descend, descend". As the Boeing aircraft increased its rate of descent, it entered the flight path of the Tupolev aircraft. The two planes collided at 34,890 ft, causing both to impact the ground north of Ueberlingen. The collision resulted in 69 fatalities (all 60 passengers and 9 crew on the Tupolev, and both pilots on the Boeing).
Findings
Investigations into the accident revealed that the primary cause was the conflicting instructions provided to the two crews. While the air traffic controller instructed the Tupolev crew to descend, the aircraft's TCAS system issued a command to climb. The crew of the Tupolev TU-154M followed the TCAS RA, while the crew of the Boeing B757-200 followed their TCAS descent command, leading the aircraft into a direct collision course.