- Status: Preliminary
Bashkirian Airlines flight 2927 (a Tupolev 154) originated in Ufa, Russia and flew to Moscow to pick up passengers. From Moscow the aircraft continued as a charter flight to Barcelona. The flight used the RNAV-Route Salzburg - Traunstein - Kempten - Trasadingen at Flight Level 360. Communications were handed over from Munich to Zürich ACC at 23.30:11. At that moment one controller was responsible for the entire traffic in the Zürich airspace. He was monitoring two frequencies and two radar scopes. On one frequency (119,925 MHz) he was guiding one traffic for an approach into Friedrichshafen and on the other frequency (128,050 MHz) he had to control four aircraft. Between 23.25:43 and 23.33:11 LT the controller tried serveral times to establish contact with Friedrichshafen by phone. Because of working on the telephone net of Skyguide, the controller was not able to reach Friedrichshafen.
At 23.34:42, 50 seconds prior collision, the Tupolev's Honeywell 2000 TCAS gave a Traffic Advisory and seven seconds later the radar controller issued the first descent instructions to flight 2927 to FL350: "descend flight level 350, expedite, I have crossing traffic". This descent was necessary for continuation of the flight to Barcelona and to achieve a vertical separation with respect to an approaching DHL Boeing 757 cargo plane. This flight was en route from Bergamo, Italy to Brussels along RNAV-Route ABESI-AKABI-TANGO at FL360. At 23.42:56 the crews of both aircraft receive a Resolution Advisory-command from their TCAS. The DHL crew complied with this and initiate a descent, when the Tupolev crew were then trying to deal with the conflicting descent (by ATC) and climb (TCAS) instructions. Seven seconds after the Resolution Advisory-command, the ATC controller repeated the instruction to descend. The Bashkirian crew then decided to follown the ATC controller's instructions. A little later the Traffic alert and Collision Avoidance System (TCAS) aboard the Boeing 757 gave the crew a Resolution Advisory (RA) to "increase descent". They then contacted ATC, telling the controller that they were doing a TCAS descent. Since both aircraft were descending, the TCAS of the Russian plane warned the crew to "increase climb" to avoid a collision. This was eight seconds before the collision. Just prior to the collision, both crews detected the other aircraft, and reacted to avoid the collision by attempting appropriate flight manoeuvres. Nevertheless, at 23.35:32 both aircraft collided at approx. FL354. The tail fin of the Boeing 757 struck the left side of the Tupolev 154 fuselage near both overwing emergency exits. The Tupolev immediately broke up in four pieces (left wing, right wing, main fuselage and tail unit including the engines). The Boeing 757 lost control and crashed 8km North of the Tupolev, just after losing both engines.
That night, from 23.00h the configuration of the radar data processing of Skyguide was modified. Thus the system was operating in FALLBACK modus. This requires among other facts, that radar separation values were increased from 5 NM to 7 NM. Also, the STCA (Short Term Conflict Alert) was not available at that time. The STCA at Karlsruhe Upper Area Control Center (UAC) however did work. From 23.33:36 on the controller of Karlsruhe UAC, tried in vain to get in contact with Zürich-ATC until 23.35:34. Between 23.33:36 and 23.34:45 the busy signal was to be heared afterwards the ringing tone. According to his statements the controller tried repeatedly to establish the connection via the priority button, but it failed.
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Aviation Safety Network; updated 26 August 2002