We Americans, at the time, tended to place the blame for this midair squarely on the Russian pilots. But that was unfair. While U.S. regulations clearly stated that in the event of a disagreement between a TCAS Resolution Alert and Air Traffic Control instructions, pilots were to follow the TCAS. The ICAO regulations at the time were silent on the subject.
A year and a half prior to this midair, on 31 January 2001, two Japan Airlines Boeing 747s nearly collided over Japan in nearly identical circumstances. Both aircraft received proper TCAS resolution alerts but a trainee air traffic controller gave one of the planes conflicting instructions. The pilots spotted each other and managed to avoid collision by only 35 feet. Nine passengers and two crew were seriously injured during the violent evasive maneuvering. The Japanese government asked ICAO for guidance. ICAO decided to leave the rules up to individual governments.
After the DHL/Bashkirian midair, ICAO rules were changed. See TCAS for more about this.
As sad as this incident is, there is an additional tragic postscript. The air traffic controller working two screens with inoperative phones and radar and warning systems off line without his knowledge was hounded by the press and eventually murdered by a distraught father of one of the victims.
Lesson learned? Well we who started our TCAS careers in the United States continue to do as we always have: follow TCAS resolutions alerts over those of air traffic control. ICAO now agrees. (ICAO terminology calls the system Airborne Collision Avoidance System, ACAS.) It is also worth noting that just because you have TCAS doesn't mean you don't need to clear the skies with your eyes and your ears. More about this: The Big Sky Theory.
Everything here is from the references shown below, with a few comments in an alternate color.
Figure: Reconstruction of the collision according to the FDR data and collision evidence, from German Federal Bureau of Aircraft Accidents Investigation, pg. 33
Photo: B757 DHL A9C-DH, (flydata2.blogspot.com)
Photo: Tu-154M RA-85816, (flydata2.blogspot.com)
Bashkirian Airlines flight 2937 (a Tupolev 154) originated in Ufa (UFA), Russia and flew to Moscow (DME) to pick up passengers. From Moscow the aircraft continued as a charter flight to Barcelona (BCN). The flight used the RNAV-Route Salzburg - Traunstein - Kempten - Trasadingen at FL360. 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. Among these four aircraft were Bashkirian 2937 and a DHL Boeing 757 cargo plane, en route from Bergamo (BGY), Italy to Brussel (BRU), Belgium along RNAV-Route ABESI-AKABI-TANGO, also at FL360.
The Zurich ACC was contracted out to a private company, Swiss Air Navigation Services. They normally had two controllers and two assistants on duty at night, but one controller and assistant normally retired to a lounge once traffic flow decreased to a point where all concerned the load could be handled by one controller team.
Between 23:25:43 and 23:33:11 LT the controller tried several 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 the Tupolev's Honeywell 2000 TCAS gave a Traffic Advisory because of the DHL 757 in the area. Seven seconds later the radar controller issued descent instructions to flight 2937: "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 the approaching DHL Boeing 757.
[German Federal Bureau of Aircraft Accidents Investigation, page 53] TU154M Flight Operations Manual
At the time of this mishap, the ICAO did not mandate a procedure to follow the Resolution Advisory. Most country rules, including the ones the DHL crew were operating under, did place a priority of the RA over ATC instructions.
At 23:42:56 the crews of both aircraft received a Resolution Advisory (RA)-command from their TCAS. The DHL crew complied with this and initiate a descent. At the same time the Tupolev crew were 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 follow the ATC controller's instructions. A little later the TCAS aboard the Boeing 757 gave the crew a Resolution Advisory to "increase descent". They then contacted ATC, reporting 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, while the Tupolev's left wing sheared off 80% of the Boeing's tail fin. 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 8 km north of the Tupolev, just after losing both engines.
That night, from 23:00 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 heard 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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