The theory of the “Swiss Cheese effect” was proven beyond any doubt on the night of 1 July 2002, when 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. All 69 passengers and crew aboard the Tupolev and the two crew members of the Boeing were killed.
Bashkirian Airlines Flight 2937 was a chartered flight from Moscow, Russia, to Barcelona, Spain, carrying sixty 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.
The aircraft, a Tupolev Tu-154M registered as RA-85816, 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 to his credit. Grigoriev, the chief pilot of Bashkirian Airlines, had 8,500 hours of flying experience 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 who was normally the first officer, did not officially serve on duty due to this being the captain's assessment flight. 50-year-old Sergei Gennadyevich Kharlov (Сергей Геннадьевич Харлов), a flight navigator with approximately 13,000 flight hours, and 37-year-old Flight Engineer Oleg Irikovich Valeev (Олег Ирикович Валеев), who had almost 4,200 flight hours, joined the three pilots in the cockpit.
DHL Flight 611, a Boeing 757-23APF cargo aircraft registered as A9C-DHL, had originated in Bahrain and was being flown by two Bahrain-based pilots, 47-year-old British Captain Paul Phillips and 34-year-old Canadian First Officer Brant Campioni. Both pilots were very experienced — the captain had logged close to 12,000 flight hours and the first officer had accumulated more than 6,600 flight hours. At the time of the accident, the aircraft was en route from Bergamo, Italy, to Brussels, Belgium.
Despite being just inside the German border, the airspace was controlled from Zürich, Switzerland, by the private Swiss airspace control company Skyguide.
The only air traffic controller handling the airspace, Peter Nielsen, was working two workstations at the same time. Partly due to the added workload, and partly due to delayed radar data, he did not realise the problem in time and thus failed to keep the aircraft at a safe distance from each other. Less than a minute before the accident he realised the danger and contacted Flight 2937, instructing the pilot to descend to flight level 350 to avoid collision with crossing traffic (Flight 611). Seconds after the Russian crew initiated the descent, their traffic collision avoidance system (TCAS) instructed them to climb, while at about the same time the TCAS on Flight 611 instructed the pilots of that aircraft to descend. Had both aircraft followed those automated instructions, the collision would not have occurred.
Flight 611's pilots on the Boeing jet followed the TCAS instructions and initiated a descent but could not immediately inform Nielsen because the controller was dealing with Flight 2937. About eight seconds before the collision, Flight 611's descent rate was about 2,400 ft/min, not quite as rapid as the 2,500 to 3,000 ft/min range advised by that jet's TCAS; as for the Tupolev, the pilot disregarded his jet's TCAS instruction to climb, having already commenced his descent as instructed by the controller. Thus, both planes were now descending. Unaware of the TCAS-issued alerts, Nielsen repeated his instruction to Flight 2937 to descend, giving the Tupolev crew incorrect information as to the position of the DHL plane, telling them that the Boeing was in their 2 o’clock when it was in fact to the left in a 10 o’clock position.
Eight seconds before the collision, Flight 2937's crew finally realised the problem when they gained visual sight of Flight 611 incoming from the left. Flight 611, in response, increased its descent rate. Two seconds before the collision, Flight 2937's pilots finally obeyed the jet's TCAS instruction to climb and attempted to put the aircraft into a climb, but the collision was now inevitable. The aircraft collided at 23:35:32 local time, at almost a right angle at an altitude of 34,890 ft, with the Boeing's vertical stabiliser slicing completely through Flight 2937's fuselage just ahead of the Tupolev's wings. The Tupolev broke into several pieces, scattering wreckage over a wide area. The nose section of the aircraft fell vertically, while the tail section with the engines continued, stalled, and fell. The crippled Boeing, now with 80% of its vertical stabiliser lost, struggled for a further our miles before crashing into a wooded area close to the village of Taisersdorf at a 70-degree downward angle. Each engine ended up several hundred metres away from the main wreckage, and the tail section was torn from the fuselage by trees just before impact. All 69 people on the Tupolev, and the two on board the Boeing, died.
The Swiss Cheese Effect
The first hole lines up
Only one air traffic controller, Peter Nielsen of ACC Zurich, was controlling the airspace through which the aircraft was flying. The other controller on duty was resting in another room for the night. This was against SkyGuide's regulations but had been a common practice for years and was known and tolerated by management.
The second hole lines up
Maintenance work was being carried out on the main radar image processing system, which meant that the controllers were forced to use a fallback system. The ground-based optical collision warning system, which would have alerted the controller to the pending collision approximately 22 minutes before it happened, had been switched off for maintenance, Nielsen was unaware of this.
The third hole lines up
There still was an aural STCA warning system, which released a warning addressed to workstation RE SUED at 23:35:00 (32 seconds before the collision); this warning was not heard by anyone present at that time, although no error in this system could be found in a subsequent technical audit — however, whether or not this audible warning is functional is not something which is technically logged. Even if Nielsen had heard this warning, at that time finding a useful resolution order by the air traffic controller is impossible.
The fourth and final hole lines up
TCAS wasn’t mandatory in Russia at the time although fitted to their aircraft, when there is a conflict European pilots are trained to comply with only the TCAS and disregard any conflicting commands from Air Traffic Controllers. Russian pilots are trained to make their own decisions in that situation, the pilots of Flight 2937 decided to obey the ATC command and disregard the TCAS.
All countries involved could add additional "deviating" statements to the official report. The Kingdom of Bahrain, Switzerland, and the Russian Federation did submit positions that were published with the official report.
The statement by the Kingdom of Bahrain, the home country of the DHL plane, mostly agrees with the findings of the report. It says that the report should have put less emphasis on the actions of individuals and more on the faults within Skyguide's organisation and management. Bahrain's statement also mentions the lack of crew resource management in the Tupolev's cockpit as a factor in the crash.
The Russian Federation states that the Russian pilots were unable to obey the TCAS advisory to climb; the advisory was given when they were already at 35,500 ft while the controller wrongly stated there was conflicting traffic above them at 36,000 ft. Also, the controller gave the wrong position of the DHL plane (2 o'clock instead of the actual 10 o'clock). Russia asserts that the DHL crew had a "real possibility" to avoid a collision since they were able to hear the conversation between the Russian crew and the controller.
Switzerland notes that the Tupolev was about 108 ft below the flight level ordered by the Swiss controller, and still descending at 1,900 ft/min. The Swiss say that this was also a cause of the accident. Switzerland also requested that the BFU make a formal finding that the TCAS advisories would have been useful if obeyed immediately; the BFU declined to do so.
The aftermath of the tragedy
Nielsen needed medical attention due to traumatic stress caused by the accident. At Skyguide, his former colleagues maintained a vase with a white rose over Nielsen's former workstation. Skyguide, after initially having blamed the Russian pilot for the accident, accepted full responsibility and asked relatives of the victims for forgiveness.
According to news reports, Skyguide did pay out compensations to the families of the dead children. Under international aviation laws, the compensation amount was about $34,087. The Swiss Federal Court turned down appeals from some relatives for higher compensation in 2011.
On 27 July 2006, a court in Konstanz decided that the Federal Republic of Germany should pay compensation to Bashkirian Airlines. The court found that Germany was legally responsible for the actions of Skyguide. The government appealed the ruling, but in late 2013 Bashkirian Airlines and the Federal Republic of Germany reached a tacit agreement, ending the court case before a decision on the legal issues was reached.
In another case before the court in Konstanz, Skyguide's liability insurance is suing Bashkirian Airlines for 2.5 million euros in damages. The case was opened in March 2008; the legal questions are expected to be difficult, as the airline has filed for bankruptcy under Russian law.
A criminal investigation of Skyguide began in May 2004. On 7 August 2006, a Swiss prosecutor filed manslaughter charges against eight employees of Skyguide. The prosecutor called for prison terms of up to 15 months if found guilty. The verdict was announced in September 2007. Three of the four managers convicted were given suspended prison terms and the fourth was ordered to pay a fine. Another four Skyguide employees were cleared of any wrongdoing.
Devastated by the loss of his wife and two children aboard flight 2937, Vitaly Kaloyev, a Russian architect, held Peter Nielsen responsible for their deaths. He tracked down and stabbed Nielsen to death, in the presence of his wife and three children, at his home in Kloten, near Zürich, on 24 February 2004. The Swiss police arrested Kaloyev at a local motel shortly after, and in 2005 he was sentenced to prison for the murder. He was released in November 2007 because his mental condition was not sufficiently considered in the initial sentence. In January 2008, he was appointed deputy construction minister of North Ossetia. In 2016, Kaloyev was awarded the highest state medal by the government, the medal "To the Glory of Ossetia". The medal is awarded for the highest achievements, improving the living conditions of the inhabitants of the region, for educating the younger generation and maintaining law and order.
The accident raised questions as to how pilots must react when they receive conflicting orders from the TCAS and from air traffic control (ATC). TCAS was a relatively new technology at the time of the accident, having been mandatory in Europe since 2000. When the TCAS issues a resolution advisory (RA), the pilot flying should respond immediately by direct attention to RA displays and manoeuvre as indicated, unless doing so would jeopardise the safe operation of the flight, or unless the flight crew can assure separation from the help of definitive visual acquisition of the aircraft causing the RA. In responding to a TCAS RA that directs a deviation from the assigned altitude, the flight crew should communicate with ATC as soon as practicable after responding to the RA. When the RA is removed, the flight crew should advise ATC that they are returning to their previously assigned clearance or should acknowledge any amended clearance issued.