Überlingen Mid-Air Collision- The Swiss Cheese Effect

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 service operating from Moscow, Russia, to Barcelona, Spain, with sixty passengers and nine crew members on board. Among the passengers were forty-five Russian schoolchildren from Ufa, Bashkortostan, participating in a school trip to Spain's Costa Dorada region, organized by the local UNESCO committee. The majority of the children's parents held high-ranking positions in Bashkortostan.

The aircraft, a Tupolev Tu-154M with registration RA-85816, was operated by a seasoned Russian crew. The team was led by Captain Alexander Mihailovich Gross, aged 52, who had accumulated over 12,000 flight hours. First Officer Oleg Pavlovich Grigoriev, aged 40, who also served as the chief pilot of Bashkirian Airlines, had 8,500 hours of flying experience and was responsible for assessing Captain Gross's performance during the flight. Although Murat Ahatovich Itkulov, aged 41, a pilot with nearly 7,900 flight hours, was typically the first officer, he did not officially serve in this capacity due to the nature of the captain's assessment flight. The crew also included 50-year-old Flight Navigator Sergei Gennadyevich Kharlov, who had approximately 13,000 flight hours, and 37-year-old Flight Engineer Oleg Irikovich Valeev, who had nearly 4,200 flight hours.

DHL Flight 611, a Boeing 757-23APF cargo aircraft with the registration A9C-DHL, departed from Bahrain and was operated by two pilots based in Bahrain. The flight crew comprised 47-year-old British Captain Paul Phillips and 34-year-old Canadian First Officer Brant Campioni. Both pilots were highly experienced; Captain Phillips had nearly 12,000 flight hours, while First Officer Campioni had over 6,600 flight hours. At the time of the incident, the aircraft was on a scheduled 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.

Peter Nielsen, the sole air traffic controller managing the airspace, was simultaneously operating two workstations. The increased workload, combined with delayed radar data, prevented him from recognizing the problem promptly, resulting in his failure to maintain a safe distance between the aircraft. Less than a minute before the accident, he became aware of the danger and contacted Flight 2937, instructing the pilot to descend to flight level 350 to avoid a collision with crossing traffic (Flight 611). Moments after the Russian crew initiated the descent, their Traffic Collision Avoidance System (TCAS) advised them to climb, while the TCAS on Flight 611 instructed its pilots to descend. If both aircraft had followed these automated instructions, the collision would have been averted.

The pilots of Flight 611, operating a Boeing aircraft, complied with the TCAS instructions and began a descent. However, they were unable to immediately inform Nielsen, as the controller was managing Flight 2937. Approximately eight seconds before the collision, Flight 611's descent rate was approximately 2,400 ft/min, which was slightly below the recommended 2,500 to 3,000 ft/min range specified by the aircraft's TCAS. Meanwhile, the Tupolev pilot ignored the TCAS directive to climb, having already initiated a descent as per the controller's instructions. Consequently, both aircraft were descending simultaneously. Unaware of the TCAS alerts, Nielsen reiterated his descent instruction to Flight 2937, providing the Tupolev crew with incorrect information regarding the DHL plane's position. He indicated that the Boeing was at their 2 o’clock position, whereas it was actually to their left at the 10 o’clock position.

Eight seconds before the collision, the crew of Flight 2937 became aware of the impending issue when they visually identified Flight 611 approaching from the left. In response, Flight 611 increased its rate of descent. Two seconds before the collision, the pilots of Flight 2937 finally complied with the jet's TCAS directive to ascend and attempted to initiate a climb, but by then, a collision was unavoidable. The aircraft collided at 23:35:32 local time, nearly at a right angle, at an altitude of 34,890 feet. The Boeing's vertical stabilizer completely severed Flight 2937's fuselage just ahead of the Tupolev's wings. The Tupolev disintegrated into several pieces, dispersing debris over a wide area. The nose section descended vertically, while the tail section, with the engines, continued forward, stalled, and fell. The severely damaged Boeing, having lost 80% of its vertical stabilizer, managed to travel an additional four miles before crashing into a wooded area near the village of Taisersdorf at a 70-degree downward angle. Each engine was found several hundred meters from the main wreckage, and the tail section was torn from the fuselage by trees just before impact. Tragically, all 69 individuals on the Tupolev and the two on board the Boeing perished.

The Swiss Cheese Effect

The first hole lines up

Peter Nielsen of ACC Zurich was the sole air traffic controller managing the airspace through which the aircraft was flying. The other controller on duty was resting in a different room for the night. This practice contravened SkyGuide's regulations but had been a common occurrence for years, with management's knowledge and tolerance.

The second hole lines up

The main radar image processing system was undergoing maintenance, requiring controllers to rely on a backup system. Meanwhile, the ground-based optical collision warning system, which could have notified the controller about the impending collision roughly 22 minutes in advance, was turned off for maintenance. Nielsen was not informed of this.

The third hole lines up

There was still an aural STCA warning system in place, which issued a warning directed to workstation RE SUED at 23:35:00, 32 seconds before the collision. However, no one present at that time heard the warning, and a subsequent technical audit found no errors in the system. It is important to note that the functionality of this audible warning is not something that is technically logged. Even if Nielsen had heard the warning, it would have been impossible for the air traffic controller to find a useful resolution order at that time.

The fourth and final hole lines up

At the time, TCAS was not mandatory in Russia, even though it was installed on their aircraft. In situations of conflict, European pilots are trained to follow only the TCAS instructions and to ignore any conflicting directives from Air Traffic Controllers. Conversely, Russian pilots are trained to make independent decisions in such scenarios. The pilots of Flight 2937 chose to follow the ATC command and disregard the TCAS alert.

All participating countries had the option to include additional "deviating" statements in the official report. The Kingdom of Bahrain, Switzerland, and the Russian Federation submitted their positions, which were published alongside the official report.

The statement from the Kingdom of Bahrain, the home country of the DHL aircraft, largely concurs with the report's conclusions. It suggests that the report should have focused less on individual actions and more on the organizational and managerial shortcomings of Skyguide. Additionally, Bahrain's statement highlights the absence of effective crew resource management in the Tupolev's cockpit as a contributing factor to the crash.

The Russian Federation claims that the Russian pilots were unable to comply with the TCAS advisory to ascend, as the advisory was issued when they were already at an altitude of 35,500 feet. At the time, the controller incorrectly indicated that there was conflicting traffic above them at 36,000 feet. Furthermore, the controller inaccurately reported the position of the DHL aircraft as being at 2 o'clock, whereas it was actually at 10 o'clock. Russia contends that the DHL crew had a "real possibility" to avoid a collision, given that they were able to hear the exchange between the Russian crew and the controller.

Switzerland observes that the Tupolev aircraft was approximately 108 feet below the flight level assigned by the Swiss controller and was continuing to descend at a rate of 1,900 feet per minute. The Swiss authorities assert that this was also a contributing factor to the accident. Additionally, Switzerland requested the BFU to formally acknowledge that the TCAS advisories would have been beneficial if followed without delay; however, the BFU declined to make such a finding.

The aftermath of the tragedy

Nielsen required medical care due to the traumatic stress resulting from the accident. At Skyguide, his former colleagues kept a vase with a white rose at Nielsen's previous workstation. Initially attributing the accident to the Russian pilot, Skyguide ultimately accepted full responsibility and sought forgiveness from the victims' families.

According to news reports, Skyguide provided compensation to the families of the deceased children. In accordance with international aviation regulations, the compensation amount was approximately $34,087. In 2011, the Swiss Federal Court rejected appeals from some relatives seeking increased compensation.

On 27 July 2006, a court in Konstanz ruled that the Federal Republic of Germany was obligated to compensate Bashkirian Airlines, determining that Germany bore legal responsibility for the actions of Skyguide. Although the government appealed this decision, a tacit agreement was reached between Bashkirian Airlines and the Federal Republic of Germany in late 2013, effectively concluding the case before a final legal determination.

In a separate legal proceeding at the Konstanz court, Skyguide's liability insurance is pursuing a lawsuit against Bashkirian Airlines, seeking damages amounting to 2.5 million euros. Initiated in March 2008, this case involves complex legal issues, particularly due to the airline's bankruptcy filing under Russian law.

In May 2004, a criminal investigation into Skyguide commenced. On August 7, 2006, a Swiss prosecutor brought manslaughter charges against eight Skyguide employees, seeking prison sentences of up to 15 months upon conviction. The verdict was delivered in September 2007, resulting in three of the four convicted managers receiving suspended prison sentences, while the fourth was fined. The remaining four Skyguide employees were exonerated of any charges.

Grieving the tragic loss of his wife and two children aboard flight 2937, Russian architect Vitaly Kaloyev held Peter Nielsen accountable for their deaths. On February 24, 2004, Kaloyev confronted and fatally stabbed Nielsen at his residence in Kloten, near Zürich, in the presence of Nielsen's wife and three children. Swiss authorities apprehended Kaloyev at a nearby motel shortly thereafter, and in 2005, he was sentenced to prison for the murder. He was released in November 2007 after it was determined that his mental condition had not been adequately considered during sentencing. In January 2008, he was appointed Deputy Construction Minister of North Ossetia. In 2016, Kaloyev received the highest state honor from the government, the "To the Glory of Ossetia" medal, which is awarded for outstanding achievements, enhancing the living conditions of the region's inhabitants, educating the younger generation, and upholding law and order.

The accident highlighted concerns regarding how pilots should respond to conflicting instructions from the Traffic Collision Avoidance System (TCAS) and air traffic control (ATC). At the time of the incident, TCAS was a relatively new technology, having been mandatory in Europe since 2000. When TCAS issues a resolution advisory (RA), the pilot in command is expected to respond immediately by focusing on the RA displays and executing the indicated maneuvers, unless such actions would compromise the safety of the flight or if the flight crew can ensure separation through clear visual identification of the aircraft causing the RA. Upon responding to a TCAS RA that requires deviation from the assigned altitude, the flight crew should communicate with ATC as soon as feasible. Once the RA is resolved, the flight crew should inform ATC of their return to the previously assigned clearance or acknowledge any new clearance provided.