About the chain of events, human factors, a momentous error, and uncertainty about TCAS, you can read here:
1. An Unusual Night Shift at ACC Zurich on 1 July 2002
1.1. Starting the Shift
When the air traffic control officers (ATCOs) started their night shift at the area control centre Zurich (ACC Zurich) on 1 July 2002, they could have read about the sectorisation work, which was to be carried out that night in order to rearrange the control sectors. Even though time was reserved for this, the air traffic controllers did not read the material.
Later, it was discovered that this did not have any consequences and would not have altered the course of events, because the information did not contain any details about how the systems used by the air traffic controllers would be affected by the work.
ACC Zurich is responsible for an airspace which also includes the German part of the Lake Constance region. During daytime, this airspace is one of the busiest air traffic crossroads in Central Europe. At night, however, the number of passing aircraft decreases substantially.
1.2. Peter Nielsen Is Left as the Sole Air Traffic Controller
Long Breaks During the Night Shift That Were Tolerated by Upper Management
Peter Nielsen, a 35-year-old Danish-born air traffic controller, had to handle the control over the airspace alone when his colleague went on his prolonged break. These breaks had become the norm at ACC Zurich during the night, when the flow of air traffic in the airspace is low. The Management of ACC Zurich knew about the breaks and had been tolerating them for years.
Working Two Workstations and in Two Roles Simultaneously
Nielsen had to handle two workstations alone. On the first, he continued to monitor the air traffic which passed through the airspace. The other one was used to guide traffic to Friedrichshafen Airport.
It is common that two air traffic controllers work together as so-called radar executive and radar planner(RE and RP). Whereas the first is in contact with the flight crews, the second, for instance, plans ahead for the aeroplanes which will enter the airspace and makes sure that the radar executive will not experience any surprises. In addition, the two air traffic controllers work as a team and control each others´ actions.
From a CRM point of view, this is an example of cross checking and using the four-eye principle. Working alone, Peter Nielsen did not have this support late at night.
1.3. The Maintenance Personell Arrived at ACC Zurich
Maintenance workers approached Nielsen and told him that they were going to work on the main radar system and that his workstation would be in backup mode. Later, they returned to him and let him know that they would also be working on the telephone system, which would also be in backup mode.
2. Two Flights Were Heading Towards the Airspace
2.1. Flight BTC2937 From Moscow, Russia, to Barcelona, Spain
The pilots of the Bashkirian Airlines flight BTC2937 took off from Moscow´s Domodedowo Airport at 20:40 hrs local time. They were on their way to Barcelona. The aeroplane carried 45 school children, amongst other passengers. For the flight, a Tupolev TU154M was used. At 21:06 hrs (CET), it cruised above Austria.
2.2. Flight DHX611 From Bergamo, Italy to Brussels, Belgium
The crew of the DHL flight DHX611 took off from Bergamo and was heading for Brussels. It was a cargo flight for which a Boeing 757-200 was employed.
At 21:21 hrs, the pilots made contact to the ATCO at ACC Zurich. They requested permission to climb to FL 360 (36 000 feet). Peter Nielsen told them to expect the permission four to five minutes later and added that he had to handle other traffic.
Note: The flight control strips (paper strips with information on each flight), which the ATCO had in front of him, did not provide any information that the two planes were in a conflict situation (1).
1 ) Bundesstelle für Flugunfalluntersuchung, BFU, German Federal Bureau of Aircraft Accidents Investigation (May 2004): Investigation Report, AX001-1-2/02, page 75
3. An Additional Aeroplane Had to Be Handled
3.1. Flight AEF1135 Was About to Land at Friedrichshafen Airport
At 21:25 hrs, Peter Nielsen received another flight control strip. It announced an unexpected delayed flight, which was about to land at Friedrichshafen Airport in Germany. Therefore, he had to switch to the other workstation in order to handle flight AEF1135. He tried to call the ATCOs in the tower of Friedrichshafen Airport to notify the approaching airbus to them. He could not get through, however.
As he still had ten minutes left until the airbus was due to land in Friedrichshafen, he switched back to the first workstation. There, he instructed flight DHX611 to climb to FL 360.
3.2. The Tasks Became Intertwined and Precious Minutes Were Lost
Peter Nielsen went back to his second workstation and tried to call ATC in Friedrichshafen a second time without success. Before he could work out a solution, the crew of flight BTC2937 called ACC Zurich and announced its entrance into the airspace at FL 360. Nielsen rolled back to his first workstation on his office chair and asked them to repeat their message.
Almost at the same time, the crew of flight AEF1135 called ACC Zurich again. Nielsen switched back to his second workstation and called Friedrichshafen a third time without success. Following that, he radioed the crew in the airbus directly and told them to make contact with Friedrichshafen as he had lost his telephone connection.
4. A Short Digression: What Is TCAS?
The traffic alert and collision avoidance system, TCAS, is installed in aeroplanes. When another plane, which is also fitted with TCAS, flies too close, TCAS alerts the flight crew with the words "traffic, traffic". It then provides the crew with a resolution advisory, RA. For this, the crew of one plane is advised to climb ("climb, climb"), whereas the other one hears the words "descend, descend". There is also a small TCAS display in the cockpit on which the information is presented visually.
TCAS is meant to offer an extra layer of safety. Aircraft should always be separated well enough from one another and never come so close to each other that the system is activated in the first place. However, the so-called separation infringements happen nonetheless. As we will see later, in 2002, the system was still new, and the rules on how to use it where not standardised internationally yet.
5. The Last Minutes Before the Mid-Air Collision
5.1. TCAS Warned the Pilots
In the Boeing 757-200, TCAS started warning the pilots of crossing traffic with its typical "traffic, traffic". As an RA, it instructed them to descend. The two pilots followed this instruction.
In the Tupolev TU154M, TCAS started to sound at exactly the same time. It advised the crew to climb.
Almost at the same time and not knowing about the TCAS action, Peter Nielsen had switched back to his first workstation and realised that an urgent situation had evolved. He could now see the two flights DHX611 and BTC2937 approaching each other fast and at the same flight level. A separation infringement had already occurred.
The short term conflict alert, STCA, which acts as an extra safety layer and warns ATCOs of separation infringements, was not available to Nielsen due to the ongoing sectorisation work.
5.2. ATC Instruction and Confusion in the Russian Crew
The ATCO immediately advised the Russian crew to descend to FL 350. His instruction reached them almost at the same time at which TCAS had instructed them to climb. Subsequently, the pilots were confused. They did not know for a moment what they should do next. Peter Nielsen repeated his message to them.
Although one team member in the Russian cockpit had pointed out that TCAS had ordered them to climb, the crew neglected this speak up and decided to follow the air traffic controller´s instruction. It did not tell the air traffic controller about its conflict, either.
The two aeroplanes were now approaching each other at a speed of 1300 km/h.
Apparently stressed, Nielsen told the Russian crew that it had crossing traffic from its right-hand side, the two o´clock position. However, this was wrong, the cargo plane approached it from the left side.
5.3. Peter Nielsen Turned His Attention to the Airbus Again
As he saw that the crew of flight BTC2937 had followed his instruction and descended to FL350, Nielsen returned to the second workstation to turn his attention to the airbus again. The crew confirmed that it had made contact with Friedrichshafen Airport. Hence, the air traffic controller could now hand over the airliner.
When he returned to the first workstation, he tried to call BTC2937, but received no answer. On the radar screen, the flight with its tag turned into a red spot. The two planes had crashed at almost right angles at 21:35:31 hrs. The Russian plane had been torn into four pieces. The Boeing 757-200 had lost 80% of its vertical stabiliser (the tail fin). Without this stabiliser, an aeroplane cannot be piloted any more. Thus, flight DHX611 spun uncontrollably towards the ground.
5.4. ATC Karlsruhe Tried to Call Eleven Times
The air traffic controller in Karlsruhe, Germany, had noticed the two planes that were approaching each other on the same flight level. He tried to call ACC Zurich eleven times, but could not reach them due to the work on the telephone system. Only in case the ATCO, who is responsible for the planes, has left his or her workplace or become incapable, an air traffic controller from another ACC may contact the airliners directly. The ATCO in Karlsruhe followed this protocol.
In this case, the investigators have come to the conclusion that a direct contact would most likely not have prevented the accident and would only have added confusion. In addition, the two aeroplanes might not have been able to receive a radio message from ATC Karlsruhe on the respective frequency.
6. The Chain of Events in the Überlingen Mid-Air Collision
In the figure below, the links of the chain of events in the Überlingen mid-air collision are shown.
7. Which of the Human Factors Do We Detect?
Below, we will discuss which of the members of the so-called "Dirty Dozen" we detect in this mid-air collision. We will look at ATC and the cockpit crew of flight BTC 2937 in particular.
7.1. Human Factors Also Contributed to Errors in Air Traffic Control
Lack of Resources
Peter Nielsen was certainly lacking vital resources, as his workstations and his telephone were in backup mode. The STCA was not available to him. His colleague had gone on a break, which is why cross checking and using the four-eye principle were not possible.
Due to the necessity to handle the additional airbus, there was a lack of time, which was also the cause for not asking the controller assistant, CA, for help. Furthermore, there was a lack of detailed information about the sectorisation work.
Lack of Awareness
Peter Nielsen realised too late that flights DHX611 and BTC2937 had come too close to one another. This was presumably and at least partially due to distraction, stress, and pressure.
Distraction
Distraction was caused by the extra airliner, having to switch between tasks, and the repeated telephone calls, which were futile.
Stress
Stress was caused by working two control stations and handling one additional, unexpected plane without being able to call the control tower at Friedrichshafen Airport. It must have been at a maximum level, when Peter Nielsen realised on his screen that the two planes were approaching one another on the same flight level.
Pressure
Pressure often exists in dynamic work environments. In this case, Nielsen put himself under pressure when he assumed additional work in difficult circumstances.
Norms
Norms are habits that might not always be beneficial. The prolonged break could be seen as a norm at ACC Zurich. It has nothing to do with a lack of teamwork, which, according to the report, was generally good at the control centre.
A Lack of Assertiveness Cannot Be Assumed
Peter Nielsen did not ask the CA, who was present in the control room, to compare the information on the control strips with the information on the radar screen. According to the investigation report, he had assumed that he could handle both tasks and also stated that a lack of time was the reason why he did not ask for help.
Help is an important resource in CRM. We should assesss our need for it carefully and ask for it early.
Lack of Communication
A Lack of communication can be detected at several points. There was no precise communication about the sectorisation works, let alone about how to compensate for the systems which would be out of order. Neither of the two cockpit crews involved communicated the TCAS action to ATC. Several times, crews had to repeat their messages for the ATCO, as he was busy switching back and forth between the two tasks.
Peter Nielsen´s Resulting Errors
Due to the handling of Flight AEF1135 and the complications involved, the precious resource of time became scarce. Contrary to his judgement, the ATCO could not handle both the separation of the flights and the airbus, which was about to land at Friedrichshafen airport.
As a result, he noticed the separation infringement too late. Most likely due to the stress and pressure, he gave the Russian crew wrong information about the crossing traffic ("at your two o´clock position").
It is mentioned in the report that Peter Nielsen had never received any team resource management training. He acquired his knowledge of human errors on the job (page 85).
7.2. Human Factors Affected the Pilots of Flight BTC2937
Stress and pressure may certainly have developed when TCAS and the ATCO provided the crew with different instructions. A speak up by one of the team members was not heard. In the end, the Russian pilots followed the air traffic controller´s guidance.
8. The Root Causes of the Überlingen Mid-Air Collision
8.1. The Causes of the Mid-Air Collision According to the Investigation Report
According to the Investigation Report, the following factors contributed to the mid-air collision near Überlingen:
- the ATCO noticed too late that the separation of aircraft had been infringed
- the Russian crew chose to follow the air traffic controller´s instruction and not the TCAS RA
- at ACC Zurich, there were too few ATCOs during the night shift due to the long breaks
- the breaks had been tolerated by the management for years
8.2. The Role of TCAS in the Überlingen Mid-Air Collision
Whereas the pilots of flight DHX611 followed the TCAS RA, the crew of flight BTC2937 did not. The investigators learned from the Russian handbooks that in case of a discrepancy, the pilots had to follow the air traffic controller´s instruction, which was contrary to Western European procedures.
In 2002, TCAS was still new in Europe, and international standards did not exist yet. Hence, in this mid-air collision, TCAS did not help to prevent the fatal accident.
9. Peter Nielsen
9.1. A Separation Infringement in 2001
In 2001, there was an separation infringement under Nielsen´s control, which was partly due to an error in judgement by him. The incident was investigated by the Swiss BFU (Büro für Flugunfalluntersuchungen, Aircraft Accident Investigation Bureau, AAIB). Also when this separation infringement occurred, he had to handle a section alone without any cross checking by another air traffic controller.
By no means, his professionalism and abilities as such were questioned by his company or the Swiss AAIB. As a consequence of the incident, an additional tool was implemented on the radar monitor of each workstation (Investigation Report, page 82).
9.2. The Aftermath of the Mid-Air Collision in 2002
Following the mid-air collision on 1 July 2002, Peter Nielsen suffered from traumatic stress and had to seek medical help. He did not work as an air traffic controller at ACC Zurich again after the accident. Before that, according to his colleagues and managers who had been interviewed for the investigation report, he had been a truly competent and team-oriented air traffic controller with a highly professional attitude.
We have covered many an aspect and looked at the chain of events, as well as the human factors which contributed to this horrible air accident. Not everyone, however, is able to see this multifaceted picture, which a meticulous investigation produces.
Vitaly Kaloyev, who had lost his wife and two children in the accident, saw Peter Nielsen as the sole scapegoat. He approached the former air traffic controller at his home in Klothen near Zurich on 24 February 2004, confronted him with the accident and stabbed him to death in front of his wife and three children.
In remembrance of the 71 victims of the Überlingen mid-air collision
In remembrance of Peter Nielsen
10. One More ATC Case in the Next Blog Article
The role of an air traffic controller is highly demanding as we have seen in the first three articles of this series. In April, we will examine an accident which happened on an airport runway. Why could the air traffic controller, who was responsible for the two aeroplanes involved, not prevent it? We will uncover an array of contributing factors, ranging from the airport layout to staffing.
Author: Eva-Maria Schottdorf
Date: March 30th, 2024
On my blog page , I have linked more blog articles for you.
After one year, the articles will be removed from the blog page, updated, augmented and turned into books. You can purchase these directly on the page "A Special CRM Book Series" . The first volume is already waiting there for you.