Überlingen 2002: the midair collision over Lake Constance and what it meant for TCAS

If you hear in a cockpit documentary today how a crew reports “TCAS RA” to air traffic control and then doggedly follows the onboard computer, you are seeing the result of a summer night in 2002. Over the north shore of Lake Constance, two jets collided back then even though both had a working collision avoidance system on board. The Überlingen midair collision is still the case instructors use to explain why, in an emergency, there may be only one authority that decides. This article shows you the sequence, the chain of causes and what changed afterward for pilots, controllers and technology.

Überlingen 2002: the short answer

On July 1, 2002, at 23:35:32, a Russian Bashkirian Airlines Tupolev Tu-154M and a Boeing 757-200PF of the DHL freight network collided at an altitude of about 34,900 feet. The responsible controller at the Swiss air navigation service Skyguide noticed the conflict late and told the Tupolev to descend. Almost simultaneously, its TCAS instructed it to climb, while the Boeing’s TCAS ordered a descent. The Russian crew followed the controller, the DHL crew its system. Both aircraft descended toward each other.

Behind this moment stood a long chain: a single controller on the night shift, switched-off support systems, a failed telephone system and a regulation that did not clearly say whether TCAS or the controller takes priority.

The two flights

Bashkirian Airlines flight 2937 was a charter flight from Moscow-Domodedovo to Barcelona. On board the Tu-154M with the registration RA-85816 were 60 passengers and 9 crew members. Most passengers were schoolchildren from Ufa, the capital of the Russian republic of Bashkortostan, on their way to a vacation in Catalonia. The exact number of children among the victims is given slightly differently in the sources; it was around 50.

The Tupolev Tu-154 was the backbone of many Russian airlines at the time, a three-engine medium-haul jet with a T-tail that you now see practically only in a museum. More on the background is in our overview of Soviet airliners.

DHL flight 611 was operated by DHL International Aviation ME, based in Bahrain. The Boeing 757-200PF with the registration A9C-DHL came from Bahrain, had made an intermediate stop in Bergamo and was on its way to Brussels, DHL’s European hub at the time. Only the two pilots were on board. You find more on the express cargo network in our article on the integrators DHL, FedEx and UPS and on the page about DHL Aviation.

Who controlled the airspace

The collision happened over German soil but not under German control. Germany had delegated the airspace over parts of southern Germany around Lake Constance to the Swiss air navigation service Skyguide – a practical solution because the approaches to Zurich run through this area anyway. How the two Alpine countries organize their air traffic control is explained in our article on Austro Control and Skyguide.

On the night of the accident, only one controller sat at the workstations for this sector in the Zurich control center. His colleague had withdrawn to another room for a break. According to the investigation, this contradicted internal rules but had long been tolerated by management during quiet night hours.

Added to this was maintenance work on the equipment:

• The radar data system was running in fallback mode, with limited display among other things.
• The optical conflict alert (STCA), which warns the controller of insufficient separation, was switched off. The controller knew nothing about it.
• The main telephone system was out of service, and the backup system did not work.

At the same time, at a second workstation the controller was handling a delayed Aero Lloyd flight on approach to Friedrichshafen. Because the telephone wasn’t working, he couldn’t simply hand the approach over to the tower and had to switch back and forth between two screens and frequencies.

How the collision came about

The Boeing checked in with Zurich around 23:21, climbed at its own request to flight level 360 and reached it around 23:29. Shortly afterward, the Tupolev also checked in – likewise at FL 360. The two aircraft’s courses crossed. The controller acknowledged the radio call without assigning a different altitude.

The Tupolev broke apart in the air, with large parts crashing near Brachenreuthe north of Überlingen. The Boeing lost its vertical stabilizer and hit the ground a few kilometers farther north in the municipality of Owingen. The wreckage was spread over around 30 square kilometers in total. Nobody survived.

A side note that heightens the tragedy: In the upper control center of DFS in Karlsruhe, the system there had detected the conflict. The controllers tried several times to warn Zurich by telephone but couldn’t get through because of the disrupted lines. They were not allowed to intervene directly in airspace controlled by someone else. What DFS does otherwise is explained in our profile of DFS, Germany’s air navigation service.

What TCAS actually does

To understand why Überlingen changed so much, a look at the technology is worthwhile. TCAS II – ACAS II in ICAO terms – is a collision avoidance system that works on board independently of air traffic control. It interrogates the transponders of surrounding aircraft, calculates from distance and altitude change how much time remains until closest approach, and warns in two stages:

Level	Callout	Meaning	What the crew does
Traffic Advisory (TA)	“Traffic, traffic”	Traffic nearby, possible danger	Look out, prepare for a maneuver
Resolution Advisory (RA)	e.g. “Climb, climb” or “Descend, descend”	Acute collision risk	Follow the vertical maneuver immediately and promptly
Coordinated RA	opposite for each aircraft	The two systems coordinate via Mode S	One aircraft climbs, the other descends
Clear of Conflict	“Clear of conflict”	Danger over	Return to the assigned altitude, inform ATC

The decisive point is in the third row: The systems of both aircraft agree with each other. If both crews follow, the aircraft separate safely. If one crew doesn’t follow, the coordinated maneuver can even make the situation worse – exactly what happened over Überlingen. The Tupolev descended although its TCAS demanded a climb, and the Boeing descended because its TCAS assumed the Tupolev would climb.

According to the BFU, the Boeing also initially descended somewhat more slowly than the recommended rate of 2,500 to 3,000 feet per minute. That alone would not have caused the accident but shows how tight the tolerances are. If you want to dig deeper into how controllers secure distances between aircraft, you find background in our articles on wake turbulence and separation and on airspace classes.

Why the crew followed the controller

From today’s perspective, the Russian crew’s decision seems incomprehensible. In 2002 it was not. The international rules at the time did not clearly establish whether a TCAS resolution advisory or the air traffic control instruction takes priority. Many Western airlines were already training to always follow an RA, but that was not binding everywhere.

According to the BFU, the Bashkirian Airlines operations manual was even internally contradictory: In one place, air traffic control was described as authoritative and TCAS as an additional aid, in another place maneuvers contrary to an RA were prohibited. The crew still had the controller’s descent instruction in their ears when the RA came, and the experienced captain decided for the controller. That the controller placed the opposing traffic on the wrong side didn’t make it easier to assess the situation correctly.

On top of that: In 2001 there had already been a near collision in Japan between two Japan Airlines aircraft, in which one crew likewise followed a controller’s instruction instead of the RA. Numerous people on board were injured by the abrupt avoidance maneuver. The Japanese report with the recommendation to regulate the order of priority unambiguously appeared only shortly after Überlingen.

The BFU investigation

Because the accident happened over German territory, the German Federal Bureau of Aircraft Accident Investigation (BFU) led the investigation. It published its final report on May 19, 2004. The BFU deliberately named no individual as culpable but described two groups of causes.

Bahrain, Switzerland and Russia submitted divergent comments on individual points, which are attached to the report. This did not change the overall picture – a chain of organizational and technical weaknesses, at the end of which two crews reacted in opposite ways.

The consequences for TCAS

The most important lesson from Überlingen was a simple rule – and today it is anchored in every cockpit.

TCAS takes priority. ICAO changed its procedures in November 2003: Pilots must follow a resolution advisory even if it contradicts an air traffic control instruction. A contrary controller instruction must not override an RA.

Report instead of discuss. The crew tells air traffic control as quickly as possible that it is following a TCAS maneuver (“TCAS RA”). From that moment, the controller gives the affected aircraft no more altitude instructions until the crew reports “Clear of conflict.” You can get a sense of how such reports sound on the radio if you listen to aviation radio online.

New logic: version 7.1. Even before the accident, a change proposal was on the table that became known as CP112E: With it, TCAS detects when the other aircraft does not follow a coordinated RA and can then reverse its own advisory – “Descend” becomes “Climb.” Eurocontrol later concluded that this reversal logic would probably have prevented the Überlingen collision if both crews had followed it. With version 7.1, the often misunderstood callout “Adjust vertical speed, adjust” was also replaced by the unambiguous “Level off, level off.”

Mandatory in Europe. The EU prescribed TCAS II version 7.1 with Implementing Regulation (EU) No 1332/2011. It applies to turboprop and jet aircraft with a maximum takeoff mass above 5,700 kg or more than 19 passenger seats: for newly certified aircraft from March 1, 2012, for older aircraft from December 1, 2015. The coordination of such Europe-wide rules lies largely with Eurocontrol.

The consequences for air traffic control

On the ground, too, hardly anything stayed the way it was. Even during the investigation, the BFU addressed recommendations to Skyguide and the supervisory authorities: Maintenance work on safety-relevant systems should be compensated by substitute measures, and at least two controllers should be present at all times. The practice tolerated for years, in which a controller alone at night handles several workstations while the colleague takes a break, was thereby fundamentally called into question.

Überlingen is still considered a teaching example for the so-called Swiss cheese model: Every individual safety barrier has holes, but only when the holes of many barriers line up does the catastrophe occur. How air traffic control as a whole has developed is told in our article on the history of air traffic control. And if you are considering working in this profession yourself, you find information under Becoming an air traffic controller.

The legal aftermath

Skyguide initially rejected responsibility but later admitted mistakes and asked the bereaved for forgiveness. A court in Bülach convicted four of the company’s managers of negligent homicide in 2007; three received suspended prison sentences, one a fine. Four other defendants were acquitted.

The Konstanz regional court ruled in 2006 that the Federal Republic of Germany is liable for the mistakes of the Swiss air traffic control because it had transferred the airspace to Skyguide. Germany appealed, and the dispute ended in 2013 without a landmark ruling. Bashkirian Airlines itself was insolvent from 2007.

A second victim

On February 24, 2004, the controller who had been on duty on the night of the accident was stabbed to death in front of his house in Kloten near Zurich. The perpetrator was a man from North Ossetia who had lost his wife and two children in the disaster. A Swiss court convicted him in 2005, and he was released early in 2007. Former colleagues placed a vase with a white rose at his old workplace. For many in the industry, the case showed how strongly individual blame can cover up a system failure – one reason why modern accident investigation deliberately does not look for culprits but for causes.

Remembering at Lake Constance

At Brachenreuthe, a district of Überlingen, a memorial was inaugurated in May 2004. The sculpture “The Torn Pearl Necklace” (Die zerrissene Perlenkette) by sculptor Andrea Zaumseil stands for the severed life paths of the victims. Relatives planted 71 trees there for the dead. There are further memorials at Skyguide in Dübendorf and in a cemetery in Ufa, where many of the victims are buried.

What this means for you as a spotter

Überlingen is not an event you see at the fence, but it shapes what you hear there and read on the radar. When you hear a terse “TCAS RA” on aviation radio, you now know why the controller falls silent afterward. When you see two aircraft approaching at the same altitude on a flight tracker map, you know the logic by which they separate. And if you are interested in the technology behind it: TCAS works with the transponders on 1030 and 1090 MHz, and on 1090 MHz aircraft also transmit the Mode S and ADS-B data you can record with a receiver of your own – a starting point is our article on flight tracking with ADS-B.

The region itself is certainly interesting for spotters. Around Friedrichshafen with the AERO trade show, the Dornier Museum and, on the Swiss side, the Altenrhein aviation museum, a visit is worthwhile. And it’s not far to Zurich.

Überlingen and Tenerife: two turning points

Much like the 1977 Tenerife disaster, Überlingen changed aviation less through new hardware than through clearer rules for communication. At Tenerife, it was about ambiguous radio calls and hierarchy in the cockpit; over Lake Constance, about the question of who has the say at the last moment. Both disasters show the same pattern: Modern technology only protects if people and procedures are clearly aligned with it.

Conclusion

The Überlingen collision was not an accident with a single culprit but the result of many small gaps: too little staff, switched-off support systems, a dead telephone line and a rule that did not say whom a crew should follow in case of doubt. The answer to it was unambiguous and applies worldwide to this day: A TCAS resolution advisory is followed, reported and not discussed. On top of that came software that detects wrong reactions by the other side and corrects for them.