Philosophy of Blame: The story of the 1976 Zagreb mid-air collision

On the 10th of September 1976, tragedy unfolded in the skies over Yugoslavia as two airliners collided at 33,000 feet, scattering burning debris over a 320-square-kilometer stretch of the Croatian countryside. The horrifying crash killed all 176 passengers and crew aboard both planes, forever traumatized two villages, and triggered a legal battle over who was to blame. As British and Yugoslav investigators sought to uncover the cause, authorities arrested the air traffic controllers who were on duty at the time of the crash and charged them with “endangering air traffic.” But a grassroots effort by other controllers contended that the real cause was not one man’s mistake, but a fatally deficient air traffic control system. A reckoning ensued with major implications for the safety of Yugoslavia’s airways: would the government get away with scapegoating controller Gradimir Tasić, or would it acknowledge that its treatment of air traffic controllers had always been a disaster waiting to happen?

An Inex-Adria Avioproment DC-9 similar to the one involved in the accident. Image source: Paul Furmanski

Yugoslavia in the 1970s existed between the two opposing worlds of the East and the West, NATO and the Warsaw Pact. Under the leadership of strongman dictator Josip Broz Tito, Yugoslavia pioneered the non-aligned movement, a group of developing countries that sought friendly relations with both sides during the Cold War. Although it professed its own form of communism, Yugoslavia welcomed Western friendship. Its skies were open to both Soviet and Western aircraft, and its beaches hosted tourists from east and west alike.

It was no surprise, then, that in 1976 a large number of tourists from West Germany chose to spend an early September holiday in the scenic city of Split on Yugoslavia’s Adriatic coast. As this was an organized tour group, the travel company chartered its own flights to transport the Germans from Cologne to Split and back again. For the return journey, it hired Inex-Adria Aviopromet, an airline based in the Yugoslav republic of Slovenia, which provided a nearly brand-new McDonnell Douglas DC-9,ready to pick up the group at Resnik Airport in Split. On the morning of the 10th of September, 108 tourists and their guides said goodbye to the historic city and boarded the plane that would take them home.

In command of the DC-9 were Captain Jože Krumpak, an experienced pilot with over 10,000 flying hours; and First Officer Dušan Ivanuš, a temporary hire with some 3,000 hours of his own. Three flight attendants rounded out the crew, bringing the total number of people on board to 113. At 10:48 a.m. local time, Krumpak and Ivanuš steered their DC-9 to the head of the runway and took off into the bright, clear sky. Under the call sign JP550, they headed north along a well-established airway toward a navigational waypoint near Zagreb, capital of the Yugoslav Republic of Croatia.

A map of the various airways passing through the Zagreb area, with the relevant ones highlighted. Image source: Jonathan McFly

JP550 was hardly the only aircraft passing over Croatia that morning. Due to its intermediate location between Western Europe and the Middle East, Yugoslavia’s skies were always busy, especially as many western airlines couldn’t fly over the Soviet-aligned nations of Eastern Europe and had to take the longer way around through the Balkans. Handling this constant flood of traffic on the 10th of September were a group of air traffic controllers based in a facility at Zagreb Airport. To more easily manage the large number of planes in the area, the airspace in Croatia was divided up into lower, middle, and upper sectors. The middle sector began at an altitude of 25,000 feet and extended up to 31,000 feet, while the lower sector handled everything below that range and the upper sector handled everything above it.

A map of the middle and upper sectors in the Zagreb control area. Image source: The Yugoslav Federal Civil Aviation Administration Aircraft Accident Investigation Commission

One full controller and one assistant controller coordinated the traffic in each of the three sectors. In charge of the middle sector that day were Bojan Erjavec and his assistant Gradimir Pelin, while Gradimir Tasić and assistant controller Mladen Hochburger manned the upper sector. At 11:00, Hochburger was scheduled to transfer to a different station, at which point his place would be taken by the relief assistant, Nenad Tepeš. But that day, Tepeš was running late, and at 11:00 he was nowhere to be found. Hochburger got up and left to go look for him. As a result, 27-year-old Gradimir Tasić was left to handle the entire upper sector by himself. Hochburger later claimed that he informed Shift Manager Julio Dayčić of his absence, but Dayčić denied that anyone ever told him Tepeš was missing or that Hochburger had left his station.

The British Airways Hawker-Siddeley Trident 3 involved in the accident. Image source: Mike McBey

At 11:04, British Airways flight 476, a three-engine Hawker Siddeley Trident operating a scheduled flight from London to Istanbul, entered Yugoslavian airspace, heading southeast from the Austrian border at 33,000 feet. On board were 9 crew and 54 passengers, many of them Turkish merchants heading to Istanbul to buy cheap goods to take back to Britain. In command of the flight were three pilots: Captain Dennis Tann, and two first officers, Brian Helm and Martin Flint. When the Trident entered the Zagreb air traffic control zone, the crew contacted upper sector controller Gradimir Tasić.

“Zagreb, Bealine 476, good afternoon.” (A bit of time zone confusion — at that moment it was still morning.)

“Bealine 476, good afternoon, go ahead,” Tasić replied.

“476, Klagenfurt at 02, 330 estimating Zagreb at 14,” the Trident replied, giving a routine position update. The message meant that flight 476 had passed the Klagenfurt beacon in Austria at 11:02 and expected to pass over the Zagreb beacon at 11:14, maintaining an altitude of 33,000 feet — “flight level” 330.

Tasić replied, “Bealine 476, roger, call me passing flight level 330, squawk alpha 2312.” Here, Tasić assigned a four-digit code to which the crew would tune their transponder, to make it easier to identify the flight on his radar screen. Flight 476 acknowledged the request and set their transponder to “squawk” 2312. This would be the last, and only, conversation between British Airways flight 476 and Zagreb control.

At 11:05, Inex-Adria flight 550, climbing through the middle sector, reached an altitude of 26,000 feet and requested permission to climb higher. Their flight plan called for a cruising altitude of 31,000 feet, but due to conflicting traffic, middle sector controller Bojan Erjavec couldn’t give them this altitude. 28,000 feet and 33,000 feet were also unavailable. Instead, Erjavec offered to clear JP550 to an altitude of 35,000 feet. The crew of the DC-9 replied, “Affirmative, affirmative, with pleasure.” JP550 was then cleared to proceed north to the Zagreb beacon and climb to flight level 350.

At 11:07, with JP550 beginning its ascent from 26,000 feet, Erjavec wanted to start the handover process to transfer the flight to the upper sector. He tried to get the attention of Tasić, but Tasić waved him off; working the entire upper sector single-handedly, he was too busy to take another plane. Instead, Erjavec sent assistant middle sector controller Gradimir Pelin to inform Tasić of the approaching plane. Pelin later claimed that he told Tasić that JP550 was climbing to flight level 350 and would soon enter his jurisdiction. Tasić recalled a different story, claiming that Pelin pointed at the blip on his radar and stated that it wanted to climb, to which he allegedly responded that it should remain at its present altitude until after it passed Zagreb. Nevertheless, Pelin believed that the flight had been handed over.

Progress of the two planes on the Zagreb control area map.

Erjavec should not have cleared JP550 to climb without a flight progress strip — a piece of paper showing the name of the flight and its intentions. He likely didn’t have a strip prepared because JP550’s filed flight plan didn’t originally take it into the upper sector. Without the flight progress strip, Tasić would not necessarily be aware that he was in charge of the plane.

At 11:12, Bojan Erjavec signed off with Inex-Adria flight 550, giving them the frequency to contact the upper sector and instructing them to “squawk standby.” The crew of JP550 set their transponder to “standby” mode while waiting for Tasić to give the flight a new four-digit code, as he had done with British Airways flight 476 some eight minutes earlier. But Tasić, apparently unaware that he was responsible for the flight, never assigned one. He had plenty of other things to worry about: at that moment, there were 11 planes in his sector, and he was actively conversing with four of them.

An example of a typical flight progress strip. Image source: Skybrary

At 11:13, the middle sector controllers finally got around to preparing a flight progress strip for JP550, which Pelin handed over to Tasić. But Tasić did not yet realize that this was not an incoming flight — it was a flight already in the upper sector. The DC-9 continued climbing toward 35,000 feet with its transponder set to standby, thus appearing on the radar only as a blip with no attached information, just like any other airplane that was not in Tasić’s sector.

For one minute and 52 seconds after signing off with the middle sector, the crew of JP550 made no attempt to contact the upper sector, even though there were several gaps in the ongoing conversations that would have enabled them to jump in. For whatever reason, they hesitated.

At 11:14 and 10 seconds, the DC-9 at last contacted Tasić and reported that they were climbing through 32,500 feet and expected to reach the Zagreb beacon within the minute. At that moment, the British Airways Trident was still cruising normally at 33,000 feet, less than a minute out from the same Zagreb beacon. In the cockpit, the pilots were relaxed; during cruise, duties were few, so they worked on a crossword puzzle, discussed the market prices of vegetables, and commented on a newspaper story about a helicopter crash. Everything was laid back and routine. Although their radio was tuned to the upper sector frequency, it’s not clear that they were listening to the conversation between Tasić and JP550 — or at least, they never fully processed its significance.

The Trident involved in the accident, pictured in flight just days before the crash. Image source: Civilaviation.co.uk user “Uuoret”

Alarmed to discover JP550 already well into his sector, Tasić asked for its present altitude, to which the crew replied that they were now at 32,700 feet. Tasić wasn’t sure exactly what altitude the Trident was at, but he recalled seeing 33,200 or 33,500 feet displayed on the radar screen. He immediately realized that the two planes were likely on a collision course. Switching from English to his native Serbo-Croatian, he frantically stuttered, “Eh… maintain that level now and report passing Zagreb!” He hoped that if the DC-9 leveled off at 32,700 feet, it would pass 500 to 800 feet below the Trident.

Unfortunately, he was wrong: the Trident was actually at 33,000 feet exactly. And by the time the pilots of the DC-9 had received and processed the order to level off, they too were at 33,000 feet. The stage was set: Tasić had made a fatal error; now the last line of defense lay with the crews of the two converging airliners.

For some 30 seconds, the crews of the northbound DC-9 and southeast-bound Trident could theoretically have seen each other coming. The weather was clear, the sun was shining, and the view wouldn’t have been blocked by the windscreen posts. But seeing an airliner coming in time to react is surprisingly difficult. The crew of the Trident didn’t know about the critical conversation between Tasić and the DC-9 because they didn’t speak Serbo-Croatian, so they were not expecting to encounter another plane in close proximity. Furthermore, the position of the sun backlit the DC-9 from their perspective, making it harder to see. The crew of the DC-9 had a better chance to spot the Trident, because it was not backlit and they were aware that there was conflicting traffic. But Tasić didn’t tell them where the traffic was coming from or how close it would pass. It is likely that, in searching a big sky for a distant airliner, they simply failed to see it until it was too late.

Artist’s rendering of the moment of the collision. Image source: Wikimedia user Anynobody

At exactly 11:14 and 41 seconds, British Airways flight 476 and Inex-Adria flight 550 collided at a closing speed of well over 1,000 kilometers per hour. Passing the Trident from left to right at 58-degree angle, the left wing of the DC-9 sliced straight into the cockpit of the British jet, cleaving it in half at the window level. The flight crew of the Trident died immediately on impact, likely without ever realizing what had hit them. Debris from the decapitated Trident continued forward and struck the left engine of the DC-9, causing an uncontained engine failure that sent shrapnel ricocheting off the control surfaces on its tail.

As the remains of its cockpit blossomed out into the sky, the Trident pitched up into a stall and began to fall like a leaf. The DC-9, missing the outer 5 meters of its left wing, immediately swung into an extreme yaw and bank. The aerodynamic forces generated by the manoeuvre were so enormous that they ripped the tail right off the plane, sending the DC-9 into a terrifying uncontrolled flat spin.

My original drawing of the collision.

Some 28 kilometers behind the Trident, the crew of a Lufthansa Boeing 737 witnessed an explosion over the approximate position of the Zagreb beacon. As they watched in horror, two airliners began to plunge downward from 33,000 feet, trailing smoke as they fell. The shocked Lufthansa captain attempted to contact Zagreb control several times, saying repeatedly, “I think there’s been a midair collision!” It took three minutes for him to finally get the message across; in the meantime, Gradimir Tasić frantically tried to contact British Airways flight 476 and Inex-Adria flight 550, but there was no response.

Meanwhile, the disintegrating airliners continued their diverging death spirals down toward the quiet Croatian countryside. The fuselage, wings, and tail of the Trident plunged almost straight down to the ground, while the out-of-control DC-9 corkscrewed and pirouetted wildly as it fell. Its systems remained powered and the cockpit voice recorder continued to function, capturing the horrific last moments of its 113 occupants. Although the recording was never released, Yugoslav investigators wrote that it contained the last words of the doomed crew.

Footage taken at the Trident crash site after the accident. Video source: The Associated press

The DC-9 crashed inverted into a forest just east of the village of Dvorišće, northeast of Zagreb. The wreckage caught fire and burned for hours, leaving little that was recognizable. The Trident, on the other hand, had already partially disintegrated by the time it struck the ground in a farmer’s field some seven kilometers southeast of the DC-9. In the nearby village of Gaj, pieces of the plane, luggage, and human remains fell from the sky like rain. Bodies slammed into rooftops, yards, and driveways. The fuselage, with one wing still attached, crashed to earth upright in a corn field and did not catch fire. After rushing to the scene, a police officer searching the wreckage of the Trident found a baby showing feeble signs of life, but there was nothing he could do; it was already too far gone. “Even if the ambulances had arrived before me,” he later said, “it would have been too late to save it.”

Footage taken at the DC-9 crash site after the accident. Video source: The Associated press

Firefighters arriving from the nearby town of Vrbovec extinguished the burning DC-9 and mounted a search for survivors, but it was soon clear that none of the 176 people aboard the two planes had survived the crash. It was the deadliest midair collision ever at the time, the deadliest plane crash in Yugoslavia, and the only fatal accident in the history of British Airways (which remains true today).

The crash scene was enormous. In addition to the two main wreckage sites, a considerable amount of large debris, including the Trident’s cockpit, fell in between the two planes. Other aircraft parts, some bodies, light materials, and papers were carried on the wind for a considerable distance, leaving a trail of debris stretching northeast over forests and fields for 62 kilometers. Yugoslavian investigators arriving on the scene found it impossible to recover all of the pieces, and some large sections — including the outer 5 meters off the DC-9’s left wing — were simply never found.

The remains of the Trident came to rest in a corn field. Image source: Jutarnji List

Yugoslavian investigators, with the help of an accredited representative from the United Kingdom, spent four months unraveling the causes of the disaster. But it took very little time at all to trace the origin of the disaster to the air traffic control center in Zagreb. Within hours of the crash, six controllers — Tasić, Hochburger, Erjavec, Pelin, Dayčić, and Tepeš — were arrested and taken in for questioning. Each was found to have committed procedural errors. Tasić failed to ensure the separation of the two aircraft. Hochburger left his station even though his replacement had not arrived. Erjavec did not prepare a flight strip in a timely manner. Pelin did not ensure that Tasić was aware he was handing over control of the DC-9. Dayčić allowed his direct subordinate, Tasić, to work the entire upper sector alone. And Tepeš was late to his shift. Under charges of “endangering air traffic,” each man faced up to 20 years in prison for his part in the events leading up to the crash. Five of the controllers were released while awaiting trial; only Tasić was held until the hearing.

The front page of a local newspaper in Vrbovec after the crash. Image source: ZGPresten

The trial began on the 11th of April 1977 with arguments from both sides. Over the course of the trial, a number of noteworthy facts came to light. First of all, each of the three sectors was supposed to be manned by three controllers, but the Zagreb center was short-staffed and had cut that to two. Occasionally an entire sector could even fall under the command of a single man, as happened to Tasić just before the critical handoff of Inex-Adria flight 550. On top of this, the controllers worked 12 hour shifts, sometimes without days off in between. At the time of the accident, Tasić was four hours into his third 12-hour shift in as many days, a schedule that was very much incompatible with the stressful nature of the job. Furthermore, the radar at Zagreb Airport was still not fully set up and frequently produced unreliable altitude readings, even though two years had passed since its installation. Despite this, there were no rules in place to govern how controllers were expected to use it. It was therefore entirely possible that the altitude for the Trident that Tasić saw on his radar screen was incorrect. All of these factors taken together suggested that the controllers were overworked, and mistakes may have been inevitable. But would the court agree?

The crash and the subsequent investigations and trials play out in a series of newspaper headlines. Image source: ZG-Magazin

There were some in Yugoslavia (and many abroad) for whom the prospect of 20-year sentences for the controllers didn’t sit well, and during the trial they found an unlikely standard-bearer: Richard Weston, an Englishman representing a British victim of the crash on behalf of the prosecution. In stark and moving terms, he laid out the hidden cost of locking up the air traffic controllers, and what it would mean for aviation safety. His speech, reproduced from court documents for a 1979 documentary, is included in full below.

“Let me take this court into the mysterious electronic world in which air traffic controllers live. Many of us here have stressful jobs, and many of us carry heavy responsibilities. I ask this court to recognize that the air traffic controller’s responsibility is a very special one, and that the stress that that responsibility induces is of an abnormal nature. The job of juggling airliners and making snap decisions on which hundreds of lives depend is a herculean task. It takes a very special individual to withstand that onslaught day after day, because it is a job that very few of us could perform at all, let alone endure. I sincerely believe that, while I respect that this is a criminal trial of the defendants, the court will more properly discharge its obligations to society if it will use its powers in an imaginative and resourceful way, and not limit itself to the punishment of those who stand before it today. They, as I have endeavoured to show, are themselves the victims of a system. Is it not a fact that the aviation community has created a system which has grown into a monster that we have not had time to learn how to control, in the real sense? On September the 10th, Gradimir Tasić was the final link in a system that failed. And yet if we single out an individual or individuals for blame and lock them up, the deeper underlying problem remains unresolved. And next week another individual in the same fallacious system may make the same error with equally disastrous consequences.

“That there was confusion, misunderstanding, mishandling, breaches of the rules, overwork, unauthorized absenteeism, shortage of skilled staff at Zagreb control, seem to be undisputed. But let me make myself clear, beyond any doubt, I do not ask this court to condone or overlook any blame attaching to the defendants. But what I do ask is that this be seen in perspective. I could, I suppose, draw a parallel — the temptation is irresistible. If I were to say that doctors, when prescribing the wrong drugs, were then subject to criminal prosecution — ask, I suggest, what kind of medical profession we would have tomorrow. The answer is only too painfully clear. It is no exaggeration to say that the aviation community around the world waits with bated breath for the decision of this court. All these defendants will have to live with the thought of that catastrophe for the rest of their lives. And that, together with the experience of this trial, would, I suggest, be wholly sufficient punishment under the circumstances. The real danger is that the effect of a prison sentence on these men will be disastrous for the morale of the air traffic control system throughout the world, and therefore directly affect the safety of the traveling public. The imprisonment of these defendants will be wholly counterproductive to the solution which is sought. Instead of being in the limelight today as the ‘architect of tragedy,’ Zagreb could tomorrow be, by virtue of an enlightened decision of this court, a model for world civil aviation. The lives of those killed on September the 10th will not have been sacrificed in vain.”

Investigators work on the wreckage of the Trident. Image source: the Bureau of Aircraft Accidents Archives

On the day of the sentencing, the court acquitted Hochburger, Erjavec, Pelin, Dayčić, and Tepeš on all charges, and due to mitigating circumstances, Tasić received seven years in prison instead of the proposed 20. For those who wanted to see the blame placed on the system, it was a partial victory.

In its final report, the Yugoslavian commission of inquiry partly blamed the controllers, but also placed some responsibility on the pilots of both aircraft. The investigators asserted that both crews had a legally codified responsibility to monitor communications and look out for other aircraft, and that because the planes collided, they must have failed to do this. In an addendum to the report, the British representative on the investigation team protested this finding, pointing out that the commission had not explored the possibility that two perfectly observant crews did not see each other for reasons outside their control. The representative also noted that regardless of how the crews handled the situation, it was air traffic control that put them in danger in the first place.

A memorial commemorates several British victims on the crash. Image source: Wikimedia user SpeedyGonsales

The story, however, was not quite over. After the court handed down the verdict against Tasić, Richard Weston helped organize an international network of air traffic controllers to petition for his release. Seemingly against all odds, the petition successfully swayed the Yugoslavian government. In a great victory for the aviation community, Gradimir Tasić was released from prison in 1978 — less than two years into his seven-year sentence. During the subsequent years, Yugoslavia’s aviation authority quietly set about overhauling the Zagreb air traffic control sector from top to bottom, ultimately leading to safer skies in southeast Europe.

Today, an accident like the Zagreb midair collision could not happen. Radar technology has vastly improved, air traffic controllers are better trained and generally have better working conditions, and most importantly, airliners are fitted with traffic collision avoidance systems that automatically warn pilots of conflicting traffic and give instructions for evasive manoeuvres without any input from controllers at all. It is therefore far less likely that a single error by a controller could end in disaster. But the broader lessons of the crash still ring true today. It is a microcosm of the shift in thinking that has made modern air travel so safe — the slow adoption of a philosophy that avoids blame and focuses on what could be improved, not who should be punished. Punishing an individual in a deficient system is folly; anyone could have been sitting in Tasić’s chair and made exactly the same mistake. It is likely that, in watching the unfolding legal drama that surrounded the crash, air traffic controllers around the world thought, “There but for the grace of god go I.”

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Analyzer of plane crashes and author of upcoming book (soon™). Contact me via @Admiral_Cloudberg on Reddit or by email at kylanddempsey@gmail.com.

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