One of the few existing photos of the Zagreb crash investigation hangar (File Photo).

In the last 30-some years, air traffic over Zagreb, Yugoslavia has been an increasing nightmare for the operators of the Zagreb Air Route Traffic Control Center (ARTCC). Since the end of World War II the majority of flights between Europe and the Far East have been routed south around Yugoslavia to skirt the Eastern Bloc nations, and the growth of popular holiday travel since the 1960s has meant that traffic between northern Europe and the eastern Mediterranean has also passed directly over the Zagreb beacon.

As a result, by 1970, five high level airways crossed the region, three of them, Upper Blue 5 (UB5), Upper Blue 9 (UB9) and Upper Red 22 (UR22) intersecting directly over Zagreb. Upper Blue 1 (UB1) ran slightly to the south, and the fifth airway, Upper Amber 40 (UA40) originated at Zagreb on a direct route to Sarajevo.

In the first five years of the decade, 760,000 aircraft movements were handled by Zagreb ARTCC, and by 1976 the Zagreb ARTCC was the second busiest in Europe, its staff of 30 desperately coping with traffic which required at least double their number. In 1973, a modern radar flight control system was installed, but had teething problems, and three years later was still being used only as back-up. Zagreb ARTCC still relied on procedural control with pilots transmitting their positions at specified points along the airways, these positions then being monitored by the radar system.

Despite the staff shortage and increased workload, Zagreb's safety record was, under the circumstances, surprisingly good, although 32 air misses had been reported in the past five years and two controllers had been dismissed for negligence. Even the negligence charges came out not through lack of skill but from underwork: lateness for duty and unauthorised absence from the control station to snatch a bite to eat.

In September 1976, the Zagreb ARTCC was working on a knife edge of nerves. At 08.32hrs GMT (09.32hrs local time) on the morning of 10th September, British Airways scheduled flight BA476 from London to Istanbul took of from Heathrow Airport, London. Captain Dennis Tann commanded the Trident 3B, registration G-AWZT, with First Officer Brian Helm as co-pilot and First Officer Martin Flint in the P3 position. Six cabin crew attended the 54 passengers in the half-full aircraft for the three and a half hour flight to Turkey.

BA476 was routed outwards over Dover and onto Brussels and Munich. After reporting its position to Munich, the aircraft settled on airway Upper Blue 1 (UB1), flying at 33,000ft (330) and crossed into Austrian Airspace at 09.48hrs GMT.

At exactly that time, in the holiday resort of Split, on the Yugoslavian coast, Flight JP550, a DC-9 of the Jugoslav charter airline Inex Adria Aviopromet, took off on the journey to Cologne. Two pilots, Captain Joze Krumpak and First Officer Dusan Ivanus were at the controls of the DC-9, registration YU-AJR, while behind them in the cabin three stewardesses coped with the demands of the 108 passengers. All but one of them were West Germans taking the two and a half hour journey home.

The south-east bound Trident and the north-west bound DC-9 were both flight-planned to traverse the Zagreb beacon. The airspace over Zagreb was divided into three layers - lower, middle, and upper - with the middle and upper directed in the short-staffed ATC center by a controller, where ideally three personel - radar controller, procederal controller, and assistant controller - would be required.

JP550, climbing over Zagreb to its planned flight level of 310, would pass through the middle control layer (from 25,000 to 31,000ft) at level 330.

By 10.00hrs GMT (11.00hrs local time) the duty shift which started at 07.00hrs local at Zagreb ARTCC, had already been on station for four hours under its supervisor, 43-year-old Julije Dajcic. Dajcic's five-man team normally worked a 12-hour duty day, with two hours at a control station controlling the upper and middle layers of airspace, followed by a one-hour break.

Bojan Erjavec, who had been at his station for an hour, controlled the middle section console with his assistant Gradimir Pelin, newly on duty. Mladen Hochberger controlled the upper section and was due, at that moment, to be relieved by Nenad Tepes, who had not so far appeared in the building and looked like being late. Hochberger's assistant was 28 year old Gradimir Tasic, who had been on duty for three and a half hours, the first two as duty controller. When Tepes arrived, Tasic was due to continue for a further hour as his assistant, monitoring procedures and co-ordinating flights with other regions on ground telephone links. Tasic, the youngest member of the Zagreb staff, was on his third consecutive day of 12-hour duty; all the others had taken at least 24-hours off.

This morning was developing into a busy one. Amongst other difficulties, Flight JP550, planned to reach level 310 in a smooth climb out of Split, was tiptoeing up the lower flight levels, on instruction from Zagreb, because levels above 260 were blocked by an east-west flow of traffic. Captain Krumpak, in the Cologne bound DC-9, estimated Zagreb at about 10.16hrs; at 10.02 he radioed passing level 220 and was instructed to transmit his next call on Erjavec's middle sector frequency. A minute later he did so, and was allocated a radar indentification "squawk" code, Alpha 2506.

At the same time, Flight BA476 was crossing the Austrian-Yugoslavian border on flight level 330, airway UB5, and was switched from Vienna control to Zagreb. The call was taken by Tasic, who was now alone at the upper level console. Controller Hochberger had impatiently left his seat to look for Tepes, the late arrival.

Tasic allocated squawk Alpha 2312 to the Trident, with a continued flight level of 330. A glance at his radar screen showed BA476's flight label to be either level 332 or 335, but the radar at Zagreb was known to be imprecise. Tasic ignored it in order to send a Turkish aircraft at level 350 on its way. Someone on BA476's flight deck exclaimed "there he is," as the Turkish aircraft passed over them. Tasic now required clearance from Belgrade for an Olympic Airways flight to Sarajevo on UB1 eastbound. He made a ground telephone call of some 25 seconds to clear this and a German aircraft, with an Iran Air flight close on their heels.

Meanwhile Erjavec, at the middle control console, had JP550 at level 260. Captain Krumpak asked for clearance to a higher level. Erjavec checked the levels available: " Are you able to climb maybe to 350?",  "Affirmative, affermative, " replied Krumpak. "With pleasure."  "Roger. Call you back." It was 10.06:14hrs.

In the corridor outside the control rooms, Supervisor Hochberger had met his overdue replacement, Tepes, and was deep in conversation with him. Through the glass windows, Erjavec could see Tasic. To move JP550 to the upper level, he needed conformation from either Hochberger or Tasic. He waved to Tasic through the glass, and Tasic impatiently waved him away. Pelin, Erjavec's assistant, went around to the busy Tasic's radar screen and indicated JP550's target, which was unlabelled, and asked Tasic for clearance. Tasic, speaking into his headset, simply thought an aircraft near Kostajnica was being indicated, and nodded impatiently. Pelin assumed clearance had been recieved. It was a disasterous misunderstanding.

BE476 was cruising at flight level 330 with a true airspeed of 480kt, though a tailwind pushed it to a groundspeed of 489kt. The Trident had tracked a little to the south, and at 10:11:41 Captain Tann made a six degree left turn to home directly over the Zagreb beacon, estimated at 10:14hrs. At the same time, the DC-9 was at level 310, passing into the upper sector.

At 10:12:06hrs, Erjavec bade Captain Krumpak goodbye, instructing him to squawk "standby" on his transponder, in order to release his middle section squawk code - 2506 - for another aircraft. This was the second mistake, for though the radar was programmed to show the flight label of any aircraft crossing into the upper section while transmitting a middle section code, the "standby" squawk would not register in the upper console. Hastily, Pelin altered the written strip showing JP550's details for the planned middle sector and handed it to Tasic, but the harassed controller had barely time to glance at the details. Virtually unannounced, JP550 appeared on Tasic's radar screen as an unlabelled target.

At that moment, Nenad Tepes, the latecomer, arrived at Tasic's elbow; as duty controller he was impatient to vacate his seat, but for the moment Tasic was still busily directing flights. He began trying to brief Tepes on the situation, which was reaching saturation point: there were 11 aircraft now in the upper sector. Tasic still had not had the time to examine JP550's flight details progress strip. In any case, Pelin, in his own haste, had ommited to indicate with an arrow the fact that the DC-9 was still climbing. Tasic was unaware of its height. In fact JP550's rate of climb and true ground speed of 470kt would mean that it crossed Zagreb, still climbing, at 10:14hrs - the same time as BA476.

Tasic's first verbal contact with JP550 came at 10:14:04. Captain Krumpak said, "Dobar dan, Zagreb," or "Good morning." Then: "three two five crossing Zagreb." The DC-9 was directly over the beacon, 500ft below the Trident, still climbing, and the two aircraft were still closing at a combined airspeed of 920kt - 1,100mph - faster than a high velocity rifle bullet.

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A frantic glance at his faulty radar screen showed BA476's flight level, incorrectly, at 335. Tasic reckoned that if he could hold the DC-9 at 327 they would whisker each other with 800ft to spare. Breaking into Croatian, Tasic begged Captain Krumpak to hold his present height. Krumpak replied in their usual native tongue:

"OK. We'll remain precisely at 330." The time was 10:14:38. Three seconds later the outer five meters of the DC-9's port wing sliced through the Tridents flight deck, killing Tann, Flint, and Helm instantly. As JP550's wing tore off, the two aircraft rolled together and fell from the sky.

The DC-9's CVR caught F/O Dusan Ivanus's last words, as he sat in the right hand seat behind his usless controls. "We are finished," he said, "Goodbye. Goodbye."

The two aircraft struck the ground about 4 miles apart near the town of Vrbovec, 16 miles north-east of Zagreb. All 176 people aboard died. Seven months after the crash, on 11 April 1977, the five Zagreb controllers, the shift supervisor and two senior officials faced charges of criminal negligence - and up to 20 years of imprisonment each. After much battling in court, Tasic was the only one to face the charges and got 7 years prison. After an appeal it was cut in half, then following a petition he was released. Investigators found that all the controllers had to cope with impossible situations using old equipment.