What happened
On the morning of the accident, a Hawker Siddeley HS-121 Trident 3B, operating as British Airways flight BE476, departed London-Heathrow for Istanbul-Yesilköy. The flight was carrying 54 passengers and 9 crew members. After departing at 08:32 GMT, the aircraft entered the Zagreb Area Control Centre airspace, following airway UB5. At approximately 10:04 GMT, the aircraft established contact with the Upper Sector controller and was instructed to report passing the Zagreb VOR at flight level 330.
Simultaneously, a McDonnell Douglas DC-9-32 operated by Inex-Adria Aviopromet departed Split Airport carrying 108 passengers. The crew was cleared to climb through various flight levels, eventually receiving clearance to FL350 from the middle sector controller. As the aircraft climbed, it maintained a heading of 353° and a speed of 273 Kts.
At 10:14:04 GMT, the crew of the DC-9 contacted the Upper Sector controller to report they were climbing through FL325. The controller, communicating in Serbo-Croatian, instructed the DC-9 to maintain its altitude and warned that another aircraft was passing from left to right at FL335. However, the British Airways aircraft was actually positioned at FL330. Three seconds after this instruction, the two aircraft collided. The left wing of the McDonnell Douglas DC-9-32 struck the cockpit area of the Trident. The impact caused an immediate loss of structural integrity in the forward section of the Trident, leading to its disintegration and a subsequent tail-first impact with the ground. The DC-9 also lost its left wing during the collision and crashed right-wing first. All 176 individuals aboard both aircraft were killed.
Findings
Investigations into the disaster identified several contributing factors, including improper air traffic control operations and a failure by the crews to maintain a proper lookout from their respective cockpits. Additionally, there was a noted lack of compliance regarding continuous monitoring of the appropriate ATC radio frequencies.