What happened
On a morning flight from London-Heathrow to Istanbul, a Hawker Siddeley HS-121 Trident 3B operated by British Airways was cruising at flight level 330. The aircraft, carrying 54 passengers and 9 crew members, was navigating airway UB5 under the direction of the Zagreb Area Control Centre. Shortly before the incident, the pilot adjusted the heading to 115 degrees to correct a slight deviation from the airway centerline.
Simultaneously, an Inex-Adria Aviopromet McDonnell Douglas DC-9-32 was climbing from Split Airport toward Cologne with 108 passengers. The crew had been cleared to climb through various flight levels and were eventually instructed by a middle sector controller to reach FL350. As the aircraft passed west of the KOS NDB, it maintained a heading of 353 degrees.
At approximately 10:14 GMT, the crew of the DC-9 contacted the Upper Sector controller to report they were climbing through FL325. During this exchange, the controller informed the crew that another aircraft was passing from left to right at FL355; however, the British Airways flight was actually positioned at FL330. Moments after the crew confirmed they were maintaining FL330, the two aircraft collided. The left wing of the DC-9 struck the cockpit area of the Trident, causing an immediate structural failure and decompression that disintegrated the forward fuselage of the larger plane. Both aircraft subsequently fell to the ground near Vrobec, resulting in 176 fatalities.
Findings
Investigations into the disaster identified several critical failures leading to the impact. The primary factor was improper air traffic control operations, specifically regarding altitude information provided to the crew. Additionally, investigators noted a failure by the flight crews to maintain a proper lookout and a lack of compliance with regulations concerning continuous monitoring of the assigned ATC radio frequencies.