Summary
On 26 November 2017, an Air Serbia ATR 72-202, registration YU-ALO, operating scheduled flight JU123 from Sofia Airport to Belgrade Airport, experienced a serious incident during initial climb. The aircraft had 36 passengers and 4 crew members on board. At a height between 8,000 and 10,000 feet, the crew heard a loud clattering noise and felt vibrations. The senior flight attendant also reported hearing the noise and feeling shaking in the upper cabin. The crew requested ATC to stop the climb and vector them back to Sofia. A longer approach was requested due to concerns about the nose landing gear. The aircraft landed safely at 16:21 local time. After landing, a panel from the central part of the wing to fuselage fairing was found missing. No injuries occurred.
Flight History
The aircraft had flown from Belgrade to Sofia without incident, landing at 15:16. A transit check performed before the departure to Belgrade revealed no faults. The aircraft took off at 16:00. During initial climb, at approximately 8,000 to 10,000 feet, the crew heard a loud clattering noise and felt vibrations. The commander was the pilot flying, and the aircraft was in a clean configuration. The crew requested a return to Sofia and was vectored to intercept the final approach 15 nautical miles from the runway threshold. Emergency services were alerted. The crew reported a safe landing gear down indication, including the nose landing gear, at 16:17.
Findings
Examination after landing revealed that a panel (designation 291 BL) from the central wing-to-fuselage fairing was missing. The panel had been removed during planned technical servicing and reattached incorrectly. Specifically, the bolts used for the panel attachment were not screwed properly.
Cause
The cause of the incident was determined to be incorrect screwing up of the bolts used for the 291 BL panel attachment during planned technical servicing performed by the staff maintaining the aircraft.