Incident Overview
On 26 November 2017, an Air Serbia ATR 72-202, registration YU-ALO, was conducting a scheduled passenger flight (JU123) from Sofia Airport, Bulgaria, to Belgrade Airport, Serbia. The aircraft departed Sofia at 16:00 local time with 36 passengers and 4 crew members on board.
During the initial climb, at an altitude between 8,000 and 10,000 feet, the flight crew heard a loud clattering noise and felt vibrations. The senior flight attendant also reported hearing unusual noises and feeling shaking in the upper cabin section. The commander, who was the pilot flying at the time, requested ATC to stop the climb and vector the aircraft back to Sofia Airport. The crew suspected a detached panel or nose landing gear issue, though no indications were present. At approximately 16:12, the crew requested a longer approach due to concerns about the nose landing gear. Airport emergency services were alerted. The aircraft was vectored to intercept the final approach 15 nautical miles from the runway threshold. At 16:17, the crew reported a safe landing gear down indication. The aircraft landed safely at 16:21 local time.
Damage and Findings
After landing, an inspection revealed that a panel from the central part of the wing-to-fuselage fairing was missing. No other damage was noted. There were no injuries to passengers or crew.
Cause
The investigation determined the cause to be incorrect screwing of the bolts used for the 291 BL panel attachment during planned technical servicing performed by the maintenance staff.
Investigation Details
The incident was classified as a serious incident by the Aircraft Accident Investigation Unit of Bulgaria. The investigation was conducted under Annex 13 to the Chicago Convention and EU Regulation 996/2010. The flight was a commercial air transport operation under instrument flight rules (IFR). The aircraft, an ATR 72-202 (MSN not specified), was airworthy at the time of the flight. The crew consisted of a commander and co-pilot, both holding valid licenses. No meteorological or communication issues were reported. The flight data recorder (FDR) and quick access recorder (QAR) were analyzed. The missing panel was not found and no debris was located. The incident occurred in daylight conditions (16:06 local time). The aircraft had undergone a transit check before departure with no faults detected.