What happened
On November 26, 2017, an ATR 72-202, registration YU-ALO, operated by Air Serbia, departed Sofia Airport for Belgrade 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 reported hearing a loud clattering noise accompanied by significant vibrations. The cabin crew also noted unusual shaking in the upper section of the cabin.
Suspecting a detached panel or potential issues with the nose landing gear, the crew requested to stop their climb and return to Sofia. The crew requested a longer approach due to concerns regarding the landing gear's condition. The aircraft landed safely at Sofia Airport at 16:21 local time. A subsequent inspection revealed that a panel from the wing-to-fuselage fairing (panel 291 BL) was missing. No injuries were reported to the passengers or crew, and no structural damage was found on the aircraft's stabilizers or control surfaces.
The investigation
The investigation examined the aircraft's maintenance history and flight data. Investigators analyzed the assembly and disassembly records for the missing panel, noting that recent maintenance activities had involved adjacent panels. Technical logs indicated that the bolts used for the attachment of the 291 BL panel had not been sufficiently tightened, with some bolt heads protruding. The investigation also ruled out external factors, such as weather or pilot error, and determined that the vibrations were caused by turbulent airflow passing through the area where the panel had detached, rather than a collision with the tail.
Findings
- The primary cause of the incident was the incorrect tightening of bolts used to attach the 291 BL panel during planned technical servicing.
- The missing panel was not recovered from the airfield or taxiways following the incident.
- There was no evidence of broken or twisted attachment hardware, suggesting the panel was improperly secured rather than forcibly ripped away.
- The aircraft's control systems and structural integrity remained intact despite the loss of the fairing component.