Preminary data based on the notification from the Bulgarian authorities:
Loss of a part of the aircraft structure in flight.
At 14:06 UTC on Nov 26, 2017, while performing a flight from LBSF to LYBE, the flight crew of an aircraft of ATR AT72 model, reg. marks YU-ALO, of "AIR SERBIA" operator decided to land back at Sofia airport due to vibrations and noise felt on board during the initial climb. The aircraft landed normally at LBSF with no consequences caused to the crew, passengers and the aircraft at 14:21 UTC.