On September 2, 2018, a Mi-8MTV-1 helicopter, registration RA-25502, was operating a flight from Ust-Kut airport to a specific coordinate (54°49′55.4′′ N, 108°41′11.2′′ E) and back. The mission, conducted under a contract between JSC 'Angara' Airline and JSC 'Siberian Research Institute of Geology, Geophysics and Mineral Resources,' was intended to transport a geological group and cargo.
Prior to departure, the crew underwent medical examinations at the Ust-Kut airport medical center and received meteorological information via the Automated Meteorological Information System. The helicopter was refueled with 2300 liters of TS-1 fuel, bringing the total fuel load to 3500 liters. Pre-flight inspections were completed, and the aircraft's takeoff weight of 10600 kg was within the permitted limits.
During the flight, the pilot-in-command attempted a turn in mountainous terrain. The accident resulted in three fatalities and no other injuries.
The investigation noted that the crew had completed pre-flight meteorological training and the aircraft was accepted without remarks during the pre-flight inspection.