Investigation Summary
The flight was a ferry operation intended to move CCCP-11000 to Syktyvkar for maintenance, combined with a cargo delivery of 12 metric tons of household goods and a piledriver to Surgut. The aircraft carried two full crews (a flight crew and a relief crew) plus two additional staff. During the descent into Surgut, the aircraft encountered severe icing conditions between 400 and 1,300 meters. As the crew performed the third turn of the landing circuit at a speed of 330 km/h, the aircraft suffered flow separation on the wing. This led to a spontaneous roll and rapid loss of altitude. The aircraft reached a bank angle of approximately 90 degrees before impacting the ground 15 km short of the runway. Investigators found that the design of the bleed air valves lacked clear indicators for the fully open position, leading the crew to believe the anti-icing system was active when it was actually restricted.
Final Conclusions
The commission concluded the primary cause was a stall during the third turn due to a sharp deterioration in aerodynamics caused by wing icing. This icing occurred because the engine bleed air valves were incompletely opened, rendering the anti-icing system insufficient for the severe conditions. Contributing factors included a lack of flight testing data regarding An-12 behavior with iced wings, the absence of crew recommendations for flying in such conditions, and an inappropriate speed selection for the aircraft's 55-ton weight while operating in icing environments.
Photographic Evidence (1)
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Wing Anti-Icing System
Uses hot bleed air from the engines to heat the leading edges of the wings to prevent ice buildup.
Engine Bleed Air Valves
Control the flow of hot air from the Ivchenko AI-20M engines to the anti-icing system; these were found to be incompletely open during the accident flight.
Interesting Facts
- 01The aircraft (c/n 5343610) was manufactured on December 3, 1965, at the Tashkent Aviation Plant.
- 02The aircraft was carrying 12 metric tons of freight, including a C-995 piledriver and plastic floor tiles.
- 03A second Aeroflot An-12 (CCCP-12996) crashed at the same airport under nearly identical icing conditions just nine days later on January 31, 1971.
- 04The investigation revealed that the bleed air valves were not fully open, which was a common issue due to the lack of valve position indicators.
- 05The aircraft had completed 5,626 flying hours and 2,578 landings at the time of the accident.
Safety Actions & Advisories
Bleed Air Control Improvements
Significant improvements were made to the air bleed control systems, including the installation of indicators to show the fully open position of the valves.
Aerodynamic Testing
Special flight tests were conducted to clarify the aerodynamic characteristics and stall behavior of the An-12 when wings are contaminated by ice.
Manual Revisions
Civil aviation governing documents and flight manuals were updated to include specific recommendations for flying the An-12 in severe icing conditions.