Investigation Summary
The aircraft was approaching Surgut in conditions conducive to severe icing, with layered clouds down to 240 meters and a temperature of -7 degrees Celsius. During the first approach, while performing the third turn of the airfield traffic pattern at an altitude of 400 meters, the aircraft began to behave abnormally. The crew reported strong icing and activated the de-icing systems, but the air bleed valves from the engines were likely not fully open. As the flaps were set to 15 degrees, the airspeed dropped from 330 km/h to 310 km/h despite increased thrust, indicating significant aerodynamic degradation. The aircraft entered a series of spontaneous rolls and a sharp left tilt, eventually crashing 16.5 kilometers north of the airport. This accident occurred just nine days after another Aeroflot An-12 (CCCP-11000) crashed under nearly identical circumstances at the same airport, leading to major revisions in An-12 icing procedures and de-icing system indicators.
Final Conclusions
The accident was attributed to a loss of control during approach in icing conditions. The accumulation of ice on the airframe significantly degraded the aircraft's handling characteristics, a known vulnerability for the type during that period.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
De-icing System
The An-12 utilized hot air bled from the engines to heat the wing leading edges and electrical heating for the tailplane and propellers.
Flaps
Extension of flaps changes the downwash over the tailplane; if the tailplane is iced, this can trigger a tailplane stall, causing the nose to pitch down violently.
Interesting Facts
- 01The accident occurred on January 31, 1971, near Surgut.
- 02The aircraft was carrying 12 metric tons of Pacific herring.
- 03Severe icing caused flow separation on the wing tips, leading to a stall.
- 04The de-icing system was ineffective because air bleed valves were not fully open.
- 05This was the second An-12 crash at Surgut due to icing in nine days.
Safety Actions & Advisories
Icing Operational Procedures
Following the two crashes at Surgut in January 1971, Aeroflot and the Soviet aviation authorities revised procedures for An-12 operations in icing conditions, including limitations on flap settings.