CCCP-11000 was not supposed to be flying to Surgut that day. The five-year-old An-12B, built at the Tashkent Aviation Plant in December 1965 and assigned to the Syktyvkar aviation department, had accumulated 5,626 flight hours and 2,578 landings. An order had come through to ferry the aircraft back to Syktyvkar for routine maintenance. Someone decided to combine the trip with a freight delivery: 12 metric tons of cargo bound for Surgut, including rolls of netting, plastic floor tiles, household goods, and a C-995 piledriver for construction. Aboard were two complete crews. Captain Sergei Bakharev commanded the flight deck, while Captain Leontiy Butov led the relief crew. Including an engineer and a loadmaster, fourteen people occupied an aircraft designed to haul cargo, not carry passengers in comfort.

The aircraft departed Omsk at 18:09 Moscow time and climbed to a cruise altitude of 6,600 meters. At Surgut, conditions were deteriorating. Solid cloud sat at 450 meters, visibility was 5.5 kilometers, a fresh northerly wind blew across the airfield, and temperatures had dropped well below freezing. Icing conditions permeated the cloud layer. The crew began their descent, passing through 4,500 meters, then 1,200 meters, before entering the landing circuit on a magnetic heading of 180 degrees. By the time they reported passing the outer marker beacon at 400 meters, the aircraft was deep inside the kind of weather that makes Siberian winter flying perilous. The crew had no reason to suspect their de-icing system was not functioning. But the engine bleed air valves, which should have channeled hot air from the turboprops to the wing and tail surfaces, were closed. Ice accumulated silently.

What makes this crash extraordinary is not just the accident itself but what followed. On January 31, 1971, nine days after CCCP-11000 went down, another An-12, registration CCCP-12996, crashed at Surgut under nearly identical circumstances. That crew, too, was executing the third turn of their landing circuit when ice-induced flow separation caused an uncontrollable roll. That crew, too, died. The parallel was too precise to ignore. Both aircraft had suffered from the same mechanical failure: bleed air valves not fully open, rendering their ice protection systems useless. Both had encountered severe icing in the cloud layer around 1,200 meters. Both lost control in exactly the same phase of flight.

The twin disasters at Surgut forced Soviet aviation authorities into action. Investigators determined that the An-12's bleed air control system lacked a critical feature: there was no cockpit indicator showing whether the valves were fully open or closed. Crews had no way of knowing their de-icing systems were inoperative until ice had already compromised the aircraft. The response was comprehensive. Engineers redesigned the bleed air control system to include a position indicator for the valves. Special flight tests were conducted to better understand how icing affected the An-12's aerodynamic characteristics, generating data that had not previously existed. The findings rippled outward through Soviet civil aviation, prompting revisions to governing documents and operational procedures. The fourteen people aboard CCCP-11000 and the crew of CCCP-12996 became, in the grim arithmetic of aviation safety, the cost of a lesson that should have been learned on the ground.