Khabarovsk United Air Group Flight 3949

Cause of crash

During the climb, the aircraft banked instantaneously to the left without any input from the crew due to a fault in one of its flight control systems, and as a result, the crew had to maintain a constant right bank input. The bothersome problem led Captain Sumarokov to opine that they should use the same method that had been applied earlier to counteract such left bank by conducting a fuel transfer, as it had resulted in multiple successes when the aircraft encountered the exact same problem in previous flights. The aircraft manual didn't specify the actions that the crew should take when such problems occur, and hence said move would be deemed a non-standard procedure. Despite this, the crew agreed with Sumarokov's idea and decided to follow through with it.

The transfer from the left tank and middle tank produced rolling forces to the right. Asymmetrical fuel transfer was not prohibited, as long as it was still within the tolerated limits. The instruments gave information that the amount of fuel imbalance between the tanks was minimal. Likely due to faulty gauge reading, the crew didn't notice that they had pumped fuel of more than the allowable amount to the right tank, which had already been filled with more than 3,600 kg of fuel. The left tank, meanwhile, was only filled with less than 1,000 kg.

The autopilot was initially able to counteract the unwanted left bank, but due to the addition of fuel into the right tank, the rolling forces to the right exceeded the forces that the system could control, and hence the aircraft started to bank right instead. When the aircraft started to bank to the right, it was unfortunately not noticed by the crew as they didn't think that there was anything wrong and thought that the autopilot was working normally, aggravated by the fact that they were focusing on the pre-landing checklist.

When the bank had reached an angle large enough to alert the pilots, they weren't able to identify which way it had banked. Add to the pitch-black sky; the crew could not make sense of their spatial orientation. Instead of working together, they kept panicking with the state of their aircraft, which was getting nearer to the ground by the minute. The crew was not trained in handling spatial orientation, which led to Cockpit Resource Management (CRM) breakdown, eventually causing them to be unable to resolve the issue.

Conclusion

The investigation listed five possible causes, whose combination might have led to the crash. It is believed that in order to counteract the left-wing low flying tendency fuel-feed was selected from left wing fuel tanks only. The fuel imbalance likely caused the aircraft to bank to the right and the autopilot was able to counteract it for 35 minutes after take-off.

The investigation report further criticized the navigational operation in the region, which was severely deficient. The radar operating the Khabarovsk region was damaged following the passage of a recent storm. The radio-technical support, however, was not used by workers. The conversation between the pilots and the ATC was not recorded due to a failure within the tape. The aircraft was not actively tracked by the worker who was on duty, and there was barely any communication from ATC and workers from Air Defence following the disappearance of the aircraft.

In response to the accident, an audible bank angle alarm would be re-installed to the Tupolev Tu-154 aircraft.