Czech investigators believe the crew of a Let Aircraft Industries L-410 did not apply sufficient rudder to counter yaw, after a propeller pitch reversal, before the aircraft fatally rolled into the ground on final approach to Nelkan.
Russia's Interstate Aviation Committee, which led the inquiry, states that the pitch setting of the blades dropped to minus 1.8°, significantly below the minimum in-flight setting of 13.5°, while the Khabarovsk Airlines aircraft was just 170m above ground.
The inquiry says it suffered an "extremely improbable" uncommanded shift of the right-hand propeller into the 'beta' range – effectively reversing its thrust – and the crew had been unprepared to respond to this unlikely scenario.
While the Russian probe says the pitch reversal and the forward setting of the throttle levers caused the aircraft to enter a "significant" roll to the right, Czech authorities have formally requested an amended interpretation.
The Czech side says the crew increased power on the left-hand engine, leading to a developing yaw which – combined with low airspeed and "only small rudder deflection" – resulted in a roll that could not be compensated by aileron deflection alone.
Within 14s of the onset of the problem the first officer had identified that a propeller had entered the beta range.
But while the pilots attempted to recover the aircraft using ailerons, elevators and rudder, as well as the left-hand throttle control, they did not feather the right-hand propeller.
Manufacturer Aircraft Industries says the certification process for the particular L-410 variant – the UVP-E20, fitted with General Electric H80-200 engines – did not consider propeller pitch reversal to be a risk.
"[This] was the reason why [no] instructions for flightcrew for this event were introduced into [the aircraft flight manual," it told the inquiry. Analysis had indicated that the probability of such an event was 10-14 and, as a result, crew training was not required.
Seven occupants, including the two pilots, had been on board the aircraft (RA-67047) and only a single passenger survived the 15 November 2017 accident.