Investigators examining the crash of a Let Aircraft Industries L-410 in eastern Russia have disclosed that the aircraft's propellers started turning at different speeds seconds before the right-hand engine entered the beta range on final approach to Nelkan.
Russia's Interstate Aviation Committee says that reliability data from the aircraft's developer suggests that a spontaneous triggering of the beta mode is a highly unlikely event – with the extraordinarily low occurrence probability of 10-14.
The Khabarovsk Airlines aircraft had been descending for runway 04 while flying at 100-115kt when the flight-data recorder started registering a difference in the rotation speeds of the left- and right-hand propellers of the General Electric H80-200 engines.
This difference continued to increase, with the right-hand propeller exhibiting the higher speed, and 14s after the onset a signal was detected to activate the right-hand engine's beta mode – effectively commanding reverse thrust. The signal continued until the end of the recording.
The inquiry says that, at about the same moment, both throttles were advanced. The aircraft began to roll to the right, banking to 20-25° within 4-5s. This bank fluctuated as the crew used the ailerons and pedals to try to rebalance the aircraft.
Two second after the beta-mode trigger signal, the aircraft's right-hand propeller increased rotation speed to 2,300rpm, above the normal maximum operating level of 2,080rpm.
"The difference in the [rotation] level of the right and left propellers continued to persist," says the inquiry.
It states that the cockpit-voice recording shows the pilots were almost immediately aware that the aircraft had entered the beta range, but were unable to counter with the left-hand engine or the control surfaces.
The situation quickly deteriorated, with the aircraft descending to 100m and rapidly losing speed, and a stall warning sounded as it slowed to 76kt.
Some 3-4s later, at 80m and 72kt, the aircraft rolled sharply to the right, making a complete rotation around its longitudinal axis as it fell, crashing into woodland 1.4km from the threshold of the runway. Just one of the seven occupants, a young child, survived.
Both engines had logged 1,693h across 1,028 cycles.
One month after the 15 November accident the aircraft developer issued a bulletin, approved by the European Aviation Safety Agency, outlining a procedure for pilots in the event of a beta-mode warning during final approach, which includes reducing the throttle and feathering the affected propeller.
Investigators are still probing the circumstances of the crash. Fuel and oil analysis is being carried out and the associated activity also includes examining the emergency beacon on the aircraft, which failed to activate.