Investigators probing the fatal crash of a Let Aircraft Industries L-410 in Russia have revealed several other incidents involving beta-range propeller pitch on the type.
The Khabarovsk Airlines aircraft (RA-67047) came down on approach to Nelkan in November 2017 after the right-hand engine entered the beta range, effectively reversing thrust and causing the L-410 to roll into the ground from low altitude.
Russia's Interstate Aviation Committee says it identified another case of in-flight beta status annunciation with a separate Khabarovsk L-410 (RA-67036) which was approaching Ayan airport in March 2015.
The turboprop was travelling at 115kt, at a height of 260m, when the annunciation occurred for the right-hand engine.
But a back-up system – a pitch lock, designed to stop the blades moving into reverse – appears to have activated and stopped the propeller, as intended, at a 9° pitch.
Investigators state that the beta status annunciation lasted 21s, the aircraft descended 110m, but it did not roll more than 10° to the right.
Subsequent ground testing of the engine, in various modes, could not replicate the beta status annunciation.
"[This case] was not classified as an incident," says the Interstate Aviation Committee. "Therefore, no preventive measures were developed at that time."
No information was provided to the aircraft, engine or propeller designers and manufacturers and the aircraft remained in operation, with "no further attempts" made to determine the cause, it adds.
Two months after the fatal Nelkan accident another beta-status annunciation incident occurred, this time involving a third Khabarovsk L-410 (RA-67040) on the ground in January 2018.
The annunciation from the right-hand powerplant – together with a pitch-lock signal – was generated after engine start, when the pitch control was moved from feather mode to minimum in-flight pitch angle.
Specialists examined the propeller governor and found that it met acceptance test requirements, but that assessment of the beta valve exhibited some "sticking", says the inquiry. This could potentially cause uncommanded propeller overspeed and blade movement below the minimum pre-set angle, it adds.
But the conclusion that the governor design is vulnerable to possible hard seizure has proven controversial. Czech authorities, representing the state of manufacture, are "positive" that the governor design "excludes" beta valve seizure.
While work has been carried out on L-410s fitted with General Electric H80-200 engines, involving modification of beta switches and adjustment of propeller and governor systems, the Interstate Aviation Committee says two more cases of abnormal Khabarovsk Airlines operations occurred in April and May last year.
After engine start on aircraft RA-67035, when the pitch control was moved into the 'fine' position, the beta-range and pitch-lock annunciations activated, followed by propeller overspeed.
Czech authorities have told the inquiry that the April occurrence related to an incorrect beta-switch adjustment procedure while that in May involved contamination of the beta switch.