Khabarovsk L410 at Nelkan on Nov 15th 2017, impacted ground short of runway, right propeller went into reverse in flight
Last Update: August 10, 2019 / 13:47:08 GMT/Zulu time
Incident Facts
Date of incident
Nov 15, 2017
Classification
Crash
Aircraft Registration
RA-67047
Aircraft Type
Let L-410 Turbolet
ICAO Type Designator
L410
The direct cause of the L410UVP-E20 RA-67047 A/C accident was the uncommanded inflight RH engine propeller blades setting to the angle of minus 1.8° which is significantly below the minimum inflight pitch angle (13.5°) with TCLs set to forward thrust. It caused the significant rolling and turning moments, the A/C loss of speed and controllability, and the subsequent with the ground collision.
The propeller blades' setting to the negative angles was caused by the failures of two systems: the BETA Feedback system and the Pitch Lock system. As the Propeller Pitch Lock system components that are to be tested during the PITCH LOCK TEST most probably did not contribute to the system malfunction, then it is unlikely that the crews' deviation of the PITCH LOCK TEST procedure could have make any difference in the detection of the said system malfunction before the flight.
The said situation had been classified as extremely improbable during the aircraft type certification, so, there was no required crew actions in AFM for such situations, and the respective crew training was not required.
Czechia's UZPLN requested the first paragraph of the conclusions to be worded as: "The negative thrust and increased power on LH ENG without using higher deflection of rudder to eliminated high yawing moment resulted in significant sideslip angle, rolling and turning moment. The A/C loss of speed and controllability and the subsequent with the ground collision."
The MAK reported the aircraft departed Khabarovsk with takeoff mass as well as center of gravity within permitted limits. Chumikan had been planned a refueling stop only. About 50 minutes prior to estimated arrival in Chumikan the crew checked their remaining fuel and requested to continue to Nelkan without stop in Chumikan, which was approved by Khabarovsk ATC.
Descending through a height of 100 meters (328 feet AGL) at a speed of 100 KIAS the aircraft developed an aggressive right roll, lost speed quickly (76 KIAS 9 seconds after the BETA range indication activated) and height and collided with the ground.
The MAK reported that no anomaly was found with the fuel, engines, propellers, propeller governors and BETA valve. The BETA valve's electrical circuitry's integrity had not been interrupted.
The MAK annotated as result of the technical investigation: "It was not determined why the BETA Valve failed to prevent the propeller in flight movement below minimum flight blade angle."
The MAK subsequently described another crucial technial examination:
The initial examination of the Pitch Lock Solenoid Valve serviceability was conducted by using the method of sound assessment of the pilot valve activation when electrically powered. The serviceability of the Pitch Lock Solenoid Valve was assessed during the four full cycles. No sound effects were observed during the first 2 cycles. The sound effects were observed during the 3rd and 4th operating cycles. Further, when the Pitch Lock Solenoid Valve assembly was fully disassembled, no damage of the Pitch Lock Solenoid Valve, sliding spool or sleeve was found.
The absence of the sound effects from the Sliding Spool (solenoid) movement could be possibly caused by presence of oil inside of the Valve which might have acted as a damper during the first 2 cycles. The solenoid functional assessment was performed two months after the accident, and during these two months the sliding spool was not properly preserved, that could also cause its stiction.
It was not determined why the Pitch Lock Solenoid Valve was not activated when the propeller had moved below minimum flight blade angle and BETA status annunciation got on.
Therefore, the examinations, including the propeller governor full disassembly, didn't let to determine the cause of the propeller movement below minimum flight blade angle (+13.5°) and of the Pitch Lock Solenoid Valve failure to be activated when the blades had reached the angle of +9°.
The MAK analysed also stating: "Therefore, it may be concluded that at least two systems (BETA Feedback system and pitch lock system) failed in the same (accident) flight.":
the BETA Feedback system faults were registered for the different airplanes (power plants) on numerous occasions. The faults were repeated ones (but also intermittent ones); the causes of the said faults were not recognized for sure (as they were intermittent ones); and the measures taken by the propeller designer/manufacturer were ineffective. Therefore, the real-life operations does not prove the declared probability of failure (10-7 for 1 flight hour).
Moreover, in all abovementioned cases of the BETA Feedback system faults (excluding the accident flight in question), the Pitch Lock system had operated and its operation prevented the propeller blades' setting to the negative angles of attack.
Incident Facts
Date of incident
Nov 15, 2017
Classification
Crash
Aircraft Registration
RA-67047
Aircraft Type
Let L-410 Turbolet
ICAO Type Designator
L410
This article is published under license from Avherald.com. © of text by Avherald.com.
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