Plain, Valley

The probable cause of the accident was the shut down of the right engine and then the left engine, possibly due to a faulty shut down on part of the crew. In the course of the subsequent emergency descent due to complete loss of power of the engines with no radio communication with air traffic controllers, as well as possibly due to the resulting confusion and nervousness, the crew unintentionally forgot to switch the pressure value on the mechanical barometric altimeter of the aircraft to the pressure on the landing airfield (704 mm Hg), leaving the pressure value of 760 mm Hg, which led to incorrect readings of true altitude of the flight. Subsequently, the crew was able to start the engines and attempt to make a forced landing at a distance of 28 km from the airfield of Almaty at night in poor weather conditions in an unlit area. Incorrect readings on the barometric altimeter of the true flight altitude (the radio altimeter showed 750 m) resulted in the aircraft colliding with a power line, causing a fire, as well as colliding with the ground, as a result of which the aircraft structure completely collapsed and burned down.

The cause of the accident was an inner mechanical defect in the left engine past its stipulated TSO. The defect caused a forced stall of the engine. For this reason, the propeller on this engine could not be feathered. During the shutdown of the right engine, the feathering of the propeller on this engine did not take place. The aircraft entered to the fall and a stall-spin in which it fell to the ground.
Contributing factors:
- The left engine TSO was over the stipulated limit of 12 years. (The last general overhaul was performed on 29 October 2001.),
- Limited experience of the pilot on this type of aircraft.

Loss of engine power due to the failure of the cylinder n°5 caused by a faulty manufacturing.

Loss of control during climb due to the combination of the following factors:
- The particular characteristics of the aircraft;
- The attention required by the transponder as it is not displayed by air traffic control;
- The detour of the planned flight at the request of the inspection department of air traffic;
- Aircraft trajectory management with autopilot switched off;
- The pilot's limited experience in instrument flight conditions. In addition, the absence of additional instruction in the aircraft type (due to the lack of specific regulations) can be considered a contributing factor, according to the probable scenario described above. In addition, the research identified the following elements with potential impact in operational safety:
- Absence of the TAWS system on the aircraft in non-conformity with the established RAAC 91;
- Lack of effective means to enable the flight plan office to quickly determine whether an aircraft has operating restrictions;
- Shortcomings in the training of staff in the units ATS and SAR in search and rescue.

The investigation determined the cause of the mishap to be an inflight departure of the number four blade from the number two propeller. This propeller blade (P2B4) liberated while the aircraft was flying at a cruise altitude of 20,000 feet . The liberation of P2B4 initiated the catastrophic sequence of events resulting in the midair breakup of the aircraft and its uncontrollable descent and ultimate destruction. Post- mishap analysis of P2B4 revealed that a circumferential fatigue crack in the blade caused the fracture and liberation. This fatigue crack propagated from a radial crack which originated from intergranular cracking (IGC) and corrosion pitting. The analysis also revealed the presence of anodize coating within the band of corrosion pitting and intergranular cracking on the blade near the origin of the crack. This finding proves that the band of corrosion pitting and intergranular cracking was present and not removed during the last overhaul of P2B4 at Warner Robins Air Logistics Complex (WR-ALC) in the fall of 2011. The investigation concluded that the failure to remediate the corrosion pitting and intergranular cracking was due to deficiencies in the propeller blade overhaul process at WR-ALC which existed in 2011 and continued up until the shutdown of the WR-ALC propeller blade overhaul process in the fall of 2017. The investigation also examined whether any operational or intermediate level maintenance inspections or maintenance actions exist which could have detected the underlying causal conditions prior to the mishap. The investigation concluded that while these inspections exist, it cannot be quantifiably determined that these inspections would have detected the causal condition. The investigation arrived at this conclusion due to the fact that the growth or propagation rate of an IGC radial crack cannot be predicted. Though no evidence exists to determine when the radial crack had grown to a detectable area, beyond the bushing, there exists a distinct possibility that it could have been detected if the radial crack had grown past the bushing and the off wing eddy current inspection was performed.

The pilot's inadvertent flight into instrument meteorological conditions and subsequent controlled flight into terrain.