Scheduled Revenue Flight

In a preliminary report, the Nepalese Authorities confirmed the following points:
- At 10:56:27, the PF disengaged the Autopilot System (AP) at an altitude of 721 feet Above Ground Level (AGL). The PF then called for “FLAPS 30” at 10:56:32, and the PM replied, “Flaps 30 and descending”. The flight data recorder (FDR) data did not record any flap surface movement at that time. Instead, the propeller rotation speed (Np) of both engines decreased simultaneously to less than 25%1 and the torque (Tq) started decreasing to 0%, which is consistent with both propellers going into the feathered condition. On the cockpit voice recorder (CVR) area microphone recording, a single Master Caution chime was recorded at 10:56:36. The flight crew then carried out the “Before Landing Checklist” before starting the left turn onto the base leg. During that time, the power lever angle increased from 41% to 44%. At the point, Np of both propellers were recorded as Non-Computed Data (NCD) in the FDR and the torque (Tq) of both engines were at 0%. When propellers are in feather, they are not producing thrust.
- When both propellers were feathered, the investigation team observed that both engines of 9N-ANC were running flight idle condition during the event flight to prevent over torque. As per the FDR data, all the recorded parameters related to engines did not show any anomaly. At 10:56:50 when the radio altitude callout for five hundred feet3 was annunciated, another “click” sound was heard. The aircraft reached a maximum bank angle of 30 degrees at this altitude. The recorded Np and Tq data remained invalid. The yaw damper disconnected four seconds later. The PF consulted the PM on whether to continue the left turn and the PM replied to continue the turn. Subsequently, the PF asked the PM on whether to continue descend and the PM responded it was not necessary and instructed to apply a little power. At 10:56:54, another click was heard, followed by the flaps surface movement to the 30 degrees position.
- When ATC gave the clearance for landing at 10:57:07, the PF mentioned twice that there was no power coming from the engines. At 10:57:11, the power levers were advanced first to 62 degrees then to the maximum power position. It was followed by a “click” sound at 10:57:16. One second after the “click” sound, the aircraft was at the initiation of its last turn at 368 feet AGL, the high pressure turbine speed (Nh) of both engines increased from 73% to 77%.
- It is noted that the PF handed over control of the aircraft to the PM at 10:57:18. At 10:57:20, the PM (who was previously the PF) repeated again that there was no power from the engines. At 10:57:24 when the aircraft was at 311 feet AGL, the stick shaker was activated warning the crew that the aircraft Angle of Attack (AoA) increased up to the stick shaker threshold.

It was determined that the crew of the fire truck entered the active runway 16L without prior permission from air traffic controller. This chain of events was caused by the lack of joint planning, poor coordination and the non-use of ICAO-standardized communication and phraseology.

The collapse of the left main landing gear during the landing roll resulted in a runway excursion due to a loss of controllability on the runway, during which the aircraft impacted a small equipment building, breaching the right-wing fuel tank and causing a post-crash fire. The performance of the crew was thoroughly evaluated during this investigation and found to be appropriate for the circumstances of the accident.
The focus of this analysis is the cause of the left main landing gear collapse. Particularly
(1) the left shimmy damper’s failure to adequately dampen vibration during landing and
(2) the failure of the left main gear downlock mechanism due to excessive vibration.
The structural failure of the left main landing gear downlock following ineffective shimmy dampening during the landing roll which caused the collapse of the left main landing gear, resulting in a runway excursion and post-flight fire.