The pilot monitoring the Yeti Airlines flight NYT-691 that crashed in Pokhara in January killing all 72 people on board appears to have inadvertently pulled a “wrong lever” to operate the wing flaps, leading the plane to crash, according to the final probe report released on Thursday.

“It is likely that the pilot monitoring the course of the flight had misidentified and moved both condition levers [which control fuel supply to the engine] to feather,” the report said.

In commercial aviation with a two-person flight crew, the pilot flying is the pilot operating the flight controls, and the other pilot is referred to as the pilot monitoring. Likewise, feathering is used when shutting down an engine to align the blades with the direction of flight, reducing drag for better efficiency.

“When the pilot flying called for flaps 30, the monitoring pilot did not make an appropriate crosscheck loop as per cockpit resource management training.”

The pilot flying did not visually crosscheck the position of flaps 30 and confirm. The term “flaps 30” refers to the angle at which the flaps are extended.

The most probable cause of the accident is determined to be the inadvertent movement of both condition levers to the feathered position in flight, which resulted in the feathering of both propellers and subsequent loss of thrust, leading to an aerodynamic stall and collision with terrain, the report said.

“During the investigation, in particular, the analysis of the flight recorder data, the recorded parameters indicated that the propellers were most likely moved to the feathered condition when the pilot flying requested flaps 30 during the approach.”

The condition levers and flap levers are located next to each other on the pedestal and close to the co-pilot’s seat. All evidence gathered from the detailed examinations appears to indicate that the propellers were manually set to the feathered position.

“It appears that human factors such as high workload and stress resulted in the misidentification and selection of the propellers to the feathered position,” said Buddhi Sagar Lamichhane, the member secretary of the probe committee.

“Our investigation shows that the pilot monitoring had lost concentration in the cockpit. He seems to have repeatedly ignored calls from the pilot flying.”

Although the report has pointed to inadequate training, it has not mentioned what led to the loss of the pilots’ concentration in that particular case.

Some sources said salary delays could be one factor causing inconvenience and stress for pilots.

Before take-off, a pilot of another aircraft of Yeti Airlines asked the pilot monitoring “Ghanti bajyo?” (“Did the bell ring?”). [meaning, has the phone pinged with salary notification?]. The pilot replies, “chhaina” (“No”), “aaula ni” (“It may come”).

The probe members, however, ruled out any link of the salary payment delay to the accident. But some experts say salary delays affect employees across all levels and result in stress.

There was no evidence of engine failure until the impact of the aircraft, the report says. Both the engines were running at idle condition till the time of impact. “Hence the possibility of engine failure is ruled out.”

There was no evidence of any systems failure either. Hence, the failure of the aircraft systems e.g. hydraulic, flight control and other major components can, too, be ruled out, it says. The probability that the power plants, systems, structural failures or any other mechanical malfunction contributing to the accident can be ruled out, the report reads.

Before reaching MANKA, a point along the Kathmandu-Pokhara route, the pilot monitoring expressed his intent to familiarise the pilot flying on Runway 12 of the new airport, traffic permitting.

The reason for a change of runway was to clear the pilot flying from Runway 12 under instructor pilot’s supervision. However, this familiarisation of the Runway 12 was not something planned by the Yeti’s operations department.

When the aircraft was at 15 miles to the Pokhara International Airport at 6,500 ft, the pilot monitoring requested and received clearance to land on Runway 12.

After joining downwind to Runway 12, the pilot flying announced “flap 15” and then “flap 30”. However, it was found the checklists were not adhered to in all phases of flight, as per standard operating procedures, the report said.

The pilot flying then asked if they should continue the turn to which the pilot monitoring responded “positive” and advised on increasing the power, but the pilot flying reported that there was no power.

The pilot flying again repeated that there was no power, no torque. The Flight Data Recorder records show the power levers were moved from the ‘flight idle’ position.

The flight was the first for the pilot flying.

During the first contact with Pokahra tower, the Air Traffic Controller (ATC) assigned runway 30 for the aircraft to land. But during the later phases of flight, the flight crew, without mentioning any reason for changing the allocated runway, requested and received clearance from the ATC to switch to runway 12 for landing.

At 10:51:36, the aircraft descended from 6,500 feet. At 10:56:12, the pilots extended the flaps to a 15-degree position and 46 seconds later they selected the landing gear lever to the down position.

At 10:56:27, the pilot flying disengaged the Autopilot System at an altitude of 721 feet. The pilot flying then called for “flaps 30” at 10:56:32, and the pilot monitoring replied, “Flaps 30 and continue descent. The flight data recorder [FDR] data did not record any flap surface movement at that time,” said the report.

Instead, the propeller rotation speed (Np) of both engines decreased simultaneously to less than 25 percent and the torque started decreasing to 0 percent, which is consistent with both propellers going into the feathered condition. When propellers are in a feathered position, they do not produce thrust.

The plane came down as a result. At 10:57:18, in the very last stage of the flight, the pilot flying handed over the aircraft’s control to the pilot monitoring. But it was already too late.

The report has pointed out serious lapses on the regulatory part. The flight operation in the unfamiliar runway was approved in haste.

Aerodrome obstacle data and information on facilities available at the new airport were published in the Aeronautical Information Publication of Nepal and became effective on December 29, 2022.

Air Traffic Services Regulation was not coordinated with that of aerodromes and not considered within the certification process, the report said.

The approval of validation flights as well as the development of standard operating procedures was a prerequisite for commencement of scheduled flight operations to the new airport.

However, according to the report, there is no evidence of the Civil Aviation Authority of Nepal approving the procedures before authorising validation flights for Yeti Airlines.

The civil aviation body neither developed the visual circuit procedure on its own nor did it ensure its development by the airline concerned.

“However, the civil aviation body approved operations from both the runways. It is the responsibility of the safety regulator to assess the service provider’s plan for change before providing operating authorisation in a changing environment to mitigate the risk and to increase the confidence of the public in air travel,” the report said.

The flight trajectories show that the flight path gradients were not as per ATR 72-212A version 500’s flight manual.