The Yeti Airlines aircraft which crashed in Nepal last month killing 71 people, including five Indians, lost thrust and fell after the propellers of both engines went into a feathered position, according to a preliminary probe report which lead investigators to suspect human error behind the deadly crash.

Yeti Airlines flight 691, after taking off from Kathmandu's Tribhuvan International Airport on January 15, crashed on the Seti River gorge between the old airport and the new airport in the resort city of Pokhara.

There were 72 people including four crew members onboard the ATR-72 aircraft when it crashed, but rescue officials have so far managed to recover only 71 bodies with the other missing passenger presumed dead.

It is rare for the propellers of both engines to come to a feathered position, said one of the members of the five-member probe committee, which is also mentioned in the 14-page preliminary report posted on the website of the Ministry of Tourism, Culture and Civil Aviation of Nepal.

“Human factor in the accident could not be disregarded. So it is an issue of investigation,” said the member on condition of anonymity.

“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,” states the preliminary report.

“As per the Flight Data Recorder (FDR) all the recorded parameters related to engines did not show any anomaly,” adds the report.

“When Air Traffic Controller (ATC) gave the clearance for landing at 10:57:07, the Pilot Flying (PF) mentioned twice that there was no power coming from the engines,” the report further says.

“During the time of the crash, the prevailing visibility was 6 km and the sky was almost clear with only a few clouds,” points out the preliminary report.

According to experts, the pilots unintentionally pulled the condition levers causing the engine to shut down and feather the propellers. Each lever starts and stops the fuel supply, and controls the idle speed for its respective engine.

One investigator said that they found the levers pulled down at the crash site.

“We are waiting for a detailed report. We cannot ascertain what happened before that,” said a probe committee member.

“Yes, there is also the issue of flaps. There are questions about why the pilots delayed extending the flaps. The routine checklists were not followed. There are many factors to look at,” Joint Secretary at the Tourism Ministry Buddhi Sagar Lamichhane - a senior member of the probe committee -- was quoted as saying by The Kathmandu Post newspaper.

As per International Civil Aviation Organisation (ICAO), the state investigating an accident or incident must produce a preliminary report within 30 days of the accident.

The final report should be produced within 12 months of the accident.

According to the preliminary report, the flight crew had made two flights between Kathmandu and Pokhara earlier in the morning. The flight that crashed was the third in a row by the same crew.

There were two captains on the plane. Captain Anju Khatiwada was in the process of obtaining aerodrome familiarisation for operating in Pokhara, and Captain Kamal KC was the instructor pilot on this training flight.

The take-off, climb, cruise, and descent to Pokhara were normal.

At 10:56:12, the pilots extended the flaps to the 15 degrees position and pushed the landing gear lever to the down position. Captain Khatiwada then disengaged the autopilot system at an altitude of 721 feet above the ground.

Captain Khatiwada then called for “FLAPS 30”, and Captain KC replied, “Flaps 30 and descending.” The flight data recorder (FDR) did not record any flap surface movement at that time.

Instead, the propeller rotation speed of both engines decreased simultaneously to less than 25 per cent and the torque, the force that causes an opposite rotation, started decreasing to zero per cent, which is consistent with both propellers going into the feathered condition, the report said.

When the propellers are feathered, they don't produce thrust that pushes the aircraft forward. As per the flight data recorder, no recorded parameters related to the engines showed any anomaly.

When the air traffic controller gave the clearance for landing, Captain Khatiwada twice mentioned that there was no power in the engines.

The report said that the aircraft's airworthiness certificate was valid till April 24, 2023.

Nepal's investigators have received analytical support from Singapore's Transport Safety Investigation Bureau.

The airport, built with China's assistance, was hastily inaugurated and operated without adequate preparations to meet the January 1 project deadline. Flight calibration, which tests all pieces of equipment at the new airport, has yet to recommend landing from the west. The calibration report is due to be published on February 26.

The flight procedure has not been published either. Airlines have very little information about airport procedures and data, according to experts.

The AAIC has stated that further investigation will focus on the circumstances under which both propellers went into the feathered condition, human factors, and visual approach procedures, including simultaneous operation of both national and international airports, according to media reports.