Feathered propellers on Yeti Airlines plane resulted in loss of thrust, says crash report

KATHMANDU – The propeller blades of the

Yeti Airlines ATR 72-500 plane that crashed in January

were in the feathered position, which caused the aircraft to lose thrust and fall, a preliminary report issued on Wednesday said.

All 72 people on board were killed in the accident in Pokhara that occurred in clear weather, as horrified residents watched from their rooftops.

According to experts, the propellers on an aircraft can “feather”, which means the propeller blades are rotated so that they lie flat towards the air flowing past it, to reduce drag when engine power is lost. In this position, the plane is not being pushed forward and it stalls.

It is rare for the propellers of both engines to come to a feathered position, according to ATR pilots. Feathering is typically done to reduce drag in the event of engine failure.

So what happened with the Yeti Airlines ATR 72-500 bearing the call sign 9N-ANC?

“We are analysing the matter. It’s a human error,” said Mr Buddhi Sagar Lamichhane, member secretary of the five-member probe commission formed by the government. “No engine failure has been reported. There are many factors that we have to analyse.”

According to experts the Post talked to, 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 idling speed for its respective engine.

One investigator told the Post that at the crash site, they found the levers pulled down.

“We are awaiting details. We cannot ascertain what happened now,” said Mr Lamichhane. “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.”

According to Annex 13 of the International Civil Aviation Organisation’s (ICAO) Convention on International Civil Aviation, the state conducting the investigation of an air accident or incident must produce a preliminary report within 30 days of the accident.

A publicly available final report should be produced within 12 months of the accident.

On Jan 15, Yeti Airlines Flight NYT 691 was approaching from the west towards runway 12 at Pokhara’s brand-new international airport, which had been inaugurated only two weeks earlier.

The plane had taken off from Kathmandu at about 10.30am, and the crash occurred at about 11am in Pokhara.

According to the preliminary report, the crew had made two flights between Kathmandu and Pokhara earlier that 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 going through aerodrome familiarisation for flying in Pokhara, and Captain Kamal KC was the instructor pilot on this training flight.

Captain Khatiwada was in the left-hand seat and was flying the plane. Captain KC sat in the right-hand seat and was monitoring her.

During the first contact with the control tower at Pokhara airport, the air traffic controller had assigned the plane runway 30 for landing. But during the later phases of the flight, the crew requested and received clearance from the air traffic controller to land on runway 12.

At 10:56:12, the pilots extended the flaps to the 15-degree position and pushed the landing gear lever to the down position.

At 10:56:27, Captain Khatiwada disengaged the autopilot system at an altitude of 220m.

Khatiwada then called for “Flaps 30” at 10:56:32, and Captain KC replied: “Flaps 30 and descending.”

The flight data recorder (FDR) did not record any flap surface movement at that time, the report said.

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 0 per cent, which is consistent with both propellers going into the feathered position, the report said.

The flight crew then carried out the “before landing checklist” before starting the left turn for the base leg of the flight. At that time, the power lever angle increased from 41 per cent to 44 per cent.

At this point, the rotation speed of both propellers was recorded as non-computed data – unreliable data output caused by a condition other than a system failure in the flight data recorder.

When the propellers are feathered, they do not produce thrust that pushes the aircraft forward.

According to the flight data recorder, no recorded parameters related to the engines showed any anomaly.

At 10:56:50, when the aircraft was at 152m, a clicking sound was heard. The aircraft reached a maximum bank angle of 30 degrees at this altitude.

The recorded propeller rotation speed and torque data remained invalid.

Captain Khatiwada asked Captain KC whether to continue the left turn and Captain KC told her to continue the turn. Subsequently, Captain Khatiwada asked Captain KC whether to continue to descend, and Captain KC said it was not necessary and told her to apply a little power.

At 10:56:54, another click was heard, followed by the flaps’ surface movement to the 30-degree position.

When the air traffic controller gave the clearance for landing at 10:57:07, Captain Khatiwada twice mentioned that there was no power in the engines.

At 10:57:11, the power levers were advanced first to 62 degrees, and then to the maximum power position. It was followed by a click at 10:57:16.

One second after the click, the aircraft was initiating its last turn 112m above ground level, and the high-pressure turbine speed of both engines increased from 73 per cent to 77 per cent.

“It is noted that Khatiwada handed over control of the aircraft to KC at 10:57:18,” the report said.

At 10:57:20, Captain Khatiwada, who was previously commanding the plane, repeated that the engines were not producing any power.

At 10:57:24 when the aircraft was 95m above ground level, the stick shaker was activated. The stick shaker device is designed to shake an aircraft’s control yoke as a warning that an aerodynamic stall could be about to occur.

At 10:57:26, a second sequence of stick shaker warnings was activated when the aircraft banked towards the left abruptly. At 61m, the stick shaker ceased.

At 10:57:32, the sound of an impact was registered by the cockpit voice recorder. The flight data recorder and cockpit voice recorder stopped recording at 10:57:33 and 10:57:35, respectively.

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

The wind speed was light at three to five knots. The prevailing visibility was 6km, and the temperature was measured at 14 deg C.

Nepal’s investigators have

received analytical support from Singapore’s Transport Safety Investigation Bureau.

This is

Nepal’s third-deadliest air accident

and its worst involving a domestic airline. More than four million passengers fly on its domestic carriers annually.

It is the second air disaster in Nepal in less than a year, highlighting the country’s consistently poor air safety record, analysts said.

The commission has issued an interim recommendation to the Civil Aviation Authority of Nepal (CAAN) to conduct a comprehensive study to determine whether runway 12 is an appropriate flight path that allows for a stabilised visual approach.

One investigator pointed out serious lapses by CAAN for allowing flights to an untested airport.

“We have recommended stopping flights from runway 12 until a comprehensive analysis is done,” said Mr Lamichhane.

The airport was hastily inaugurated without adequate preparations to meet the Jan 1 project deadline. The flight calibration process, where all equipment at the new airport has to be tested, has yet to show that landing from the west is recommended. The calibration report is due to be published on Feb 26.

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

An evaluation as required by the safety management system was not carried out by either CAAN, as a service provider of the airport, or the airlines, a former chief of CAAN told the Post in a recent interview.

“The civil aviation body has misused its regulator role to operate the airport, neglecting safety requirements. This is a serious crime and mass murder, and subject to a criminal investigation.”

Air traffic controllers at a new airport should also have a specific rating.

But in Pokhara, no one has this rating. THE KATHMANDU POST/ASIA NEWS NETWORK