Yeti crash probe report suggests fixing flight path for landing

By A Staff Reporter,Kathmandu, Feb. 17: The Civil Aviation Authority of Nepal (CAAN) must conduct a comprehensive study to determine the appropriate flight path that allows the criteria for a stabilised visual approach to be met before resuming visual approach on Runway 12 of the Pokhara International Airport (PIA). This is the immediate recommendation made by the Aircraft Accident Investigation Commission looking into the Yeti Airlines plane crash that occurred in Pokhara on January 15.

The Commission, led by former Secretary Nagendra Prasad Ghimire, based this recommendation on the data it recovered from the flight data recorder of the crashed 9N-ANC plane. From the data, members of the probe team were able to piece together the path the ill-fated flight carrying 72 people took on its approach to PIA. They also recovered the path taken by another flight, piloted by a different crew that landed on Runway 12 at PIA three days before the accident under investigation.

In both these flights, due to the shortened final approach leg for Pokhara’s Runway 12, the Commission found that the stabilisation criteria for a visual approach could not be stabilised at the height of 500 feet above ground level. Therefore, through the Preliminary Accident Investigation Report it submitted to the Ministry of Culture, Tourism and Civil Aviation on Wednesday, the Commission has asked CAAN to look into this issue before allowing visual approach landings at this particular landing strip.

However, investigation is still ongoing and the Commission stressed that its findings could change as it looks at multiple facets of the incident. 

Last month, on the day of the Nepali festival of Maghe Sankranti, the unfortunate Yeti Airlines flight, numbered NYT691 (Yeti 691), took off from Kathmandu at 10.32 am. It was bound for Pokhara and was supposed to land at PIA, inaugurated only two weeks before, at 10.58 am. However, one minute before its expected landing time, it fell from the sky while making a visual approach towards the landing track, killing everyone on board.

It was the third flight between Kathmandu and Pokhara made by the plane that day. It was operated by two pilots – one was obtaining aerodrome familiarisation for operating into Pokhara and other was instructing her. As per the preliminary report, the first pilot Anju Khatiwada, who was the learning captain, was flying the aircraft and the second pilot Kamal KC, who was 

the instructor, was monitoring her performance. The take-off, climb, cruise and descent to Pokhara were all normal. At first contact with the Air Traffic Controller at Pokhara, the plane was assigned Runway 30. But later, the flight crew requested and were cleared to land on Runway 12.

What cockpit voice recorder lodged 

At 10.51.36 am, the aircraft descended from 6,500 feet at five miles away from PIA and joined the downwind track for Runway 12 to the north of the airstrip.

At 10.56.12 am, the pilots extended the flaps to the 15 degrees position and selected the landing gears lever to the down position. The take-off setting was selected on the power management panel.

At 10.56.27 am, the flying pilot disengaged the Autopilot System when the plane was at an altitude of 721 feet above ground. She called for “Flaps 30” at 10.56.32 am and the instructor pilot replied, “Flaps 30 and descending”. However, the flight data recorder did not record any flap surface movement at that time.

Instead, the propeller rotation speed of both engines dropped simultaneously to less than 25 per cent and the torque started dropping to zero. This is a situation that indicates both the propellers ‘feathered’. 

As per the Investigation Commission, the feathering of a propeller on this particular type of plane can be commanded automatically by aircraft systems or manually by the pilot. It is usually performed when an engine is shut down so that the leading edge of the propeller is parallel to the oncoming airflow to reduce drag. Also, when propellers are feathered, they do not produce thrust.

On the cockpit voice recorder, a single Master Caution chime was recorded at 10.56.36 am. The flight crew then implemented the ‘Before Landing Checklist’ before starting the left turn onto the base leg. In that time, the power lever angle increased from 41 per cent to 44 per cent. At that point, the speed of both propellers was recorded as non-computed data and the torque of the engines was zero.

At 10.56.50 am, when the radio altitude callout for 500 feet (meaning the aeroplane was 500 feet above the ground) was enunciated, another click was heard. This suggests that some action by the crew was inhibiting the master caution light. The aircraft reached a maximum bank angle of 30 degrees at this altitude. The yaw damper (a system used to reduce the undesirable tendencies of an aircraft to oscillate in a repetitive rolling motion) disconnected four seconds later. 

Khatiwada asked KC whether or not to continue the left turn and KC told her to continue. Then, she asked him whether to continue descending and he told her it was not necessary and directed her to apply a little power. 

At 10.56.54 am, another click was heard, followed by the flaps surface movement to the 30 degrees position. 

When Pokhara’s Air Traffic Controller cleared the plane for landing at 10.57.07 am, Khatiwada noted twice that there was no power coming from the engines. At 10.57.11 am, the power levers were advanced first to 62 degrees and then put to maximum power. It was followed by yet another click sound at 10.57.16 am. 

Khatiwada handed over the controls to KC at 10.57.18 am and at 10.57.20 am, KC too noted that there was no power from the engines. 

At 10.57.24 am, when the aircraft was at 311 feet, the stick shaker was activated, warning the crew that the aircraft’s angle of attack increased to the stick shaker’s maximum threshold. 

At 10.57.26 am, a second sequence of stick shaker warning was activated when the aircraft abruptly banked left. Thereafter, the radio altitude alert for two hundred feet was enunciated and the stick shaker ceased. At 10.57.32 am, the sound of impact was heard. The flight data recorder and the cockpit voice recorder stopped at 10.57.33 and 10.57.35 respectively.

This series of events that unfolded in less than 10 minutes led to the worst accident in the history of Nepal’s domestic aviation, obliterating six dozen precious lives.