Description

On 15 January 2023, an ATR 72-500 (9N-ANC) was being operated by Yeti Airlines on a scheduled domestic passenger flight from Kathmandu to the recently-opened Pokhara International airport as NYT691. It was being used for a Line Captain’s familiarisation with the new airport under supervision of a type-experienced Training Captain. The aircraft was about to turn onto a visual final approach for runway 12 at destination in day VMC when it was observed to suddenly depart controlled flight and begin a rapid descent before impacting uninhabited terrain. The impact and post crash fire completely destroyed the aircraft and all 68 passengers and the four crew members on board were killed. There were no injuries to people on the ground or damage to ground structures.  

Investigation

An Aircraft Accident Investigation Commission was established on the day of the accident in accordance with the Civil Aviation (Accident Investigation) Regulation (2016) of Nepal. The CVR and FDR were both recovered from the accident site and their data were successfully downloaded under supervision by the Singapore Transport Safety Investigation Bureau (TSIB) and all relevant data were able to be accessed. A Preliminary Report on initial progress with the Investigation was completed on 13 February 2023.

The 58 year-old male Training Captain in command, who was acting as PM for the accident flight and occupying the right hand seat, had a total of 21,901 hours of flying experience which included 3,300 hours on type. He had also flown the two types previously operated by the airline, the BAe Jetstream 41 and the De Havilland Canada DHC6-300. The 44 year-old female Captain occupying the left hand seat as PF had a total of 6,396 hours flying experience of which 186 hours were on type. She had previously flown the same operator’s BAe Jetstream 41 aircraft. The accident flight was being used for familiarisation training of the junior Captain in respect of the recently opened (1 January 2023) new airport at Pokhara. The accident sector was both pilots’ third flight of the day and had been preceded by a Kathmandu-Pokhara-Kathmandu rotation which had used the opposite direction of the runway (30) at the new Pokhara airport for the earlier landing.

What Happened

Recorded flight data showed that the flight proceeded normally until the approach phase. ATC initially assigned 2,500 metre-long runway 30 for a straight-in approach landing but given the light and variable reported surface wind, the crew subsequently requested and received clearance to use the equivalent-length runway 12 for approach familiarisation purposes (although their reason for this request was not provided with it).

The flight began descent from 6,500 feet when approximately fifteen miles away from the destination and joined the left hand downwind leg for Runway 12. The aircraft was visually identified by ATC during the approach and when on the downwind left hand leg, after the flaps had been set to 15°, the landing gear was extended. The PF then disengaged the AP at 721 feet agl and called for ‘flaps 30’ and the PM replied, “flaps 30 and continue descent”. No movement of the flap position was recorded on the FDR and instead, the propeller rotation speed (Np) of both engines (manually controlled using the condition levers which were located between the power levers and the flap position lever) simultaneously began to decrease, eventually going below 25%, the minimum relative speed which is recorded on the FDR. As a direct consequence of this, as the power output from both engines (torque) began to reduce, a Master Caution chime was recorded on the CVR. It was subsequently considered that this and subsequent Master Cautions were likely to have been activated because of one or more secondary system status conditions - electrical, hydraulic or anti icing - rather than being directly related to the dual engine feathering action.

The Central Pedestal controls - from right to left the flap lever, the RH & LH condition levers, the RH & LH Power Levers and the Parking/Emergency Brake. [Reproduced from the Official Report]

Without either pilot identifying that the flaps were not at 30°, the ‘Before Landing’ Checklist was then completed and the left turn from downwind to base leg was as the automatic call at 500 feet agl was annunciated. The PF was recorded asking the PM “whether to continue the left turn” and was advised to do so. She then asked whether the descent should be continued and the PM responded by saying this was not necessary but instructed her to “apply a little power”. The power levers were moved very slightly forward at this time but this had no effect on the engine torque indications which had reached zero - engine torque decreases to flight idle by design if the corresponding propeller has been feathered. However, another ‘Master Caution’ source “click” was then recorded as the flaps were moved from 15° to the previously requested 30° without comment.

Ten seconds later, with the aircraft still on left base leg, the TWR controller gave a landing clearance but there was no readback as the PF “mentioned twice that there was no power coming from the engines”. Almost immediately, the power levers were rapidly advanced initially to 62° and then to maximum over two seconds but with both propellers feathered, this had no effect. Another “click” indicating a ‘Master Caution’ activation occurred and one second later, the left turn onto final approach was commenced at a recorded 368 feet agl and almost immediately the PF had handed over control of the aircraft to the Training Captain and followed this by repeating that there was no power coming from the engines. Six seconds after control had been handed over, with the aircraft at a recorded 311 feet agl, the stick shaker activated twice and after the second activation, the aircraft banked left abruptly. Three seconds after this, as the automatic 200 feet agl call occurred and after a further three seconds the sound of impact was recorded on the CVR.

A post crash fire began and the wreckage was subsequently located in one of the few unpopulated parts of the area, the gorge of the Seti Gandaki River which was very close to the original (and still open) Pokhara Airport. The accident site is shown on the illustration below with the ground track of the flight closely following that of a previous flight which also positioned for a successful final approach at the new airport (see below).

The accident aircraft ground track (yellow) and the accident site close to the old airport which is just 2nm to the west southwest of the new one. [Reproduced from the Official Report]

Why It Happened

Despite the combination of a comprehensive reduction of the aircraft to many small pieces and the extensive fire damage, the Investigation was able to show that the aircraft had been unaffected by any relevant defects. The loss of control had been solely a consequence of the failure of either pilot to notice that the power output from both engines had been reduced to flight idle or that the cause of this had been the selection of both condition levers to the propeller feathering position instead of the flaps 30 call being correctly actioned despite a confirmatory call that it had. Even when - 20 seconds later - the PM Training Captain realised that he had not set the flaps as requested and confirmed and therefore did so, he did not verbalise this corrective action or notice that the abnormal in-flight feather position of both condition levers was the cause. During the 30 seconds which followed before the stick shaker signalled imminent loss of control, the only attempt to obtain power was to move the power levers forward to no effect.

The tentative conclusion was that the Training Captain had allowed his advice to and monitoring of the PF Captain’s visual circuit procedure to obscure his core responsibility to carry out his normal PM duties as instructed unless unsafe. The Investigation therefore identified the immediate human factors cause of the accident as potentially being the Training Captain’s self-imposed excessive workload and therefore proceeded to look at the procedural context in which this had occurred.

It was noted that on 7 August 2022, the Board of the Civil Aviation Authority Nepal (CAAN) had decided to open the newly constructed Pokhara International Airport to scheduled passenger flights from January 1, 2023. All domestic flight operations except STOL flights to mountain aerodromes were therefore instructed to prepare to move their operations from the old Pokhara Airport to the new one.

The Authority was found to have conducted a Safety Risk Management assessment of the airport which identified the proximity of terrain as the main unsafe event that could potentially lead to an accident and proposed that this risk should be mitigated by all operators developing SOPs for the new airport and that “a Validation Flight should be conducted by each operator”. It was recommended that the Authority’s Flight Safety and Standards Department should “put these control measures in place before the commencement of operations at the new airport”.

Operators were required to perform these demonstration flights under observation by a CAAN Flight Operations Inspector based on their awareness that the initial opening of the airport was going to be for VFR flights only. Some airlines noted that approach and landing on runway 12 was “difficult and not advisable” and one identified the lack of published data for Pokhara International in the AIP as “a Hazard of High risk category” which could lead to “crew confusion and disorientation” and ultimately a CFIT outcome. Yeti Airlines used runway 30 for landing and runway 12 for departure during their demonstration flight and were permitted by the CAAN Inspector observing to operate it as a “day IFR flight” instead of under “full VFR conditions” including left and right hand circuits to both runways. They were then granted CAAN approval for both runways, despite having failed to even document a visual circuit procedure for landings on runway 12.

It was found from FDR data that neither the flight path of the accident approach nor that of another visual approach made to the same runway at the new Pokhara airport three days earlier by a different crew had met the normal requirement for a visual approach to be stabilised at and below 500 feet agl because of the close-in base leg flown. The operational safety implications of such ‘tight’ visual circuits to runway 12 were found to have been completely overlooked by both the CAAN and the aircraft operator.

It was also noted that only “general aerodrome information” for the new airport had been was published in the AIP at the time the accident occurred and even that only a couple of days prior to commencement of operations. Detailed flight procedures only came into effect from 23 February 2023, over a month after the accident.

In respect of the flights that morning being used to familiarise a Line Captain with approaches to the new airport, it was noted that although this task had been intended by the operator to include approaches to runway 12, it had not been properly planned by their operations department or by any detailed pre flight briefing given by the Training Captain.

The Probable Cause of the accident was determined as “the inadvertent movement of both condition levers to the feathered position in flight, which resulted in feathering of both propellers and subsequent loss of thrust, leading to an aerodynamic stall and collision with terrain”.

Six Contributory Factors were also identified as:

1. High workload due to operating into a new airport with surrounding terrain and the crew missing the associated flight deck and engine indications that both propellers had been feathered.
2. Human factor issues such as high workload and stress that appears to have resulted in the misidentification and selection of the propellers to the feathered position.
3. The proximity of terrain requiring a tight circuit to land on runway 12. This tight circuit was not the usual visual circuit pattern and contributed to the high workload. This tight pattern also meant that the approach did not meet the stabilised visual approach criteria.
4. The use of a visual approach circuit to runway 12 without any evaluation, validation or resolution of the corresponding threats which had been highlighted by both the CAAN Safety Risk Management team and in a proposed flight procedures design report prepared by a consultant, or the prior development (by the operator) and approval (by the regulator) of a (procedure) chart.  
5. The lack of appropriate flight crew technical and skill based training (including simulator exercises and proper classroom briefings) to support safe operation of visual approaches to runway 12.
6. Non-compliance with aircraft operating SOPs, ineffective CRM and a lack of sterile flight deck discipline.

A total of 9 Safety Recommendations were made as a result of the findings of the Investigation as follows:

• that Yeti Airlines should take into consideration the (standard approach) stabilisation criteria when designing or proposing approaches for approval by Civil Aviation Authority Nepal.
• that Yeti Airlines should ensure that sufficient skill-based training for their (flight) crew is provided before beginning operations at (a) new airport.
• that Yeti Airlines Safety Department should act effectively to monitor and take prompt corrective actions against the violations related to CRM discipline and failure to follow applicable Checklists during flight.
• that Yeti Airlines should conduct an in-depth Change Management study with consideration of human factors and other probable safety issues before establishing criteria for aircraft clearance and approval of crew in a new operating environment.
• that Yeti Airlines should ensure (that) sufficient technical ground training is provided for (flight) crew.
• that the Civil Aviation Authority Nepal (CAAN) Aerodrome Safety Standards Department should, while conducting the aerodrome certification process, consider all the safety critical parameters including the ATC procedures, visual circuits and stabilisation criteria for particular aircraft (types) which conform with the aircraft design criteria and (the) local operational environment to ensure the safety of flight operations.
• that the Civil Aviation Authority Nepal (CAAN) should ensure that (any approved) visual flight path allows the criteria for a stabilised visual approach to be met by the operator and facilitated by the ANSP prior to the start of commercial operation in any new airport or before introducing any new fleet at an existing airport.
• that the Civil Aviation Authority Nepal (CAAN) should evaluate and manage the impact of change in the aviation system so that no operation takes place in a changed system or operational context until all the safety risks (have been) evaluated and controlled prior to implementing the significant changes.
• that the Government of Nepal Ministry of Culture, Tourism and Civil Aviation should establish a permanent investigation entity with sufficient financial, human and technical resources to meet the international obligation as per Annex 13 of the Chicago Convention.

The Final Report was submitted by the Investigating Commission and released on 28 December 2023.

Related Articles

• Loss of Control
• Feathering
• Stall
• Stabilised Approach
• Visual Approach
• Crew Resource Management (CRM)
• Sterile Flight Deck
• Standard Operating Procedures (SOPs)
• Pre Flight Preparations
• Pilot Workload
• Stress
• Risk Management
• Risk Assessment