Levers of Power: The crash of Yeti Airlines flight 691

Admiral Cloudberg
33 min readJan 8, 2024
The wreckage of Yeti Airlines flight 691 teeters on the brink of a gorge in Pokhara, Nepal. (PTI)

On the 15th of January 2023, an ATR-72 operating into a brand new airport in Pokhara, Nepal suddenly stalled and plunged into a gorge on final approach, killing all 72 people on board. It did not take long for investigators to identify the proximate cause: in a horrible and untimely error, an instructor captain in the right seat accidentally feathered the propellers instead of extending the flaps. With no thrust from the engines, the airplane slowed until it stalled, without the crew noticing that something was gravely amiss — by all accounts a tragic failure of cross-checking and crew resource management. But what really happened behind the scenes? What was the story behind the instructor’s inexplicable lever confusion? One year later, the release of the final report on the accident has answered some of these questions, but not all, hinting at a deeper story involving an airport that was rushed into operations too early, a dangerous approach procedure, corruption on a national scale, and a regulatory body that threw caution to the wind at the expense of lives.

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A lakefront neighborhood in Pokhara lies in the shadow of the striking summit of Machapuchare (6,993 m). Three of the world’s 10 highest mountains can be seen from Pokhara, although this is not one of them. (Intrepid Travel)

Flying in the Himalayan nation of Nepal has always presented certain unique challenges. Home to eight of the world’s ten highest mountains, Nepal possesses terrain of unparalleled ruggedness, which often combines with fickle weather to create downright dangerous flying conditions. These risks could be more easily overcome were it not for the fact that Nepal is also among the world’s poorest countries, with limited capacity to implement the expensive safety measures needed to cope with its hair-raising geography. Nevertheless, the local economy is dependent on tourists who almost all arrive by air via the single-runway Tribhuvan International Airport in the capital, Kathmandu.

Seeking to alleviate this bottleneck, successive Nepalese governments have pondered the notion of opening additional international airports for more than half a century. Plans for a major airport in Pokhara, Nepal’s second largest city, were drafted by a German consulting firm as early as 1971, and the government acquired land for the purpose in 1975, but the project never got off the ground, in part due to the question of where the money would come from in a country that was then struggling to provide basic services such as electricity and running water. In the meantime, domestic flights continued to pour into what is now known as Old Pokhara Airport, a tiny, ill-equipped aerodrome unbefitting Pokhara’s significance as a city of 600,000 and a tourist gateway to western Nepal.

Construction workers lay pavement at Pokhara International Airport in 2019. (Kathmandu Post)

This situation finally began to change in 2009, when the project’s sails were suddenly filled by the lucrative wind of Chinese development money. China’s proposal came with plenty of strings attached, including a requirement to select a bid from a Chinese company, and Nepal would have to pay back the full sum plus interest within 20 years, which would later be viewed as a debt trap. At the time, however, the offer to make Pokhara International Airport a reality seemed too good to pass up, and the deal was signed in 2011.

Construction on the new airport began in 2016, with the works carried out by the Chinese construction firm CAMC Engineering. According to New York Times reports, Nepalese officials exercised little to no oversight of the process, and subsequent allegations of poor build quality have since left the government wondering whether they were ripped off, especially considering that CAMC’s bid for the project cost almost twice Nepal’s estimate. Nevertheless, the airport was on track to open on schedule in 2021 — until the Covid-19 pandemic struck, forcing authorities to delay the opening until July 2022.

But as this new deadline approached, it became clear that the opening would need to be pushed back again, this time not due to a global pandemic, but because of a lack of foresight. A forested hill near the east end of the runway had been found to impinge upon the obstacle-free margin surrounding the approach path, and it would not be safe to fly into the airport unless the height of the hill could be reduced by 40 meters. This would require cutting down around 600 trees, necessitating an environmental assessment, which had not been done in advance. July 2022 therefore came and went with the airport physically complete, but unable to accept traffic due to the continued presence of the hill. Opening was then further delayed to January 1st, 2023.

Before and after the intruding hill was decapitated. (Own work, map by Google)

It ultimately took until the second half of 2022 for the environment ministry to grant permission to lower the hill. And only once the work was well underway could the project proceed to the next phase, in which a specially equipped airplane decked out in sensors would have to fly repeated approaches to calibrate ground-based equipment and determine precise airport data. This was carried out on November 21st by AEROTHAI, a state-owned company from Thailand that specialized in such calibration flights. The already-installed navigational aids, including the instrument landing system, or ILS, could not be used until AEROTHAI issued its final report on the calibration flight. Officials estimated that the ILS would thus not be ready until February 26, 2023. As you may have already noticed, this date was after the planned opening date for the airport on January 1st. But instead of delaying the opening until the ILS was approved, Nepalese authorities instead decided that Pokhara International Airport would operate for its first two months as a visual flight rules (or VFR)-only airfield.

Air traffic generally falls under one of two basic rulesets — instrument flight rules (IFR), or visual flight rules (VFR). On the most basic level, VFR traffic navigates by sight, while IFR traffic navigates using defined relationships to ground-based or GPS-based navigational aids. All airline traffic normally operates under IFR, unless approaching an airport that lacks instrument approach procedures. In the case of Pokhara International Airport, since there would be no instrument approach procedures until the final report was received from AEROTHAI, only visual approaches would be allowed, conducted under visual flight rules, and only in daytime due to the proximity of unlighted terrain.

The benefits of using runway 30 over runway 12. (Own work, map by Google)

At the same time, officials working for the Civil Aviation Authority of Nepal (CAAN) conducted a safety risk analysis prior to commencing operations at the new airport, during which they noted that this same proximity to terrain created an unacceptable hazard in light of the fact that the proposed procedures for approaches, go-arounds, and departures had not yet been validated in the actual aircraft that would be using the airport. Among the recommendations from their report was that validation flights be carried out by each airline that would operate into Pokhara International Airport in order to verify that the proposed procedures met their theoretical specifications and would not create an undue burden on flight crews.

At that time, the CAAN had not developed any visual approach procedures for the new airport. Although visual approaches are normally quite straightforward, high mountains around Pokhara meant that more detailed procedures were needed, especially for runway 12. Pokhara International Airport has one runway, designated runway 30 or runway 12 depending on the direction of approach, but the airport was envisioned with runway 30 as the primary runway due to the open valley east of the airport, as shown above. By contrast, high terrain on both sides of the approach path to runway 12 meant that a straight-in approach from this direction would be impractical. Instead, a circling approach would have to be used, in which an inbound aircraft initially descends in line with runway 30, then breaks off the approach, circles the airport, and lands from the opposite direction.

Despite this, the CAAN did not publish any specific visual approach procedures for either runway, instead allowing airlines to develop their own interim procedures while waiting for the airport to become IFR compliant. Various Nepalese airlines subsequently carried out the recommended “validation flights” in late 2022, during which some operators noted that a visual circuit to runway 12 would be “difficult and not advisable,” prompting these companies to restrict landings to runway 30 only. However, a few airlines developed procedures for landing on runway 12 anyway, including Yeti Airlines, Nepal’s second-largest domestic carrier.

A Yeti Airlines ATR-72 superimposed over the city of Pokhara was featured in this anniversary poster uploaded by the airline on Facebook just two weeks before the accident. (Yeti Airlines)

Yeti Airlines is a part of the Yeti World tourist conglomerate, which bills itself as “Nepal’s largest travel and tourism group.” Founded in 1998 by Ang Tshering Sherpa, the airline got its start flying tourists to Mount Everest, but eventually spun off these flights to a subsidiary, Tara Air, in order to focus on trunk routes between major Nepalese cities. For this purpose, in 2017 the company acquired a fleet of six 72-passenger ATR-72–500 twin turboprop airliners, built in France by French-Italian manufacturer Avions de Transport Régional. This was a massive step up for a company that never previously operated anything bigger than the 34-passenger Saab 340.

Although the company sells itself as a champion of carbon neutrality and the advancement of the Sherpa people, its leadership has been embroiled in a number of corruption scandals involving bribes and kickbacks from government officials as high up the chain as the Prime Minister himself. The airline is also infamous for having one of the world’s worst safety records. Despite the fact that Yeti Airlines and its subsidiary Tara Air can barely scrape together a dozen aircraft between them, the two companies suffered no less than six fatal accidents between 2004 and 2022, in addition to several non-fatal accidents that resulted in airframe write-offs. By 2023 there was no sign that this trend had reversed, and in fact as recently as May 2022, a Tara Air DHC-6 Twin Otter had crashed in Jomsom with the loss of all 22 souls aboard. In general, these accidents have been the result of poor pilot training and oversight in combination with an unforgiving operating environment.

A basic overview of the visual circuit that Yeti Airlines developed for runway 12. (Own work, map by Google)

In late 2022 Yeti Airlines conducted a validation flight into Pokhara International Airport with a CAAN inspector on board, during which a landing was made on runway 30 with a subsequent departure from runway 12. Although Yeti Airlines had also developed a procedure for the visual circuit to land on runway 12, this procedure was not validated nor was it demonstrated to the CAAN prior to the commencement of airport operations. Nevertheless, the CAAN granted Yeti Airlines the right to operate under VFR into either runway without having examined the airline’s in-house visual circuit procedure in any way.

The procedure developed by Yeti Airlines called for a downwind leg parallel to the runway at a distance of about 1.5 nautical miles laterally, passing over two prominent forested hills, followed by a left turn onto a base leg while in a descent, in line with the north-south-oriented runway at the old Pokhara Airport. While over the old airport, another left turn onto final was to be conducted at a distance of less than 1.5 nautical miles from the threshold of runway 12, followed by an immediate landing. Although it would play no role in the accident, it’s also worth noting that this procedure was not officially coordinated with the air traffic services at the old Pokhara Airport, even though it involved a low-altitude overflight of the field.

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The pilots of flight 691: at left, Captain Kamal KC; at right, Captain Anju Khatiwada. (Khabarhub, BBC)

Despite the questionable nature of the whole operation, Pokhara International Airport opened as planned on January 1st, 2023, with Nepal’s Prime Minister arriving to much fanfare aboard the first official flight. Regular flights by Buddha Air and Yeti Airlines followed, almost all of which landed straight in from the east on runway 30.

Two weeks later, on January 15th, a two-person cockpit crew arrived at Tribhuvan International Airport for Yeti Airlines flight 691, a regularly scheduled 30-minute hop to the new Pokhara Airport. Their ATR-72 was almost fully booked, with four crew and 68 passengers aboard who collectively hailed from eight different countries. In command was 58-year-old instructor captain Kamal KC,* a highly experienced pilot with 21,900 flying hours, including 3,300 on the ATR-72. He was providing instruction that day to another captain, 44-year-old Anju Khatiwada, who had around 6,400 flying hours. Khatiwada had never previously flown to the new Pokhara airport and was receiving airport familiarization training from KC, although the airport was so new that KC had only been there twice himself.

Captain Khatiwada originally took up a career in aviation after her husband Dipak Pokhrel lost his life in the 2006 crash of a Yeti Airlines Twin Otter in Jumla, in which he was the copilot. In honor of his memory, she followed in his footsteps, eventually reaching the rank of captain on a much bigger aircraft than her husband ever flew — an achievement he surely would have been proud of. Unfortunately, however, with a carrier as sketchy as Yeti Airlines, tragedy can and does strike twice.

*Note: Despite its appearance, “KC” is a common Nepalese last name. It was originally a caste signifier that stood for “Khatri Chhetri,” but has since been abbreviated to KC and turned into a surname. It is therefore correct to refer to someone as “KC,” just like any other surname.

9N-ANC, the aircraft involved in the accident, pictured at the old Pokhara Airport. (Bhupendra Shrestha)

KC and Khatiwada had already flown once to Pokhara and back when they departed Kathmandu again, lifting off at 10:33 a.m. local time. Although KC was the senior crewmember, he sat in the right seat and acted as pilot monitoring, providing support and instruction to Khatiwada, who sat in the left seat.

The brief flight was uneventful until nearing the Pokhara area, when air traffic control indicated that runway 30 was in use. However, Captain KC planned to familiarize Captain Khatiwada with runway 12 instead, a maneuver that was apparently not specifically planned by the airline’s operations department. The reasons for this discrepancy have not been clearly elucidated. Regardless, what is known is that KC requested, and received, permission to circle to runway 12 as they were descending through 6,500 feet, at which point he briefed Khatiwada on the route of the visual circuit and the maneuvering techniques that would be required, in particular for the final turn through the base leg, which would need to be especially steep.

A pair of stills from a Facebook Live video shot from inside the accident airplane. (Sonu Jaiswal)

As near-continuous instructional conversation filled the cockpit, air traffic control declared the aircraft in sight, and the pilots began configuring the airplane for landing. Proceeding onto the downwind leg parallel to the runway, they extended the flaps to 15 degrees and lowered the landing gear.

Back in the cabin, Indian passenger Sonu Jaiswal was streaming the view live on Facebook, capturing the smiling faces of his fellow passengers, unaware that he was about to earn a tragic distinction as the first person to live stream a fatal plane crash from inside the plane.*

Moments after lowering the landing gear, Khatiwada disengaged the autopilot, taking manual control. Five seconds later, seeking to establish the full landing configuration, she called for the flaps to be set to 30 degrees. “Flap 30 and continue descent,” KC replied. Using his left hand, he instinctively reached for the center pedestal to move the flap lever — and inexplicably grabbed the wrong lever entirely.

*Note: I will not be including his footage in this article as it isn’t suitable for all audiences. If you want to watch it, it’s not hard to find.

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The layout of levers on the center pedestal of an ATR-72, courtesy of Magnar Nordal’s YouTube channel.

On modern aircraft, every aspect of the design is carefully considered, all the way down to the shapes and positions of each individual cockpit lever. In fact, to avoid confusion, many airplanes have the same set of levers distributed in the same order across the center pedestal between the pilots. Traditionally, jets have the thrust levers on the left. In the middle is the speed brake lever, which activates the speed brakes in order to slow the plane or increase descent rate, and on the right is the flap lever, which determines the position of the flaps, used to increase the lift generated by the wings at low speeds during takeoff and landing.

On turboprop aircraft, a fourth set of levers is added to this sequence between the speed brake and the flaps, in the center-right position, or in place of the speed brakes, in the center position. Known as the “condition levers,” these control the overarching operating mode of the propellers. On the ATR-72–500 involved in the accident, the condition levers had four possible positions, consisting of “100% override,” “Auto,” “Feather,” and “Shutoff.”

A visualization of propeller blade pitch and feathering. (Skybrary, Pilots of America)

In simple terms, turboprop engines work by using a turbine to spin a propeller, which generates thrust by forcing air backward over the wings. The propeller generates thrust in much the same way a wing generates lift. Each blade is an airfoil generating “lift” in an aft direction, with the amount of lift (or thrust) depending primarily on rotation speed and blade angle of attack. Rotation speed, measured in rotations per minute (RPM), is self-explanatory, and much like the airplane itself, a higher speed means more lift/thrust. The blade angle of attack, meanwhile, is the angle of the chord line of the blade relative to the airflow passing over it — again, much like a wing. A higher blade angle of attack also results in increased lift/thrust, up to a point. You can think of the angle of attack as the size of the “bite” that the propeller takes out of the air. If the propeller blades are all aligned with the plane of rotation, then they will take no bite at all, and that position is considered to represent a blade pitch of zero degrees. As pitch increases from zero degrees — or, imagine the blades slowly angling away from the plane of rotation — then the angle of attack increases, and thrust increases, up to some optimal value that varies depending on the conditions. And if the pitch increases even further, reaching 90 degrees, or perpendicular to the plane of rotation, then the blades will not direct any airflow aft, and thrust reduces back to zero again. This is known as the “feathered” position, which is normally used in flight in the event of an engine failure, because it greatly reduces the drag generated by an unpowered propeller “windmilling” in the airstream.

Finally, it must be noted that blade pitch and RPM have an inverse relationship, such that a given thrust level can be maintained at a lower RPM by increasing blade pitch, and vice versa, with one parameter compensating for the other. Furthermore, RPM can be directly controlled using blade pitch, because a higher blade pitch slows the rotation of the propeller, and a lower blade pitch accelerates it.

A page from the ATR systems guide describes the various condition lever positions. (ATR)

On the ATR-72, control of thrust is achieved using the power levers, which do not directly control either blade pitch or RPM while in flight. Instead, the power lever position determines the pilot’s desired torque output, which is a facsimile for thrust. Fuel flow to the engine is determined based on the power lever position and the position of the power management selector switch (e.g., “climb” or “cruise”). When the condition levers are set to “Auto,” computers then calculate the RPM required to produce the desired torque output, and a command is then sent to the propeller control units, which adjust the blade pitch to achieve the requested RPM. The full extent of the relationship between torque, RPM, blade pitch, power lever position, and condition lever position is a fair bit more complicated than this, but this description should be sufficient for the purposes of this story.

Alternatively, if the condition levers are set to “100% override,” then the propeller RPM will be fixed at its maximum allowable value. At the opposite end, the “shutoff” position is self-explanatory — that’s where the condition levers should be when the engines are off.

Finally, in between “Auto” and “Shutoff” is “Feather,” which is the position we’re most interested in. As you probably guessed, moving the condition lever to feather commands a blade pitch increase to about 90 degrees, or the feathered position. This is part of the normal shutdown process after landing, where feathering the propellers allows the engine to idle without generating thrust, and it’s also part of the engine failure procedure, for the reasons discussed earlier. Moving the condition levers from “Auto” to “Feather” is therefore something that happens on every flight, albeit only when the plane is on the ground.

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In this clip from Magnar Nordal’s YouTube channel, Magnar demonstrates what it looks like to move the flap lever from 15 to 30, versus moving the condition levers from auto to feather. You can watch his full video about the ATR-72’s cockpit lever design in relation to this crash here: https://www.youtube.com/watch?v=p5m8kvZqwC4

With all this in mind, imagine the scene in the cockpit of flight 691 as instructor Captain KC called out “flaps 30,” reached over, and, apparently without looking, grabbed both condition levers and moved them straight back to the “Feather” position.

Needless to say, this did not cause the flaps to extend. Instead, both propellers immediately moved to the feathered position, orienting their blades into the oncoming airflow to reduce drag. With the blades no longer forcing any air backwards, torque immediately dropped to zero in both engines, and RPM decreased dramatically within seconds. Incredibly, neither pilot noticed.

Instead, the pilots launched into the before landing checklist, which consisted of seven points including but not limited to checks of the landing gear and flaps. Since KC had not actually extended the flaps to 30 degrees as requested, the flap position indicator should have continued to show 15 degrees, but neither pilot commented on this discrepancy, nor did anyone extend the flaps. However, it seems that Khatiwada did notice a change in airplane performance — which was expected because the flaps increase drag — and she consciously added power to make up the difference. But this had no effect, because with the propellers feathered, the turbine is limited to idle power regardless of power lever position in order to prevent engine overtorque. The engines were never designed to operate above idle with the blades in the feathered position because the torque required to spin the propeller in that configuration is beyond what is safe to produce, and even if torque could be commanded, a feathered propeller would not convert that torque into thrust. Therefore, when she moved the power levers forward, Khatiwada might as well have been moving toy power levers instead.

A visualization of flight 691’s approach based on the actual FDR data. (Nepal Aircraft Accident Investigation Commission, annotations mine)

This situation immediately began to create secondary problems as the engines spooled down. Four seconds after the propellers were feathered, the propeller RPM decreased below the threshold required to run the ATR-72’s two AC electrical generators, triggering a master caution chime and a pair of warning lights on the dash. Khatiwada commented on these, but KC didn’t reply. All cockpit systems continued to work normally due to the continued availability of DC power.

At almost that same moment, Khatiwada began turning left onto the base leg, banking steeply to make the tight turn roughly into line with the runway at the old Pokhara Airport. An automated voice called out “five hundred,” announcing that they were at 500 feet above ground level, and someone reached up and cancelled the master caution light without comment. Instead the pilots remained focused on the execution of the turn, as Khatiwada asked how long to hold the bank, then whether to continue the descent, to which KC replied that they could level off for the moment. He also suggested adding a little power, and Khatiwada obliged, moving the useless power levers slightly forward again.

Immediately after that, KC seemingly noticed that the flaps were not set to 30 degrees, fully 20 seconds after he had supposedly extended them. In response to this discovery, he promptly moved the flap lever to the 30-degree position, but it doesn’t seem he took any interest in the question of what he had originally moved instead, nor did he bring the discrepancy to Khatiwada’s attention. Instead, Khatiwada continued navigating through the base leg, watching for the point where they would turn to final, even as their airspeed kept dropping lower and lower, bleeding away as she held the airplane aloft without engine power.

A visualization of why more lift is required to maintain altitude in a banked turn.

Thirteen seconds after KC applied flaps 30, and about 33 seconds after the propellers were feathered, Khatiwada finally recognized that something was wrong, at which point she twice called out that she was getting no power from the engines. Air traffic control cleared the airplane to land, but the crew did not reply. Khatiwada advanced the power levers to maximum torque in an attempt to extract some response, any response, from the engines, but there was none.

At that point, against all good sense, the pilots began their last, steep turn onto final approach, from a height of just 368 feet above the ground. The final report on the accident does not explain whether Khatiwada initiated this turn herself, or did so at KC’s urging, nor does it explain why this decision was made just moments after Khatiwada discovered such a serious problem. We might speculate that with no engine power, she saw completing the turn onto final approach for an immediate landing as her least dangerous option. After all, they were in a heavily urbanized valley surrounded by mountains, with no way out. But even so, making a steep turn at low speed is risky. At high bank angles more lift is required to remain airborne because the lift vector does not directly counteract the downward pull of the aircraft’s weight, and because lift is partially a factor of airspeed, insufficient speed during a turn can easily result in a loss of altitude. An attempt by the pilot to prevent this loss of altitude by pulling up can end in disaster. Pulling up without enough airspeed to climb simply increases the angle of attack instead, and if the angle of attack exceeds the critical point, then the wings will cease to generate lift and the aircraft will stall and plunge from the sky. (By the way, you can find a much less oversimplified explanation of stalls in banked turns in my article on Braniff flight 352.)

Unfortunately, in the heat of the moment, the pilots of flight 691 never managed this level of foresight. Despite their plunging airspeed and lack of engine power, they initiated the turn anyway — and they almost made it. But by then the events were outrunning them. Part way through the turn and growing increasingly alarmed by the lack of thrust, Khatiwada abruptly asked KC to take control, repeating that there was no power from the engines. KC did take over, but he had no time to assess the situation. Within four seconds, the stick shaker stall warning activated, vibrating the pilots’ control columns to warn that they were about to fall out of the sky. The airplane began to lose altitude, dropping from 311 feet to only 200 feet in the space of ten seconds; KC attempted to level the wings, but he also raised the nose, slowing the plane even more. On the ground, a Pokhara resident filmed the airplane from his balcony as it wallowed along, on the verge of disaster, its nose high in the air — until, nine seconds after the control handover, the left wing stalled.

A bystander filmed the final moments of flight 691 from his balcony. He didn’t capture the moment of impact as he ducked for cover immediately after the freeze frame, believing that the plane was headed in his direction. Fortunately neither he nor anyone else on the ground was hurt. (Avherald)

From that point, the end came with shocking swiftness. As soon as the left wing ceased to generate lift, the plane banked dramatically to the left, passing through 90 degrees and rolling inverted in a matter of seconds. From such a low altitude, there was no hope of recovery. And in a horrifying epitaph for a flight gone awry, Sonu Jaiswal’s livestream abruptly captured a chorus of panicked shouts amid blurry images of seats and walls, followed by the sound of a boom, a few frames of darkness, and a sea of scorching flame.

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The investigation commission’s breakdown of the wreckage distribution. (Nepal Aircraft Accident Investigation Commission)

Yeti Airlines flight 691 impacted the ground in a left wing low attitude along the brush-covered bank of the Seti Gandaki River, which winds its way through central Pokhara at the bottom of a precipitous gorge. By some stroke of luck, the plane came down within the narrow green strip alongside the river, missing residential neighborhoods by mere meters. As the left wingtip struck the ground, the entire aircraft pivoted about the impact point and plunged nose-first into the slot-like canyon, which was only a few meters wide in that area. The right wing and aft fuselage slammed into the opposite bank and were sheared off, remaining atop the cliff, while the majority of the airplane careened off the narrow rock walls as it plunged to the bottom of the murky chasm, where water trickled over giant boulders in the half-darkness.

Although locals rushed to the scene of the crash, where a large explosion had set fire to surrounding brush, those passengers they found atop the cliff were already dead, having been killed instantly on impact. Only a few bodies were intact — most had fallen to the bottom of the gorge and then burned, resulting in a delay of several days before authorities could confirm that everyone on the flight manifest had indeed been aboard the plane, and that none had survived. With 72 people dead, it was the deadliest crash involving a Nepalese airline, and the worst in the 34-year operating history of the ATR-72.

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Rescuers and bystanders pack the crash site as firefighters attempt to extinguish the flames. (PTI)

The aftermath of the crash was consumed by wide-ranging speculation, but within a rather short time, investigators determined that instructor Captain Kamal KC had inadvertently moved both condition levers to “feather” while attempting to set the flaps to 30 degrees. This revelation caused consternation within the aviation industry, where many were struck by the simplicity and silliness of the error. How was it possible for an instructor captain with over 20,000 flying hours not only to make such a bizarre mistake, but to fail even to notice until it was too late?

One year later, the Nepalese investigation commission has released its final report on the accident, answering some of our most burning questions, but not all. What follows is a summary of the experts’ best attempts to explain the seemingly inexplicable sequence of events, from the smoldering wreckage all the way back to the construction of Pokhara International Airport, long before the accident flight ever departed.

Rescuers work at the crash site as bystanders observe from nearby apartment balconies. (Kathmandu Post)

Any analysis of the events must start with the lever error that set the disaster in motion. On the surface, there are plenty of reasons why this shouldn’t have happened — and in particular, it’s worth discussing ergonomic aspects of the cockpit design that guard against this kind of mistake. Specifically, all the major levers in the cockpit have different knob designs in order to reduce the probability that one could be mistaken for another — for example, on the ATR-72, the power lever handles are cylindrical, the landing gear handle is shaped like a wheel, the flap handle is shaped like an airfoil, and the condition lever handles are square with ribbed edges. These varied designs give each lever a distinct feel that discourages confusion between them by conferring a sense of wrongness if the wrong lever is grabbed.

Regarding the condition levers and the flap lever in particular, another key difference must be noted. To move the flap lever, one must pull the entire lever upward, raising it out of a detent and permitting movement along the lever track to the desired position. By contrast, there are two condition levers, as opposed to only one flap lever, and each requires the pilot to pull a trigger beneath its respective handle in order to release a stop gate. Only then can the levers be moved along their individual tracks.

All of these are conscious, proper design elements that collectively form a reasonable safeguard against lever confusion. There’s certainly no reason to believe that the design of either lever was flawed. But at the same time, there are a number of factors that worked in the opposite direction, reducing Captain KC’s awareness of which lever he was grabbing.

Rescuers search for bodies in the bottom of the Seti Gandaki gorge. (AFP)

The first thing to note is that KC was a Captain who normally flew from the left seat. While he did occupy the right seat periodically in order to evaluate other pilots, the vast majority of his ATR-72 experience was in the left seat, and he would have been accustomed to reaching across the center pedestal to move the flap lever with his right hand. Conversely, actuating the flap lever with his left hand from the right seat would have been less thoroughly ingrained in his muscle memory, increasing the likelihood that he could misjudge the location of the lever if he reached for it without looking. Furthermore, having grasped the wrong lever, the different feel of the handles would have had less effect when transmitted through his left hand, since it was less accustomed to grasping any lever at all.

Then, once his hand was in place, he may have instinctively carried out the action required to move it, even if that action was wrong. All the levers are designed to be intuitive to use, including the condition levers, whose triggers are positioned so as to be depressed with one swift, natural motion. Furthermore, the distance between the 15 and 30-degree flap positions is almost equal to the distance between the “Auto” and “Feather” positions on the condition lever, so the feeling of the stop would have arrived at the expected time, eliminating another potential sensation of “wrongness.”

None of these factors was so powerful as to logically overcome the inherent difficulty of selecting the wrong lever. But they are sufficient to raise the probability of such a mistake to a believable level, even though its occurrence was unlikely. The truth is that such boneheaded errors can happen to anyone without any immediately obvious reason. Have you ever shifted your car into the wrong gear, or taken your dishes to the bathroom instead of the kitchen? Then you should be able to relate on some level.

First responders strung hoses into the gorge in an attempt to extinguish the flames. (PTI)

This was, in fact, not the first time that the pilot of an ATR aircraft had accidentally feathered one or both propellers instead of lowering the flaps — ATR pilot and Youtuber Magnar Nordal has identified at least two other previously unreported cases. (I want to shout out his channel, which is full of very technical descriptions of ATR systems that proved useful in writing this article). Neither of those prior cases was particularly noteworthy, however, because the pilots immediately recognized the error, reversed their inputs, and continued the flight without incident. What turned a silly but forgivable mistake into a tragedy was the pilots’ failure to notice the problem, and it’s here that we really have to get critical of their decision-making and performance. I don’t like to make those kind of judgments — it’s not really my place, and forgiveness is free — but it’s undeniable that both pilots made serious errors of airmanship. If you want to hear trained NTSB investigators cast much harsher judgment than me, you can watch Greg Feith and John Goglia discuss this accident on “Flight Safety Detectives.”

The first thing that must be noted is that both pilots are required to cross-check the flap setting during the before landing checklist, which was not done. Secondly, according to the manufacturer’s procedures, after the flying pilot calls for flaps, the monitoring pilot is supposed to watch the flap position indicator and call out when the flap position reaches the desired value. And third, it’s simply good practice for pilots to consciously identify what they’re grabbing before making any kind of control selection. Unfortunately, none of these techniques was used. If they had been, then the wrong lever selection probably wouldn’t have occurred, and even if it did, Captain KC should have immediately noticed that the flap position indicator didn’t budge from the 15-degree position, at which point he could have discovered his mistake. As Greg Feith suggests, it never hurts to “trust but verify.” And as these pilots found out, it can hurt quite a lot if one doesn’t.

Large amounts of debris were left suspended in hard-to-reach places. (AFP)

After the propellers were feathered, it then took 33 seconds for either pilot to notice that the engines were not generating any thrust, despite the fact that a master caution chime and caution light were triggered after only four seconds. Captain Khatiwada did notice these indications, but she didn’t immediately understand their meaning, and in an abject failure of crew resource management, Captain KC failed to even acknowledge her questions. It goes without saying that when one pilot observes an abnormal indication, the other pilot should evaluate the indication and contribute an opinion!

It has also been noted that propellers sound different when feathered, and that this difference might have been noticeable even to experienced passengers, to say nothing of the pilots. However, the final report notes that Captain Khatiwada was wearing a particularly effective noise cancelling headset that might have masked the change. But even so, Captain KC wasn’t wearing such a headset, and he seemingly didn’t notice either.

When Khatiwada finally recognized that her power lever inputs had no effect, another opportunity presented itself to nip the problem in the bud. Magnar Nordal argues, and I agree, that the first instinct of any ATR-72 pilot upon observing zero torque on the engine gauges should be to check the position of the condition lever. If the engines are running but producing no torque, the most obvious explanation is that the propellers are feathered — in fact, it’s essentially the only explanation. And yet, Khatiwada did not notice that the condition levers were set to “feather,” and she did not restore them to “auto.” Neither did Captain KC. Instead, he finally applied flaps 30, but made no apparent attempt to determine why flaps 30 had not already been set, and he did not specifically reply to Khatiwada’s entreaties about the lack of engine power either.

People examine debris hauled back from the edge of the gorge. (AFP)

Taken together, all of these errors had by now amounted to a complete breakdown of cockpit discipline and communication. And then, just to top everything off, the pilots attempted the steep left turn onto final approach despite being aware that they had no engine power. Neither pilot specifically commented on airspeed, but Khatiwada at least was probably aware that they were too slow, and as pilot monitoring, KC should have noticed, too. Multiple expert commentators have suggested that the pilots ought to have considered a forced landing at this point, but satellite imagery of the area doesn’t suggest that there were any good options. Still, maneuvering to the runway from such a tight position was not advisable under the circumstances either. Attempting the turn onto final at such a low speed would have required extraordinary concentration, if it was even possible. But instead of summoning her best flying skills to save the airplane, Khatiwada instead initiated a last-minute control handover to Captain KC, who clearly had no idea what was going on. Twelve seconds later, everyone on board was dead.

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Rescuers and bystanders pack the clifftops as recovery crews search for victims. (Reuters)

In the opinion of the investigation commission, a number of factors could have contributed to this tragic performance breakdown. One aspect hovering in the background throughout the flight was the pilots’ excessive workload during the visual circuit to runway 12. The problem went all the way back to the very design of the procedure, with its extremely compact circuit featuring two sharp, low-altitude turns only 1.5 nautical miles from the runway threshold, while descending steeply into a mountainous valley. This sequence of maneuvers would have distracted the crew during the relatively limited timeframe available in which to carry out critical checklists and configure the airplane for landing. In fact, the cockpit voice recording showed that the pilots spent most of their time during the approach discussing when and how to make the turns into and out of the base leg, diverting their attention first from Captain KC’s input error, and subsequently from the aircraft’s deteriorating energy state. This workload was further exacerbated by the fact that Captain Khatiwada was undergoing airport familiarization and needed to receive instruction on the fly, occupying KC’s attention during periods when he would normally have been monitoring the instruments. Expert commentators have pointed out that this type of flight should be accomplished with an observer pilot in the cockpit jumpseat to assist the instructor, but Yeti Airlines did not provide one.

The ATR-72’s aft fuselage was flattened and unfolded by the impact. (AP)

The investigation commission also discovered that Yeti Airlines’ visual approach procedure for runway 12 actively undermined the so-called stabilized approach concept, a universal set of criteria designed to help pilots determine when an approach can be safely continued. As laid out by the International Civil Aviation Organization, a visual approach is considered stabilized if, at an altitude of 500 feet, the aircraft is configured for landing, the power levers are in the correct position, the runway is in sight, and no major course adjustments will be needed in order to land. Ideally, crews should abandon the approach if these criteria can’t be met. But Yeti Airlines’ visual approach to runway 12 required major maneuvers below 500 feet, and the timeline also made it difficult to configure the aircraft for landing before reaching this altitude. Data from earlier flights retained in the accident airplane’s cockpit voice recorder revealed that another flight crew who previously approached runway 12 followed the same timeline as flight 691, and neither crew was able to establish the airplane in the landing configuration by 500 feet. Unstable approaches such as these are inherently more dangerous because they force the pilots to make more inputs and control selections in close proximity to the ground.

This suboptimal approach procedure came about due to Yeti Airlines’ attempt to develop a visual circuit that would avoid busting terrain clearance minimums as incoming aircraft skirted the edges of the valley. The fact that the airline’s solution precluded the establishment of a stabilized approach escaped notice in large part because the procedure was not reviewed by the Civil Aviation Authority of Nepal prior to its implementation. As mentioned in the beginning of this article, the CAAN approved operations into both runways at Pokhara International Airport without examining Yeti Airlines’ visual approach procedure for runway 12 or observing any real-world demonstration of its feasibility. Furthermore, the CAAN did not impose any restrictions on the use of runway 12 despite the fact that multiple operators had found that it was “not advisable” to land on that runway. The reasons for these apparently negligent decisions were not discussed in the final report.

Rescuers search the charred remains of the aft fuselage. (AFP)

It bears mentioning that the actions of the CAAN were not the only aspect of this crash that the final report glossed over. The report didn’t include a cockpit voice recorder transcript, which makes it very difficult to understand what the pilots were thinking, and the quality of Yeti Airlines’ training is not even mentioned. The report does state that the pilots were not given “proper classroom briefings” or simulator training on the runway 12 approach procedure, but this fact only appears in a single sentence in the “Contributing Factors” section. Further discussion in the body of the report is nowhere to be found. This abject failure to examine the deeper causes of the accident within the organizational cultures of Yeti Airlines and the CAAN raises questions about the commission’s ability to prevent future accidents — which is something the commission itself appears to have been aware of, as its members recommended that Nepal establish a dedicated, independent air accident investigation body instead of appointing a commission from scratch after every plane crash. Such a body could wield greater authority to collect evidence and could prove more resistant to institutional pressures.

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Several passenger seats and a window lie stacked near the crash site. (Bijaya Neupane)

Beyond the matters covered in the accident report, it’s worth circling back around to the considerable context I provided in the beginning. One first of all must wonder whether, after two major delays, high-ranking officials endorsed the airport’s dubious VFR-only scheme in order to prevent a third, potentially embarrassing setback. The airport was widely considered a “national pride project” of great personal significance to the powerful chairman of the CAAN, Pradip Adhikari, who has been accused of autocratic behavior and blatant corruption. One Nepali newspaper quoted Adhikari declaring that he “was the state,” and anonymous officials told the Kathmandu Post that the airport was indeed opened prematurely because Adhikari wanted it to start operations on New Year’s Day. All declined to identify themselves for fear of retaliation. Others have found out the hard way that Adhikari does not hesitate to use his power for personal retribution, as Nepal Airlines discovered in October 2022 when the CAAN arbitrarily grounded a flight to Delhi after the company declined to utilize the newly revamped Bhairahawa International Airport, another of Adhikari’s very expensive pet projects. The move was slammed by the Nepalese press, which highlighted a conflict of interest within the structure of the CAAN, where the same officials who are responsible for commissioning aviation infrastructure projects also receive profits from their operation while simultaneously overseeing their compliance with regulations. Nepal’s parliament has drafted numerous bills that would split off these responsibilities to different departments, but every attempt has been quashed before being put to a vote.

The mountain peak of Machapuchare towers over Pokhara International Airport. (Nepal News)

Following the crash, these fundamental problems within Nepal’s aviation industry have only grown more apparent as Pokhara International Airport teeters on the brink of financial disaster. A New York Times investigation found that Nepal may have grossly overcompensated the Chinese construction companies that built it, and Nepal may be unable to repay China’s 20-year loan because the airport has failed to attract any international flights. As of October 2023, only one international flight had landed there, which was a Sichuan Airlines charter flight carrying Chinese goodwill ambassadors. Nepalese officials had hoped to attract airlines from India, and Nepalese airline Buddha Air also hoped to fly to Indian cities from the new airport, but India has so far not granted permission for any of these flights because of its geopolitical rivalry with China. According to various reports, India views Pokhara International Airport as part of China’s Belt and Road initiative, an international influence program that it seeks to undermine. Allowing flights to an airport constructed by India’s rival in order to gain influence over a mutual neighbor is unacceptable to the Indian government and will likely remain so for the foreseeable future. As a result, Nepal has been left with no way to pay back the airport’s $216 million construction cost. The Times of India has referred to the project as Nepal’s “economic albatross,” and in late 2023 Nepal’s anti-corruption watchdog initiated an investigation into the airport, which is still ongoing as of this writing.

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Nepalis hold a candlelight vigil for the victims in Kathmandu. (AP)

In an ideal world, the crash of Yeti Airlines flight 691 would be a wakeup call for Nepal’s aviation authority, which, in the words of one Kathmandu Post columnist, “only seems interested in building swanky new airports while paying little attention to aviation safety.” However, at this stage there’s no reason to believe that Nepal’s dismal safety record will improve overnight. In fact, the most recent ICAO audit of the country’s aviation industry found that the quality of organizational structures required to meet safety standards actually fell relative to the global average, which is especially concerning given that Nepal was already near the bottom in this particular metric. Unfortunately, improvement in this sector can only come from the top, and all evidence suggests that the current CAAN leadership is not committed to implementing reforms on the scale required — and in fact, the agency in its current form may be an impediment to progress. Nepal is a country blessed with great beauty and promise, but its mountains will continue to be marred by tragedy until those who control the levers of power recognize the need to change course.

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Don’t forget to listen to Controlled Pod Into Terrain, my new podcast (with slides!), where I discuss aerospace disasters with my cohosts Ariadne and J! Check out our channel here, and listen to our latest episode, in which we discuss how American Airlines flight 965 went tragically awry. Alternatively, download audio-only versions via RSS.com, or look us up on Spotify!

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Admiral Cloudberg

Kyra Dempsey, analyzer of plane crashes. @Admiral_Cloudberg on Reddit, @KyraCloudy on Twitter and Bluesky. Email inquires -> kyracloudy97@gmail.com.