On the 16th of August 2015, a regional flight carrying 54 passengers and crew into a remote village in Indonesia’s Papua province disappeared on approach to the airport. For more than a day, no trace of it could be found. Finally, search planes discovered the wreckage of the ATR 42 scattered across a steep forested mountainside more than 8,000 feet above sea level, in an area so inaccessible that few humans had ever set foot there before. It was in this inhospitable location that Indonesian investigators had to set about finding the cause of the crash. The details were troubling: the pilots had not been flying the normal approach path into Oksibil Airport. Aeronautical charts of the poorly explored area were incorrect. No ground proximity warning sounded before the plane slammed into the mountain. And this wasn’t the first time the crew had flown this dangerous, unpublished route. As the facts came together, investigators were able to paint a picture of a company which allowed its pilots to take risky shortcuts that lured them into a place where no airliner should ever have gone.
The island of New Guinea is a land of extremes. The second largest island in the world, New Guinea is home to 11 million people, as well as vast rainforest reserves, towering mountains, and some of the world’s only equatorial glaciers. Its population is incredibly diverse; the island is home to one out of every seven human languages, as the rugged terrain has historically prevented contact between villages located just a few kilometers apart. It is this same impenetrable terrain that has made air travel a critical economic lifeline for the island’s population, of which a significant percentage lives in isolated valleys with no access to the wider road network.
New Guinea is divided politically into two roughly equal halves, of which the eastern half is occupied by the independent state of Papua New Guinea. The western half is part of Indonesia, a country with which the local people have always had a complicated relationship. But in most material respects, the region differs little from its eastern neighbor, particularly in its reliance on air transportation. Regular commercial flights are most inland communities’ only connection to the outside world — and beyond a bubble of modern life surrounding each airport, people still live largely as they have for thousands of years, growing local vegetables and hunting wild animals with bows and arrows.
One of the many isolated communities in Papua Province is the district of Oksibil, in which some 4,000 people live scattered across several steep-sided valleys within New Guinea’s mountainous central spine. Life in Oksibil revolves around Oksibil Airport, which is served by several regional airlines offering multiple daily flights to the provincial capital of Jayapura, some 260 kilometers to the north. In 2015, one of these airlines was Trigana Air Services (commonly known as Trigana Air), a medium sized airline specializing in short flights in three different areas of Indonesia. For its operations in Papua Province, Trigana Air relied on the ATR 42, a twin turboprop high-wing airliner produced jointly by France and Italy. Capable of seating 48 passengers, the ATR 42 was just about the largest plane that could be flown into Oksibil.
The 16th of August 2015 began as a day like any other for First Officer Aryadin Falani and Captain Hasanuddin (who, like many Indonesians, used only one name). The 60-year-old Hasanuddin was probably the most experienced ATR 42 pilot at Trigana Air, with more than 25,000 flight hours under his belt. Falani was also no rookie, with over 3,000 hours of his own. Both were well versed in the intricacies of flying in Papua, and they certainly got a lot of practice — by 2:00 that afternoon, they had already completed four flights and were gearing up for their fifth. Their next two legs would take them from Sentani Airport in Jayapura to Oksibil and back, a round trip journey that spanned approximately two hours. This would be their second trip to Oksibil that day.
At Sentani Airport, 49 passengers boarded the plane for flight 267 to Oksibil, including two lap infants. Most of the passengers were local businesspeople and officials, as the general population of subsistence farmers usually did not fly, and the area had no tourism industry to speak of. Besides the pilots, there were three other crew members: two flight attendants, and a mechanic who would service the plane in Oksibil, which lacked a permanent maintenance station. In total, there were 54 people on board the ATR 42 when it departed Jayapura at 2:22 p.m. local time.
At 2:55, with First Officer Falani at the controls, Captain Hasanuddin made contact with the Aerodrome Flight Information Services (AFIS) officer in Oksibil. Unlike a true controller, the AFIS officer only had the authority to provide information to pilots and could not give them orders. Hasanuddin told the AFIS officer that they were descending from 11,500 feet; the AFIS officer acknowledged and asked that he confirm when flight 267 was positioned over Oksibil.
Oksibil Airport has almost no navigational aids. There is no instrument landing system and only one non-directional beacon, which identifies the location of the airport but otherwise provides no assistance to pilots. (This beacon was inoperative at the time.) All aircraft landing in Oksibil must therefore execute a visual approach — an approach flown manually while maintaining continuous visual contact with the airfield. However, airport authorities had not published an official visual approach procedure for pilots to follow. As a result, Trigana Air devised its own procedures, which called for approaching aircraft to overfly the airfield, make a loop to the southeast, fly past the airport to the west, then turn around and land on runway 11.
Expecting flight 267 to follow this route, the AFIS officer asked the crew to report when they were over the field. But Captain Hasanuddin had other plans. When landing at Oksibil, he liked to follow a much simpler route: instead of flying a complicated loop around the airport, he (and some other Trigana pilots) often flew directly to the west end of the airfield, made a single left turn, and lined up with runway 11. This approach, referred to as a “left base leg,” saved considerable time and effort. As a result, Hasanuddin told the AFIS officer that he intended to perform a left base leg approach to runway 11 and would not be reporting over Oksibil, as he had done on his first flight to Oksibil that morning. While a regular controller could deny clearance to perform such an approach, the AFIS officer lacked the authority to do this.
A left base leg approach to runway 11 will take an approaching aircraft near mountains northwest of the airport, many of which exceed 9,000 feet (2,750m) in height. The highest terrain in the area lies 44 kilometers northwest of Oksibil, where the towering Puncak Mandala rises to 15,620 feet (4,760 meters), the second tallest mountain in Australasia. The ability to see these mountains is critical to making a safe approach to Oksibil, especially via a left base leg, which (unlike the published procedure) does not make any attempt to avoid the high ground.
But as flight 267 descended from 11,500 feet, the weather over Oksibil was not clear. The AFIS officer had reported that there were broken clouds at 8,000 feet obscuring more than half of the sky, with surface visibility fluctuating between 4 and 5 kilometers. Flying under Visual Flight Rules (VFR) below 10,000 feet requires visibility of at least 5 kilometers, and VFR aircraft must keep at least 1,000 feet away from clouds at all times. Given the conditions, a visual approach to Oksibil at that time was probably not possible. Nevertheless, Hasanuddin and Falani pressed onward with their plan.
Although they didn’t specifically describe their reasoning, it is thought that several factors led the crew to believe that a visual approach could be accomplished. Most critical to their decision-making was the chart of the Oksibil area provided to the crew by Trigana Air, which included the minimum safe altitudes in five sectors surrounding the airport. In the area northwest of the airport where they planned to fly, the chart specified a minimum safe altitude of 8,000 feet. To Hasanuddin, this suggested an obvious course of action: since the cloud base was at 8,000 feet, and this was also the minimum safe altitude, he thought he could descend to 8,000 feet and remain clear of the terrain while also giving himself a chance to catch sight of the airport. While the plan made sense, he was missing a critical piece of information: the minimum altitudes on the chart were wrong. In reality, the highest peaks in the northwest sector were well over 9,000 feet tall.
As flight 267 descended toward 8,000 feet, it entered the cloud bank and visibility dropped to zero. But the pilots kept right on going, confident that they were at a safe altitude. They extended the flaps and lowered the landing gear, preparing for an imminent landing. They didn’t know that they were on a collision course with a mountain — and the alarm that should have warned them had been deactivated.
Trigana Air had fitted its ATR 42s with the Enhanced Ground Proximity Warning System, or EGPWS, the most advanced terrain avoidance equipment available in 2015. EGPWS continuously checks an airplane’s position against a computerized terrain database to predict when it is in danger of striking terrain. When a conflict is detected, it issues a “TERRAIN” warning one minute before impact, followed by “PULL UP” callouts at 30 seconds. The database is divided into grid squares of varying resolutions, each of which is assigned an altitude based on the highest point within its borders. The resolution of the database is typically higher — around 15 arcseconds — near airports where planes must fly near terrain, and lower — around 30 arcseconds — in less developed areas. (One arcsecond is equivalent to about 30 meters.) The resolution in most of New Guinea was low enough that EGPWS would frequently activate even if a plane was not actually in danger of striking terrain, such as when flying down a valley narrower than the width of the unit squares. Often, these “nuisance alarms” occurred when the crew already had the airport in sight. In order to silence the annoying alarms, some Trigana pilots had developed a dangerous habit of pulling the EGPWS circuit breaker to disable the system. When Hasanuddin and Falani first flew to Oksibil that morning, their improvised left base leg approach took them into an area where the EGPWS sounded an alert, even though they could see the runway and knew they could land safely. Hasanuddin therefore pulled the circuit breaker to silence the warning — but after departing Oksibil, he forgot to reset it. Now, as flight 267 descended straight toward a mountain, the EGPWS was unable to sound the alarm.
At 2:58 and 14 seconds, Trigana Air flight 267 slammed into a fog-bound ridge of Mount Tanggo at an elevation of 8,300 feet. The clouds were so thick and the impact so sudden that neither the crew nor the passengers ever knew what hit them. The ATR-42 struck the mountainside and disintegrated utterly, instantly killing all 54 people on board and scattering burning debris upward through the dense jungle.
Two minutes later, the AFIS officer became concerned when the crew failed to contact him as expected. After trying repeatedly to raise the flight without success, he sounded the alarm, and a major search and rescue operation kicked into gear. At first, no one knew whether the plane had crashed or merely landed at an alternate airport, but as the hours passed with no word about its fate, hopes began to fade. An initial air search had to be called off later that night due to bad weather, and relatives of the victims were forced to endure an agonizing wait for news. Eventually, villagers in a remote farming community reported that they had seen the plane strike Tanggo mountain. Another Trigana flight was sent to scout the area, and the crew reported smoke rising from tangled wreckage visible through a gash in the jungle canopy. Finally, more than 24 hours after the crash, flight 267’s last resting place had been found.
Reaching the wreckage proved to be extremely difficult. Perched on a steep slope far from the nearest road, the crash site probably had never been touched by human feet. After hacking their way through the jungle, the first exhausted rescuers arrived at the site on the 18th of August, where they immediately concluded that there were no survivors.
Once search and rescue personnel had made the site ready, investigators with Indonesia’s National Transportation Safety Committee (KNKT) began to arrive on foot and by helicopter. Unable to stay at the site for long, they could do little more than photograph the wreckage and extract the black boxes. When they left, they brought very little wreckage with them, leaving most of the debris to be reclaimed by the jungle.
Unfortunately, the effort expended to recover the flight data recorder proved to be in vain, as no useful information could be extracted from the device. It had been working incorrectly since 2013, and Trigana’s maintenance technicians kept reinstalling it in the airplane without fixing the problem, only for it to be flagged again days or weeks later. This seriously negligent maintenance led investigators to wonder what other poor practices might be occurring at Trigana Air.
The cockpit voice recorder revealed more about what happened in the cockpit before the crash. Investigators were astonished to discover that the pilots did not use any checklists or conduct any briefings at any point during the flight. The approach briefing is one of the most critical elements involved in a safe landing, as it forces the pilots to think about and discuss items like visibility minimums and areas of high terrain. On the accident flight, the pilots never said a word about either of these things. Instead, they flew straight into the clouds without any acknowledgement of the fact that they were flying VFR and were supposed to stay in clear air at all times. Although they were misled by the incorrect minimum safe altitude in their chart, they never should have flown into conditions where they relied on this figure in the first place. Oksibil Airport only allowed visual approaches specifically because the surrounding terrain is poorly mapped and there are no navigational aids. By choosing to fly into clouds when they were cleared only for VFR flight, they removed one of the only safeguards keeping them away from the mountains.
The other system supposedly keeping them safe was the ground proximity warning, which was not heard on the cockpit voice recording. Investigators were left stunned when Trigana Air informed them that some of its pilots had been pulling the EGPWS circuit breaker to silence nuisance alarms. In fact, first officers who had flown with Captain Hasanuddin reported that they had personally seen him do this, and the airline told the KNKT that they had already planned to talk to him about this tendency when the accident occurred. Investigators felt that Trigana Air had not displayed sufficient urgency in its attempts to correct the habit, which greatly increased the risk of a crash.
In terms of human psychology, one single factor underpinned both the deactivation of the EGPWS and the decision to fly into clouds on an improvised approach path: simple overconfidence. Pilots at Trigana Air freely admitted that Captain Hasanuddin performed a left base leg approach almost every time he landed at Oksibil, and up until the accident flight he had gotten away with it. This tendency was understandable; after all, the approach procedure published by the airline was extremely complicated and difficult to fly. After numerous successful left base leg approaches to Oksibil, Hasanuddin overestimated his ability to fly the route safely, leading him to believe that he could perform it even in conditions which should have precluded any attempt to land. Similarly, his confidence in his knowledge of the terrain led him to disable the terrain warning, which came back to bite him when he forgot to turn it back on. In a certain sense, his conduct on flights to Oksibil suggested a “bush pilot” mentality, where the driving factor behind his decision-making was his personal assessment of the situation rather than the standard procedures. This mentality developed due to the rudimentary flying aids in the Oksibil area, but an ATR 42 is no bush plane — the procedures that did exist were the minimum necessary to prevent disaster.
Among human factors specialists, this problem is known as normalization of deviance. Coined by sociologist Diane Vaughan in the aftermath of the Space Shuttle Challenger disaster, normalization of deviance is, according to Vaughan, a process by which “people within [an] organization become so much accustomed to a deviation that they don’t consider it as deviant, despite the fact that they far exceed their own rules for elementary safety.” This concept applies perfectly to the crash of Trigana Air flight 267. Due to a complete lack of oversight by the airline, the extremely experienced captain began to violate established procedures because they presented some form of inconvenience, and the positive outcomes that occurred despite this risky behavior reinforced his deviant practices. Before long, these deviations had become part of the company culture, until they were no longer seen as deviant. This insidious decline in safety margins eventually stripped away every barrier preventing Captain Hasanuddin from flying his plane into a mountain.
The severe normalization of deviance that occurred at Trigana Air had resulted in several accidents even before the crash of flight 267. Between its founding in 1991 and the crash in 2015, Trigana Air had written off no less than eight aircraft in various accidents, including two which resulted in fatalities. But while the KNKT’s findings in the flight 267 disaster could and should have been used to permanently shut down the airline, it continues to fly passengers in Indonesia today. Airlineratings.com, which releases yearly lists of both the safest and the most dangerous airlines in the world, included Trigana Air in its list of the 20 least safe airlines for 2018, alongside such esteemed carriers as North Korea’s Air Koryo. Although it hasn’t had any fatal accidents in the five years since the crash, in 2016 it did lose a Boeing 737 cargo plane when an extremely hard touchdown caused the main landing gear to collapse.
It is also worth noting that Trigana Air is not the only airline to have lost a plane on approach to Oksibil. In 2009, Merpati Nusantara Airlines flight 9760, a de Havilland Canada DHC-6 Twin Otter operating the same route from Jayapura to Oksibil, flew into a mountain north of the airport after the pilots entered a cloud while under visual flight rules. All 15 passengers and crew were killed. And in 2018, a privately chartered Pilatus PC-6 also crashed on approach to Oksibil, killing eight of the nine people on board. As long as the Oksibil area remains so underdeveloped, it might not be possible to entirely eliminate the factors which make landing there so risky.
After the crash of flight 267, Trigana Air did take voluntary steps to improve safety. It gathered its pilots together to discuss issues like adherence to standard operating procedures and crew resource management, started making more objective assessments of pilots’ skill levels during training, issued guidance on how to reduce approach and landing accidents, revised its instructions for the approach to Oksibil, introduced new simulator training related to VFR flying and EGPWS responses, conducted a fleet-wide inspection of flight data recorders, and provided its pilots with an internal publication about the factors leading to the crash. Additionally, EGPWS manufacturer Honeywell updated the terrain database for Papua Province, and Trigana Air fixed its charts for the Oksibil region to include accurate minimum safe altitudes. On top of these changes, the KNKT recommended that Oksibil Airport publish its own visual approach procedure, and that Honeywell update the EGPWS terrain database around several other Indonesian airports where resolution is low.
Despite the changes it made after the crash, Trigana Air remains an unsafe airline, and flying into Oksibil is still rather dangerous. This situation will change with time as Indonesia slowly upgrades its infrastructure and develops a culture of safety in its aviation industry. But until then, the residents of Oksibil will have to keep taking risks every time they want to leave their isolated valley.
However, the crash of Trigana Air flight 267 can serve as a teachable moment for pilots both commercial and private. While pilots in much of the world work for airlines where normalization of deviance is properly managed, there remain many companies where more work needs to be done to prevent standards of safety from slipping. And private pilots, who often haven’t worked for an organization with robust standard operating procedures, must be especially vigilant when it comes to problems like overconfidence and experience-based risk-taking. Unless one knows the symptoms of normalization of deviance, it is difficult to detect; by its very nature, it is a silent killer, because when the deviation is normalized it is no longer seen as deviant. Recognizing when this is occurring is a skill that everyone, especially pilots, should have. As Richard Feynman wrote in an appendix to the Challenger report, “When playing Russian roulette, the fact that the first shot got off safely is little comfort for the next.” In real life, he added, “[Sometimes] we don’t even know how many bullets are in the gun.”
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