Down in Chuuk Lagoon: The crash of Air Niugini flight 73

Rescuers approach Air Niugini flight 73 following its crash landing in the water near Chuuk International Airport. (US Navy)

On the 28th of September 2018, a Boeing 737 on an island-hopping journey across Micronesia unexpectedly slammed into the bright blue waters of Chuuk Lagoon while on final approach, coming to rest 140 meters short of the runway. The unplanned ditching caught the occupants by surprise, and one passenger who didn’t wear his seat belt struck his head and died, but the remaining 46 passengers and crew all miraculously escaped with their lives. Stunned by the sudden crash and glad to be alive, only later would they ask how it was that a modern airplane equipped with numerous state-of-the-art safety systems could simply miss the runway and fly into the sea. Investigators from Papua New Guinea, the airplane’s state of registry, found themselves asking much the same question. What they discovered was shocking: as a mechanic stood in the cockpit with his smartphone camera rolling, the pilots flew into a cloud, initiated an excessively steep descent, and ignored thirteen separate alarms warning them that they were about to crash. The findings would underscore the importance of thorough pilot training in a world where advanced aviation safety technologies are increasingly providing an illusion of invulnerability.

A still from an Air Niugini promotional video. (AIr Niugini)

Air Niugini is the state-owned flag carrier of the Western Pacific island nation of Papua New Guinea. (“Niugini” is the standard spelling of “New Guinea” in Tok Pisin, an English-based creole that serves as the diverse country’s most widely used lingua franca.) The airline was founded in 1973, two years before Papua New Guinea’s independence, and has continued operating ever since, providing flights to underserved destinations within New Guinea, as well as numerous countries around the Pacific via its hub in Port Moresby, the capital. Although Papua New Guinea is one of the poorest and least developed countries on earth, Air Niugini has a good safety record; by 2018, it had operated for 45 years without a single fatality, thanks in no small part to the airline’s unselfish acceptance of help from Australian experts.

P2-PXE, the aircraft involved in the accident. (Randall Munro)

Among the planes in Air Niugini’s fleet in 2018 was a next-generation Boeing 737–800 registered as P2-PXE. The plane’s owner was actually Loftleiðir Icelandic Airlines, but the Icelandic carrier had been leasing the plane to Air Niugini since it acquired it in the summer of 2013.

On the 28th of September 2018, this plane was scheduled to operate a flight from the island of Pohnpei in the Federated States of Micronesia, to Port Moresby with a stopover in the Chuuk Islands. The Federated States of Micronesia is a country in Oceania, primarily located north and northeast of New Guinea, consisting of around 600 small islands and coral atolls, which amount to a land area a little over half the size of New York City, but spread out over a distance of 2,700 kilometers. The country’s population is only about 100,000 and it lacks any domestic scheduled airlines. However, airplanes are the only convenient means of transport between the islands and to and from neighboring countries, so Air Niugini and US carrier United Airlines have stepped in to offer domestic flights connecting the islands of Pohnpei, Kosrae, and Chuuk.

The route of Air Niugini flight 73, in Micronesia and Papua New Guinea. (Google + own work)

Under the command of a 52-year-old Papua New Guinean captain and his 35-year-old Australian first officer, P2-PXE arrived on Pohnpei, home to Micronesia’s capital and administrative center, sometime after 22:00 on the night of September 27th. By 9:00 a.m. the next morning, the pilots were back on the plane, overseeing the boarding of 35 passengers for Air Niugini flight 73 to Chuuk and Port Moresby. As this flight was a public service rather than an attempt to generate revenue, it was no great matter that the plane was only one fifth full. However, despite the low number of passengers, the plane carried an unusually large crew of twelve, including the two pilots, a cargo loadmaster, a ground engineer, four regular flight attendants, a trainee flight attendant and instructor, a check stewardess monitoring the training process, and an observer monitoring the check stewardess. The engineer set himself up in the cockpit jump seat, but the loadmaster and one of the check staff sat in first class (which contained only one fare-paying passenger) because there weren’t enough crew seats on the airplane to handle so many crewmembers.

The approximately one hour flight proceeded normally, as the Boeing 737 climbed to its cruising altitude of 40,000 feet, stayed there for a few minutes, then began to descend toward its stopover in Chuuk.

A map of Chuuk Atoll. (Wikimedia user “Aotearoa”)

The Chuuk atoll consists of 57 hilly islands rising from the waters of Chuuk Lagoon, an area of shallow water enclosed within a 225-kilometer-long coral breakwater. The atoll’s main transport hub is Chuuk International Airport, a small single-runway airstrip in Weno, a village on the island of the same name, which at 13,700 inhabitants is the largest town in the Federated States of Micronesia. Due to its isolated location, the area receives relatively few tourists, but within the international diving community it is well known for its abundance of Japanese WWII shipwrecks.

The difference between an RNAV approach and a traditional instrument approach. (FAA)

Aboard flight 73, the pilots decided to approach Runway 4 at Chuuk International Airport from the southwest using the published RNAV approach. An RNAV approach, a relatively modern innovation, allows a plane to fly an approach using GPS without reference to ground-based navigational aids, such as non-directional beacons and instrument landing systems. Flying an RNAV approach is as simple as calling up the appropriate set of GPS waypoints and their crossing altitudes in the flight computer; the approach can then be flown automatically by the autopilot, or manually by following computer-generated instructions based on the GPS plan. The pilots had apparently grown so confident in the technique that they never conducted a formal approach briefing, which would have covered the required parameters in considerable detail.

During cruise, the First Officer used Boeing’s “Operational Performance Tool,” an app which he had installed on his iPad, to calculate the required landing configuration. The app had not been approved by the airline for use in line operations, but the First Officer had apparently gotten used to it anyway. He advised the Captain they would need to land with the flaps extended to 40 degrees, the maximum allowable, in order to slow down for landing on the very short runway in Chuuk, and the Captain agreed without hesitation.

Unlike the rest of the Air Niugini fleet, P2-PXE also came with an Integrated Approach Navigation System, which could simulate the components of an instrument landing system (ILS) in order to provide the crew with more information about their position during an approach which lacked an actual ILS. In a regular ILS approach, ground-based beacons produce a localizer and glide slope, a pair of signals that can be picked up by the plane’s instruments to produce a precise descent path leading directly to the runway threshold. But with IANS installed, pilots flying an RNAV approach into an airport without ILS (such as Chuuk) could receive instrument indications showing their location relative to an imaginary localizer and glide slope, allowing them to more easily maneuver the plane onto the precise landing trajectory.

As flight 73 descended toward Chuuk on autopilot, the plane turned to follow the GPS waypoints to align with the runway, in the process intercepting the imaginary localizer displayed on the pilots’ instruments. As they passed through 4,100 feet, the Captain said, “Okay, we on RNAV at 041, and I’ll go to 1,000.” In the jump seat, the engineer pulled out his smartphone and began to film the landing, capturing a crystal-clear view of the instrument panel as the plane began its final approach.

A still from the engineer’s cockpit video. (Papua New Guinea Accident Investigation Commission)

At first, all seemed to be going according to plan. Following the programmed vertical navigation data, the autopilot brought the plane onto a steady three-degree descent path toward the runway, in the process aligning with the imaginary glide slope generated by the IANS. Although their weather radar showed a small but intense storm cell moving between them and the airport, the pilots seemed unconcerned. “That must be some storm, but it’ll be out soon,” the Captain commented.

“Ah, we’ll probably just go down on the PAPIs,” the First Officer said. The PAPI, or Precision Approach Path Indicator, was a set of four lights on the left side of the runway that would show white if they were too high, red if they were too low, and both if they were on course.

The storm cell would have been clearly visible to the crew on radar during the approach. (Papua New Guinea Accident Investigation Commission)

“Alright, flaps 30, flaps 40,” the First Officer continued, making the final configuration changes for landing.

“Landing checks,” said the Captain.

“ONE THOUSAND,” said an automated voice, calling out their altitude.

“Okay, stable,” said the First Officer. They were on course and configured to land.

“Continue,” the Captain declared.

“And visual, 900 cloud base,” said the First Officer, catching sight of the runway as they broke through a low overcast layer at 900 feet.

At this point, the extension of the flaps to 40 degrees increased the amount of lift generated by the wings and caused the plane to rise slightly above the imaginary 3-degree glide slope. With the airport in sight, the Captain decided to take manual control to get back on course. “I’m going back on profile,” he announced, disconnecting the autopilot at a height of 677 feet above the ground. In order to return to the glide slope, he pitched down and increased their rate of descent.

A clip from the cockpit video, with a concurrent animation of the airplane. (Papua New Guinea Accident Investigation Commission)

Around the edge of the rain shaft, the pilots were still able to see the runway edge lights and the PAPI lights, but their field of view was narrowing. “Okay, landing,” said the Captain.

“Visual, one red, three whites,” the First Officer announced. The PAPI was showing three white lights, indicating that they were slightly too high.

Suddenly, the plane entered the rain shaft and all visual references disappeared. Intense rain and wind swept over the plane, but the pilots barely reacted.

“MINIMUMS,” said the automated voice of the Enhanced Ground Proximity Warning System (EGPWS), informing the crew that they had reached the minimum descent altitude for the approach. At this point they were required to abandon the approach if they could not see the runway, but the pilots kept descending. They just had the runway a moment ago — surely it would come back into view soon, they must have thought. But it did not.

Descending at more than 1,000 feet per minute from a height of less than 500 feet above the water, flight 73 passed back through the imaginary glide slope and began to fall below it. “SINK RATE! SINK RATE! SINK RATE!” blared the EGPWS.

But the pilots ignored it. “I just wanna get on profile,” said the Captain.

“GLIDESLOPE,” pronounced the EGPWS. “GLIDESLOPE! GLIDESLOPE!” The plane was trying frantically to warn the pilots that they were descending below the 3-degree descent path. Their instruments clearly showed that they were too low. The words “PULL UP” appeared in red on their primary flight displays. But the pilots seemed oblivious to the danger.

As the plane hurtled toward the waters of Chuuk Lagoon, the EGPWS continued to blare, “SINK RATE! SINK RATE!”

“That’s fine, I’ll just go a little bit more,” said the Captain, expecting to break out the other side of the storm at any moment.

“GLIDESLOPE! GLIDESLOPE!”

“See the runway?” the First Officer suddenly asked. The Captain didn’t respond.

“ONE HUNDRED,” said the EGPWS, again calling out their altitude in feet. “GLIDESLOPE!”

“Monitor airspeed, okay, got it,” said the Captain.

“GLIDESLOPE! SINK RATE! SINK RATE!”

Suddenly the First Officer realized that they were in mortal danger. “Too low!” he shouted. “We’re too low! We’re too low! We’re too low!”

But it was already too late. Two seconds later, Air Niugini flight 73 slammed into Chuuk Lagoon with an almighty splash.

The plane came to rest 140 meters short of the runway in water approximately 25 meters deep. (US Navy)

At first, some of the passengers thought they had merely landed hard, but those illusions were quickly shattered when the bottom of the fuselage split open near row 22 and water poured into the back of the economy class cabin. Still moving forward under its own momentum, the plane plowed through the water for several hundred meters, slewed more than 90 degrees to the right, then came to a stop about 140 meters short of the runway threshold. Almost immediately, the water in the back of the cabin rose to knee height, and with a chorus of clicks the passengers unfastened their seatbelts and rushed for the exits.

Due to the very low passenger volume, no one was seated in the emergency exit rows, and the queue quickly blocked access to doors, forcing the flight attendants to push through the crowd to reach the overwing exits. Farther back, a number of passengers near the break in the fuselage had suffered serious injuries and were in dire need of assistance. The situation could have gotten ugly fast if not for the actions of quick-thinking witnesses.

US Navy divers approach the plane using an inflatable boat. (US Navy)

Chuuk Airport had no water rescue services, but a group of US Navy divers witnessed the accident, as did numerous local residents, who rushed to the scene in their own motorized vessels. The Navy divers arrived by boat just as the first overwing exits were opened, disembarking onto the partially submerged wing to assist. On the left side of the plane, 28 passengers and two flight attendants climbed off the wing and into private boats, while the Navy divers took on six passengers, four flight attendants, and the loadmaster from the right side. A further five crewmembers were pulled by boaters from the L1 door behind the cockpit. Within a couple of minutes, everyone appeared to be off the plane; the US Navy divers entered the cabin and found no apparent stragglers, but considered it too dangerous to enter the now-submerged rear economy class cabin, which was quickly becoming a death trap as the plane slipped deeper into the water and jet fuel leaked from the wing tanks.

Scenes from the rescue were captured on camera. (US Navy)

Because Chuuk Airport lacked a designated rendezvous area, the transportation of the victims to the local hospital was extremely chaotic, and no one was able to conduct a head count until many hours after the crash. Despite this, officials initially reported that all 47 passengers and crew had survived, a figure which was quoted by news media around the world. But when a count was finally undertaken that evening, those officials came to a disturbing conclusion: one passenger, an Indonesian man seated in row 23, was missing. Rescuers initially hoped that he might be found wandering somewhere nearby, but on such a small island there were not very many places he could have gone, and it was soon clear that he had not made it to safety. Divers began searching for his body in the area near the sunken airplane, but after three days, no trace of him was found.

Local residents rushed to the scene in their boats and were able to evacuate all the passengers with remarkable speed. (James Yaingeluo)

Eventually, local authorities called in a team of expert Japanese divers to search the inside of the plane, which was lying under about 25 meters of water. Upon entering the fuselage, the Japanese divers made a grim discovery: the body of the missing man was lying between rows 22 and 23, right by the break in the fuselage, having made it only a few meters from his seat. He had in fact been on board the plane the entire time.

Initial suspicions were that the man had drowned while attempting to escape the plane, but an autopsy revealed no signs of drowning. In fact, he had suffered traumatic facial and cranial injuries on impact which led to death within about three minutes after the crash. Although his traveling companions had seen him stand up from his seat, he apparently collapsed and died shortly afterward, and as the passengers rushed to escape the sinking tail section, nobody noticed. Based on the lack of seat belt bruising, which was present on the six passengers who suffered serious injuries, pathologists concluded that he was not wearing his seat belt at the moment of impact, as a result of which he was thrown into the seat in front with force sufficient to kill him.

Inside the plane after it sank. (Papua New Guinea Accident Investigation Commission)

Meanwhile, investigators hurried to the Chuuk Islands to begin an inquiry into the crash. Initially, the investigation was handled by the Division of Civil Aviation of the Federated States of Micronesia, with assistance from the Papua New Guinea Accident Investigation Commission, whose investigators arrived on the scene the day after the accident. However, by February 2019 it had become clear that Micronesia lacked the facilities and expertise to run an investigation of a major air accident, so the country delegated this responsibility to Papua New Guinea.

Diagram showing the relative positions of the plane, the runway, and the CVR. (Papua New Guinea Accident Investigation Commission)

Within a short time after the accident, investigators learned that the engineer — on board to service the airplane while it was at various poorly equipped airports in Micronesia — had filmed the entire approach from 3,000 feet to impact while seated in the cockpit jump seat. In addition to the evidence from the cockpit voice recorder and flight data recorder, the video helped provide investigators with an unusually complete picture of the final moments of flight 73.

Upon hitting the sea floor, the wreckage fully split into two pieces. (The Guam Daily Post)

The basic sequence of events resembled that of numerous previous accidents, albeit mostly from the 1960s and 1970s. After rising slightly above the 3-degree glide path, the captain took manual control, increased the rate of descent, and simply failed to correct it upon reaching the proper trajectory. The plane subsequently descended into the sea short of the runway.

The problem is that this was not the 1960s, when the only thing stopping a plane from flying into the water was the pilot’s vigilance. This was 2018, and the plane was equipped with all manner of state-of-the-art equipment, from the Enhanced Ground Proximity Warning System to Area Navigation (RNAV) to the Integrated Approach Navigation System. At a height of 1,000 feet the plane was on course and ready to land with all systems working perfectly. There was no reason for the flight to have ended the way that it did.

The rescue as seen from the shore of nearby Weno Island. (The Guam Daily Post)

From the cockpit voice recording it was apparent that the pilots had ignored 13 “SINK RATE” and “GLIDESLOPE” alerts in the moments leading up to the crash. Because the pilots survived the accident, investigators were able to ask them why they had done this. The answer was tragically simple: the pilots thought they were nuisance warnings. Rewinding the CVR and FDR back to the previous day’s approach into Pohnpei confirmed that similar warnings were generated on that approach, even though it landed successfully. From the very beginning, the pilots flew the approach to Pohnpei below the standard 3-degree glide path, triggering no less than 24 “GLIDESLOPE” alerts, but they simply talked right over them, engaging in an irrelevant conversation even as the plane tried desperately to get their attention. Not only did this conversation continue through numerous warnings, it also violated the sterile cockpit rule, which bans non-pertinent discussions below 10,000 feet. Investigators did not explicitly look into why the pilots flew this abnormally low approach, but it might have been an improvised technique that Air Niugini pilots had developed in order to touch down closer to the start of the runway, increasing the available landing distance — an important consideration at Micronesia’s cramped airports that lacked clear overrun areas.

The plane took some time to fill up with water and sink. (The Guam Daily Post)

Because of these findings, the fact that P2-PXE was equipped with IANS suddenly became rather important. The IANS creates an imaginary glide slope where no real glide slope exists, which is what allowed “GLIDESLOPE” alerts to be generated by the EGPWS during RNAV approaches into Pohnpei and Chuuk. But P2-PXE was the only plane in the entire Air Niugini fleet equipped with IANS, and the pilots were not explicitly trained to use it, so the fact that this particular plane produced “GLIDESLOPE” alerts on RNAV approaches while other planes didn’t could have led the pilots to believe that the warnings were a bug rather than a feature. In fact, in 2016 Air Niugini had ordered the system removed from P2-PXE in order to standardize its fleet, but the retrofit was never carried out, and no one at the airline knew that it was still installed. In light of this fact, and the apparent habit of conducting low approaches in Micronesia, it seemed that the pilots of flight 73 had become conditioned to believe that the persistent “GLIDESLOPE” alerts on P2-PXE were nothing more than nuisance warnings.

As the plane sank it started to heel over to the left. (The Guam Daily Post)

However, there were a number of other cues that should have informed the pilots that something was amiss, such as the “SINK RATE” alerts, the “PULL UP” indications on their displays, and the simple fact of their low altitude. Their apparent obliviousness to these warning signs suggested that the pilots were suffering from a case of tunnel vision. Just before things went awry, the plane was stabilized on the approach, the runway was in sight, and the pilots had already mentally shifted into the expectation of an imminent landing. So when the plane suddenly flew into a rainstorm and the runway disappeared from view, the pilots became stuck in their previously established, but now outdated, modes of thought. The Captain had already switched to visual flying, but when he lost sight of the runway, he did not look back at his instruments, causing him to lose situational awareness. The same thing happened to the First Officer, who had been monitoring their position by watching the PAPI lights. The last time he saw the PAPI, it was showing three white lights — indicating slightly too high — and when the crew lost sight of it, its last known indication stuck in their minds even after it became obsolete. The cumulative effect of these factors was that the pilots became fixated on regaining visual reference and finishing the landing, at the expense of everything else, including instrument scans and decision-making. Having lost overall situational awareness, they failed to stop their descent until the plane hit the water.

Another view of the sinking plane, with the runway in the background. (Loop PNG)

Obviously the correct thing for the pilots to have done when they lost sight of the runway would have been to abandon the approach. It is forbidden to descend below minimums when the runway is not visible. To continue descending under such conditions is reckless and irresponsible. Furthermore, they should have anticipated the possibility of losing visual contact when they saw the storm cell on radar, but the Captain inexplicably dismissed its significance. Although various reasons for their decision to continue the approach under these conditions could be proposed, at the end of the day this remains the oldest mistake in the book, the single decision that has killed the most aviators, and the fact that a pair of trained pilots would do this in 2018 suggests that their education perhaps failed to sufficiently emphasize this point.

View of the plane from atop a nearby hill. (Reuters)

As a result of the accident, Air Niugini stopped flying the Boeing 737 to Chuuk and Pohnpei, and stricter training requirements were introduced for pilots flying to those airports. The airline also introduced “sudden loss of visual reference on final approach” scenarios in simulator training, improved its EGPWS response training, and moved to ensure that an able-bodied passenger is always seated in each emergency exit row, along with several other points. Investigators also recommended that Honeywell, the manufacturer of the EGPWS, amend the warning regime so that pilots in similar situations will receive aural “pull up” warnings, or at least a flashing visual “pull up” indication that is more likely to gain pilots’ attention than the static version which currently exists. However, Honeywell replied that in a case like Air Niugini flight 73, an aural “pull up” warning would violate federal guidelines for ground proximity warning systems, which are designed to ensure that a “pull up” warning is only generated if the situation is extremely dire (thus ensuring that pilots don’t become conditioned to ignore it). The US National Transportation Safety Board, which assisted with the investigation, also pointed out that in a case where pilots ignored 13 warnings before flying into the ground, adding more warnings was probably not the solution.

The plane starts to tip over before it finally slipped beneath the waves. (the Guam Daily Post)

Ultimately, those on board Air Niugini flight 73 were fortunate that the outcome was not worse. Despite the lack of preparation for a water landing, the plane stayed intact and 46 out of 47 passengers and crew walked away from the wreck. But the crash was also a cautionary tale about the limitations of technology. No amount of iPad apps, RNAV approaches, and fancy warnings prevented this classic controlled flight into terrain accident, and the lack of context for the Integrated Approach Navigation System arguably helped cause the crash in the first place. So while technology can and does reduce accidents, much still depends on the way it is used, and pilots and airlines alike had best not take that for granted. In the right circumstances, with the right combination of conditioning, hubris, and inattention, modern pilots can still fly their planes into the ground, and it is still the responsibility of pilots and those who train them to ensure that a needless crash like Air Niugini flight 73 doesn’t happen again.

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Analyzer of plane crashes and author of upcoming book (soon™). Contact me via @Admiral_Cloudberg on Reddit or by email at kylanddempsey@gmail.com.