In the Shadows of Giants: The crash of Thai Airways International flight 311
Note: this accident was previously featured in episode 41 of the plane crash series on June 16th, 2018, prior to the series’ arrival on Medium. This article is written without reference to and supersedes the original.
On the 31st of July 1992, a Thai Airways Airbus A310 on approach to Kathmandu flew off course into the Himalayas and slammed into a precipitous mountainside, taking with it the lives of 113 people. Amid the peaks of the world’s tallest mountain range, it took search crews two days to find the plane, and by the time they did, little remained, save for a smear of black across a cliff face and a pile of debris on a hillside far below. But the most surprising aspect of the discovery was not the extent of the destruction, but its location. Why, investigators asked, did a plane which controllers thought was south of Kathmandu strike a 16,000-foot mountain 40 kilometers north of the airport?
The story of how Thai Airways flight 311 reached its final resting place in fact played out over about 45 minutes of increasingly frustrating conversations between the pilots and air traffic control, as each sought to reconcile ever more baffling contextual clues with their mental models of the situation. Somewhere amid the chaos, flight 311 started flying in the wrong direction, and nobody noticed until it was much too late. How could such an error have been made? And more worrying, how could it have gone undetected? The answers lay somewhere in the intersection of human nature and technology, illustrating what not to do when a situation becomes too complex to properly understand — a lesson not just for pilots, but for anyone who must interact with people and the systems that they build.
Home to eight of the world’s fourteen peaks over 8,000 meters, the small Himalayan nation of Nepal is a country defined more so than almost any other by its topography. Getting from one place to another can be an arduous trek even over short distances, especially outside of the densely populated lowlands near the border with India. Some countries with difficult terrain, such as Indonesia with its thousands of islands, embraced air travel as an effective solution, but in Nepal, flying was slow to take off, due to the difficulty of building airports and serving them safely. Some of the world’s hairiest mountain airfields are in Nepal, most notably Tenzing-Hillary Airport in Lukla, the gateway to Mount Everest and by some accounts the most dangerous airport on earth.
Even Tribhuvan International Airport, the main port of entry to Nepal, has long had a reputation for tricky approaches and landings. Located in the capital city of Kathmandu, the runway sits in a valley at over 4,000 feet (1200m) above sea level, with peaks rising to over 9,000 feet (2,750m) immediately to the south and 20,000 feet (6,100m) a short distance to the north. To make matters worse, in the 1990s Tribhuvan Airport lacked basic infrastructure such as radar or an instrument landing system, either of which would have been expensive to set up due to interference from nearby terrain. The result was that international flights to Nepal were forced to come in over the southern ranges, then descend steeply to the runway without the benefit of a glide slope or radar guidance from air traffic control. If the weather was bad, which it was during much of the summer monsoon season, then landing was simply impossible.
One of the relatively small number of airlines which routinely flew large jets into Kathmandu in 1992 was Thai Airways International, the state-owned flag carrier of Thailand. The airline operated a regular Bangkok-Kathmandu service using a wide body Airbus A310, a long range twin jet designed for medium-to-long-haul flights with a lower passenger capacity than other wide bodies.
One of these A310s, registered as HS-TID and nicknamed “Buri Ram,” was scheduled to carry out Thai Airways International flight 311 from Bangkok to Kathmandu on the 31st of July 1992. On board were 99 passengers and 14 crew, including the two pilots, 41-year-old Captain Preeda Suttimai and his 52-year-old First Officer Phunthat Boonyayej. Although Boonyayej had slightly more experience, both pilots had well over 13,000 flying hours, and Suttimai was not only a captain but an instructor as well. Boonyayej had tried to upgrade to captain in the past, but was deemed unsuitable for the position; he was capable enough as a first officer, but lacked the qualities needed to assume command. Some of those limitations would become apparent hours later, when flight 311 began its fateful approach to Kathmandu.
After an uneventful takeoff, climb, and cruise, flight 311 made contact with Kathmandu area control at 6:26 UTC, or 12:11 local time in Nepal. With Captain Suttimai on the controls, First Officer Boonyayej reported their position over a waypoint called Monda, and estimated that they would arrive over the Romeo waypoint at 12:28, followed by landing in Kathmandu at 12:39. The controller replied with the latest weather observation, taken at 11:45, which indicated scattered clouds at 2,500 and 4,000 feet, overcast at 10,000, and visibility greater than 10 kilometers, more than enough for a successful approach.
At 12:18, First Officer Boonyayej again called air traffic control and asked, “Thai three one one, request descent.” But there was no reply. He tried again two more times, without success. Wondering whether the First Officer’s radio might be faulty, Captain Suttimai tried himself, but was only able to get through on his third attempt. Recordings would later prove that the calls were heard in the control center, but for whatever reason the controller wasn’t paying attention. Only now, after six calls, did he reply, reporting that the only other plane in the area was a Royal Nepal flight inbound to Kathmandu at 37,000 feet. He then cleared flight 311 to descend from 35,000 feet, and Boonyayej replied, “Copy, leaving three five zero.”
Five minutes later, at 12:23, the controller called again with bad news: rain had moved over the airport, and visibility to the south was only 1,500 meters. This presented an issue because flight 311, like most flights to Kathmandu, was planning on a straight-in approach to runway 02 which required a visibility of at least 3,000 meters.
Tribhuvan International Airport has one runway, which is referred to as runway 02 when landing from the south and runway 20 when landing from the north. When visibility was poor, flights to Kathmandu normally landed on runway 02 using what is known as a VOR/DME approach. In this type of approach, the pilots must tune in to a VOR, or Very high frequency Omnidirectional Range, located at the airport. The VOR receivers in the plane can then determine where the plane is in relation to the VOR, expressed in terms of its radial. For example, a plane directly east of the VOR is said to be “on the 90 [degree] radial of the VOR,” while a plane directly to its south is on the 180-degree radial, and so on. Simultaneously, a set of Distance Measuring Equipment, or DME, co-located with the VOR, determines the distance between the plane and the DME in nautical miles; this distance is then displayed on the pilots’ DME indicators. Using these two sources of information, pilots can align with the radial corresponding to the runway heading, and then descend to the runway by hitting certain target altitudes at certain DME distances, as indicated on their approach chart.
At Tribhuvan International Airport, a VOR/DME approach procedure existed for a straight-in landing on runway 02, beginning at a point 16 nautical miles from the VOR (henceforth, 16 DME) and following the 202-degree radial to the runway. This procedure was known as the Sierra approach, after waypoint “Sierra,” located at 10 DME. There was no corresponding approach procedure for runway 20, due to high terrain north of the airport; consequently, the only way to land on this runway was to perform the Sierra approach to runway 02, break out below the cloud base, and then circle the airport in visual conditions to land from the other direction. Performing this so-called circling approach at Kathmandu could be stressful, especially if visibility was marginal, and pilots generally tried to avoid it if possible.
However, today, the visibility to the south of the airport was too low to land directly on runway 02, while the visibility in other directions was above 3,000 meters, so the controller offered flight 311 a circling approach to runway 20. The pilots were not satisfied with the proposal, so First Officer Boonyayej requested that they continue to runway 02 instead. But the controller simply repeated that runway 02 was unavailable. “Roger, runway 20, Thai 311,” Boonyayej replied.
“I guess we can’t make it,” said Captain Suttimai.
“Got to go in first,” said Boonyayej, apparently suggesting that they continue their descent to see if conditions might improve.
Suttimai wasn’t going to count on it, however, so he asked Boonyayej how much fuel they would need to divert to their alternate airport in Calcutta, India. In reply, Boonyayej remarked that they had enough fuel to go all the way back to Bangkok, let alone Calcutta. But this wasn’t the response Suttimai wanted: “Calcutta, how much?” he snapped.
This time, Boonyayej provided the fuel required to divert to Calcutta. Apparently Captain Suttimai simply wanted to know how long he could wait over Kathmandu before fuel would force a diversion.
Having gotten that matter out of the way, the pilots finished briefing the Sierra approach. Notably, when discussing the minimum on-course altitude (or MOCA) applicable to the segment between waypoint Romeo and the start of the approach, Captain Suttimai said, “We can descend to 10,500 [feet] if we are on radial 022.” This was a slip of the tongue — the minimum actually applied on radial 202, previously mentioned as the radial used to approach runway 02. Radial 022 of the Kathmandu VOR was the extension of the same line to the north, in the opposite direction, and was not relevant to the Sierra approach, or any other approach. It was, however, the magnetic heading on which they would fly while following the 202 radial (as shown above), which may have been the source of his confusion. The significance of this slip-up, in light of later evidence, is up for debate.
Minutes later, at 12:27, flight 311 reported that it was over the Romeo waypoint, located on the 202 radial of the Kathmandu VOR at a distance of 41 DME. In response, the controller said, “Clear Sierra on radial 202 inbound, descend to 11,500… expect Sierra approach, no delay, report 25 DME [at] 11,500.”
The pilots acknowledged the clearance, then began discussing the missed approach procedure, in case the approach failed. According to their charts, the normal procedure in such a case was to overfly the VOR, climb to 11,500 feet, make a 270-degree right turn, roll out on a heading of 291 degrees, and proceed to a holding pattern at waypoint “Whiskey,” northwest of the airport.
Hoping that they might avoid having to actually use this procedure, at 12:30, First Officer Boonyayej again requested the visibility. But instead of providing a value, the controller replied that “runway 02 [is] also available,” indirectly suggesting that visibility had improved.
Boonyayej keyed his mic and started to say, “Ah…”
But before he could get any words out, Captain Suttimai jumped on the radio and asked, “Confirm runway 02 available?”
“Affirm,” said the controller.
Captain Suttimai now added another request for the visibility, which the controller again ignored, asking for their DME distance instead. “25 DME now, request present visibility,” Suttimai repeated.
In response, the area controller told him to contact the tower, responsible for operations in the immediate vicinity of the airport. With the plane now inside 25 DME, flight 311 was the tower’s responsibility.
At 12:31, the crew contacted the tower, which cleared them for a Sierra approach to runway 02 and asked them to report crossing 10 DME at 9,500 feet. Meanwhile, the pilots began configuring their plane for the approach. “Speed brake in, flaps 15,” Suttimai ordered.
First Officer Boonyayej moved the flap handle to the 15-degree position, but an unexpected problem occurred: due to excessive friction in the system, the torque limiter engaged and stopped the flaps slightly short of the 15 degree position. A chime sounded, and a “flap fault” message appeared on their computerized display.
“The flaps have not extended!” Boonyayej exclaimed. Truly this was the last thing they needed. He tried moving the flap lever farther, to the 20 degree position, but the flaps still didn’t move.
Captain Suttimai knew that it was impossible to land on runway 02 if they couldn’t fully extend the flaps. Most approaches have a 3-degree descent gradient, but due to the steep terrain, the Sierra approach to Kathmandu required descent gradients as high as 6.6 degrees. On such a steep descent, the extra drag from fully extended flaps was required in order to prevent the plane’s speed from increasing. In fact, company procedure barred pilots from flying the Sierra approach with anything less than full flaps.
Instead of continuing their descent, Suttimai leveled off at 10,500 feet to help Boonyayej troubleshoot the issue. However, there didn’t seem to be a procedure for fixing it — the quick reference handbook simply provided instructions for landing with less than full flaps, which was inapplicable to their situation. As they debated what to do, the plane passed over the point where it was supposed to descend to 9,500 feet, placing it at least a thousand feet too high.
“Take it easy,” Suttimai said. “Too late to make a landing. We’ve got to have the configuration… we cannot land.”
At 12:33, Captain Suttimai called the tower and said, “Thai 311, request to maintain 10,500 and request, ah, back to Calcutta, due to, ah, technical.”
“Understand Thai 311, like to divert to Calcutta due to technical,” the tower replied.
“Affirm,” said Suttimai. But at that very moment, First Officer Boonyayej’s troubleshooting efforts paid off: by retracting the flaps to 0 degrees and then moving the handle back to 20 degrees, the flaps extended properly. Now they didn’t have to divert — they could still land at Kathmandu, but since they were too high and too close to the airport to salvage their approach, they would have to circle back to the start of the approach and try again. And so, just four seconds after his affirmation that they wanted to divert, Suttimai jumped on the radio again and said, “Ah we like to… left turn out, left turn out again and back to, to join Sierra approach again, ah back to normal now. Can we make a left turn to Romeo now?”
“Understand operation normal and you would like to make an approach?” the tower asked.
“Affirm, affirm!” said Suttimai.
His intention was to turn back to waypoint Romeo, which he considered to be the start of the approach. He probably requested a left turn because the terrain was lower in that direction. But here, a critical misunderstanding occurred: although Suttimai seemed to believe that the approach began at Romeo, this was not actually the case. The Sierra approach procedure began at the 16 DME point, while Romeo lay at 41 DME, much further away from the airport. He may have gotten this impression from his approach chart, which showed Romeo and the 16 DME point right next to each other, albeit with a break in the line and a label reading “scale distorted” (shown above). As a result, Suttimai thought he wanted to go all the way back to Romeo — something which made no sense to the air traffic controllers. In fact, Romeo was so far from the airport that it wasn’t even on the charts in the tower, and flying back to it would require traveling the wrong direction in a one-way airway.
Compounding their confusion was another misunderstanding related to the role of air traffic control at Kathmandu. Tribhuvan International Airport did not have radar, so controllers could not tell pilots to turn in specific directions or to specific headings. Instead, controllers in a non-radar environment normally clear flights to perform a particular published procedure, such as the Sierra approach, and ensure separation by keeping inbound flights at different altitudes. As for how to navigate to the desired waypoint, that was the flight crew’s problem. So when Captain Suttimai asked for a left turn back to Romeo to start the approach again, the controller simply cleared him for a new approach, expecting him to get to the starting point on his own: “Thai 311,” he said, “clear Sierra approach, report one zero DME, leaving 9,500.”
But this was not what Suttimai wanted to hear. He interpreted this clearance as authorization to continue the approach he was already on, which was impossible, seeing as they were now too high to land. Calling the tower, he said, “We can’t land at this time. We have to… left turn back to Romeo again, and start our approach again.”
“Roger,” said the controller. “Go ahead with your DME distance.”
“We are 9 DME, 10,500 feet,” said Suttimai.
“Understand 9 DME, 10,500 feet?” said the controller.
Suttimai was getting fed up with what he perceived as the controller’s inability or unwillingness to acknowledge his request for a left turn. “Answer please, answer please!” he said to Boonyayej, sounding frustrated.
“We’ll maintain 10,500 feet,” Boonyayej said to the tower. Turning to his captain, he asked, “Are we cleared over MOCA [minimum on-course altitude] okay? We can see on our right hand side. How about on the left?”
“Yes, I can see it,” said Suttimai. Taking over the radio again, he asked, “Left turn now?”
“Go ahead your DME distance,” the tower replied.
“Ah… 7 DME now,” Boonyayej said, jumping onto the radio. “Request left turn back.”
“Roger, 10 DME copy sir, report 16 DME leaving 11,500,” the tower replied. There was still no acknowledgement of the request to turn. Most likely, the controller did not have the English language skills he needed to explain his confusion, so he simply kept repeating similar clearances over and over, hoping that the pilots would eventually decide to proceed on their own.
At this point, Captain Suttimai gave up on trying to get the clearance he wanted. “We’ll climb ahead,” he said. At that moment, he reached over to the autopilot control panel and rotated the heading knob a few turns to the right, commanding the autopilot to assume a heading of 045˚, or northeast. Simultaneously, he started to climb, apparently mimicking the missed approach procedure, although they were still at 5 DME and not over the VOR, where their charts showed the procedure beginning. As the plane embarked on the maneuver, he called the tower and said, “We’re climbing.”
Again, the tower reiterated their clearance: “Report 10 DME leaving 11,500 feet for Sierra approach runway 02.”
Flight 311 started to roll out on its new heading of 045˚, so Captain Suttimai rotated the heading knob some more. The autopilot stopped leveling the plane and started turning to the right again. “Report 10 DME, one one five,” First Officer Boonyayej said, reading back the clearance incorrectly.
“Negative… report one zero DME leaving 9,500 feet,” said the tower.
“Affirm, report 10 DME, 9,500,” said Boonyayej.
“Affirm, affirm,” the tower replied.
But Captain Suttimai still seemed to think the tower wanted him to report 10 DME and 9,500 feet on his current approach, which he was already abandoning. “We cannot make approach now,” he said, trying to explain clearly. “We right turn back to Romeo and climb to 18,000 feet… to start our approach again.”
The report that flight 311 was climbing to 18,000 feet without permission caught the controller off guard. “Roger, ah, standby for the time being,” he said. “Maintain 11,500 feet due to traffic.” At that time, Royal Nepal flight 206 was descending toward 15,000 feet, and since neither flight was currently following a published procedure, he could not allow their altitudes to cross without violating the principles of separation in a non-radar environment.
In response, First Officer Boonyayej looked at their altimeter — they were already at 13,500 feet and climbing. “Okay, we maintain one three thousand now,” he said.
“Roger,” said the tower.
“Is that okay?” Boonyayej asked, hesitating.
“Initially maintain 11,500 due to traffic, RA206, estimating Simara [at] 57 from Delhi, descending to flight level 150,” the controller repeated.
“Descend to 11,500,” Boonyayej acknowledged. “We are now 8 DME.” The plane started to level out at a southeasterly heading of 130 degrees, so Suttimai turned the heading knob several more times. Then, in response to the order to remain at 11,500 feet, Suttimai stopped climbing and put the plane into a descent. “Okay, we are now in descent to 11,500, 9 DME from Kathmandu,” he reported.
“Roger,” said the tower. “Thai 311, understand you like to proceed Romeo to make an approach?”
Finally the controller seemed to be catching on to what Suttimai wanted. “Affirm, affirm,” he said.
All the while, the plane continued to turn to the right, passing clear through 202 degrees — the heading back to Romeo — and onward toward 340 degrees, or northwest. This was not the way to get to Romeo, but nobody seemed to notice. “Kathmandu, Thai 311, confirm we can proceed to Romeo now?” Suttimai asked.
“Roger, proceed to Romeo now, and contact one two six decimal five,” said the tower, handing them back to the area controller, whose jurisdiction included Romeo.
The flight started to roll out on a heading of 340˚, so Captain Suttimai reached up and turned the heading knob even more, to 025˚. In the background, Boonyayej could be heard saying, “Romeo oh, direct Romeo oh…”
At Tribhuvan Airport, the tower controller called the area control center and informed them that flight 311 was “unable to make approach” and was proceeding back to Romeo. In response, the area controller asked why the flight was being sent to Romeo, since this wasn’t the start of the approach procedure, to which the tower replied that this is what the flight crew wanted. One must imagine that there were some shrugs and raised eyebrows, but who were they to deny the request?
Meanwhile on board flight 311, the pilots discussed the minimum altitudes in the area, trying to make sure that an altitude of 11,500 feet would keep them clear of terrain on the way back to Romeo. The plane rolled level once again, now on a heading of 025˚, or northeast — almost exactly the same direction they were traveling before Captain Suttimai started his series of right turns. To air traffic control, he reported, “We are heading 025, maintain 11,500, we like to proceed to Romeo to start our approach again.”
Whether Suttimai purposefully chose a heading of 025, and if so why, would later become a subject of considerable debate. This heading would take the flight not south toward Romeo, but north, toward the vast bulk of the Himalayas. It was hard to imagine he would choose this heading on purpose, and yet he read it out to air traffic control without a second thought. Some would later argue that he must have been aware of where he was going, but as for why he would do this, answers are elusive. The more compelling argument seems to be that he had somehow become disoriented.
One possibility is that he was mixing up his headings and his radials. When flying directly away from a VOR, an aircraft’s heading is the same as its radial of the VOR — i.e., a plane flying outbound on the 202-degree radial of the VOR will also be flying on a heading of 202 degrees. But if one is flying toward the VOR, as one would during an approach to land, the heading and the radial are reciprocal. Therefore, while performing the Sierra approach, an aircraft is on the 202-degree radial of the Kathmandu VOR, but its heading is 022 degrees, or 180 degrees off from the radial. Before all the confusion, during the approach briefing, Captain Suttimai had accidentally referred to the “022 radial” instead of the “202 radial,” apparently switching the heading and the radial, although he later referred to the 202 radial correctly. So did he simply suffer a mental lapse, somehow getting the numbers backwards in his head? Did he accidentally input the 022 radial instead of the 202 radial on his instruments, and then blindly select whatever heading would take him there? Given that he was an experienced pilot who worked with these subjects every day, it seems hard to imagine. And yet, if this was the case, then his actions make some sense: after all, a heading of 025 degrees would put them on course to intercept the 022 radial of the Kathmandu VOR, and it would explain why Suttimai was able to read off their heading without apparently realizing that it was not the heading he wanted. Still, it somehow must not have registered that 025 was a northeasterly heading that would take them straight into the Himalayas, or else he surely would have reconsidered.
Following Suttimai’s report to air traffic control, the controller asked to confirm their altitude, then reiterated, “Maintain 11,500, and go ahead your intention.” He didn’t seem to notice Suttimai’s remark that they were flying on heading 025.
“We like to proceed to Romeo, we got some technical problems… concerned with the flight,” Suttimai replied.
“Thai 311, proceed to Romeo, maintain 11,500,” the controller said again.
One minute later, at 12:41, the controller called again to ask for the flight’s DME.
“We are five DME from Kathmandu,” Suttimai replied.
This was not what the controller expected. If the flight had turned around and flown back toward Romeo, then their DME should be increasing. After all, their last report was at 9 DME, and in the four minutes since, the flight should have been able to cover 20 nautical miles. Suspecting that he had misheard, the controller asked, “Confirm two five DME?”
“Five!” Suttimai emphatically answered. “Zero five!” Because the plane had made a 360-degree orbit before resuming flight in the same direction as before, it was not 25 nautical miles south of the VOR, but 5 nautical miles north of it.
The controller, however, did not have the words to express his confusion. “Five DME, roger, Thai 311, maintain 11,500, report over Romeo,” the controller said.
“Report over Romeo, 11,500,” First Officer Boonyayej acknowledged.
Meanwhile, it seemed Boonyayej was trying to program their course back to Romeo into the plane’s computerized Flight Management System, or FMS. The FMS featured a handy navigation display which highlighted waypoints in the area ahead of the airplane, and which could be programmed with a desired course by selecting additional waypoints from an internal database. By simply selecting Romeo from a list and pressing the DIR TO (“direct to”) button, they could command the autopilot to fly the plane to Romeo, wherever it was. But because the display only showed waypoints in front of the plane, not behind it, Romeo was not on the screen, nor were any other waypoints for that matter, as none had been established in the area north of Kathmandu — normally, planes simply do not go there.
To his first officer, Captain Suttimai said, “Please find Romeo for me.”
Boonyayej typed in “Romeo,” and a list of waypoints called Romeo appeared on the screen. Boonyayej asked if they wanted the one located at a latitude of 27 degrees north; Suttimai checked his chart and confirmed that this was the correct waypoint. Boonyayej selected it and pressed DIR TO, expecting it to appear on the screen with a white line from the aircraft’s position to the waypoint. What it actually showed is not known, but if everything was working correctly, then it would have displayed a sharp curve back the way they came and out of the field of view. If Captain Suttimai knew he was flying north, this should have been what he was expecting. But his reaction indicates that the result was, in fact, unexpected. And so, instead of pressing “execute” and letting the autopilot fly them back to Romeo, he decided to keep trying until he could get Romeo to appear where he thought it should be.
His next course of action was to try a waypoint located near Romeo, to see if it produced similar results. The waypoint they selected was called Simara, but when they entered Simara, the result was the same. “It doesn’t show Simara at all,” said Boonyayej.
Suttimai, meanwhile, was noticing another thing that was odd: their distance from Romeo. If he thought Romeo was close to the 16 DME point, then he would have expected the display to show a distance of 16 nautical miles or less with Romeo selected, but since Romeo was actually 41 nautical miles from the VOR, the number displayed would have been much greater. “Romeo is how many miles from here!?” he exclaimed.
“We get it, it works direct,” said Boonyayej. The line to Romeo was showing up, Suttimai just didn’t like where it went.
“Romeo radial two zero two, sixteen DME — wait, I will follow the line,” said Suttimai.
This answered some questions about Suttimai’s thought process, but raised at least one new one. First of all, it showed that he knew Romeo lay along the 202 radial of the VOR; that is, to the south. Second, it suggested that he thought Romeo was at the 16 DME point, seemingly confirming that he believed it to be the start of the approach. And third, he referred to following a line — but which line? Certainly it was not the line which appeared when selecting direct to Romeo, because if they had followed that line, they would have turned around. Instead, he might have been referring to the 022 radial of the VOR, which also may have appeared as a line on his display. If he had gotten himself turned around and was under the impression that he was flying south, then he might have thought this line was the 202 radial, and that following it would take him to Romeo even if he couldn’t get the waypoint to appear on the FMS.
Meanwhile, First Officer Boonyayej again selected Romeo and pressed DIR TO. “It’s coming — already coming direct,” he said.
“Wait, wait, we will return back, correct?” Suttimai said, in disbelief at the resulting indication. “We will return to Romeo again… can you punch in Romeo again.”
“Here’s Simara again, right?” Boonyayej said.
“Okay, we’ll return to our initial approach point again… punch in for position Romeo — load Romeo again,” Suttimai repeated.
All the while, flight 311 continued flying north at over 230 knots, headed straight for the high peaks of the Himalayas. Surely the pilots could not have realized this — if they had, they would have feared for their lives.
At 12:43, Captain Suttimai radioed air traffic control and reported, “Thai 311, 14 DME.”
This report with a new DME greater than the previous one assuaged the controller’s concerns — the plane was now proceeding away from the VOR, as he expected. “Thai 311,” he said, “maintain 11,500, report over Romeo.”
“11,500, report over Romeo, Thai 311,” Suttimai read back yet again.
At this point they were approaching 16 DME, where the Sierra approach begins, or rather would have begun, if they were on the right radial. That meant they needed to make a hairpin turn back the way they came, reversing course to begin the approach. This would normally be accomplished by turning slightly to the left, then making a sweeping 180-degree right turn, as shown above. In preparation for this turn, Captain Suttimai used the heading knob to adjust their heading slightly left, from 025 to 005, or almost due north.
First Officer Boonyayej, meanwhile, still couldn’t get the FMS to display what he wanted. “This thing fails again,” he commented.
At that moment, Royal Nepal flight 206 called the controller on the same frequency as Thai 311, and a back-and-forth conversation ensued for about 30 seconds, during which time the Thai pilots refrained from speaking. After the conversation ended, Suttimai said to Boonyayej, “Transfer it from my side,” suggesting that they use the data in his FMS.
“Yes, we had transferred it, but it is gone, same as before,” said Boonyayej. No matter which waypoints they entered, none would appear — again, because they were all behind the plane, and the display only showed an 85-degree slice of airspace in front of them.
Captain Suttimai decided to forget about the FMS; it was time to turn in. To the controller, he said, “Request visibility from Thai 311.”
“Standby for tower observation and visibility,” the controller replied.
Suddenly, some cue caught First Officer Boonyayej’s attention, and he was struck by an overwhelming sensation that something was wrong. Most likely, he had simply torn his eyes away from the FMS for long enough to glance at a compass, but we will never know for sure. In any case, he abruptly exclaimed, “Hey, we are going… we are going north!?”
His questioning tone might have failed to convey proper urgency, as Captain Suttimai seemingly believed that Boonyayej was asking when they would turn back to the north to begin the approach. “We will turn back soon,” he said. Keying his mic, he said to air traffic control, “Kathmandu, Thai 311, request right turn back to the airfield.”
Just as the controller started to reply, the plane’s ground proximity warning system, or GPWS, burst into life. “TERRAIN, TERRAIN!” the robotic voice called out. “WHOOP WHOOP, PULL UP! WHOOP WHOOP, PULL UP!”
“Level change,” Suttimai said, calmly ordering Boonyayej to set up the autopilot for a climb to a higher altitude.
“Turn back, turn back!” Boonyayej shouted.
“It’s false, it’s false!” Suttimai exclaimed, apparently still believing they were south of the field. The southern ranges didn’t reach the plane’s present altitude of 11,500 feet, but there were places where the terrain rose steeply enough beneath the flight path to impinge on the GPWS warning envelope; Suttimai might have thought this was the cause of the alarm. If so, it would explain his muted response. But the warning did not stop: “WHOOP WHOOP, PULL UP!” it repeated, over and over. “WHOOP WHOOP, PULL UP! WHOOP WHOOP, PULL UP!”
Unfortunately, the time for action had already passed. Seconds later, at just past 12:45, the pilots spotted the massive bulk of a mountain lurking in the clouds, directly in their path. There was just enough time for someone to shout, “Oh no!” And then it was over.
Still cruising at 11,500 feet, Thai Airways flight 311 plowed headlong into the side of a nearly vertical cliff at a speed of 425 kilometers per hour. The impact resulted in the instantaneous and total destruction of the aircraft and everyone on it, reducing both to shredded fragments in the blink of an eye. The plane’s remaining fuel atomized and ignited with a mighty flash, and then the riven debris began to fall back to earth, tumbling down the 70-degree face of the cliff and into a valley 800 meters below. Within seconds, all that remained was a black smear across the cliff face, and a field of wreckage strewn like snow across an alpine mountainside. So complete was the devastation that there was hardly anything left to burn.
When Thai Airways International flight 311 failed to respond to further radio calls, and then failed to arrive in Kathmandu, Nepalese authorities launched one of the largest search and rescue missions in the country’s history. Without any radar records to reveal the plane’s last known location, the rescuers had to base their search grid on the statements of the controllers, who reported that as far as they knew, the plane was south of the airport, and last reported its distance as 14 DME. Searchers set out to this area to look for the plane, and some sightings of smoke were reported in the area, but bad weather and darkness moved in before they could be properly investigated.
The following day, as searchers continued their fruitless efforts in the hills south of Kathmandu, a specially appointed commission of inquiry was already sitting down to listen to the air traffic control tapes in the hope of finding some clue as to the location of the airplane. The eureka moment came when they heard Captain Suttimai report his heading as “025,” a detail which the controllers themselves seemed to have missed. The search area was immediately expanded to cover the area north of Kathmandu, where there were few roads and even fewer means of communication. As rescue teams entered the area on foot, following narrow trails between villages clinging to precipitous mountainsides, they received word that people in the town of Gyangphedi had heard the sound of a plane, followed by a loud explosion, on the day of the accident.
This clue narrowed the search area to a particular valley, where a more comprehensive mission was organized the next morning. The efforts quickly paid off, and at midday on August 2nd, 48 hours after the crash, the wreckage was spotted almost simultaneously by teams on the ground and in the air. The crash site was found on the side of a descending ridge of a 16,000-foot (4,800-meter) peak in the Langtang National Park, deep in a steep-sided valley surrounded by inhospitable terrain. The impact point itself was inaccessible; only the wreckage field below the cliff could be reached on foot, and even this required navigating treacherous slopes choked with razor-like bamboo tussocks. Furthermore, it was obvious that none of the 113 people on board had survived, and now their remains were scattered throughout the area, mixed in with dirt, plants, and bits of metal.
The investigation into the crash proved to be one of the most physically demanding on record. As experts from Nepal, Thailand, and France, home to Airbus, converged on the scene, they found that the only ways to get to the site were to trek five hours round trip from a base camp in Gyangphedi, or to fly by helicopter to a makeshift landing pad at 12,500 feet (3,800 m) and then scramble 1,000 feet (300 m) down a slope to the wreckage. The high altitude and high level of physical exertion took their toll — a deadly one, in fact. Days into the mission, 62-year-old British test pilot Gordon Corps, who was on scene to represent Airbus, was attempting the trek from base camp to the crash site when he was struck by severe altitude sickness. He soon collapsed and died as he was being rushed to safety.
In light of the conditions, it proved impossible to recover the majority of the debris; instead, investigators went home with little more than the remains of the victims and the two flight recorders, which they hoped would reveal the cause of the crash. In the end, what they did reveal was perhaps not so clear-cut — but they did provide a detailed look at the baffling sequence of events which caused flight 311 to slam into a mountainside 40 kilometers north of Kathmandu, when it should have been far to the south.
The basic sequence of events, revealed by the black boxes, built up slowly through a series of compounding miscommunications. Everything was initially normal as flight 311 began its descent and prepared for the straight-in Sierra approach to runway 02, and despite a momentary disruption when the visibility appeared to drop below the minimum, the plane remained on course to land until it was about 23 nautical miles from the airport, when the flaps failed to properly extend. The pilots spent about 90 seconds troubleshooting the issue, during which time they remained at 10,500 feet instead of continuing to descend. As a result, they ended up above the proper descent profile, and were forced to discontinue the approach to try again. Captain Suttimai seemed to think that this required him to fly back to Romeo, which was not the case. Nepalese investigators believed he misread his approach chart and thought the approach began at Romeo, while Thai investigators argued that Thai Airways pilots were taught to start preparing for the approach at Romeo, and it would have been natural for him to return there. Nevertheless, some of Suttimai’s later remarks, which implied that Romeo was at 16 DME when it was not, appear to support the Nepalese view that he was mistaken as to its location.
At this point, Suttimai’s belief that Romeo was the start of the approach and that he needed explicit permission to turn around caused a misunderstanding between the crew and air traffic control, which was then exacerbated by a lack of English proficiency on the part of the controllers. Although they did not understand Suttimai’s request, their language skills were so limited that they were reduced to repeating the same clearance over and over, without directly addressing the captain’s questions or explaining the source of their confusion. In an English speaking country, a controller might have said, “Sir, you may turn whenever you want, you are clear back to the start of the approach by any route,” but the controllers in Kathmandu apparently possessed no such ability.
It is also worth noting that as time went on, Captain Suttimai assumed more and more of the radio duties, even though he was the pilot flying, and handling the radio was supposed to be First Officer Boonyayej’s job. He most likely did this because he was unsatisfied with Boonyayej’s ability to extract the answers he wanted from air traffic control, but he proved no more effective than the First Officer had been. Instead, he found himself doing the jobs of both flying and non-flying pilot, while flying in clouds in a mountainous area without radar coverage and without any understanding of his present clearance. His workload was immense, and conditions were ripe for a mistake.
After four unsuccessful attempts to request a left turn, which the controllers were not even authorized to grant, Captain Suttimai made the unilateral decision to turn to the right instead. Investigators speculated about why he chose to change the direction of the turn, but the best explanation they could come up with was that he was basing his maneuver off of the official missed approach procedure, albeit 5 nautical miles early. However, in his book “Air Disaster: Volume 3,” Macarthur Job rightly points out that what Suttimai should have done was continue straight at 10,500 feet until reaching the VOR, then follow the missed approach procedure to a T, climbing out on radial 291 before holding at the Whiskey waypoint. The flight could then have remained in the prescribed holding pattern at a known location and altitude while the crew tried to work out their situation with air traffic control. Instead, Suttimai forged ahead into undefined airspace without articulating a clear plan for what would come next. This set the stage for his greatest error of all: while making the turn back to the right, he simply kept rotating the heading select knob until the plane had turned a full 360 degrees and rolled back out on the same heading as before.
Investigators spent many long hours debating among themselves why he did this. One theory was that he was distracted by his continued efforts to figure out his clearance with air traffic control, and kept mindlessly rotating the knob every time he felt the plane level off, not realizing that they were turning too far. Another theory was that he wanted to fly northwest to intercept the 291-degree radial and join the official missed approach procedure, but overdid it. Thai investigators, on the other hand, argued that the turn was completely intentional, although it is hard to imagine why. This author at least can think of no conceivable reason why Captain Suttimai would have knowingly wanted to fly a northeasterly heading into the mountains at 11,500 feet, even if he thought he would receive clearance to go back to Romeo soon. The area north of Kathmandu had a minimum sector altitude of 21,000 feet, and the pilots had previously been quite cognizant of the various minimums in the area, so it is hard to imagine that they were unaware of the 21,000-foot limit, or that they didn’t care about it. It makes much more sense to believe that when Captain Suttimai made the 360-degree turn, he thought he was only turning 180 degrees, and was so distracted that he didn’t notice this gross overcorrection.
If, in his mental model, Suttimai had turned only 180 degrees, not 360, then he would have believed he was flying south, away from the high terrain, explaining his lack of concern. Furthermore, once this belief was established, confirmation bias would have caused him to reject cues which suggested he was actually flying north. For instance, he did not appear to register that his heading of 025 was incorrect — he probably just read it off without thinking. Furthermore, he seemed to be aware of a line on his navigation display, which may have been the 022 radial of the VOR, but if he thought it was the 202 radial, then it would appear to him that he had selected the correct heading.
All things considered, these were errors which a trained and experienced pilot like Preeda Suttimai could only have made in the absence of proper situational awareness. This loss of awareness occurred due to the high workload he had placed upon himself, while communicating in a foreign language with an apparently unhelpful air traffic controller and simultaneously attempting to perform complex maneuvers in low visibility.
This loss of awareness placed flight 311 on a collision course with a mountain, but an accident was not yet guaranteed. The flight in fact continued north for over five more minutes and could have turned around at almost any point without hitting anything. But to rectify the situation, the pilots would first have needed to recognize that they had lost situational awareness in the first place, and this critical step never happened. Instead, both pilots became narrowly focused on trying to make the FMS show the waypoints and routes that they wanted. The final minutes of the flight were consumed by their fruitless efforts to make Romeo appear on the display, when it could not in fact appear, since it was behind them. It is not certain that the two FMS displays worked correctly throughout this period — in fact, it was known that malfunctions could sometimes occur if both pilots entered information using their separate keypads simultaneously. But some comments made by the crew appear to be consistent with an FMS display showing an arc back the way they came, which would suggest normal operation. In all likelihood, even if the displays were working properly, the results of selecting “Romeo” were sufficiently incongruous with their expectations that they believed a problem existed.
The prudent thing to do at this point would have been to revert to more straightforward methods of navigation — their VOR receivers, automatic direction finders, and magnetic compass. These surely would have revealed that they were not where they thought they were. But by the time First Officer Boonyayej finally looked at an analog instrument for long enough to notice that they were flying north, only 30 seconds remained until impact. If they had reacted to this discovery by immediately executing a steep, climbing turn, they might have just barely missed the mountains — emphasis on “might.” But the mountains were so high, and their sides so steep, that within seconds the plane entered an unrecoverable situation, where there was neither enough room to climb nor enough to turn around. And by the time the GPWS activated 17 seconds before impact, it was certainly much too late.
This sequence of events highlighted a number of key decision-making pitfalls that humans often encounter in complex situations. In general, we tend not to recognize when the level of complexity has exceeded our ability to keep track of all relevant parameters, especially if the complexity builds up slowly over time, as it did on flight 311. In such cases, we tend to lurch from one problem to the next, focusing on each in turn, without recognizing that the situation itself has become a problem which is beyond our ability to resolve. The correct action, then, is to abandon the situation. On flight 311, this would have meant climbing to a high altitude, picking a southerly heading, and getting as far away from any mountains as possible before starting over with a clean slate. It certainly did not mean getting bogged down trying to troubleshoot a computer. In fact, the best advice in a situation like this is to ditch the computers, ditch the automation, and fly by raw data until you can confirm with your eyes that you are not in danger. But to do this, one needs to be trained to recognize the circumstances in which it is necessary, and these pilots evidently were not.
Much could be said about their lack of crew resource management, or the tower controller’s inexperience (he had been on the job for 9 months), or the controllers’ remarkable incuriosity toward the increasingly strange reports from flight 311. Indeed, no one did a particularly good job that day. But focusing on specific mistakes does not help us learn how to avoid similar situations, which involve different errors, different miscommunications, and different problems, but the same fundamental circumstances that cause a complex situation to spiral out of control. The lesson of flight 311 is that one must learn to recognize such a “fog of confusion” from the inside, because as soon as one knows it’s there, it’s easy enough to escape.
Still, this is easier said than done. As soon as the official report was released, Thai Airways flight 311 became a case study on situational awareness in training for airline pilots around the world, including at American Airlines. Three years later, however, two American Airlines pilots who had recently studied flight 311 were on approach to Cali, Colombia, when they flew their Boeing 757 into a mountain, killing 159 people under very similar circumstances. They too were flying amid high mountains to an airport without radar; suffered a miscommunication with a controller; unknowingly turned off course while in a high-workload situation; became overly focused on their FMS; and descended into a fog of confusion from which they never emerged. Had the pilots recognized these similarities before the sudden activation of the GPWS, American Airlines flight 965 might not have crashed.
There is of course another way to approach the problem, which is to put in place technological safeguards which will prevent a fog of confusion from developing in the first place. The necessity of this approach was made plain just two months later, on the 28th of September 1992, when tragedy struck Nepal a second time. This time there was no fog of confusion, only a simple error in an unforgiving environment. As Pakistan International Airlines flight 268, a regular flight from Islamabad, lined up for the same Sierra approach to runway 02 at Kathmandu, the pilots apparently misread their approach chart and began to descend one step out of phase with the proper approach profile. At each DME point, they descended to the altitude prescribed for the following point, and so on, until the Airbus A300 abruptly slammed into the top of an 8,000-foot mountain south of the airport. All 167 people on board were killed in what remains Nepal’s worst air disaster.
This crash, and indeed the Thai airways crash as well, could have been prevented quite easily if Tribhuvan International Airport had been equipped with radar. One plane was off course, the other was too low, but either would have been apparent on radar. In the case of flight 268, an astute controller did pick up on a problem when the Pakistani pilot reported “10 DME at 8,200 feet,” which he knew was too low, but even though he asked the flight for an immediate altitude check, it was already too late. Radar, on the other hand, would have revealed the discrepancy much earlier.
These twin disasters in 1992 ultimately led to the installation of modern radar at Tribhuvan International Airport in 1997. Due to the steepness of the surrounding terrain, the airport still does not have an instrument landing system, but it does now offer an RNAV approach in addition to the old VOR/DME approach, allowing flights to navigate to the runway using GPS. These improvements have undoubtedly made it safer to fly to Nepal, and with the country’s tourism industry having boomed over the last 30 years, resulting in greatly increased air traffic, the extra safeguards have probably prevented additional accidents.
Despite everything, however, flying in Nepal today is still considered unsafe. The country maintains one of the worst safety records in the world, particularly among its domestic airlines, although international carriers have not escaped disaster either — in 2018, for instance, 51 people died when a Bangladeshi airliner careened off the runway on landing at Tribhuvan and burst into flames. That said, the crash was hardly Nepal’s fault, as the pilot suffered a mental breakdown while on final approach, leading directly to the accident. Much more concerning for anyone hoping to travel to Nepal is the recent crash of Yeti Airlines flight 691, which abruptly plunged into a gorge on approach to Pokhara on January 16th, 2023, killing all 72 people on board. Once again, infrastructure may have been involved: in fact, the destination airport had just opened two weeks earlier, and an official procedure for the approach chosen by the crew had not yet been published, a concerning discrepancy which the investigation will hopefully elucidate.
For those of us who have no plans to fly to Nepal, the more useful lesson is to learn when to back out of a complex situation. One does not need to be a pilot to find value in this. If you’re driving down the motorway, you’ve missed your exit, your phone is ringing, and the check engine light is on, consider pulling over to deal with each problem one at a time. The pilots of flight 311 found themselves in a situation like this, but they kept driving. The question is: will you?
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