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On the 26th of April 1994, a China Airlines Airbus A300 was on final approach to Nagoya, Japan when it climbed steeply, stalled, and plunged suddenly to earth, slamming belly-first into the ground beside the runway with a great burst of flame. Although rescuers rushed to the scene, the aircraft had disintegrated utterly, and from the shattered fuselage just seven of the 271 passengers and crew would emerge alive.
The crash turned out to be the bleak apotheosis of a prior series of near misses involving unfavorable interactions between humans and automation on Airbus A300 flight decks. In each case, some seemingly minor trigger escalated into a complete loss of control because the pilot and the autopilot began fighting each other, only for the human pilot to win a pyrrhic victory, leaving the aircraft in a dangerous and precarious configuration. The extent to which these events were connected, as well as the allocation of responsibility, became major subjects of debate among experts assigned to the case, and because no accident ever has a single cause, it would be equally inappropriate to say that there was one right answer. But the story of China Airlines flight 140, and the final 100 seconds in which it went awry, nevertheless hold valuable lessons as a sobering example of the fatal feedback loop that can develop when pilot and plane find themselves on radically different trajectories.
Between 1985 and 2007, Taiwan’s flag carrier China Airlines maintained a wide-body fleet consisting primarily of Boeing 747s for its heaviest routes and Airbus A300s for the next tier below. In service since 1974, the A300 was Airbus’s first production aircraft and the first ever wide body airliner with only two engines, pioneering a format that has since come to dominate the long-range market. China Airlines began experimenting with the model in the second half of the 1980s with the purchase of two A300s, followed by two more in 1990 and 1991, for a total of four; a major buying spree between 1996 and 2000 would ultimately expand its cumulative fleet to 18 (although the first several had already been retired or written off by the time the 18th was added). Despite their long-range capabilities, China Airlines most often used the A300s on high-volume routes within East Asia, where soaring demand allowed them to reliably fill the 264-seat airplanes on flights as short as two hours.
In 1994, one of four A300s in China Airlines’ fleet was B-1816, purchased in February 1991. At just over three years old, it was still early in its lifespan when, on the 26th of April, 1994, it was called upon to operate China Airlines flight 140 from Taipei to Nagoya, Japan, a metropolitan center southwest of Tokyo that is home to some 10 million people. The evening flight that day was almost full, as every economy class seat had been sold, leaving only a smattering of vacant seats in business class; in total, 256 passengers boarded. Also scheduled to fly that night were 13 flight attendants and a cockpit crew of two, consisting of 42-year-old Captain Wang Lo-chi and a young, 26-year-old First Officer, Chuang Meng-jung. Captain Wang had a respectable 8,340 hours of experience, including 1,350 on the A300, but the bulk of his career — over half — had been spent flying Douglas C-47 Skytrains for the Taiwanese Air Force. His First Officer had no such background: his total flight time was about 1,600 hours, including about 1,000 on the A300, and he had come up through China Airlines’ in-house training program with no prior military or airline experience.
At 16:53 local time (17:53 Japan time), flight 140 departed Chiang Kai-shek International Airport in Taipei and climbed to its cruising altitude of 33,000 feet, with an estimated total en-route time of two hours and 18 minutes. First Officer Chuang was at the controls, which is normal practice in order to help first officers gain experience, and Captain Wang provided him with a nearly constant stream of handy tips and suggestions intended to improve his flying, although he was not an instructor. Nevertheless, the continuous instruction did not distract the crew from their normal duties, and First Officer Chuang completed a normal descent briefing before they were cleared to begin descending into Nagoya at 19:47 Japan time. Captain Wang thereafter continued to provide advice on descent and landing techniques, ranging from how to deal with optical illusions on touchdown to the benefits of flying the final approach manually. “The more you fly and practice, the better you can fly,” he commented.
Amid clear weather with good nighttime visibility, the descent proceeded normally, and Captain Wang made it clear that he expected First Officer Chuang to see the landing through himself using the advice he had given up to that point. “You do it by yourself,” he said. “I will not bother you. Don’t ask me, do it yourself. Make decision. I will remind you just before the situation reaches the point that I cannot cover.”
“Yes,” Chuang agreed.
“You do it by yourself, okay?” Wang repeated.
“Yes sir,” said Chuang.
Continuing the descent, flight 140 successfully achieved the speeds asked of it by air traffic control, aligned with the runway, and commenced an instrument landing system (ILS) approach to runway 34. Both pilots commented on wake turbulence from preceding aircraft, but the bumpiness did not rise above the level of a minor annoyance, and at 20:11 they successfully intercepted the glideslope down to the runway. At that point, First Officer Chuang disconnected the autopilot to fly the approach manually, following the flight director overlay on his attitude indicator, which displayed the control inputs needed to keep the plane on the glideslope. The flight was on course and properly configured, so he had no difficulty hitting the bullseye, and by 20:12 they were number one in line and cleared to land. The pilots completed the landing checklist and lowered the landing gear, and the flight attendants made the final cabin announcement, expecting to be on the ground in less than two minutes. Ironically, it was at that very moment that the seemingly perfect approach began to go off the rails, because at precisely 20:14 and 6 seconds, at a height of 1,070 feet, First Officer Chuang accidentally triggered the go-around lever.
The go-around lever or switch, a feature of all modern airliners, is a small lever located on each of the A300’s thrust levers where it can be quickly and instinctively actuated in the event that the crew decides to abandon an approach and “go around.” On the A300, pulling the go-around lever, or “go lever” as pilots normally call it, immediately configures the autoflight systems for a swift climb away from the approach to a preset go-around altitude. If the autopilot is engaged, it will automatically hold the wings level and pitch up into a climb at a predetermined angle, and if it is not engaged, then the flight director will instruct the pilot to climb manually. In either case, the autothrottle system will also automatically engage and increase thrust in both engines.
It is unknown how exactly First Officer Chuang managed to trigger the lever accidentally, although the fact that it was inadvertent is certain. One possibility is that he bumped it with his fingertips while pulling the thrust levers back to reduce power; a second is that his hand struck the lever due to a jolt from the wake turbulence; and yet another hypothesis holds that he intended to press the nearby autothrottle disconnect button in order to control thrust manually, but accidentally hit the go lever instead. But whatever the reason, the result was the same: the autoflight system entered go-around mode, the flight directors began commanding a pitch up, and the autothrottle began increasing thrust in order to climb.
On the Airbus A300, the term “autoflight system” refers collectively to the autopilot, autothrottle, and flight director, which work in unison according to an overarching mode, which in turn engages sub-modes applicable to each component. (The A300 actually has two autopilots, but this had no effect on the following events, and the phrase “the autopilot” will be used throughout this article even though both autopilots were active during the accident flight.) During the ILS approach to Nagoya, the active mode was “LAND,” which tracks the instrument landing system and guides the aircraft all the way through the landing flare just before touchdown. In this mode, with the autopilot disengaged, the flight director was providing guidance to hold the glideslope and maintain alignment with the runway, and would continue to do so until just above the ground, at which point it would instruct the pilot to pitch up for the landing flare, enabling a smooth and successful touchdown. However, pressing the go lever immediately changed the active mode to “GO-AROUND,” which resulted in all of the effects described in the previous paragraph.
As thrust began to increase, Captain Wang exclaimed, “Eh, eh, ah,” and then said, “You, you triggered the go lever.” Evidently he had checked his flight mode annunciator and observed that the active mode was now “GO-AROUND.”
“Yes, yes, yes. I touched a little,” said First Officer Chuang
Because the A300’s engines are mounted below the wings, placing them below the center of gravity, an increase in engine power tends to cause a pronounced increase in pitch. Intuitively, you can imagine a 2-D airplane pinned to a wall, with the pin through its center of gravity. If you grab the 2-D plane below the pin and push forward, the plane will rotate about the pin and the nose will rise. (Conversely, if you grab the plane above the pin and push forward, the nose will drop.) Therefore, as the autothrottle increased thrust for the go-around, the plane began to pitch up, straying above the glideslope to the runway. Instinctively, First Officer Chuang countered by reducing thrust slightly and pushing forward on his control wheel to pitch down. But these actions were insufficient, and the plane started to level off at approximately 1,000 feet.
“Disengage it,” said Wang. With the autothrottle still engaged in go-around mode, it would quickly increase thrust again unless Chuang disengaged it or switched out of go-around mode; which course of action Wang meant is uncertain, but it was surely one of these two.
“Aye,” said Chuang. But he did not disengage the autothrottle, nor did he switch the autoflight system out of go-around mode.
Instead, moments later, at 20:11 and 18 seconds, Chuang re-engaged the autopilot. Why he did this is unknown. After all, the active autoflight mode at this point was still GO-AROUND, so engaging the autopilot would naturally cause the plane to automatically pitch up, which was not what he wanted. The most likely explanation is that Chuang pressed the “LAND” button in an attempt to re-engage LAND mode, and then immediately engaged the autopilot to help him regain the glideslope, but if so, then he must have been unaware that LAND mode cannot be selected while in GO-AROUND mode. The design logic is presumably rooted in the fact that once a go-around has been declared, no airline on earth provides pilots with the option of changing their minds, so there is no need to enable disengagement of GO-AROUND mode by pressing the “LAND” button. Furthermore, initiating a go-around usually causes the plane to rise above the glideslope, while LAND mode is designed with the assumption that the glideslope will be intercepted from below, not above, rendering it useless once a go-around has begun. Instructions for correctly cancelling GO-AROUND mode were contained in The Flight Crew Operations Manual (or FCOM), but at 1,000 feet on approach, nobody was going to be flipping through manuals, and Chuang had likely never needed to disengage GO-AROUND mode at any point in his career, so it seems plausible that he did not know how. However, there is no direct evidence that he ever pressed the LAND button, so we will never know his thought process for certain.
In any case, as soon as the autopilot was engaged, it began working to make the aircraft climb. However, First Officer Chuang was still pushing forward on his control wheel in an attempt to descend, and on the A300 pilot inputs on the yoke override the autopilot. However, in GO-AROUND mode, these inputs would not disengage the autopilot — a distinction that was about to become very important.
The problem here originates from the fact that all jet airliners, including the A300, have not one, but two means of pitch control. Jet pilots are intimately familiar with this concept and can skip this paragraph, but for everyone else, some basic background is helpful. When a pilot pushes forward on the controls, they are making inputs using the elevators, which are hinged to the back of the horizontal stabilizer. When the pilot lets go, the elevators are aerodynamically forced back into the neutral position, and the plane resumes whatever trajectory it was on before. However, on jets, pitch stability is controlled by tilting the horizontal stabilizer itself, which alters the neutral point for the elevators, and thus the stable pitch angle of the aircraft. Therefore, in order to establish a stable climb (for example), a pilot will pull up using the elevators to initiate the climb, and then move the horizontal stabilizer in a nose up direction so that the climb will continue after they let go of the controls. On the A300, the horizontal stabilizer can be adjusted automatically by the autopilot, or manually by the human pilot using either the manual trim wheels attached to the center pedestal, or the electric trim switches, located on the pilots’ control wheels. Inputs via these mechanisms move the horizontal stabilizer up for nose down, or down for nose up, via a jackscrew that resists aerodynamic forces, thus the stabilizer remains wherever it is set, unlike the elevators. This overarching stability control concept is referred to as “pitch trim;” adjusting the horizontal stabilizer is known as “trimming;” and the autopilot’s pitch trim control function is often called “autotrim.”
Now, returning to the cockpit of flight 140, recall that First Officer Chuang was pushing forward on his control wheel, pitching down using the elevators and overriding the autopilot. However, because the control wheel does not control the horizontal stabilizer, his actions overrode only the elevators and did not override the autotrim function, so even as he continued to push the nose down, the autopilot began trimming the horizontal stabilizer in the opposite, nose up direction in order to establish a climb configuration. Then, with the horizontal stabilizer increasingly forcing the nose up, Chuang had to apply even more nose down pressure using the elevators to prevent the plane from climbing, which in turn prompted the autopilot to trim even farther nose up, in a rapid feedback loop.
Recognizing that his control wheel inputs were proving insufficient, Chuang tried several times to trim nose down using the electric trim switches, but these are inhibited when the autopilot is engaged. If he wanted to override the autotrim function, he should have either disconnected the autopilot (ideally), or used the manual trim wheel, but he did not appear to recognize this, and so the situation only continued to escalate.
“Push down, push it down, yeah,” Captain Wang said, urging Chuang to return to the glideslope. “You, that [unintelligible] disengage the throttle,” he added, again suggesting that Chuang disengage the autothrottle to prevent thrust from increasing, although by this point the nose up trim inputs by the autopilot were the bigger issue. He almost certainly was not aware that the autopilot was engaged, because Chuang had not told him.
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