Falling from the Sky: The near crash of China Airlines flight 006

Damage to the tail of China Airlines flight 006 after its in-flight upset. (Code 7700)

On the 19th of February 1985, a China Airlines Boeing 747 cruising high above the Pacific Ocean suffered a malfunction of its number four engine. As the pilots worked to troubleshoot the problem, the plane slowly turned over until it suddenly entered a dive, spiraling down out of the sky as the confused and terrified pilots struggled to regain control. Pieces began to tear off the jet as it tumbled toward the sea. And then, just when it seemed all hope was lost, the captain regained his bearings and leveled the plane — after falling more than 30,000 feet in two and a half minutes, they pulled out just in time. The crew nursed the crippled 747 to an emergency landing in San Francisco, and the 274 passengers and crew disembarked with only a few serious injuries. The media hailed the captain as a hero. But what was the real story? A jumbo jet shouldn’t just fall out of the sky. After reviewing the evidence, investigators with the National Transportation Safety Board came to an uncomfortable conclusion: the pilots who ended the terrifying plunge had themselves caused the emergency.

N4522V, the Boeing 747 SP involved in the accident. (Andrew Hunt)

On many of its marathon trans-Pacific routes, Taiwanese flag carrier China Airlines used an unusual Boeing 747 variant: the 747 SP, a shorter, stubbier version of the jumbo jet intended for extreme long haul flights. One such route frequented by the slightly odd-looking 747-SP was the 11-hour trip from Taipei, Taiwan to Los Angeles, California, a popular route connecting the United States with East Asia. Designated flight 006, on the 19th of February 1985 there were 251 passengers and 23 crewmembers on board this flight, including a relief captain and a relief flight engineer who would step in during the middle of the trip so the others could rest. In command was veteran pilot Captain Ho Min-Yuan, who was assisted by First Officer Chang Ju-Yue, Flight Engineer Wei Kuo-Pin, Relief Captain Liao Chien-Yuan, and Relief Flight Engineer Su Shih-Lung.

The route of China Airlines flight 006. (Google)

The takeoff and cruise were normal for the first nine and a half hours of the flight. After flying the takeoff and climb, Captain Ho, First Officer Chang, and Flight Engineer Wei left the cockpit for an extended rest break, and were replaced by Relief Captain Liao and Relief Flight Engineer Su, who were the only crewmembers needed to pilot the 747 during cruise flight. But after two hours of fitful sleep, Captain Ho returned to the cockpit and sat in the first officer’s seat until he and the other primary flight crewmembers officially went on duty again three hours later. By the time flight 006 approached the coast of California at 10:00 a.m. local time, Ho, Chang, and Wei were all back in the cockpit, and their five hour rest break was rapidly fading in the rearview mirror.

Before the flight, the pilots had noted a couple of recent write-ups in the technical log describing an occasional problem with the number four engine not providing enough thrust. The issue, which maintenance workers had yet to discover, was incredibly minor: the main fuel control valve had worn down by one tenth of a millimeter. This slowed down the introduction of fuel into the engine, resulting in sluggish acceleration. The engine also tended to “hang” when accelerating above idle. This was due to the way the plane’s bleed air system worked. “Bleed air” is air that is diverted from the engines in order to pressurize the cabin, supply the air conditioning system, along with several other applications.

How bleed air works. (Aviation Stack Exchange)

While it is critical for maintaining pressurization on the 747, bleed air reduces airflow through an engine and negatively affects its ability to generate power. When the engines are at higher power settings, the bleed air system doesn’t have to divert as much air to maintain pressure, because the air is already more pressurized. But when the number four engine failed to accelerate as quickly as the other engines, it remained on the wider bleed valve (drawing more air) after the other engines had transitioned to the narrower valve (drawing less air), causing the already weak engine to assume a larger proportion of the total bleed air load. This would cause it to “hang,” unable to generate more power due to the disproportionate bleed air loading in combination with the reduced fuel flow, and it would fail to accelerate when commanded to do so.

At 10:11 a.m., as Captain Ho attempted to accelerate through some turbulence, the number four engine became stuck at a setting where it produced only about two thirds as much power as the other engines. Observing the thrust discrepancy, Flight Engineer Wei moved the number four throttle lever back and forth to see if he could “unstick” the engine, but its power output did not change. Although the engine was still producing some power, he informed Captain Ho that the engine had flamed out. The proper procedure for dealing with a hung engine was to close the bleed air valves on that engine before attempting to increase power, and this probably would have fixed the problem; however, Wei was allowed to perform the procedure without reference to the checklist, and while working from memory he forgot to perform this minor but critical step.

The low power produced by the number four engine, located on the outboard part of the right wing, began to result in a gentle yaw to the right as the higher thrust on the left wing pushed that side through the air faster. Because it was moving faster, the left wing also generated more lift than the right wing, resulting in a small but increasing right bank. The autopilot immediately began to compensate for this bank by applying left-wing-down commands using the ailerons. However, this only addressed a symptom — the desire to bank — without correcting the underlying yaw. Yaw must be countered using the rudder, but on the Boeing 747 SP, the autopilot was not allowed to use the rudder. Compensating for the pull of the weak engine was supposed to be the responsibility of the human behind the controls.

As the plane began to yaw (or drift) more sharply to the right, the increased air resistance and reduced engine power caused the aircraft’s speed to decrease, which was the first thing Captain Ho noticed after Flight Engineer Wei mentioned the engine flameout. Concerned about their falling speed — a serious issue at 41,000 feet — he instructed First Officer Chang to contact the Oakland air traffic control center and request a cruising altitude more suited to three-engine operations. He then instructed Flight Engineer Wei to restart the number four engine, even though procedures advised that above 30,000 feet there was not enough air to initiate combustion. Wei flipped the ignition switch to “flight start,” but needless to say, nothing happened.

Somewhere around this time, the number four engine really did flame out, as the amount of power demanded of it proved too great for the restricted fuel flow, causing combustion to cease. The yaw and bank to the right became heavier, forcing the autopilot to make even larger left-wing-down aileron inputs to keep the plane flying level. However, Captain Ho remained focused on their decreasing speed. The only indication of the increasing difficulty faced by the autopilot was the position of his control column, which was not part of his regular instrument scan. With no obvious indication of what was happening under the hood, the decreasing airspeed appeared to be the most urgent aspect of the situation. In an attempt to increase their airspeed, he disconnected the automated Performance Management System, which was coordinating with the autopilot to control the plane’s pitch, and applied a larger nose down input manually. However, this failed to halt the continued deceleration.

As a plane’s airspeed decreases, the effectiveness of the flight controls also decreases, due to reduced airflow over the control surfaces. Consequently, the autopilot had to apply ever larger left-wing-down aileron inputs in order to compensate for the drag of the windmilling number four engine attempting to pull the aircraft to the right. Eventually, the autopilot commanded 22.9 degrees left wing down, the most it was allowed to apply; however, the speed of the airplane continued to decrease, meaning even larger inputs were needed. Unable to apply anything more than this value, the autopilot could only maintain 22.9 degrees left wing down, and as a result, the plane slowly began to bank to the right. Captain Ho noticed a slight right bank on his attitude indicator, and First Officer Chang said, “We’re banking right,” but Ho’s main focus remained the airspeed.

Over the next minute or so, the 747 continued to roll until it was banked 45 degrees to the right. Still, Ho kept trying to increase their airspeed. Dissatisfied with the performance of the autopilot in this area, he decided to disconnect it and take full manual control of the airplane. He reached out and flipped the autopilot disconnect switch — a move which would have immediate and catastrophic consequences.

Simulation of the moment flight 006 first went out of control. (Mayday)

As soon as Captain Ho disconnected the autopilot, it stopped applying the 22.9-degree left wing down aileron input. With the plane already banking through 45 degrees to the right, the sudden removal of this counteracting force caused the plane to roll rapidly rightward, passing through 90 degrees and turning inverted. The 747 began to lose altitude at an alarming rate as it rolled clear over onto its roof, catching the pilots completely by surprise. In the confusion of the moment, neither Captain Ho nor First Officer Chang was able to comprehend what was happening; during the engine troubleshooting process, the plane had flown into a bank of clouds and all references to the horizon had been lost. As the plane embarked on an extreme aerobatic maneuver which neither pilot had experienced before, they became profoundly disoriented, and First Officer Chang exclaimed that the attitude indicators must have malfunctioned!

At the moment of the upset, Relief Flight Engineer Su had come up to the cockpit to assist Wei, and Relief Captain Liao was on his way up as well. The violent maneuver threw Liao against the floor of the forward galley; in the cabin, passengers screamed in terror as loose objects were suddenly hurled against one wall. Captain Ho instinctively pulled back on his control column, which put the airplane into a terrifying inverted dive. Massive G-forces crushed passengers down into their seats; Flight Engineer Wei was pinned against the center console. In a moment of panic, he cried out that engines one, two, and three had all lost thrust; however, he didn’t realize that Ho had set the throttles to idle in an attempt to arrest their rapidly increasing rate of descent. If he had looked behind him at the engineer’s panel, he would have been able to see that all electrical systems were functioning and the engines could not have failed, but he was physically unable to turn around.

The terrifying plunge continues. (Mayday)

In just 33 seconds, flight 006 plunged 10,000 feet as the pilots fought to regain control. As the plane approached 30,000 feet, it rapidly rolled level and began to pull up, subjecting the occupants to a soul-crushing, face-melting 4.8 vertical G’s. After executing a full 360-degree aileron roll, the 747 slowed down to less than 100 knots (185km/h), possibly stalling the airplane, before it rolled hard to the right and entered a second, even steeper plunge. Once again, the 747 rolled inverted, throwing unsecured objects and crewmembers in every direction. Captain Ho and First Officer Chang had no idea which way or how much the plane was banking; Ho swung his control column back and forth in a futile attempt to figure out how his inputs were affecting his aircraft. Behind them, passengers held on for dear life, convinced that they were about to die.

The plane begins to tear itself apart. (Mayday)

As the plane fell toward 19,000 feet, it pulled up steeply with a force in excess of 5 G’s, then dived again, hurtling toward the ocean at unbelievable speeds. At this point, the extreme stress of the maneuvers began to rip the airplane apart. G-forces pulled the main landing gear out of its mountings and slammed it against the inside of the gear doors, causing the doors to tear right off the plane. Huge chunks ripped off of both horizontal stabilizers, leaving mangled pieces of the elevators flailing in the wind. Inside the plane, the auxiliary power unit sheared off its mountings and slammed into an interior wall; in the cabin, overhead bins popped open and showered luggage onto the passengers. The left aileron cracked and lost one of its surface panels. Under such punishment, the 747 was on the verge of disintegrating in midair.

Overview of the plunge. (Tom Bearden)

Suddenly, at an altitude of 11,000 feet, flight 006 fell through the base of the clouds and into the open air. As the wind-tossed whitecaps of the Pacific raced up to meet them, the pilots realized that they could see the horizon, and within seconds Captain Ho regained his bearings. He rolled the plane level, accelerated the engines, and pulled out of the dive in a single, masterful maneuver. After falling 31,400 feet in less than three minutes, flight 006 leveled off at an altitude of just 9,600 feet, saving all 274 occupants from what had seemed like certain death. Because they had never failed, engines one, two, and three returned to full power immediately, and after a little finagling, so did engine four. In the cockpit, the mood could be described as a strange mixture of confusion and relief, and it was clear at first that the pilots had not fully processed the severity of the situation. They initially requested a higher altitude from air traffic control so that they could proceed to Los Angeles, as though nothing had happened! However, as flight 006 climbed back away from 9,000 feet, Flight Engineer Wei discovered that the main landing gear was down and locked and would not retract, and the number one hydraulic system had lost all its fluid. Although they didn’t know the true extent of the damage, it was clear that they would need to make an emergency landing, and the crew now accepted the Oakland controller’s suggestion that they divert to San Francisco.

Damage to the airplane, photographed after its arrival in San Francisco. (Code 7700)

Less than an hour later, China Airlines flight 006 landed safely at San Francisco International Airport, despite the severe damage to the elevators and other control surfaces. Of the 274 people on board, only two suffered serious injuries: a passenger who fractured their foot, and a flight attendant who badly strained his back. Another 24 people suffered minor injuries, but overall, they were lucky: the turbulence through which the plane had been flying meant that almost everyone had their seat belts fastened when the upset occurred. The outcome had to be considered miraculous, especially after seeing the aircraft. Most of the main gear doors were missing. The outboard 3.4 meters of the left horizontal stabilizer had been ripped away, and the damage on the right side was almost as bad: the outboard 1.5 meters were entirely gone, as well as areas stretching inboard up to 3.4 meters in the area aft of the center stabilizer box, including most of the right outboard elevator. It was clear to the National Transportation Safety Board that this had been an extremely serious incident which demanded a full investigation.

The NTSB’s series of computer-generated stills provides snapshots of the aircraft’s attitude during the initial plunge. (NTSB)

The investigation turned out to be more difficult than anticipated. Because the plane flew on for quite some time after the accident, the cockpit voice recording from the period of the upset had already been taped over by the time they landed. To understand how the crew lost control of the plane, investigators had to rely on the testimony of the pilots themselves, then cross-check it against the information on the flight data recorder. It turned out that the pilots’ understanding of the events was rather different from what actually happened. All three pilots told the NTSB that as they lost control of the plane, their attitude indicators failed, and that their three good engines all failed during the dive. However, there was nothing wrong with the attitude indicators; in reality, the plane was embarking on an extreme maneuver that the pilots were unable to spatially relate to the indications that they were seeing. And there was nothing wrong with the engines — someone had merely set them to idle. Having lost trust in their attitude indicators and without reference to a horizon outside the cockpit, the pilots flailed helplessly as the plane fell from sky, until they finally broke out of the clouds and almost instantly regained their bearings. As for what triggered it all, the NTSB found a single worn valve that caused the number four engine to hang at a low power setting — a fault so minor that the pilots barely needed to take any action to resolve it. How could such a tiny fault cause an experienced crew to lose control of their plane?

Captain Ho Min-Yuan explains the incident to reporters in San Francisco. (Der Spiegel)

By reconciling Captain Ho’s testimony with the FDR data, investigators realized that Ho was not aware of the increasing right bank caused by the asymmetric thrust until the plane had practically turned on its side. He had become singularly fixated on his falling airspeed, which he interpreted to be the most significant safety threat because the autopilot was successfully handling the pull to the right. However, this interpretation persisted even after the autopilot reached the limits of its control authority and the plane began to roll over. It was apparent that Captain Ho was “out of the control loop,” as the NTSB put it. He trusted the autopilot to handle the situation and wasn’t aware of the escalating inputs that it was having to make, because he did not have his hands on the control column. Thus, when he disconnected the autopilot, he was not prepared to take over the actions that it had been performing, causing an extremely rapid loss of control. The NTSB noted, however, that as a veteran pilot with over 10,000 flying hours and a good training record, he should have known to steer against the failed engine using the rudder. In fact, he had done so many times in the simulator without any issues.

An overview of the damage t the aircraft. (Mayday)

In trying to explain why Ho never took this critical step and subsequently failed to notice the plane’s increasing bank, the NTSB looked at two areas: fatigue, and overreliance on automation. Regarding the latter, investigators noted that during cruise flight, the job of a Boeing 747 pilot is to monitor the automation, not to fly the airplane. Studies have shown that humans are naturally poor monitors of automation, because it’s boring and does not actively engage our brains and bodies. As a result, when something goes wrong, the brain has to “wake up” before it can assess the situation and take corrective action. Therefore, when flying on autopilot pilots have increased reaction times to unexpected events, as opposed to flying manually, when a sudden change in the state of the aircraft can be instinctively assessed using physical cues transmitted via the control column. This was why Captain Ho failed to steer left after the engine failure: he was used to receiving a physical cue that the plane was pulling right, and in the absence of any such cue, he simply forgot to step on the rudder.

Damage to the landing gear bay doors. (Mayday)

The other possible contributor was fatigue. Within the 24 hours preceding the accident, the pilots had crossed numerous time zones and received only limited sleep. Captain Ho told the NTSB that he only slept two hours out of his five-hour break, and all the pilots agreed that the quality of sleep while on the plane is always very poor. Furthermore, the accident occurred at around 2:00 a.m. Taiwanese time, which was the time zone to which the pilots’ internal body clocks would have been adjusted. This internal clock is known as a circadian rhythm. The time during which a person is normally asleep is known as the “window of circadian low,” and being awake during this period can cause elevated levels of fatigue, decreased perceptiveness, poor peripheral vision, and increased risk of fixation. Considering that the accident occurred during Captain Ho’s window of circadian low, these symptoms could explain many of his actions. But at the time of the China Airlines accident, research into the effects of disruptions to circadian rhythms was only just beginning, and the NTSB declined to state that fatigue and a disrupted circadian rhythm were the reasons for his behavior. If the investigation had been conducted with access to modern knowledge about the effects of fatigue, this conclusion would almost certainly have been different — as would the NTSB’s decision not to issue any safety recommendations.

Damage to the horizontal stabilizer. (Der Spiegel)

After the accident, China Airlines repaired the aircraft and it returned to service later that year. After changing hands twice, it eventually flew for a private Christian missionary organization until it was retired in 2005. Unfortunately, the plane never got a dignified end: to this day it sits rotting in a field in Tijuana, Mexico.

The pilots of flight 006, and Captain Ho in particular, were simultaneously the heroes and the villains of the story. Somehow, Ho lost control of a perfectly good aircraft and nearly killed 274 people — but his remarkable recovery from the plunge using just 2,000 feet of altitude was an incredible feat of airmanship. No one knows how long the plane had until it would have become uncontrollable or broken apart in flight — a minute? Thirty seconds? Fifteen? Regardless of exactly how much time remained, Ho saved the plane. Most of the passengers still think of him as a sort of flawed hero — he caused the situation, but he also corrected it, which is far more than many pilots can say. However, it is worth noting that outcome bias might be in play: had there been clouds all the way down to the ocean surface, he probably would not have regained control, and our view of his actions would be very different. But regardless of whether we praise or condemn Captain Ho, his error should serve as a lesson and a warning to all airline pilots: no matter your skill as an airman, you must remain vigilant, because it can happen to you.

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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.