Shattered in Seconds: The crash of China Airlines flight 611

Note: this accident was previously featured in episode 34 of the plane crash series on April 28th, 2018, prior to the series’ arrival on Medium. This article is written without reference to and supersedes the original.

The cockpit of China Airlines flight 611 is recovered from the Taiwan Strait in 2002. (Aviation Safety Council)

On the 25th of May 2002, a China Airlines Boeing 747 abruptly disintegrated at 35,000 feet over the Taiwan Strait, killing all 225 people on board in the latest blow to the airline’s already troubled safety record. But while China Airlines was infamous for accidents due to poor airmanship, this one immediately struck experts as different. Indeed, everything was normal aboard flight 611 up until the moment it abruptly split in two, spewing debris across hundreds of square kilometers as the crippled remains of the plane plummeted toward the sea. And yet there was no explosion, no collision, nothing that would cause a 747 to simply fall apart. Unlocking the secret of its demise would require painstaking forensic analysis of the debris, piecing together when and how each ravaged strip of metal broke away from the plane, until finally investigators zeroed in on the source of the rot which had been eating through the aircraft’s structure until it could no longer hold itself together. There they would learn that China Airlines flight 611 was a slow-moving catastrophe, building incrementally for 22 years, out of sight and out of mind, until it reached an invisible tipping point, and at last with a great burst of violence the Boeing 747 was shattered in seconds.

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A CGI image of B-1866 striking its tail on the runway in 1980. (Mayday)

In July 1979, Taiwan’s flag carrier China Airlines took delivery of one of its first second-generation Boeing 747–200s, a brand new wide body jet which had just rolled off Boeing’s assembly line in Everett, Washington. Registered initially as B-1866 and later as B-18255, the plane, along with its sister ships, helped propel China Airlines into new and more distant markets, including its first flights to Europe. But this particular plane would not get off to an auspicious start.

In February 1980, just eight months into its service with China Airlines, B-1866 was landing in Hong Kong when the pilots flared for touchdown too aggressively, causing the tail to strike the runway in a shower of sparks. The plane rolled out safely and taxied to the gate, but damage was heavy. Long, ugly scratches had been gouged into the skin on the underside of the empennage, and a drain mast and a valve door had been destroyed. With such damage, pressurizing the plane would be unsafe if not outright impossible, and immediate repairs would be needed to ensure the integrity of the pressure vessel.

The location of the doubler placed over the tail strike damage. (Aviation Safety Council)

The only place where China Airlines could carry out such a repair was at its base of operations at Chiang Kai-shek International Airport in Taipei. Airline management quickly resolved to bring it there, and on the same day as the incident the plane was ferried, unpressurized and without passengers, from Hong Kong back to Taipei, flying at low altitude the whole way across the Taiwan Strait.

After arriving in Taipei, engineers assessed the damage, and the following day a temporary repair was carried out. Intended to render the plane airworthy only until downtime could be found to implement a more permanent solution, the temporary repair consisted of a series of aluminum doubler plates riveted over the scratched fuselage skin.

The skin of an airplane is a key structural element and the primary absorber of the stresses associated with pressurization. The stress placed on the skin when the cabin is pressurized is considerable, on the order of 9 pounds per square inch, and any section unable to withstand the repeated application of this force will quickly fail. If an area of fuselage skin is damaged, the most reliable way to prevent it from failing under pressurization loads is to cover it with a doubler plate — an extra layer of skin which redirects these stresses around the damaged area. Doubler plates are ubiquitous in aviation maintenance, and if you look closely at any plane with an extensive service history you will find at least a few, and sometimes dozens.

The temporary repair to B-1866 remained in place for about three and a half months, before the plane was taken in for more extensive servicing in May 1980. The damaged area was sanded down to remove any sharp edges, and the aluminum plates installed in February were swapped for a new, larger doubler. An inspector signed off on the work, and B-1866 was cleared to fly. No one at the workshop that day could have realized that they had just set in motion a chain of events that would not bear its bitter fruit until long after every one of them had retired.

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B-18255, the aircraft involved in the accident, seen here in 2000. (Tommy Lo)

For 22 years, B-1866 (later re-registered as B-18255) flew passengers around the world, becoming a mainstay of China Airlines’ fleet as countless other aircraft came and went. In 1980, it had been one of the newest planes at the airline, but by 2002, it was the oldest, the last of its kind. China Airlines still operated four other Boeing 747–200s, but all of them were configured for cargo, and B-18255 was the only one still carrying passengers, its sister ships having been superseded by later, more modern versions of the 747. And its time, too, was coming: by May of that year, China Airlines had finalized a deal to sell the plane to low cost carrier Orient Thai, where it would presumably operate for a few years yet before eventually finding itself in a boneyard in some forgotten stretch of desert.

On the 25th of May 2002, B-18255 taxied to the runway at Chiang Kai-shek International Airport and took to the air, bound for Hong Kong — the same route on which it suffered the tail strike back in 1980. The New York Times later reported that this was the plane’s last scheduled flight before it would be transferred to Orient Thai, although official confirmation of this assertion has proven elusive. Other than that, China Airlines flight 611 was to be an utterly routine flight, both for the crew, and for the 206 passengers, mostly Taiwanese citizens connecting through Hong Kong to visit mainland China. Joining them were 19 crewmembers, including a flight crew consisting of 51-year-old Captain Yi Ching-fong, 52-year-old First Officer Shieh Yea Shyong, and 54-year-old Flight Engineer Chao Sen Kuo. Normally some words would be said about their experience and backgrounds, but sadly none are needed — it suffices to say that they were perfectly fine pilots who never stood a chance.

The route and crash site of flight 611. (Google + own work)

At 15:07 that afternoon, flight 611 took off, contacted departure control, and turned southwest over the Taiwan Strait. Climbing steadily over the ocean, Flight Engineer Chao made a routine progress report to company operations, and at 15:16 the flight was cleared to ascend to its cruising altitude of 35,000 feet. The crew acknowledged the clearance and signed off. This would be their last communication with air traffic control.

On the flight deck, the pilots were not very talkative. For the most part they worked in silence as radio transmissions from other planes played over the speakers. At 15:24, somebody yawned. Two minutes later, Captain Yi called out “Two thousand,” indicating that they were 2,000 feet from their cleared altitude. Other than that, nobody said a word.

As the clock ticked past 15:27, an altitude alert chime sounded, indicating that they were approaching their selected altitude of 35,000 feet. The voice of a different flight crew on the radio echoed again in the background. And then, at 15:28 and three seconds, disaster struck like lightning from a clear sky.

This still from Mayday season 7 episode 1 approximately captures the appearance of the breakup. I chose not to show the full animation because it includes further breakup of the forward section which did not actually occur.

With a thunderous boom, a hole opened up in the tail of the aircraft, circling all the way around the fuselage until the entire tail section, including all the flight controls and a sizeable chunk of the economy class cabin, simply fell off. A tremendous burst of wind swept through the plane, tearing away anything and anyone that was not strapped down. Debris spewed rearward into the wide open sky as the crippled plane abruptly pitched down into an irrecoverable dive. For fifteen more seconds, a radar station in Xiamen, China continued to receive transponder returns from the plane, indicating a rapid descent, before powerful G-forces ripped off all four engines at a height of 29,000 feet, cutting power to the avionics. The pilots probably kept trying to fly, but we will never know for sure. Their efforts would have been futile, as the shuddering hulk of what had once been their plane spiraled down toward the sea, missing its engines, missing its flight controls, a gaping hole staring out into the infinite blue where the tail section used to be. The pilots probably never knew that everything aft of the wings was gone, and even if they had, it would have made no difference, for two and a half minutes later, what was left of flight 611 slammed into the Taiwan Strait and disappeared beneath the waves.

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The location of the debris field and actual flight path of flight 611. (Aviation Safety Council)

At air traffic control centers in Taiwan, the plane disappeared from secondary radar, while primary radar detected multiple objects falling from its last known position. When attempts to contact the plane failed, a massive search and rescue operation was launched, zeroing in on the suspected crash site, some 43 kilometers off Magong in the Penghu Islands.

Arriving at the scene, rescuers encountered the grim flotsam of a flight gone suddenly awry. Small pieces of debris lay floating on the surface, mixed with the bodies of victims, all of them outwardly intact but suffering from massive internal injuries, having fallen from a great height into the sea. Hope for survivors soon faded. Rescuers hauled in dozens of lifeless bodies left adrift on the strait, each one weighing the odds ever more steeply against finding the next alive. Within hours, they were forced to come to the conclusion that none of the 225 passengers and crew had survived.

: Officials carry a piece of flight 611 on shore in Magong. (CBS News)

The disaster immediately sent Taiwan into an uproar. Although some commentators advanced an early theory that the People’s Republic of China had shot down the plane, most Taiwanese knew that China Airlines itself was a far more likely culprit. In fact, the crash of flight 611 had made China Airlines the first and to date only carrier to suffer three different crashes with over 200 fatalities, a distinction which rightly placed it among the world’s most unsafe airlines. And to make matters worse, all of those disasters had occurred within the last eight years, beginning with the loss of a fully loaded Airbus A300 in Nagoya, Japan in 1994, followed by the crash of another A300 in Taipei in 1998. Each time, the airline pledged sweeping reforms, and yet like clockwork, another plane full of people would again be lost. How long would the carnage continue? Pressure on officials to find out would be intense.

: A wing is retrieved from the sea by the specialized salvage platform. (Aviation Safety Council)

It was against this background that Taiwan’s independent investigative body, the Aviation Safety Council, or ASC, launched the largest and most important investigation in its history. The task facing them was immense: the plane had clearly broken apart at high altitude, scattering wreckage over a vast area of ocean, and even on land, as some light objects such as documents and seat back safety cards were carried on the wind as far away as mainland Taiwan, where they drifted down from an empty sky, no doubt to the bafflement of onlookers. Therefore, before the ASC could even begin determining the cause, they had to deal with the equally enormous challenge of finding all of the wreckage.

After the initial recovery of debris floating on the surface, a series of salvage teams were brought in, first to recover what could be retrieved by divers from the shallow sea bed, followed by a specialized salvage platform which hauled up large pieces of the plane, including the sections containing the cockpit voice recorder and flight data recorder, which were found on June 18th and 19th, respectively.

Color-coded diagram of the three main wreckage zones, and which parts of the plane were found within them. (Aviation Safety Council)

As the salvage crews brought up one piece after another, ASC investigators sought to understand how the debris was distributed on the ocean floor. They soon began to sense a pattern. Except for the wreckage recovered from the surface, the remains of the plane fell into three distinct zones, designated red, yellow, and green, based on their distance from the flight’s final radar return. The first of these zones, known as the red zone, contained the widely scattered remains of the tail section and aft cabin, up to but not beyond the trailing edge of the wings. The wide distribution of the individual pieces of debris, and the field’s position close to the point of breakup, indicated that this massive section of the plane came off first, and was ripped away with significant violence.

The second zone, the yellow zone, was many times smaller, but contained the majority of the airplane, including the cockpit, forward cabin, and center wing section with both wings still attached. The concentrated nature of the wreckage here indicated that these sections fell in one piece. This assumption was further substantiated by data given to the ASC by authorities in mainland China, which revealed that the plane’s transponder continued to broadcast for 15 seconds after the initiation of the breakup. This would only have been possible if the cockpit, forward fuselage, wings, and engines all remained connected to one another for at least those 15 seconds, enabling continued operation of the plane’s electrical system.

Finally, there was the green zone, containing the debris which traveled the furthest. All four engines and parts of their pylons were found in this area. Aware that one of B-18255’s sister ships had crashed during a cargo run in 1991 following the in-flight separation of two of the plane’s engines, investigators examined the engine attachment points for signs of damage consistent with such a failure, but found nothing. All indications were that the engines had been ripped off by massive G-forces during the descent; after that, their relatively high density gave them additional momentum, which propelled them beyond the main wreckage zone. Later, ballistic analysis would suggest that the engines broke off after the main event as the plane descended through approximately 29,000 feet.

Flight 611’s cockpit, remarkably intact after its high-speed plunge, is pulled from the sea. (Aviation Safety Council)

Within the first weeks, other external reasons for the breakup were ruled out, including the possibility that the plane had been downed by an explosive device. Had there been an explosion, scorching and pitting would have been found on nearby items of wreckage, but none of the recovered pieces showed any signs of having been exposed to fire. Although early comparisons were made with the 1996 crash of TWA flight 800, another 747 which broke up in flight after the center wing fuel tank exploded, this theory was also ruled out after the fuel tanks were all found intact on the ocean floor. That left only one real suspect in the catastrophic midair disintegration: the structure of the fuselage itself.

The results of the ballistic trajectory study. (Aviation Safety Council)

In an effort to understand where the structural breakup began, investigators labeled every piece of wreckage (thousands in total), scanned significant pieces using a 3-D laser scanner, and fed the resulting data into a ballistic trajectory analysis model originally developed by the United States NTSB during the investigation into TWA flight 800. The results confirmed what was already suspected, namely that the initiating event began in the tail.

Evidence from the black boxes provided additional clues. Both flight recorders, which are located at the back of the plane, lost power simultaneously at 15:28:03, but the transponder continued to broadcast until 15:28:18, indicating that the failure occurred between the black boxes and their power supply, but not between the transponder and its power supply.

Additionally, the cockpit voice recorder had captured the first half second of a loud noise before cutting out. A spectral analysis of the sound showed that it captured a “precursor” vibration, transmitted to the microphone via the aircraft structure, followed by a much louder sound transmitted via the air inside the cabin. The fact that this latter sound was louder than the precursor indicated that it did not have to pass through the structure from outside the plane before reaching the microphone, and that in turn meant that the source of the noise was inside the pressurized portion of the aircraft — in other words, this was a failure of the pressure vessel itself.

This was further supported when investigators recovered several of the plane’s dado panels — vents in the floor of the passenger cabin which open automatically to relieve the pressure differential between the cabin and the cargo hold below it in the event of a rapid decompression of the cargo area. Witness marks on the vents showed that those in the front of the cabin were closed, but that some near the back of the cabin had opened before the crash, indicating that the failure originated below the floor level near the back of the plane.

A detailed overview of the relevant portion of item #640. (Aviation Safety Council)

At a dock in the nearby Penghu Islands, investigators carefully examined each piece of recovered fuselage structure, and in particular the fuselage skin, for some indication of damage associated with such a failure. What they were looking for was metal fatigue — the incremental breakdown of a metal component across many repetitive load applications. Signs of metal fatigue would include a flat fracture surface, as opposed to a ragged one, and the presence of numerous striations in the fracture surface, like the rings of a tree.

Necessarily, a plane shattered into thousands of pieces contains thousands of fracture faces, and nearly all of them will be jagged and uneven, indicating that the piece was forcibly ripped from the structure during the breakup. The piece that originated the breakup, by contrast, had probably failed slowly over time due to metal fatigue or corrosion. It would be there on the dock in Magong, weeks into the investigation, that investigators finally found what they were looking for on the edge of what had been labeled item #640.

Item #640 was a large section of fuselage skin and supporting structure from the lower right side of the tail, wrapping around to include part of the aft baggage door. It also contained the doubler plate which had been placed over the tail strike damage back in May 1980. And it was along the edge of this doubler, in the repaired skin lying underneath it, that they found exactly what they were looking for.

When investigators removed the doubler, they noticed that the damage from the 1980 tail strike was still present in the underlying skin. Over a large area more than two meters long and half a meter wide, hundreds of long scratches were visible despite a mechanic’s attempt to sand them down. Almost all of these scratches looked just as they did in 1980, having incurred no further damage. But investigators noticed one baffling problem: some of the scratches under the left edge of the doubler plate were outside the outermost row of rivets holding the doubler in place.

Locations of the fatigue cracking on the edge of the doubler plate. (Aviation Safety Council)

As described earlier, the purpose of a doubler plate is to redirect stress on the fuselage skin around a damaged region. This stress passes from the undamaged skin, through the rivets, and into the doubler plate. It’s not hard to see, then, why damage which is under the doubler plate but outside the outermost row of rivets would not actually be protected by the doubler plate. Any such damaged skin would have continued to bear the full pressurization load every time the plane took off, leaving it at risk of metal fatigue if the damage was sufficiently severe.

Indeed, this is what had happened on B-18255. The repair in 1980 had left some damage stranded outside the area protected by the doubler, causing metal fatigue to take root in the scratches. Fatigue initiated at numerous sites on the surface off the scratch, causing shallow cracks which grew infinitesimally deeper with every pressurization cycle — over 20,000 in total between 1980 and 2002.

This was something which metallurgists call “multiple-site damage” — cracking which originates in many locations, only to slowly link up over time to create a much larger degraded area. In total, fatigue cracks were found running under the edge of the doubler across an area 1.8 meters in length. Some of these cracks were connected, others were not; the longest continuous area of fatigue damage measured 38.3 centimeters, and the total length of all the fatigue cracks added up to 65.4 centimeters. The remainder of the 1.8 meter zone consisted of overload damage rather than fatigue, indicating that the series of fatigue cracks had linked up suddenly and violently.

The thin red line represents the extent of the crack along the edge of item #640. The thick red bars on the scale below show which portions of that crack were caused by metal fatigue. (Aviation Safety Council)

There was and still is some uncertainty as to whether this final “linking up” occurred on flight 611 or on one of the flights immediately before it. In their submission to the investigation, Boeing metallurgists noted a few points of evidence which suggested that the fatigued areas may have linked up to form a single crack between 1.80 and 2.36 meters in length before flight 611 even took off. Some of their evidence included rubbing, or fretting, marks on the underside of the doubler above the crack, which suggested that the entire 1.8-meter stretch may have opened and closed several times before the final failure, leaving telltale scrape marks where the doubler and the adjacent skin slid past one another. Boeing also noted faint markings in the fracture surface beyond the fatigued area which they believed represented signs of “quasi-stable” overload cracking — that is, that during the last few pressurization cycles, the growth of the crack ceased to behave like metal fatigue, but instead began jumping multiple centimeters at a time before hanging up, or stabilizing. This could have extended the length of the crack to 2.36 meters, at which point it would actually have emerged out from under the forward end of the doubler by the time flight 611 departed.

On the other hand, a metallurgist working for China Airlines contended that quasi-stable overload cracking was not a real phenomenon; that the faint striations probably represented places where the crack stopped momentarily during the final breakup, not before it; and that in any case there was no evidence that this growth occurred prior to the departure of flight 611. They also challenged the meaning of the fretting marks, arguing that these scrapes were too widely scattered and too fresh to be evidence of repeated opening and closing of the crack along its entire 1.8-meter length. In their opinion, the marks were probably made by motion in one direction only, as the two sides of the crack permanently drew apart from one another during the structural failure. In their opinion, the individual areas of fatigue probably remained separate until they suddenly linked up on flight 611, precipitating the breakup immediately.

The implication, largely left unstated, was that if the crack extended beyond the edge of the doubler before the crash, then it could potentially have been detected during the pre-flight walkaround inspection. In the end, however, it remains unknown whether this opportunity actually existed.

The actual entry in the major repair and overhaul record concerning the tail strike damage repair. (Aviation Safety Council)

The more important questions were why the repair was conducted this way, and why China Airlines never detected the problem over the following 22 years. But here again, there are conflicting theories.

Regarding the first question, investigators needed to find records explaining how the repair was conducted. But when they asked China Airlines to provide these records, they were met with an unfortunate surprise: the airline had almost none of the documents that were expected to exist. The main reference to the permanent tail strike damage repair in 1980 was an entry in the plane’s “major repairs and overhauls” log (shown above), which simply stated that the repair was accomplished on May 24th, 1980, that the damaged skin was cut out and replaced, that a doubler was placed over the cut-out area, and that the repair was accomplished in accordance the relevant section of the Boeing Structural Repair Manual, or SRM. But investigators already knew that practically everything about this entry was false.

For one, the damaged area evidently was not cut out and replaced — it was sitting right in front of them, having been pulled off the ocean floor at great expense, and the scratches from the tail strike were clearly still there. And that in turn meant that the repair was not conducted in accordance with the Boeing SRM. According to the manual, scratches on the fuselage skin could only be patched by a doubler if the distance between the scratch and any fastener, hole, or skin edge was more than 20 times the depth of the scratch. This criterion was clearly not met, as the scratches ran directly over numerous fasteners. In that case, the SRM clearly called for the damaged area to be completely cut out, replaced with a filler plate, and only then covered by a doubler. But this obviously had not been done, even though the entry in the log claimed that it was.

In this photo of the skin underneath the doubler, you can still see the longitudinal scratches caused by the tail strike. (Aviation Safety Council)

It is often repeated, rather incorrectly, that the failure to cut out the damaged skin is what led to the fatigue cracking. Here we should stop to note that this is not true — even with the damage still present, the fatigue probably would not have occurred, as long as the doubler was actually big enough to cover all of it. The single error which led directly to the accident was the failure to make the doubler big enough, leaving some scratches stranded outside the protected area. On the other hand, had the damaged area been cut out, there is no doubt that China Airlines engineers would have fabricated a doubler which was big enough, seeing as a smaller one would not cover the hole.

To try to add clarity about why the repair was done this way, investigators tracked down several retired engineers who had worked on the repair, in the hope that they might remember something, even after more than two decades. The ASC also managed to find the Boeing Field Service Representative, or FSR, who was stationed at China Airlines in 1980 to answer the airline’s questions about maintaining Boeing airplanes. According to the FSR, he knew about the tail strike, but was not informed about the permanent repair, nor would it have been common practice to inform him unless difficulties were encountered following the procedures in the SRM. His job was to liaison with Boeing to find an acceptable solution if the SRM procedures did not adequately cover a repair scenario. Because China Airlines never asked him to weigh in, he assumed that the repair was conducted according to the SRM.

The wreckage of the aft section was reconstructed on a frame. Item 640 is in the center foreground. (Aviation Safety Council)

On the other hand, a China Airlines engineer who oversaw the work gave investigators a different story. According to him, the maintenance department initially intended to conduct the repair according to the SRM, but ran into difficulty because the damaged area was too large to be cut out easily. As a result, a plan was developed to patch the damaged area without cutting it out. The engineer claimed that the department informed the Boeing FSR about this decision, but that no reply was ever received, which the engineers took to mean that Boeing had no objections. The FSR, for his part, said he never received any such notification. No records could be found which would corroborate either story, and no one who was interviewed could explain why the doubler was too small.

This lack of records was in itself a serious problem. Investigators couldn’t find paper evidence of a formal damage assessment, engineering diagrams of the proposed repair, or a work card listing the steps used to complete the repair. China Airlines explained that the lack of records was in part due to the way the repair was classified — despite an entry about the repair having been made in the “major repairs and overhauls” log, the repair was officially classified as “minor,” so records related to it were not required to be kept longer than two years. This classification was clearly incorrect, as common sense ought to dictate that any repair affecting the structure of the airplane is “major.” Still, many more records were missing than just those associated with this particular repair, a fact which China Airlines put down to inadequate record-keeping practices in the 1980s, as well as files being lost during relocations.

The largest recovered section of the tail contained part of the vertical stabilizer and all of the right horizontal stabilizer. (Aviation Safety Council)

Although the specifics of how and why have been lost to time, it was thus apparent that the airplane was dispatched for service with what was, in effect, unrepaired damage to the skin. Hidden behind the doubler plate, but not protected by it, scratches from the tail strike turned into fatigue cracks, which began to grow, slowly spreading across the skin for 22 years, until finally, the skin could no longer sustain the pressurization forces associated with normal flight. As flight 611 climbed through 34,900 feet, the crack suddenly leapt outward and upward across the left side of the fuselage, looped over the top of the plane, and met up with itself on the opposite side. With the skin no longer providing structural stability, the plane could not stay together. One after another, key structural members failed in overload, until the whole tail section simply broke away. The entire process couldn’t have taken more than three or four seconds.

The other big question, then, was why no one noticed the growing damage to the fuselage skin during the two decades leading up to the accident.

The most immediate barrier to detection of the fatigue cracking was obviously the doubler plate which covered it. That meant that the only way inspectors could see the cracks was by looking in from the other side while inspecting the bilge area in the bottom of the empennage.

The fatigue cracking on item 640 vs. normal fatigue cracking. (Aviation Safety Council)

But here investigators noticed something highly unusual about the way the fatigue cracks grew. Most fatigue cracks, having initiated at the surface of the skin, quickly penetrate the skin’s entire thickness and then spread laterally through the material. In contrast, the multiple-site damage under the doubler on B-18255 grew in a totally different fashion: the cracks started out very long but also very shallow, and then slowly increased in depth with every cycle, rather than increasing in length. Although the cracks did get longer over time, this was not their primary direction of growth.

The implication of this discovery was that the cracks may not have been detectable from inside the plane either, because they might not have penetrated the entire thickness of the skin by the time of the last inspection. In fact, many areas still hadn’t fully penetrated the skin by the time of the accident. The main exception was in the largest of the fatigue cracks, the one measuring 38 centimeters, which had achieved 100% penetration along most of its length. The question, then, was when it achieved this level of penetration. On this matter, there are two prevailing theories.

Pieces of the fuselage structure and cabin interior furnishings were found embedded in the tailplane, providing a window into the moment of the breakup. (Aviation Safety Council)

The theory advanced by Boeing metallurgists, and accepted by the ASC, was that it was not possible to determine when the crack fully penetrated the skin. By counting the striations on the fracture surface, it was possible to say that in various areas, the time from crack initiation to full penetration lasted anywhere from 2,400 to 11,000 flight cycles, but because the cracks stopped when they ran out of material to grow into, it could not be said when during the lifetime of the aircraft these cycles occurred.

On the other hand, the aforementioned China Airlines metallurgist argued that the crack had only reached 75% penetration at the time of the last inspection of the bilge area in 1998. The exact reasoning behind this determination was not entirely clear.

In any case, unable to prove that the cracks were not visible, the ASC did a deep dive into the various inspections which may or may not have detected the cracks. As it turned out, however, there were very few such opportunities.

The aftermath of the structural failure aboard Aloha Airlines flight 243. (Honolulu Star-Advertiser)

The problem was that structural inspections of aircraft at that time did not involve pulling off repair doublers to look underneath them — the system was built on the assumption that repairs would be conducted properly, and that even if they were not, the damage would become obvious before progressing to the point of failure. However, this assumption was challenged in the wake of the near crash of Aloha Airlines flight 243 in April 1988, an accident which was blamed on extensive fatigue cracking of the Boeing 737’s fuselage skin. In response, the United States Congress mandated that the Federal Aviation Administration launch a massive study on the maintenance of aging airplanes.

As a result of this program, during the early 1990s the FAA’s Airworthiness Assurance Working Group became aware of the issue of older aircraft flying with inadequate structural repairs dating to time periods with less stringent safety standards. To learn more, the group conducted a study of past repairs on 65 older airplanes that had recently been withdrawn from service, and found that only 40% of these repairs met modern standards.

These findings led the FAA to introduce the concept of a Repair Assessment Program, or RAP. The purpose of a RAP would be to provide specific timelines and procedures for inspecting the quality of past repairs, which represented a blind spot in existing inspection regimes. In 1997, the FAA announced its intention to require operators of certain older aircraft types, including the 747–200, to have a RAP, and the final rule, including detailed guidelines for how to create and run such a program, went into effect in 2000. Among its requirements was that any aircraft with over 20,000 flight cycles undergo a complete examination of all structural repairs before reaching 22,000 cycles, or within the next 1,200 cycles, whichever was later.

The doubler involved in the accident, seen 6 months before the crash. The brown streaks emerging from the side of the doubler plate were indicative of cabin air leakage. (Aviation Safety Council)

Although the FAA has no direct authority over airlines registered abroad, Taiwan’s Civil Aviation Authority strongly encouraged Taiwanese airlines to follow the regulations put in place in their fleet’s country of manufacture. As a result, China Airlines decided to implement a Repair Assessment Program for its Boeing 747–200s in May 2001. At that time, B-18255 had accumulated 20,400 cycles.

One of the challenges tackled by a Repair Assessment Program is the fact that every repair is unique, and the action required to assess it varies on a case by case basis. As such, the RAP contained an initial overview phase in which all structural repairs would be identified and documented, and provided guidelines to help airlines develop a plan for inspecting each repair individually.

In the case of B-18255, China Airlines decided to align the initial overview phase and the inspection phase with the plane’s scheduled C-checks. A C-check is a yearly heavy maintenance session lasting several days in which all major portions of the plane are inspected, including the structure. Because the plane would be on the ground with inspectors present, a C-check provided an ideal opportunity to conduct the type of work called for in the RAP.

China Airlines conducted the initial overview phase of the RAP when B-18255 went in for a C-check in November 2001. This overview revealed the presence of 31 doubler plates on the fuselage, including the one involved in the accident. Of these, only 22 had any accompanying documentation. That alone made it clear that this plane was in urgent need of a RAP inspection. Furthermore, one of the most problematic observations concerned the doubler which covered the 1980 tail strike damage. Photographs taken during the November 2001 overview, shown above, revealed the presence of brown streaks on the fuselage skin originating from underneath the doubler, a telltale sign that cabin air containing contaminants such as dirt and nicotine had been leaking from under the doubler, most likely for many years. This should have set off alarm bells in the engineering department, but it appears that no one there recognized at the time that these streaks could be indicative of a serious structural problem.

Having completed the initial overview phase, the repair inspection phase was scheduled for B-18255’s next C-check in November 2002, which would fall before the 22,000-cycle deadline. This inspection surely would have discovered the damage under the doubler plate. But the plane never made it to November — it broke apart on May 25th, six months before the next C-check.

Cleaning the bilge area during the corrosion inspection would have made it much easier to spot the crack incidentally. (Aviation Safety Council)

Other than this inspection which never happened, it was debatable whether there were any chances for China Airlines to have detected the crack. A detailed examination of every inch of the skin for fatigue cracks only occurred at scheduled D-checks, which were more extensive than C-checks but took place only once every 25,000 flight hours. During the latter part of its life, B-18255 was not being heavily utilized, so it hadn’t undergone a D-check since 1993, and the next one wasn’t due for some time yet. It was doubtful that the crack was already so large as to be detectable from inside the plane by 1993, so this probably did not present a meaningful opportunity to discover the damage.

The only other opportunity was when the plane went in for a Mid-Period Visit (MPV) check, a structural inspection half way between each D-check, in 1998. This check did not specifically include a fatigue inspection of the aft fuselage skin, but it did include a corrosion inspection of the inside of the bilge area, including the area where the fatigue cracks were found.

As mentioned earlier, China Airlines and Boeing disagreed about whether the main crack could have fully penetrated the skin by 1998. If it hadn’t, then the MPV inspection had no chance of detecting it. But even if it had, the odds of detection might have been long. For one, the inspection was aimed at detecting corrosion, and the inspectors were not looking for fatigue cracks. Secondly, maintenance personnel did not clean the bilge area before the inspection, so the crack could have been covered by dirt. Furthermore, the inspector did not use a magnifying glass, even though this was required. And finally, the lighting inside the bilge area was poor, with most illumination coming from a handheld flashlight. All of these factors stacked the odds against the inspector noticing the crack, assuming it was even there to be seen.

How another opportunity to inspect the bilge area was missed before the accident. (Aviation Safety Council)

Investigators noted that China Airlines should have conducted an additional inspection of the bilge area before the accident, but this was not done. Corrosion inspections of this area were supposed to happen every four years, according Boeing’s Corrosion Prevention and Control Program (CPCP); and since the first such inspection was conducted in 1993, subsequent inspections should have taken place in 1997 and 2001. Had such a check been conducted in 2001, the crack might have been so large as to be incidentally discovered during the corrosion inspection. But this opportunity was missed because China Airlines had tied certain elements of the CPCP, including bilge inspections, to the D-check and MPV check intervals, which were based on flight hours, not years. As such, the 1998 bilge inspection should have happened in 1997, and the next inspection, scheduled for late 2002, should have been conducted in 2001.

China Airlines had actually discovered this problem in the 1990s, and the Maintenance Planning department thought they fixed it by changing the C-check interval to every 12 months instead of every 13 months, ensuring that CPCP items tied to the C-check interval were actually being inspected once per year, as called for by Boeing. But this didn’t solve the problem for items connected to the D-check and MPV check intervals, a fact which was apparently overlooked due to a miscommunication between Maintenance Planning and Maintenance Operations.

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Investigators examine the cockpit of flight 611 after it was hauled from the sea. (Bureau of Aircraft Accidents Archives)

Taken in total, the ASC’s findings revealed that an improper repair, conducted during a time of less stringent safety standards, had been allowed to precipitate a failure decades later due to known blind spots in the system of structural inspections for aging aircraft. The solution to these gaps in the system was already being implemented in the form of the Repair Assessment Program, but it came ever so slightly too late to prevent the crash of flight 611. In that sense, the accident was the result of bad luck and poor timing more than anything else. Investigators found it hard to harshly criticize the conduct of China Airlines — while the original repair was indeed performed negligently, the airline had come a long way since then. Everyone involved in the repair had retired and almost every applicable maintenance procedure had since been revised.

Such a botched repair certainly wouldn’t have happened at the airline in 2002, but that didn’t mean that there wasn’t room for improvement. As a result of the accident, China Airlines overhauled its maintenance record-keeping process, improved its inspection procedures, and introduced a new Engineering Planning Department to improve oversight of complex engineering tasks. Taiwan’s CAA initiated a program of close cooperation with the FAA intended to enhance its institutional knowledge of problems related to aging airplanes. China Airlines, and indeed many other airlines around the world, began using the case of flight 611 to teach techniques for detecting hidden damage, such as fatigue cracking behind a doubler plate. The accident also prompted the FAA to create procedures which would help airlines identify past repairs which might be hiding serious structural damage, and increased industry awareness of the existence of depth-oriented fatigue cracks.

Investigators survey the cockpit after it was taken to storage. (Sam Yeh)

In its comments on the final report, the NTSB praised the ASC for conducting a thorough investigation and for issuing recommendations that would improve aviation safety around the world. Indeed, such an accident would be unlikely to happen today, thanks to the numerous programs which now help ensure that there are no blind spots in structural inspections of aging aircraft. The crack which brought down flight 611 may have found refuge behind a doubler plate, but today there is no part of a transport-category aircraft where a crack of such magnitude would be able to hide. And if such an accident does happen again, it will almost certainly be due to negligent inspection practices, not ignorance of the problem, as happened at China Airlines.

At the time of this writing, however, flight 611 represents a milestone in aviation safety, as both the last fatal accident for the once-maligned China Airlines, and the last fatal structural failure of a large commercial aircraft.

Members of the press view the reconstructed fuselage of flight 611. (Sam Yeh)

To go 20 years without a major crash due to structural failure is unprecedented in aviation history. However, to say that this modern safety record would not have been achieved without the crash of flight 611 would be disingenuous. The disaster over the Taiwan Strait was almost prevented, and need not have happened at all, if only the next inspection had been scheduled a little sooner, or if only the structure had held on just a little longer. These what-ifs will always haunt both the investigators who solved the case, and the families of the 225 people whose lives were so abruptly ended. Investigators wrote that if we must learn one lesson from this tragedy, perhaps it should be that safety need not always wait until the deadline. A deadline, like the 22,000-cycle limit for the RAP, is a number chosen based on general, not specific, data. Nothing prevents a deadly failure from occurring before the deadline for fixing it. Airlines should take that to heart — the next structural failure might be avoided if they do.

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Admiral Cloudberg

Analyzer of plane crashes. Contact me via @Admiral_Cloudberg on Reddit, @KyraCloudy on Twitter, or by email at kyracloudy97@gmail.com.