Qiao Shanxun, Wen
The China Airlines Flight 611 accident in 2002 triggered a mysterious discussion, that is, the location of the crash - the mysterious airspace known as Penghu's "Devil's Sea".
In fact, the term "Devil Sea" is not new. As early as when Japan occupied Taiwan, Japanese navy and army pilots had already listed a sea and air space 30 nautical miles southwest of Penghu as a "less time to visit" area, because There are always some strange things happening nearby, such as magnetic compasses spinning crazily, white lights appearing on the sea surface, clouds appearing spherical and other supernatural phenomena.
In modern times, the Taiwan Air Force has had many tragic accidents reoccur here, so this area is also known as the "Bermuda Triangle of the Taiwan Strait." Once a plane crashes in the sea, various rumors about the accident in the mysterious sea become rampant, and the China Airlines Flight 611 accident is no exception.
Figure 1: Computer simulation of China Airlines passenger plane
On May 25, 2002, Flight 611 took off from Taipei Chiang Kai-shek Airport (now Taoyuan Airport). After the pilot finished talking to the tower, he turned on the automatic flight program. The flight was bound for Hong Kong International Airport, carrying a total of 225 passengers.
The route between Hong Kong and Taipei is one of the busiest routes on earth. This route is also known as the "golden route". The aircraft climbed smoothly under the control of the autopilot, but when it was about to approach the normal flight altitude (35,000 feet), it suddenly disappeared from the radar of the Taipei Area Control Center.
Air traffic control personnel frequently called Flight 611, but received no response. Air traffic control information shows that the plane's last coordinates were approximately 10 nautical miles north of Mudou Island in the Penghu Sea. The Taiwan authorities immediately launched the largest rescue mission in Taiwan's aviation history, involving more than a thousand people.
Rescuers found a large number of wreckage in the sea, including landing gear wheels, napkins, knives and forks, etc. Because the accident occurred at high altitude, some items fell to central Taiwan, more than 100 kilometers away. The Taiwan Aircraft Safety Commission took over the subsequent investigation.
China Airlines was founded in 1959 and is the largest state-owned airline in Taiwan. However, its operating history is very poor. Major air crashes occurred in 1994 and 1998 that killed more than 100 people.
Figure 2. Approximate location of the crash of Flight 611
The period before the crash was prime time to rescue survivors, but unfortunately all 225 passengers on the plane died. The bodies and wreckage of the plane were transported back to the Penghu Accident Management Center to investigate the cause of their China Airlines crash. Because the manufacturer of the aircraft is from the United States, the U.S. National Transportation Safety Board (NTSB) also sent personnel to join the accident investigation team. The 747 passenger aircraft is a product that Boeing is proud of in its history. Why did such a tragic accident suddenly occur?
ground radar tracked and recorded the flight record of China Airlines 611 in detail. The above data is more detailed and accurate than the data from the air traffic control center. Investigators obtained the first important piece of information in this case: the radar tracked that the flight lost information while climbing, and three or four large pieces of debris appeared on the screen, and began to spread out and fall towards the sea. These characteristics verified that the aircraft was a signal of disintegration at high altitude. .
How could a passenger plane suddenly disintegrate in the air?
Investigators began to focus on the investigation of the plane's disintegration. Generally, air crashes are always accompanied by "conspiracy theories." The media suspected that a mainland missile shot down China Airlines 611.
Historically, the mainland and Taiwan have indeed had shelling incidents at each other, but those are all in the past. Coincidentally, on the day of the Flight 611 accident, the mainland was conducting military exercises in the Taiwan Strait. But no matter what, there is no reason for the mainland to fire a missile at a civilian aircraft. The investigation evidence also shows that there are no traces of the aircraft being shot down, and the radar monitoring records do not show that a missile hit the aircraft.
The external factors of the plane crash have been ruled out, that is, this accident is China Airlines' own fault.
The most likely thing that caused the plane to disintegrate in the air was a bomb. Investigators found no relevant evidence after inspection.If there is an explosion on the plane, there will be specific distortions on the metal skin, and there will be no such traces on the salvaged wreckage.
In order to ensure safety and until the results of the investigation are clear, China Airlines grounded all 747-200 aircraft. The disintegration of the
passenger plane in the air is not an isolated case. In 1996, TWA Flight 800 also had an in-flight disintegration event during the climb-out phase. It crashed into the Atlantic Ocean shortly after taking off from New York. This accident was also the largest investigation in the history of the NTSB, which lasted for Four years, so every air crash investigation must be prepared for a protracted battle.
25 days later, investigators found the two "black boxes" of Flight 611, which recorded cockpit recordings and flight data respectively. Investigators began analyzing the recordings, hoping to discover how the pilots responded or whether there were other unusual sounds.
Figure 3. The wreckage of the aircraft nose salvaged by the search and rescue team
Before the accident, everything was calm.
The last call in the cockpit came from Captain Yi Qingfeng reporting the current flight altitude. Then a whistle was heard, alerting the crew that the aircraft was approaching cruising altitude. What follows is the sudden sound of the plane breaking up, and the recording stops after half a second. Flight
800 was caused by heavy damage to the central fuel tank, so the investigation team focused on the aircraft's central fuel tank. They discovered a flaw in a Boeing 747 where a line near a fuel tank could cause a spark, igniting the fuel in the tank.
Investigators found that the two accidents were similar in appearance: the two passenger planes were of the same model, both were in a climbing state, the disintegration was not expected, and both took off in hot weather.
On July 12, investigators discovered the aircraft's central fuel tank on the seabed. The fuel tanks and other debris were salvaged and placed on the dock. After investigation, the center fuel tank of Flight 611 did not leave any traces of explosion.
The plane did not crash due to an explosion, and investigators can only look for the cause from other aspects.
As the wreckage of the plane was further salvaged, investigators discovered that the passenger plane had an unusual series of small exhaust holes.
Figure 4, Boeing 747-209B China Airlines painting
There will be dozens of such exhaust holes (pressure relief valves) on the floor of all passenger aircraft. If the cargo hold is suddenly depressurized, such as if a hole occurs, the pressurized oxygen will exert a large amount of pressure on the floor, causing the floor to collapse and destroy important aircraft components.
In 1974, a similar accident occurred on a Turkish Airlines passenger plane. The cargo door suddenly separated from the aircraft body, and the captain was unable to control the aircraft normally, causing it to crash. If the floor of the passenger plane hadn't collapsed, the captain might have saved the day.
After the accident, the NTSB recommended that the aircraft manufacturer install a pressure relief valve between the upper and lower cabins of the passenger aircraft. If the aircraft depressurizes the cargo compartment again, the pressure relief valve will automatically open and release the floor. pressure. There were 65 pressure relief valve devices on Flight 611. Investigators recovered 19 from the sea, 4 of which were open.
This shows that there was a sudden loss of pressure in the lower part of the cabin. The existing evidence is not enough to discover the cause, but these make the chain of evidence of the investigation a big step forward.
investigators decided to adopt the method used in the United States' study of Flight 800 and use flight trajectory analysis technology to understand the disintegration of the aircraft. After the wreckage was recovered, each piece was numbered for inspection.
investigators selected 18 pieces of wreckage and entered the weight and shape data into a specific computer program for analysis. The components came from three major parts: the front fuselage, the middle fuselage and the tail.
Based on the location where the wreckage fell, the computer calculated which of the 18 pieces of wreckage broke away from the aircraft first. The computer uses this data to simulate the events when the plane breaks up. The results showed that the tail of the aircraft separated from the aircraft first, and the aircraft's tail immediately became the focus of investigators' research.
NTSB sent metallurgist Frank Zucker to help with the investigation. When he walked to the wreckage of No. 640, he found something unusual about this part. This piece of metal had been severely damaged.From an aerodynamic point of view, when the aircraft is flying in the air, air with a speed of more than 800 kilometers per hour will be poured in when the aircraft disintegrates. This will cause the structure of the passenger aircraft to disintegrate, which is called overload rupture in the industry.
Figure 5. The wreckage of the 747 passenger plane stacked at the dock
The early salvaged parts confirm this. When metal breaks due to overloading or excessive stress, it separates at an angle. The cracks in the No. 640 wreckage have no angles and are flat, which shows that it is not excessive stress, but another natural enemy of the aircraft structure-metal fatigue.
Metal is repeatedly stressed over time and eventually metal fatigue occurs. Imagine we want to break an iron wire. Investigators learned that the debris did not break away suddenly when the plane fell, but gradually separated from the aircraft body.
Another feature of this wreckage is that there is a metal patch on it, also called a cover reinforcement plate. Its function is similar to the piece of rubber we use to repair bicycle tires.
This type of reinforcement plate is very common on older aircraft. For example, if cracks, damage, cracks, etc. appear on the skin of an aircraft, this reinforced version can restore the original strength of the skin of the aircraft.
Components of this Boeing 747 had been repaired, and investigators retrieved the original maintenance data. Frank found that there were long longitudinal streaks on the reinforcing plate of the cladding, which seemed to emanate from between the reinforcing plate and the skin. Something was leaking out from under the metal plate.
Investigators cut out this part of the material and sent it to the laboratory for further study.
Frank found an obvious clue in the cracks, an area covered in aluminum oxide, like traces of rust on a car. Years of exposure to oxygen cause the metal to oxidize and change its color.
Investigators judged from the color of the metal that the cracks on the tail of the plane had been there for a long time. After the cladding stiffeners were removed, investigators finally saw the aluminum skin underneath. Investigators saw some long marks on the skin that were damaged beyond the scope of the reinforcement plate repairs.
Figure 6. The maintenance records of the No. 640 wreckage
accident can be traced back to 22 years ago, when the aircraft was only half a year old.
The tail of the passenger plane hit the runway when it was landing. The controller saw smoke and sparks appearing from the tail of the plane when it was landing, and they notified the pilot. This situation generally occurs when the take-off and landing angle is too large, and is also called tail scraping.
This kind of thing happens very commonly. Some aircraft will even place a bumper on the tail. If the tail is too close to the ground, the bumper will be sacrificed to protect the skin of the aircraft.
The damage to the aircraft's tail scrape is usually subject to regular maintenance. Investigators only found simple records for maintenance in the aircraft's logbook. After in-depth research, investigators found that the quality of China Airlines' maintenance was worrying. The existing documents only recorded that temporary repairs were made the day after the tail was wiped. The maintenance technician used a large aluminum plate to stick to the damaged area. carried out within four months.
investigators inquired through the original records and found that the maintenance program did not follow the manual provided by Boeing to repair .
The structural maintenance manual provided by Boeing shows that if there are many deep scratches on the aircraft that cannot be repaired, the damaged area can only be replaced with as a whole.
22 years later, investigators discovered that the damaged metal had not been replaced, and the original plane scratches remained in their original locations.
China Airlines’ technicians told investigators that the scratched area was too large to be disassembled, so they resorted to grinding away the scratches.
The maintenance technician then made the final mistake. They covered the scratched metal directly with the reinforcement plate. The additional reinforcement plate did not exceed 30% of the damaged area.
Although the maintenance technicians did not perform maintenance according to Boeing's instructions, their maintenance records were flawlessly written..
The biggest danger of this method is that the reinforcement plate hides all the damage. If you are not there to watch the entire repair process, you will not find the source of the problem.
So for 22 years, everyone thought that the damaged parts were repaired according to normal procedures. In this way, a plane with hidden dangers flew for more than 20 years.
Any change in anything is a quantitative change that accumulates into a qualitative change. Every takeoff and landing of an airplane brings one step closer to danger..
And we know that airliners at high altitudes will undergo a pressurization process. When the aircraft climbs, a large amount of air will be poured into the cabin, making the pressure inside the aircraft greater than the outside world. Just like inflating a tire, the air inside will compress the skin, and the crack may also expand some distance. The skin of
will expand and contract slightly every time. Since it is not properly repaired, the cracks will become larger and larger. Finally, the cracks will expand to an irreparable 2.3 meters. This distance has exceeded Yao Ming's height!
A small scratch turned into a big problem over time. In May 2002, as the aircraft climbed and the pressure on the fuselage gradually increased, the expanding crack received enough pressure and exceeded the safety threshold. Then it It spread like a spider web until the entire tail was separated from the main structure of the aircraft. The pressure exerted by the air on the remaining fuselage caused the aircraft to rapidly disintegrate, triggering a major accident that killed 225 people.
Figure 7, patched skin sample
html Flight 0611 has been flying normally for 22 years with more than 20,000 takeoffs and landings despite improper skin repairs. If maintenance technicians strictly follow Boeing's standards to inspect the aircraft, accidents can be avoided. happened.In 1988, an air crash in Egypt should have attracted China Airlines' attention. At that time, the roof of an old Boeing 737 detached during flight. To this end, the Federal Aviation Administration has specially developed a set of inspection procedures for older aircraft, and airlines around the world are inspecting aircraft for corrosion and cracks more frequently.
The new regulations require airlines to re-evaluate how existing maintenance will affect the structure of the aircraft. In 2001, China Airlines began implementing new inspection procedures.
Investigators found a dark brown stain on the outside of the aircraft and learned that China Airlines did not prohibit passengers from smoking during previous operations. The cabin is filled with nicotine due to the presence of pressurized air.
Smoke was expelled from the cracks in the tail of the plane and gradually left traces of nicotine stains on the exterior of the plane. However, China Airlines had already announced a ban on smoking on flights in 1995, which proves that the cracks on the aircraft had existed for more than seven years. China Airlines maintenance personnel discovered the stains, but did not investigate the source of the stains.
Figure 8. Restored passenger plane wreckage
China Airlines received new inspection procedures from Boeing and scheduled a comprehensive inspection on November 2, 2002. However, China Airlines 611 did not survive this day.
Investigators hope to prevent similar crashes from happening again by recommending that airlines around the world implement new comprehensive maintenance inspection regulations immediately.
A compromised maintenance caused an air tragedy 22 years later. China Airlines learned a profound lesson and immediately revised its aircraft inspection and maintenance methods.
Through the correction of this matter, China Airlines' operating record has been greatly improved, and it has also removed the "hat" of having one air crash in four years. Investigators also recommended the development of new tools to more conveniently detect damage under aircraft reinforcement panels. Subsequently, Boeing developed a non-destructive inspection tool that uses the characteristics of ultrasonic waves to penetrate metal and can show damage to components under reinforcement panels. However, it has its own shortcomings, which is that it greatly increases the time for maintenance and inspection.
As time goes by, existing airliners become more and more worn out, requiring more thorough and detailed inspections. Six years after the crash of Flight 611, Southwest Airlines set a new record fine of $10.2 million for failing to conduct crack inspections.
Regulators hope that through sky-high fines, they will remind everyone to put the correct maintenance of aircraft at an important level. Although correct maintenance and inspection procedures are expensive, the lessons of Flight 611 tell everyone that the cost of not maintaining the aircraft is even higher.
Figure 9. The Flight 611 accident has sounded a bell for the aviation industry.
The truth of the accident has also earned the reputation of the Penghu Sea as the "Devil's Sea". To borrow a sentence from the National Geographic Channel's "Return to the Crisis" " Accidents do not happen out of thin air, but are the chain effects of key events." The same applies to air crashes.
