I chose to open this part with the Japan Airlines flight 123 crash, which is one of the worst crash ever known in "modern" aviation, and which has impressed me most. CVR (voice recorder in the cockpit) is in the following video, which is rather chilling.
Happy reading!
On August 12, 1985 is a national holiday in Japan. However, the worst air disaster of all time is about to occur. The JAL flight 123 is performed by a crowded Jumbo jet. It must fly a domestic route between Tokyo and Osaka. 524 people took place this aircraft for a flight expected to last a little over an hour. This is a Monday. The aircraft is a Boeing 747-SR for Short Range, which is a modified version of the jumbo, designed to carry more than 550 passengers on domestic Japanese flights.
The aircraft lines up and take off at 18:30 local time. A few minutes later, when it is climbing to 24,000 feet, a distress message is transmitted. The pilots first speak English as required by the international phraseology. Then panic takes over, they begin to share in Japanese with ATC. JAL crews are sturdy characters and fly with white gloves. This is not the kind of people who loose control for a trifle. As they describe, the situation seems very serious. They heard an explosion, then they have lost control of the aircraft. The 4 independent hydraulic circuits are disabled and the Boeing no longer responds to commands. Only throttles remain working. The cabin is depressurized, multiple alarms sound and pilots think they have lost a cargo door. The sky is cleared around the JAL flight 123.
Several alternate airports are planned but the aircraft is not able to join them anymore. Then began a macabre dance called phugoid oscillation typical of aircraft in perdition. The aircraft loses altitude as it was going to crash and then, suddenly, changed its attitude and starts to climb. Reached a certain height, it plunges again. This movement is known as one of the aerodynamic instability modes of which aircraft are subject. The most famous being the dutch roll, which is countered by the Yaw Damper system installed on all airliners. The plane, which went down to the south-west along the coast back to earth while continuing its roller coaster motion. Inside, pilots are struggling with the rest of the functional systems to try to find any opportunity to control their aircraft. Passengers understand that the situation is serious and many of them begin to write farewell letters to their parents. The controller is terrified by what he sees on his radar screen.
The aircraft flies below 7,000 feet and comes in a rugged area where many mountains above this altitude. The end seems near. On the ground, a person takes a picture which will tour the world in 1985. After 30 minutes of a long perdition, the plane begins a steep dive toward the ground from an altitude of 13,000 feet.
Upon impact, the 747 is sprayed and nearly 520 passengers are instantly killed. Miraculously, four people sitting at the back are safe after being projected upon impact against a pile of upholstered seats. All were sitting in the same row. A girl is found alive at the top of a tree. Less than 20 minutes after the crash, a helicopter from a U.S. Navy base finds the wreckage and offer assistance. The Japanese military ordered him to return to his base and leave the field open to their own army. Do not mess with these sensitivities. On the ground, survivors regain consciousness on a turbine noise and see the lights of the helicopter getting closer to their position and suddenly away leaving room for the night. Japanese equipment will be on that area only in the morning, while the cries of most survivors have snuff out long ago.
The investigation
The black boxes are found and quickly stripped. They do not say much more than what we already knew. The study of the last moments of CVR shows that there were two impacts on the ground separated by a few seconds. Between them, the recording continues with automatic voice GPWS announcing the approach of the ground. By cons, it is the analysis of the maintenance log and the various testimonies of staff who worked on this plane that will explain the causes of the accident. We must return in 1978, about 7 years earlier, June 12 exactly. The 747 had an accident on the ground at Osaka Airport, and the tail was damaged. All the rear of the passenger cabin, just after the last door is a hemispherical dome representing the boundary between the pressurized area and the area that is not. Against this dome are often installed cupboards meal trays and is not visible when looking at the back of the plane. This part is subject to mechanical stress in view of the huge difference of pressure it receives from each side.
In the incident of 1978, the pressure dome was damaged and Boeing commissioned a technician to repair it. He was instructed to use a single metal plate and fix the crack by two rows of rivets. Once there, he decided to do better than what he was asked to do, he will use two plates. Once it's set up, the whole is more difficult to fix and it will stick to a single line of rivets instead of two.
On this type of repair added plates work in extension and rivets in shear. These are the limiting factor because the metal has a lower resistance to shear forces as expansion stress. Superimposing many plates to reduce the number of rivets will decrease the strength of the whole assembly. This is not the plaques that are most important in this arrangement, but the rivets. Using his own method of repair, the technician had obtained a 70% lower assembly than it should achieve.
In airmanship, what the Boeing technician committed is called optimization error. That is to say that a person will cause damage by seeking to improve the functioning of things as it was explained. Pilots and technicians are normally sensitive to this kind of fault.
This poor repair holds up, but for years, every flight of this 747 is a lottery. Worse still, long time before the crash, repair starts to show signs of weakness and this is manifested in the form of whistling heard in the back of the cabin during flights. Air escapes through the slot that opens gradually. Everyone knows at JAL, but no one takes measure. The slot is too small to prevent pressurization of the cabin, then let it.
The fateful day when the aircraft arrived at 24,000 feet, typical altitude for decompression accidents, the pressure dome tears explosively. The cabin air is blown inside the empennage and rudder, vertical stabilizer, half of the elevator and stabilizer, and the APU. These parts will be found after the crash, off the coast of Osaka.
JAL is singled out in the press the shortcomings and mistakes that led to the tragedy and spread. Several officials commit suicide. The technician who performed the famous repair will end his days also.
Unbelievable..
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