Aging Aircraft and Structural Failures

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  • Topic: Air safety, Aloha Airlines Flight 243, Aloha Airlines
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  • Published : October 23, 2012
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Aloha Airlines Flight 243:
Structural Failure of an Aging Aircraft

Safety 335
aloha Airlines Flight 243:
Structural Failure of an Aging Aircraft

The age of the United States' commercial aircraft fleet is a serious problem. The average age of commercial airline fleets is continuing to increase. As of year 2000, more than 2,500 commercial aircraft in the United States were flying beyond their original design lives. In 1988, a major incident in which the top peeled off an Aloha Airlines Boeing 737 in flight, sweeping a flight attendant to her death, was blamed on weak maintenance of the old aircraft's structure. The flight attendant was swept overboard at 24,000 feet after a spontaneous failure of one of the aircraft's longitudinal joints. The aircraft involved, a Boeing 737, had been subjected to the severe operating environment particular to inter-island service during its 19-year lifespan. The Aloha Airlines 737 was the second oldest aircraft still in service. The aircraft, which had been designed for 75,000 flight cycles, had actually accumulated 89,680 cycles with stage lengths of 20 to 40 minutes. This intensive use also inflicts the loads associated with repeated pressurization and de-pressurization of the aircraft's cabin. Fuselage fatigue damage is primarily caused by the application of the pressurization cycle that occurs on each flight. Typically, the inter-island carriers fly at 23,000 ft while the cabin is pressurized to 8,000 creating a 5 psi differential. The fuselage of this aircraft suffered from extensive Multiple Site Damage (MSD). MSD occurs when stress factors are fairly uniform, so that small cracks appear and grow at roughly the same rate. Each individual crack is difficult to see and by itself poses little problem; however, the small cracks can join together to form a large crack (Oster, Clinton, Strong, Zorn, 1992). The Aloha 737's MSD's were cracks extending on both sides of rivet holes along the upper row of the lap joints along the fuselage. Two other major fuselage failures existed on the upper row of rivets on the S10L lap joint. Near the forward entry door, the MSD cracks had joined to form a single crack about 6-8 inches long. Two passengers noticed this crack as they boarded the aircraft in Hilo, HI. The crack was long enough and wide enough that the internal fiberglass insulation was being extruded from it. The passengers did not report the crack, feeling that if the aircraft was not safe, the airline would obviously not fly it (NTSB, 1988). The focus of the National Transportation Safety Board's (NTSB) hearings were the failure of the Boeing 737's design to Safely Decompress. Contrary to the NTSB findings, the fuselage did tear open a Safe Decompression Flap as designed. If the Flight Attendant had not been standing directly underneath the Flap when it occurred, the plane would probably not have suffered an explosive decompression (Hinder, 2000). The forces exerted on the fuselage by leveling of the aircraft was the final blow that caused a link up of MSD cracks at BS500 (Approximately Row 5) which were arrested by the Safe Decompression design causing the Flap to open. At the instant in time represented by Figure 1, the aircraft is in the process of rotating from climb to level flight, there is a tear in the S10L lap joint at approximately in front of row 1 and a Safe Decompression Flap at approximately Row 5. [pic]

Figure 1
The cabin was pressurized. With the approximately 10" x10" opening, the internal cabin air began to escape at over 700 mph. The Flight Attendant who was reaching to pick up a cup from Passenger 5B was immediately sucked into but not through the Safe Decompression Flap. Only the Flight Attendant's right arm and head were forced through the opening. This effectively slammed the door shut on a 700 mph jet stream. The resultant reaction to corking a high velocity fluid...
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