New Materials for Next-Generation Commercial Transports (1996) / Chapter Skim
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7 Aircraft Maintenance and Repair
7 Aircraft Maintenance and Repair
Pages 57-66
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From page 57... ...
The lessons learned from the aging of metal and composite structure are discussed. Chapter 8, "Nondestructive Evaluation," describes current aircraft inspection practices and identifies needs for improved nondestructive evaluation techniques and promising technologies for the future.
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From page 59... ...
Structural Maintenance Any new aircraft program is based on assessing structural design information, fatigue and damage tolerance evaluations, service experience with similar aircraft structures, and pertinent test results. Generally, the maintenance task evaluates sources of structural deterioration including accidental damage, environmental deterioration, and fatigue damage; susceptibility of the structure to each source of deterioration; the consequences of structural deterioration to continuing airworthiness includin;, effect on aircraft (e.a., loss of function and reduction of residual strength, multiple-site or · 59 multiple-element fatigue damage, the effect on aircraft flight or response characteristics caused by the interaction of structural damage or failure with systems or power plant items, or in-flight loss of structural items)
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From page 60... ...
Generally, most airlines classify specific component maintenance tasks as follows: · lubrication or servicin a, where the replenishment of the consumable reduces the rate offunctional deterioration; · operational or visual check, where identification of the failure must be possible; inspection or function check, where reduced resistance to failure must be detectable and the rate of reduction in failure resistance must be predictable; · restoration, where the item must show functional de; radation characteristics at an identifiable age, have a large proportion of units survive to that ace, and be able to be restored to a specific standard of failure resistance; and . "Walk-around" visual check of aircraft exterior and engines for damage, leakage, and brake and tire wear Specific checks on engine oils, hydraulics, oxygen, and specified unique aircraft requirements "A" check-detailed check of aircraft and engine interior, services and lubrication of systems such as ignition, generators, cabin, air conditioning, hydraulics, structure, and landing gear "B" check (or "L" check~torque tests, internal checks, and flight controls "C" check~etailed inspection and repair program on aircraft engines and systems Inspection and reapplication of corrosion protective coatings Major structural inspections with attention to fatigue damage, corrosion, etc.
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From page 61... ...
Ramp and Maintenance Damage An International Air Transport Association survey esti mates that 36~0 percent of damage to aircraft is from ramp and maintenance damage, sometimes called friendly foreign object damage (IATA, 1991~. Figure 7-1 shows a diagram of the Boeing 777 aircraft interfaces with servicing and other equipment (Boeing, 1994b)
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From page 62... ...
to develop and verify advanced NDE technology that can reliably and economically detect disbands, small MSD fatigue cracks, and corrosion and characterize their effect on the residual strength; and (2) to develop and verify advanced fracture mechanics and structural analysis methodology to predict fatigue crack growth and residual strength of airframe structures to determine in-service inspection thresholds and repeat intervals, quantitatively evaluate inspection findings, and design and certify structural repairs.
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From page 63... ...
Aging aircraft repairs have typically involved upper-skin lap fastener replacement, nonbonded skin panel replacement, skin lap doubler repairs, frame reinforcement, entryway door and scuff-plate doublers, replacement bushings and clevis joints, bulkhead forging replacement, and selected landing gear component replacement. Based on service experience, the airlines have expectations that manufacturers of new aircraft will (DeRosa, 1995~: include stress corrosion prevention in all design re .
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From page 64... ...
In general, the service experience with composites indicates that damage occurs because of discrete sources such as impacts, lightning strikes, and handling rather than progressive growth caused by a fatigue condition (Blohm, 1994~. In addition to groundhandling damage, a recent survey by the International Air Transport Association, summarized in table 7-3, lists the particular causes of damage that occur in the current generations of composite structure (IATA, 1991~.
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From page 65... ...
Previous service experience with metallic and composite structures supports the importance of a maintainable design. The experience of the aging fleet with metallic structures provides lessons in corrosion prevention and control as well as detection and control of multiple-site fatigue damage through appropriate analysis methods, improved component designs, and focused inspection and maintenance.
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From page 66... ...
i' _ ! Fastener Locations for Frame Splices FIGURE 7-2 Bolted splice repair of a composite primary structure panel.
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