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Component surfaces that are deformed, or yielded, during non-normal landing conditions may develop tensile residual stresses. Hardened high-strength steel components that are deformed but still functionally satisfactory require special salvage procedures to ensure continued structural integrity. Otherwise, these parts may develop stress corrosion or fatigue cracks in service.

When component yielding or damage to the adjacent airplane structure is found, all the gear or one or two legs should be replaced on the airplane and sent to an overhaul shop for complete overhaul, beginning with close dimensional inspections. Hardened high-strength steel parts, which are slightly deformed (including those dropped or damaged during maintenance) or suspected of being overloaded in service but are still serviceable, may need special salvage processing. The parts would be completely stripped, baked at 50 below tempering temperature, and stored for a period of six months. Parts that exhibit no cracking after six months should be magnetic-particle-inspected, shot-peened, and completely refinished according to overhaul procedures before return to service. This procedure also may be needed for some nondeformed parts on the same gear as a conservative measure.

Parameters that indicate when to perform this salvage procedure have not been added to Boeing AMMs to date, nor has this salvage process been added to the Boeing Standard Overhaul Processes Manual. Special salvage procedures are currently used only after technical review of each component affected. This will result in some deformed components being removed from service, even if functionally satisfactory.

Airframe damage and FDR data may indicate gear overload and likely detrimental residual stresses for some of the gear components, even though they may not be deformed. Special processing may be warranted for these components because they maintain shape within design tolerances even when loaded in excess of design. Circumstances permitting special processing to salvage a gear component depend on review of the airframe damage, specific gear component damage (including noted deformations or estimated detrimental residual stresses), and may include review of FDR data.

Parts that have the potential to develop fatigue or stress corrosion cracking after overload are generally high-strength steel landing gear components. Following are examples of cracking caused by residual stresses from overload and subsequent crack initiation:

  • A 737 nose landing gear inner cylinder cracked at the upper surface of the integral axle adjacent to the vertical barrel during the recommended six-month storage period. A hard nose gear landing had resulted in fuselage damage at the nose wheel well aft corner (i.e., the fuselage skin, strap, and frame had buckled). The fuselage was repaired and the nose landing gear removed for overhaul and close examination. Special salvage processing was initiated after slight upward deformation of the axle was noticed. Axle cracking was noted following the six-month storage period, and the inner cylinder was scrapped.
  • A 737 main landing gear inner cylinder was found slightly buckled by an overhaul agency during hard landing salvage procedures. The part was set aside; it cracked suddenly within a week.
  • A 737 nose landing gear was removed for special salvage processing after a hard landing. The outer cylinder was overhauled normally, without dimensional checks or special processing. When placed in service, the outer cylinder cracked during towing approximately 100 flight cycles after overhaul (fig. C). Metallurgical analysis verified the crack was initiated by fatigue. The fatigue was the result of residual stresses from apparent overload during the hard landing followed by normal cyclic loads. Further propagation of the initial crack was caused by stress corrosion that occurred before the fracture.

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