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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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