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Secondary structure is classified as Category 1.  Airplane safety is assured  by demonstrating that a flight can be completed safely after the component has lost its ability to function or has separated from the airplane.  Structural maintenance is dictated by the economic consequences of major damage to or loss of the component compared to those of an inspection program required to detect damage at an early stage.
Damage-tolerant structure is classified as Category 2 if damage becomes obvious or malfunction is evident before reaching critical size. An example of this is significant fuel tank leakage.  In this category, safety is assured by providing adequate residual strength with extensive damage that is obvious to personnel whose primary responsibility may not be structural inspection.  Economics again dictate structural maintenance where it may be desirable to detect damage at an early stage.
Most damage-tolerant primary structure is classified as Category 3.  Structural integrity is assured by timely damage detection with a planned inspection program. Inspection program requirements for detecting corrosion, stress corrosion and accidental damage are based on operator experience on the same or similar structure. For fatigue damage, inspection program requirements are matched to structural characteristics that include residual strength, crack growth rate and damage detection.
Safe-life primary structure is classified as Category 4.  Safety is assured by conservative fatigue life limitations. Operator experience is used to develop a maintenance program for preventing or detecting and repairing any corrosion, stress corrosion or accidental damage.
All Category 3 items are included in the 737-300/-400/-500 Supplemental Structural Inspection Program.  The parameters considered in determining Category 3 SSIs include:
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Consequence of cracking

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Inspectability

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Stress environment

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Multiple cracking

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 Structural redundancy

For each SSI several cracking patterns were considered to determine the most critical.  This generally resulted in a crack origin being considered in a location difficult to inspect.  In some cases, a single SSI may have multiple cracking patterns due to various parts of the item being hidden from view.  For example a spar chord can be visible from both sides, hidden on either side, or hidden on both sides. A separate DTR Check Form is provided for SSIs with more than one critical cracking pattern.
Original D6-82669 4.0 STRUCTURAL SIGNIFICANT ITEMS

A list of SSIs and the corresponding DTR Check Forms for the wing, fuselage and empennage are provided in respective subsections of Sections 8.0 and 11.0.
STRUCTURAL CLASSIFICATION 
STRUCTURAL CATEGORY  SAFETY ANALYSIS REQUIREMENTS  STRUCTURAL CLASSIFICATION EXAMPLES 
TECHNIQUE OF ASSURING SAFETY  TECHNOLOGY CONTROL METHOD 
　
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本文鏈接地址：737-300/400/500 補充結構檢查文件 SUPPLEMENTAL STRUCTURAL INSPECTION DOCUMENT 1(7)