3.0 IMPLEMENTATION
Airplanes with the highest number of flight cycles are most likely to experience initial fatigue cracking in the fleet. Therefore, supplemental inspection programs for these airplanes, coupled with the reporting of discrepancies found and, where necessary, follow-up action will maintain structural airworthiness in the total fleet if fatigue cracking should occur. To maintain adequate fleet surveillance, each operator with airplanes approaching the initial inspection threshold listed below must provide for those airplanes a supplemental inspection program which meets the requirements established by this document.
This document is effective for all model 737-300/-400/-500 airplanes.
NOTE: Historically, the 737-100/-200 Supplemental Structural Inspection Document, D6-37089 was effective for a specific group of airplanes known as the Candidate Fleet. Airworthiness directive 98-11-04 R1 expands the program to include all 737-100/-200 airplanes at a specific initial threshold. This document is applicable to all 737-300/-400/-500 airplanes that are at or beyond the specific initial thresholds listed below.
Boeing recommends inspection of all Structural Significant Items (SSIs) listed in this document be accomplished:
Initial Inspection
At, or prior to, 66,000 cycles.
Repeat Inspections
At intervals in accordance with the Damage Tolerance Rating (DTR) Check Forms in this document. Application of the DTR system is discussed further in Sections 5.0 and 7.0.
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4.0 STRUCTURAL SIGNIFICANT ITEMS (SSIs)
The two design principles for obtaining structural safety are "damage tolerance" and "safe life." Damage tolerance relies on some means of detecting damage before airplane safety is jeopardized. This is the preferred design principle whenever practical. A safe-life principle is used for items where there is little or no chance of detecting damage before residual strength is reduced below acceptable limits. A conservative fatigue damage threshold is used for these items to limit the service life. On modern commercial jet transports, the safe life design concept is generally limited to landing gear components.
In order to determine safety analysis requirements and corresponding maintenance considerations, airplane structure can be categorized as primary (structurally significant) or secondary (see Page 4.0.2). This breakdown is a determination of an item's function in relation to structural integrity. Any detail, element, or assembly, which contributes significantly to carrying flight, ground, pressure, or control loads and whose failure could affect the structural integrity necessary for the safety of the airplane is classified as an SSI. Items not in this category are classified as secondary or other structure.
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