The 737 program makes use of the Damage Tolerance Rating (DTR) System developed by Boeing (Section 10.3). This rating system is used to determine the probability of detecting fatigue damage, which is presumed to have occurred in a fleet of airplanes and has not been reported, before the residual strength of any airplane falls below regulatory fail-safe strength levels.  A detailed explanation of the DTR System concept is presented in Section 10.3.
Airplanes with a high number of flight cycles are the most likely to experience initial fatigue damage in the fleet. Therefore, this program is based on supplemental inspection of all airplanes at a specific threshold.  See Section 3.0 for threshold information.
The supplemental inspection program developed for Models 737-300/-400/-500 permits operator flexibility by adjusting or supplementing existing maintenance programs, where necessary, to meet the required damage tolerance rating.  The program was developed in cooperation with members of the airline industry who participated with Boeing in working group sessions.  These meetings were also attended by engineering and maintenance specialists from the FAA and CAA.
All SSIs which require directed inspections for maintaining structural integrity and with no history of in-service fatigue cracking, are included in this document.  An explanation of SSIs is provided in Section 4.0 and a summary listing of the SSIs is given in Sections 8.0 and 11.0, respectively.
DTR Check Forms are provided in Section 11.0 to select the inspection options which will meet the damage tolerance rating requirements for each SSI. If an operator's existing maintenance program meets or exceeds the required damage tolerance rating, no additional inspection requirements are necessary for that SSI. If the current maintenance program falls below the required damage tolerance rating, the operator must improve or supplement the program.  To achieve this, the operator may increase the frequency or intensity of the normal maintenance for eligible airplanes, or establish a supplemental inspection program, including Non-Destructive Testing (NDT) techniques.  Feasible inspection options were used to complete an example rating form for each
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SSI.  These forms are provided in Section 8.0 and are for reference purposes only.  Each operator must fill out the DTR Check Forms in Section 11.0 of the SSID to determine the specific inspection options and intervals that will be used in their fatigue inspection program.
Known fatigue problems, which could affect the structural integrity necessary for safety of the airplane, that are covered by existing Service Bulletins (SBs), have been reassessed and a flight safety addendum/inspection program added to each as required.  A list of SBs with flight safety addenda/inspection programs is given in Section 9.0.  Fatigue related SBs which are subsequent to, or result from, the supplemental structural inspection program and which require a flight safety addendum/inspection program, will also be referenced in Section 9.0.  When these SBs are applicable to eligible airplanes, the SSI, or part of an SSI, directly related to the reported fatigue problem will be removed from this document.
　
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