Development of a “Low Stress Criterion” That Eliminate a Large Portion of the Aircraft from Damage Tolerance Based Maintenance Program for Structural Repairs Required by the New Part 26 Federal Regulation

The main purpose of the paper is to outline a major criterion developed for the new Part 26 Code of Federal Regulation (CFR) that require aircraft manufacturers to support operator compliance to damage tolerance based maintenance program for aircraft structural repairs and alterations. The paper focuses on this newly developed “low Stress Criterion” that eliminates large areas of the aircraft from damage tolerance evaluation and the resulting damage tolerance inspections (by the operators) if stresses are below a certain level. In order to comply with this Aging Airplane Safety Rule (AASR) Aircraft manufacturers are required to provide damage tolerance data for repairs and alterations to the operators so that they can develop inspection program to maintain airworthiness of their aircrafts. As the focal of this activity, Structural Damage Technology (SDT) group of Boeing is actively engaged in developing methods and criteria to support the Part 26 rule. The “Low Stress Criterion” is one of the major criteria developed recently to support the Part 26 rule. This paper describes in detail this criterion that eliminates large areas of the aircraft from damage tolerance evaluation by Boeing and resulting inspections by the operators if stresses in the repaired structure are below a certain level.

Low Stress Criterion is based on both fatigue crack initiation and propagation considerations. The criterion provides a stress level below which the fatigue life of the part will be many times the design service objective of the airplane. At the same time this stress level will also provide a large crack growth life and the required damage tolerance based inspection interval will be much longer that the routine inspection already being carried out on the airplane.

The paper describes in a comprehensive manner how the criteria has been developed which in the long run will save the world airlines significant maintenance costs by eliminating inspections on repairs in less critical low stress areas, thereby allowing them to focus on more critical structural repairs