A new approach to accidental damage on aircraft metallic structure

icaf2023 Tracking Number 52

icaf2023 presentation

A new approach to define the inspection requirements for Accidental Damage (AD) on aircraft metallic structure is described in this paper. Accidental Damage is characterized by the occurrence of a random discrete event (e.g. impact by foreign object, hail, runway debris, human error during manufacturing, operation or maintenance of the aircraft) which may reduce the inherent level of residual strength of the structure. The Accidental Damage inspection tasks were defined by Airbus through a crack growth calculation from detectable to critical on the structural areas prone to this type of damage. These inspection tasks were part of the Airworthiness Limitation Section. Some years ago, it was decided to change from a calculation based approach towards a more realistic and pragmatic approach. To achieve this, test and in-service experience accumulated on AIRBUS aircraft was exploited. It appeared that very few non-detected accidental damages turned into fatigue cracking in service. Test results showed that readily detectable damages such as dents take a long time, providing many detection opportunities, before a fatigue crack initiates and grows. The philosophy of the approach was changed from crack detection to accidental damage detection. An MSG-3 evaluation is applied to identify the Significant Structural Items (SSIs) susceptible to AD. The likelihood of AD occuring is evaluated and an Accidental Damage Rating (ADR) is determined. The fatigue, crack growth and residual strength performance of the SSI are used to derive a Stress Sensitivity Rating (SSR), assessing the likelihood of AD to turn into growing fatigue crack. The inspection frequency, expressed in calendar time, is determined through the combination of the ADR and the SSR. The inspection level, general or detailed visual, is determined through a standard MSG-3 assessment. Finally, inspection requirements are consolidated with the Zonal and Structure Programs of the Maintenance Review Board (MRB) Report. This new approach was developed in a joint effort between the Maintenance Engineering and Stress departments in Airbus. While being more realistic than the previous one, it also optimized the overall number of inspections in the scheduled maintenance program.