The AP-3C Orion aircraft is the oldest aircraft in the Royal Australian Air Force (RAAF) inventory. The planned fleet withdrawal has been extended far beyond the original design service objective. Continued safe and effective operation has required the development of a robust ageing aircraft management approach. A fundamental aspect was supplementing the structural certification basis with appropriate standards in the form of fatigue management requirements from Federal Aviation Regulations (FAR) 25.571 and Federal Aviation Administration Advisory Circular (FAA AC) 120-93. To develop and underpin the ageing aircraft management plan and transition to the supplementary fatigue management standards, the RAAF collaborated with the Original Equipment Manufacturer, Lockheed Martin Aeronautics Company, the United States Navy (USN) and other operators to form the P-3C Service Life Assessment Program (SLAP). This program provided Full Scale Fatigue Test (FSFT) data, associated analyses and analysis tools to support management in accordance with FAR 25.571. An important element of the ageing aircraft management plan included the introduction of a rigorous Safety By Inspection (SBI) maintenance regime to assure structural airworthiness. FAA AC 120-93 requires assessment of structural repairs to determine revised fatigue management and inspection requirements. Often, this information is derived using tailored analysis tools and detailed models on a case-by-case basis. This approach is specialized, expensive and usually occurs after the repair has been designed and installed. To avoid these limitations, the AP-3C Repair Assessment Manual (RAM) was developed to provide the repair designer with a design handbook approach to fatigue analysis. In conjunction with some simple Finite Element (FE) models, the RAM supports complete repair analysis prior to an aircraft leaving the maintenance venue. This paper will present the history of the SBI program, the genesis of the RAM and actual examples of assessing structural repairs on the P-3 platform using the RAM.
Abstract. Aft pressure bulkhead ring frames on the RAAF P-3C Orion aircraft are being replaced due to in-service fatigue cracking. Installation of the new 7075-T6511 extrusion L-section circular frame requires local adjustment of the included angle at various positions around the circumference. The adjustment should be carried out under a controlled temperature in the range of 135 -160° C. However in some recent cases the temperature was either not well controlled and/or was below that range, raising concerns about the potential for adverse residual stresses in a region already known to be susceptible to fatigue. This paper details an investigation including 3-D non-linear finite element (FE) modelling and X-Ray diffraction based residual stress measurements on representative frame segments. The FE modelling was performed with NASTRAN using 3-D solid elements. Plastic behaviour was modelled using a Ramberg-Osgood approach. The X-Ray diffraction was performed using a Proto iXRD Combo instrument with a Cobalt X-ray source and a rectangular aperture (1.0, 1.5 or 2.0 x 5.0 mm depending on the location) and also electro-polishing for depth data. Permanent resultant angles of the frame segments were also measured using a "MarSurf CD 120" contact profiler. Typically low residual stresses were found for both the within and the below specification temperature cases. The FE and measurements were in reasonable agreement giving confidence in the results. The results from this work supported a decision to clear affected aircraft for a return to service. IntroductionThe aft pressure bulkhead structure at FS 1117 on the P-3 Orion aircraft, Figure 1, includes an approximately "L" shaped forward frame segment, Figure 2. The frame, manufactured from 7075-T651 alloy extrusion has a nominal included angle of 100°. Due to cracking which has occurred in service at the location of interest shown in Figure 2, the frames are being replaced on some RAAF aircraft. During installation of the new frames, the included angle needs to be adjusted in some areas. In the upper region of the fuselage the angle is tightened to about 94°, and in the lower end the angle is increased to about 102°. The angle adjustments are performed mechanically as shown in Figure 3 with locally applied heating [1]. The temperature range for the heating should be carefully controlled and thermocouple measurements are to be taken to ensure that the temperature range is controlled to the range 135 -160° C. In some cases however, thermocouple measurements were not taken and it was thought that the hand forming in those cases may have occurred at a temperature below the specified range.
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