Structural Analysis of Ageing Pilatus P-3 Engine Mount

Abstract

The Pilatus Trainer P-3 full scale fatigue test was performed in 1960 to validate a service life of 2’500 FH for the Swiss Air Force. The test showed minor damages after 5’000 FH which were all addressed in modifications for the whole fleet. During the military service life, no cracks were discovered. In 1975, a structural integrity study was done to assess the fatigue life of the fleet based on the Air Force usage. The Swiss fleet was cleared to 3’000 FH and 12’000 flights due to the less severe usage compared to the full scale fatigue test. The Swiss Air Force retired the P-3 fleet in 1995. The P-3 were sold as ̈oldtimers̈ to private people. In Switzerland, 17 aircraft received a civil registration by FOCA. The P-3 had between 3’000 and 3’400 FH during military usage. The current fleet leader has accumulated 4’270 FH. The P-3 full scale fatigue test demonstrated no fatale structural failure at 5’000 FH and 10’000 flights. Due to the high number of FH compared to the test, further investigations are needed to ensure the continuing safe operation of the civil registered P-3 airplanes. First, the available Nz spectrum from the military usage were analyzed and compared to the full scale fatigue test spectrum. Second, two Swiss operators were interviewed to understand the civil usage. The civil usage seems to be heavily depending on the operators, so a conservative engineering approach is necessary to ensure the structural integrity. An assessment was done with collecting all the data from the military usage and reviewing the available information. Some structural critical areas were identified after the first review. Most concern showed the engine mount which was never tested, and only limited static analysis was done in 1958 with no fatigue calculation. The engine mount is a tube structure with welded connection of steel material AISI / SAE 4130. A detailed FE model using ANSYS Workbench Platform was developed and 6 load cases based on EASA CS23 certification standard were applied. The mesh size was reduced to 1mm at the critical areas to model the welded structure. The static analysis was based on FKM criteria. The horizontal spin maneuver exceeds the tensile strength at two local positions. Based on the spectrum information from the test and the Nz exceedance data for the Swiss Air Force usage, a Rainflow Counting Matrix was developed. With the corresponding results for the Nz values, the spectrum could be converted to stresses at the critical locations based on principal stresses of the sub model FE data. For fatigue investigations the flying steady maneuver loads are contributing to the fatigue damage. In the first approach MIL-HDK-5J stress life data was used for max min stress level to see if the fatigue life is in the area of durability life of 107 cycles. Further analysis was done using strain life fatigue analysis which showed at two locations a much shorter life. Inspections at every 100 FH were scheduled to ensure the safe operation under civil usage.