Hawk Mk 51/51A/66 Tailplane Full-Scale Fatigue Tests

Until 2017, there was no certainty about the fatigue life of Hawk tailplanes in FINAF’s flight conditions. Then full-scale fatigue tests were performed to determine if the FINAF is required to procure more tailplanes, and to extract evidence, which could be used to increase the structural inspection interval times. The tests were executed with two 4000 FH flown tailplanes and the goal was to achieve additional 2000 FH with a scatter factor of 5. Test loads were applied with actuators feeding both buffeting and maneuvering symmetrically at the same time. Test’s spectrum was based on the FINAF OLM strains and on the usage spectrum of the FINAF flights 2014–2015. Limited NDIs were done after every 200–340 EFH and full inspections after every 1000 EFH. Several damages, such as broken rivets and cracks in spars and angles, arose. Following the testing, the tailplanes were subjected to RSTs with the load corresponding the ultimate design load. The tailplanes passed the RSTs without noticeable additional damages. Centre sections were torn down for more detailed inspections. Some fault indications were obtained from the buttstraps, but all the defects were very small. Seven cracks were found on the skins and one location could be determined as the critical location. The centre joint survived the test period. The residual strength was sufficient with a 20 mm crack at the skin rivet hole, which was estimated to be the most loaded. The tests gave solid basis for increasing the TP’s acceptable usage life by 1000 FH. It was possible to determine the crack propagation rate to verify the structural inspection period to be applied. Considerable cost savings will be achieved, because the inspections can now be optimized. In addition, now it is known that the current number of TPs is sufficient with the additional 1000 FH for the targeted HW life cycle, and no additional procurement is required.