A new study describes how Southwest Research Institute’s risk and damage tolerance analyses helped determine a more effective inspection schedule, allowing the Air Force to find cracks before they grow to critical size.
Extending the Life of Aging Aircraft
For several decades, Southwest Research Institute has provided engineering support to the Air Force to extend the life of aircraft that have exceeded their original design life. The Institute has been working to sustain the T-38, first introduced in 1961, for more than 40 years. This activity includes using SwRI-developed models to predict crack growth to determine optimal inspection and maintenance schedules. The Air Force also uses finite element models to help predict the potential crack locations of the aircraft.
“Typically, our role involves predicting structural life and providing analysis used to help determine an inspection or repair schedule,” said SwRI Lead Engineer Laura Hunt. “In the spring of 2017, when a large, unexpected crack was found by a crew chief in a longeron, a key structural component along the aircraft’s fuselage, we were equipped for a rapid response.”
Rapid Response to Critical Structural Issues
This discovery resulted in the immediate grounding of the entire T-38 fleet for visual inspections, which were completed within four days. SwRI assisted with performing risk, damage tolerance, and failure analyses.
While the crack appeared in a location that full-scale fatigue tests and finite element models did not predict, SwRI responded swiftly, minimizing downtime and maintaining safety, resulting from the Institute’s prior analysis work with the T-38 and other aging aircraft.
“A new problem was identified and the entire team supporting the T-38 reacted quickly to ensure the safety of the T-38 and return it to service in a manner of days,” said David Wieland, who oversees SwRI’s Aerospace Structures Section.
Destructive Failure Analysis and Findings
Senior Research Engineer Mirella Vargas led the destructive failure analysis for the cracked T-38 longeron. This entailed photographing the as-received component at the macro scale and examining specific areas with optical and electron microscopes at the micro scale, as well as identifying factors that contributed to the cracking. This analysis uncovered an additional crack in the longeron and determined that structural fatigue, the gradual weakening and eventual failure of a material due to cyclic stresses, caused both cracks.
“With this new information, we updated the model to improve the risk analyses,” Vargas said. “Consequently, the frequency of inspections was adjusted, though not drastically changed. To maintain aircraft structural integrity, models may need periodic updates to ensure the safety and functionality of the aircraft components.”
Optimised Inspection Schedules for Long-Term Safety
The updated inspection schedule ensured any risk remained low, balancing safety and efficiency until the remaining longerons could be replaced.
“This experience demonstrated our ability to quickly respond to unexpected issues,” Hunt said. “It has reinforced the importance of being prepared and the need for robust analysis methods.”
