image: Southwest Research Institute (SwRI) has received a contract from the U.S Air Force Academy to characterize inspection methods for bushing repaired bolt holes in aging aircraft. Assessing bolt hole condition through inspection supports wider efforts that inform repair decisions and ensure aircraft safety.
Credit: Southwest Research Institute
SAN ANTONIO — March 16, 2026 — Southwest Research Institute (SwRI) has received a contract from the U.S Air Force Academy to characterize inspection methods for bolt holes in aging aircraft to inspect through bushings without removing them. Assessing bolt hole condition inspection supports wider efforts that inform repair decisions and ensure aircraft safety.
For decades, SwRI has supported the U.S. Air Force’s Aircraft Structural Integrity Program (ASIP) and the U.S. Air Force Academy Center for Aircraft Structural Life Extension (CAStLE), which ensures that aging military aircraft can continue to fly safely. These programs use damage tolerance analysis, aircraft usage data, and the results of nondestructive inspections to establish regular inspection intervals to extend the life of aging structures.
“It’s important to inspect locations where stress can lead to damage,” said SwRI Senior Research Engineer Nathan Richter, who oversees the project. “We use nondestructive evaluation (NDE) methods in areas where cracks may form. NDE helps detect damage without altering or removing a part but its sensitivity varies with the flaw size.”
Bolt holes require careful monitoring, because they exist in high-stress areas of the aircraft. When cracks or wear develop around a bolt hole, the damaged material is often replaced with a cylindrical metal sleeve called a bushing.
“Inspecting bushing-repaired holes is challenging because the bushing must be removed, risking further damage to the hole during removal and reinstallation,” Richter said.
To address this, SwRI is using low frequency eddy current testing, an NDE technique that applies electromagnetic fields to detect and characterize subsurface flaws and cracks; using low frequencies allows the inspection to detect flaws without removing the bushing. The method will allow the inspector to more easily inspect Air Force repairs. Its performance to flaws of different sizes is characterized by inspecting coupons with purposefully generated flaws produced by SwRI. These results will then be used to develop probability of detection, or POD, curves to characterize the flaw detection capability of the method.
Characterizing the performance of these methods will provide aircraft maintainers with the confidence to assess damage progression and make repair decisions to keep the aircraft flying safely.
For more information, visit https://www.swri.org/markets/defense-security/defense-aerospace-aircraft/aerospace-structures or https://www.swri.org/markets/chemistry-materials/materials/sensor-systems-nondestructive-evaluation-nde.