Inspecting aging Air Force jets without pulling apart their repairs

Every bolt hole in a military aircraft is a potential crack site. The holes sit in high-stress areas where metal flexes under aerodynamic loads, and over decades of service, fatigue cracks can propagate from these stress concentrators outward. When damage is found, maintainers often install a metal bushing - a cylindrical sleeve pressed into the hole to replace compromised material.

But here is the problem: to inspect the hole again later, you traditionally have to pull the bushing out. And removing a press-fit metal sleeve from a bolt hole risks causing the very damage you are trying to detect.

Seeing through the sleeve

Southwest Research Institute (SwRI) in San Antonio has received a contract from the US Air Force Academy to develop inspection methods that bypass this dilemma. The approach uses low-frequency eddy current testing, a nondestructive evaluation (NDE) technique that applies electromagnetic fields to detect subsurface flaws. By operating at low frequencies, the electromagnetic field penetrates through the bushing to reach the underlying hole surface, making it possible to check for cracks without any disassembly.

Nathan Richter, a senior research engineer at SwRI overseeing the project, explained that conventional NDE methods work well on exposed surfaces but struggle when intervening material blocks access. The bushing creates exactly that barrier. Low-frequency eddy currents solve the access problem by pushing the inspection field through the metallic sleeve.

Quantifying what the method can catch

Detection alone is not enough for aircraft maintenance. Engineers need to know the smallest flaw the method can reliably find. To characterize this, SwRI is fabricating test coupons - metal samples with intentionally created flaws of known sizes. By inspecting these coupons with the eddy current system and comparing results to the known flaw dimensions, the team will generate probability of detection (POD) curves that quantify the method's sensitivity.

These POD curves are the critical deliverable. Aircraft structural integrity programs set inspection intervals based on how quickly cracks grow and how small a crack the inspection method can catch. A method with well-characterized detection limits lets maintainers set confident inspection schedules - not too frequent (which wastes resources) and not too sparse (which risks missing a growing crack).

Decades of keeping aging jets flying

The work feeds into the US Air Force's Aircraft Structural Integrity Program (ASIP) and the Air Force Academy's Center for Aircraft Structural Life Extension (CAStLE). These programs use damage tolerance analysis, usage monitoring, and inspection results to keep military aircraft operational well beyond their original design life. Many airframes in the current fleet are decades old, and structural inspections are the primary mechanism for ensuring they remain safe.

SwRI has supported these programs for decades, developing and validating NDE techniques tailored to the specific inspection challenges that aging aircraft present. Bolt hole inspections are among the most common and most important, given that structural joints are both numerous and structurally critical.

What the project will not address

The contract covers characterization of the inspection method, not deployment across the fleet. The immediate output will be data on detection capability - the POD curves and supporting analysis. Whether the method gets adopted for routine maintenance inspections will depend on how those results compare to existing techniques and on the Air Force's assessment of operational practicality.

The approach is also specific to bolt hole geometries with press-fit bushings. Other structural inspection challenges - corrosion under paint, fatigue in composite panels, damage in honeycomb structures - require different NDE approaches. But for the specific and common problem of inspecting bushing-repaired holes, the low-frequency eddy current method could eliminate a significant source of maintenance-induced damage and reduce inspection time.