SWRI To Characterize New Inspection Methods for Us Air Force Aircraft

Photo credit: Courtesy of SwRI

Nondestructive evaluation (NDE) is central to fracture control for spaceflight hardware, but it also adds cost and schedule pressure. This has led to questions about whether NDE requirements can be selectively reduced, or “descoped,” without increasing risk.

Southwest Research Institute (SwRI) has received a contract from the US Air Force Academy to characterize inspection methods for bolt holes in aging aircraft that allow evaluation through bushings without removing them. Assessing bolt hole condition during inspection supports broader efforts to guide repair decisions and maintain aircraft safety.

For decades, SwRI has supported the US Air Force’s Aircraft Structural Integrity Program (ASIP) and the Center for Aircraft Structural Life Extension (CAStLE), which help ensure aging military aircraft remain safe to fly. These programs combine damage-tolerance analysis, aircraft usage data, and nondestructive inspection results to determine inspection intervals and extend service life.

“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 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 subsurface discontinuities and cracks. Using lower frequencies allows inspection through the bushing without removal. The team is evaluating performance using test samples with engineered discontinuities created at SwRI and will develop probability of detection (POD) curves to quantify the detectability of these discontinuities.

Characterizing these methods will help aircraft maintainers better assess damage progression and make informed repair decisions to support safe operation.