A case study in the limitations of crack detection with respect to an aircraft brake system piston rod

A brake assembly piston rod from a RAAF aircraft had fractured as a result of tensile fatigue that had initiated within a relatively pronounced machining mark at the change in section from a stress relief groove to piston rod body. While cracking could be detected under Scanning Electron Microscope (SEM) examination, crack width was such that high sensitivity fluorescent dye penetrant inspection methods did not highlight cracking and ultra-high sensitivity fluorescent dye penetrant indicated the presence of cracking but not its full length. At the time of writing the OEM recommendations are for high sensitivity fluorescent dye penetration inspection to determine the presence of cracking within the stress relief groove. Approximately 360 similar aircraft (which includes a number of different variants), were understood to be in service, operating in fourteen (14) countries around the world. The risk to the aircraft and crew should a piston rod with an undetected or undetectable crack fail during an aborted take-off or during landing is potentially catastrophic.