Propellers that have been in operation for more than five years must be inspected using which technique?

Liquid penetrant testing is the correct answer because it is widely used to detect surface-breaking defects in non-porous materials, such as those commonly found on propellers. This method is particularly effective for inspecting components that may have experienced wear and tear after extended periods of operation, like the more than five years specified in the question.

Liquid penetrant testing involves applying a liquid that can seep into defects on the surface of the material. After allowing sufficient time for the penetrant to dwell, the excess is removed, and a developer is applied, which serves to draw the penetrant out of any defects. This process highlights the flaws against the surface, making them visible for inspection.

While other techniques, such as radiographic testing, magnetic particle testing, and ultrasonic testing, can also be used for different types of inspections, liquid penetrant testing is specifically suited for surface defects and is cost-effective and easy to perform. This is why it is often the preferred method for inspecting propellers that have been in service for several years.