The challenge NASA faced in the early 1970s was to test its equipment on Earth (e.g. landing vehicles), but then also to ensure, that after the tests all components were in order. The result of this study was published in 1972 and the result was unambiguous:
No other technology can detect failures on bearings earlier than ultrasound. The somewhat bulky study can be viewed here. NASA STUDY
The results of the NASA were confirmed in numerous follow-up studies, a somewhat more recent study can be found here: Ultrasound as an early warning system
The portfolio of modern condition monitoring offers many technologies. The P/F diagram provides a frequent comparison of the technologies, where P stands for "Potential Failure" and F for "Functional Failure". The "P" therefore indicates a potential failure, since problems in components can be detected, while the "F" stands for the actual failure of the component's function.
Ultrasound therefore allows the planning time to be maximized.
