Abstract: Method and apparatus for continuously detecting cracks in aircraft blades while in flight. A hollow blade is sealed with a pressure differential, either above or below atmospheric. A reduction in the pressure differential such as results from cracks, is detected by a pressure sensor having a member which moves between a high pressure differential position and a low pressure differential position. A radiation source, typically a beta source, is carried on the member and moves from a shielded position to an unshielded position when the pressure differential drops. A radiation detector is mounted on a fixed portion of the aircraft and detects radiation from the source when the member moves to the unshielded position. The detector output may be used to operate a go-no go indicator providing a warning signal when the pressure differential falls below a predetermined value, or to operate an analog indicator providing an output varying as a function of the pressure differential.