CVM™ offers a novel method for in-situ monitoring of crack initiation and/or propagation. CVM™ is a measure of the differential pressure between fine galleries containing a low vacuum alternating with galleries at atmosphere in a simple manifold. If no flaw is present, the vacuum will remain at a stable level. If a flaw develops, air will flow through the passage created from the atmosphere to the vacuum galleries. Sensors may either take the form of self-adhesive polymer sensors or may form part of the component. A transducer measures the fluid flow between the galleries. CVM™ has been developed for use in both a laboratory environment or on operating equipment. The laboratory testing equipment has rapidly gained acceptance in testing facilities within the aviation industry; often used to control a fatigue testing program due to its proven reliability and built-in fail-safe properties. A portable unit is available to allow monitoring of components in operating equipment where access to critical components is difficult, time consuming and costly. This product is capable of being integrated with an aircraft to provide an in-flight structural health monitoring system (SHM). The In-flight structural health monitoring system developed using the CVM™ technology is able to continuously monitor the development of any cracks in predefined areas on an aircraft that are deemed to have a high risk of crack formation. The implementation of this technology will reduce the cost of mandatory structural integrity maintenance inspections, increase aircraft performance, and provide an increase in the safety of air travel. In-flight SHM using CVM™ system produced by SMS will consist of a number of different types of sensors placed at strategic locations throughout an aircraft that are linked to onboard instrumentation to detect and report on the initiation and growth of cracks.