Continuous vacuum monitoring (CVM) sensors have been found to be effective means for detecting fatigue cracks growing on surfaces. The principle behind their operation is that a crack growing beneath the adhered sensor creates a leak path beneath tiny galleries. These galleries in the sensor pad are connected to a vacuum source and a very accurate flow meter. Associated electronics provide a low-skilled user with one of four unambiguous readings: no crack, crack present, vacuum line pinched, or sensor disbonded. The system is fail-safe in that a disbonded sensor will “flat-line”, triggering investigation of the condition with other methods.

A team involving Northwest Airlines, Boeing, Structural Monitoring Systems, and the FAA’s NDI Validation Center is currently conducting flight testing of CVM sensors in a DC-9 fuel cell, to provide an alternate means of compliance to mandated internal eddy current inspections. Further applications on DC-9 landing gear and aft pressure bulkheads are also being explored and will be described.