There are “tens of thousands of aging military aircraft” flying with the world’s air forces and many of those aircraft are more than 25 years old, according to Rob McDonald, ASIS marketing director. “Many of these fleets are set to be still flying in 20 years or more, with some aircraft, such as the USAF KC-135 tankers, topping 80 years old before the last aircraft is withdrawn from service,” he said.

“As aircraft age, knowing where, when and how fast cracks might occur becomes an increasing burden on maintainers. A system that informs ground staff on the crack activity within structures that might otherwise not be routinely inspected for months, or even years, is a valuable tool,” McDonald said.

To that end, his company offers ASIS, a passive acoustic detection system that identifies cracks that can be caused by fatigue, corrosion or abrasion. Such cracks can be extremely small, below the surface, and in inaccessible areas that would make the use of existing non-destructive testing techniques difficult, time consuming or even impossible. ASIS can be retrofitted to any aircraft.

Sensors attached to the structure detect the acoustic emissions created by developing cracks. The precise moment that each sensor detects each emission is analyzed and the noise of the crack is compared with the ASIS system’s permanent memory of the acoustic signature of the structure. The ASIS user interface then displays a 3-D illustration of the structure, the crack’s location and potential severity.

The system’s built-in solid-state gyroscope, accelerometers, GPS and aircraft air data inputs allow ASIS to show the correlation of any crack emissions with the aircraft’s usage.

The on-aircraft components include an array of sensors and their associated pre-amplifiers, one or more remote data concentrators and a single data storage unit. Everything operates passively, simply listening for the noise created by cracks and storing that data for downloading when the aircraft is on the ground. The off-aircraft components consist of a tablet PC and a hand-held locator tool that, along with the PC displaying the location of the tool, can refine the crack position from within a few inches to the precise location.