Ultra cracks fatigue issues with ASIS

 - July 18, 2010, 10:45 PM

With military aircraft are working harder and longer, the task of managing their service life safely and cost efficiently is becoming ever more critical. This has prompted Ultra Electronics Controls to conceive the ASIS aircraft structural integrity system, providing an innovative approach to monitoring and maintaining them.
ASIS, which is being launched here at the Farnborough airshow this week, is expected to provide more reliable and less time-consuming way to detect cracks anywhere in the airframe. It is based on passive acoustic detection technology that can detect extremely small cracks below the surface of aircraft structures in inaccessible places that would be difficult to access through existing nondestructive testing methods, such as eddy current and X-ray.

Sensors attached to the structure detect the acoustic emissions created by developing cracks, using piezo-electric sensors to listen for acoustic signals within a specific frequency bandwidth. The precise moment in time when each sensor detects each emission is analyzed and the noise generated by the crack is compared with the ASIS system’s permanent memory of the acoustic signature of that structure. ASIS then displays a 3-D illustration of the structure, showing the location and potential severity of each crack.

ASIS consists of a data storage unit and remote data concentrator developed by Appareo Systems, pre-amplifiers to amplify and digitize readings and a network of sensors, installed at intervals of around two feet. The system runs on the aircraft’s 28 VDC bus and draws minimal power.

The system is mainly intended to find cracks in aluminum structures. In theory, it could also be used to test composite surfaces, but this would require more work because composites have a significant different acoustic profile to metals.

“The way aircraft and structures are maintained hasn’t changed for decades,” said Ultra’s marketing director Rob McDonald. “Checks are based simply on flying hours since last check and no one knows if cracks are there until they strip and test an area.”

The cost of installing ASIS on larger military aircraft such as P-3 Orions would be in the range of $120,000 to $140,000, dropping to around $60,000 for a helicopter. The weight of the equipment is largely determined by the amount of harnessing required, so for a P-3 it would be around 50 pounds, but much lighter for a helicopter.

According to McDonald, civil aircraft operators will not be willing to accept this weight penalty, and, in any case, would have to change their maintenance philosophy fairly radically to adopt the ASIS approach. He believes that significant quantities of data generated from using the system in the military environment may change this attitude. In fact, ASIS is not intended to replace existing inspection techniques, but rather to complement them by providing earlier detection of cracks to avoid the need for more costly repairs once the damage has worsened.