The Problem
Aircraft structures are prone to fatigue. Metal structures will crack and if these cracks are left un-checked, they can lead to failure of the airframe. This ultimately drives the life of the aircraft, and results in heavy maintenance and inspection costs. Typical Non-Destructive Test methods employed are labor intensive and limited in operation. Sub-surface cracks are hard to detect.

The result is early grounding of aircraft and unnecessary shortening of the aircraft life. Many operators wish to continue to operate aircraft which have accrued a large number of flight hours and are unable to do so for these reasons.

Using technology originally designed for the monitoring of marine vessels, Appareo is working with Ultra Electronics (Middlesex, UK) to develop the Airborne Acoustic Integrity Monitoring System (AAIMS). This technology allows operators to detect cracks in the airframe at a very early stage, using real-time flight data.

The System Solution
A propagating crack releases a burst of energy in to the structure called and acoustic emission. In order to detect these acoustic emissions, the system deploys an array of piezoelectric sensors fixed to the aircraft structure.

These sensors feed signals into Data Concentrators which process this information to detect and store the acoustic emission events. This information is transmitted to the data storage unit which also gathers associated non-acoustic data (such as airspeed, altitude and any other aircraft parameters), in order to assist with mission profiling.

The data storage unit contains built-in GPS, gyros and accelerometers to provide non-acoustic data without the need for access to other aircraft systems. The unit also contains an SD card for the removal of data for analysis.

The architecture is designed to be very flexible to allow a user to define a system size suitable to their specific requirements. The system also uses triangulation to pin-point sensitive the location of cracks. The system is sensitive to different types of crack, such as fatigue cracks, stress corrosion cracks, sub-surface and surface breaking cracks. Versions of the system are being developed to operation on composite structures and in intrinsically safe applications (such as fuel tanks).