**Acknowledgements**

relatively small (20% of the total cost saved). It is also noted that cost saved by the

Inspection cost

SHM replacement Fuel cost Total

Crack repair cost

Total

<sup>3</sup> <sup>10</sup><sup>5</sup> 1.2 <sup>10</sup><sup>6</sup> 1.5 <sup>10</sup><sup>5</sup> 1.65 106

Removing/installing cost

1.1 107 7.56 105 1.62 <sup>10</sup><sup>6</sup> 4.9 104 1.34 <sup>10</sup><sup>7</sup>

1.18 <sup>10</sup><sup>7</sup>

In addition to the cost savings calculated, some further potential benefits may be gained by using SHM system on condition-based maintenance. Firstly, skipping the removing/installing surrounding structure procedures in SHM systems not only saves time and labor but also prevents potential damage to structures caused by the removing/installing process. Although the fasteners can be replaced after each removing/installing, the fastener holes, taking rivet holes, for example, will be enlarged after each repair work. This is an irreversible damage and might also be the source of new cracks. Even worse, some accidents might occur during the removing/installing, such as drilling through unrelated structures. All these are troublesome issues reported by MRO companies and airlines frequently. With the

Secondly, maintenance is more predictable with the SHM system. In scheduled maintenance, damages are detected by manual inspection in the hangar. For some unexpected damages, several days are wasted on preparing special equipment, tools, and/or materials. Sometimes, it even takes a week or so to confirm a repair

In condition-based maintenance, however, by monitoring the cracks continuously using the SHM system combined with the Paris-Erdogan model and the MCS to model the growth, crack growth and size are more predictable, thus stepping up

Furthermore, with the ongoing research on sensors and actuators, its detection ability will not only be confined on cracks; it can also be used for detecting other typical structural damages such as corrosion, dents, holes, delamination, etc. By collecting and analyzing all the data from the SHM system, the structures on which certain damage frequently occurred affecting the safety of aircraft could be found. These can be posted in Airworthiness Directives (AD), Service Bulletins (SB), or Service Letters (SL), to help eliminate the potential safety issues in the whole fleet

Two maintenance approaches are discussed in this chapter. Traditionally, scheduled maintenance is carried out at predetermined intervals to maintain a

reduced number of cracks repaired is negligible.

*Summary of cost increased and decreased for two maintenance approaches.*

Manufacturing cost

*Reliability and Maintenance - An Overview of Cases*

Net revenue saved

Cost increased

Cost decreased

Total cost saved

(\$)

(\$)

(\$)

**Table 3.**

**6. Potential advantages of condition-based maintenance**

introduction of an SHM system, this problem may be eliminated.

plan by consulting the manufacturer of the aircraft.

maintenance and repair work.

of same aircraft model.

**7. Conclusions**

**38**

This research was partly supported by NASA Langley Research Center (Contract No. NNX08AC33A). The authors gratefully acknowledge this support.
