**4. Parameters assumed for scheduled and condition-based maintenance**

simulations. A fleet of 20,000 airplanes with 500 initial cracks per airplane due to manufacturing or previous maintenance are considered. These cracks are distributed on fuselage skins. The initial crack size and crack growth parameters ð Þ *m; ath* are randomly sampled for each crack. Pressure is also assumed to vary in each flight. The fraction of cracks that cause fuselage skins to fail is computed for different values of skin thickness, and the variation is plotted in **Figure 4**. Based on **Figure 4**, a fuselage skin with a minimum thickness of 0.06 in (1.53 mm) is required to

*Advantages of Condition-Based Maintenance over Scheduled Maintenance Using Structural…*

is the most common thickness of a typical fuselage skin, this calculation provides a

in Appendix C, and is proven to satisfy the desired level of safety.

**5. Cost comparison between two maintenance processes**

tural Repair Manual and estimated the cost in the maintenance field.

In condition-based maintenance, the threshold for scheduling aircraft to maintenance must be chosen in such a way so as to satisfy the reliability constraint until the next maintenance assessmentð Þ *Nshm* . The latter has been chosen as 180 flight cycles, which is equivalent to the current A check interval. If say the threshold for requesting maintenance ð Þ *ath* is fixed at 1.57 in (40 mm), the reliability for the given value of *ath* and *Nshm* can be computed using a direct integration procedure, detailed

In this chapter, the lifecycle cost of an airplane is considered to be the sum of manufacturing cost, fuel cost incurred during lifecycle, and maintenance cost. Other costs that remain constant for two different approaches are not considered. Cost comparison of two maintenance approaches is discussed in two aspects: cost increase and cost decrease. **Table 1** summarizes the parameters that are used for cost calculation for the two maintenance processes based on Boeing 737-300 Struc-

Based on the Structure Repair Manual of a Boeing 737-300, the fuselage skin in the pressurized area is not a regular cylinder. However, it was assumed to be a cylinder to simplify calculation, by using the average diameter *D* ¼ 148in. In addition, the length of the cylinder can be calculated as *L* ¼ 977in. As already stated, the thickness of the fuselage skin varies from station to station; however, the most common thickness of *t* ¼ 0*:*063in is used herein. In addition, the density of fuselage skin, which is made of aluminum alloy 2024-T3, is about *<sup>ρ</sup>* <sup>¼</sup> <sup>0</sup>*:*1lb*=*in3. Therefore, the total weight of fuselage skin in the pressurized area is *W* ¼ *πDLtρ* ¼ 2957lb.

Weight of fuselage skins 2957 lb. [10] Interval of C check 2800 flight cycles Life cycles 50,000 flight cycles Net revenue lost due to downtime \$27,000/airplane/day Labor cost in hangar \$60/h

. Considering that 0.063 in (1.6 mm)

achieve the target probability of failure of 10�<sup>7</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.83614*

reasonable estimate.

**5.1 Cost increased**

**Table 1.**

**35**

(1) Manufacturing cost.

*Parameters for maintenance cost calculation.*

Manufacturing cost with SHM system: \$600/lb. Manufacturing cost without SHM system: \$500/lb.

Cracks that are missed or intentionally left unattended during maintenance and grow to critical size before the next maintenance interval affect the safety of the aircraft structure. In the case of scheduled maintenance, the thickness of the fuselage skin ð Þ*t* , the interval of scheduled maintenance ð Þ *Nman* , and the threshold for repair *agvi* affect the aircraft's safety, which is influenced by the thickness of the fuselage skin ð Þ*t* , the frequency of maintenance assessment ð Þ *Nshm* , and the threshold for requesting maintenance ð Þ *ath* .

This section deals with quantifying the range of parameters for scheduled and condition-based maintenance. As such, each damage instance is modeled as a through-the-thickness center crack in an infinite plate subject to Mode-I fatigue loading, as shown in Appendix A. The uncertainty in the loading condition and material parameters are summarized in **Table 4**. A crack grows due to pressure differential between the cabin and atmosphere, which is modeled by the Paris-Erdogan model, as shown in Appendix A. From fracture mechanics, the critical crack size (Eq. (3)) to cause failure of a fuselage skin depends on the pressure load and, hence, may also be modeled as a probability distribution. This chapter considers a fuselage skin to be failed if the crack grows undetected beyond the 10�<sup>7</sup> percentile of critical crack size distribution.

In the scheduled maintenance of a B737-300/400/500, the C check is carried out at about every 2800 flight cycles ð Þ *Nman* ¼ 2*;* 800 [4] for an airplane life of 50,000 flights. The threshold for repair is equal to the detection capability of GVI, *agvi* ¼ 0*:*5 in (12.7 mm). The fraction of cracks which cause failure of fuselage skins due to excessive crack propagation until the end of life is computed by Monte Carlo

#### **Figure 4.**

*Variation of lifetime (50,000 flight cycles) probability of failure as a function of fuselage skin thickness for scheduled maintenance at every 2800 flight cycles.*

*Advantages of Condition-Based Maintenance over Scheduled Maintenance Using Structural… DOI: http://dx.doi.org/10.5772/intechopen.83614*

simulations. A fleet of 20,000 airplanes with 500 initial cracks per airplane due to manufacturing or previous maintenance are considered. These cracks are distributed on fuselage skins. The initial crack size and crack growth parameters ð Þ *m; ath* are randomly sampled for each crack. Pressure is also assumed to vary in each flight. The fraction of cracks that cause fuselage skins to fail is computed for different values of skin thickness, and the variation is plotted in **Figure 4**. Based on **Figure 4**, a fuselage skin with a minimum thickness of 0.06 in (1.53 mm) is required to achieve the target probability of failure of 10�<sup>7</sup> . Considering that 0.063 in (1.6 mm) is the most common thickness of a typical fuselage skin, this calculation provides a reasonable estimate.

In condition-based maintenance, the threshold for scheduling aircraft to maintenance must be chosen in such a way so as to satisfy the reliability constraint until the next maintenance assessmentð Þ *Nshm* . The latter has been chosen as 180 flight cycles, which is equivalent to the current A check interval. If say the threshold for requesting maintenance ð Þ *ath* is fixed at 1.57 in (40 mm), the reliability for the given value of *ath* and *Nshm* can be computed using a direct integration procedure, detailed in Appendix C, and is proven to satisfy the desired level of safety.
