# Through-Life Maintenance Cost of Digital Avionics
## 7 Results and Discussion
### Post-Warranty Maintenance  
Let's calculate the optimal quantity of spare LRU for different PWM options described in Section 5 when \(T_{PW}=40,000\ \mathrm{h}\), \(q=3\), \(t_{M}^{O-level}=0.5\ \mathrm{h}\), \(t_{TE}^{I-level}=t_{ATE}^{I-level}=3\ \mathrm{h}\), \(t_{FP,D}^{I-level}=t_{TE,D}^{I-level}=2\ \mathrm{h}\), \(\lambda=1\times 10^{-4}\mathrm{h}^{-1}\), \(T_{RS}=120\ \mathrm{h}\), \(T_{ED}=40\ \mathrm{h}\), and \(t_{STOP}=1\ \mathrm{h}\). Figures 11 and 12 show the dependences of the optimal quantity of planned spare LRU on the quantity of aircraft owned by the airline for the first and second PWM option, respectively. As we can see in Figure 11, the optimal quantity of spare LRU strongly depends on the conditional probability of in-flight false-positive and intermittent fault rate. Moreover, the dependency of \(PS\) on \(\theta\) (b) is more substantial than on \(P_{flight}^{FP}\) (a). Figure 12 shows the dependency of the optimal quantity of spare LRU on the quantity of aircraft for the second PWM option. Figure 12 corresponds to the case when \(P_{flight}^{FP}=\ 1\times 10^{-4}\ \mathrm{and}\ E\big{(}T_{repair}\big{)}=60.4\ \mathrm{h}\). (red circles), \(P_{flight}^{FP}=\ 1\times 10^{-3}\ \mathrm{,}\ E\big{(}T_{repair}\big{)}=25.4\ \mathrm{h}\) (black squares), and \(\theta=1\times 10^{-4}\ \mathrm{h}^{-1}\). Figure 12b matches with \(\theta=1\times 10^{-4}\ \mathrm{h}^{-1}\), and \(E\big{(}T_{repair}\big{)}=60.4\ \mathrm{h}\) (red circles), \(\theta=1\times 10^{-3}\ \mathrm{h}^{-1}\), \(E\big{(}T_{repair}\big{)}=14.3\ \mathrm{h}\) (black squares), and \(P_{flight}^{FP}=\ 1\times 10^{-4}\). Figure 10: The dependency of the optimal quantity of spare LRU on the quantity of aircraft for the second WM option; (**a**) Red circles: \(P_{flight}^{FP}=1\times 10^{-4}\), black squares: \(P_{flight}^{FP}=1\times 10^{-3}\); (**b**) Red circles: \(\theta=1\times 10^{-4}\ \mathrm{h}^{-1}\), black squares: \(\theta=1\times 10^{-3}\ \mathrm{h}^{-1}\). Comparing the graphs in Figures 11 and 12, we state that the optimal quantity of spare LRU is significantly less for the second PWM option than for the first. Figure 13 shows the dependency of the optimal quantity of spare LRU on the quantity of aircraft for the third PWM option. Figure 13a corresponds to the case when \(P_{flight}^{FP}=1\times 10^{-4}\) and \(E\big{(}T_{repair}\big{)}=3.8\) h (red circles), \(P_{flight}^{FP}=1\times 10^{-3}\) and \(E\big{(}T_{repair}\big{)}=3.3\) h (black squares),  
and \(\theta=1\times 10^{-4}\) h\({}^{-1}\). Figure 13b meets with \(\theta=1\times 10^{-4}\) h\({}^{-1}\) and \(E\big{(}T_{repair}\big{)}=3.8\) h (red circles), \(\theta=1\times 10^{-3}\) h\({}^{-1}\) and \(E\big{(}T_{repair}\big{)}=3.2\) h (black squares), and \(P_{flight}^{FP}=~{}1\times 10^{-4}\). As seen in Figure 13a, the optimal quantity of spare LRUs for the third PWM option is independent of the conditional probability of in-flight false-positive for \(N_{PW}=\overline{1,3,~{}9,20}\). When \(N_{PW}=\overline{4,8}\), the number of spare LRU for \(P_{flight}^{FP}=~{}1\times 10^{-3}\) is twice than that for \(P_{flight}^{FP}=~{}1\times 10^{-4}\). We should also note that the maximum number of spare LRU for the third PWM option (Figure 13a) is two times less than for the second option (Figure 12a). Comparing Figure 13a,b, we note that for given flight duration, the rate of intermittent faults has a greater impact on the quantity of spare LRU than the conditional probability of in-flight false-positive. Figure 14 shows the dependency of the optimal quantity of spare LRU on the quantity of aircraft for the fourth and fifth PWM options. Figure 14a corresponds to the case when \(P_{flight}^{FP}=~{}1\times 10^{-4}\) and \(E\big{(}T_{repair}\big{)}=4.7\) h (red circles), \(P_{flight}^{FP}=~{}1\times 10^{-3}\) and \(E\big{(}T_{repair}\big{)}=3.7\) h (black squares), and \(\theta=1\times 10^{-4}\) h\({}^{-1}\).