# Through-Life Maintenance Cost of Digital Avionics
## 5 Post-Warranty Maintenance Models of Avionics Systems
Conventional ATE may not be used to test intermittent faults for the following reasons [35]: ATE does not check simultaneously all circuits or functional paths of the LRU, including all connection paths to SRU, ATE do not test the LRU in the appropriate operating environment, and ATE are incapable of detecting short-duration intermittent faults that cause NFF. Given the conventional ATE disadvantages, as stated above, specific specialized tools for intermittent fault diagnostics are currently in development. For instance, Universal Synaptics Corporation (USA) produces a Voyager Intermittent Fault Detector (IFD) [37] and an Intermittent Fault Detection and Isolation System (IFDS) [38]. Therefore, in the third variant of the maintenance system with O- and I-level, both ATE and IFD are used at the I-level to check the LRU and detect faulty SRU. The combination of ATE and IFD allows checking the dismounted LRU for permanent failures and intermittent faults and detecting faulty SRU. The repair of LRU is performed by replacing the identified faulty SRU. Further, the airline will ship the SRU with detected defects to the outsourcing company or manufacturer for repair. We should note that IFD can also be used to detect intermittent faults in SRU with the depth up to non-repairable elements. The expected maintenance cost for this PWM option comprises the same cost components as for the last variant, as well as the additional cost of IFD and operations to detect SRU with intermittent faults. We determine EMCPW by the following formula:  
\[EC_{PWA}=q\left[LC\left(t_{M}^{O-level}+t_{ATE}^{I-level}\right)+\left(C_{PF,R}+ LC\times t_{PF,D}^{I-level}\right)P_{PF}+\left(C_{IF,R}+LC\times t_{IF,D}^{I-level} \right)P_{IF}+\right.\]  
\[\left.C_{TR,SRU}\right]ER(T_{PW})+C_{ATE}^{I-level}/\left(N_{PW}\times F_{ATE}^{ I-level}\right)\ +C_{IFD}/\left(N_{PW}\times F_{IFD}^{I-level}\right)\ + \tag{38}\]  
\[\left[C_{LRU}(PS+US)+\sum\limits_{j=1}^{n}C_{j}SRU_{j}\right]/N_{PW}\,\]  
where \(C_{IF,R}\) is the mean cost of repairing the SRU, because of detected intermittency, at the outsourcing company or manufacturer, \(t_{IF,D}^{I-level}\) is the mean time of detecting the place of an intermittent fault in the dismounted LRU with depth to SRU using IFD, \(C_{IFD}\) is the IFD cost, and \(F_{IFD}^{I-level}\) is the quantity of LRU types in which IFD can detect intermittent faults. For the fourth PWM option the average LRU repair time we calculate as follows:  
\[EC_{PW,2}=q\Big{[}(C_{TR}+C_{PF})P_{PF}+LC\Big{(}t_{M}^{O-level}+t_{TE}^{I-level }\Big{)}\Big{]}ER(T_{PW})+\]0Analogically to (37), by satisfying the following condition, we determine the optimum quantity of the \(j\)-th type spare SRU:  
\[EC_{PW,2}=q\Big{[}(C_{TR}+C_{PF})P_{PF}+LC\Big{(}t_{M}^{O-level}+t_{TE}^{I-level }\Big{)}\Big{]}ER(T_{PW})+\]1  
where \(\theta_{j}\) is the intermittent fault rate of the \(j\)-th type SRU. We remind you that level D maintenance is possible to implement if a repair shop has diagnostic equipment for detecting the place of failures and faults in printed circuit boards judged as faulty at the I-level. Maintenance at level D will operate successfully if the warehouse provides the repair shop with spare electronic components. The expected maintenance cost of this PWM option comprises the following constituent elements: the cost of dismounting and mounting the LRU onboard the airplane within the time-interval \(T_{W}\) concerning O-level maintenance, the cost of checking the dismounted LRU and identification of faulty SRU by ATE concerning I-level maintenance, the cost of troubleshooting and repairing SRU concerning D-level maintenance, the cost of ATE, IFD, spare LRU, SRU, electrical and electronic elements recalculated per one aircraft.