# Through-Life Maintenance Cost of Digital Avionics
## 1 Introduction  
Modern avionics comprises redundant electronic systems. Each such system may contain two or three identical line replaceable units (LRU). Each LRU operates to a safe failure, which is recorded in-flight or after landing. Since LRU is recovered after a failure, this maintenance strategy is called run-to-failure. The choice of the implementation of the maintenance of avionics systems largely determines the aircraft operation's efficiency. After the start of its service, the entire life cycle of the aircraft can divide into two stages: warranty and post-warranty period. It is desirable for each period to select the optimal maintenance option, thus minimizing the maintenance cost over the life cycle. If the airline buys a new plane, it is necessary to choose a warranty service option and assess the possible fines imposed on the supplier if the warranty is not complied with. One of the most critical tasks that must be performed when purchasing a new aircraft is to prove the warranty service's reasonableness and evaluate the buyer's cost upon the warranty period. During the operation period after the warranty, the aircraft owner pays all maintenance costs. Modern avionics systems are characterized by a high level of testability and maintainability. Shop replaceable units (SRU) are the interchangeable components of an LRU. An SRU is generally a printed circuit board assembly that can be replaced at a backstop. Modern electronic LRU usually has built-in test equipment (BITE) for in-flight testing. The described design of avionics systems gave rise to the following three levels of maintenance: organizational maintenance (O-level), intermediate maintenance (I-level), and depot maintenance (D-level). LRU rejected in-flight is dismantled from aircraft andthen serviced at the O-Level, also called a flight line maintenance. Specialized backshops located at the airline base usually conduct maintenance at the I-level. I-level support is more particularized, allowing testing and recovering rejected units at the O-level. The repairing of the faulty LRU at the I-level is carried out by replacing the defective SRU. Consequently, the spare parts at the I-level are SRU. D-level maintenance provides the repair of SRU rejected at the I-level. The spare parts at this maintenance level are electrical and electronic components. At present, some airlines eliminate D-level and occasionally even I-level maintenance by outsourcing the heavy work to repair stations. Major US airlines often use outsourced repair stations, with about 24% of heavy aircraft servicing done outside the US [1]. However, American Airlines, one of the country's largest airlines, fulfills most of the support internally. Therefore, there are many options for using one-, two-, or three-level systems to service avionics. We need to have an objective function for comparing alternatives among possible maintenance options. The objective function can be a maintenance cost, considering the essential technical characteristics. Avionics failures cause many situations like \"aircraft on the ground\", which lead to significant economic losses [2]. Therefore, the maintenance cost function should include characteristics of operational reliability. A study by the FAA found that flight delays in 2007 cost airlines an estimated $31 billion [3]. Permanent failures and intermittent faults represent most of the avionics refusals. The major causes of unconfirmed failures in civil and military aviation are intermittent faults and false alarms. We should note that intermittent faults and false alarms are a significant root of so-called no fault found (NFF) occasions in onboard electronic systems. In avionics, the estimated level of NFF is 20% to 50% [4; 5], leading to about 27% of unscheduled removals of avionics LRU [6]. The aviation industry's false alarm rate is very high, achieving 28% of all incidents related to alarms [7].