# Development of a Maintenance and Spare Parts Distribution Model for Increasing Aircraft Efficiency
## 3 Conceptual Model Design
### Model Elements  
The elements of the conceptual model refer to those physical objects that are part of a spare parts distribution network. These elements are needed to analyze, develop, and improve the existing distribution network and to design and implement a new distribution network for spare parts. Figure 1 shows the elements of the conceptual model and their characteristics.  
The elements are not always available or known when defining the distribution network and maintenance management of a new maintenance system. The number and values of the elements develop over time. Moreover, the characteristics and their values have expected values that should be validated or adjusted over time withcomparisons of plan or predicted and actual or recorded data.  
\begin{table}
\begin{tabular}{c c c} \hline
**No.** & **Key Indicator** & **Formula** \\ \hline
1 & \(\mathcal{O}\) spare parts demand (units) & (\(\sum\) Spare parts demand)/Total number of weeks \\ \hline
2 & On-time delivery (OTD) (\%) &
\begin{tabular}{c} Sparte parts on time \\ Total spare parts ordered \\ \end{tabular} \(\times\) 100\% \\ \hline
3 & Service level (\%) &
\begin{tabular}{c} \(\sum\) Days without product backlog \\ Total number of. weeks \(\times\) number of locations \(\times\) 100\% \\ \end{tabular} \\ \hline
4 & \(\mathcal{O}\) spare parts backlog (units) & (\(\sum\) spare parts backlog)/Total number of weeks \\ \hline
5 & \(\mathcal{O}\) age of backlog (weeks) & [\(\sum\) backlog (units) \(\times\) age of spare parts (weeks)]/Total number of weeks \\ \hline
6 & \(\mathcal{O}\) stock in network (units) & (\(\sum\) Stock central and regional warehouses)/Total number of weeks \\ \hline
7 & \(\mathcal{O}\) stock in Central Warehouse (CW) & (\(\sum\) Stock central warehouse)/Total number of weeks \\  & (units) & (\(\sum\) Stock regional warehouses)/Total number of weeks \\ \hline
8 & \(\mathcal{O}\) stock in Regional Warehouses (RW) & (\(\sum\) Stock regional warehouses)/Total number of weeks \\  & (units) & (\(\sum\) Spare parts production)/Total number of weeks \\ \hline
9 & \(\mathcal{O}\) production (units) & (\(\sum\) Spare parts production)/Total number of weeks \\ \hline
10 & \(\mathcal{O}\) WIP (Work-In-Progress) (units) & (\(\sum\) Spare parts units in transportation process)/Total number ofweeks \\ \hline
11 & \(\mathcal{O}\) airplane availability (\%) & (\(1-\sum\) Aircraft on ground) \(\times\) 100\%/Total number of weeks \\ \hline
12 & \(\mathcal{O}\) airplane on ground (\%) & (\(\sum\) Spare parts demand)/Total number of weeks \\ \hline
14 & \(\mathcal{O}\) interval for planned maintenance & (\(\sum\) Maintenance interval for planned maintenance)/Number of planned \\  & (operating hours) & maintenance intervals \\ \hline
15 & \(\mathcal{O}\) interval for unplanned maintenance & (\(\sum\) Maintenance interval for unplanned maintenance)/Number of unplanned \\  & (operating hours) & maintenance intervals \\ \hline
16 & Inventory costs (mil. USD) & \(\sum\) (Procurement + Storage + Stock-out) costs \\ \hline
17 & Production costs (mil. USD) & \(\sum\) (Production cost per unit \(\times\) units produced) \\ \hline
18 & Transport costs (mil. USD) & \(\sum\) (Transport cost per unit \(\times\) units transported) \\ \hline \end{tabular}
\end{table}
Table 1: Key performance indicators (KPIs) for the target system (own elaboration).