# Development of a Maintenance and Spare Parts Distribution Model for Increasing Aircraft Efficiency
## 2 Materials and Methods
### Methods
#### 2.2.2 Maintenance and Spare Parts Management  
Industrial maintenance is defined according to DIN 31051 as the \"combination of all technical and administrative measures and management measures during the life cycle of an observation unit in order to maintain the functional state or the return of the same so that it can fulfill the required function\" [2]. In addition to the basic measures of maintenance, inspection, repair, and improvement, the maintenance area also includes the analysis of failure behavior, the improved detection of potential failures, and theprevention of accidents [4]. In this way, industrial maintenance encompasses the set of all measures that are necessary for the maintenance of a system [1]. Adopting a maintenance strategy is the method used to achieve the maintenance objectives. The maintenance strategy regulates when maintenance activities are carried out and what measures are included. When determining the maintenance strategy, legal, safety, technical, and production-relevant and economic aspects should be considered. The frequency of activities can be non-systematic or systematic. In the case of a non-systematic approach, maintenance measures are carried out when the need arises or damage occurs. With a systematic approach, measurements are taken periodically according to time or use intervals or depending on a determined or predicted status of maintenance objects [4]. With a periodic preventive maintenance strategy, the repair or replacement ofsystem components is carried out regardless of the actual condition of the component according to the defined use intervals. These can be determined based on timing (calendar time, hours of operation) or events (takeoffs and landings, distance travelled). Maintenance measures are carried out as scheduled in time windows and downtime is optimized by high repeatability and predictable processes. This contrasts with the fact that with a periodic preventative maintenance strategy, component replacement is generally carried out too soon, increasing the consumption of spare parts and materials [4]. Maintenance policies affect demand for spare parts. Preventive replacements often reduce the rate of random failures but can lead to wasted resources, so it is advantageous to coordinate maintenance activities and inventory control policies. Decision models should lead to maintenance and provisioning periods [2]. Therefore, increased use of preventive methods leads to a reduction in the cost of downtime while increasing the costs of prevention. The challenges in implementing a periodic preventive maintenance strategy are that all components of a technical system have different failure behavior and therefore expected lifespans. To optimize the utilization of component wear, regular maintenance measures should be carried out at different intervals. To this end, previous damages must be well documented to determine replacement intervals [4]. The spare parts identification process generally starts with technical considerations. However, the efficiency of this process is affected by the quantity and quality of information available. At the equipment acquisition stage, not much information is available to the buyer. Therefore, spare parts sourcing decisions are based on spare part kits, or similar equipment failure rates and expert estimates. A complete set of service life data is assumed to be available from accelerated tests performed or from sufficiently long operation of the equipment. Based on assumptions for each component, the lifespan function, reliability function, failure rate, and mean time between failures (MTBF) can be calculated. Once the selection step is applied to all components of a technical system, the quantities required during a given period can be determined [3]. In this context, the typical decisions that are made in maintenance related to the inventory are related to the items to be stored, quantity of items, place of storage, suppliers of materials, maximum and minimum levels of supply, when to buy and pay, and price.