The concept of “serviceability” is related only to the technological maintenance works, so only the serviced product must have the qualities of serviceability, which determine the adaptability of the aircraft to the expected operating conditions and are not related to the airworthiness and serviceability of the aircraft. The ET properties are introduced into the aircraft TE system directively in the form of restrictions, which determine the technical perfection of an aircraft in relation to the expected conditions of its operation and correspondence of the expected and actual operating conditions of the aircraft. In case of their non-compliance, an unattended product may have a need for technological maintenance works, and in order to avoid this, a reasonable and reliable assessment of the expected operating conditions is required at the stages of aircraft design and manufacturing.

The concept of “recoverability” is related only to the works on maintenance and restoration of reliability, so only the recoverable product must have the qualities of repair manufacturability, which determine the adaptability of the product as a component of AC to perform these works in the expected conditions of AC operation, which also requires a qualitative and quantitative assessment of the influence of these operating conditions on the technical condition of products. The properties of RT are introduced in the system of TE of an aircraft in the form of limitations on the aircraft airworthiness in the process of its operation before decommissioning.

The notion of “testability” is related only to the works on technical condition control, which are common for all the articles of an aircraft. The need for these works is determined by the requirements for airworthiness of the aircraft during operation, which, depending on the characteristics of reliability and consequences of failures, form requirements for control of the depth, reliability, modes, methods and technologies. According to the current regulations the aircraft allowed to fly must be fully operational or if there are failures and malfunctions they must be included in the list of acceptable failures and malfunctions (MMEL/MEL), but in both cases the decision to allow the aircraft to fly requires information on the technical condition of the aircraft, obtained from the control results.

The complex of the above definitions from the unified methodological positions allows forming the need for maintenance and repair work on the aircraft. From these positions the aircraft as a whole is considered as an object of operation, the airworthiness and readiness of which for use is provided by the performance of maintenance and repair work on its individual elements (systems, products, components, parts). There are two groups of elements in aircraft as an object of maintenance and repair: structural and non-constructive:

1. Constructive element (E) is a separate item (unit, block, assembly, part), which is represented by physically separable samples.
2. Non-constructive element (G) – physically non-separable elements providing necessary active connection or functioning of constructive elements and represented in the form of common material entity or fixed labor.

For structural elements, the criterion of their fitness for use is the normative value of the reliability function (Rn)> which in the general case decreases monotonically with the increase of operating time. The serviceability of nonconstructive elements is characterized by the normative value of the quality parameter (77n), the dependence of which on the operating time can generally be of an arbitrary kind.

At the stage of aircraft operation the categories of elements are determined at the level of replaceable units, restorable parts, operational and technological connectors, docking and mounting assemblies, as well as input and output parameters of functioning (regulated and unregulated) systems and products, which allows to adapt the actual need for aircraft maintenance and repair works to the actual conditions of their operation and production activity of airlines.

Generalized classification of E and G elements according to their need in MRO works and corresponding requirements for ensuring the characteristics of ET, PT and CP of these elements are given in Table 2.4, according to which the initial performance qualities of the elements can be set a priori at the stage of aircraft design.

For example, serviced elements E have no need in target repair and replacement works, but they have a need in works on serviceability control, accordingly, qualities of ET and CP of these elements must be provided. The serviced element Eg. has a need for all kinds of target work and must have good ET, PT and CP characteristics, and the unattended element E4 does not need ET characteristics, etc.