7.
In addition to scheduled overhauls, there are problems that arise from damage and defects. A proportion of these, which are uneconomic or impractical to rectify in the aircraft, necessitate unscheduled removals and require the engine to be returned to an engineering base or an overhaul shop for partial or complete overhaul.

8.
The purpose of overhaul is to restore an engine enabling it to complete a further life by complying with new engine performance acceptance limitations and maintaining the same reliability. This is achieved by dismantling the engine in order that parts can be inspected for condition and to determine the need for renewal or repair of those parts whose deterioration would reduce the performance, or would not remain in a serviceable condition until the next overhaul.

9.
The design of the modular constructed engine (Part 22, fig. 22-1) makes it particularly suited to a different technique of overhaul/repair. This technique is based on 'on-condition' monitoring (Part 24). This means that a life is not declared for the total engine but only certain parts of the engine. On reaching their life limit, these parts are replaced and the engine returned to service, the remainder of the engine being overhauled 'on-condition'.

Overhaul

10. Modular construction, together with associated tooling, enables the engine to be disassembled into a number of major assemblies (modules). Modules which contain life limited parts can be replaced by similar assemblies and the engine returned to service with minimum delay. The removed modules are disassembled into mini-modules for life limited part replacement, repair or complete overhaul as required.
OVERHAUL/REPAIR
11.
The high cost of new engines has a consider-able influence on the overhaul/repair arrangements, as the number of spare engines normally bought by the operator is kept to an absolute minimum. This means that an unserviceable engine must be quickly restored to serviceability by changing a module, or a part if the modular construction will permit it, or by careful scheduling of planned removals for overhauls at time expiry. This scheduling, through the workshop, of engines or modules that require the use of specialized equipment for repair is important, both to keep the flow of work even and to stagger removals to avoid aircraft being grounded by shortage of serviceable engines or modules.

12.
Because the work that is to be implemented must be planned and subsequently recorded, the engine or module is received in the workshop with documents to show its modification standard and its reason for rejection from service. The planning will include a list of the modifications that can or must be incorporated to improve engine reliability or performance or to reduce operating costs.

13.
The layout of the overhaul/repair workshop is designed to facilitate work movement through the complete range of operations, to achieve maximum utilization of floor space and to allow special equipment to be sited in positions that will suit the general flow pattern. All these considerations are aimed at achieving a quick turnround of engines. As an example of how shop layouts may be planned, a typical arrangement is shown in fig, 25-2.

Disassembly
14. The engine can be disassembled in the vertical or horizontal position. When it is disassembled in the vertical position, the engine is mounted, usually front end downwards, on a floor-fixture stool or a ram-top fixture. To enable it to be disassembled horizontally, the engine is mounted in a special turnover stand.
15.
When the floor-fixture stool is used, the personnel use a mobile work platform to raise themselves to a reasonable working position around the engine. When the ram-top fixture is used, the ram and engine are retracted into a pit, so enabling the workmen to remain at floor level.

16.
The engine is disassembled into main sub-assemblies or modules, which are fitted in trans-portation stands and despatched to the separate areas where they are further disassembled to individual parts. The individual parts are conveyed in suitable containers to a cleaning area in preparation for inspection.

Cleaning
17.
The cleaning agents used during overhaul range from organic solvents to acid and other chemical cleaners, and extend to electrolytic cleaning solutions.

18.
Organic solvents include kerosine for washing, trichloroethane for degreasing and paint stripping solutions which can generally be used on the majority of components for carbon and paint removal. The more restricted and sometimes rigidly controlled acid and other chemical cleaners are used for corrosion, heat scale and carbon removal from certain components. To give the highest degree of cleanliness to achieve the integrity of inspection that is considered necessary on certain major rotating parts, such as turbine discs, electrolytic cleaning solutions are often used.
　
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