Page 275 - The Jet Engine

Overhaul

10. Modular construction, together with associated 15. When the floor-fixture stool is used, the
tooling, enables the engine to be disassembled into personnel use a mobile work platform to raise
a number of major assemblies (modules). Modules themselves to a reasonable working position around
which contain life limited parts can be replaced by the engine. When the ram-top fixture is used, the ram
similar assemblies and the engine returned to and engine are retracted into a pit, so enabling the
service with minimum delay. The removed modules workmen to remain at floor level.
are disassembled into mini-modules for life limited 16. The engine is disassembled into main sub-
part replacement, repair or complete overhaul as assemblies or modules, which are fitted in trans-
required.
portation stands and despatched to the separate
areas where they are further disassembled to
OVERHAUL/REPAIR individual parts. The individual parts are conveyed in
suitable containers to a cleaning area in preparation
11. The high cost of new engines has a consider- for inspection.
able influence on the overhaul/repair arrangements,
as the number of spare engines normally bought by Cleaning
the operator is kept to an absolute minimum. This 17. The cleaning agents used during overhaul
means that an unserviceable engine must be quickly range from organic solvents to acid and other
restored to serviceability by changing a module, or a chemical cleaners, and extend to electrolytic
part if the modular construction will permit it, or by cleaning solutions.
careful scheduling of planned removals for overhauls
at time expiry. This scheduling, through the 18. Organic solvents include kerosine for washing,
workshop, of engines or modules that require the use trichloroethane for degreasing and paint stripping
of specialized equipment for repair is important, both solutions which can generally be used on the
to keep the flow of work even and to stagger majority of components for carbon and paint
removals to avoid aircraft being grounded by removal. The more restricted and sometimes rigidly
shortage of serviceable engines or modules. controlled acid and other chemical cleaners are used
for corrosion, heat scale and carbon removal from
12. Because the work that is to be implemented certain components. To give the highest degree of
must be planned and subsequently recorded, the cleanliness to achieve the integrity of inspection that
engine or module is received in the workshop with is considered necessary on certain major rotating
documents to show its modification standard and its parts, such as turbine discs, electrolytic cleaning
reason for rejection from service. The planning will solutions are often used.
include a list of the modifications that can or must be
incorporated to improve engine reliability or 19. Aircraft which operate at high altitudes can
performance or to reduce operating costs. become contaminated with radio-active particles
held in the atmosphere, this radio-activity is retained
13. The layout of the overhaul/repair workshop is in the dirt and carbon deposits in the engine.
designed to facilitate work movement through the 20. If contamination is suspected the radio-activity
complete range of operations, to achieve maximum level of the engine must be determined to ensure the
utilization of floor space and to allow special limitations agreed by the health authorities are not
equipment to be sited in positions that will suit the exceeded, Evidence of contamination will entail
general flow pattern. All these considerations are additional cleaning in a designated region, separate
aimed at achieving a quick turnround of engines. As from the overhaul area, to safeguard the health of
an example of how shop layouts may be planned, a personnel in the workshop. Arrangements have to
typical arrangement is shown in fig, 25-2. made with the health authorities for disposal of the
waste radio-active cleaning material.
Disassembly
14. The engine can be disassembled in the vertical
or horizontal position. When it is disassembled in the Inspection
vertical position, the engine is mounted, usually front 21. After cleaning, and prior to inspection, the
end downwards, on a floor-fixture stool or a ram-top surfaces of some parts, e.g. turbine discs, are
fixture. To enable it to be disassembled horizontally, etched. This process removes a small amount of
the engine is mounted in a special turnover stand. material from the surface of the part, leaving an even

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