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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 subassemblies
or modules, which are fitted in transportation
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.
19. Aircraft which operate at high altitudes can
become contaminated with radio-active particles
held in the atmosphere, this radio-activity is retained
in the dirt and carbon deposits in the engine.
20. If contamination is suspected the radio-activity
level of the engine must be determined to ensure the
limitations agreed by the health authorities are not
exceeded, Evidence of contamination will entail
additional cleaning in a designated region, separate
from the overhaul area, to safeguard the health of
personnel in the workshop. Arrangements have to
made with the health authorities for disposal of the
waste radio-active cleaning material.
Inspection
21. After cleaning, and prior to inspection, the
surfaces of some parts, e.g. turbine discs, are
etched. This process removes a small amount of
material from the surface of the part, leaving an even
Overhaul
265
Overhaul
266
Fig. 25-2 Typical overhaul workshop layout.
matt finish which reveals surface defects that cannot
normally be seen with the naked eye. The metal
removal is normally achieved either by an electrolytic
process in which the part forms the anode, or by
immersing the part for a short time in a special acid
bath. Both methods must be carefully controlled to
avoid the removal of too much material.
22. After the components have been cleaned they
are visually and, when necessary, dimensionally
inspected to establish general condition and then
subjected to crack inspection. This may include
binocular and magnetic or penetrant inspection
techniques, used either alone or consecutively,
depending on the components being inspected and
the degree of inspection considered necessary.
23. The non-dimensional inspections can be
divided into visual examination for general condition
and inspection for cracks. The visual examination
depends on the inspector’s judgement, based on
experience and backed by guidance from the manufacturer.
Although the visual examination of many
parts of the engine conform to normal engineering
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