3.11.
OVERHAUL AND REPAIR
The time between overhauls (TBO) varies considerably between
different engines. The TBO is established by the Army and the engine
manufacturer who take into account the kind of operation and use
expected for the engine, also the environment it will be operating
in. As a specific model engine builds up operating time, it is
inspected for signs of wear and impending failure of parts. If the
engine is wearing well, the TBO is extended. The large improvement
of TBO has been accomplished mainly through improvements in engine
design, metallurgy, manufacturing, overhaul, inspection, and
maintenance procedures. The use to which the engine is put is
especially important in determining the TBO. For example, if the
mission the aircraft is designed for calls for frequent starts and
stops, or for power changes as in a helicopter, the resultant rapid
temperature changes will shorten the allowed TBO. The following
paragraphs cover disassembly, assembly, and repair procedures.
a. Disassembly. Engine disassembly can be accomplished on a
vertical or horizontal disassembly stand as shown in figure 3.8.
Some engines can be disassembled either horizontally or vertically,
while others have to be done in only one position. After the engine
is disassembled, the major components and section assembly are
mounted on individual stands. To disassemble an engine, instructions
in the TM must be followed, and a large number of special tools is
required. A set of these tools may cost as much as the engine.
b. Assembly. Engine assembly also follows instruction in the
TM; it is done on the same stand as disassembly. During assembly,
care must be taken to prevent dirt and other foreign materials from
entering the engine. The procedures and use of special tools as
outlined in the maintenance manual must be followed to minimize
possible injury to the mechanic and damage to the engine.
c. Repair. All engine parts must be repaired using methods
approved by the engine TM. Figure 3.9 shows an illustration of
typical repair limits for compressor rotor blades on the Lycoming T53
series engines.
3.12.
MAINTENANCE ALLOCATION CHART
Maintenance function assignments are determined by the
maintenance allocation chart found in the aircraft 20 technical
manual. The maintenance allocation chart assigns functions to the
lowest capable maintenance level based on past experience, and the
skills, tools,
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