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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.

Engine Disassembly Stand
Figure 3.8. Engine Disassembly Stand.

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.

Compressor Rotor Blade Damage Before and After Repair
Figure 3.9. Compressor Rotor Blade Damage Before and After Repair.



David L. Heiserman, Editor

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Revised: June 06, 2015