PRELIMINARY LABORATORY MODULE:
GAS TURBINE ENGINE OVERHAUL

It is most important that the cost of maintaining an engine in service is considered at the design stage. All aspects of engine repairability are also considered, both to reduce the requirement for overhaul or repair and to avoid, where possible, designs which make repairs difficult to effect. Engine construction must allow the operator to complete the overhaul or repair work as quickly and cheaply as possible.

In service, the engine is inspected at routine periods based on manufacturers' recommendations and agreed between the operator and the relevant airworthiness authority. In the past, most engines had specified numbers of hours they could operate before they needed to be overhauled. This period became known as the time between overhauls (TBO). The length of time between overhauls varies widely with different types of engines. When a new type of engine enters service, its TBO is fairly short, but as condition monitoring, the engine's service record, and inspections prove the engine to be reliable, the TBO is generally extended. Many engines have proven to be so reliable that they are overhauled only when they need major maintenance. This concept is a form of "on condition" maintenance or overhaul.

Modular construction is a concept that treats an engine as a set of separate modules assembled together. The inspection, line maintenance, and overhaul requirements of each module can be addressed separately. The modular concept reduces down time and expense for operators because maintenance and overhaul activities are spread over a longer period of time. In most cases, replacement of a module is considered to be a minor repair, requiring no FAA Form 337. On the other hand, a form 337 is required for the overhaul of an engine module since it is considered a major repair.

The length of time between overhauls varies considerably between different engine types, being established as a result of discussions between the operator, the airworthiness authority and the manufacturer, when such considerations as the total experience gained with the particular engine series, the type of operation, the utilization, and sometimes climatic conditions, are taken into account. In improving the overhaul period the airworthiness authority may take into consideration the background of the operator, his servicing facilities and the experience of his maintenance personnel.