Optimal regulation

Customization of engine overhaul scope

Aircraft engines are not only responsible for providing propulsion, but they also supply energy for all the important aircraft functions, from control to supplying the cabin. Despite the immense demands made on them, they are required to be as reliable in operation as the proverbial Swiss watch. Being highly complex and particularly cost-intensive, engines are continuously monitored in operation and serviced at regular intervals. And when an engine is taken off the wing and sent to the shop for an overhaul, it will normally have spent many thousand hours in the air and flown millions of kilometers. The engine overhaul itself is very time-consuming since, depending on the engine type, up to 40,000 parts may need to be inspected and maintained. 

To permanently improve efficiency in the overhaul of aero engines, detailed knowledge of the internal interactions in the engine is essential. Lufthansa Technik is already using a maintenance concept that is based on continuous monitoring of important components. In this way, parts can be replaced as a function of their condition. A further efficiency improvement can be achieved when not only is the condition of individual components known but their effect on the behavior of the engine as a whole can be exactly stated. By establishing this link between component condition and the operating behavior of the engine it is possible to identify and then target critical components to work on. At the same time the actual condition of an engine and the individual modules can be ascertained more accurately through a suitable test run.

However, engine manufacturers are more interested in developing technologies for new engine types than in innovative methods that have the potential to improve the efficiency of maintenance. This is especially true of engine types no longer in production such as the CFM56-5A and CFM56-5C, which are used, respectively, on older aircraft in the Airbus A320 family and the A340-300.

Under the three-year Engine Performance Analysis innovation project Lufthansa Technik has therefore concentrated on optimizing engine maintenance for the CFM56-5C. The aim was to obtain information of its own about the influence of particular components on engine performance through targeted investigations and as a result to make itself no longer dependent on data supplied by the engine manufacturer. The insights gained in this way make it possible to increase thermodynamic performance and hence the efficiency of overhauled engines. Greater efficiency in turn leads to reduced exhaust gas temperatures (EGT), lower fuel consumption, reduced damage frequency and as a result to greater intervals between overhauls and hence cost savings on the maintenance side. The intention is to apply the results of the project in the future to other engine types as well as the CFM56-5C.

During the innovation project a multi-level thermodynamic model of the engine was developed that would not only enable detailed analysis of the actual condition but also elucidate the desired cause-and-effect link between component condition and engine operating behavior. This thermodynamic model consists of three levels. The first of these is the overall engine level, in which general engine parameters such as thrust, fuel consumption and EGT are ascertained. The second level is the module level, for which a simplified flow simulation is used. The third level consists of detailed computational fluid dynamics (CFD) flow models for individual groups of components. To improve the accuracy of analysis during test runs, improved instrumentation was also developed for the test rig.

The insights gained as a result of the work carried out so far make it possible to work selectively on the major components of the engine during an overhaul or to develop improved repairs. In addition, it is now also possible, based on an incoming inspection run to specify a customized overhaul scope in close consultation with the customer, with the aim of operating components that still function reliably such as expensive turbine blades for longer or to get the maximum use out of the available life limited parts (LLP).

The results of the Engine Performance Analysis are giving customers the option of ordering a customized overhaul for every engine. In the past many engine parts had to be replaced at predetermined points in time, irrespective of their technical condition. This is no longer the case. In addition, the innovation project is also contributing in a major way towards achieving greater efficiency, lower fuel consumption and hence lower exhaust gas emissions.