Understanding the Engine

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1 Source: Lufthansa Technik Understanding the Engine Engines rely on so many complex, interconnected components that a perfect understanding of how each affects the other over thousands of cycles will always remain elusive. Nonetheless, significant strides are being made in data analysis, inspection technology and industrial research. Susi Debus talks to Lufthansa Technik about how these are being used to optimise engine maintenance processes and planning. ommercial aircraft take off in their C thousands every day. Safety is always the most important consideration, but the enormous cost pressures faced by operators and maintenance companies are playing an increasingly significant role. Operators must keep a tight lid on running costs to survive cut-throat competition from budget and other airlines, while MRO providers must cope with flat-rate contracts and other aftermarket plays by manufacturers. A number of information systems, such as the Aircraft Condition Monitoring System (ACMS) and Aircraft Communications Addressing and Reporting System (ACARS), assist MRO providers with maintenance of the engine and aircraft. ACMS monitors the status of on-board systems and components and collects a broad range of flight data, sending it under certain conditions to ground stations using the ACARS data communication system. The frequency of transmission varies according to the operator. Two different types of engine report, one during take-off and one during cruise, provide information on the status of the systems. MRO providers can receive these inflight reports from their customers engines. Lufthansa Technik, for instance, receives them at its central data center in Frankfurt, Germany. These reports are used for what is termed engine condition monitoring (ECM). Key parameters such as engine speed, exhaust gas temperature, fuel flow, vibrations and oil consumption are monitored in ECM and their development trends tracked over time. Both normal and abnormal aging processes can be established from the changes. What must be avoided in any case is that a healthy engine is removed and sent to the shop. Christian Werner-Spatz, project manager for innovation, engine services, Lufthansa Technik 2

2 Moreover, sudden engine damage can be identified from jumps in the trend curves. This procedure has been standard for decades and is an important requirement for the oncondition maintenance that is typical today. When an engine from long-haul operation comes into the shop, for example for overhaul, it has performed around 3,000 cycles, has spent more than 25,000 hours in the air and has been in operation for nine hours per day on average. Such longevity between shop visits would not be conceivable without the continuous monitoring of ECM. One of the major advantages of ECM is to pre-empt major problems with engines and the interruptions to flight operations they cause. First, any minor damage must be reliably identified and diagnosed. This means that the ECM system unequivocally identifies the existence of damage, and assigns it to a module or component in the engine. Such diagnosis is the prerequisite for taking an appropriate counter-measure, be it replacing the engine or an on-wing repair. The speed and extent to which damage can be identified and diagnosed is therefore a quality criterion for an ECM system and is an area where a certain level of development potential still exists, says Christian Werner- Spatz, engineer and project manager for innovation in the engine services product division at Lufthansa Technik. However this should not be limited to damage that occurs suddenly, but should also cover gradual wear that exceeds normal thresholds, he adds. HOW ENGINES AGE Werner-Spatz s second condition is difficult to fulfill, as it requires the ability to differentiate between wear attributable to normal aging processes, and uncommon wear that results from problems in the engine. It is important in this respect to understand the aging processes in the engine, says Werner-Spatz. And this in turn is dependent on having all of the available data as well as a solid, physics-based model of the engine to use as a basis. Understanding the aging processes in engines is a necessary basis for not only identifying and diagnosing existing problems, but also for predicting the ongoing development of the condition. At this point we are no longer simply talking about engine condition monitoring, rather proper engine health management, says Werner-Spatz. The speed and extent to which damage can be identified and diagnosed is a quality criterion for an ECM system and is an area where a certain level of development potential still exists. Christian Werner-Spatz AST repair at Santiago de Chile 4

3 A CFM56 overhaul Engine health management means identifying and classifying problems at an early stage, and then using a reliable projection of progressive development to decide how and when to respond. This can sometimes mean that an engine has to be removed prematurely to avoid serious damage. On the other hand, not all damage automatically means that a shop visit is necessary. Should an unforeseen incident occur with an engine, for example as a result of a bird strike, Lufthansa Technik offers a 24/7 airline support service. Following preliminary diagnosis of the incident, an Airline Support Team (AST) is dispatched to the incident location within the shortest possible time along with tools and replacement parts. And with new repair processes, damage that might once have necessitated a shop visit can be addressed at the customer s location. For instance, Lufthansa Technik s Blending Scope can fix damage to compressor blades overnight by means of regrinding. In the past, though, this problem might have required complete dismantling of the engine. Lufthansa Technik s positive experience with mobile services, such as AST, has led it to question the expensive engine overhauls of the past. The more diagnosis and repair we can do on site and on wing during ongoing flight operation, the less work is required in the shop. This saves a lot of time and money in the long term, comments Werner-Spatz. BORESCOPE INSPECTIONS Engine problems cannot always be reliably diagnosed with ECM data alone, as its analysis often leaves several options open as to the precise cause of the damage. These options can be narrowed down by borescope insepctions on the ground. These visual inspections use a flexible hose fitted with a lens, which can be introduced into the particular cavity of the engine that is to be examined. While optical fibers were formerly used to transmit the image from the lens to the viewer, the image is now typically displayed on a screen using digital technology. Thus the borescope supplies images of locations that would never be accessible without the removal of larger components. 5

4 EVE project s experimental engine in test cell Source: Jan Brandes When an engine from long-haul operation comes into the shop it has performed around 3,000 cycles, has spent more than 25,000 hours in the air and has been in operation for nine hours per day on average. The ECM generally provides initial findings and more detailed investigations are then carried out on site. The borescope is already very important for many actions carried out by mobile service teams, since the data obtained allows binding statements to be made concerning the condition inside the engine, and improves forecasting. Nevertheless, the efficiency of borescope procedures needs to be improved to allow even broader application. Optimizing existing diagnostic procedures is just one possibility, however. It is even more efficient to collect as much data as possible in a separate system, and then to correlate this data in order to obtain reliable predictions concerning the actual condition of an engine. However, a better understanding of all processes and interworkings of the engine is also necessary so that reliable predictions can be made. APOSEM AND EVE Whatever advances are in store, every engine must be overhauled or repaired in shop at some point. What is most important here is to ensure that reliable planning can be carried out as early as possible, says Werner-Spatz. With this in mind, Lufthansa Technik has developed physical models under the APOSEM (Advanced Prediction Of Severity Effects on Engine Maintenance) research project models that can be used to make predictions regarding the operation-related aging of engines. This concerns a physics-based model of the engine, which responds sensitively to operational constraints, says Werner-Spatz. To put it another way: You tell me how you fly and I ll tell you when your engine has to visit the shop. APOSEM was carried out between January 2014 and December 2016 together with ANSYS, the DLR, the University of Stuttgart and TU Braunschweig, and was funded by Germany s Federal Ministry of Economics and Technology (BMWI). The three-year project developed detailed models through thermomechanical lifetime consumption analyses for a number of critical components and validated these with existing data. It produced a methodology for simulating flight operation and for predicting general aging of an engine. 6

5 In order to validate under real conditions the aging behavior of engines simulated by APOSEM, a decommissioned CFM56-5C engine was modified as part of the EVE (Engine for Validation Experiments) research project with extensive additional instrumentation. Up to 310 additional parameters can now be established using an independent data acquisition system corresponding to a tripling of the existing measured values. A detailed impression of the engine can therefore be gained in a test cell under the most varied conditions. This provides for a better understanding of the response of the secondary air system and also enables its impact on the air flow and aging behavior in the turbine to be recorded. The more parameters whose impact we can identify the better. This will allow potential damage, faults or problems in the engine to be detected early on. And this will therefore allow a wider selection of measures and better ability to influence costs than without these findings, says Werner-Spatz. With the results from the APOSEM and EVE projects, it will be possible to make long-term predictions regarding the operation-related aging of engines. This will allow reliable predictions about service life and extent of intervention, thus influencing factors such as scrap rates and workscopes. Moreover, the research results can be used by Lufthansa Technik in the future as a basis for improved costing in long-term, flat-rate maintenance contracts. And the findings derived will help to acquire a deeper and more comprehensive understanding of engines. RELIABLE PREDICTIONS A major goal of engine support is to identify the precise effect of stresses on different components to make predictions for maintenance. For example, many defects in the low-pressure turbine can already be resolved on-wing, up to replacement of the entire module. On the other hand, a shop visit is often inevitable if there are faults in the high-pressure system. Yet with reliable prognoses, concrete indications of the scope of the work required and precise pre-planning, maintenance costs can be calculated much more precisely for individual operators. We will always have to bring the engine into the shop for overhaul at some point over its lifetime, be it for damage diagnosis or for repair, says Werner-Spatz. Yet, if we can familiarize ourselves with the engine to such an extent that we can also reliably diagnose many problems directly on-wing, we will reduce unnecessary maintenance costs. For example, if a bleed air leak occurs, kerosene consumption can be more than one per cent higher than normal and a traditional analysis of ECM data would conclude that the high-pressure compressor was inefficient. In fact, however, the fault is not in the engine, but in the aircraft, and can be resolved very simply in line maintenance. It is critical that the fault is diagnosed accordingly so that troubleshooting can be initiated, explains Werner-Spatz. What must be avoided in any case is that a healthy engine is removed and sent to the shop. The scope of shop events will consequently be reduced in future, while the number of on-wing repairs will increase. Furthermore, more precise planning in the shop will be possible by knowing exactly when something is going to break and what the resulting costs will be. For maintenance organizations, this means more efficient spare parts planning and, for operators, it means shop visits are shorter and can be planned better. Thus significant savings can be made across an engine fleet. Positive developments in predicting service lifetimes, scrap rates and extent of intervention will therefore be to the benefit of both airline customers and MROs like Lufthansa Technik. from left: System Engineers and members of APOSEM-Project team Stefan Kuntzagk, Christian Werner-Spatz, Marc Zschieschank INSIGHT SAVES MONEY As stated, a prioity is to reduce costs per flight hour for the operator not only in the shop but also on wing. Both aspects must dovetail and be considered together, while planning needs to be adapted to each individual operator and its fleet. The earlier a higher-quality diagnosis can be made, the greater the scope to find the the most balanced and timely package of measures. In addition, this generally also allows a decision as to whether to perform a repair on wing, on site or in the shop. Innovative companies like Lufthansa Technik want to use optimized diagnostic tools to identify potential work- and cost-intensive faults in engines earlier, faster and more precisely lowering the risk of unexpected engine failure. And it is the company s extensive knowledge of damage mechanisms in the engine that allows it to develop new repairs or even to completely redefine maintenance measures. 8