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The examples discussed illustrate that the implementations of condition monitoring systems as stand-alone systems are numerous. However, the trend is moving towards and thus the capabilities that a condition monitoring system provides are being used as the core of a large and very sophisticated maintenance management system. Such systems are known as health management systems, and they cover the entire range of operations involved in the maintenance of an aircraft.

The final objective of a modern health management system is to achieve maximum fleet availability and optimum safety and airworthiness, combined with maximum degradation of costs resulting from factors such as maintenance and logistics. Depending on the manufacturer and the participants, various terms can be used to describe the implementation of health management systems and methods. A number of examples are in use under the generic name of Integrated Vehicle Health Management (IVHM), mainly referring to Boeing and the US Navy, although that doesn’t mean that there are no other names for similar applications. Like all of these systems, IVHM uses the OSA-CBM architecture and a complete system is able to incorporate the philosophy, methodology and the processes required to improve safety, operability, maintainability, reliability, and finally testing (Keller et al., 2001; Dunsdon and Harrington, 2008).

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The desired objective is achieved by dividing the related processes into either on-board or off-board, each having separate but also interacting functions. Every on-board architecture is able to assess the component or system health for both operational and maintenance purposes. The on-board functions are grouped under the term Operational Maintenance Program.

Examples of on-board capabilities can be found across many military and civil uses. The source of data is always on-board sensors. The primary processing of these data takes place on board in order to fulfil more important needs, while the aircraft is still in flight. These needs are expressed by means of warning signals that the pilot sees, which inform him of a malfunction in one or more systems. These signals are also indications that a maintenance action has to be done when the aircraft lands (Keller et al., 2001).

By adding new technology systems these sensors are capable of receiving signals for multiple uses. In practice, after the sensor module, the systems that receive these data can use them for two purposes. The first, as previously mentioned, is for on-board warnings and alerts, while the second is for performing long term trend analysis of the monitored system or equipment, which is equally useful for maintenance purposes. Both types of data can be stored in on-board memories and also be transmitted to the ground, depending on their importance.

The health management systems available for commercial aircraft illustrate their rapid evolution since the time that the first on-board alerting systems were introduced. These first generation systems used light bulbs which lit up when an interruption of signal or power was observed. Their evolution began with the second generation systems that were first introduced by Boeing and Rockwell-Collins and Airbus on the 757/767 and A320 respectively, and with the integration of built-in test equipment, whose responsibility it was to detect anomalies on their dedicated system, the process was simplified considerably. All the warnings that were generated were collected from the Maintenance Control and Display Panel (MCDP), for Boeing aircraft, and from the Centralized Fault Display System (CFDS) for Airbus. Due to the fact that the CFDS was introduced later, it has a very sophisticated capability (for those times) to translate the cryptic codes into English text, and thus much decoding time was saved.

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In the early nineties the establishment of the Central Maintenance Computers (CMC) or On- board Maintenance System (OMS) gave a true enhancement to the mechanics because these systems were not only able to indicate which LRU was faulty, but also which maintenance procedures were required. With continued improvement in the CMC concept, the 777 and A340 are equipped with the next step of the initial CMC. An extensive analysis of maintenance enhancements systems follows in Chapter 3. Improved diagnostic capabilities using model-based techniques are now available on these systems. Additional help is now provided by portable devices that enable the mechanic to be as close as possible to the root of failure, without needing to return again and again to the cockpit (Bird et al., 2005).

In terms of maintenance activities, the on-board systems are capable of diagnosing the failure and provide significant enhancement to the troubleshooting process. However the sampling undertaken by the on-board systems generates the off-board process, wherein lies the true evolution of health management systems. By performing trend analyses, the off- board devices are capable of predicting incipient failures and planning the maintenance activities according to when these failures are likely to happen. The fact that prognostics capabilities are very sophisticated keeps unexpected failures to a minimum, such that they only arise as a result of some external event, such as a bird hitting the engine. The data can be downloaded either via transmission from the aircraft to the ground or by extracting them from the memory modules located on the aircraft via portable ground systems.

The IVHM architecture incorporates two modules within the off-board domain. The first is the Maintenance Data Warehouse, where all the information is collected and stored. The loaded data are processed by a Ground Based Reasoner which produces diagnostic/prognostic data which is fed into the Maintenance Reasoner. The latter supports the maintenance personnel by indicating precisely which component needs to be repaired. In addition, the process is completed by means of a Dynamic Resource Manager function, which takes account of the availability of resources, but also of the future operational profile of the aircraft, and according to these, indicates the optimum time at which to perform the maintenance activity.

The on-board/off-board partition architecture provided by IVHM is in use in several ongoing applications, which include the Navy/Boeing Dual Use Reconfigurable Control and Fault Identification and OSA/CBM programs, as well as the DARPA/USAF Unmanned Combat Air Vehicle (UCAV) program, with the latter targeting the fuel system of the aircraft. In addition, a huge effort has been made for the helicopter Health Usage Monitoring System (HUMS) in

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order to predict incipient failures on intermediate gear boxes, while under the Dual Use Science & Technology program (DUST) the US Navy is collaborating with General Electric for the Integrated Engine Prognostics and Health Management system (IEPHM), for aircraft fitted with GE engines. General Electric is also collaborating with Boeing for the Aircraft Electrical Power System Prognostic Health Management (AEPHM) (Butcher, 2000; Hess, 2002; Dunsdon and Harrington, 2008).

The state-of-the-art in health management programs, however, is being implemented on the modern JSF F-35 fighter jet, with its own dedicated program, Joint Strike Fighter Prognostic Health Management (PHM). This is being developed as a collaboration between Lockheed Martin, Boeing Aerospace, Pratt and Whitney and General Electric. The complete system is able to reduce life cycle costs, and consequently the maintenance man hours needed per flight hour. The key features of this program are the advanced technologies that are implemented both on-board and off-board.

The on-board system is able to perform fault detection and fault isolation with a very high level of accuracy, and also provides very accurate prognostic data. Every subsystem on the aircraft is equipped with a dedicated PHM Area Management system, all of which are connected to the central Air Vehicle PHM Manager. The flight critical events are processed in the air and only the most necessary operations are left to the pilot. The non-flight critical events are used for long term analysis on the ground. The off-board systems take account of all the operational usage, not only of the aircraft, but of the whole fleet as well. Finally, the connection of the system with the Autonomic Logistic Information System (ALIS) provides the extra advantage for the operational usage of the aircraft (Brown et al., 2007; Ferrell, 2000).

To summarize, the improvements and benefits that result from a condition based monitoring maintenance methodology were quickly recognized by the industry. From the point of view of the US Army CBM team, this technique offers many advantages. First and foremost, due to the lack of unscheduled corrective maintenance actions, the availability and airworthiness of the fleet are improved significantly, thus addressing the two major dimensions of aircraft maintenance. Firstly, safety is not compromised because the flight critical systems are monitored, and secondly, thanks to the prognostic capability, no unexpected problems occur, thus unscheduled downtime is eliminated. The whole procedure also helps to improve the troubleshooting process, which increases the confidence of the crew by making them feel safer. Moreover, the economic advantages are also considerable, as savings are accrued by eliminating the downtime throughout the life of each component.

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