This paper is a continuation of the authors' work on the development of a systematically derived PACK simulation for accurate fault diagnostics, utilising a model-based approach. In practise the PACK simulation accuracy is dependent on a number of factors, which include the understanding of its control system. This paper addresses this by taking an in-depth look at the factors controlling the operation of the PACK in order to enable the gap between the theoretical understanding of the PACK and the engineering design of the system to be bridged, and accurate simulations under healthy and degraded scenarios obtained. The paper provides a comprehensive explanation of the PACK control system elements (principally valves) and verifies their operation based on experimental test data acquired from a B737-400 aircraft. A discussion of the control used in the simulation is then given, resulting in the correct temperature, pressure and flow being delivered to the cabin. The overall simulation results are then presented to demonstrate the importance of employing a systematically derived control logic. They are then further used to assess the impact of degradation in the main PACK valves.
The Environmental Control System (ECS) of an aircraft is responsible for regulating and conditioning the airflow into the cockpit, cabin and avionics bay. The ECS is composed of several complex sub-systems and components that are reported as key unscheduled maintenance drivers for legacy aircraft by aircraft operators. Furthermore, the incorporated temperature and flow control valves in these sub-systems have the capability to mask potential faults at the component level, making the diagnostic process very challenging. To overcome this challenge, the aviation industry is currently proactively exploring the predictive maintenance approach that allows real-time monitoring of the key systems, sub-systems and components. In the context of the ECS, this necessitates the requirement to equip the system with appropriate condition monitoring capabilities. To do this, the performance characteristics of the ECS at sub-system and component level needs to be well understood under a wide-range of aircraft operating scenarios. Existing literature provides component level and system level analyses of the ECS. However, it lacks an experimentally verified and validated ECS sub-system and component level simulation tool (ECS Digital Twin), capable of simulating the thermodynamic performance and component health state parameters under wide-ranging aircraft operational scenarios. The ECS Digital Twin (DT) developed by the Cranfield University IVHM Centre offers the capability to simulate healthy and faulty cases of the Passenger Air Conditioner (PACK). This paper proposes a methodology for full-scale experimental Verification & Validation (V&V) of the developed ECS DT, to enable component level simulation, and enabling accurate diagnostics, of the civil aircraft ECS. The paper reports on progress to date in this project.
In this paper the experimental investigation of a Boeing 737 aircraft Environmental Control System (ECS) passenger air conditioner (PACK) has been reported. The PACK is the heart of the ECS that conditions bleed air prior to supplying it to the cabin and avionics bay. Its capability to mask fault occurrences has resulted in increased unscheduled maintenance of the system. As such it has been a key research topic to understand PACK performance characteristics in order to support an accurate diagnostic solution. This paper is a continuation of the authors' work on the development of a systematically derived PACK simulation model and reports the overall development and qualification of a novel in-situ ground test facility (GTF) for the experimental investigation of a B737-400 aircraft PACK under various operating modes, including the effect of trim air system. The developed GTF enables the acquisition of the temperature, pressure and mass flow data throughout the PACK. The overall process of instrumentation selection, installation, sensor uncertainty, and testing in terms of data repeatability and consistency has been reported. The acquired data is then employed to conduct a V&V of the SESAC simulation framework. The reported research work therefore enables the advancement in the level of scientific understanding corresponding to the ECS PACK operation under real operating conditions, and therefore supports the development of a robust simulation framework.
The aircraft Environmental Control System (ECS) enables the aircraft to maintain a comfortable and safe environment for its passengers throughout its operating envelope. The Pressurised Air Conditioner (PACK) is the heart of the ECS, and is composed of multiple sub-systems: heat exchangers, valves, compressor, turbine, and a water separator. The PACK’s principle function is to enable conditioning of the hot, high pressure bleed air from the engine or APU, for temperature, pressure and humidity against the cabin requirements. The operation of the PACK is governed by a control system which has the ability to mask degradation in its component during operation until severe degradation or failure results. The required maintenance is then both costly and disruptive. The PACK has been reported as major driver of unscheduled maintenance by the operators. The aviation industry is currently proactively exploring innovative health management solutions that aid the maintenance of aircraft key systems based on predictive based maintenance approaches using online condition monitoring techniques. This paper presents a comprehensive review of the simulation and diagnostic methodologies applicable to fault diagnostics of the ECS PACK. The existing literature suggests that model-based and data-driven methods are effective for conducting fault detection and isolation of the PACK system. The conceived findings indicate that the model-based diagnostic approach have been extensively employed to conduct PACK diagnostics at component level only. Their successful implementation requires robust experimental verification and validation against the actual data under healthy and faulty conditions. Although a substantial amount of work has been reported on developing first principles based simulation models and diagnostic strategies for the ECS, the acquired findings suggest that there is a compelling need for a verified and validated ECS simulation model to enable accurate PACK system-level diagnostics based on single and multiple component level degradation scenarios. It has also been identified that the existing literature lacks the evaluation of humidity regulation and the effect of the control system on the PACK performance characteristics. Finally, a taxonomy of diagnostic techniques and simulation models is compiled based on the available literature.
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