## 2 Background  
### Environmental Control System Overview and Simulation Platform  
The role of an ECS is to supply conditioned air to the aircraft cabin and cooling air to the avionics bay [16]. To ensure that the supplied air is at an appropriate temperature and humidity, Passenger Air Conditioners (PACK) are the primary subsystems within the ECS that condition air. A schematic of the PACK for a Boeing 737-800 is shown in Figure 1[16], and a brief description of its working principles is given below.  
The high-temperature and high-pressure air taken from the engine bleed first passes through the PACK Valve (PV), which controls the total mass flow rate into the system. A proportion of this hot air stream passes directly to the merge outlet through the Temperature Control Valve (TCV), while the rest passes through a cooling cycle and becomes a cold air stream. The PACK outlet temperature can be controlled by regulating the opening degree of the TCV, which regulates the proportion of the hot and cold air streams. The proportion of bleed air that passes through the cooling cycle first  
Figure 1: Schematic of a Boeing 737–800 PACK with the air flow sequence labelled [16].  
Exchanger (PHX), where its temperature drops by transferring heat to the cold ram air. Next, its temperature and pressure are recovered through the compressor of the Air Cycle Machine (ACM) to allow a higher heat exchange rate and hence better effectiveness during the subsequent heat exchange in the Secondary Heat Exchanger (SHX), where it is cooled down again. The outlet air from the SHX then enters the High Pressure Water Separator (HPWS), where water is condensed and extracted before the air enters the turbine. The air expands through the ACM turbine, which drives the compressor, and its temperature and pressure are reduced. After merging with the hot stream at the TCV outlet and passing through the HPWS again, the air reaches the PACK outlet.  
For ground operation, the PACK cycle works in the same manner, except that the bleed air comes from the Auxiliary Power Unit (APU) rather than the engines.  
The PACK cycle described above can be simulated by Simscape Environmental Control System Simulation under All Conditions (SESAC) [17], an ECS simulation platform validated against actual data from Boeing 737-800 aircraft. The configuration of the PACK cycle in SESAC was identical to that of the Boeing 737-800 described above.  
Five fault modes were simulated: ACM fault, PHX fault, SHX fault, TCV fault, and ram air intake (RI) blockage. Different degradation levels were simulated for each fault mode, as summarised in Table 1. The ACM fault is simulated by assigning degraded mechanical efficiency to the component, and the degradation degree in Table 1 corresponds to the percentage reduction in mechanical efficiency from the healthy-state. PHX and SHX faults were simulated by inputting the degraded heat exchanger effectiveness for PHX or SHX, representing both fouling and blockage in the heat exchangers [18]. The degradation degree for PHX and SHX in Table 1 corresponds to the percentage reduction in the heat exchanger effectiveness compared with healthy PHX and SHX values. RI blockage was also simulated at three levels, by simulating 25%, 50%, and 75% reductions in ram air mass flow rate compared to the commanded value. The TCV fault is simulated in a different way owing to the nature of the fault. While the TCV opening angle was found to be between 15\({}^{\circ}\) and 18\({}^{\circ}\) for the cases studied, faults can occur if the valve is jammed at a lower or higher angle than the commanded angle, resulting in an undershoot or overshoot, respectively. In the case of the undershoot, the TCV opening angle was fixed at 10\({}^{\circ}\), and in the case of the overshoot, the TCV opening angle was fixed at 23\({}^{\circ}\). Any fault with the HPWS is not considered as the simulation was carried out assuming no water in the bleed air.