## 4 FCE Model for Landing Gear Hydraulic R/E System
### Determining the Severity of the Failure Modes
#### 4.3.2 Fault Simulation and Severity Assessment  
Traditional severity evaluation is too reliant on expert determinants. Various failure modes of the set of fault type index factors will be simulated and simulated to increase the accuracy and confidence of the final findings in order to reduce the effect of this subjective human element on the final evaluation results. In the field of fault diagnosis [36; 37], time-domain and frequency-domain signals are often used for analysis and signal transformation. In this study, time-domain characteristics are used to measure the working state deviation. Under the assumption of no catastrophic failure, the general failure mode will vary as the severity of the failure increases, the duration of landing gear extension will change, and the actuator's working speed will become more unstable. The degree of variation between these two indications is a key metric for determining the severity of the failure mode. Multiple hydraulic cylinders are used as actuators in the hydraulic system. As an example, consider the landing gear extension. The succeeding upload actuator is the evaluation item for assessing different failure mechanisms since the upload actuator functions first in the sections of the landing gear. Define the action time change value as \(\gamma\):  
\[\gamma=|T-T_{0}| \tag{5}\]  
\(T\) is the operating time after being affected by the fault, and \(T_{0}\) is the operating time without fault. Define the percentage of actuation speed fluctuation as \(\eta\):  
\[\eta=\left(\frac{V-V_{0}}{V_{0}}\right)\times 100\% \tag{6}\]  
\(V\) is the operating speed that the fault affects at a specific moment, and \(V_{0}\) is the operating speed that the fault does not affect at the exact moment. The time domain signal acquisition frequency is 1000 Hz. In all simulated faults, working condition i is an absolutely ideal working condition, but such a situation often does not exist. Working condition iv is an extremely bad working condition, which is more conducive to data comparison. Because of the nonlinear characteristics of hydraulic system, working conditions ii and iii can be regarded as general fault conditions. In the following fault simulation, the final system can complete the work, but the completion degree is different. At present, aviation hydraulic oil is generally divided into petroleum-based hydraulic oil and synthetic aviation hydraulic oil. In the hydraulic oil characteristics of the model, synthetic aviation hydraulic oil is used, and all aspects are more ideal. Its purpose is to change the fault characteristics of hydraulic oil only through fault simulation, not because it has defects. (1) Oil contains wear particles. Because of the mutual friction of the reciprocating motion of different mechanical components in the hydraulic system, various metal abrasive particles are formed. The size and concentration of abrasive particles grow as the wear degree of mechanical components rises, adversely influencing the system's pressure and flow and potentially leading to mechanical failure [38]. In this fault simulation, the filter was removed, and different number of equal inner diameter throttle valves were connected in series to simulate a fault with a greater concentration of abrasive particles multiplied by the number of throttle valves, assuming uniform distribution of the abrasive particles. The inner diameter of the throttle valve is 5 mm. When there is only one throttle valve, the hydraulic oil will only produce one pressure drop. With the increase of the number of throttle valves, the number of pressure drops will also increase, which will eventually affect the actuation of the actuator. Table 4 shows the parameters for establishing the number of throttles for fault setting. Figure 6 shows the results of the simulation after it has been conducted. (2) Oil mixed with air. The volume percentage of the air contained in the hydraulic oil is called the air content.