## 2 Basic Description  
FCS consists of several parts: a fuel pump for supplying required fuel, an electronic control unit(ECU) for receiving the command signal, and a FMU for transferring required fuel to combustor and returning the excess fuel to tank. The typical FCS is shown in Figure 1. The chemical energy generated by combustion can be converted into kinetic energy to generate thrust of the aeroengine, and mechanical energy to drive airborne accessories and fuel pump to operate through accessory gearbox (AGB). The heat transfer conduction of high temperature fuel gas in the jet pipe and the power loss of airborne accessories and fuel pump will be dissipated by the fuel system.  
Taking fuel as a heat sink to absorb the thermal loads in the airborne electro-mechanical system is an important aspect of airplane thermal management. However, the fuel control system usually recirculates the hot fuel to the return tank, thus increasing the fuel temperature in the tank [27]. Thus, the control of the fuel-based thermal topology network, including valves, pumps, heat exchangers, and coolers, and planning of aircraft trajectories for optimal thermal performance have become the present challenging issues [28]. The schematic of fuel thermal management system is shown in Figure 2.  
The dissipating thermal load of hydraulic system is realized by the immersed heat exchanger in the fuel tank or the hydraulic oil-fuel forced-convection heat exchanger in the fuel feed line. In order to restrain the temperature rise of the fuel in the tank caused by the recirculated fuel, a cooler of fuel-ram air is used to absorb the thermal load of the fuel and control the outlet temperature of cooler [29]. At the same time, the corresponding heat exchangers also dissipate the waste heat energy generated by the lubrication system and environment control system. The main layout approaches of heat dissipation of the hydraulic control system are as follows [30; 31]:  
1. The first approach is to make return line of hydraulic control system across the fuel tank, thus, produces the convective heat exchange through the area of heat transfer.
2. The second approach is to arrange a fuel-hydraulic heat exchanger on the return circuit of fuel pump housing. The heat exchanger can be immersed in fuel tank or connected in series with the fuel supply circuit of aeroengine.  
Figure 1: Typical aeroengine control system.  
3. The third arrange method is to set a fuel-hydraulic heat exchanger on the main hydraulic return pipe. The fuel after heat exchanging can be transmitted into a combustor or fuel tank.
4. The fourth method is to set a hydraulic-ram air heat exchanger on the return circuit of hydraulic pump housing and a hydraulic-fuel heat exchanger on the bypass line to be connected in parallel with the fuel supply line. Note that the cold side of the heat exchanger should be arranged on the stamping air channel after the diffusion of the stamping air inlet. In these two heat exchangers, priority is given to operation hydraulic-ram air heat exchanger. When the heat exchange capacity of hydraulic-ram air heat exchanger is insufficient, both operate at the same time.
5. The fifth method is to arrange the heat exchanger in the hydraulic booster tank so as to absorb the dissipating thermal loads through the ram air, cold air, fuel, or other heat sinks.