Wide-body aircraft fuel jettison system design and analysis

In order to design the highly integrated fuel jettison system, the hydrodynamic performance design flow chart for the highly integrated fuel system has been proposed, and the 1-D fuel line network performance model has been developed. An innovative fuel jettison system for wide-body aircraft has been designed. The system performance has been simulated and analyzed. The results show that the proposed flow chart can be used for the highly integrated fuel system design, and the fuel jettison performance requirements can be met. The system parameters including the wing tank main pump characteristic, the flow area of the orifice set in the fuel line connecting the feeding line and the refueling line, and the flight height have significant effects on the fuel jettison performance. Besides, the aircraft total fuel jettison performance is also affected by the central tank override pump characteristic, the engine fuel consumption, and the flow resistance of fuel line network.


Introduction
The wide-body aircraft is widely used in the long range airlines, and its onboard fuel quantity is quite large.When some emergency cases happen, the aircraft is necessary to reroute and land off as soon as possible.To ensure the aircraft safety during the landing off process, it can reduce the aircraft weight within the limited time by jettisoning fuel in the airspace which is permitted by the government [1][2].However, the frequency of jettisoning fuel is quite low in an aircraft full life cycle.To jettison fuel quickly and safely is the key problem to design the fuel jettison system.How to control the weight and cost increment caused by setting the fuel jettison system need to be paid attention and studied.
Many scholars have studied the aircraft fuel jettison system.Li M Z proposed a fuel jettison system design for transport aircraft, and analyzed the layout of the fuel line and jettison nozzle [ 3].Chen Z B analyzed the requirements of the airworthiness article and studied the flight test method for the fuel jettison system of civil aircraft [4].Wang P studied and summarized the legislative background, the airworthiness requirements, and recommended compliance method for civil aircraft jettison system [5].Chen G H used a numerical method based on discrete phase model to simulate the wake pattern of fuel jettison, and studied the wake shape and motion trajectory under different pitch and yaw angles [6].Yang A J discussed the gravity fuel jettison of wing tank, established the fuel jettison mathematical model based on the Flowmaster platform, and studied the key factors that affect the fuel jettison efficiency [7].Luo J J designed an aircraft fuel jettison test system which can be used to test and verify the performance of the fuel jettison system [8].Gong H raised a system concept design for correcting fuel imbalance between the aircraft wing tanks, the system can also support to jettison fuel [9].
This article proposed a hydrodynamic performance design flow chart for the highly integrated fuel system, and designed an innovative fuel jettison system for wide-body aircraft application based on

System description
Figure 1 showed a typical three tank layout wide-body aircraft engine feeding system and fuel jettison system.It reuses the components and pipelines of the engine feeding system and refueling system for fuel jettison.When jettisoning, all the wing tank main pumps and the central tank override pumps are involved in the fuel jettison process.To ensure the fuel supplied to the engine can meet the requirements, the orifices which are marked as 1 and 2 in Figure 1 are installed in the wing tank fuel line which connects the feeding line and the refueling line.The advantage of the design shown in Figure 1 is that the aircraft weight gained by the installation of the fuel jettison system can be limited.However, the engine feeding system and the fuel jettison system can not been isolated.In the event that the jettison shut-off valve cannot be closed due to a malfunction, there is a risk to shut off all the fuel pumps in one wing tank to stop jettison, which will affect the safety of engine feeding.
In response to the shortcomings of the system scheme shown in Figure 1, an innovative system architecture is proposed, which is shown in Figure 2. The two designs are based on the same engine feeding architecture and pressure refueling system, and the fuel jettison of the central tank is the same.The differences between them are that, the innovative design shown in Figure 2 has adjusted the connection position of the wing tank fuel jettison pipeline and the engine feeding pipeline, and a check valve is added at downstream of the main pump output pipeline.Due to the check valve, only one of the two main pumps in one wing tank can be used for jettison, while the other main pump is only used for engine feeding.Therefore, the isolation between engine feeding and fuel jettison can be achieved.

System modeling
For the highly integrated fuel system, there is a close cross-linking relationship between the hydrodynamic performance of different functions, as shown in Figure 3.This article proposed a design flow chart for the highly integrated fuel fluid system, as shown in Figure 4.In order to design and analyze the system performance, simulation models have been developed based on AMESim software for the typical architecture fuel system shown in Figure 1 and the innovative architecture fuel system shown in Figure 2. The two models are almost the same except for the connection position of the fuel jettison pipeline and the engine feeding pipeline in the wing tank, and a check valve is added at downstream of a main pump output pipeline for the innovative system.The AMESim model shown in Figure 5 is for the innovative system shown in Figure 2. According to the process shown in Figure 4, the system design iteration was carried out to obtain the main parameter values of the hydrodynamic performance model for the fuel system.The design result is shown in Table 1.During the design and calculation process, the environmental parameters were based on the ISA standard day, and the fuel type was Jet A1.The characteristic curves of the main pump and the override pump assumed by similar equipment are respectively shown in Figure 6 and Figure 7.If the performance of the override pumps and the main pumps are properly matched, all the tanks can jettison fuel simultaneously.The flow ratio between the driving flow and the output flow (i.e. the sum of the driving flow and the input flow) of the injection pump was assumed to be 1:3.The output flow of the injection pump was set as similar to the engine fuel consumption.Considering that the research object of this article is only the engine feeding system and fuel jettison system, the design of other subsystems are not discussed in this article.The institute the author works for has built an aircraft engine feeding and fuel jettison test bench which can be referred to [8].Based on the test bench and the parameters shown in Table 1, engine feeding and fuel jettison tests for the system architecture shown in Figure 1 have been conducted.The experimental data and the simulation results was compared in Table 2.According to engineering experience, the calculation accuracy of this simulation model can support the system design.

Fuel jettison performance at the design condition
The fuel jettison performance of the typical architecture system and the innovative architecture system were simulated, the results are shown in Table 3.The parameter values used in the calculation referred to Table 1.From the data in Table 3, it can be seen that the performance of innovative architecture system is relatively close to that of typical architecture system.Taking fuel jettison at 10668 meter flight altitude as an example, the aircraft weight reduction target can be achieved by simultaneously jettisoning fuel from three fuel tanks in less than 50 minutes based on the data in Table 3.

Effects of the main pump characteristic and the wing tank jettison orifice flow area on the fuel jettison performance
The main pump pressure rise shown in Figure 6 is regarded as the reference value.In order to analyze the impact of the main pump characteristic on fuel jettison performance, five set of main pump pressure rise characteristic were formed by scaling based on the reference value.The scaling factor were 50%, 75%, 100%, 125%, and 150%.Simulation was conducted based on the main pump characteristic in conjunction with the override pump characteristic shown in Figure 7.It should be noted that, simulation analysis has shown that all five set of the main pump pressure rise can meet the engine feeding requirements, and ensure the central tank override feeding performance when there is no tank jettisoning.The orifice discussed in this section refers to the flow limiting component which is installed on the fuel line connecting the engine feeding line and the refueling line in the wing tank, and it is used to limit the fuel jettison flow rate of the wing tank.The impact of the ratio of the flow area of the orifice to the cross-sectional area of the fuel line on fuel jettison performance was discussed in this section.
Figure 8 and Figure 9 show the fuel jettison performance of the innovative architecture system under different main pump pressure rise and wing tank jettison orifice sizes.The simulated flight altitude was 10668m Among them, Figure 8 shows the situation where all the tanks are simultaneously jettisoning, and Figure 9 shows the situation that only the wing tanks are jettisoning.As shown in Figure 8(a), when the ratio of the orifice flow area to the fuel line cross-sectional area increases, i.e. the flow area of the orifice increases, the wing tank fuel jettison flow rate increases correspondingly.When the ratio is small (such as between 0.05 and 0.30), the wing tank fuel jettison rate rapidly increases with the increase of the ratio.When the ratio is large, the wing tank fuel jettison flow rate increases slightly with the increase of this ratio.As reflected in Figure 8, the slope of the curve decreases with the increase of the x-axis parameter, that is, the larger the ratio, the smaller the corresponding increasing degree of the wing tank fuel jettison flow rate.In addition, as shown in Figure 8, under the same orifice flow area, the larger the main pump outlet pressure rise, the stronger the corresponding wing tank fuel jettison performance.
In the case of three fuel tanks jettison simultaneously, due to the convergence of the wing tank jettison flow and the central tank jettison flow at the downstream of the fuel jettison line, the fuel flow in the different fuel lines affects each other.Figure 8 Figure 9 shows the situation where only the wing tanks jettison fuel.At this time, the central tank override pumps do not work, and the change of the fuel jettison performance is only caused by the change of the wing tank fuel jettison performance.The parameter variation pattern in Figure 9(a) is the same as that in Figure 8(a).However, the fuel jettison flow rate in Figure 9(a) is slightly higher than that in Figure 8(a).That is because when only wing tanks jettison fuel, the jettison performance is affected by the wing tank fuel jettison line flow resistance, and is not affected by the central tank override pump performance.Compared to the three fuel tanks simultaneous fuel jettison, the downstream flow resistance is smaller at this time.

Effects of the flight height and the wing tank jettison orifice flow area on the fuel jettison performance
The purpose of the fuel jettison is to quickly reduce the weight of the aircraft, but it should be noted that the actual target is the reduction of the total weight of the aircraft, rather than the weight reduction caused only by fuel jettison.Figure 10 and Figure 11 show the innovative architecture jettison system performance at different flight altitudes with the reference fuel pump pressure rise characteristic.When only the wing tanks jettison, there is no cross-linking effect of the central tank fuel jettison on the wing tank fuel jettison, but the basic pattern is the same with that of all tanks jettison situation, as shown in Figure 12.Under the given orifice flow area, the rate of weight reduction of the aircraft decreases with the increase of flight altitude.

Conclusion
This article proposed an innovative fuel jettison system architecture for a three tank layout wide-body aircraft and the performance has been analyzed.The main conclusion are as following.
(a) The performance of the innovative architecture fuel jettison system is similar to that of the typical architecture system.Besides, the innovative architecture system can improve the safety margin of the fuel system when the wing tanks jettison.
(b) The main pump characteristic and the wing tank jettison orifice flow area have significant impacts on the jettison performance.The overall fuel jettison performance is jointly influenced by the main pump characteristic , override pump characteristic , and fuel line network flow resistance.
(c) The flight altitude has a significant impact on the fuel jettison performance.The fuel jettison performance mainly depends on the fuel pump characteristics at different flight altitudes and the engine fuel flow requirement.

Figure 1 .
Figure 1.Typical engine feeding and fuel jettison system scheme.

Figure 2 .
Figure 2. Innovative engine feeding and fuel jettison system scheme.

Figure 3 .
Figure 3. Relationship between the hydrodynamic performance of highly integrated fuel system.

Figure 4 .
Figure 4. Flow chart of the hydrodynamic performance design for highly integrated fuel system.

Figure 5 .
Figure 5. Engine feeding and fuel jettison performance model for the innovative design.

Figure 6 .
Figure 6.Characteristic curve of main pump.Figure 7. Characteristic curve of override pump.

Figure 7 .
Figure 6.Characteristic curve of main pump.Figure 7. Characteristic curve of override pump.

Figure 8 .
Figure 8. Jettison performance with different main pump characteristic when all the tanks jettison.

Figure 9 .
Figure 9. Jettison performance with different main pump characteristic when only wing tanks jettison.
(a) and Figure 8(b) show that, as the flow area of the wing tank jettison connecting line orifice increases, the wing tank fuel jettison flow rate increases, but the central tank fuel jettison flow rate decreases.The relationship between the wing tank jettison flow rate and the central tank jettison flow rate is influenced by the flow resistance of the fuel jettison line.Besides, it is also related to the output performance of the main pump and the override pump.As shown in Figure 8(b), under the same orifice flow area, the larger the main pump pressure rise, the weaker the central tank fuel jettison performance.The total fuel jettison flow rate under the different orifice flow area is shown in Figure 8(c).Under the characteristics of the main pump and override pump in this article, the total fuel jettison flow rate increases with the increase of the orifice flow area, that is because the increase of the wing tank fuel jettison flow rate is stronger than the decrease of the central tank fuel jettison flow rate.Under the same orifice flow area, the larger the main pump pressure rise, the stronger the corresponding total fuel jettison performance.
Figure 10 is for the situation of the three fuel tanks simultaneously jettison, and Figure 11 is for the situation of only the wing tanks jettison.The total aircraft weight reduction for the two situations is comprehensively shown in Figure 12.

Figure 10 .
Figure 10.The innovative architecture jettison system performance at different flight altitudes when all the tanks jettison simultaneously.

Figure 11 .
Figure 11.The innovative architecture jettison system performance at different flight altitudes when only the wing tanks jettison.

Figure 12 .
Figure 12.The total aircraft weight reduction at different flight altitudes.

Table 1 .
Model parameter at design condition.The flight height is chosen as the design condition, while the aircraft can jettison fuel at a range of flight height.
bThe aircraft weight reduction is caused by both fuel jettison and engine fuel consumption.c It is the time-cost for reducing aircraft weight.

Table 2 .
Comparison the design condition simulation result with the test data for the typical design.

Table 3 .
Fuel jettison performance of typical design and innovative design.