Aircraft Fuel Tank Pressurization System and Method

Abstract

This document provides an aircraft fuel tank pressurization system and method, specifically for a system and method of storing aircraft fuel in at least one on-board fuel tank at a predetermined pressure and internal ambient tank temperature. A passive heat exchanger having an exchanger inlet is fluidly connected to the aircraft's propulsion engine to receive heated pressurized bleed air while the exchanger outlet is fluidly connected to the fuel tank. The passive heat exchanger is configured to cool the heated pressurized bleed air from the engine by heat transfer to the surrounding environment by means of radiation and convection in order to supply pressurized air to the fuel tank at a predetermined internal tank pressure and internal ambient tank temperature. A system controller is provided to supply pressurized air at a predetermined temperature to the fuel tank during various aircraft flight phases.

Description

Technical Field

[0001] The embodiments disclosed herein generally relate to aircraft having an on-board auxiliary fuel tank. In a particularly preferred form, the embodiments of the present disclosure relate to an aircraft auxiliary fuel tank that is provided with a fuel tank pressurization system to provide a reduced risk of flammability and to allow pressurized fuel to be transferred to another on-board fuel tank (e.g., a less-loaded main fuel tank and / or another auxiliary fuel tank). Background Art

[0002] It is well known that on-board aircraft fuel tanks pose an explosion risk due to the highly flammable fuel carried therein. Accordingly, several proposals have been made in the art to reduce the flammability risk associated with on-board aircraft fuel tanks, as evidenced, for example, by U.S. Pat. Nos. 2,749,062 and 9,758,255 and U.S. Patent Application Publications 2006 / 0021652, 2015 / 0151845, and 2015 / 0217153 (the entire contents of each of these patents and published applications are hereby expressly incorporated by reference).

[0003] For example, one technique known in the art for reducing the flammability risk associated with on-board aircraft fuel tanks is to pressurize the fuel tank using bleed air from an aircraft turbofan engine (e.g., a main propulsion turbofan engine or a turbofan associated with an on-board auxiliary power unit (APU)). Pressurizing the fuel tank reduces the highly volatile layer of fuel vapor inside the fuel tank, thereby reducing the flammability of the fuel and thus its explosiveness. However, since the engine bleed air is too hot (typically over 200° C.) to be introduced directly into the fuel tank for fuel tank pressurization, it must first be cooled. Accordingly, existing proposals have focused on using an active heat exchanger that employs cooled air as a working heat exchange fluid obtained from an on-board cooler unit (e.g., an on-board air conditioning unit) and / or cooler aircraft external ram air. Thus, such existing proposals typically draw cold air from the ram air stream and direct it to a pre-cooler where it is heat exchanged with the engine bleed air. Some form of active control valve system is typically employed to control the cold air stream, and temperature sensors are used to measure the temperature of the output air stream. However, disadvantageously, the implementation of these conventional systems is complex and expensive.

[0004] Some other known methods for reducing flammability also rely on using an inert gas, such as nitrogen, inside the fuel tank to displace the oxygen content therein. However, providing an on-board nitrogen source is an expensive solution that is not typically employed in commercial aviation.

[0005] Accordingly, there is a desire to provide systems and methods for pressurizing an aircraft fuel tank to reduce the risk of flammability and allow pressurized fuel transfer to another aircraft fuel tank, which systems and methods are less complex and thus more economical to implement. Embodiments disclosed herein relate to providing such systems and methods. SUMMARY OF THE INVENTION

[0006] Broadly, embodiments disclosed herein relate to systems and methods by which an aircraft fuel tank can be pressurized at normal fuel tank internal pressure and temperature conditions. According to some embodiments, there are provided systems and methods in which at least one aircraft fuel tank is capable of storing aircraft fuel at a predetermined internal tank pressure and internal ambient tank temperature, the fuel tank having a tank inlet for receiving pressurized air, a tank outlet for discharging fuel from the fuel tank, and a tank vent line. A passive heat exchanger having a heat exchanger inlet is fluidly connected to the propulsion engine of the aircraft to receive heated pressurized bleed air from the propulsion engine of the aircraft while being fluidly connected to the fuel tank at an exchanger outlet. The passive heat exchanger is configured to cool the heated pressurized bleed air from the engine by transferring heat to the surrounding environment by means of radiation and convection so as to supply pressurized air to the fuel tank at a predetermined internal tank pressure and internal ambient tank temperature.

[0007] Inlet and outlet fixed calibration orifices may be positioned in the inlet and outlet of the passive heat exchanger to controllably restrict the flow of bleed air from the engine to the heat exchanger and the fuel tank, respectively. A pressure regulating / shutoff valve (PRSOV) is positioned in the inlet of the heat exchanger downstream of the inlet orifice and has an open state and a closed state to fully supply and completely stop the flow of pressurized bleed air to the heat exchanger, respectively. A system controller is operatively connected to the PRSOV to regulate the PRSOV between the open state and the closed state, thereby maintaining the pressure and temperature conditions of the pressurized air supplied from the heat exchanger to the fuel tank at a predetermined pressure and temperature setpoint condition.

[0008] Pressure and temperature sensors may be positioned in the tank inlet to sense the pressure and temperature conditions of the pressurized air supplied from the exchanger outlet to the tank inlet and to send corresponding pressure and temperature condition signals to the system controller when the pressure and temperature conditions exceed a predetermined range. Thereby, in response to the receipt of the pressure and / or temperature condition signals, the system controller may issue a command signal to regulate the PR / SOV so as to maintain the pressure and temperature conditions of the pressurized air supplied from the heat exchanger to the tank inlet within a predetermined range.

[0009] Certain embodiments will provide a normally closed positive pressure outflow relief valve (ORV) and a normally closed negative pressure inflow relief valve (IRV) for the fuel tank. Each of the ORV and IRV is preferably a normally closed relief valve that opens in response to the internal tank pressure state exceeding its mechanically preset pressure state setpoint, thereby maintaining the internal tank pressure within its maximum and minimum design pressure states. For example, the ORV can have a preset pressure state setpoint at the maximum operating internal tank pressure, while the IRV can have a preset pressure state setpoint at the minimum operating internal tank pressure (e.g., less than 0 psig).

[0010] The system controller can be programmed to include first and second pressure setpoints that respectively establish a first internal pressure state of the fuel tank and a second internal pressure state of the fuel tank, the second internal pressure state being greater than the first internal pressure state. Thus, these first and second internal pressure states establish the upper and lower pressure limits of the normal range of the internal tank pressure state during operation. During aircraft climb (when the pressure inside the fuel tank will increase relative to the ambient pressure state), the system controller will keep the PR / SOV in the open position until a pressure signal is received from the pressure sensor that the internal tank pressure has reached the first pressure setpoint. At this time, the system controller responsively issues a command signal to move the PR / SOV to its closed position. In such an embodiment, the mechanically preset pressure state of the ORV will preferably correspond to the second pressure setpoint such that when the internal tank pressure reaches the second setpoint pressure, the normally closed ORV opens, thereby maintaining the internal tank pressure within the normal range of the internal tank pressure state.

[0011] The system controller can also be programmed to include a third pressure setpoint that is less than the first pressure setpoint, thereby establishing a third internal pressure state of the fuel tank, the third internal pressure state being below the lower pressure limit of the normal range of the internal tank pressure state established by the first pressure setpoint. During aircraft descent (i.e., where the internal tank pressure decreases relative to the ambient pressure state), the system controller will keep the PR / SOV in the closed position until a pressure signal is received from the pressure sensor that the internal tank pressure has reached the third pressure setpoint, at which time the system controller issues a command signal to move the PR / SOV to its open position.

[0012] According to certain embodiments, the fuel tank will include a fuel transfer valve located at the tank outlet and operatively connected to the system controller. The system controller will also preferably be programmed to include a third pressure setpoint below the first pressure setpoint, thereby establishing a third internal pressure state of the fuel tank, the third internal pressure state being below the lower pressure limit of the normal range of the internal tank pressure state established by the first pressure setpoint. During a fuel transfer mode (e.g., when fuel is transferred from the fuel tank to another on-board fuel tank), the system controller will issue a command signal to open the fuel transfer valve, thereby allowing fuel to flow through the tank outlet under the pressure within the fuel tank, which in turn causes the internal pressure within the fuel tank to decrease. Thus, during such a fuel transfer mode, the system controller can issue another command signal in response to the internal pressure within the fuel tank decreasing to the third pressure setpoint to cause the PR / SOV to open, thereby increasing the pressure within the fuel tank.

[0013] These and other aspects and advantages of the present invention will become more apparent after a careful consideration of the following detailed description of the preferred illustrative embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The disclosed embodiments of the present invention will be better and more fully understood by reference to the following detailed description of the illustrative non-limiting exemplary embodiments in conjunction with the accompanying drawings, in which:

[0015] Figure 1 is a schematic diagram of an on-board aircraft fuel tank pressurization system according to an embodiment of the present invention; and

[0016] Figure 2 is Figure 1 a graphical representation of a control scheme employed by an embodiment of the aircraft fuel tank pressurization system shown in. DETAILED DESCRIPTION

[0017] As schematically shown in the attached Figure 1 the aircraft AC is provided with an on-board fuel tank pressurization system PS for supplying pressurized air to the on-board auxiliary fuel tank 10 from the port engine Ep and the starboard engine Es via the bleed air duct. Importantly, in the illustrated embodiment, the pressurization system PS is provided with a passive heat exchanger 12, the passive heat exchanger including a heat exchange duct 12a arranged in a sinusoidal curve between its inlet 12a-1 and outlet 12a-2. The effective length of the heat exchange duct 12a between its inlet 12a-1 and outlet 12a-2 is determined by the air flow rate and residence time therein and is controlled as will be discussed in more detail below.

[0018] Pressurized and heated bleed air via ducts 14a, 14b is respectively withdrawn from the port engine Ep and the starboard engine Es and introduced into the heat exchanger 12 at the inlet 12a-1 of the heat exchanger 12. The main function of the heat exchanger 12 is to cool the hot bleed air in the passive heat exchange duct 12a by radiation / convection to the surrounding environment so that the pressurized bleed air can be cooled to the ambient air temperature and introduced into the auxiliary fuel tank 10.

[0019] To effect cooling of the hot pressurized bleed air, the passive heat exchanger 12 is provided with a pressure regulating / shut-off valve (PR / SOV) 16 which is capable of fully blocking or regulating the air flow entering the inlet 12a-1 of the heat exchanger 12 from the bleed air ducts 14a, 14b. The PR / SOV 16 is commanded by an appropriate signal from the on-board system controller 17 which activates the pressurization system PS. When acting as a pressure regulator, the PR / SOV 16 will reduce the pressure of the hot bleed air obtained via ducts 14a, 14b and introduced into the inlet 12a-1 of the heat exchanger 12 to an intermediate pressure state as compared to the higher pressure state of the bleed air when withdrawn from the engines Ep, Es. When acting as a shut-off valve, the PR / SOV 16 will completely cut off the bleed air flow from ducts 14a, 14b to the inlet 12a-1 or allow all of the bleed air flow from ducts 14a, 14b to be diverted to the inlet 12a-1.

[0020] A fixed inlet orifice plate 18a is disposed upstream of the PR / SOV 16 in the inlet 12a-1 of the heat exchanger 12, while a fixed outlet orifice plate 18b is disposed in the outlet 12a-2 of the heat exchanger. The outlet orifice plate 18b is calibrated to provide a desired air flow for pressurizing the auxiliary fuel tank 10, while the fixed inlet orifice plate 18a is calibrated to limit the air flow in the event of failure of the PR / SOV 16 and to limit the air flow to avoid an increase in temperature within the auxiliary fuel tank 10. Thus, the diameter of the orifice plate 18a is designed not to allow the air flow temperature to exceed the maximum allowable temperature within the auxiliary fuel tank 10. As a non-limiting example, the diameter of the orifice plate 18a is designed to be approximately + / -25% of the diameter of the heat exchange duct 12a. This dimensional relationship between the diameter of the orifice plate 18a and the heat exchange duct 12a is determined based on the pressure and air flow levels of the withdrawn bleed air introduced into the pressurization system PS, the effective length of the heat exchange duct 12a, and the maximum design inlet temperature of the auxiliary fuel tank 10 when considering a fault condition.

[0021] To avoid reverse air flow from the auxiliary fuel tank 10 to the heat exchanger 12, a pair of check valves 20a, 20b may be provided downstream of the orifice plate 18b. The auxiliary fuel tank 10 will include a tank inlet 10a in fluid communication with the outlet 12a-2 of the heat exchanger 12, and a pressure sensor (transducer) 22 and a temperature sensor 24 are operably arranged in the tank inlet. The auxiliary fuel tank 10 will also be provided with: a vent pipe 26a in which a vent valve (VV) 26 is operably positioned; and a fuel transfer pipe 28a in which a fuel transfer valve 28 is operably positioned. As shown, the vent pipe 26a is used to discharge air pressure to the surroundings of the fuel tank 10, and the fuel transfer pipe is in fluid connection with another on-board fuel tank (not shown), such as the main fuel tank or the second auxiliary fuel tank of an aircraft with a low fuel level therein.

[0022] The fuel tank 10 is provided with a positive pressure outflow relief valve (ORV) 30 and a negative pressure inflow relief valve (IRV) 32, which ensure that the pressure within the fuel tank 10 remains between the appropriate maximum and minimum design pressure states. Each of the valves 30, 32 is a normally closed mechanically preset relief valve that opens when the pressure within the fuel tank 10 exceeds its mechanically preset pressure state set point.

[0023] During operation, the system controller 17 will send a command signal to the PR / SOV 16 to pressurize the auxiliary fuel tank 10 according to the sensed pressure and temperature states within the tank inlet 10a sensed by the pressure and temperature sensors 22 and 24 respectively.

[0024] During climb, the ambient external pressure of the aircraft will decrease due to the decrease in atmospheric pressure, so the pressure difference between the pressure within the fuel tank 10 and the ambient pressure will increase. Therefore, the system controller will keep the PR / SOV 16 in the open position until the fuel tank pressure reaches Figure 2 the set point 2. During the climb mode, the fuel tank pressure will rise to a value higher than the set point 2 even if the PR / SOV 16 is in the closed position. In this flight phase, the ORV 30 will function to release the fuel tank pressure to prevent the fuel tank pressure from exceeding Figure 2 the set point 3.

[0025] During descent, the internal pressure within the fuel tank 10 will of course decrease due to the increase in atmospheric pressure outside the aircraft AC. The system controller 17 will command the PR / SOV 16 to be in the open position when the fuel tank pressure drops below Figure 2 the value of the set point 1, thereby keeping the fuel tank pressure higher than Figure 2 the set point 1.

[0026] The pressurization system 10 also has a fail-safe mode. In this regard, in the event of a failure of the ORV 30, the pressure within the fuel tank 10 can increase during the climb of the aircraft AC such that a value above the upper pressure limit for normal operation ( Figure 2 the set point 3 in Figure 2 is reached). In such a case, the system controller will issue a command signal to the VV valve 26 when the pressure sensor 22 senses the maximum pressure condition within the fuel tank 10 (identified as set point 4 in

[0027] ), thereby causing the VV 26 to open, thus allowing the excess pressure to be discharged from the fuel tank 10 to the surrounding environment. The system controller 17 can also issue an audible or visual notification signal to the flight crew to warn them that the pressure relief valve ORV 30 has failed. Figure 2 During the descent mode, if the PR / SOV 16 fails in the closed position, the pressure inside the fuel tank 10 may drop below 0 psig. Thus, if such a state exists, when the mechanical preset pressure state point of the IRV 30 (identified as set point 5 in

[0028] is reached), the IRV 32 will be caused to open, thus allowing the pressure within the fuel tank 10 to be repressurized until there is no negative pressure state, at which time the IRV 32 will mechanically close again. If the IRV 32 fails, the controller 17 will issue a command signal to the VV 26 when the pressure within the fuel tank 10 reaches the maximum negative pressure state identified by the set point 6, thereby causing the VV 26 to open, thus allowing the internal pressure state of the fuel tank 10 to equalize with the ambient pressure outside the fuel tank 10. The system controller 17 can also issue an audible or visual notification signal to the flight crew to warn them that the pressure relief valve IRV 32 has failed. By way of example only, the maximum allowable pressure within the fuel tank can be between 12 psig (set point 4) and -1 psig (set point 6) and is determined based on the structural design of the fuel tank. Figure 2The setpoint 1 therein is raised to setpoint 2, at which time, the command PR / SOV reaches the closed position. Thus, this repetitive cycling of the PR / SOV 16 between the closed position and the open position will maintain the internal pressure within the fuel tank 10 within the fuel transfer pressure operating range determined by the pressure range between setpoints 1 and 2. Thus, this cycling of the PR / SOV 16 serves as an example of an "on-off" valve with hysteresis and allows fuel to be transferred from the fuel tank 10 to another on-board fuel tank without a pump (i.e., fuel transfer is achieved only through the pressure state within the fuel tank 10, thereby eliminating the need for a fuel transfer pump and its associated systems).

[0029] Accordingly, while reference has been made to particular embodiments of the present invention, those skilled in the art can envision various variations. Thus, it should be understood that the present invention is not limited to the disclosed embodiments, but rather, the present invention is intended to cover various variations and equivalent arrangements included within the spirit and scope of the present invention.

Claims

1. An aircraft fuel tank pressurization system, comprising: at least one on-board fuel tank configured to store aircraft fuel at a predetermined internal tank pressure and at an internal ambient tank temperature, the fuel tank having: a tank inlet for receiving pressurized air; a tank outlet for discharging fuel from the tank; and a tank vent line; a passive heat exchanger having: an exchanger inlet fluidly connected to a propulsion engine of the aircraft to receive heated pressurized bleed air from the propulsion engine; and an exchanger outlet fluidly connected to the fuel tank, the passive heat exchanger being configured to cool the heated pressurized bleed air from the engine by heat transfer to the surrounding environment by means of radiation and convection so as to supply pressurized air to the fuel tank at the predetermined internal tank pressure and the internal ambient tank temperature; inlet and outlet fixed calibration orifices positioned in the inlet and outlet of the passive heat exchanger for controllably restricting the flow of bleed air from the engine to the heat exchanger and the fuel tank respectively; a pressure regulating / cut-off valve located in the inlet of the heat exchanger downstream of the inlet orifice and having an open state and a closed state to fully supply and fully stop the flow of the pressurized bleed air to the heat exchanger respectively; a system controller operatively connected to the pressure regulating / cut-off valve for regulating the pressure regulating / cut-off valve between the open state and the closed state so as to maintain the pressure and temperature state of the pressurized air supplied to the tank through the heat exchanger within a predetermined pressure and temperature set point state; and a pressure and temperature sensor located in the tank inlet for sensing the pressure and temperature state of the pressurized air supplied from the exchanger outlet to the tank inlet and, if the pressure and temperature state is outside a predetermined range, sending a corresponding pressure and temperature state signal to the system controller, wherein the system controller issues a command signal for regulating the pressure regulating / cut-off valve in response to receipt of the pressure state signal and / or the temperature state signal and thereby maintains the pressure and temperature state of the pressurized air supplied from the heat exchanger to the tank inlet within the predetermined range; wherein the fuel tank further includes a normally closed positive pressure out-flow relief valve and a normally closed negative pressure in-flow relief valve, wherein each of the out-flow relief valve and the in-flow relief valve is a normally closed relief valve that opens in response to the internal tank pressure state exceeding its mechanically preset pressure state set point, thereby maintaining the internal tank pressure state within its maximum and minimum design pressure states; and wherein The system controller includes a first pressure setpoint and a second pressure setpoint, the first pressure setpoint and the second pressure setpoint respectively establishing a first internal pressure state of the fuel tank and a second internal pressure state of the fuel tank, the second internal pressure state being greater than the first internal pressure state, thereby establishing an upper pressure limit and a lower pressure limit of a normal range of the internal tank pressure state; and wherein during aircraft climb, the system controller holds the pressure regulating / cutoff valve in the open position until a pressure signal is received from the pressure sensor that the internal tank pressure state has reached the first pressure setpoint, at which time the system controller issues a command signal to cause the pressure regulating / cutoff valve to move to its closed position; and wherein the mechanical preset pressure state of the outflow pressure relief valve corresponds to the second pressure setpoint such that the outflow pressure relief valve opens when the internal tank pressure state reaches the second pressure setpoint, thereby maintaining the internal tank pressure state within the normal range of the internal tank pressure state; and wherein the fuel tank further includes a vent valve, the vent valve being located in the tank vent pipe and operatively connected to the system controller; and wherein the system controller issues a command signal to open the vent valve in response to receiving from the pressure sensor a pressure signal that the internal tank pressure state has reached a pressure state greater than the second internal pressure state, thereby allowing the internal pressure within the fuel tank to be discharged to the surrounding environment through the tank vent pipe.

2. The aircraft fuel tank pressurization system according to claim 1, wherein: the system controller includes a third pressure setpoint, the third pressure setpoint being lower than the first pressure setpoint, thereby establishing a third internal pressure state of the fuel tank, the third internal pressure state being lower than the lower pressure limit of the normal range of the internal tank pressure state established by the first pressure setpoint; and wherein during aircraft descent, the system controller holds the pressure regulating / cutoff valve in the closed position until a pressure signal is received from the pressure sensor that the internal tank pressure state has reached the third pressure setpoint, at which time the system controller issues a command signal to cause the pressure regulating / cutoff valve to move to its open position.

3. The aircraft fuel tank pressurization system according to claim 1, wherein: the fuel tank includes a fuel transfer valve, the fuel transfer valve being positioned in the tank outlet and operatively connected to the system controller; and wherein the system controller includes a third pressure setpoint, the third pressure setpoint being lower than the first pressure setpoint, thereby establishing a third internal pressure state of the fuel tank, the third internal pressure state being lower than the lower pressure limit of the normal range of the internal tank pressure state established by the first pressure setpoint; and wherein During a fuel transfer mode, the system controller issues a command signal to cause the fuel transfer valve to open and thereby allow fuel to flow through the tank outlet under the pressure within the fuel tank, which in turn causes a reduction in the internal pressure within the fuel tank; and wherein during the fuel transfer mode, the system controller issues another command signal to cause the pressure regulating / shutoff valve to open in response to the internal pressure within the fuel tank decreasing to a third pressure setpoint, the third pressure setpoint establishing a third internal pressure state that is lower than the first internal pressure state.

4. The aircraft fuel tank pressurization system according to claim 1, wherein, the mechanical preset pressure state setpoint of the inlet relief valve is set below 0 psig.

5. The aircraft fuel tank pressurization system according to claim 1, wherein, the fuel tank further includes a vent valve that is located in the tank vent line and is operatively connected to the system controller; and wherein the system controller includes a fourth pressure setpoint that is higher than the second pressure setpoint, thereby establishing a fourth internal pressure state of the fuel tank, the fourth internal pressure state being greater than the upper pressure limit of the normal range of the internal tank pressure state established by the second pressure setpoint; and wherein in the event of a failure of the outlet relief valve, the system controller issues a command signal to open the vent valve in response to receiving a pressure signal from the pressure sensor that the internal tank pressure state has reached the fourth internal pressure state, thereby allowing the internal pressure within the fuel tank to be vented to the surrounding environment through the tank vent line.

6. The aircraft fuel tank pressurization system according to claim 5, wherein, the mechanical preset pressure state setpoint of the inlet relief valve establishes a fifth pressure setpoint that is lower than 0 psig, such that the inlet relief valve opens in response to the internal tank pressure state reaching the fifth pressure setpoint to allow ambient air to enter the fuel tank.

7. The aircraft fuel tank pressurization system according to claim 6, wherein, the system controller includes a sixth pressure setpoint that is lower than the fifth pressure setpoint, thereby establishing a sixth internal pressure state of the fuel tank, the sixth internal pressure state being lower than the mechanical preset pressure state setpoint of the inlet relief valve; and wherein in the event of a failure of the inlet relief valve, the system controller issues a command signal to open the vent valve in response to receiving a pressure signal from the pressure sensor that the internal tank pressure state has reached the sixth internal pressure state, thereby allowing ambient air pressure to enter the fuel tank through the tank vent line.

8. An aircraft, the aircraft including the aircraft fuel tank pressurization system according to claim 1.

9. A method of pressurizing an on-board fuel tank of an aircraft, comprising: (a) establishing a fluid connection between an exchanger inlet of a passive heat exchanger and a propulsion engine of the aircraft to receive heated pressurized bleed air from the propulsion engine; (b) Establish a fluid connection between the exchanger outlet of the heat exchanger and the tank inlet of the fuel tank; (c) Allow the passive heat exchanger to cool the heated pressurized bleed air from the engine by heat transfer to the surrounding environment by means of radiation and convection to supply pressurized air to the fuel tank at a predetermined internal tank pressure state and internal ambient tank temperature; (d) Position inlet and outlet fixed calibration orifices in the exchanger inlet and exchanger outlet of the passive heat exchanger and controllably restrict the flow of bleed air from the engine to the heat exchanger and the fuel tank respectively through the orifices; (e) Provide a pressure regulating / cut-off valve which is located in the inlet of the heat exchanger downstream of the inlet orifice and has an open state and a closed state to fully supply and completely stop the flow of pressurized bleed air to the heat exchanger respectively; (f) Provide pressure and temperature sensors which are located in the tank inlet for sensing the pressure and temperature state of the pressurized air supplied from the exchanger outlet to the tank inlet and, if the pressure and temperature state is outside a predetermined range, send corresponding pressure and temperature state signals to the system controller; And (g) Cause the system controller to send a command signal to the pressure regulating / cut-off valve in response to the pressure and temperature signals from the pressure and temperature sensors to regulate the pressure regulating / cut-off valve between the open state and the closed state, thereby maintaining the pressure and temperature state of the pressurized air supplied to the fuel tank through the heat exchanger within a predetermined pressure and temperature set point state; (h) Further provide a positive pressure out-flow relief valve and a normally closed negative pressure in-flow relief valve to the fuel tank; And (i) Allow each of the out-flow relief valve and the in-flow relief valve to open in response to the internal tank pressure state exceeding its mechanically preset pressure state set point, thereby maintaining the internal tank pressure within the maximum and minimum design pressure states; (j) Provide a first pressure set point and a second pressure set point within the system controller, the first pressure set point and the second pressure set point establishing a first internal pressure state of the fuel tank and a second internal pressure state of the fuel tank respectively, the second internal pressure state being greater than the first internal pressure state, thereby establishing an upper pressure limit and a lower pressure limit of the normal range of the internal tank pressure state; (k) During aircraft climb, cause the system controller to hold the pressure regulating / cut-off valve in the open position until a pressure signal is received from the pressure sensor that the internal tank pressure has reached the first pressure set point, and then cause the system controller to send a command signal to cause the pressure regulating / cut-off valve to move to its closed position; And (l) Provide the out-flow relief valve with a mechanically preset pressure state corresponding to the second pressure set point and allow the out-flow relief valve to open when the internal tank pressure reaches the second pressure set point, thereby maintaining the internal tank pressure within the normal range of the internal tank pressure state; (m) Provide a vent valve, which is located in the tank vent pipe of the fuel tank and operatively connect the vent valve to the system controller; and (n) Cause the system controller to issue a command signal for opening the vent valve in response to a pressure signal received from the pressure sensor that the internal tank pressure has reached a pressure state greater than the second internal pressure state, thereby allowing the internal pressure in the fuel tank to be discharged to the surrounding environment through the tank vent pipe.

10. The method according to claim 9, the method further includes: (o) Provide a third pressure set point to the system controller, the third pressure set point being lower than the first pressure set point, thereby establishing a third internal pressure state of the fuel tank, the third internal pressure state being lower than the lower pressure limit of the normal range of the internal tank pressure state established by the first pressure set point; and (p) During the descent of the aircraft, cause the system controller to maintain the pressure regulating / cut-off valve in the closed state until a pressure signal is received from the pressure sensor that the internal tank pressure has reached the third pressure set point, and then cause the system controller to issue a command signal to cause the pressure regulating / cut-off valve to move to its open position.

11. The method according to claim 9, the method further includes: (o) Position the fuel transfer valve in the tank outlet and operatively connect the fuel transfer valve to the system controller; (p) Provide a third pressure set point to the system controller, the third pressure set point being lower than the first pressure set point, thereby establishing a third internal pressure state of the fuel tank, the third internal pressure state being lower than the lower pressure limit of the normal range of the internal tank pressure state established by the first pressure set point; and (q) During the fuel transfer mode, allow the system controller to issue command signals to the fuel transfer valve and / or the pressure regulating / cut-off valve, thereby causing: (1) The fuel transfer valve opens, thereby allowing fuel to flow through the tank outlet under the pressure in the fuel tank, which in turn causes the internal pressure in the fuel tank to decrease, and (2) The pressure regulating / cut-off valve opens in response to the internal pressure in the fuel tank decreasing to the third pressure set point, the third pressure set point establishing a third internal pressure state lower than the first internal pressure state.

12. The method according to claim 10, the method includes providing a mechanical preset pressure state set point lower than 0 psig to the inflow pressure relief valve.

13. The method according to claim 9, the method further includes: (o) Provide a fourth pressure set point to the system controller, the fourth pressure set point being higher than the second pressure set point, thereby establishing a fourth internal pressure state of the fuel tank, the fourth internal pressure state being greater than the upper pressure limit of the normal range of the internal tank pressure state established by the second pressure set point; and (p)In the case of a failure of the outlet pressure relief valve, the system controller is caused to issue a command signal for opening the vent valve in response to receiving a pressure signal from the pressure sensor that the internal tank pressure state has reached the fourth internal pressure state, thereby allowing the internal pressure in the fuel tank to be discharged to the surrounding environment through the tank vent pipe.

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