Method for removing hydraulic fluid from an aircraft hydraulic system, aircraft hydraulic system and aircraft

Abstract

A method of removing hydraulic fluid from an aircraft hydraulic system. The hydraulic system includes: a hydraulic actuation mechanism actuated by an electro-hydraulic servo valve; a hydraulic fluid port through which the hydraulic fluid can escape; and a hydraulic fuse having a closed state and an open state, the hydraulic fuse being between the electro-hydraulic servo valve and the hydraulic fluid port. The hydraulic fluid port is opened, and activation of the electro-hydraulic servo valve is then controlled to force the hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port, the control being such that the hydraulic fuse does not enter and remains in a closed state.

Description

Technical Field

[0001] The present invention relates to a method of removing hydraulic fluid from an aircraft hydraulic system, an aircraft hydraulic system and an aircraft. More particularly, but not exclusively, the present invention relates to removing hydraulic fluid from an aircraft hydraulic system comprising a hydraulic fuse which reduces undesired loss of hydraulic fluid. The present invention is particularly, but not exclusively, applicable to hydraulic brake systems for aircraft landing gear. Background Art

[0002] It is often desirable to remove hydraulic fluid from an aircraft hydraulic system to flush and replace the hydraulic fluid, or to bleed a hydraulic brake system to, for example, remove air from the hydraulic fluid. Traditionally, this has been done manually by a human operator. This is time consuming and may also result in unintentional system effects, such as the closure of hydraulic fuses (as discussed in detail below), requiring more complex maintenance and potential removal and replacement of parts.

[0003] It would be advantageous to be able to automate or partially automate the removal of hydraulic fluid from an aircraft hydraulic system. However, it is undesirable to require additional components in order to do so, particularly in the case of an aircraft and landing gear, particularly as the additional weight and space required for the additional components may be disadvantageous.

[0004] The present invention aims to alleviate the above mentioned problems.Alternatively or additionally, the present invention aims to provide an improved method of removing hydraulic fluid from an aircraft hydraulic system, an aircraft maintenance system and an aircraft. Summary of the invention

[0005] According to a first aspect of the invention, there is provided a method of removing hydraulic fluid from an aircraft hydraulic system, wherein the hydraulic system comprises:

[0006] an electro-hydraulic servo valve connected to a fluid path for hydraulic fluid;

[0007] a hydraulic actuation mechanism connected to the fluid path, the hydraulic actuation mechanism being arranged to be actuated when a force is applied to the hydraulic fluid via the electro-hydraulic servovalve;

[0008] a hydraulic fluid port through which hydraulic fluid can escape from the hydraulic system when the hydraulic fluid port is open;

[0009] A hydraulic fuse in the fluid path, the hydraulic fuse being between the electro-hydraulic servo valve and the hydraulic fluid port, the hydraulic fuse being arranged to move towards a closed state through which hydraulic fluid cannot pass when the fluid pressure differential across the hydraulic fuse is above a closing threshold, and being arranged to move towards an open state through which hydraulic fluid can pass when the fluid pressure differential across the hydraulic fuse is below an opening threshold; the method comprising the steps of:

[0010] Open the hydraulic fluid ports;

[0011] Activation of the electro-hydraulic servo valve is controlled to force hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port, wherein the electro-hydraulic servo valve is controlled such that a hydraulic fuse does not enter and remains in a closed state.

[0012] This will allow hydraulic fluid to escape from the hydraulic fluid port despite the electro-hydraulic servo valve being enabled and the hydraulic fluid port being open. However, this will also create a fluid pressure differential across the hydraulic fuse, thereby causing the hydraulic fuse to move to the hydraulic fuse's closed state to prevent any further hydraulic fluid from escaping. By controlling the activation of the electro-hydraulic servo valve so that the hydraulic fuse does not enter and remains in the closed state, the process of removing hydraulic fluid from the hydraulic system can be automated and therefore does not suffer the disadvantages of manual removal caused by the user. This can advantageously be accomplished using existing components of the hydraulic system.

[0013] It will be appreciated that the opening threshold of the hydraulic fuse will be the same as or lower than the closing threshold of the hydraulic fuse.

[0014] Advantageously, the electro-hydraulic servo valve has a maximum activation level, and in the activation step, the electro-hydraulic servo valve is activated at a level below the maximum activation level. For example, the maximum activation level may be 40 mA and the activation level used is 20 mA. Although this will reduce the speed at which hydraulic fluid is removed from the aircraft hydraulic system when the hydraulic fuse is in the open state, because the electro-hydraulic servo valve will act as a limiter on the speed at which the liquid fluid can flow and therefore escape, the electro-hydraulic servo valve also reduces the speed at which the hydraulic fuse moves to the closed state. In addition, the electro-hydraulic servo valve can be activated at a level that prevents the hydraulic fuse from moving completely to the closed state at any time, for example, at a level that causes a pressure difference across the hydraulic fuse and is lower than the closing threshold of the hydraulic fuse. The activation level used may be a predetermined and / or fixed level, or may be a level that varies over time.

[0015] Advantageously, the sub-step of controlling the activation of the electro-hydraulic servo valve comprises:

[0016] activating the electro-hydraulic servo valve during a first period of time to force hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port;

[0017] deactivating the electrohydraulic servovalve during a second time period to allow a fluid pressure differential across the hydraulic fuse to drop below an opening threshold;

[0018] The activation and deactivation steps are repeated a desired number of times. In this manner, the hydraulic fuse is prevented from moving completely to the closed state because, although the hydraulic fuse will move more toward the closed state when the electro-hydraulic servo valve is activated, the electro-hydraulic servo valve is deactivated before the hydraulic fuse reaches the closed state, allowing the fluid pressure differential across the hydraulic fuse to drop below the opening threshold, thereby causing the hydraulic fuse to move more toward the open state. Alternatively, if the hydraulic fuse does enter the closed state completely, the second period during which the electro-hydraulic servo valve is deactivated will allow the hydraulic fuse to move back toward the open state.

[0019] The desired number of times may be a predetermined number calculated and / or determined from a test system, or may be determined from the properties of the system, such as using sensors, that the desired number of times has been reached. The desired number of times may be a sufficient number of times that all or a desired proportion of the hydraulic fluid has been removed from the hydraulic system with the desired probability of a predetermined number of times.

[0020] With the electro-hydraulic servo valve activated at a level below the maximum activation level and alternately activated and deactivated, this can allow more hydraulic fluid to pass before a closed state occurs and reduce the number of times the electro-hydraulic servo valve is deactivated to allow the hydraulic fuse to move from a closed state to an open state, and this can therefore be optimally set to reduce the overall time required to remove the hydraulic fluid.

[0021] Advantageously, when hydraulic fluid escapes from the hydraulic system via the hydraulic fluid port, the fluid pressure differential across the hydraulic fuse is above the closing threshold. A high fluid pressure differential across the hydraulic fuse is associated with a high flow rate, which can reduce the overall time required to remove the hydraulic fluid.

[0022] The first time period and the second time period can be predetermined time periods. The first time period and the second time period can be determined using a test system. For example, the first time period and the second time period can be around 5 seconds. For example, the first time period can be between 4.9 seconds and 5.1 seconds, or between 4.5 seconds and 5.5 seconds, or between 4 seconds and 5 seconds. For example, the second time period can be between 4.9 seconds and 5.1 seconds, or between 4.5 seconds and 5.5 seconds, or between 4 seconds and 5 seconds.

[0023] The first time period may end when the hydraulic fuse is in the closed state. The first time period may end once the hydraulic fuse is in the closed state, or a period after the hydraulic fuse is in the closed state. Alternatively, the first time period ends before the hydraulic fuse is in the closed state.

[0024] Advantageously, during the activation step, the electro-hydraulic servo valve is controlled such that the hydraulic fuse does not enter a closed state.

[0025] The second time period may end when the hydraulic fuse is in the open state. The second time period may end once the hydraulic fuse is in the open state, or a period after being in the open state. Alternatively, the second time period ends before the hydraulic fuse is in the open state. The second time period may end when the hydraulic fuse is in the fully open state or the partially open state, or before the hydraulic fuse is in the fully open state or the partially open state.

[0026] The step of opening the hydraulic fluid port may be performed by a human operator.

[0027] Advantageously, the method further comprises the step of checking whether the hydraulic fluid port is open after the hydraulic fluid port is opened. This may be done by means of a sensor on or near the hydraulic fluid port.

[0028] Advantageously, the hydraulic system further comprises a position sensor to determine the position of an internal component of the hydraulic fuse, and the electro-hydraulic servo valve is controlled using the output of the position sensor. In this way, the hydraulic system can use the position sensor to directly determine the state of the hydraulic fuse, and use this to enable / disable the electro-hydraulic servo valve so that the hydraulic fuse does not enter and remain in a closed state. In the case where the electro-hydraulic servo valve is enabled at a level below the maximum enable level, the output of the position level can be used to determine the level at which the electro-hydraulic servo valve is enabled. (For example, if it is determined that the hydraulic valve is close to a closed state, the level can be reduced.) In the case where the electro-hydraulic servo valve is enabled in a first time period and disabled in a second time period, the first time period and the second time period can be determined using the output of the position sensor, rather than using a predetermined time period. The position sensor can be a linear variable differential transducer. The internal component can be a piston or a poppet valve, etc. in the hydraulic fuse.

[0029] Advantageously, the hydraulic system further comprises an accelerometer which measures the movement of the hydraulic fuse and the electro-hydraulic servo-valve is controlled using the output of the position sensor. In this way, the state of the hydraulic fuse can be determined from the accelerometer, since the vibration of the hydraulic fuse will change as the hydraulic fuse changes mode. In addition, the accelerometer can determine when - even when the hydraulic fuse is in an open state - no hydraulic fluid is passing through the hydraulic fuse, thus enabling a determination when no more hydraulic fluid is removed from the hydraulic system. As with the position sensor, the output of the accelerometer can be used to determine the level of activation of the electro-hydraulic servo-valve and / or the time periods during which the electro-hydraulic servo-valve is activated and deactivated.

[0030] The first time period may end when the hydraulic fuse is in the closed state. The first time period may end once the hydraulic fuse is in the closed state or a period after being in the closed state. Alternatively, the first time period ends before the hydraulic fuse is in the closed state.

[0031] The second time period may end when the hydraulic fuse is in the open state. The second time period may end once the hydraulic fuse is in the open state, or a period after being in the open state. Alternatively, the second time period ends before the hydraulic fuse is in the open state. The second time period may end when the hydraulic fuse is in the fully open state or the partially open state, or before the hydraulic fuse is in the fully open state or the partially open state.

[0032] Advantageously, the hydraulic actuation mechanism is a hydraulic brake. In this case, advantageously, the hydraulic system is an aircraft landing gear brake system.

[0033] According to a second aspect of the invention there is provided an aircraft hydraulic system arranged to perform any of the methods described above.

[0034] According to a third aspect of the present invention, there is provided an aircraft comprising an aircraft hydraulic system as described above.

[0035] According to a fourth aspect of the invention, there is provided a method for removing hydraulic fluid from an aircraft hydraulic system, wherein the hydraulic system comprises: a fluid path for the hydraulic fluid; a hydraulic fluid port through which the hydraulic fluid can escape from the hydraulic system; and a hydraulic fuse in the fluid path, wherein the hydraulic fuse comprises a fuse member configured to move between a cut-off position and a reset position, wherein the fuse member prevents the hydraulic fluid from escaping from the hydraulic system via the hydraulic fluid port, and wherein the fuse member does not prevent the hydraulic fluid from escaping from the hydraulic system via the hydraulic fluid port in the reset position, the method comprising: generating a series of pulses in the hydraulic fluid, wherein each pulse causes the hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port, each pulse causes the fuse member to move away from the reset position of the fuse member, and after each pulse, the fuse member returns to the reset position of the fuse member.

[0036] Advantageously, each pulse moves the fuse element away from a reset position of the fuse element without reaching an off position away from the fuse element.

[0037] The fuse member may include a piston.

[0038] In one embodiment, the fuse member is arranged to move towards the off position of the fuse member when the flow rate or fluid pressure difference across the hydraulic fuse is above a closing threshold, and is arranged to move to the reset position of the fuse member when the flow rate or fluid pressure difference across the hydraulic fuse is below an opening threshold, and each pulse causes the flow rate or fluid pressure difference across the hydraulic fuse to rise above the closing threshold so that the fuse member moves away from the reset position of the fuse member. In another embodiment, the hydraulic fuse is a volumetric fuse.

[0039] Of course, it will be appreciated that features described in relation to one aspect of the invention may be incorporated into other aspects of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0041] Figure 1 is a schematic diagram of an aircraft landing gear system according to an embodiment of the present invention;

[0042] Figure 2a is in fully open state Figure 1 A cross-sectional view of a hydraulic fuse shown in;

[0043] Figure 2b is partially open. Figure 1 A cross-sectional view of a hydraulic fuse shown in;

[0044] Figure 2c is in closed state Figure 1 A cross-sectional view of a hydraulic fuse shown in;

[0045] Figure 3 It shows Figure 1 A flowchart of the operation of an aircraft landing gear system;

[0046] Figure 4 is a schematic diagram of an aircraft landing gear system according to another embodiment of the present invention;

[0047] Figure 5 It shows Figure 4 A flowchart of the operation of an aircraft landing gear system;

[0048] Figure 6 is a schematic diagram of an aircraft landing gear system according to another embodiment of the present invention;

[0049] Figure 7 Shows Figure 6 A flowchart of the operation of an aircraft landing gear system;

[0050] Figure 8 is a perspective view of an aircraft including an aircraft landing gear system according to an embodiment of the present invention;

[0051] Fig. 9 is a graph showing a first series of pressure pulses;

[0052] Fig.10 The piston is shown in Fig. 9 A graph of the movement during a pressure pulse;

[0053] Fig.11 is a graph showing a second series of pressure pulses; and

[0054] Fig.12 The piston is shown in Fig.11 Graph of the movement during a pressure pulse. DETAILED DESCRIPTION

[0055] Now refer to Figure 1 1 is a schematic diagram of an aircraft landing gear system for an aircraft according to an embodiment of the present invention. The aircraft landing gear system 1 comprises a set of wheels 2a and 2b, which are mounted on legs (not shown) which are in turn mounted on the lower side of the aircraft. The wheels 2a and 2b have respective hydraulic brakes 3a and 3d, which are part of a hydraulic brake system 3.

[0056] The hydraulic brake system 3 also includes a hydraulic fluid 4 in a hydraulic hose 5. The end of the hydraulic hose 4 is connected to both of the hydraulic brakes 3a and 3d in a T-shaped branch. The other end of the hydraulic hose 5 is connected to the electro-hydraulic servo valve 6. The electro-hydraulic servo valve 6 can apply a variable pressure to the hydraulic fluid 4, depending on the input current supplied to the electro-hydraulic servo valve 6. In the present embodiment, the electro-hydraulic servo valve 6 provides a maximum pressure to the hydraulic fluid 4 when a maximum input current of 40 mA is supplied, so when an input current of 20 mA is supplied as an example, the electro-hydraulic servo valve 6 will provide a lower pressure to the hydraulic fluid 4.

[0057] In the path of the hydraulic hose 5 close to the electro-hydraulic servo valve 6 is a selector valve 7 , which is a binary switch valve that can allow or prevent the flow of the hydraulic fluid 4 .

[0058] A hydraulic fuse 8 is located in the path of the hydraulic hose 5 close to the selector valve 7 , and thus on the side of the selector valve 7 farthest from the electro-hydraulic servo valve 6 , and the hydraulic fuse 8 is described in detail below.

[0059] Finally, hydraulic fluid port 9 is in the path of hydraulic hose 5 near hydraulic fuse 8, and thus between hydraulic fuse 8 and hydraulic brakes 3a and 3d, via which hydraulic fluid 4 can be removed from hydraulic brake system 3 when hydraulic fluid port 9 is open.

[0060] The aircraft landing gear system 1 includes a brake and steering control unit (BSCU) 11 to which the electro-hydraulic servo valve 6 and the selector valve 7 are electrically connected so that the electro-hydraulic servo valve 6 and the selector valve 7 can be controlled by the BSCU 11. The BSCU 11 is interconnected with a multifunction control and display unit (MCDU) 10 of the aircraft, of which the aircraft landing gear system 1 is a part, as described in more detail below. The MCDU 10 has a control panel in the cockpit of the aircraft, but the MCDU 10 is only used for maintenance and is independent of the cockpit control system used by the pilot to fly / taxi the aircraft, etc.

[0061] Now refer to Figure 2a to Figure 2c Description Hydraulic fuse 8, Figure 2a to Figure 2c 2 are cross-sectional views of the hydraulic fuse 8 in different states. The hydraulic fuse 8 is a complete mechanical device for reducing an undesired loss of hydraulic fluid 4 from the hydraulic brake system 3 , for example due to a leak in the hydraulic hose 5 .

[0062] The hydraulic fuse 8 includes a body 50 having an inlet port 51, an outlet port 52 and a cylindrical internal cavity 53. In the internal cavity 53 there is a movable piston 60, which includes a plunger 54, a flange 55 and a ball 56. A compression spring 57 is located in the cavity between the outlet port 52 and the cylindrical flange 55, so that the plunger 54 passes through the middle of the spring 57, and the spring 54 exerts a force on the flange 55 to push the piston 60 toward the inlet port 51. The diameter of the flange 55 is smaller than the diameter of the cavity 53, so that there is a gap around the circumference of the flange 55.

[0063] Figure 2a The hydraulic fuse 8 is shown in a fully open state, which is the state to which the hydraulic fuse 8 is or will return when there is no pressure differential on the hydraulic fluid 54 across the hydraulic fuse 8, and therefore no pressure differential applied by the hydraulic fluid 54 from the inlet port 51 or the outlet port 52. In this state, the spring 57 is fully extended, so that the piston 60 is in a position where the piston 60 is closest to the inlet port 51. When the piston 60 is in this position, the ball 26 of the piston 60 is pushed against the entrance of the inlet port 51 to the cavity 53. Therefore, in effect, in the fully open state, the inlet port 51 is closed by the ball 56, which prevents the hydraulic fluid 4 from passing from the cavity 53 through the inlet port 51 (i.e., in the wrong direction).

[0064] When the hydraulic fuse 8 is in a fully open state, wherein the inlet port 51 is closed by the ball 56, the pressure differential on the hydraulic fluid 54 from the inlet port 51 to the outlet port 52 will force the ball 56 away from the inlet port 51, thus opening the inlet port 51. This causes the hydraulic fuse to be in a fully open state. Figure 2b 53. In this state, the hydraulic fluid 54 passing through the cavity 53 of the hydraulic fuse 8 exerts a force on the piston 60, and in particular, the flange 55 of the piston 60, toward the outlet port 52. This force counteracts the force exerted by the spring 57 on the piston 60 toward the inlet port 51. As a result, the piston 60 moves from the inlet port 51 toward the outlet port 52, and the hydraulic fluid can pass through the hydraulic fuse 8, in particular, by bypassing the outside of the piston 60 within the cavity 53.

[0065] However, when the hydraulic fluid 4 exerts too much force for too long a period of time, the piston 60 will move far enough toward the outlet port that the plunger 54 of the piston 60 enters the outlet port 52. This puts the hydraulic fuse in a Figure 2c In this state, the hydraulic fluid 54 can no longer pass through the hydraulic fuse 8 because the outlet port 52 is blocked by the plunger 54.

[0066] In this way, the hydraulic fuse 8 can reduce the undesired loss of hydraulic fluid 4 from the hydraulic brake system 3, for example due to a leak in the hydraulic hose 5. When a leak or the like occurs, a large pressure differential in the hydraulic fluid 4 will develop across the hydraulic fuse 8 due to the lower pressure caused by the leak. Although the hydraulic fluid 4 will initially be able to pass through the hydraulic fuse 8, thereby allowing some hydraulic fluid to be lost through the leak, the hydraulic fuse 8 will quickly enter a closed state, i.e., where the piston 60 will move completely toward the outlet port 52, such that the plunger 54 of the piston 60 prevents any more hydraulic fluid 4 from being able to pass through the outlet port 52.

[0067] As described above, once there is no longer any pressure differential on the hydraulic fluid 54 across the hydraulic fuse 8, the spring 57 will move the piston 60 back towards the inlet port 51, thus automatically returning the hydraulic fuse 8 to a fully open piston.

[0068] Now refer to Figure 3 The flow chart describes the operation of the hydraulic brake system 3.

[0069] In a first step, the maintenance operator selects on the MCDU 10 that the maintenance operator wishes the hydraulic fluid 4 to be flushed from the hydraulic brake system 3 (step 101). The MCDU 20 then instructs the operator to place the aircraft in the required configuration (step 102), including displaying any required safety messages. The hydraulic fluid port 9 is then opened (step 103). This may be done by the operator, and the opening of the hydraulic fluid port 9 may be part of the configuration indicated by the MCDU 10 in the previous step 102.

[0070] Then, the MCDU 10 instructs the BCDU 11 to open the selector valve 7 (step 104), and activate the electro-hydraulic servo valve 6 (step 105). The electro-hydraulic servo valve 6 is activated by supplying an input current of 20 mA to the electro-hydraulic servo valve 6, so that the electro-hydraulic servo valve 6 provides a pressure to the hydraulic fluid 4 that is lower than the maximum pressure of the electro-hydraulic servo valve 6. This forces the hydraulic fluid 2 to escape from the hydraulic brake system 3 via the hydraulic fluid port 9. The hydraulic brake system 3 then waits for 5 seconds (step 106).

[0071] During the 5 second waiting period, as the hydraulic fluid 2 escapes via the hydraulic fluid port 9, there will of course be a pressure differential across the hydraulic fuse 8. This will cause the hydraulic fuse 8 to move from the fully open state to the partially open state, and depending on the intensity of the conditions in the hydraulic brake system 3 and the properties of the hydraulic fuse 8, including in particular the strength of the spring 57, the hydraulic fuse 8 may enter the closed state within the 5 second waiting period.

[0072] When the 5 second waiting period has ended, the MCDU 10 instructs the BCDU 11 to deactivate the electro-hydraulic servo valve 6 (step 107) and close the selector valve 7 (step 108). The hydraulic brake system 3 then waits for another 5 seconds (step 109). During this time, since there is no longer a pressure difference across the hydraulic fuse 8, the hydraulic fuse 8 returns to the fully open state.

[0073] Then, the hydraulic brake system 3 determines whether the number of times the electro-hydraulic servo valve 6 has been activated has met a threshold value (step 110). If the threshold value has been met, the process of flushing the hydraulic fluid 4 from the hydraulic brake system 3 is completed. If the threshold value is not met, return to step 104 and repeat the two 5-second waiting periods.

[0074] In this way, the process of removing hydraulic fluid 4 from the hydraulic brake system 3 can be automatically performed by "pulsing" the electro-hydraulic servovalve 6 at 5 second intervals to first force hydraulic fluid 4 out of the hydraulic brake system 3 and then allow the hydraulic fuse 8 to reset.

[0075] It will be appreciated that the number of times the two 5 second waiting periods are repeated may be calculated and / or predetermined using a test system so that the two 5 second waiting periods are repeated a sufficient number of times for all of the hydraulic fluid 4 to be removed. Additionally, it will be appreciated that other aspects of operation may be calculated and / or predetermined using a test system to attempt to provide optimal removal of the hydraulic fluid 4. For example, it may be determined that a current other than 20 mA is ideal and / or that one or both of the waiting periods is a period other than 5 seconds.

[0076] exist Figure 4 A schematic diagram of an aircraft landing gear system applicable for an aircraft according to another embodiment of the present invention is shown in FIG. The aircraft landing gear system 199 is very similar to the aircraft landing gear system 1 of the previous embodiment, except that the hydraulic brake system 198 of this embodiment includes a position sensor 200 connected to the BSCU 11, which is arranged to measure the position of the piston 60 of the hydraulic fuse 8. The position sensor 200 is a linear variable differential transducer, but it will be understood that in other embodiments of the present invention, other types of position sensors may be used.

[0077] Now refer to Figure 5 The flow chart describes the operation of the hydraulic brake system 198.

[0078] Similar to the previous embodiment, in the first step, the maintenance operator selects to flush the hydraulic fluid 4 to be flushed from the hydraulic brake system 3 (step 201), the MCDU 20 instructs the operator to put the aircraft in the required configuration (step 202), and opens the hydraulic fluid port 9 (step 203). The selector valve 7 is then opened by the BSCU 11 (step 204), and the electro-hydraulic servo valve 6 is enabled (step 205), thereby forcing the hydraulic fluid 2 to escape from the hydraulic brake system 3 via the hydraulic fluid port 9 again.

[0079] However, in contrast to the previous embodiment, in the present embodiment, instead of the electro-hydraulic servo valve 6 being simply enabled for a predetermined period of time, such as 5 seconds, the position sensor 200 is instead used by the BSCU 11 to determine when the hydraulic fuse 8 has moved to the closed state (step 206), i.e. the piston 60 has moved to block the outlet port 52. Once this has occurred, the electro-hydraulic servo valve 6 is deactivated (step 207) and the selector valve 7 is closed (step 208) as in the previous embodiment. Again unlike the previous embodiment, the hydraulic brake system 198 then again uses the position sensor 200 to determine when the hydraulic fuse 8 has moved back to the fully open state (step 209).

[0080] Finally, the brake system 198 then determines whether the number of times the electro-hydraulic servo valve 6 has been activated has met a threshold (step 210). If the threshold has been met, the process of flushing the hydraulic fluid 4 from the hydraulic brake system 198 is completed, otherwise the steps starting from step 204 are repeated.

[0081] Thus, again, the process of removing hydraulic fluid 4 from the hydraulic brake system 198 may be automatically performed by "pulsing" the electro-hydraulic servo-valve 6. However, where the state of the hydraulic fuse 8 is directly sensed by the position sensor 200, the flushing may be more efficiently effected because the activation and / or deactivation of the electro-hydraulic servo-valve 6 may be initiated when the electro-hydraulic servo-valve 6 is measured to be in the desired state, rather than merely when it is predicted that this should be the case based on calculations and / or determinations made using a test system.

[0082] It will again be appreciated that various other aspects of operation may be calculated and / or predetermined using a test system or otherwise to attempt to provide optimal removal of the hydraulic fluid 4. For example, in this embodiment, activation and / or deactivation of the electro-hydraulic servo-valve 6 may be initiated only before the electro-hydraulic servo-valve 6 moves to a fully open state or closed state, respectively, rather than when / after these states have actually been reached.

[0083] exist Figure 6 A schematic diagram of an aircraft landing gear system applicable for an aircraft according to another embodiment of the invention is shown in FIG. The aircraft landing gear system 299 is very similar to the aircraft landing gear system of the previous embodiment, except that the hydraulic brake system 298 of this embodiment includes an accelerometer 300 connected to the BSCU 11, which is arranged to measure the movement, in particular the vibration, of the hydraulic fuse 8.

[0084] Now refer to Figure 7 The flowchart of FIG. 29 describes the operation of the hydraulic brake system 298. Similar to the previous embodiment, in the first step, the maintenance operator selects to flush the hydraulic fluid 4 to be flushed from the hydraulic brake system 3 (step 301), the MCDU 20 instructs the operator to put the aircraft in the required configuration (step 302), and opens the hydraulic fluid port 9 (step 303). Then the selector valve 7 is opened (step 304) and the electro-hydraulic servo valve 6 is enabled (step 305), thereby forcing the hydraulic fluid 2 to escape from the hydraulic brake system 3 through the hydraulic fluid port 9 again.

[0085] However, in contrast to the previous embodiment, in this embodiment, the accelerometer 300 is then used to determine whether any hydraulic fluid 4 is passing through the hydraulic fuse 4 (step 306). If no hydraulic fluid 4 is passing, there must be no hydraulic fluid 4 in the hydraulic brake system 298 (or at least no hydraulic fluid 4 that can be removed by activating the electro-hydraulic servovalve 6), and therefore the process ends.

[0086] If the hydraulic fluid 4 initially passes through the hydraulic fuse 8, the accelerometer 300 is used to determine when the hydraulic fuse 8 has moved to the closed state (step 307). This will be determined based on the change in vibration of the hydraulic fuse 8 caused by the movement of the hydraulic fluid 4 within the hydraulic fuse 8 as the piston 60 moves within the cavity 53 of the hydraulic fuse 8 to eventually block the outlet valve 52.

[0087] Once the hydraulic fuse 8 is moved to the closed state, again as in the previous embodiment, the electrohydraulic servo valve 6 is deactivated (step 308) and the selector valve 7 is closed (step 309). The accelerometer 300 is then used to determine when the hydraulic fuse 8 has moved back to the fully open state (step 310), and when this occurs, the steps forward from step 304 are repeated.

[0088] Thus, again, the process of removing hydraulic fluid 4 from the hydraulic brake system 298 may be performed automatically by "pulsing" the electro-hydraulic servovalve 6. However, in this case, the accelerometer 300 may be used to directly sense the state of the hydraulic fuse 8, and also when all hydraulic fluid 4 has been removed.

[0089] It will again be appreciated that various other aspects of operation may be calculated and / or predetermined using a test system or otherwise to attempt to provide optimal removal of the hydraulic fluid 4, for example again, activation and / or deactivation of the electro-hydraulic servo valve 6 may be initiated only shortly before the electro-hydraulic servo valve 6 moves to a fully open or closed state, respectively, rather than when / after these states have actually been reached.

[0090] Figure 8 5 is a perspective view of an aircraft according to an embodiment of the present invention. Aircraft 500 includes an aircraft landing gear system according to the above-mentioned embodiment.

[0091] As described above, the servo valve 6 is controlled such that the servo valve 6 performs a repeated series of activation and deactivation steps. Fig. 9 is a graph showing a series of pressure pulses 600, each pulse 600 corresponding to an activation step and the interval between each adjacent pair of pulses 600 corresponding to a deactivation step. Fig. 9 The Y-axis of represents the fluid pressure difference across the hydraulic fuse 8 , or equivalently, the flow rate through the hydraulic fuse 8 .

[0092] The hydraulic fuse includes a piston 60, which is configured to be in a closed position (eg Figure 2c ) and reset position (in Figure 2a ), in which the piston 60 prevents the hydraulic fluid from escaping from the hydraulic system via the hydraulic fluid port 9, and in which the piston 60 does not prevent the hydraulic fluid from escaping from the hydraulic system via the hydraulic fluid port 9 in the reset position.

[0093] Each pulse 600 causes hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port 9. Each pulse 600 also causes the piston 60 to move away from the reset position of the piston 60 and during a deactivation step after each pulse 60 the piston 60 returns to the reset position of the piston 60.

[0094] Fig.10 The piston 60 is shown Fig. 9 6. The movement corresponding to the series of pulses 600 shown in FIG. 6. During each pulse 600, the piston moves towards the piston's cut-off position, as indicated at 601, because the fluid pressure difference across the hydraulic fuse is above the closing threshold. The fluid pressure difference during each pulse 600 has a level L(1) above the closing threshold L(T). Therefore, when hydraulic fluid escapes from the hydraulic system via the hydraulic fluid port 9, the fluid pressure difference across the hydraulic fuse 8 is above the closing threshold L(T).

[0095] like Fig.10 As shown in , each pulse moves the piston 60 away from the reset position of the piston 60 without reaching the cut-off position of the piston 60. In other words, in the activation step corresponding to each pulse 600, the electro-hydraulic servo valve 6 is controlled so that the hydraulic fuse 8 does not enter the closed state. This is beneficial because if the piston 60 reaches the cut-off position ( Figure 2c If the piston 60 is moved as close to the closed position of the piston 60 as possible without reaching the closed position, the piston 60 may become locked in place and difficult to force back to the reset position of the piston 60. Ideally, the piston 60 moves as close to the closed position of the piston 60 as possible without reaching the closed position.

[0096] The speed of the movement 601 of the piston in the activation step depends on the characteristics of the hydraulic fuse 8 (eg the stiffness of the spring 57) and the magnitude of the fluid pressure difference across the hydraulic fuse 8 - higher fluid pressures are associated with higher speeds.

[0097] At the end of each pulse 600, the fluid pressure difference across the hydraulic fuse 8 drops below the opening threshold and this causes the piston to be forced back to its reset position by the spring 57. In this case, the opening threshold is the same as the closing threshold L(T).

[0098] The speed of the movement 602 of the piston in each deactivation step also depends on the characteristics of the hydraulic fuse 8 (e.g. the stiffness of the spring 57). Fig. 9 and Fig.10 In this case, the speed is higher during the deactivation step than during the activation step, so the first time period T(1) of each pulse 600 is longer than the second time period T(2) between pulses.

[0099] Fig.11 is a graph showing a series of pressure pulses 600a, each pulse 600a corresponding to an activation step and the interval between each adjacent pair of pulses 600a corresponding to a deactivation step. Fig. 9 Compared with the pulse 600, Fig.11 The pulse 600a in the example has a higher pressure level L(2), and the time T(2) between the pulses 600a is Fig. 9 The time T(2) in the pulse sequence is the same.

[0100] like Fig.12 As shown in Fig.10 The speed of the movement 601a of the piston in the activation step is higher than in the activation step because the pressure difference (and the associated flow rate) is higher. For this reason, the first time period T(3) is shorter so that the hydraulic fuse 8 does not enter the closed state. In this case, the first time period T(3) for each pulse 600a is approximately the same as the second time period T(2) between pulses.

[0101] By comparison Fig. 9 and Fig.10 , it can be seen that selecting the optimum pressure level (i.e. the pressure level that minimizes the total time required to remove the hydraulic fluid) involves a compromise between a high pressure level and a long first time period for each pulse. In other words, the higher the pressure level, the higher the flow rate out of the hydraulic fluid port 9, but the hydraulic fuse 8 must also be reset more frequently. The optimum pressure level, first time period and second time period may be determined by laboratory testing and stored in a computer memory that is part of the BSCU 11.

[0102] While very high pressures may not minimize the overall time required to remove hydraulic fluid (due to the need to frequently reset hydraulic fuses), they may be beneficial because the higher flow rate will excite or energize air bubbles and reduce the risk of these bubbles becoming trapped in the hydraulic system.

[0103] In the above-described embodiment, the hydraulic fuse 8 is a rate fuse or a pressure fuse. In other words, the piston is arranged to move towards the piston's cut-off position when the flow rate or fluid pressure difference across the hydraulic fuse is above a closing threshold, and is arranged to move towards the piston's reset position when the flow rate or fluid pressure difference across the hydraulic fuse is below an opening threshold. In an alternative embodiment of the present invention, the hydraulic fuse 8 may be replaced by a volume fuse that enters a closed state when a threshold amount of hydraulic fluid has flowed through the fuse.

[0104] An example of such a volume fuse is described in US5507466, the contents of which are incorporated herein by reference. The piston moves between a reset position and a closed position. When the piston moves toward the closed position, the piston forces hydraulic fluid to leave the hydraulic system via the hydraulic fluid port 9. When the piston retracts to the reset position, the piston again draws hydraulic fluid into the hydraulic line via a refill line (not shown) between the volume fuse and the hydraulic fluid port 9.

[0105] While the present invention has been described and illustrated with reference to specific embodiments, those skilled in the art will appreciate that the invention lends itself to many different variations not specifically illustrated herein.

[0106] For example, in other embodiments of the present invention, the electro-hydraulic servo-valve may not be deactivated during the removal of hydraulic fluid, but instead may be continuously enabled, but continuously enabled at a low enough level that the hydraulic fuse does not enter the closed state (or does not move toward the closed state at all), or the electro-hydraulic servo-valve may be continuously enabled, but continuously enabled at varying levels so that the hydraulic fuse is still able to move toward the fully open state over multiple time periods to allow the hydraulic fuse to move from the closed state, or prevent the hydraulic fuse from entering the closed state entirely.

[0107] In the case of wholes or elements with known, obvious or foreseeable equivalents mentioned in the foregoing description, these equivalents are incorporated herein as if separately set forth. The true scope of the invention should be determined by reference to the claims, which should be interpreted as including any such equivalents. The reader will also understand that the wholes or features of the invention described as preferred, advantageous, convenient, etc. are optional and do not limit the scope of the independent claims. In addition, it will be understood that although these optional wholes or features that may be beneficial in some embodiments of the invention may be undesirable in other embodiments and therefore may not exist.

Claims

1. A method of removing hydraulic fluid from an aircraft hydraulic system, wherein: The hydraulic system comprises: an electro-hydraulic servo valve connected to a fluid path for hydraulic fluid; a hydraulic actuation mechanism connected to the fluid path, the hydraulic actuation mechanism being arranged to be actuated when a force is applied to the hydraulic fluid via the electro-hydraulic servo valve; a hydraulic fluid port through which the hydraulic fluid can escape from the hydraulic system when the hydraulic fluid port is open; a hydraulic fuse located in the fluid path between the electro-hydraulic servo valve and the hydraulic fluid port, the hydraulic fuse being arranged to move toward a closed state through which hydraulic fluid cannot pass when a fluid pressure differential across the hydraulic fuse is above a closing threshold, and to move toward an open state through which hydraulic fluid can pass when a fluid pressure differential across the hydraulic fuse is below an opening threshold; The method comprises the following steps: opening the hydraulic fluid port; Activation of the electro-hydraulic servo valve is controlled to force hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port, wherein the electro-hydraulic servo valve is controlled such that the hydraulic fuse does not remain in the closed state if the closed state is entered.

2. The method according to claim 1, wherein: The electro-hydraulic servo valve has a maximum activation level, and in the activation step, the electro-hydraulic servo valve is activated at a level lower than the maximum activation level.

3. The method according to claim 1 or 2, wherein: The step of controlling the activation of the electro-hydraulic servo valve comprises the following sub-steps: activating the electro-hydraulic servo valve during a first period of time to force hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port; deactivating the electro-hydraulic servovalve for a second period of time to allow a fluid pressure differential across the hydraulic fuse to drop below the opening threshold; The activating step and the deactivating step are repeated a desired number of times.

4. The method according to claim 3, wherein: The first time period and the second time period are predetermined time periods.

5. The method according to claim 4, wherein: The first time period and the second time period are determined using a test system.

6. The method according to claim 3, wherein: The first time period ends when the hydraulic fuse is in the closed state.

7. The method according to claim 3, wherein: The first time period ends before the hydraulic fuse is in the closed state.

8. The method according to claim 3, wherein: The second time period ends when the hydraulic fuse is in the open state.

9. The method according to claim 3, wherein: The first time period is longer than the second time period.

10. The method according to claim 3, wherein: During the first time period, the electro-hydraulic servo valve is activated at a predetermined level.

11. The method according to claim 1 or 2, further comprising the following steps: After the hydraulic fluid port is opened, it is checked that the hydraulic fluid port is open.

12. The method according to claim 1 or 2, wherein: The hydraulic system also includes a position sensor that determines a position of an internal component of the hydraulic fuse, and the electro-hydraulic servo valve is controlled using an output of the position sensor.

13. The method according to claim 12, wherein: The hydraulic system also includes an accelerometer that measures movement of the hydraulic fuse, and the electro-hydraulic servo valve is controlled using an output of the position sensor.

14. The method according to claim 1 or 2, wherein: The hydraulically actuated mechanism is a hydraulic brake.

15. The method according to claim 14, wherein: The hydraulic system is an aircraft landing gear brake system.

16. The method according to claim 1 or 2, wherein: In the activation step, the electro-hydraulic servo valve is controlled so that the hydraulic fuse does not enter the closed state.

17. The method according to claim 1 or 2, wherein: When the hydraulic fluid escapes from the hydraulic system via the hydraulic fluid port, a fluid pressure differential across the hydraulic fuse is above the closing threshold.

18. An aircraft maintenance system arranged to perform the method of any one of claims 1 to 17.

19. An aircraft comprising an aircraft maintenance system according to claim 18.

20. A method of removing hydraulic fluid from an aircraft hydraulic system, wherein: The hydraulic system comprises: a fluid path for the hydraulic fluid; a hydraulic fluid port through which the hydraulic fluid can escape from the hydraulic system; and a hydraulic fuse in the fluid path, wherein the hydraulic fuse includes a fuse member configured to move between an off position in which the fuse member prevents the hydraulic fluid from escaping from the hydraulic system via the hydraulic fluid port and a reset position in which the fuse member does not prevent the hydraulic fluid from escaping from the hydraulic system via the hydraulic fluid port, The method comprises: A series of pulses are generated in the hydraulic fluid, wherein each pulse causes hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port, each pulse causes the fuse member to move away from the fuse member's reset position, and after each pulse the fuse member returns to the fuse member's reset position.

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