GB2588383A

GB2588383A - A method of removing hydraulic fluid from an aircraft hydraulic system, an aircraft hydraulic system, and an aircraft

Info

Publication number: GB2588383A
Application number: GB1914918.6A
Authority: GB
Inventors: Bidmead Ashley
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2019-10-15
Filing date: 2019-10-15
Publication date: 2021-04-28
Anticipated expiration: 2039-10-15
Also published as: GB2588383B; GB201914918D0

Abstract

An aircraft hydraulic system 199 has an electrohydraulic servo valve 6 connected to a hydraulic fluid path 5; a hydraulically actuated mechanism 3a,3b connected to the fluid path, actuated when force is applied to the fluid by the valve; a hydraulic fluid port 9 to allow fluid to escape the system when opened; and a hydraulic fuse 8 in the fluid path between the value and the port, arranged to move towards a closed state when fluid pressure differential across the fuse is above a threshold, and towards an open state allowing fluid to pass when below the threshold. The method of removing the fluid from the system involves opening the port and controlling the activation of the valve to force fluid to escape the system without the hydraulic fuse entering, and remaining in, the closed state. Preferably, the valve is activated, below a maximum level, and deactivated repeatedly for periods of time, alternately forcing fluid to escape and allowing the pressure differential across the fuse to fall. The time periods for activation/deactivation may be predetermined using a test system. The valve may be controlled using the output of a fuse internal component position sensor 200 and/or an accelerometer (fig.6,300).

Description

A METHOD OF REMOVING HYDRAULIC FLUID FROM AN AIRCRAFT HYDRAULIC SYSTEM, AN AIRCRAFT HYDRAULIC SYSTEM, AND AN

AIRCRAFT

FIELD OF THE INVENTION

[0001] The present invention concerns a method of removing hydraulic fluid from an aircraft hydraulic system, an aircraft hydraulic system, and an aircraft. More particularly, but not exclusively, the invention concerns removing hydraulic fluid from an aircraft hydraulic system which includes a hydraulic fuse to mitigate unwanted loss of hydraulic fluid. The invention is particularly, but not exclusively, applicable to hydraulic braking systems for aircraft landing gear.

BACKGROUND OF THE INVENTION

100021 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 braking system to remove air in the hydraulic fluid, for example. Conventionally this is done manually by a human operator. This is time-consuming, and can also lead to inadvertent system effects such as closure of hydraulic fuses (as discussed in detail below), requiring more complex maintenance and potentially removal and replacement of parts.

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

100041 The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide improved methods of -2 -removing hydraulic fluid from an aircraft hydraulic system, aircraft maintenance systems, and an aircraft.

SUMMARY OF THE INVENTION

[0005] In accordance with 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: an electrohydraulic servo valve connected to a fluid path of hydraulic fluid; a hydraulically actuated mechanism connected to the fluid path, arranged to be actuated when a force is applied to the hydraulic fluid by the electrohydraulic servo valve; a hydraulic fluid port through which, when opened, the hydraulic fluid can escape from the hydraulic system; a hydraulic fuse in the fluid path between the electrohydraulic servo valve and the hydraulic fluid port, arranged to move towards a closed state in which hydraulic fluid cannot pass when the fluid pressure differential across the hydraulic fuse is above a closing threshold, and to move towards an open state in 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: opening the hydraulic fluid port; controlling the activation of the electrohydraulic servo valve to force hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port, wherein the electrohydraulic servo valve is controlled so that the hydraulic fuse does not enter and remain in the closed state.

100061 While the electrohydraulic servo valve is activated and the hydraulic fluid port is open, this will cause hydraulic fluid to escape from the hydraulic fluid port. However, this will also create a fluid pressure differential across the hydraulic fuse, causing the hydraulic fuse to moved to its closed state to prevent any further hydraulic fluid escaping. By controlling the activation of the electrohydraulic servo valve in such a way that the hydraulic fuse does not enter and remain in the closed state, the process of removing hydraulic fluid from the hydraulic system can be automated, so does not suffer from the disadvantages manual removal by a user involves. This can advantageously be done using the existing components of the hydraulic system.

[0007] It will be appreciated that the opening threshold of the hydraulic fuse will be the same as or lower than its closing threshold.

[0008] Advantageously, the electrohydraulic servo valve has a maximum activation level, and in the activating step, the electrohydraulic servo valve is activated at a level below the maximum activation level. The maximum activation level may be 40mA and the activation level used 20mA, for example. While this will reduce the speed at which hydraulic fluid is removed from aircraft hydraulic system while the hydraulic fuse is in an open state, as the electrohydraulic servo valve will act as a restrictor on the rate at which the hydraulic fluid can flow and so escape, it will also reduce the speed at which the hydraulic fuse moves to the closed state. Further, the electrohydraulic servo valve may be activated at a level that prevents the hydraulic fuse from ever moving frilly to the closed state, for example a level that results in a pressure differential across the hydraulic fuse that is lower than its closing threshold. The activation level used may be a predetermined and/or fixed level or may be a level that varies over time.

[0009] Advantageously, the step of controlling the activation of the electrohydraulic servo valve comprises the substeps of: activating the electrohydraulic servo valve for a first time period to force hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port; deactivating the electrohydraulic servo valve for a second time period, to allow the fluid pressure differential across the hydraulic fuse to fall below the opening threshold; repeating the activating and deactivating steps a desired number of times. In this way, the hydraulic fuse is prevented from moving fully to the closed state, as while when the electrohydraulic servo valve is activated the hydraulic fuse will more towards the -4 -closed state, before it reaches the closed state the electrohydraulic servo valve is deactivated, allow the fluid pressure differential across the hydraulic fuse to fall below the opening threshold, so causing the hydraulic fuse to more back towards the open state. Alternatively, if the hydraulic fuse does fully enter the closed state, the second period during which the electrohydraulic servo valve is deactivated will allow it to move back towards the open state.

[0010] The desired number of times may be a predetermined number of times calculated and/or determined from a test system, or it may be determined from the properties of the system, for example using sensors, that the desired number of times has been reached. The desired number of times may be a sufficient number of times to have removed all or a desired proportion of hydraulic fluid from the hydraulic system, with a required likelihood where the number is predetermined.

[0011] Where the electrohydraulic servo valve is activated at a level below the maximum activation level as well as being alternately activated and deactivated, this can allow more hydraulic fluid to pass before the closed state occurs and reduce the number of times the electrohydraulic servo valve is deactivated to allow the hydraulic fuse to move from the closed state to the open state, and so can be optimally set to reduce the overall time require to remove the hydraulic fluid.

100121 The first time period and second time period may be predetermined time periods. The first time period and second time period may be determined using a test system. The first time period and second time period may be around 5 seconds, for example. The first time period may between 4.9 and 5.1 seconds, or between 4.5 and 5.5 seconds, or between 4 and 5 seconds, for example. The second time period may between 4.9 and 5.1 seconds, or between 4.5 and 5.5 seconds, or between 4 and 5 seconds, for example. 100131 The first time period may end when the hydraulic fuse is in the closed state. It may end as soon as the hydraulic fuse is in the closed state, or a period thereafter. Alternatively, the first time period ends before the hydraulic fuse is in the closed state. 100141 The second time period may end when the hydraulic fuse is in the open state. It may end as soon as the hydraulic fuse is in the open state, or a period thereafter. Alternatively, the second time period ends before the hydraulic fuse is in the open state It -5 -may end when, or before, the hydraulic fuse is in a fully open state, or a partially open state.

100151 The step of opening hydraulic fluid port may be performed by a human operator. 100161 Advantageously, the method further comprising the step, following the hydraulic fluid port being opened, of checking that the hydraulic fluid port is open. This may be done by means of a sensor on or near then hydraulic fluid port.

[0017] Advantageously, the hydraulic system further comprises a position sensor to determine the position of an internal component of the hydraulic fuse, and the electrohydraulic 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 activate/deactivate the electrohydraulic servo valve using that so that the hydraulic fuse does not enter and remain in the closed state. Where the electrohydraulic servo valve is activated at a level below the maximum activation level, the output of the position level can be used to determine the level at which to activate the electrohydraulic servo valve. (For example, if it is determined that the hydraulic valve is close to the closed state, the level could be reduced.) Where the electrohydraulic servo valve is activated and deactivated for first and second time periods, the first time period and second time period can be determined using the output of the position sensor, rather than using a predetermined period. The position sensor may be a linear variable differential transducer. The internal component may be a piston or poppet or the like within the hydraulic fuse.

[0018] Advantageously, the hydraulic system further comprises an accelerometer to measure the movement of the hydraulic fuse, and the electrohydraulic 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 accelerator as its vibrations will change as it changes mode. Further, the accelerometer can determine when no hydraulic fluid is passing though the hydraulic fuse even though it is in the open state, so enabling it to be determined when there is no more hydraulic fluid to remove from the hydraulic system. As with the position sensor, the output of the accelerometer can be used to determine the -6 -level of activation of the electrohydraulic servo valve and/or the time periods for which it is activated and deactivated.

100191 The first time period may end when the hydraulic fuse is in the closed state. It may end as soon as the hydraulic fuse is in the closed state, or a period thereafter. Alternatively, the first time period ends before the hydraulic fuse is in the closed state. [0020] The second time period may end when the hydraulic fuse is in the open state. It may end as soon as the hydraulic fuse is in the open state, or a period thereafter. Alternatively, the second time period ends before the hydraulic fuse is in the open state It may end when, or before, the hydraulic fuse is in a fully open state, or a partially open state.

[0021] Advantageously, the hydraulically actuated mechanism is a hydraulic brake. In this case, advantageously the hydraulic system is an aircraft landing gear brake system.

[0022] In accordance with a second aspect of the invention, there is provided an aircraft hydraulic system arranged to perform any of the methods described above.

100231 In accordance with a third aspect of the invention, there is provided an aircraft comprising an aircraft hydraulic system as described above.

100241 It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention.

DESCRIPTION OF THE DRAWINGS

[0025] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 is a schematic diagram of an aircraft landing gear system in accordance with an embodiment of the invention; -7 -Figure 2a is a cross-sectional view of the hydraulic fuse shown in Figure 1 in a fully open state; Figure 2b is a cross-sectional view of the hydraulic fuse shown in Figure I in a partially open state; Figure 2c is a cross-sectional view of the hydraulic fuse shown in Figure 1 in a closed state; Figure 3 is a flow chart showing the operation of the aircraft landing gear system of Figure 1; Figure 4 is a schematic diagram of an aircraft landing gear system in accordance with another embodiment of the invention; Figure 5 is a flow chart showing the operation of the aircraft landing gear system of Figure 4; Figure 6 is a schematic diagram of an aircraft landing gear system in accordance with another embodiment of the invention; Figure 7 is a flow chart showing the operation of the aircraft landing gear system of Figure 6; and Figure 8 is a perspective view of an aircraft comprising the aircraft landing gear system of an embodiment of the invention.

DETAILED DESCRIPTION

[0026] A schematic diagram of an aircraft landing gear system for an aircraft in accordance with an embodiment of the present invention is now described with reference to Figure I. The aircraft landing gear system I comprises a set of wheels 2a and 2b, which are mounted on a leg (not shown) which is in turn mounted on the underside of the aircraft. The wheels 2a and 2b have respective hydraulic brakes 3a and 3d, which are part of a hydraulic braking system 3.

100271 The hydraulic braking system 3 further comprises hydraulic fluid 4 within a hydraulic hose 5. An end of the hydraulic hose 4 is connected to both hydraulic brakes 3a -8 -and 3d, in a T-shaped branch. The other end of the hydraulic hose 5 is connected to an electrohydraulic servo valve 6. The electrohydraulic servo valve 6 can apply a variable pressure to the hydraulic fluid 4, depending on the input current supplied to the electrohydraulic servo valve 6. In the present embodiment, the electrohydraulic servo valve 6 provides a maximum pressure to the hydraulic fluid 4 when supplied with a maximum input current of 40mA, so when example be supplied with an input current 20mA will provide a lower pressure to the hydraulic fluid 4.

[0028] In the path of the hydraulic hose 5 next to the electrohydraulic servo valve 6 is a selector valve 7, which is a binary on-off valve that can either allow or prevent hydraulic fluid 4 flowing.

[0029] In the path of the hydraulic hose 5 next to the selector valve 7, so on the side of the selector valve 7 furthest from the electrohydraulic servo valve 6, is a hydraulic fuse 8. The hydraulic fuse 8 is described in detail below.

[0030] Finally, in the path of the hydraulic hose 5 next to the hydraulic fuse 8, so between the hydraulic fuse 8 and the hydraulic brakes 3a and 3d, is a hydraulic fluid port 9, through which the hydraulic fluid 4 can be removed from the hydraulic braking system 3 when the hydraulic fluid port 9 is opened.

100311 The aircraft landing gear system 1 comprises a Braking and Steering Control Unit (BSCU) 11, to which the electrohydraulic servo valve 6 and selector valve 7 are electrically connected so that they can be controlled by the BSCU 11. The BSCUll is interfaced with the Multi-Function Control and Display Unit (MCDU) 10 of the aircraft of which the aircraft landing gear system 1 is part, as described in more detail below. The MCDU 10 has a control panel in the cockpit of the aircraft, but is used for maintenance only, and is independent from the cockpit control systems used by the pilots to fly/taxi the aircraft etc. [0032] The hydraulic fuse 8 is now described with reference to Figures 2a to 2c, which are cross-sectional views of the hydraulic fuse 8 in different states. The hydraulic fuse 8 is an entirely mechanical device, which is used to mitigate unwanted loss of the hydraulic -9 -fluid 4 from the hydraulic braking system 3, for example due to a leak in the hydraulic hose 5 100331 The hydraulic fuse 8 comprises a body 50, with an inlet port 51, outlet port 52 and a cylindrical internal cavity 53. In the internal cavity 53 there is a movable piton 60 comprising a plug 54, flange 55 and ball 56. A compression spring 57 is in the cavity between the outlet port 52 and cylindrical flange 55, so that the plug 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 towards 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. [0034] Figure 2a shows the hydraulic fuse 8 in a fully open state, which is the state it will be in, or will return to, when there is no pressure differential on the hydraulic fluid 54 across the hydraulic fuse 8, so no pressure being exerted by the hydraulic fluid 54 from either the inlet port 51 or the outlet port 52. In this state, the spring 57 is fully extended, so that the piston 60 is at its closest position 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. So in fact in the fully open state the inlet port 51 is closed by the ball 56, which prevents hydraulic fluid 4 passing from the cavity 53 through the inlet port 51 (i.e. in the wrong direction).

100351 When the hydraulic fuse 8 is in the fully open state with the inlet port 51 closed by the ball 56, a 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, so opening the inlet port 51. This results in the hydraulic fuse being in the partially open state shown in Figure 2b. 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 thereof, towards the outlet port 52. This force counteracts the force exerted by the spring 57 on the piston 60 towards the inlet port 51. Thus, the piston 60 is moved away from the inlet port 51 towards the outlet port 52, and hydraulic fluid can pass through the hydraulic fuse 8, in particular by passing around the outside of the piston 60 within the cavity 53.

100361 However, when too great a force is exerted by the hydraulic fluid 4 over too great a period of time, the piston 60 will be moved sufficiently far towards the outlet port that -10 -the plug 54 of the piston 60 enters the outlet port 52. This results in the hydraulic fuse being in the closed state shown in Figure 2c. In this state, hydraulic fluid 54 is no longer able to pass through the hydraulic fuse 8, as the outlet port 52 is blocked by the plug 54. 100371 In this way, the hydraulic fuse 8 is able to mitigate unwanted loss of the hydraulic fluid 4 from the hydraulic braking 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 be created across the hydraulic fuse 8, due to the lower pressure caused by the leak. While the hydraulic fluid 4 will initially be able to pass through the hydraulic fuse 8, allowing some to be lost through the leak, the hydraulic fuse 8 will quickly enter the closed state, i.e. with the piston 60 will be moved fully towards the outlet port 52, so that the plug 54 of the piston 60 prevents any more hydraulic fluid 4 being able to pass through the outlet port 52.

[0038] As noted 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, so automatically returning the hydraulic fuse 8 back to the fully open piston.

100391 The operation of the hydraulic braking system 3 is now described with reference to the flow chart of Figure 3.

100401 In a first step, a maintenance operator selects on the MCDU 10 that they want the hydraulic fluid 4 to be flushed from the hydraulic braking system 3 (step 101). The MCDU 20 then instructs the operator to put the aircraft in required configuration for this to occur (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 required configuration instructed by the MCDU 10 in the preceding step 102.

[0041] The MCDU 10 then instructs the BCDU I I to open the selector valve 7 (step 104), and to activate the electrohydraulic servo valve 6 (step 105). The electrohydraul c servo valve 6 is activated by supplying it with an input current of 20mA, so that it provides a pressure lower than its maximum pressure to the hydraulic fluid 4. This forces hydraulic fluid 2 to escape from the hydraulic braking system 3 via the hydraulic fluid port 9. The hydraulic braking system 3 then waits 5 seconds (step 106).

100421 During the 5-second waiting period, while hydraulic fluid 2 is escaping 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 strength of the conditions in the hydraulic braking 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.

[0043] When the 5-second waiting period has ended, the MCDU 10 instructs the BCDU 11 to deactivate the electrohydraulic servo valve 6 (step 107), and to close the selector valve 7 (step 108). The hydraulic braking system 3 then again waits 5 seconds (step 109). During this time, as there is no longer a pressure differential across the hydraulic fuse 8, it moves back to the fully open state.

100441 The hydraulic braking system 3 then determines if the number of times the electrohydraulic servo valve 6 has been activated has met a threshold (step 110). If the threshold has been met, the process of flushing hydraulic fluid 4 from the hydraulic braking system 3 is complete. If not, step 104 is returned to, and the two 5-second waiting periods are repeated.

100451 In this way, the process of removing the hydraulic fluid 4 from the hydraulic braking system 3 can be automatically performed, by "pulsing" the electrohydraulic servo valve 6 at 5-second intervals, to first force hydraulic fluid 4 from the hydraulic braking system 3 and then to allow the hydraulic fuse 8 to reset.

[0046] It will be appreciated that number of times to repeat the two 5-second waiting periods can be calculated and/or predetermined using a test system, so that they are repeated sufficient times for all of the hydraulic fluid 4 to be removed. Further, it will be appreciated that other aspects of operation can be calculated and/or predetermined using a test system to try to give optimal removal of hydraulic fluid 4. For example, it may be determined that a current other than 20mA is desirable and/or that one or both of the waiting periods is for a period other than 5 seconds.

100471 A schematic diagram of an aircraft landing gear system for an aircraft in accordance with another embodiment of the present invention is applicable is shown in Figure 4. The aircraft landing gear system 199 is very similar to the aircraft landing gear system 1 of the previous embodiment, except that the hydraulic braking system 198 of the present embodiment comprises a position sensor 200 connected to the BSCU 11, 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 appreciated that in other embodiments of the invention, other types of position sensor could be used. [0048] The operation of the hydraulic braking system 198 is now described with reference to the flow chart of Figure 5.

[0049] Similarly to the previous embodiment, in a first step a maintenance operator selects to flush the hydraulic fluid 4 to be flushed from the hydraulic braking system 3 (step 201), the MCDU 20 instructs the operator to put the aircraft in required configuration (step 202), and the hydraulic fluid port 9 is opened (step 203). The selector valve 7 is then opened by the BSCU 11 (step 204) and the electrohydraulic servo valve 6 is activated (step 205), again forcing hydraulic fluid 2 to escape from the hydraulic braking system 3 via the hydraulic fluid port 9.

100501 However, in contrast to the previous embodiment, in the present embodiment instead of the electrohydraulic servo valve 6 simply being activated for a predetermined period of time such as 5 seconds, instead the position sensor 200 is 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, again as in the previous embodiment the electrohydraulic servo valve 6 is deactivated (step 207) and the selector valve 7 is closed (step 208). Again in contrast to the previous embodiment, the hydraulic braking system 198 then again use the position sensor 200 to determine when the hydraulic fuse 8 has moved back to the fully open state (step 209).

100511 Finally, the braking system 198 then determines if the number of times the electrohydraulic servo valve 6 has been activated has met a threshold (step 210), if so the -13 -process of flushing hydraulic fluid 4 from the hydraulic braking system 198 is complete, otherwise the step from step 204 onwards are repeated.

100521 Thus, again, the process of removing the hydraulic fluid 4 from the hydraulic braking system 198 can be automatically performed by "pulsing" the electrohydraulic servo valve 6. However, as the state of the hydraulic fuse 8 is being directly sensed by the positon sensor 200, the flushing can be made more efficient, as the activation and/or deactivation of the electrohydraulic servo valve 6 can be initiated when it is measured that the electrohydraulic servo valve 6 is in the desired state, rather than only when it is predicted that it should be based on calculation and/or determination using a test system. [0053] It will again be appreciated that various other aspects of operation can be calculated and/or predetermined using a test system, or in other ways, to try to give optimal removal of hydraulic fluid 4. For example, in the present embodiment, the activation and/or deactivation of the electrohydraulic servo valve 6 could be initiated just before electrohydraulic servo valve 6 moves to the fully open or closed states respectively, rather than when/after those states have actually been reached.

100541 A schematic diagram of an aircraft landing gear system for an aircraft in accordance with another embodiment of the present invention is applicable is shown in Figure 6. The aircraft landing gear system 299 is very similar to the aircraft landing gear systems of the previous embodiments, except that the hydraulic braking system 298 of the present embodiment comprises an accelerometer 300 connected to the BSCU 11, arranged to measure the movement, particularly vibration, of the hydraulic fuse 8 [0055] The operation of the hydraulic braking system 298 is now described with reference to the flow chart of Figure 7. Similarly to the previous embodiments, in a first step a maintenance operator selects to flush the hydraulic fluid 4 to be flushed from the hydraulic braking system 3 (step 301), the MCDU 20 instructs the operator to put the aircraft in required configuration (step 302), and the hydraulic fluid port 9 is opened (step 303). The selector valve 7 is then opened (step 304) and the electrohydraulic servo valve 6 is activated (step 305), again forcing hydraulic fluid 2 to escape from the hydraulic braking system 3 via the hydraulic fluid port 9.

100561 However, in contrast to the previous embodiments, in the present embodiment the accelerometer 300 is then used to determine if any hydraulic fluid 4 is passing through the hydraulic fuse 4 (step 306). If no hydraulic fluid 4 is passing then there must be none left in the hydraulic braking system 298 (or at least, none that can be removed by activating the electrohydraulic servo valve 6), and so the process is finished.

[0057] If hydraulic fluid 4 is initially passing thought the hydraulic fuse 4, the accelerometer 300 is used to determine when the hydraulic fuse 8 has moved to the closed state (step 307). This will be determined from the changes in the vibration of the hydraulic fuse 8 due to the movement of the hydraulic fluid 4 within it, as the piston 60 moves within the cavity 53 of the hydraulic fuse 8 to eventually block the outlet valve 52. [0058] Once the hydraulic fuse 8 has moved to the closed state, again as in the previous embodiments 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 that occurs the steps from step 304 onwards are repeated.

100591 Thus, again, the process of removing the hydraulic fluid 4 from the hydraulic braking system 298 can be automatically performed by "pulsing" the electrohydraulic servo valve 6. However, in this case the accelerometer 300 can be used to directly sense the state of the hydraulic fuse 8, and also when all of the hydraulic fluid 4 has been removed.

[0060] It will again be appreciated that various other aspects of operation can be calculated and/or predetermined using a test system, or in other ways, to try to give optimal removal of hydraulic fluid 4, for example again the activation and/or deactivation of the electrohydraulic servo valve 6 could be initiated just before electrohydraulic servo valve 6 moves to the fully open or closed states respectively, rather than when/after those states have actually been reached.

100611 Figure 8 is a perspective view of an aircraft in accordance with an embodiment of the invention. The aircraft 500 comprises a aircraft landing gear system an embodiment described above.

-15 - 100621 While the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. [0063] For example, in other embodiments of the invention the electrohydraulic servo valve may not be deactivated during the removal of hydraulic fluid, but instead continuously activated but at a level low enough that the hydraulic fuse does not enter the closed state (or does not move towards the closed state at all), or the electrohydraulic servo valve may be continuously activated but at a level that is varied so that the hydraulic fuse is nevertheless for periods of time able to move towards the fully open state, either to allow it to move from being in the closed state, or to prevent it from entering the closed state at all.

[0064] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

-16 -CLAIMS1. A method of removing hydraulic fluid from an aircraft hydraulic system, wherein the hydraulic system comprises: an electrohydraulic servo valve connected to a fluid path of hydraulic fluid; a hydraulically actuated mechanism connected to the fluid path, arranged to be actuated when a force is applied to the hydraulic fluid by the electrohydraulic servo valve; a hydraulic fluid port through which, when opened, the hydraulic fluid can escape from the hydraulic system; a hydraulic fuse in the fluid path between the electrohydraulic servo valve and the hydraulic fluid port, arranged to move towards a closed state in which hydraulic fluid cannot pass when the fluid pressure differential across the hydraulic fuse is above a closing threshold, and to move towards an open state in 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: opening the hydraulic fluid port; controlling the activation of the electrohydraulic servo valve to force hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port, wherein the electrohydraulic servo valve is controlled so that the hydraulic fuse does not enter and remain in the closed state.
2. A method as claimed in claim 1, wherein the electrohydraulic servo valve has a maximum activation level, and in the activating step, the electrohydraulic servo valve is activated at a level below the maximum activation level.
3. A method as claimed in claim 1 or 2, wherein the step of controlling the activation of the electrohydraulic servo valve comprises the substeps of: -17 -activating the electrohydraulic servo valve for a first time period to force hydraulic fluid to escape from the hydraulic system via the hydraulic fluid port; deactivating the electrohydraulic servo valve for a second time period, to allow the fluid pressure differential across the hydraulic fuse to fall below the opening threshold; repeating the activating and deactivating steps a desired number of times.
4. A method as claimed in claim 3, wherein the first time period and second time period are predetermined time periods.
5. A method as claimed in claim 4, wherein the first time period and second time period are determined using a test system.
6. A method as claimed in any of claims 3 to 5, wherein the first time period ends when the hydraulic fuse is in the closed state.
7. A method as claimed in any of claims 3 to 5, wherein the first time period ends before the hydraulic fuse is in the closed state.
8. A method as claimed in any of claims 3 to 7, wherein the second time period ends when the hydraulic fuse is in the opened state.
9. A method as claimed in any preceding claim, further comprising the step, following the hydraulic fluid port being opened, of checking that the hydraulic fluid port is open.
I 0. A method as claimed in any preceding claim, wherein the hydraulic system further comprises a position sensor to determine the position of an internal component of the hydraulic fuse, and the electrohydraulic servo valve is controlled using the output of the position sensor.
11. A method as claimed in any preceding claim, wherein the hydraulic system further comprises an accelerometer to measure the movement of the hydraulic fuse, and the electrohydraulic servo valve is controlled using the output of the position sensor.
12. A method as claimed in any preceding claim, wherein the hydraulically actuated mechanism is a hydraulic brake.
13. A method as claimed in claim 12, wherein the hydraulic system is an aircraft landing gear brake system.
14. An aircraft maintenance system arranged to perform the method of any of claims 1 to 13.
15. An aircraft comprising an aircraft maintenance system as claimed in claim 14.