GB2586790A

GB2586790A - Aircraft hydraulic actuation system

Info

Publication number: GB2586790A
Application number: GB1912510.3A
Authority: GB
Inventors: Kerr Sean; Marles David; Gordon L'allier Paul
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2019-08-30
Filing date: 2019-08-30
Publication date: 2021-03-10
Also published as: GB201912510D0

Abstract

An aircraft hydraulic actuation system comprises a supply line carrying pressurised hydraulic fluid, a line returning hydraulic fluid to a reservoir, and a hydraulic actuator moving a load (e.g. door, landing gear, wheel brake). A first hydraulic line 126 supplies a first actuator chamber 118. A second hydraulic line 128 supplies a second actuator chamber 120. A hydraulic fluid flow path 132 connects the first and second lines. A circulation valve 134 (234, figure 8) selectively opens the flow path allowing fluid circulation from the supply to the return lines through the first and second lines (e.g. passing hydraulic fluid through a filter for cleaning). The circulation valve may be a check valve having a cracking pressure or threshold pressure that fluid in first hydraulic line must reach before the valve opens. Alternatively, a control unit (246) may determine actuator reduced performance based on operation speed or a pressure drop and open a command circulation valve (234).

Description

AIRCRAFT HYDRAULIC ACTUATION SYSTEM

IliCHNICAL FIELD

100011 The present invention concerns an aircraft hydraulic actuation system. More particularly, but not exclusively, this invention concerns an aircraft hydraulic actuation system that provides for the recirculation of hydraulic fluid for cleaning. The invention also concerns a method of cleaning hydraulic fluid in an aircraft hydraulic actuation system.

BACKGROUND OF THE INVENTION

[0002] Aircraft hydraulic systems may utilise hydraulic components, such as actuators, valves, lines and the like, that have relatively small orifices through which the hydraulic fluid passes. For example, hydraulic lines can be around 5mm in diameter, and the input/output ports on hydraulic components can be as small as lmm in diameter. Such elements of the hydraulic system may clog if the hydraulic fluid becomes contaminated, for example due to the accumulation of debris.

100031 Contaminants can enter the hydraulic system from various sources. For example, various parts of the hydraulic system may degrade over time, such as seals and moving parts, which may release small amounts of debris into the hydraulic fluid. Contaminants can also enter the system when the hydraulic system is connected to an external supply of pressurised hydraulic fluid. An example of such an external supply is a ground cart, which typically uses a diesel generator to produce power. Such a connection may be made to power and test the hydraulic system of the aircraft when the aircraft is on the ground and the engines cannot be run, for example during maintenance. Such external supplies may not be subject to the high maintenance and control standards of the aircraft itself, and may therefore be more likely to contain contaminants and introduce those contaminants into the aircraft hydraulic system when connected thereto.

[0004] Typically, the aircraft hydraulic system contains filters, such as screens, to capture contaminants and prevent them from circulating within the hydraulic system. However, the filters can themselves have a performance impact on the hydraulic system; -2 -for example, they may slow the flow of hydraulic fluid and/or create a pressure drop and therefore a reduction in power. In some cases the filters may themselves become clogged.

[0005] It may be particularly difficult to effectively filter hydraulic fluid which is present in parts of the hydraulic system where the hydraulic fluid does not circulate from supply to return during normal use. This may be the case in long pipe runs that end in a dead end, such as at an actuator, and particularly where the volume of hydraulic fluid in the hydraulic line is greater than the maximum volume of the corresponding chamber of the actuator.

[0006] Figure I shows an example of an arrangement in which a double acting hydraulic actuator I has chambers which are each connected to a selector valve 2 via two long stretches of hydraulic line 3. Due to the length of the hydraulic lines 3, each hydraulic line contains a volume of hydraulic fluid (the volume closest to the hydraulic actuator 1) which may not move through the selector valve 3 as the hydraulic actuator I extends and retracts. Said volumes of hydraulic fluid may essentially be stuck moving backward and forward in the hydraulic lines 3, and may therefore not get circulated around the hydraulic system where they can be filtered by the aircraft's central filter system. It may therefore be difficult to remove contaminates from these volumes of hydraulic fluid and therefore the risk of clogging is increased.

[0007] Furthermore, in parts of the aircraft system with little or no circulation of hydraulic fluid, the risk of air collecting is increased, a build-up of air in the hydraulic system may reduce the efficiency of the hydraulic system and/or prevent it from working correctly.

[0008] The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved hydraulic actuation system.

SUMMARY OF THE INVENTION

[0009] The present invention provides, according to a first aspect, an aircraft hydraulic actuation system comprising: a supply line arranged to carry hydraulic fluid pressurised to -3 -a supply pressure by a pump, a return line arranged to return hydraulic fluid to a reservoir, a hydraulic actuator for moving a load, the actuator comprising a first chamber and a second chamber separated by a piston, a first hydraulic line arranged to supply hydraulic fluid into the first chamber of the actuator and a second hydraulic line arranged to supply hydraulic fluid into a second chamber of the actuator, a valve arrangement arranged to control extension and retraction of the hydraulic actuator, the valve arrangement having a first mode in which the first hydraulic line is fluidly connected to the supply line and the second hydraulic line is fluidly connected to the return line, a hydraulic fluid flow path connecting the first hydraulic line to the second hydraulic line, wherein a circulation valve is arranged to selectively open the fluid flow path so as to allow hydraulic fluid to circulate from the supply line to the return line through at least part of the first hydraulic line and at least part of the second hydraulic line.

[0010] The hydraulic fluid flow path connecting the first hydraulic line to the second hydraulic line provides a route for the hydraulic fluid to circulate within the hydraulic actuation system. Particularly for hydraulic actuation systems that include long pipe runs, the present invention may reduce the chance that a volume of hydraulic fluid remains within one part of the hydraulic system and is not sent through a filter. The present invention may be particularly advantageous when the volume of fluid containable within the (first and/or second) hydraulic line between the valve arrangement and the hydraulic actuator is greater than the maximum volume of the respective (first or second) chamber of the hydraulic actuator.

[0011] The circulation valve may be arranged to open when the hydraulic fluid at one side of the circulation valve is above a threshold pressure, optionally relative to the pressure on the other side of the circulation valve. The circulation valve may be a mechanically operated valve. The circulation valve may be arranged to open under the influence of hydraulic fluid pressure. The threshold pressure may be the cracking pressure of the circulation valve. The circulation valve may be arranged to open when the hydraulic fluid in the first hydraulic line, the second hydraulic line, the first chamber and/or the second chamber is above a threshold pressure. -4 -

[0012] The actuator may be arranged to move the load in a first direction when the valve arrangement is configured in the first mode. When the actuator is used to move the load in the first direction, the pressure of the hydraulic fluid in the first chamber may reach a working pressure during movement of the load. The working pressure may depend on the force required to be output by the actuator to move the load. The working pressure is preferably below the supply pressure. It may be preferable for the working pressure to be below the supply pressure even in extreme operating conditions which may make it more difficult to move the load.

[0013] When the actuator has reached the end of its range of motion, the pressure in the first chamber may begin to increase from the working pressure towards the supply pressure. Preferably, the threshold pressure is above the working pressure so that the actuator can compete its motion before the circulation valve opens.

[0014] In practice, the working pressure may fluctuate during movement of the load.

The working pressure may depend on external conditions, for example temperature and/or aircraft speed. Additionally, the working pressure may not be constant during movement of the load, for example the working pressure may increase or decrease. The working pressure may fall within a working pressure range. The working pressure range may be defined by the maximum and minimum working pressures that may be required to move the load, preferably under all anticipated operating conditions. The threshold pressure preferably exceeds the maximum pressure of the working pressure range; the actuator preferably does so under all conditions the actuator may experience in use. This may help ensure no hydraulic fluid is diverted to the return line when it is required for operation of the actuator.

[0015] The threshold pressure may be at or above 80% of the supply pressure. The threshold pressure may be at or above 90% of the supply pressure.

100161 The circulation valve may be operated by a control unit. The opening and closing of the circulation valve may be electrically and/or electronically controlled. There may be one or more sensors in communication with the control unit. The control unit may command the circulation valve to open or close on the basis of the output of the sensors. -5 -

[0017] The sensors may determine the position of the load. The sensors may determine the position of the load on the basis of the extent to which the actuator is extended or retracted, for example the actuator may comprise a piston rod and the sensors may measure the position of the piston rod. The control unit may instruct the circulation valve to open on the basis of the load being determined to have attained a predetermined position. The predetermined position may be an end position, i.e. a position at which the load can travel no further in that direction. The control unit may also command the circulation valve to close when the load is moved away from the predetermined position.

[0018] It may be that the control unit is configured to determine that the actuator has reached the predetermined position by monitoring outputs from sensors which indicate the movement and/or position of other, related, components of the aircraft. Preferably, such indications are those which occur when force from the actuator is no longer needed and so the flow of hydraulic fluid to the working chamber can be safely bypassed. For example, in embodiments where the hydraulic actuator is a landing gear retraction actuator. The control unit may command the circulation valve to open or shut in dependence on signals from sensors indicating the engagement/disengagement of a landing gear up-lock and/or down-lock. For example, the control unit may be configured to command the circulation valve to open when it is sensed that the down-lock has engaged.

[0019] The sensors may be pressure sensors. The control unit may command the circulation valve to open when the pressure in the first line, the second line, the first chamber, and/or the second chamber exceeds the threshold pressure. The control unit may also command the circulation valve to close when the pressure in the respective line and/or chamber falls below the threshold pressure.

[0020] The control unit may be configured to monitor the performance of the actuator.

The control unit may be configured to command the circulation valve to open in response to a reduced performance of the actuator being detected.

100211 The performance of the actuator may be related to the speed at which the actuator operates. A reduced speed may be indicative of reduced performance. A reduction in the speed of the actuator may be due to a reduction in the flow rate of hydraulic fluid in the hydraulic actuation system. A reduction in the flow rate may be associated with a build- -6 -up of contaminants in the hydraulic actuation system. Thus, the control unit may be configured to monitor the speed of the actuator (e.g. the speed at which the actuator extends and/or contracts). The control unit may be configured to command the circulation valve to open if the speed of the actuator drops below a threshold value.

[0022] The performance of the actuator may be related to the pressure of the hydraulic fluid at certain points in the hydraulic system. In particular, the performance of the actuator may be related to a pressure drop (pressure difference) between two points in the hydraulic system. An increased pressure drop may be indicative of reduced performance. An increased pressure drop may be due to a build-up of contaminants between those points of the hydraulic actuation system, which may cause a reduction in the flow rate of hydraulic fluid between those points. Thus, the control unit may be configured to monitor the pressure difference between two points in the hydraulic actuation system. For example, the pressure different may be measured between two points on the first or second hydraulic line feeding the actuator, the pressure difference may be measured between the supply line and an inlet/outlet port of the actuator, and/or the pressure difference may be measured between the valve arrangement and an inlet/outlet port of the actuator. The pressure difference may be measured during extension and/or retraction of the actuator (i.e. when hydraulic fluid is flowing through that part of the system). The control unit may be configured to command the circulation valve to open if the pressure difference exceeds a threshold value.

[0023] The control unit may be configured to command the circulation valve to open after a predetermined number of extension and retraction cycles of the actuator. The control unit may command the circulation valve to open every two or more (e.g. every ten) cycles.

[0024] The control unit may be configured to command the circulation valve to open only when the aircraft is in a certain flight phase, for example when the aircraft is on the ground, or when the aircraft is in cruise.

100251 In such cases, the control unit may only command the circulation valve to open when other parameters are also met, for example the actuator has reached the end of its range of movement and/or the pressure has reached the threshold pressure.

100261 The valve arrangement may have a second mode in which the valve arrangement fluidly connects the second hydraulic line to the supply line and the first hydraulic line to the return line. The actuator may be arranged to move the load in a second direction when the valve arrangement is configured in the second mode. The second direction may be opposite to the first direction. When the actuator is used to move the load in the second direction, the pressure of the hydraulic fluid in the second chamber may reach a second working pressure during movement of th e load. The second working pressure may be within a second working pressure range.

[0027] The hydraulic actuation system may be arranged so as to permit only one way fluid flow through the hydraulic fluid flow path connecting the first hydraulic line to the second hydraulic line. Hydraulic fluid may only be allowed to flow from the first hydraulic line to the second hydraulic line, or vice versa. The circulation valve may be configured to only allow one way fluid flow; this may be due to the mechanical arrangement of the valve, for example the circulation valve be a check valve with a cracking pressure at the threshold pressure, and/or due to the configuration of the control unit commanding the valve. The hydraulic fluid flow path may comprise an additional check valve in addition to, for example, a control unit operated circulation valve that permits fluid flow in either direction when opened.

100281 It may be that movement of the load in the second direction requires the actuator to exert more force than during movement of the load in the first direction. This may be the case where the load is moved against an external force (e.g. aerodynamic forces, gravity, a resilient bias, etc.) in the second direction, and moved in the same direction as that force in the first direction. For example, the load may be a component such as a landing gear. When the landing gear retracts, the actuator may be required to lift the weight of the landing gear, and possibly also move the landing gear against aerodynamic forces acting on the landing gear during flight, whereas when the landing gear is extended, the gravity and/or the aerodynamic forces may act with the actuator as the landing gear extends. Hence, the actuator may be required to exert significantly more force in the second direction, which retracts the landing gear, than in the first direction, which extends the landing gear. Therefore the working pressure of the hydraulic fluid in the second chamber may need to be higher in order to move the load in the second direction, than the working pressure of the hydraulic fluid in the first chamber to move the load in the first direction. -8 -

[0029] It may be advantageous for the hydraulic fluid flow path connecting the first hydraulic line to the second hydraulic line to be openable when the actuator moves the load in the lower power demand (lower force) direction of operation, rather than in the higher power demand (higher force) direction of operation. Doing so may make it possible to provide for a greater difference between the threshold pressure and the working pressure. Hence, there may be a lower chance that the pressure in the actuator may reach the threshold pressure, and cause the valve to open, whilst the actuator is attempting to move the load.

[0030] The valve arrangement may comprise one or more valves, for example selector valves. The valve arrangement may be a selector valve arrangement. The first hydraulic line and/or the second hydraulic line may each directly connect the valve arrangement to the actuator. There may be no other valves within the fluid flow path between the valve arrangement and the actuator. The valve arrangement may be controlled (i.e. its operating mode may be selected) by a flight control system and/or by an operator of the aircraft (e.g. a pilot).

[0031] The fluid flow path may connect to the first hydraulic line at a distance from the actuator in which the volume of fluid in the first hydraulic line in said distance is less than the maximum volume of the first chamber. The fluid flow path may connect to the second hydraulic line at a distance from the actuator in which the volume of fluid in the second hydraulic line in said distance is less than the maximum volume of the second chamber. This may improve the chance that the hydraulic fluid that enters and exits the actuator will circulate to the return line.

[0032] The fluid flow path connecting the first hydraulic line to the second hydraulic line may be provided by a third hydraulic line that directly connects the first hydraulic line to the second hydraulic line. The fluid flow path connecting the first hydraulic line to the second hydraulic line may be provided within the piston of the actuator, for example by a through hole in the piston.

[0033] The supply pressure may be sufficient for the operation of hydraulically actuated components such as flight control surfaces (e.g. slats, flaps, stabilisers, elevators, etc.), brakes, landing gear retraction actuators and the like, which may also use the same -9 -hydraulic system. The supply pressure may be 100 Bar or greater, 150 Bar or greater, or 200 Bar or greater. The supply pressure may be greater than the pressure in the return line, for example five time greater or ten times greater than the pressure in the return line [0034] The supply line may be arranged to carry the pressurised hydraulic fluid directly from the pump, or indirectly from the pump via one or more components, lines, valves, accumulators, filters and/or the like. The pump may be an engine driven pump. The pump may be an electric pump. A plurality of pumps may be provided, at least one of the pumps may be provided as a backup in case of failure of a primary pump.

[0035] The return line may be arranged to return hydraulic fluid directly to the reservoir, or indirectly to the reservoir via one or more components, lines, valves, accumulators, filters and/or the like. The hydraulic actuation system may be arranged such that the pressure of hydraulic fluid in the return line is 20 Bar or less, 10 Bar or less, or 5 Bar or less. The reservoir may be arranged to supply hydraulic fluid to the pump.

[0036] The hydraulic actuation system preferably includes a filter arranged to remove contaminants from the hydraulic fluid.

[0037] The hydraulic actuation system may have a first mode of operation in which the valve arrangement is in the first mode. The hydraulic actuation system may have a second mode of operation in which the valve arrangement is in the second mode. The hydraulic actuation system may be arranged such that in the first mode of operation the piston is urged in a direction so as to extend the overall length of the actuator. In the second mode of operation the piston may be urged in a direction so as to reduce the overall length of the actuator. Alternatively, the hydraulic actuation system may be arranged such that in the first mode of operation the piston is urged in a direction so as to reduce the overall length of the actuator. In the second mode of operation the piston may be urged in a direction so as to extend the overall length of the actuator.

100381 The valve arrangement may have a third mode in which the first hydraulic line and the second hydraulic line are both isolated from the supply line and the return line, and may also be isolated from each other. The hydraulic actuation system may have a third mode of operation in which the valve arrangement is in the third mode.

-10 - [0039] The hydraulic actuator may comprise two opposing ends. The hydraulic actuator may be pivotally mountable at either or both of its ends. The piston may be connected to a piston rod. The piston rod may be pivotally mountable at its free end. The hydraulic actuator may be mounted, for example pivotally mounted, at a first end to a fixed aircraft structure (e.g. to a fixed component within the fuselage). The hydraulic actuator may be mounted, for example pivotally mounted, at a second end to a movable aircraft component (e.g. a landing gear, a brake, a door (e.g. a cargo door or a landing gear bay door), etc.) [0040] The present invention may have particular application in situations where the actuator is required to move a load between a first predetermined position and a second predetermined position, preferably without requiring the load to stop in an intermediate position. In other words, the hydraulic actuator may be arranged to only move a load between two discrete positions. It may be that the hydraulic actuation system does not comprise a servo valve or proportional control valve associated with the hydraulic actuator. The hydraulic actuator may be a "bang-bang" actuator. It may be that the hydraulic actuation system does not provide positional control of the load between the end positions of the range of movement. However, in alternative arrangements, positional control may be provided for, for example the valve arrangement may comprise a servo valve or a proportional control valve.

[0041] The present invention may provide, according to a second aspect, an aircraft comprising a hydraulic actuation system according to the first aspect of the invention. The aircraft may further comprise the load movable by the hydraulic actuator. The load movable by the actuator may be an aircraft component. The component may be arranged such that the actuator exerts more force to move the component from a first position to a second position, than from the second position to the first position. The component may be an aircraft landing gear, a door (e.g. a cargo door or landing gear bay door) or a brake.

100421 The landing gear may be arranged such that it may extend under gravity and/or under the action of aerodynamic forces. The landing gear may be a nose landing gear. The landing gear may be a main landing gear. The landing gear may comprise a shock absorber strut (i.e. an oleo strut). The landing gear may comprise a wheel mounted to the shock absorber strut. The landing gear may comprise a bogie attached to the shock absorber strut. The landing gear may comprise a plurality of wheels mounted to the bogie.

[0043] The hydraulic actuator may form part of a landing gear extension and retraction system. The landing gear extension and retraction system may further comprise a landing gear up-lock mechanism and/or a landing gear down-lock mechanism.

[0044] The landing gear and hydraulic actuator may be arranged such that retraction of the hydraulic actuator causes retraction of the landing gear, and extension of the hydraulic actuator causes extension of the landing gear. Alternatively, the landing gear and hydraulic actuator may be arranged such that extension of the actuator causes retraction of the landing gear, and retraction of the hydraulic actuator causes extension of the landing gear. There may be a mechanical linkage to transfer movement between the hydraulic actuator and the landing gear.

[0045] The aircraft is preferably a passenger aircraft. The passenger aircraft preferably comprises a passenger cabin comprising a plurality of rows and columns of seat units for accommodating a multiplicity of passengers. 'The aircraft may have a capacity of at least 20, more preferably at least 50 passengers, and more preferably more than 50 passengers. The aircraft is preferably a powered aircraft. The aircraft preferably comprises an engine for propelling the aircraft. The aircraft may comprise wing-mounted, and preferably underwing, engines.

[0046] The present invention may provide, according to a third aspect, a method of cleaning hydraulic fluid in an aircraft hydraulic actuation system according to any preceding aspect of the invention, the method comprising the steps of: configuring the valve arrangement in the first mode, supplying pressurised hydraulic fluid to the actuator from the supply line such that the actuator moves a load to a predetermined position, when the load has reached the predetermined position, opening the circulation valve so as to create a fluid flow path from the supply line to the return line through at least part of the first hydraulic line and at least part of the second hydraulic line, filtering the hydraulic fluid sent to the return line through at least part of the first hydraulic line and at least part of the second hydraulic line.

-12 - [0047] 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. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

[0048] The term 'or' shall be interpreted as 'and/or' unless the context requires otherwise.

DESCRIPTION OF THE DRAWINGS

[0049] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: [0050] Figure 1 shows a prior art hydraulic actuation system; [0051] Figure 2 shows a plan view of an aircraft according to a first embodiment of the invention; [0052] Figure 3 shows hydraulic actuation system according to a first embodiment of the invention; 100531 Figure 4 shows a hydraulic actuator and connections thereto according to a first embodiment of the invention; 100541 Figure 5 shows a plot of hydraulic pressure in a chamber of the hydraulic actuator vs time; 100551 Figure 6 shows a landing gear and landing gear extension and retraction system comprising a hydraulic actuator according to the first embodiment; [0056] Figure 7 shows a plot of hydraulic pressure in chambers of the hydraulic actuator as the landing gear is extended and retracted; [0057] Figure 8 shows a hydraulic actuator and connections thereto according to a second embodiment of the invention.

DETAILED DESCRIPTION

[0058] Figure 2 shows an aircraft I 02 according to a first embodiment of the invention.

The aircraft 102 comprises a hydraulic actuation system comprising an engine driven pump -13 - 104 arranged to pressurise hydraulic fluid to a supply pressure of 200 Bar. The engine driven pump 104 is supplied with hydraulic fluid from a reservoir 106. A back-up electric pump 108 is provided in parallel to the engine driven pump 104 to pressurise the hydraulic fluid in the hydraulic system in the event that the engine driven pump 104 cannot operate. In alternative embodiments, only an electric pump or pumps are provided.

[0059] A hydraulic fluid supply line 110 extends from the pumps 104, IOS and carries the pressurised hydraulic fluid to various actuators in the aircraft 102, for example, actuators associated with flight control surfaces, brakes and landing gear retraction. The hydraulic actuation system further comprises an accumulator 11 I that feeds into the supply line 110 downstream of the hydraulic pumps 104, 108. A return line 112 returns hydraulic fluid to the reservoir 106 via a filter system 113. The pressure of hydraulic fluid in the return line 112 is approximately 5 to 10 Bar.

[0060] Figures 3 and 4 show a hydraulic actuator 114 that forms part of the hydraulic actuation system. The hydraulic actuator 114 comprises a housing 116 containing a first chamber 118 and a second chamber 120 separated by a piston 122. 'The piston 122 is attached to a piston rod 124, the free end of which is pivotally mounted to a movable aircraft component (not shown). The end of the housing 116 that is distal from the free end of the piston rod 124 is pivotally mounted to a fixed structural part of the aircraft 102.

[0061] In an embodiment, the aircraft component is a landing gear and the hydraulic actuator 114 is a landing gear retraction actuator arranged to extend and retract the landing gear. In an alternative embodiment, the aircraft component is a cargo door. In an alternative embodiment, the aircraft component is a component of the aircraft brake system, the actuator being coupled to a brake pad.

[0062] A first hydraulic line 126 feeds into the first chamber 118 and a second hydraulic line 128 feeds into the second chamber 120, The first hydraulic line 126 and the second hydraulic line 128 each directly connect their respective chamber of the actuator 114 to a respective port of a selector valve 130.

[0063] The actuator 114 and the selector valve 130 are remote from each other. The first hydraulic line 126 and the second hydraulic line 128 each have a length wherein the volume of hydraulic fluid contained in the hydraulic lines 126, 128 is greater than the -14 -volume of fluid contained in the respective chamber 118, 120 of the actuator 114 to which they are connected.

[0064] The selector valve 130 is further fluidly connected to the supply line 110 and the return line 112. The selector valve 130 has a first mode in which it is configured to place the first hydraulic line 126 into fluid communication with the supply line 110 and place the second hydraulic line 128 into communication with the return line 112. The selector valve 130 has a second mode in which it is configured to place the first hydraulic line 126 into fluid communication with the return line 112 and place the second hydraulic line 128 into communication with the supply line 110. The selector valve 130 also has a third mode in which both the supply line 110 and return line 112 are taken out of fluid communication with the first and the second hydraulic lines 126, 128.

100651 The selector valve 130 is controlled by a flight control system of the aircraft 102. The system is arranged to command the selector valve 130 to move to a fixed number of discrete positions, the selector valve 130 does not provide for proportional control of the pressure being delivered to the first and second hydraulic lines 126, 128.

[0066] A third hydraulic line 132 connects the first hydraulic line 126 to the second hydraulic line 128. The third hydraulic line 132 is provided proximate the actuator 114, such that the volume of fluid in each of the hydraulic lines 126, 128 between (i) the point at which the third hydraulic line 132 connects thereto, and (ii) the respective port of the hydraulic actuator 114, is less than the maximum volume of the respective chamber 118, 120 of the hydraulic actuator 114 [0067] A circulation valve 134 is provided in the third hydraulic line 132. The circulation valve 134 is openable and closable so as to open and close the hydraulic fluid flow path between the first hydraulic line 126 and the second hydraulic line 128.

[0068] In the first embodiment of the invention, the circulation valve 134 is a mechanical pressure operated check valve. The circulation valve 134 is arranged so as to permit only one way fluid flow from the first hydraulic line 126 to the second hydraulic line 128. The circulation valve 134 has a cracking pressure (a threshold pressure) that the fluid in the first hydraulic line 126 must reach before the circulation valve 134 opens, if the pressure is below the threshold pressure, the circulation valve 134 closes.

-15 - [0069] When the selector valve 130 is in the first mode, when the pressure in the first hydraulic line 126 is above the threshold pressure, and therefore when the circulation valve 134 is open, hydraulic fluid can circulate from the supply line 110 to the return line 112 via the first hydraulic line 126 and the second hydraulic line 128. The fluid therefore eventually makes its way through the filter system 113.

[0070] In order to move the aircraft component, the hydraulic actuator 114 is required to supply a certain level of force in order to overcome the external forces acting on the aircraft component. To supply this force, the pressure applied to the piston of the actuator must reach a certain level, termed the working pressure, at which point the force provided by the actuator I 14 is sufficient to move the aircraft component.

100711 Figure 5 is a plot showing how the pressure in the first chamber 118 of the actuator 114 varies over time when the selector valve 130 is placed in the first mode in order for the actuator 114 to move the aircraft component in a first direction against an external force (e.g. against the weight of the aircraft component). As can be seen, the pressure increases from an initial pressure P1, which may be equal to the pressure in the return line, towards the working pressure Pw. The pressure initially exceeds the working pressure Pw slightly in order to overcome the inertia of the system and start the aircraft component moving. As the aircraft component is moved, the pressure in the first chamber 118 fluctuates slightly around the working pressure Pw, this may be due to slight changes to the external force being applied to the aircraft component as it moves.

[0072] Once the aircraft component has reached its end position at time ti and can move no further, the pressure in the first chamber quickly increases towards the supply pressure Ps. When the pressure reaches the threshold pressure PT (the cracking pressure of the circulation valve 134), the circulation valve 134 opens allowing hydraulic fluid to flow from the supply to the return. The flow of hydraulic fluid flushes out the previous volume of hydraulic fluid from the first and second hydraulic lines 126, 128. The hydraulic fluid can thereafter be filtered by the filter system 113 of the aircraft 102.

[0073] Depending on the conditions (e.g. temperature, aircraft speed, etc), the working pressure may vary slightly, the expected range of working pressures PWR is shown on the plot. The threshold pressure is selected to be comfortably above the upper limit of the -16 -working pressure range PWR. The risk of the circulation valve 134 opening during movement of the aircraft component is thereby reduced. In this embodiment, the threshold pressure is set to be 180 Bar, which is 90% of the supply pressure of 200 Bar.

[0074] In embodiments of the invention, the hydraulic actuator 114 requires less force to move the aircraft component in one direction, than it requires to move the aircraft component in the opposite direction. As such, the working pressure in one direction is lower than the working pressure in the opposite direction An aircraft landing gear is an example of such a component where this is the case [0075] Figure 6 shows an example landing gear arrangement according to an embodiment of the invention. The landing gear 136 comprises a landing gear leg 138 comprising a shock absorber 140 and a torque link 142. A set of wheels 144 are mounted to the bottom of the leg 138. Ita alternative embodiments, the landing gear may comprise a bogie pivotally mounted to the landing gear leg, the bogie comprising a plurality of sets of wheels.

100761 A landing gear extension and retraction system 146 is coupled to the landing gear 136. The landing gear extension and retraction system 146 comprises various components that also form part of the aircraft's hydraulic actuation system, including the hydraulic actuator 114 according to the first embodiment which acts to extend and retract the landing gear 136. The landing gear extension and retraction system 146 also comprises a landing gear up-lock mechanism and a landing gear down-lock mechanism (not shown), which each comprise their own hydraulic actuators.

[0077] In order to extend the landing gear 136, the first chamber 118 of the actuator 114 is fluidly connected to the supply line 110 and the second chamber 120 is fluidly connected to the return line 112 by the selector valve 130. The weight of the landing gear 136 and the aerodynamic forces on the landing gear 136 both act in the same direction as the force provided by the actuator 114. In this direction of motion, the actuator 114 therefore needs to exert only minimal force. In order to retract the landing gear, the second chamber 120 of the actuator 114 is fluidly connected to the supply line 110 and the first chamber 118 is fluidly connected to the return line 112 by the selector valve 130. When the landing gear 136 is retracted, the actuator 114 must act against both the weight and -17 -aerodynamic forces acting on the landing gear 136. Therefore the actuator 114 must provide a significantly larger force in this direction.

[0078] Figure 7 shows an example plot of the pressure in the first chamber Pi vs time as the landing gear is extended, and the pressure in the second chamber P2 vs time as the landing gear is retracted (assuming that the force required of the actuator during extension and retraction is approximately constant). As can be seen, the working pressure Pvvi of the first chamber 118 is significantly lower than the working pressure Pvv, of the second chamber 120. The circulation valve 134 is arranged so as to open in the low force (low power) extension direction of the actuator 114. This allows for more leeway between the working pressure of the chamber and the cracking pressure PT of the circulation valve 134, reducing the chance that the circulation valve 134 will inadvertently open before movement of the landing gear is compete due to a spike in pressure.

[0079] Figure 8 shows a second embodiment of the invention. Parts equivalent to the first embodiment are given like reference numerals. In the second embodiment, the circulation valve 234 is a command valve controlled by a control unit 246. the control unit 246 is connected to a position sensor that is arranged to sense the position of the aircraft component being moved by the actuator 214. When the control unit 246 determines that the aircraft component has completed a movement in the first direction and has reached an end position, the control unit 246 instructs the circulation valve 234 to open so as to allow circulation of hydraulic fluid from the supply to the return via the first hydraulic line 226 and the second hydraulic line 228.

[0080] In an alternative to the second embodiment of the invention, the control unit is connected to a pressure sensor arranged to sense the pressure in the first chamber, and is arranged to only open the circulation valve when the pressure in the first chamber is above a threshold pressure.

100811 Whilst 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. By way of example only, certain possible variations will now be described.

-18 - [0082] In embodiments, the control unit may take into account data from more than one source when determining whether to open the circulation valve. For example, the control unit may also be in communication with the selector valve and be arranged to only open the circulation valve when the selector valve is in a particular mode of operation. In another example, in which the actuator is a landing gear retraction actuator, the control unit is configured so as to only open the circulation valve when the landing gear down-lock is engaged. In embodiments, the circulation valve is arranged such that the circulation valve is not opened every cycle of the actuator, instead the valve is only opened every, for example, tenth cycle. In embodiments, the control unit is configured to open the circulation valve when the aircraft is in a certain flight phase.

100831 In embodiments, the hydraulic fluid flow path connecting the first and second hydraulic lines may comprise two parallel oppositely facing pressure sensitive check valves such that the hydraulic fluid flow path may allow hydraulic fluid to flow in either direction, provided the threshold pressure of the respective check valve is reached.

[0084] It is envisaged that the present invention may have non-aircraft applications.

References in the description and claims to aircraft hydraulic actuation systems could be replaced by references to hydraulic actuation systems in the general sense.

[0085] 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

-19 -CLAIMSAn aircraft hydraulic actuation system comprising: a supply line arranged to carry hydraulic fluid pressurised to a supply pressure by a pump, a return line arranged to return hydraulic fluid to a reservoir, a hydraulic actuator for moving a load, the actuator comprising a first chamber and a second chamber separated by a piston, a first hydraulic line arranged to supply hydraulic fluid into the first chamber of the actuator and a second hydraulic line arranged to supply hydraulic fluid into a second chamber of the actuator, a valve arrangement arranged to control extension and retraction of the hydraulic actuator, the valve arrangement having a first mode in which the first hydraulic line is fluidly connected to the supply line and the second hydraulic line is fluidly connected to the return line, and a hydraulic fluid flow path connecting the first hydraulic line to the second hydraulic line, wherein a circulation valve is arranged to selectively open the fluid flow path so as to allow hydraulic fluid to circulate from the supply line to the return line through at least part of the first hydraulic line and at least part of the second hydraulic line.
2 An aircraft hydraulic actuation system according to claim 1, the wherein the circulation valve is arranged to open when the hydraulic fluid in the first hydraulic line and/or the first chamber is above a threshold pressure.
3. An aircraft hydraulic actuation system according to claim 2, wherein the aircraft hydraulic system is arranged such that, in use, with the valve arrangement in the first mode, the pressure of hydraulic fluid in the first chamber reaches a first working pressure during movement of the load in a first direction, the first working pressure being below -20 -the supply pressure, wherein the threshold pressure is above the first working pressure and below the supply pressure.
4. An aircraft hydraulic actuation system according to claim 2 or 3, wherein the threshold pressure is at or above 80% of the supply pressure, and preferably at or above 90% of the supply pressure.
5. An aircraft hydraulic actuation system according to any preceding claim, wherein the circulation valve is a pressure operated valve, the valve being opened in response to hydraulic pressure at the valve.
6. An aircraft hydraulic actuation system according to any of claims 1 to 4, wherein the circulation valve is operable by a control unit.
7. An aircraft hydraulic actuation system according to claim 6, wherein the control unit uses sensors to determine whether the load has reached a predetermined position, the control unit being configured to command the circulation valve to open when the load is at the predetermined position.
8. An aircraft hydraulic actuation system according to claim 7, wherein the predetermined position is at an end position of a range of motion of the load.
9. An aircraft hydraulic actuation system according to any of claims 6 to 8, wherein the control unit is configured to command the circulation valve to open in response to reduced performance of the actuator.
An aircraft hydraulic actuation system according to claim 9, wherein the control unit is configured to determine reduced performance of the actuator on the basis of the speed of operation of the actuator and/or on the basis of a pressure drop between two points in the hydraulic actuation system.
-21 - 11. An aircraft hydraulic actuation system according to any of claims 6 to 10, wherein the control unit is configured to command the circulation valve to open after a predetermined number of extension and retraction cycles of the actuator.
I 2. An aircraft hydraulic actuation system according to any preceding claim, wherein the hydraulic system is arranged so as to permit only one way fluid flow through the hydraulic fluid flow path connecting the first hydraulic line to the second hydraulic line.
13. An aircraft hydraulic actuation system according to any preceding claim, wherein the valve arrangement has a second mode in which the valve arrangement fluidly connects the second hydraulic line to the supply line and the first hydraulic line to the return line.
14. An aircraft hydraulic actuation system according to claim 13, when dependent on claim 3, wherein the aircraft hydraulic system is arranged such that, in use, with the valve arrangement in the second mode, the pressure of hydraulic fluid in the second chamber reaches a second working pressure during movement of the load in a second direction opposite to the first direction, wherein the second working pressure is above the first working pressure, and the circulation valve remains closed when the valve arrangement is in the second mode.
15. An aircraft hydraulic actuation system according to any preceding claim, wherein the volume of fluid containable within the first hydraulic line between the valve arrangement and the hydraulic actuator is greater than the maximum volume of the first chamber of the hydraulic actuator, and/or the volume of fluid containable within the second hydraulic line between the valve arrangement and the hydraulic actuator is greater than the maximum volume of the second chamber of the hydraulic actuator.
16. An aircraft hydraulic actuation system according to any preceding claim, wherein the fluid flow path connects to the first hydraulic line at a distance from the actuator in which the volume of fluid containable in the first hydraulic line in said distance is less -22 -than the maximum volume of the first chamber, and/or the fluid flow path connects to the second hydraulic line at a distance from the actuator in which the volume of fluid containable in the second hydraulic line in said distance is less than the maximum volume of the second chamber.
17. An aircraft hydraulic system according to any of claim 1 to 15, wherein the fluid flow path connecting the first hydraulic line to the second hydraulic line is provided within the piston of the actuator.
18. An aircraft comprising an aircraft hydraulic actuation system according to any preceding claim.
19. An aircraft according to claim 18, wherein hydraulic actuator is arranged to move the load, the load being selected from a group of: an aircraft landing gear, a door, a cargo door, an aircraft wheel brake.
20. A method of cleaning hydraulic fluid in an aircraft hydraulic actuation system according to any of claims 1 to 17, the method comprising the steps of: configuring the valve arrangement in the first mode, supplying pressurised hydraulic fluid to the actuator from the supply line such that the actuator moves a load to a predetermined position, when the load has reached the predetermined position, opening the circulation valve so as to create a fluid flow path from the supply line to the return line through at least part of the first hydraulic line and at least part of the second hydraulic line, filtering the hydraulic fluid sent to the return line through at least part of the first hydraulic line and at least part of the second hydraulic line.
21. An aircraft comprising a hydraulic actuation system, the hydraulic actuation system comprising: a supply line arranged to carry hydraulic fluid pressurised to a supply pressure by a pump, -23 -a return line arranged to return hydraulic fluid to a reservoir, a hydraulic actuator arranged to move an aircraft component directly from a first position to a second position, the hydraulic actuator comprising a first chamber and a second chamber separated by a piston connected to the aircraft component, a first hydraulic line arranged to supply hydraulic fluid directly into the first chamber of the actuator and a second hydraulic line arranged to supply hydraulic fluid directly into a second chamber of the actuator, a selector valve operated by a flight control system, the selector valve having a first mode in which the selector valve fluidly connects the first hydraulic line to the supply line and the second hydraulic line to the return line so as to move the aircraft component from the first position to the second position, and a second mode in which the selector valve fluidly connects the first hydraulic line to the return line and the second hydraulic line to the supply line so as to move the aircraft component from the second position to the first position; a hydraulic fluid flow path connecting the first hydraulic line to the second hydraulic line, the hydraulic fluid flow path comprising a selectively openable circulation valve wherein the circulation valve is arranged to open the hydraulic fluid flow path in response to the aircraft component reaching the second position following a movement from the first position, the opening of the valve allowing hydraulic fluid to circulate from the supply line to the return line through at least part of the first hydraulic line and at least part of the second hydraulic line.