GB2584310A

GB2584310A - An aircraft landing gear drive system

Info

Publication number: GB2584310A
Application number: GB1907646.2A
Authority: GB
Inventors: Colosimo Antonio
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2019-05-30
Filing date: 2019-05-30
Publication date: 2020-12-02
Anticipated expiration: 2039-05-30
Also published as: GB2584310B; GB201907646D0

Abstract

An aircraft landing gear drive system 1000 comprises a pinion gear 1202, a hydraulic actuator 1210 for moving the pinion gear between engaged and disengaged positions with respect to a driven gear of the landing gear 1201 and a controller 1400 for controlling the hydraulic actuator. The controller comprises a hydraulic fluid input line 1501, at least one hydraulic fluid return line 1602 for returning hydraulic fluid, and an engagement command line 1503 for providing a hydraulic engagement command to the hydraulic actuator. In addition, a shut-off valve 1420 has a first state 1421 where the engagement command line is connected to a hydraulic fluid return line 1602 via a shut-off valve return line 1505, and a second state 1422 where it is not. The controller comprises a relief valve 1430 in the shut-off valve return line such that the hydraulic pressure in the engagement command line is less than the pressure of the hydraulic fluid in the hydraulic fluid input line.

Description

AN AIRCRAFT LANDING GEAR DRIVE SYSTEM

BACKGROUND OF THE INVENTION

[0001] The present disclosure relates to an aircraft landing gear drive system.

100021 The present invention concerns aircraft provided with a landing gear drive system for driving the aircraft during ground movement (e.g. taxiing) using the drive system.

100031 More particularly, but not exclusively, this invention an aircraft landing gear drive system, the drive system comprising a pinion gear mountable on an aircraft landing gear such that it is moveable in relation to the landing gear between a disengaged position, where a driven gear of the landing gear is not engaged by the pinion gear, and an engaged positon, where the driven gear is engaged by the pinion gear, a hydraulic actuator for moving the pinion gear in relation to the landing gear between the engaged and disengaged positions, and a controller for controlling the hydraulic actuator.

[0004] The invention also concerns an actuation system for an aircraft landing gear drive system, a controller for controlling a hydraulic actuator, a method of operating an aircraft landing gear drive system and a method of driving a wheel of an aircraft landing gear.

[0005] Various e-taxi systems have been described and proposed (for example, in W02014/023941) where a landing gear drive system (comprising a motor, a pinion gear connected to the motor, and a driven gear that can be driven by the pinion gear and being connected to the wheel) is used to drive one or more wheels of a landing gear of an aircraft in a forwards or backwards direction. The pinion gear is moved in and out of engagement with the driven gear, according to whether or not the drive system is to be used to drive the wheel, or not. The engagement movement of the pinion gear is achieved by using a hydraulic actuator to actuate engagement movement of the pinion gear towards the driven gear.

100061 It has been found that the pinion gear and driven gear can "bounce" off each other during the engagement process, because the teeth/rollers of the gears are not synchronised with each other. This "bouncing" is illustrated by Figure la, where it can be seen that a roller 11 of the pinion gear 10 is bouncing off a tooth 21 of the driven gear 20. It has been found that a characteristic of this "bouncing" phase is that pressure peaks are generated in the hydraulic system.

[0007] This is shown in the graph 100 of Figure 2. In Figure 2, the x axis 101 represents the time passing (in milliseconds), and the y axis represents the pressure (in bar) of the parts of the hydraulic system 102 and the displacement of the pinion gear in relation to the driven gear (in mm) 103. The system pressure (available from a hydraulic manifold) is shown by line 104. The pressure of the hydraulic actuator for controlling movement of the left hand drive system pinion is shown by line 105 and the (very similar) pressure of the hydraulic actuator for controlling movement of the right hand drive system pinion is shown by line 106. Line 107 shows the displacement of the pinion gear 10.

100081 The "bouncing" phase (Phase 1) is shown in region 111 to the left of the divider line 110. Phase 2 (which represents a "synchronised" phase that will be described later) is shown in region 112 to the right of the divider line 110.

[0009] In Phase 1, it can be seen that the pinion roller and driven gear teeth first make contact at a displacement of just under 40mm. However, as they bounce in and out from each other (as explained above), the displacement varies, as can be seen by peaks and troughs of 107 at 132. These displacement peaks and "bouncing" cause the hydraulic pressure in the actuator to vary, as can be seen by peaks and troughs of 105 and 016 at 131. The magnitude of these pressure peaks is proportional to the base level of hydraulic pressure of the actuators. Hence, this is kept at a low pressure (about 20 bar) in order to minimises load and damage to the system.

[0010] Once the rollers and teeth have become synchronised (as can be seen in Figure 1 b), the pressure peaks and displacement peaks die off and the drive system reaches a steady state. However, at this state (as at divider line 110), the pinion gear and driven gear are not fully meshed, as the displacement is only just over 40mm. Therefore, it is desired for the drive system to then enter a phase 2, in order to fully mesh the two gears 10, 20. In this second "synchronised" phase, the pressure in the left hand and right hand actuators 105, 106 needs to be increased, as shown by 141. This causes the displacement of the pinion gear 10 to increase, as shown by 142. As the displacement increases, the actuator pressures vary slightly, as shown by 144, at around 100 bars of pressure, but these variations are much smaller than at 131. Once the gears are fully meshed (at maximum displacement of just over 80mm, as shown by 143), the pressure in the right and left hand actuators is increased again, in order to maintain that engagement, as shown by 145, to reach a maximum pressure level 146, of around 200 bars, similar to the pressure 104 provided by the system.

[0011] Hence, it can be seen that there is a need to be able to achieve the pressure pattern of Figure 2. In other words, there is a need to achieve a system where the hydraulic actuators can act at a low pressure during phase 1 (in order to minimise the magnitude of the pressure peaks) but can act at a high pressure in phase 2 (in order to maintain engagement of the pinion and driven gears).

100121 In particular, there is a need to achieve the required pressure pattern using simple, relatively inexpensive and reliable components.

100131 The present invention seeks to mitigate the above-mentioned problems.

Alternatively or additionally, the present invention seeks to provide an improved aircraft landing gear drive system.

SUMMARY OF THE INVENTION

100141 The present invention provides, according to a first aspect, an aircraft landing gear drive system, the drive system comprising a pinion gear mountable on an aircraft landing gear such that it is moveable in relation to the landing gear between a disengaged position, where a driven gear of the landing gear is not engaged by the pinion gear, and an engaged positon, where the driven gear is engaged by the pinion gear, a hydraulic actuator for moving the pinion gear in relation to the landing gear between the engaged and disengaged positions, and a controller for controlling the hydraulic actuator, the controller comprising a hydraulic fluid input line for providing hydraulic fluid from a hydraulic manifold, the hydraulic fluid being at a manifold pressure, at least one hydraulic fluid return line for returning hydraulic fluid, an engagement command line for providing a hydraulic engagement command to the hydraulic actuator to move the pinion gear towards the engaged positon, wherein the controller further comprises a selector valve having two selection states i) a disengaging state where the selector valve blocks the hydraulic fluid input line from the engagement command line, such that a hydraulic engagement command is not provided to the actuator, and ii) an engaging state where the selector valve connects the hydraulic fluid input line to the engagement command line, such that a hydraulic engagement command is provided to the actuator, a shut-off valve having two states i) a first state where the shut-off valve connects the engagement command line to a hydraulic fluid return line via a shut-off valve return line, and a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line in i), wherein the controller comprises a relief valve in the shut-off valve return line such that, when the selector valve is in the engaging state and when the shutoff valve is in the first state, the hydraulic pressure in the engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve.

[0015] In the above statement, and throughout the specification, the terms connect, connection etc. when in relation to fluid lines mean "fluidly connected" etc. so that fluid can flow between the connected elements.

[0016] Such a system is able to achieve the pressure pattern required (as seen as one example in Figure 2). Furthermore, the selector valve, shut-off valve and relief valve are much simpler than, for example servo valves, and are also more reliable and less expensive to design, make, tune and install.

[0017] The selector valve may have only two states. The shut-off valve may have only two states. The pinion gear and the driven gear may be provided with meshing elements, such as teeth or rollers.

[0018] Preferably, the relief valve is configured to open at or above a given relief valve pressure so that when the selector valve is in the engaging state and when the shut- -5 -off valve is in the first state, the hydraulic pressure in the hydraulic pressure engagement command line is limited to the given relief valve pressure. For example, the system manifold pressure may be approximately 200 bar and the given pressure of the relief valve may be approximately 50 bar. The relief valve may be a passive vale that is not actively controlled.

[0019] Preferably, when the selector valve is in the disengaging state, the selector valve connects the engagement command line to a hydraulic fluid return line via a selector valve return line.

[0020] Preferably, when the shut-off valve is in the second state, the shut-off valve connects a portion of the shut-off valve return line upstream of the relief valve to a hydraulic fluid return line via a relief valve return line.

[0021] Preferably, the pinion gear is configured to be pivotally mountable to and pivotally moveable in relation to the landing gear.

[0022] Preferably, the hydraulic actuator is also connected to a disengagement command line for providing a hydraulic disengagement command to the hydraulic actuator to move the pinion gear towards the disengaged positon.

[0023] More preferably, the disengagement command line is connected to a second hydraulic fluid input line for providing hydraulic fluid from the hydraulic manifold, the hydraulic fluid being at the manifold pressure.

[0024] Preferably, the drive system comprises a biasing element to urge the pinion gear towards the disengaged position. The biasing element may be a spring.

[0025] Preferably, the engagement command line splits to provide a first engagement command line, for the (first) hydraulic actuator, and a second engagement command line for a second hydraulic actuator, such that the hydraulic engagement command can be provided to both the first and second hydraulic actuators. The first and second actuators may move two different pinion gears, for example one that is part of drive system for a left hand side landing gear and one that is part of drive system for a right hand side landing gear.

[0026] Preferably, the disengagement command line splits to provide a first disengagement command line, for the (first) hydraulic actuator, and a second engagement -6 -command line for a second hydraulic actuator, such that the hydraulic disengagement command can be provided to both the first and second hydraulic actuators. The first and second actuators may move two different pinion gears, for example one that is part of drive system for a left hand side landing gear and one that is part of drive system for a right hand side landing gear.

[0027] Preferably, the controller comprises a pressure measurement input, and wherein the state of the shut-off valve is controlled on the basis of the pressure measurement input.

[0028] More preferably, the pressure measurement input is an indication of a level of synchronisation of the pinion gear and the driven gear. For example, the pressure measurement input may be derived from the hydraulic pressure of the hydraulic pressure engagement command line or a hydraulic pressure of the hydraulic actuator, or similar. The pressure measurement input may indicate a level of variation of the pressure, for example, it may indicate whether or not the pressure is relatively stable or whether or not it significantly varies (has significant peaks and troughs). If the pressure measurement input indicates that the pressure significantly varies, this is an indication that the pinion gear is not sufficiently synchronised with the driven gear (and that the pinion gear and driven gear are "bouncing off' each other to a significant extent), and thus the shut-off valve remains in the first state to keep the pressure in the hydraulic pressure engagement command line at a relatively low level, so as to minimise the peaks and troughs of the hydraulic pressure and thus reduce the load on components of the landing gear drive system. If the pressure measurement input indicates that the pressure is relatively stable, this is an indication that the pinion gear is sufficiently synchronised with the driven gear, and thus the shut-off valve may be moved to the second state to increase the pressure in the hydraulic pressure engagement command line so as to provide a suitable force of the hydraulic actuator to maintain full engagement of the pinion gear and driven gear.

[0029] Preferably, the pinion gear and driven gear form a roller gear and sprocket gear pair. More preferably, the pinion gear is the roller gear.

[0030] Preferably, the drive system comprises a motor in driving connection with the pinion gear. -7 -

100311 According to a second aspect of the invention there is also provided an aircraft landing gear drive system, the drive system comprising a pinion gear mountable on an aircraft landing gear such that it is moveable in relation to the landing gear between a disengaged position, where a driven gear of the landing gear is not engaged by the pinion gear, and an engaged positon, where the driven gear is engaged by the pinion gear, a hydraulic actuator for moving the pinion gear in relation to the landing gear between the engaged and disengaged positions, and a controller for controlling the hydraulic actuator, the controller comprising a hydraulic fluid input line for providing hydraulic fluid from a hydraulic manifold, the hydraulic fluid being at a manifold pressure, at least one hydraulic fluid return line for returning hydraulic fluid, an engagement command line for providing a hydraulic engagement command to the actuator to move the pinion gear towards the engaged positon, wherein the controller further comprises a shut-off valve having two states i) a first state where the shut-off valve connects the engagement command line to a hydraulic fluid return line via a shut-off valve return line, and ii) a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line in i), wherein the controller comprises a relief valve in the shut-off valve return line such that, when the shut-off valve is in the first state, the hydraulic pressure in the engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve.

[0032] Such a system is able to achieve the pressure pattern required (as seen as one example in Figure 2). Furthermore, the shut-off valve and relief valve are much simpler than, for example servo valves, and are also more reliable and less expensive to design, make, tune and install.

[0033] The shut-off valve may have only two states. The pinion gear and the driven gear may be provided with meshing elements, such as teeth or rollers.

[0034] According to a third aspect of the invention there is also provided an actuation system for an aircraft landing gear drive system, the actuation system configured for moving a pinion gear of the drive system in relation to the landing gear between a disengaged position, where a driven gear of the landing gear is not engaged by the pinion gear, and an engaged positon, where the driven gear is engaged by the pinion -8 gear, the actuation system comprising a hydraulic actuator, and a controller for controlling the hydraulic actuator, the controller comprising-a hydraulic fluid input line for providing hydraulic fluid from a hydraulic manifold, the hydraulic fluid being at a manifold pressure, at least one hydraulic fluid return line for returning hydraulic fluid, an engagement command line for providing a hydraulic engagement command to the actuator to move the pinion gear towards the engaged positon, wherein the controller further comprises a selector valve having two selection states i) a disengaging state where the selector valve blocks the hydraulic fluid input line from the engagement command line, such that a hydraulic engagement command is not provided to the actuator, and ii) an engaging state where the selector valve connects the hydraulic fluid input line to the engagement command line, such that a hydraulic engagement command is provided to the actuator, a shut-off valve having two states i) a first state where the shut-off valve connects the engagement command line to a hydraulic fluid return line via a shut-off valve return line, and ii) a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line in i), wherein the controller comprises a relief valve in the shut-off valve return line such that, when the selector valve is in the engaging state and when the shut-off valve is in the first state, the hydraulic pressure in the hydraulic pressure engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve.

[0035] Such a system is able to achieve the pressure pattern required (as seen as one example in Figure 2). Furthermore, the selector valve, shut-off valve and relief valve are much simpler than, for example servo valves, and are also more reliable and less expensive to design, make, tune and install.

[0036] The selector valve may have only two states. The shut-off valve may have only two states. The pinion gear and the driven gear may be provided with meshing elements, such as teeth or rollers.

[0037] According to a fourth aspect of the invention there is also provided a controller for controlling a hydraulic actuator of an aircraft landing gear drive system, the controller comprising a hydraulic fluid input line for providing hydraulic fluid from a hydraulic manifold, the hydraulic fluid being at a manifold pressure, at least one hydraulic fluid return line for returning hydraulic fluid, an engagement command line for providing a hydraulic engagement command to a hydraulic actuator to move a pinion gear of the landing gear towards an engaged positon, wherein the controller further comprises a selector valve having two selection states i) a disengaging state where the selector valve blocks the hydraulic fluid input line from the engagement command line, such that a hydraulic engagement command is not provided to the actuator, and ii) an engaging state where the selector valve connects the hydraulic fluid input line to the engagement command line, such that a hydraulic pressure engagement command is provided to the actuator, a shut-off valve having two states i) a first state where the shutoff valve connects the engagement command line to a hydraulic fluid return line via a shut-off valve return line, and ii) a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line in i), wherein the controller comprises a relief valve in the shut-off valve return line such that, when the selector valve is in the engaging state and when the shut-off valve is in the first state, the hydraulic pressure in the engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve.

[0038] Such a controller is able to achieve the pressure pattern required (as seen as one example in Figure 2). Furthermore, the selector valve, shut-off valve and relief valve are much simpler than, for example servo valves, and are also more reliable and less expensive to design, make, tune and install.

[0039] The selector valve may have only two states. The shut-off valve may have only two states. The pinion gear and the driven gear may be provided with meshing elements, such as teeth or rollers.

[0040] According to a fifth aspect of the invention there is also provided a method of operating an aircraft landing gear drive system, the method comprising the steps of i) moving a selector valve of a hydraulic actuator controller from a disengaging state to an engaging state so that the selector valve connects a hydraulic fluid input line at a manifold pressure to an engagement command line, ii) for a first period of time after step i), providing a shut-off valve of the hydraulic actuator controller in a first state so that the shut-off valve connects the engagement command line to a hydraulic fluid return line via -10 -relief valve in a shut-off valve return line, such that the hydraulic pressure in the engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve, and iii) for a second period of time, after step ii), providing the shut-off valve in a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line, such that the hydraulic pressure in the engagement command line is substantially the same as the manifold pressure.

[0041] Preferably, the method further comprises the step of measuring a hydraulic pressure and wherein the start of step iii) occurs after hydraulic pressure measurement data indicates substantial synchronisation of the pinion gear and driven gear. For example, the pressure measurement may be derived from the hydraulic pressure of the hydraulic pressure engagement command line or a hydraulic pressure of the hydraulic actuator, or similar. The pressure measurement may indicate a level of variation of the pressure, for example, it may indicate whether or not the pressure is relatively stable or whether or not it significantly varies (has significant peaks and troughs). If the pressure measurement indicates that the pressure significantly varies, this is an indication that the pinion gear is not sufficiently synchronised with the driven gear (and that the pinion gear and driven gear are "bouncing off' each other to a significant extent), and thus the shutoff valve remains in the first state to keep the pressure in the hydraulic pressure engagement command line at a relatively low level, so as to minimise the peaks and troughs of the hydraulic pressure and thus reduce the load on components of the landing gear drive system. If the pressure measurement input indicates that the pressure is relatively stable, this is an indication that the pinion gear is sufficiently synchronised with the driven gear, and thus the shut-off valve may be moved to the second state to increase the pressure in the hydraulic pressure engagement command line so as to provide a suitable force of the hydraulic actuator to maintain full engagement of the pinion gear and driven gear.

[0042] According to a sixth aspect of the invention there is also provided a method of driving a wheel of an aircraft landing gear by moving a first gear from a disengaged configuration to an engaged configuration in which the first gear is synchronised with a second gear, the method comprising the steps of i) a hydraulic actuator moving the first gear towards the second gear during a first phase of movement in which the hydraulic actuator operates in a low pressure mode, the first gear being partially meshed but not fully synchronised with the second gear during the first phase, ii) fully synchronising the first gear with second gear during a second phase of movement during which the hydraulic actuator operates in a high pressure mode, and iii) the hydraulic actuator being switched from the low pressure mode of the first phase to the high pressure mode of the second phase by operation of a valve to block off access to a hydraulic fluid return line via a relief valve.

[0043] 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, a method of the invention may incorporate any of the features described with reference to an apparatus of the invention and vice versa.

DESCRIPTION OF THE DRAWINGS

100441 Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 100451 Figure la shows a schematic view of a roller of a pinion gear bouncing off a tooth of a driven gear; 100461 Figure lb shows a schematic view of a roller of the pinion gea n synchronisation with teeth of the driven gear; 100471 Figure 2 shows a graph demonstrating the pressure pattern desired of a hydraulic system according to embodiments of the present invention; 100481 Figure 3a shows a system diagram of a landing gear drive system according to a first embodiment of the present invention (including a controller and a hydraulic actuator), the system being shown in a disengaged configuration; -12 - 100491 Figure 3b shows a system diagram of the landing gear drive system of Figure 3a, the system being shown in an engaged (phase 1) configuration; [0050] Figure 3c shows a system diagram of the landing gear drive system of Figure 3a and Figure 3b, the system being shown in an engaged (phase 2) configuration.

DETAILED DESCRIPTION

[0051] Figure 3a shows a system diagram of a landing gear drive system 1000, the system being shown in a disengaged configuration.

[0052] The system 1000 comprises a hydraulic system manifold 1100, which is able to provide hydraulic pressure at a pressure of 200 bar, a controller 1400, a left hand landing gear 1200, a right hand landing gear 1300 and a hydraulic fluid return circuit 1600.

100531 The left hand landing gear 1200 will now be described. It comprises a landing gear structure 1201 and a pinion gear 1202 pivotally mounted by pinion gear structure (at 1203) to the landing gear structure 1201. The pinion gear 1202 is moved on the pivot 1203 in relation to a driven gear (not shown) of the landing gear 1200.

100541 The pinion gear 1202 is moved by a double-ended actuator 1210 that is pivotally mounted 1211 at a first end to the landing gear structure 1201 and pivotally mounted at its second opposite end 1212 to the pinion gear structure. Towards the first end of the actuator is a disengagement command line input 1213. Towards the second end of the actuator is an engagement command line input 1214. When the actuator 1210 receives pressurised hydraulic fluid in the disengagement command line input 1213, the fluid causes the piston in the actuator to be pushed towards the second end of the actuator. Thus, a pivot rod of the actuator 120 is caused to extend so as to move the pinion towards its disengaged position (away from the driven gear). When the actuator 1210 receives pressurised hydraulic fluid in the engagement command line input 1214, the fluid causes the piston in the actuator to be pushed towards the first end of the -13 -actuator. Thus, a pivot rod of the actuator 120 is caused to retract so as to move the pinion towards its engaged position (towards the driven gear).

[0055] The actuator 1210 also comprises a spring 1215 to urge the piston towards the second end so as to urge the pinion gear into the disengaged position. The left hand landing gear 1200 also comprises a lock link mechanism 1204 to maintain the pinion gear in the disengaged position.

[0056] The right hand landing gear 1300 corresponds to the arrangement of the left hand landing gear 1200 and will not be separately described. The same numbering applies to the right hand landing gear but with "13" instead of "12" at the beginning of the reference numeral.

[0057] The controller 1400 comprises a hydraulic fluid input line 1501, a hydraulic fluid return line 1507 and an engagement command line 1503. The hydraulic fluid return line 1507 of the controller 1400 is connected to the hydraulic fluid return circuit 1600. The engagement command line 1503 splits into a left hand engagement command line 1701 and a right hand engagement command line 1702, which are connected to the left hand landing gear engagement command line input 1214 and corresponding right hand landing gear engagement command line input 1314, respectively. The controller 1400 also has a number of valves and fluid lines and will be described in more detail later.

[0058] The hydraulic system manifold 1100 has three output lines (all at 200 bar); a left hand landing gear disengagement command line 1110, a right hand landing gear disengagement command line 1120 and a controller hydraulic fluid line 1130.

[0059] The left hand landing gear disengagement command line 1110 is connected to the disengagement command line input 1213 of the left hand landing gear 1200, the right hand landing gear disengagement command line 1120 is connected to the corresponding disengagement command line input 1313 of the right hand landing gear 1300 and the controller hydraulic fluid line 1130 is connected to the hydraulic fluid input line 1501 of the controller 1400.

[0060] The controller 1400 will now be considered in more detail: [0061] The hydraulic fluid input line 1501 (at 200 bar) is connected to a selector valve 1410, which is shown in a first state 1411 in Figure 3a. In this state 1411, the -14 -hydraulic fluid input line 1501 is blocked off from the rest of the fluid lines and valves of the controller 1400. This means that there is no pressurised hydraulic fluid in the engagement command line 1503 and so no pressurised hydraulic fluid in the left hand landing gear engagement command line input 1214 or the corresponding right hand landing gear engagement command line input 1314.

[0062] In addition, with the selector valve 1410 in state 1411, the engagement command line 1503 is connected to a selector valve return line 1502, which is connected to the hydraulic fluid return line 1507, which is connected to the hydraulic fluid return circuit 1600. This means that the pressure of the hydraulic fluid in engagement command line 1503 is low.

[0063] As there is only low pressure hydraulic fluid in the engagement command line 1503, this means that, even without pressurised hydraulic fluid being provided at the disengagement command line inputs 1213, 1313, the springs of the landing gear 1215, 1315, urge the pinion gears 1202, 1302 to the disengaged positions.

[0064] Figure 3b shows a system diagram of the landing gear drive system 1000 of Figure 3a, the system being shown in an engaged (phase 1) configuration. Importantly, however, for simplicity, the manifold 1100 has not actually been adjusted from its configuration of Figure 3a (disengaged). In actuality, the selector valves of the manifold 1100 (shown at the bottom) would have moved to an off position so pressurised hydraulic fluid was no longer provided in the disengagement command line inputs 1213, 1313.

[0065] Here, the selector valve 1410 has been moved up to its second state 1412. In this state 1412, the hydraulic fluid input line 1501 is connected to the engagement command line 1503.

[0066] There is a shut-off valve branch 1504 branching off from the engagement command line 1503 and this branch is connected to a shut-off valve 1420. The shut-off valve is shown in a first state 1421 in Figure 3b. In this state 1421, the hydraulic fluid shut-off valve branch 1504 is connected to a shut-off valve return line 1505. In this shutoff valve return line 1505 is a relief valve 1430.

[0067] The relief valve 1430 is a passive valve that simply relieves the pressure in the shut-off valve return line 1505 above the set pressure of the relief valve 1430. In this -15 -example, the set pressure of the relief valve 1430 is 50 bar. This means that the pressure in the shut-off valve branch 1504 and hence in the engagement command line 1503 is 50 bar. Any excess hydraulic fluid (i.e. at a pressure of above 50 bar) is returned via the shut-off valve return line 1505 to the hydraulic fluid return line 1507, and then to the hydraulic fluid return circuit 1600.

[0068] The 50 bar pressurised hydraulic fluid in the engagement command line 1503 is provided to the left hand landing gear engagement command line input 1214 and the corresponding right hand landing gear engagement command line input 1314. This provides the hydraulic pressure for the actuators 1210, 1310 to move the pinions 1202, 1302 towards the engagement position (shown by arrows above the actuators 1210, 1310), until they are synchronised with the corresponding driven gears.

[0069] Figure 3c shows a system diagram of the landing gear drive system 1000 of Figure 3a and Figure 3b, the system being shown in an engaged (phase 2) configuration. Importantly, however, for simplicity, the manifold 1100 has not actually been adjusted from its configuration of Figure 3a (disengaged). In actuality, the selector valves of the manifold 1100 (shown at the bottom) would have moved to an off position so pressurised hydraulic fluid was no longer provided in the disengagement command line inputs 1213, 1313.

[0070] Here, it has been sensed that the pinion gear and driven gear are synchronised and so the shut-off valve 1420 has been moved down to its second state 1422.

[0071] In this state 1422, the shut-off valve branch 1504 is blocked from the shut-off valve return line 1505. This means that there is no pressure relief on the engagement command line 1503 and so the pressure here is 200 bar. The 200 bar pressurised hydraulic fluid in the engagement command line 1503 is provided to the left hand landing gear engagement command line input 1214 and the corresponding right hand landing gear engagement command line input 1314. This provides the hydraulic pressure for the actuators 1210, 1310 to maintain the pinions 1202, 1302 in the engagement position (shown by arrows above the actuators I 210, 1310).

[0072] In addition, with the shut-off valve 1420 in state 1422, the shut-off valve return line 1505 (upstream of the relief valve) is connected to a relief valve return line -16 - 1506, which is connected to the hydraulic fluid return line 1507, which is connected to the hydraulic fluid return circuit 1600.

[0073] When it is not desired to drive aircraft wheels using the drive system, the pilot has the driving system switched off (from a cockpit control). This causes the system manifold 1000 to provide pressurised hydraulic fluid (at 200 bar) to the left hand disengagement command line 1110 and the right hand disengagement command line 1120. This causes the actuators 1210, 1310 to move the pinion gears 1202, 1302 away from the driven gears to the disengaged positions. In addition, the springs 1215, 1315 urge the pinion gears 1202, 1302 towards the disengaged positions. The selector valve 1410 is in its first state 1411 and the shut-off valve 1420 is in its first state 1421.

[0074] When the pilot wishes to drive the aircraft wheels with the drive system the pilot switches the driving system switched on (from the cockpit control). This causes the system manifold 1000 to stop providing pressurised hydraulic fluid (at 200 bar) to the left hand disengagement command line 1110 and the right hand disengagement command line 1120 (by moving the selector valves at the bottom of the manifold 1100). In addition, the selector valve 1410 is moved to its second state 1412 so that hydraulic fluid (at 50 bar, because the shut-off valve is in its first state) is provided in the engagement command line 1503 and provided to the engagement command lines 1214, 1314 of the left and right hand actuators 1210, 1310. This moves the pinion gears 1202, 1302 towards the driven gears (against the force of the springs 1215, 1315). This causes the pinion and driven gears to bounce of each other, due to the gears not being synchronised. This causes pressure peaks in the actuators 1210, 1310, which are sensed by pressure transducers (not shown).

[0075] Once the pressure transducers have sensed that the pressure peaks have died down (i.e. that the pinion and driven gears are synchronised), the shut-off valve 1420 is moved to its second state 1422. As explained above, this causes the pressure in the engagement command line 1503 (and thus provided to the engagement command lines 1214, 1314 of the left and right hand actuators 1210, 1310) to be at 200 bar. This urges the pinion and driven gears into a fully meshed configuration and maintains that -17 -configuration, despite the rotation of the driven gear acting to resist the pinion engagement.

[0076] 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.

[0077] In the above example, pressures of 200 bar and 50 bar are used. However, any suitable pressures for the system manifold pressure and relief valve set pressure can be used.

[0078] The disengagement command lines 1 I I0 and I 120 from the system manifold 1000 may actually be provided as a single line that then splits into two to connect to the disengagement command line inputs 1213, 1313.

[0079] The disengagement command line (or lines) may be connected to the disengagement command line inputs 1213, 1313 via the controller 1400, rather than directly from the system manifold 1100.

100801 In the above example, pressure transducers are used to measure the pressure at the actuators 1210, 1310. However, any other suitable sensing devices and/or locations may be used.

[0081] 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.

[0082] It should be noted that throughout this specification, "or-should be interpreted as "and/or".

Claims

-18 -CLAIMSAn aircraft landing gear drive system, the drive system comprising: - a pinion gear mountable on an aircraft landing gear such that it is moveable in relation to the landing gear between a disengaged position, where a driven gear of the landing gear is not engaged by the pinion gear, and an engaged positon, where the driven gear is engaged by the pinion gear, -a hydraulic actuator for moving the pinion gear in relation to the landing gear between the engaged and disengaged positions, and - a controller for controlling the hydraulic actuator, the controller comprising: - a hydraulic fluid input line for providing hydraulic fluid from a hydraulic manifold, the hydraulic fluid being at a manifold pressure, - at least one hydraulic fluid return line for returning hydraulic fluid, -an engagement command line for providing a hydraulic engagement command to the hydraulic actuator to move the pinion gear towards the engaged positon, wherein the controller further comprises: - a selector valve having two selection states: i) a disengaging state where the selector valve blocks the hydraulic fluid input line from the engagement command line, such that a hydraulic engagement command is not provided to the actuator, and ii) an engaging state where the selector valve connects the hydraulic fluid input line to the engagement command line, such that a hydraulic engagement command is provided to the actuator, -19 - -a shut-off valve having two states: i) a first state where the shut-off valve connects the engagement command line to a hydraulic fluid return line via a shut-off valve return line, and ii) a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line in i), wherein the controller comprises a relief valve in the shut-off valve return line such that, when the selector valve is in the engaging state and when the shut-off valve is in the first state, the hydraulic pressure in the engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve.
2. An aircraft landing gear drive system as claimed in claim 1, wherein the relief valve is configured to open at or above a given relief valve pressure so that when the selector valve is in the engaging state and when the shut-off valve is in the first state, the hydraulic pressure in the hydraulic pressure engagement command line is limited to the given relief valve pressure.
3. An aircraft landing gear drive system as claimed in claim 1 or claim 2, wherein when the selector valve is in the disengaging state, the selector valve connects the engagement command line to a hydraulic fluid return line via a selector valve return line.
4. An aircraft landing gear drive system as claimed in any preceding claim, wherein when the shut-off valve is in the second state, the shut-off valve connects a portion of the shut-off valve return line upstream of the relief valve to a hydraulic fluid return line via a relief valve return line.
5. An aircraft landing gear drive system as claimed in any preceding claim, wherein the pinion gear is configured to be pivotally mountable to and pivotally moveable in relation to the landing gear.
-2 0 - 6. An aircraft landing gear drive system as claimed in any preceding claim, wherein the hydraulic actuator is also connected to a disengagement command line for providing a hydraulic disengagement command to the hydraulic actuator to move the pinion gear towards the disengaged positon.
7. An aircraft landing gear drive system as claimed in claim 6, wherein the disengagement command line is connected to a second hydraulic fluid input line for providing hydraulic fluid from the hydraulic manifold, the hydraulic fluid being at the manifold pressure.
8. An aircraft landing gear drive system as claimed in any preceding claim, wherein the drive system comprises a biasing element to urge the pinion gear towards the disengaged position.
9. An aircraft landing gear drive system as claimed in any preceding claim, wherein the engagement command line splits to provide a first engagement command line, for the (first) hydraulic actuator, and a second engagement command line for a second hydraulic actuator, such that the hydraulic engagement command can be provided to both the first and second hydraulic actuators.
10. An aircraft landing gear drive system as claimed in any preceding claim, wherein the controller comprises a pressure measurement input, and wherein the state of the shutoff valve is controlled on the basis of the pressure measurement input.
11. An aircraft landing gear drive system as claimed in claim 10, wherein the pressure measurement input is an indication of a level of synchronisation of the pinion gear and the driven gear.
12. An aircraft landing gear drive system as claimed in any preceding claim, wherein the pinion gear and driven gear form a roller gear and sprocket gear pair.
-21 - 13. An aircraft landing gear drive system as claimed in any preceding claim, wherein the drive system comprises a motor in driving connection with the pinion gear.
14. An aircraft landing gear drive system, the drive system comprising: - a pinion gear mountable on an aircraft landing gear such that it is moveable in relation to the landing gear between a disengaged position, where a driven gear of the landing gear is not engaged by the pinion gear, and an engaged positon, where the driven gear is engaged by the pinion gear, -a hydraulic actuator for moving the pinion gear in relation to the landing gear between the engaged and disengaged positions, and - a controller for controlling the hydraulic actuator, the controller comprising: - a hydraulic fluid input line for providing hydraulic fluid from a hydraulic manifold, the hydraulic fluid being at a manifold pressure, - at least one hydraulic fluid return line for returning hydraulic fluid, -an engagement command line for providing a hydraulic engagement command to the actuator to move the pinion gear towards the engaged positon, wherein the controller further comprises: -a shut-off valve having two states: i) a first state where the shut-off valve connects the engagement command line to a hydraulic fluid return line via a shut-off valve return line, and ii) a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line in i), -2 2 -wherein the controller comprises a relief valve in the shut-off valve return line such that, when the shut-off valve is in the first state, the hydraulic pressure in the engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve.
15. An actuation system for an aircraft landing gear drive system, the actuation system configured for moving a pinion gear of the drive system in relation to the landing gear between a disengaged position, where a driven gear of the landing gear is not engaged by the pinion gear, and an engaged positon, where the driven gear is engaged by the pinion gear, the actuation system comprising: - a hydraulic actuator, and - a controller for controlling the hydraulic actuator, the controller comprising: -a hydraulic fluid input line for providing hydraulic fluid from a hydraulic manifold, the hydraulic fluid being at a manifold pressure, - at least one hydraulic fluid return line for returning hydraulic fluid, - an engagement command line for providing a hydraulic engagement command to the actuator to move the pinion gear towards the engaged positon, wherein the controller further comprises: -a selector valve having two selection states: i) a disengaging state where the selector valve blocks the hydraulic fluid input line from the engagement command line, such that a hydraulic engagement command is not provided to the actuator, and -23 -ii) an engaging state where the selector valve connects the hydraulic fluid input line to the engagement command line, such that a hydraulic engagement command is provided to the actuator, -a shut-off valve having two states: i) a first state where the shut-off valve connects the engagement command line to a hydraulic fluid return line via a shut-off valve return line, and ii) a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line in i), wherein the controller comprises a relief valve in the shut-off valve return line such that, when the selector valve is in the engaging state and when the shut-off valve is in the first state, the hydraulic pressure in the hydraulic pressure engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve.
16. A controller for controlling a hydraulic actuator of an aircraft landing gear drive system, the controller comprising: -a hydraulic fluid input line for providing hydraulic fluid from a hydraulic manifold, the hydraulic fluid being at a manifold pressure, -at least one hydraulic fluid return line for returning hydraulic fluid, -an engagement command line for providing a hydraulic engagement command to a hydraulic actuator to move a pinion gear of the landing gear towards an engaged pos ton, wherein the controller further comprises: -24 - -a selector valve having two selection states: i) a disengaging state where the selector valve blocks the hydraulic fluid input line from the engagement command line, such that a hydraulic engagement command is not provided to the actuator, and ii) an engaging state where the selector valve connects the hydraulic fluid input line to the engagement command line, such that a hydraulic pressure engagement command is provided to the actuator, -a shut-off valve having two states: i) a first state where the shut-off valve connects the engagement command line to a hydraulic fluid return line via a shut-off valve return line, and ii) a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line in i), wherein the controller comprises a relief valve in the shut-off valve return line such that, when the selector valve is in the engaging state and when the shut-off valve is in the first state, the hydraulic pressure in the engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve.
17. A method of operating an aircraft landing gear drive system, the method comprising the steps of: i) moving a selector valve of a hydraulic actuator controller from a disengaging state to an engaging state so that the selector valve connects a hydraulic fluid input line at a manifold pressure to an engagement command line, ii) for a first period of time after step i), providing a shut-off valve of the hydraulic actuator controller in a first state so that the shut-off valve connects the engagement -2 5 -command line to a hydraulic fluid return line via relief valve in a shut-off valve return line, such that the hydraulic pressure in the engagement command line is less than the manifold pressure due to the connection to the hydraulic fluid return line through the relief valve, and iii) for a second period of time, after step ii), providing the shut-off valve in a second state where the shut-off valve blocks the engagement command line from the hydraulic fluid return line, such that the hydraulic pressure in the engagement command line is substantially the same as the manifold pressure.
18. A method of operating an aircraft landing gear drive system as claimed in claim 17, wherein the method further comprises the step of measuring a hydraulic pressure and wherein the start of step iii) occurs after hydraulic pressure measurement data indicates substantial synchronisation of the pinion gear and driven gear.
19. A method of driving a wheel of an aircraft landing gear by moving a first gear from a disengaged configuration to an engaged configuration in which the first gear is synchronised with a second gear, the method comprising the steps of: i) a hydraulic actuator moving the first gear towards the second gear during a first phase of movement in which the hydraulic actuator operates in a low pressure mode, the first gear being partially meshed but not fully synchronised with the second gear during the first phase, ii) fully synchronising the first gear with second gear during a second phase of movement during which the hydraulic actuator operates in a high pressure mode, and iii) the hydraulic actuator being switched from the low pressure mode of the first phase to the high pressure mode of the second phase by operation of a valve to block off access to a hydraulic fluid return line via a relief valve.