Abstract:
A hydraulic control system for an aircraft landing gear system comprises a hydraulic pressure source connected to a hydraulic fluid pressure transmission line, and at least one hydraulic actuator for extending and/or retracting a landing gear, wherein first and second valves are provided in series along the hydraulic fluid pressure transmission line, the first and second valves being adjustable to control the hydraulic pressure supplied to the at least one hydraulic actuator. The control system also controls uplock actuators.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(a)-(d) or (f) to prior-filed, co-pending British application number 0906464.3, filed on Apr. 15, 2009, which is hereby incorporated by reference in its entirety. 
       FIELD OF INVENTION 
       [0002]    This invention relates to actuation control systems for aircraft landing gear, and in particular to valve arrangements for controlling the flow of hydraulic fluid around such actuation systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    Many aircraft are equipped with retractable landing gears, which typically can be raised following take-off and stowed in a closable compartment on the underside of the aircraft, eg. in the wings. A landing gear may often be provided under either wing and under the nose of the aircraft. 
         [0004]    When the landing gears have been raised, the gears are locked in place with uplocks, to prevent unintended extension of the landing gears, until such time as the landing gears are to be used again. It is desirable to include safety measures to ensure that when the landing gears are extended they do not accidentally retract, and that in the event of a malfunction of the hydraulic system, the landing gears can always be extended as necessary for landing. 
         [0005]    A known hydraulic landing gear system  1  is shown schematically in  FIG. 1  of the drawings, wherein a system including three landing gears is shown. A hydraulic fluid pressure line  5  and a hydraulic fluid return line  6  are connected to a hydraulic source (not shown) which is usually provided as part of the aircraft hydraulics system. A selector valve  2  is provided along the pressure and return lines  5 , 6  which is operable to direct the hydraulic pressure along first and second conduits  17 , 18  which lead to a plurality of hydraulic actuators  8 , 9 , 10 , 11 , 12 , 13 . The actuators numbered  8 , 9  and  10  comprise first, second and third gear downlock actuators respectively, which are operable to lock the landing gears in the extended position, whilst the actuators numbered  11 ,  12  and  13  comprise first, second and third gear actuators, for extending and retracting the landing gears, and optionally the doors of the landing gear compartments. Alternatively, the doors may be actuated by means of a separate hydraulic system. 
         [0006]    An uplock control valve  4  is hydraulically connected to the first conduit  17  for controlling first, second and third uplocks  14 , 15 , 16 . The uplocks lock the landing gears in the retracted position to prevent unintended extension of the landing gears. A corresponding uplock system can be provided to control opening and closing of the doors to the closable compartments in which the landing gears are housed. 
         [0007]    In one example, the first uplock  14 , the first downlock actuator  8  and the first gear actuator  11  correspond to the right wing landing gear, while the second uplock  15 , the second downlock actuator  9  and the second gear actuator  12  correspond to the left wing landing gear and the third uplock  16 , the third downlock actuator  10  and the third gear actuator  13  correspond to the nose landing gear. 
         [0008]    The selector valve  2  comprises first and second solenoids  19 ,  20  for moving the valve between first, second and third settings  23 , 24 , 25 . First and second springs  21 ,  22  centre the valve, whereby the second setting  24  is the default status of the valve. When it is desired to extend the landing gears, the first solenoid  19  is operated to bias the valve elements to the right, thereby putting the valve in the first setting  23 . Pressure is thus applied to the actuators  8 , 9 , 10 , 11 , 12 , 13  so as to extend the landing gears. In order to retract the landing gears, the second solenoid  20  is operated to bias the valve elements to the left, thus placing the valve in the third setting  25 . When neither solenoid  19 , 20  is operated, the second setting  24  is selected. In the second setting  24 , the valve inhibits the pressure from the pressure line  5  and connects both conduits  17 , 18  to the return line  6 , thereby removing hydraulic pressure from the system. 
         [0009]    The landing gear system also comprises a fail-safe free-fall system including a cut out valve  3  and a vent valve  7 , which in the event of a selector valve  2  failure, are operable to ensure that the landing gears can be extended. In free-fall operation, the cut out valve  3  isolates the system from the hydraulic pressure, whilst the vent valve  7  provides an interconnection between the fluid conduits  26 , 27  downstream thereof. This results in a status of the system where the landing gears can extend under the influence of gravity. 
         [0010]    The prior art hydraulic landing gear system  1  is complicated and backup systems are needed to operate the cut out and vent valves. Further, it is possible for inadvertent retractions of the landing gears to take place if a fault occurs in the selector valve  2 . For example, if the second solenoid  20  is erroneously operated, eg. due to a malfunction, the selector valve will move so that the valve element  25  is operative and the landing gears will retract. When the aircraft is on the ground, such a fault is both expensive and dangerous. Complex control and backup systems have to be provided to cater for the potential erroneous operation or failure of the solenoids  19 , 20 . The prior art system is therefore expensive and is heavy due to the relatively large number of components needed. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a hydraulic control system for an aircraft landing gear system comprising a hydraulic pressure source connected to a hydraulic fluid pressure transmission line, and at least one hydraulic actuator for extending and/or retracting a landing gear, wherein first and second valves are provided in series along the hydraulic fluid pressure transmission line, the first and second valves being adjustable to control the hydraulic pressure supplied to the at least one hydraulic actuator. 
         [0012]    If one of the first or second valves malfunctions, for example by jamming open, the other of the first and second valves can be operated to ensure that the landing gears can still be extended. It is unlikely that the first and second valves will both malfunction at the same time and advantageously, the system therefore offers a particularly simple way to provide fail-safe operation with improved reliability. The system according to the invention removes the need for the cut-out and vent valves and therefore utilises a reduced number of valve elements, reducing the mass of the system and the overall cost of production of the aircraft. 
         [0013]    Preferably, the first and second valves comprise 2-way valves, rather than the 3-way valves used in the prior art. An advantageous cost saving is thereby provided as well as improved reliability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    There follows a detailed description of embodiments of the invention, by way of example only, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  shows schematically a hydraulic landing gear system according to the prior art; 
           [0016]      FIG. 2  shows schematically a hydraulic landing gear system embodying the invention; 
           [0017]      FIGS. 3A ,  3 B,  3 C and  3 D show schematically various configurations of a control valve of the system shown in  FIG. 2 ; and 
           [0018]      FIG. 4  is a schematic cross-sectional view of a hydraulic landing gear system according to a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 2  shows a hydraulic landing gear system  30  embodying the invention, wherein like reference numerals are used to denote features which are present in the system shown in  FIG. 1 . 
         [0020]    The hydraulic pressure and return lines  5 , 6  lead to a first valve  33  and from there, directly to a second valve  34 . First and second hydraulic conduits  17 , 18  lead from the second valve  34  directly to the hydraulic actuators  8 , 9 , 10 , 11 , 12 , 13 . The hydraulic landing gear system  30  comprises a control valve block  31  in which the first valve  33  and the second valve  34  are disposed in series with respect to one another. By virtue of their serial arrangement no single valve failure can cause an inadvertent retraction of the landing gears. The valves of the system can be arranged as “line replaceable units”, ie. can be connected to one another via hydraulic lines permitting individual installation and removal of the valves. The first and second valves  33 , 34  take the place of the selector valve  2  and the cut out valve  3  and vent valve  7 . Thus, rather than having three valves, only two valves are required by the present invention to control the flow of hydraulic fluid to the actuators  8  to  13 . The first valve  33  comprises a supply valve, which connects the system to the aircraft hydraulic supply, and the second valve  34  comprises a direction valve which controls whether the gear is being extended or retracted. The control valve block  31  also includes an uplock control valve  4  to unlock the uplocks  14 , 15 , 16  at the appropriate point in the extension sequence. The uplock control valve  4  is located downstream of the supply  33  and direction  34  valves to provide the small flow of hydraulic fluid necessary for operating the uplock actuators. 
         [0021]    In the embodiment shown in  FIG. 2 , the supply and direction valves  33 , 34  have the same internal structure as one another. Both include a solenoid  38 , 39  for opening the valve, and a return spring  36 , 37  biasing the valve to a default position. Each valve is a 2-way valve having first and second settings  40 , 41 ;  42 , 43 , the first setting  40 , 42  allowing hydraulic fluid to flow through the valve and the second setting  41 , 43  inhibiting the hydraulic pressure from the pressure line  5 . The second setting is configured to provide a direct hydraulic flow path between the fluid conduits  17 , 18  which are on the downstream side of the control valve block  31 . A hydraulic connection shown by a dashed line is provided between the solenoids  38 , 39  of the supply and direction valves and the hydraulic pressure line  5 . The purpose of this connection is to provide a relatively low amount of hydraulic power to the solenoids to allow them to move the valve bodies of the valves  33 ,  34 . Further, the solenoid  44  of the uplock valve is also hydraulically connected to the subsidiary source of hydraulic pressure that operates the uplock actuators  14 , 15 , 16 , as shown by a dashed line. 
         [0022]    The valve block  31  is designed incorporating rip-stop principles, wherein physical barriers are provided between the supply and direction valves  33 , 34  and the uplock valve  4 , which prevent the propagation of cracks through the valve block  31  which could potentially affect the operation of the valves. 
         [0023]    The uplock valve  4  is a two-way valve having two settings  45 , 46 , respectively on and off. A solenoid  44  is operable to bias the valve to the left, thereby selecting the “off” setting  46 . A spring  47  biases the valve to the right whereby the “on” setting  45  is the default selection. A hydraulic conduit  35  leads from the uplock valve  4  to the uplocks  14 , 15 , 16 . The uplocks hold the landing gears and landing gear doors in the retracted position while the aircraft is airborne. Each uplock comprises a hydraulic uplock actuator  50  for effecting movement of the uplocks as well as an optional latch member  49  for holding the uplocks in the locked position, the latch member  49  being manually releasable. In normal operation, only the hydraulic uplock actuator  50  is needed to control the uplocks. A solenoid  48  is also provided in connection with each uplock for unlocking the uplock in the event of a hydraulic failure. When the supply valve  33  is in the open setting  40 , hydraulic pressure is transmitted therefrom to the uplock valve  4  via a subsidiary conduit  51 . 
         [0024]    Referring to  FIGS. 3A to 3D , the supply and direction valves  33 , 34  are shown schematically in the different possible settings. The hydraulic pressure  5  and return  6  lines are arranged the opposite way round to that shown in  FIG. 2 . 
         [0025]    In  FIG. 3A , the system is in a neutral configuration with both the supply valve  33  and the direction valve  34  in the “off” positions  41 , 43 . In this configuration, the hydraulic conduits  17 , 18  are interconnected and the landing gears are free to extend under freefall conditions, ie. under the influence of gravity. 
         [0026]      FIG. 3B  illustrates the system with both the supply and direction valves  33 , 34  energised to the “on” positions  40 , 42 . In this configuration, the landing gears retract. 
         [0027]      FIG. 3C  illustrates the system with the supply valve in the “on” position  40  and the direction valve  34  in the “off” position. This may correspond to the situation where the supply valve  40  has jammed open due to a fault. The landing gears extend in this configuration. 
         [0028]      FIG. 3D  shows the system with the supply valve  33  closed (setting  41 ) and the direction valve  34  open (setting  42 ). This may correspond to the situation where the direction valve  34  has jammed open due to a fault. Again, the landing gears will extend in this configuration. 
         [0029]      FIG. 4  shows an alternative embodiment of the invention including a variation on the way in which the supply and direction valves  33 , 34  are arranged. The supply and direction valves  33 , 34  are arranged in a back-to-back layout, wherein the solenoid  38  of the first valve  33  is generally aligned with the return spring  37  of the second valve  34  and the solenoid  39  of the second valve  34  is generally aligned with the return spring  36  of the first valve  33 . As shown, the solenoid  38  of the supply valve  33  is positioned on the left hand side of the supply valve and the solenoid  39  of the direction valve  34  is arranged on the right hand side of the direction valve as shown in the drawing. The springs  36  and  37  which bias the valves closed are provided at the opposite ends of the valves  33 ,  34  to the solenoids  38 ,  39 . The valve settings in  FIG. 4  are equivalent to  FIG. 3A , ie. both of the valves are in the off positions and the landing gears are free to extend under the influence of gravity. 
         [0030]    The configuration of the gear downlock actuators and the gear actuators described herein is an example of many different forms that the actuators can take in practice, and the actuation control system of the present invention is not limited to the actuator arrangements disclosed above.