Abstract:
The invention provides an aircraft landing gear arrangement, comprising a bogie including a steerable axle pivotally mounted to said bogie. A locking mechanism is provided to lock the steerable axle in the straight ahead position, where the locking mechanism includes an over-center linkage arrangement.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns aircraft landing gear. More particularly, but not exclusively, this invention concerns multi-wheel landing gears comprising at least one steerable axle. The invention also concerns a locking mechanism for a multi-wheel landing gear bogie including a steerable axle. 
     Multi-wheel landing gears are often used on large aircraft, for example the Airbus A380. The multi-wheel landing gears are used to support the large loads exerted on the landing gears by the aircraft when taxiing, at take-off, and landing. In order to prevent excess tyre wear caused by tyre scrubbing during taxiing, the multi-wheel landing gear may include at least one pair of wheels mounted on a steerable axle. When not taxiing, and during take-off and landing, it is necessary to lock the steerable axle in the straight ahead position. An example of such a locking system is disclosed in US2010/0078517. US2010/0078517 discloses a spring loaded, wedge shaped, locking mechanism that is arranged to engage with a receiving portion formed in the steerable axle. 
       FIGS. 1 and 2  show a landing gear arrangement according to the prior art. As can be seen, the steerable axle  10  is pivotally mounted to the landing gear bogie  12 . The steerable axle  10  is steerable by an actuator  14  which may extend and retract in order to change the orientation of the steerable axle. The landing gear arrangement includes a locking mechanism that comprises a wedge-shaped locking member  16  that is pivotally mounted to the landing gear bogie  12 . The wedge-shaped locking member  16  is arranged to be received by a receiving portion  18  of the steerable axle  10  in order to lock the steerable axle  10  in a ‘straight ahead’ position for taxi, take off, and landing. The wedge-shaped locking member is secured in the locking position by two springs  20  that act to pull the wedge-shaped locking member into the receiving portion. An actuator  22  is arranged to act against the springs  20  in order to move the wedge-shaped locking member from being received within the receiving portion  18 , such that the steerable axle  10  is unlocked and pivotal movement, away from the straight ahead position, is allowed. 
     However, such an arrangement is reliant on the strength of the springs to maintain the steerable axle in a lock configuration, and as the springs contract into the locked position, the force they exert on the locking member decreases. Additionally, if the locking member is not precisely lined up with the receiving portion when attempting to lock the steerable axle, the friction created between the locking member and the receiving section may be too great for the springs acting on the locking member to overcome, such that the steering axle is not fully locked. If the steering axle is not fully locked, the aircraft is not able to take off until the problem is fixed, which may cause delays to the aircraft departure time or the grounding of the aircraft. 
     The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved landing gear mechanism. 
     SUMMARY OF THE INVENTION 
     The present invention provides an aircraft landing gear comprising a bogie; 
     at least one steerable axle pivotally mounted to the bogie; and 
     a locking mechanism; 
     wherein the locking mechanism comprises a first locking member arranged to be movable between a locked position in which the first locking mechanism is engaged with the steerable axle such that pivotal movement of the steerable axle is restricted, and an unlocked position in which pivotal movement of the steering axle is allowed; 
     and an over-centre linkage arrangement arranged to move the first locking member between the locked position and unlocked position. 
     Advantageously, providing an embodiment of the invention with an over-centre linkage arrangement provides a locking mechanism that cannot be back-driven when in the locked position. 
     The over-centre linkage arrangement may comprise a spring, the spring arranged to bias the over-centre linkage to move the first locking member to the locked position. Advantageously, according to an embodiment of the invention, the over-centre linkage arrangement magnifies the return forces of the spring. The over-centre linkage arrangement may comprise an actuator arranged to drive the movement of the over-centre linkage arrangement to move the first locking member between the locked position and unlocked position. The actuator may be arranged to act against the bias of the springs in order to drive the over-centre linkage arrangement such that the first locking member is moved to the unlock position. The actuator may be a pneumatic, hydraulic, or electric actuator. The actuator may be arranged such that when the actuator is unpowered, the spring acts to move the first locking member into the locked position. Advantageously, such an embodiment according to the invention provides a mechanical lock even when power is unavailable. 
     Preferably, the over-centre linkage arrangement is in geometric lock when the first locking member is in the locked position. Preferably, the over-centre linkage arrangement includes an amount of “lost motion” such that the linkage is driven to break the geometric lock with no immediate movement of the first locking member. 
     The locking mechanism may include a sensor or plurality of sensors. A sensor may be arranged to monitor the condition of the locking mechanism, in particular whether the first locking member is in a locked or unlocked state. A sensor may be arranged to monitor the condition of the over-centre linkage arrangement. A sensor may be arranged to provide an indication of whether the over-centre linkage arrangement is in geometric lock. Advantageously, according to an embodiment of the invention, in order to move the first locking member from a locked position to an unlocked position, the over-centre linkage arrangement is arranged such that the over-centre linkage arrangement must move a greater distance than first locking member is moved between the locked position and unlocked position. A sensor may be arranged to indicate that the over-centre linkage arrangement has moved from a geometric lock position before the movement of the over-centre linkage arrangement has affected the position of the first locking member. Such an arrangement may advantageously indicate that the locking mechanism is in an ‘unsafe’ position before the first locking member is actually out the locked position. 
     According to an embodiment of the invention, the first locking member may be a wedge-shaped member. The first locking member may be arranged to be received in a receiving portion of the steerable axle such that pivotal movement of the steerable axle is restricted or prevented. The first locking member may be pivotally connected to the bogie and move between the locked position and unlocked position by movement about the pivot. 
     An embodiment of the invention also provides a method of locking a landing gear, the landing gear substantially as described above, wherein a first locking member is moved from an unlocked position to a locked position by an over-centre linkage arrangement moving into geometric lock. 
     An embodiment of the invention also provides a method of unlocking a landing gear, the landing gear substantially as described above, wherein a first locking member is moved from a locked position to an unlocked position by an over-centre linkage arrangement moving out of geometric lock. 
     A further embodiment of the invention provide a kit of parts suitable for retrofitting to an aircraft landing gear, the kit of parts comprising an over-centre linkage arrangement, such that the retrofit produces a landing gear as described above. 
     A further embodiment of the invention provides an aircraft comprising a landing gear as described above. 
     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. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG. 1  shows a schematic view of a landing gear steered axle arrangement according to the prior art; 
         FIG. 2  shows a side view of a landing gear steered axle arrangement according to the prior art; 
         FIG. 3  shows a side view of a landing gear steered axle according to a first embodiment of the invention, the landing gear being in the fully locked position; 
         FIG. 4  shows a side view of a landing gear steered axle according to the first embodiment of the invention, the landing gear with the over-centre linkage arrangement partially broken but with the first locking member remaining in the locked position—only the locking mechanism is shown for clarity; 
         FIG. 5  shows a side view of a landing gear steered axle according to the first embodiment of the invention, the landing gear shown in the unlocked position—as with  FIG. 4 , only the locking mechanism is shown for clarity; 
         FIG. 6  shows a side view of a landing gear steered axle according to the first embodiment of the invention, the landing gear shown in the returning to lock position—as with  FIGS. 4 and 5 , only the locking mechanism is shown for clarity; and 
         FIG. 7  shows an aircraft which includes a landing gear steered axle according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  shows a partial view of a landing gear according to a first embodiment of the invention, the view corresponding to that shown in  FIG. 2  for the prior art landing gear. Other than the locking arrangement which will be described below, the landing gear arrangement, for example the steering actuator, is substantially as described for the prior art. 
       FIG. 3  shows a landing gear arrangement  100  comprising a landing gear bogie  102  and a steerable axle  104  pivotally connected to the bogie  102  at pivot point  106 . The landing gear arrangement also comprises a first locking member  108  that is pivotally mounted to the landing gear bogie  102  and arranged to be received within a receiving portion  110  of the steerable axle  104  such that it restricts or prevents pivotal movement of the steerable axle  104  and locks the steerable axle  104  in the straight ahead position. The first locking member  108  is associated with an over-centre linkage arrangement  112 . The over-centre linkage arrangement  112  is arranged such that it may actuate the first locking member  108  about the pivotal connection between the landing gear bogie  102  and the first locking member  108 , moving the first locking member  108  from the locked position, as described above, and shown in  FIG. 3 , and an unlocked position as shown in  FIG. 5 . In the unlocked position as shown in  FIG. 5 , the first locking member  108  is no longer received within the receiving portion  110  and does not prevent the pivotal movement of the steerable axle  104 . The over-centre linkage arrangement  112  comprises a bell crank  114 , a short link  116 , and a long link  118 . The bell crank  114  is pivotally mounted at pivot B to the first locking member  108  and rotation of the bell crank about the pivot B either pulls the first locking member  108  towards the receiving portion  110 , or pushes the first locking member  108  away from the receiving portion  110 . The bell crank  114  is connected to an actuator  120  at pivot E, and the actuator is arranged to rotate the bell crank about pivot B by extension or contraction. The actuator  120  is connected to the landing gear bogie at pivot point A. The actuator  120  may be a hydraulic actuator or pneumatic actuator. Alternatively, the actuator may be electrically powered. 
     The short link  116  is pivotally mounted to the bell crank  114  and the long link  118 , and the first locking member  108 . The pivot C indicates the connection between the short link  116  and the long link  118 . The short link  116  is also connected to a spring  122  at point D. The long link  118  is also pivotally mounted to the landing gear bogie at pivot point A as indicated in the figures. It should be noted that the figures show only one side of the landing gear locking mechanism. In order to provide an independent dual load path, and to equalise the forces on the first locking member  108 , the landing gear comprises two over-centre linkage and spring arrangements, one each side of the first locking member  108 . However, as the arrangements are symmetrical, only one side will be described. 
     As can be seen in  FIG. 3 , when the first locking member  108  is in the locked position, the three pivots, A, B, and C of the over-centre linkage arrangement  112  are in an approximately straight line, such that the over-centre linkage arrangement is in a geometric lock. The spring  122  acts on the short link  116  and biases the over-centre linkage arrangement towards the geometric lock. When the over-centre linkage arrangement is in the geometric lock position, the first locking member  108  is securely located within the receiving portion  110  and the geometric lock of the over-centre linkage arrangement means that the first locking member cannot be back driven without the geometric lock of the over-centre linkage arrangement being deliberately broken. The actuator  120  may be locked in position, may be driven to pull backwards and act to assist the spring  122  in keeping the over-centre linkage arrangement in place, or may be unpowered, with the security of the over-centre linkage arrangement depending only on the spring  122  and the geometric lock configuration. 
     In order to move to the first locking member  108  to the unlocked position, the actuator  120  is powered to drive the bell crank  114  to rotate about the pivot B and push the first locking member  108  out of engagement with the receiving portion  110 . The bell crank  114  is designed to rotate a small angle breaking the geometric lock of the over-centre linkage arrangement  112  prior to coming into contact with the first locking member  108 . This can be considered “lost motion” and  FIG. 4  shows the bell crank  114  as it comes into contact with the first locking member  108 . It can be seen in  FIG. 4  that the first locking member is still in the locked position, though the over-centre linkage arrangement is no longer in geometric lock as shown in  FIG. 3 . A sensor  109  may be mounted to detect the lost motion and indicate that the landing gear is not fully locked (i.e. the first locking member  108  in the locked position and the over-centre linkage arrangement in geometric lock, as shown in  FIG. 3 ), while the first locking member remains in a safe, locked position. The sensor  109  may be located to monitor the movement of the short link  116  in order to provide this function. The sensor  109  may be arranged to communicate with the cockpit of the aircraft to which the landing gear is attached. This is desirable to increase the safety of the landing gear arrangement and comply with aircraft safety regulations. 
     As the actuator  120  continues to drive the bell crank  114  the first locking member  108  is pushed out of engagement with the receiving portion  110 , thus moving the first locking member  108  into the unlocked position, thereby allowing pivotal movement of the steerable axle  104 . In order to return the first locking member  108  to the locked position, thereby preventing pivotal movement of the steerable axle  104 , the actuator is reversed so that it pulls the first locking member  108  towards the receiving portion  110  as shown in  FIG. 6 . The over-centre linkage arrangement  112  returns to the over-centre position, with the over-centre linkage arrangement amplifying the force exerted by the spring  122  on the first locking member  108 . The spring  122  is arranged to constantly act on the short link  116 , attempting to move the short link  116  back to the locked position. Therefore, in order for the first locking member  108  to remain in the unlocked position, the actuator  120  must constantly be powered to act against the spring  122 . If the actuator becomes unpowered, the spring  122  acts to return the over-centre linkage arrangement  112  to the geometric lock position and return the first locking member  108  to the locked position. The amplification of the spring force by the over-centre linkage arrangement may act to locate the first locking member  108  in the receiving portion  110  even when the steerable axle  104  is not in the straight ahead position. The receiving portion  110  and/or the first locking member  108  may include angled side portions in order to assist with the location of the first locking member  108  into the receiving portion  110  when the steerable axle  104  is not in the straight ahead position. 
     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. 
     The steerable axle may be the front axle on a multi-axle landing gear, or the rear axle on a multi-axle landing gear. A landing gear may comprise a plurality of steerable axles and locking mechanisms as described above. 
     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.