Patent Publication Number: US-9405312-B2

Title: Active control column with manually activated reversion to passive control column

Description:
FIELD OF THE INVENTION 
     This invention generally relates to control columns for aircrafts and more particularly fly-by-wire control columns for aircrafts. 
     BACKGROUND OF THE INVENTION 
     As the performance requirements of both civil and military aircraft increases, conventional control technologies using mechanical linkages cannot relieve the pilot from higher mental and manual control activity. As such, today&#39;s high performance aircraft as well as some transport aircraft use “fly-by-wire” sidesticks and center sticks also referred to as “control columns”. 
     These fly-by-wire control columns simulate tactile feedback relating to the control surfaces of the aircraft to the control columns. 
     In a “passive” control column, the pilot feels spring or damper forces according to the applied deflection of a stick of the control column relative to mechanical ground. The deflection of the stick is the control input from the pilot to a flight control computer (FCC) relating to the desired pitch and/or roll The tactile forces are realized by a spring and damper package operably acting on the stick. In such a passive control column, the pilot&#39;s controller forces (i.e. tactile feel) are usually fixed. 
     A drawback of this passive control concept, as opposed to conventional controllers, is that the pilot loses the contact with the control surfaces of the aircraft and loses contact with the second pilot in the cockpit. As such, the pilot loses tactile information and can only use visual cues to inform him about the actual flight state and available trim control power as well as what the other pilot is doing. Further drawbacks relate to the fact that the feedback profile cannot be adjusted to compensate for other changes in the flight state of the aircraft or control surfaces such as due to changes in, for example, altitude, weather, or mechanical failures of the control surfaces. 
     In a “direct drive active” control column, the pilot experiences a simulated control force through the use of elaborate servo systems alone. In the direct drive active control system, a motor, drive electronics, and a high bandwidth closed loop force and damping control algorithm are used to provide the tactile feedback directly to the stick simulating the tactile feedback of the control surfaces of aircraft. By using this high bandwidth system, the system is expensive and bulky due to the increased number of sensors, and the complexity of the control system. Further, it is contemplated that in these direct drive active systems, that if the motor fails or locks-up, the stick can become locked thereby preventing the pilot from controlling the aircraft. Alternatively, if the control electronics fail, no resistance may act against manipulation of the stick such that the pilots cannot properly or easily manipulate the deflection of the sticks to provide the desired inputs to the FCC. To correct for this, unnecessary redundancy must be built into the system to avoid the mechanical and/or electrical failures. 
     It is desired to provide an adjustable tactile feedback system for a control column that incorporates the benefits of an active system but that can be transitioned to a passive system in the event of failure in the active portion of the control column. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the invention provides a control column that has improved abilities to manually convert the control column from an active control column into a passive control column that provides tactile feedback in the event of failure of an active portion of the control column. In a more particular embodiment, the control column can be reverted from an active indirect drive control column to a fully passive control column. 
     In one embodiment, a control column for an aircraft including a passive feedback arrangement, a stick and a ground lock mechanism is provided. The passive feedback arrangement is moveable relative to a mechanical ground to adjust a feedback profile provided to the stick. The stick is movable relative to the mechanical ground and the passive feedback arrangement. The ground lock mechanism has a locked state in which the passive feedback arrangement is maintained in a fixed position relative to the mechanical ground. The ground lock mechanism also has a normal state in which the passive feedback arrangement is permitted to move relative to the mechanical ground. 
     In one embodiment, the control column further includes an actuator operably coupled to the passive feedback arrangement when the ground lock mechanism is in the normal state to move the passive feedback arrangement relative to the mechanical ground. 
     Additionally, the ground lock mechanism is not an actuator configured to move the passive feedback arrangement relative to the mechanical ground during normal operation. 
     In a further embodiment, the actuator is operably de-coupled from the passive feedback arrangement when the ground lock mechanism is in the locked state such that the actuator does not inhibit movement of the passive feedback arrangement relative to the mechanical ground. 
     In a further embodiment, the ground lock mechanism further includes an intermediate transition state in which the passive feedback arrangement is permitted to move relative to the mechanical ground and the actuator is operably de-coupled from the passive feedback arrangement such that the passive feedback arrangement can be moved relative to the mechanical ground independent of the actuator. This allows the passive feedback arrangement to be moved relative to the mechanical ground in the event of mechanical failure of the actuator and the actuator is locked. Thus, if such mechanical failure occurs, the ground lock mechanism can be moved to the locked state. 
     In one embodiment, the ground lock mechanism and the passive feedback arrangement include a catch arrangement therebetween. In the normal state and the intermediate state the catch arrangement is decoupled such that the ground lock mechanism does not inhibit movement of the passive feedback arrangement relative to mechanical ground and in the locked state the catch arrangement is coupled such that the ground lock mechanism fixes the position of the feedback arrangement relative to the mechanical ground. 
     In one embodiment, the catch arrangement includes a lock member and a cooperating receiving member that includes a receiving cavity. The lock member is maintained out of the receiving cavity in the normal and intermediate states. The lock member is inserted into the receiving cavity in the locked state fixing the position of the passive feedback arrangement relative to the mechanical ground. 
     In one embodiment, a biasing mechanism acts on the locking member. The biasing member biases the locking member toward the receiving member. A blocking mechanism prevents the biasing mechanism from biasing the locking member into the receiving cavity in the normal state. Manipulation of the blocking member allows the biasing member to bias the locking member into the receiving cavity in the locked state. 
     In one embodiment, the receiving member includes an abutment surface surrounding the receiving cavity. In the intermediate state, the locking member is biased against the abutment surface. The locking member slides along the abutment surface as the ground lock mechanism transitions from the intermediate state to the locked state. 
     In one embodiment, the stick is indirectly coupled to the actuator through the passive feedback arrangement such that the stick is permitted to move relative to the actuator and/or the mechanical ground through the passive feedback arrangement in both the normal and locked states. 
     In one embodiment, the passive feedback arrangement includes a cam providing a cam surface and a resistance arrangement acting on the cam. The stick includes a cam follower that interacts with the cam surface. Movement of the cam follower along the cam surface varies the biasing force applied to the stick via the resistance arrangement. 
     In one embodiment, the passive feedback arrangement includes a gimbal arrangement that is movable relative to mechanical ground. The stick and gimbal arrangement are pivotable about a common axis. The cam member is carried by the gimbal arrangement and is movable relative thereto. The resistance arrangement acts between the gimbal arrangement and the cam member when the stick is moved along the cam surface. 
     In one embodiment, the passive feedback assembly is fixed at a ground neutral position when the ground lock mechanism is in the locked state. However, the stick remains movable relative to the mechanical ground as well as a feedback neutral position of the passive feedback mechanism. 
     In one embodiment, the passive feedback assembly is displaced from the ground neutral position when the ground lock mechanism is in the intermediate state. 
     Embodiments of the present invention provide methods of converting, typically manually, a control column from being an active control column to a passive control column that provides, typically only, passive feedback to a stick of the control column. The method includes transitioning a ground lock mechanism of the control column between a normal state in which a passive feedback arrangement is permitted to move relative to a mechanical ground and a locked state in which the passive feedback arrangement is maintained in a fixed position relative to the mechanical ground. The stick is permitted to move relative to the mechanical ground and the passive feedback arrangement in both the normal and the locked states through the passive feedback arrangement. 
     In a particular method, the method further comprises the steps of decoupling an actuator, which is operably coupled to the passive feedback arrangement when the ground lock mechanism is in the normal state to move the passive feedback arrangement relative to the mechanical ground, from the passive feedback arrangement such that the passive feedback arrangement can be transitioned relative to the mechanical ground independent of the actuator. 
     In one embodiment, the method further comprises the step of engaging a catch arrangement provided by the feedback arrangement and the ground lock mechanism to transition the ground lock mechanism into the locked state. 
     In one embodiment, the catch arrangement includes a first catch member and a second catch member. The step of transitioning the ground lock mechanism between the normal and locked states includes an intermediate state wherein the actuator is de-coupled from the actuator and the passive feedback arrangement is permitted to move relative to the mechanical ground. 
     In another embodiment, the first and second catch members abut one another in the intermediate state. The method further comprises the step of sliding the first and second catch members relative to one another in abutted contact until the catch arrangement is engaged and the ground lock mechanism is in the locked state. 
     In one embodiment, the step of engaging the catch arrangement includes inserting a lock member into a receiving cavity of a receiving member. When the lock member is inserted into the receiving cavity, the lock member prevents movement of the passive feedback arrangement relative to the mechanical ground. 
     In another embodiment, the method further includes the step of biasing the lock member toward the receiving cavity with a biasing member and blocking movement of the lock member toward the receiving cavity using a blocking member when in the normal state and unblocking movement of the lock member toward the receiving cavity by transitioning the block member to a new state when in the locked state. 
     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a schematic representation of a control system including a pair of control columns according to an embodiment of the present invention with the control columns in a normal state where both passive and active feedback is provided to the sticks of the columns; and 
         FIG. 2  is a schematic representation of the control system of  FIG. 1  with one of the control columns in a locked state such that it provides only passive feedback to the stick of the control column. 
     
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a simplified schematic illustration of an aircraft control system  100  for controlling pitch, roll or both pitch and roll of an aircraft. The aircraft control system  100  generally includes first and second control columns  102 ,  104  (referred to generically as “control columns  102 ,  104 ”). The control columns  102 ,  104  are used by the pilots (e.g. pilot and co-pilot) to control various operation of the aircraft such as pitch, roll and/or pitch and roll. 
     The control columns  102 ,  104  are considered fly-by-wire control columns because the manipulation of the control columns to adjust the pitch and/or roll of the aircraft is not translated directly to the control surfaces of the aircraft by mechanical devices. Instead, the deviations of the control columns from neutral positions are sensed and then converted into electrical signals. These signals are then sent to actuators which use the electrical signals to make proportional changes in the control surfaces of the aircraft. 
     Because the control columns  102 ,  104  are not mechanically linked to the control surfaces, the control system  100  incorporates tactile feedback that is applied to the control columns  102 ,  104  to simulate the feeling that a pilot would get if the control columns  102 ,  104  were in fact mechanically coupled to the control surfaces. For instance, if the pilots request a large degree of pitch or roll, the tactile feedback would increase the amount of force the pilots would have to apply to the control columns to implement that change in the control surfaces. As such, a large degree of deviation in the current control of the aircraft would be executed by applying a large force to the corresponding control column by the pilots. 
     The control columns  102 ,  104  generally include first and second sticks  108 ,  110  (i.e. pilot and copilot sticks) with which the pilots input control signals relating to desired pitch and/or roll. The first and second sticks  108 ,  110  interact with first and second feedback assemblies  112 ,  114  to provide tactile feedback. The columns  102 ,  104  are coupled to an electronic control arrangement  106  that controls the dynamic adjustments of the feedback assemblies  112 ,  114 . 
     Each feedback assembly  112 ,  114  provides the tactile feedback to its corresponding stick  108 ,  110 . In some embodiments, this tactile feedback has two components, a passive component and an active component. 
     Typically, the passive component, i.e. a first portion of the tactile feedback, relates to the flight state, i.e. the amount of pitch or roll the pilot is requesting due to the amount of stick deflection from a neutral position. In one embodiment, the active component, i.e. the second portion of the tactile feedback, relates to conflicts between the two different control columns  102 ,  104 . More particularly, the feedback assemblies  112 ,  114  provide tactile feedback when the two sticks  108 ,  110  are not at the same position relative to a mechanical ground, i.e. the pilots are providing conflicting control commands to the aircraft. However, other systems could provide active tactile feedback based on other inputs such as changes in characteristics of the aircraft such as changes in altitude, icing of the control surfaces, failure or reduced functionality of the actuators controlling the control surfaces, etc. 
     The control columns  102 ,  104  of this embodiment are substantially identical. Stick  108  generally includes a first grip portion  116  and stick  110  includes a second grip portion  118 . The pilots manually manipulate the grip portions  116 ,  118  to control the desired amount of pitch and/or roll. Grip portion  116  is operably coupled to a first connecting rod  120  and grip portion  118  is operably coupled to second connecting rod  122 . The connecting rods  120 ,  122  are operably coupled to or include one of first and second cam followers  124 ,  126 , respectively (illustrated as rollers in the present embodiment). The cam followers  124 ,  126  interact with the corresponding feedback assembly  112 ,  114  to provide a variable tactile feedback profile to the sticks  108 ,  110 . 
     The sticks  108 ,  110  pivot about a corresponding one of a first or a second common pivot point  128 ,  130  relative to a corresponding one of a first and a second ground neutral position  132 ,  134 . The angular displacement of the sticks  108 ,  110  relative to the corresponding ground neutral position  132 ,  134  is proportional to the amount of pitch or roll that the pilot is requesting, i.e. proportional to the amount of change in the position of the corresponding control surfaces of the aircraft. 
     In general, the feedback assemblies  112 ,  114  provide tactile feedback to the pilot by providing resistance to the movement of the sticks  108 ,  110  from the ground neutral position  132 ,  134 . In one embodiment, the feedback assemblies  112 ,  114  are indirect drive active feedback assemblies. This allows the system to provide both the active and passive feedback. The feedback assemblies  112 ,  114  utilize passive feedback as the first form of tactile feedback, which, as mentioned above, typically, relates to the control state of the sticks  108 ,  110 . This relates to the amount of pitch and/or roll requested and simulates attachment to the control surfaces of the aircraft. This passive feedback is provided by resistance arrangements  136 ,  138  (i.e. a spring and damper package) that opposes the rotational movement of stick  108 ,  110  from a feedback neutral position by using one or more springs and/or dampers or other biasing devices. 
     In typical embodiments, the resistance profile of the resistance arrangement increases the greater the amount of angular displacement or deflection of the sticks  108 ,  110  from the feedback neutral position, which happens to be neutral position  132 ,  134  in the illustrated embodiment. This resistance provides feedback to the pilot such that when the pilot requests a certain amount of pitch or roll, the pilots muscle memory will tend to apply a certain amount of pushing or pulling force to overcome the force of the springs and dampers of the resistance arrangements  136 ,  138 . Thus, the pilots will “learn” how much force is needed for control of the aircraft, i.e. how much force is used to adjust the position of the sticks  108 ,  110  relative to ground neutral  132 ,  134  for a given amount of pitch and/or roll. 
     The feedback assemblies  112 ,  114 , in the illustrated embodiment, include a profiled first or second cam  144 ,  146  that has first and second V-shaped cam surfaces  148 ,  150 , respectively, with which the cam followers  124 ,  126  interact. As the cam followers  124 ,  126  transition away from the center, i.e. bottom of the “V”, of the cam surfaces  148 ,  150 , the resistance arrangements  136 ,  138  increase the angular force applied to the corresponding stick  108 ,  110  to provide tactile feedback to the pilot. 
     The center point of cam surfaces  148 ,  150  can also be referred to as a “feedback neutral position” or a “gimbal neutral position”, because in this position, no rotational force is being applied to the sticks  108 ,  110  by the feedback assemblies  112 ,  114 . In one embodiment, in the feedback neutral position, the cam followers  124 ,  126  will contact both sides of the corresponding V-shaped cam surface  148 ,  150 , such that no rotational force is applied to the sticks  108 ,  110  by the feedback assemblies  112 ,  114 . In  FIG. 1 , the feedback neutral position is illustrated as being substantially aligned with the ground neutral positions  132 ,  134 . 
     The first and second cams  144 ,  146  in combination with the first and second resistance arrangements  136 ,  138  can be referred to as passive centering mechanisms as the forces generated thereby attempt to always drive the sticks  108 ,  110  toward the center of the cams  144 ,  146 , which correspond to the feedback neutral positions. 
     In some embodiments, the aircraft control system  100  is also configured to provide active tactile feedback to the pilots when there is a discrepancy of the control input between the two different sticks  108 ,  110 . A discrepancy occurs when one pilot is trying to request a different degree of pitch and/or roll than the other pilot. This can be represented using the second form of tactile feedback identified above, the active tactile feedback. 
     In one embodiment, the feedback assemblies  112 ,  114  are configured to attempt to maintain the first and second sticks  108 ,  110  in a same position relative to mechanical ground  159  when one pilot&#39;s actions cause a discrepancy in position between the two sticks  108 ,  110 . 
     To provide active tactile feedback to one stick  108 ,  110  relating to the operation of the other stick  110 ,  108 , the feedback assemblies  112 ,  114  include one of movable first and second gimbals  152 ,  154  that are driven by a corresponding one of first and second actuators  156 ,  158  to adjust the position of first and second cams  144 ,  146  relative to the mechanical ground  159 . The adjustment of the position of the cams  144 ,  146  relative to mechanical ground  159  actively adjusts the force feedback profile relative to mechanical ground  159 . Thus, different force can be applied to the corresponding sticks  108 ,  110  by the corresponding feedback assembly  108 ,  110  when the sticks  108   110  are moved relative to mechanical ground. 
     In the illustrated embodiment, actuators  156 ,  158  are illustrated as linear actuators pivotally coupled to the mechanical ground  159  and pivotally coupled to gimbals  152 ,  154 . However, other actuators could be used such as rotary actuators positioned, for example, at pivot points  128 ,  130  or motors having gears that act on corresponding gearing of gimbals  152 ,  154 . Other types of drive mechanisms could be used for adjusting the position of the gimbals  152 ,  154  relative to mechanical ground  159 . 
     Further, because the passive feedback portion, i.e. the resistance arrangements  136 ,  138 , corresponding gimbals  152 ,  154 , cams  144 ,  146  are interposed between the actuators  156 ,  158  and sticks  108 ,  110 , this provides an indirect drive because the actuators  156 ,  158  are not directly coupled to the sticks  108 ,  110 . Thus, the sticks  108 ,  110  may move, to at least some degree, independent of the actuators  156 ,  158 . Thus, there is at least a limited or biased degree of freedom between the sticks  108 ,  110  and their corresponding feedback assemblies  112 ,  114 . As such, if the actuators  156 ,  158  were to lock up or to be controlled to a fixed state, the sticks  108 ,  110  are still be able to move relative to mechanical ground  159  allowing the pilots to still make adjustments in the control state of the aircraft. 
     Gimbals  152 ,  154  are rotationally mounted to the mechanical ground  159  for rotation about first and second common pivot points  128 ,  130 , respectively. As such, the stick  108 ,  110  and the gimbal  152 ,  154  of a given control column  102 ,  104  are permitted to rotate about a corresponding common axis provided by the respective common pivot point  128 ,  130 . 
     In the illustrated embodiment, the gimbals  152 ,  154  include gimbal frames  160 ,  162 . The first and second cams  144 ,  146  are movably carried by gimbal frames  160 ,  162 . In the illustrated embodiment, the first and second cams  144 ,  146  include cam connecting arms  164 ,  166  that pivotally connect to first and second gimbal frame arms  168 ,  170 . The first and second cams  144 ,  146  and first and second gimbal frames  160 ,  162  rotate relative to one another through the pivotal connections  172 ,  174  therebetween to adjust the amount of force being applied to the first and second sticks  108 ,  110  due to adjustments in the compression or expansion of the biasing mechanisms within resistance arrangements  136 ,  138 . 
     However, other means for allowing the cams  144 ,  146  to move relative to gimbal frames  160 ,  162  could be provided. For instance, the cams  144 ,  146  could be free floating and merely attached to the end of the resistance arrangements  136 ,  138 . Alternatively, the cams  144 ,  146  could be mounted for linear sliding along the gimbal frames  160 ,  162 . 
     The resistance arrangements  136 ,  138  provide dampers  174 ,  176  that add damping to the system. In the illustrated embodiment, the resistance arrangements  136 ,  138  and particularly the dampers thereof are interposed between the sticks  108 ,  110  and the gimbals  152 ,  154 . While other embodiments could interpose portions of the resistance arrangements  136 ,  138 , and particularly the dampers  174 ,  176 , between the mechanical ground  159  and the sticks  108 ,  110 , this embodiment does not do so as it provides the added benefit of isolating the effect of the dampers  174 ,  176  from actuators  156 ,  158 . As such, in this embodiment, the resistance arrangements  136 ,  138  and particularly the dampers  174 ,  176  thereof do not work against actuators  156 ,  158  as the actuators  156 ,  158  adjust the position of the gimbals  152 ,  154  relative to mechanical ground  159 . 
     By placing the resistance arrangement between the sticks  108 ,  110  and the gimbals  152 ,  154 , the actuators  156 ,  158  drive the sticks  108 ,  110  through the passive feedback portions of the feedback assemblies  112 ,  114 , but, absent pilot input, the resistance arrangements  136 ,  138  and particularly dampers  174 ,  176  thereof do not oppose the motion of the actuators  156 ,  158 . 
     The dampers  174 ,  176  could be rotary fluidic damping modules. Alternatively, they could be linear style fluid dampers. Other dampers, such as electronic dampers could also be incorporated. 
     More particular control of the positioning of the cams  144 ,  146  relative to mechanical ground is described in co-pending application assigned to the assignee of the instant application, entitled Position Control System for Cross Coupled Operation of Fly-By-Wire Control Columns, application Ser. No. 12/844,867 filed on Jul. 28, 2010, the teachings and disclosure of which are incorporated herein by reference thereto. 
     In one embodiment, by actively adjusting the position of the gimbals  152 ,  154 , and consequently the corresponding cams  144 ,  146  thereof about the common pivot points  128 ,  130 , the resistance or feedback profile relative to neutral positions  132 ,  134  and mechanical ground  159  applied to the corresponding sticks  108 ,  110  is actively altered providing modified tactile feedback to the pilot. 
     This active adjustability can be used to indicate a discrepancy between the commands provided by the two sticks  108 ,  110 . This adjustability in the force profile can also be used to attempt to maintain the two sticks  108 ,  110  in a common location when one pilot inputs such a control discrepancy by providing a corrective force to the moved stick that compensates for the increased force applied by the pilot trying to deviate from the other stick. Further, this active adjustability in the resistance or feedback profile can also be used to simulate other changes in the aircraft such as changes in the control surfaces, changes or failures in the actuators that control the control surfaces, icing of the control surfaces, changes in altitude, etc. 
     The illustrated embodiment o  FIGS. 1 and 2  illustrate a further feature. The illustrated control columns  102 ,  104  are configured to be able to transition from providing active feedback and passive feedback to only providing passive feedback. As noted above to provide active feedback, the gimbals  152 ,  154  are moved relative to mechanical ground  159  to adjust the feedback profile provided by the cams  144 ,  146 . 
     In the fully passive configuration where only passive feedback is provided, a ground lock mechanism  180 ,  182  locks the position of the gimbal, and consequently feedback assemblies  112 ,  114  relative to mechanical ground  159 . 
     This allows passive resistance to be applied to the sticks  108 ,  110  in the event of failure in the position control of the gimbals  152 ,  154 . Typically, these systems are configured such that the mean time between failure is based on a determination that the electrical components will fail first. In such a situation, if the electrical components, such as control system  106  fails first, no power would be provided to actuators  156 ,  158  such that the gimbals  152 ,  154  will not be fixed relative to ground. Instead, movement of sticks  108 ,  110  will be substantially uninhibited as the actuators will provide substantially zero resistance to movement of the gimbals  152 ,  154 . With out the gimbals  152 ,  154  grounded, movement of the sticks  108 ,  110  will not be opposed by the resistance arrangements  136 ,  138  but will simply cause movement of the gimbals  152 ,  154  along with the sticks. As such, it will be very difficult of the pilots to control pith and/or roll because they will not have the “learned” resistance or tactile feel acting against their adjustments in position of the sticks  108 ,  110  relative to mechanical ground  159 . 
     Alternatively, if the actuators mechanically fail, the actuator could lock the position of the gimbals  152 ,  154  in an undesirable position. If the actuators  156 ,  158  stall, the ground lock mechanisms  180 ,  182  can be configured to operably decouple the actuators  156 ,  158  from the gimbals  152 ,  154  such that the gimbal  152 ,  154  coupled to the failed actuator  156 ,  158  can be transitioned to a desired location, typically where the feedback neutral position aligns with the ground neutral positions  132 ,  134 , and then the gimbal  152 ,  154  can be locked in that position. 
     In a normal state, the ground lock mechanisms  180 ,  182  are disengaged such that the gimbals  152 ,  154  are free to move relative to mechanical ground, such as under operation by actuators  156 ,  158 . However, upon failure, the ground lock mechanisms  180 ,  182  can both or independently be transitioned to a locked state in which the ground lock mechanisms  180 ,  182  lock the position of the gimbals  152 ,  154  relative to mechanical ground (see control column  102  in  FIG. 2 . 
     In the illustrated embodiments, the ground lock mechanisms  180 ,  182  and the gimbals  152 ,  154  include a catch arrangement  184 ,  186  that operably couples and decouples to transition between the locked state (see column  102  in  FIG. 2 ) and the normal state (see  FIG. 1 ). 
     The catch arrangements  184 ,  186  include first and second catch members illustrated in the forms of lock members  188 ,  190  and receiving members  192 ,  194  that include receiving cavities  196 ,  198 . In the locked state the corresponding lock members  188 ,  190  engages the corresponding receiving cavities  196 ,  198  of which ever column  102 ,  104  fails. When the lock members  188 ,  190  engage the receiving members  192 ,  194  the gimbals  152 ,  154  are grounded and prevented from moving relative to mechanical ground  159  no matter the operational state of actuators  156 ,  158 . 
     In the event of failure when the gimbals  152 ,  154  are offset from ground neutral positions  132 ,  134  or otherwise such that the locking members  188 ,  190  will not align with receiving cavities  196 ,  198 , the ground lock mechanisms  180 ,  182  are configured such that the actuators  156 ,  158  can be decoupled from the gimbals  152 ,  154 , in the event of failure. This allows the pilot to manually position the gimbals, by using the control sticks  108 ,  110 , to align the locking members  188 ,  190  with the receiving cavities  196 ,  198  even if the actuators  156 ,  158  have mechanically stalled. 
     This decoupling is illustrated by break  199  in  FIG. 2  for control column  102 . While the break is illustrated as completely disconnecting the actuators  156 , from gimbal  152 , other mechanisms could be implemented. For instance, the coupling between mechanical ground  159  and actuator  156  could be removed such that the actuator  156  itself could be moved relative to mechanical ground. However, this will be considered to be decoupling the actuator from the gimbal because this decouples the grounding effect that the motor has by grounding the gimbal to mechanical ground  159 . Alternatively, other means decoupling the gimbal  152  from actuator  156  could occur such that the drive shaft of the actuator can be disconnected from actuator  156 . As such, any means of removing the link between the gimbal and the mechanical ground through the actuator  156  is to be considered as “decoupling the actuator  156  from the gimbal  152 . 
     Once the actuator  156  is decoupled from the gimbal  152 , the pilot can manipulate the gimbal  152  to the ground neutral position by moving the stick  108 . This occurs because of the interaction of the cam follower  124  with the cam surface  148  of cam  144 . As the pilot deflects stick  108 , a load will be applied to cam  144  causing the gimbal  152  to transition as well. Once the receiving cavity  196  and the locking member  188  align, the two will engage locking the gimbal  152  in the desired position and only passive feedback will be available as provided by the resistance arrangement  136  and cam  144 . 
     In some embodiments, the ground lock mechanisms  180 ,  182  may have an intermediate state where the gimbals  152 ,  154  are free to move relative to mechanical ground  159  and they are decoupled from actuators  156 ,  158 . During this intermediate state, the gimbals  152 ,  154  are transitioned toward the locked state. In some embodiments, the catch members may be biased into one another. In such an intermediate state, in the illustrated embodiment, the lock members  188 ,  190  may abut against the receiving members  192 ,  194  and slide across abutment surfaces  202 ,  204  thereof. This will allow the pilot to move the sticks  108 ,  110  back and forth until the lock members  188 ,  190  actually get inserted into receiving cavities  196 ,  198 . 
     The ground lock mechanisms  180 ,  182  include biasing members  206 ,  208  configured to bias the two catch members towards one another, i.e. lock members  188 ,  190  and receiving members  192 ,  194  toward one another. In the illustrated embodiment the biasing members  206 ,  208  are illustrated in the form of coil springs in tension. However other springs or biasing members in tension or compression could be used. 
     To prevent undesirable engagement between the lock members  188 ,  190  and the receiving members  192 ,  194  during normal operation and to maintain the ground lock mechanism in the normal state, block members  210 ,  212  interfere with the movement of the receiving members  192 ,  194  toward the lock members  188 ,  190  via biasing members  206 ,  208 . 
     To transition the control columns  102 ,  104  between the active state and a fully passive state, the pilots can manually move the blocking members  210 ,  212  so that the receiving members  192 ,  194  are no longer prevented from moving towards locking members  188 ,  190  under load provided by biasing members  206 ,  208 . 
     As noted above, if the locking members  188 ,  190  are not aligned with receiving cavities  196 ,  198 , the biasing members will bias receiving members  192 ,  194  into abutment with locking members  188 ,  190 . The pilots deflect the sticks  108 ,  110  back and forth about common axes  128 ,  130  to align the components. The abutted components will slide relative to one another and when the locking members  188 ,  190  align with the receiving cavities  196 ,  198  the biasing members  206 ,  208  will cause the components to engage and lock the position of gimbals  152 ,  154 . Once engaged, the sticks  108 ,  110  will only experience passive feedback and the feedback profile cannot be adjusted. 
     While it is illustrated as the gimbals  152 ,  154  include the pin or peg like locking members  188 ,  190 , the catch arrangements  184 ,  186  could be switched such that the gimbals  152 ,  154  are the receiving members and include the receiving cavities. 
     Further, the receiving members  192 ,  194 ,  196  could be tapered proximate the receiving cavities  196 ,  198  so as to assist in biasing or directing the locking members  188 ,  190  into the receiving cavities  196 ,  198 . 
     Typically, the blocking members  210 ,  212  will be manually removed from to prevent the interference with the engagement between the catch members. This manual actuation may be provided by a push-push type cable arrangement. In one embodiment, the handle of the push-push cable can include indication devices for clearly illustrating to the pilot that the individual column  102 ,  104  has been transitioned into a fully passive state. This could be done by having the pilot rotate the handle of the push-push cable 90 degrees so that it has a different appearance. Also, this rotation could cause the handle to be aimed at an indicator that states that the column  102 ,  104  is in the fully passive mode. 
     While typically the blocking members  210 ,  212  are manually removed, the removal of blocking members  210 ,  212  could be automated. 
     Additionally, while the receiving members  192 ,  194 , i.e. members used to lock the gimbals  152 ,  154 , are illustrated as being pivotally coupled to mechanical ground  159 , they could be linearly slidable relative to mechanical ground. For instance, the movable mechanism of the ground lock mechanisms could be provided by a linearly movable pin that engages an receiving cavity formed in the gimbals  152 ,  154 . In a more particular embodiment, the biasing members could be a compression spring around the pins biasing the pin towards gimbals  152 ,  154 . In such an implementation, the blocking members have been contemplated to be c-clips that are generally fork shaped that partially extend around the pin and interfere with the action of the compression springs biasing of the pin. 
     All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.