Patent Publication Number: US-2019186512-A1

Title: Hydraulic system characteristics

Description:
TECHNICAL FIELD 
     The present invention relates to hydraulic systems of an aircraft. 
     BACKGROUND 
     An aircraft may comprise hydraulic systems for various purposes, such as for actuating various component of the aircraft, for example. The hydraulic systems of an aircraft may be intended to function in certain operating conditions including, for example, a temperature lower limit, a temperature range, a range of gas pressures inside the reservoir of the hydraulic system, etc. 
     SUMMARY 
     A first aspect of the present invention provides an apparatus for modifying characteristics of a hydraulic system of an aircraft. The apparatus comprises a heat providing element arranged to provide heat to the contents of a reservoir of the hydraulic system; and a processor arranged to control the operation of the heat providing element. 
     Optionally, the processor is arranged to control the heat providing element based on one or more characteristics of the hydraulic system as detected by one or more instruments associated with the hydraulic system. 
     Optionally, the one or more characteristics of the hydraulic system comprise a temperature as detected by a temperature sensor included in the reservoir. 
     Optionally, the processor is arranged to activate the heat providing element to provide heat if the temperature as detected by the temperature sensor included in the reservoir is lower than a temperature threshold value. 
     Optionally, the one or more characteristics of the hydraulic system comprise a gas pressure as detected by a gas pressure sensor included in the reservoir. 
     Optionally, the processor is arranged to activate the heat providing element to provide heat if the gas pressure as detected by a gas pressure sensor included in the reservoir is lower than a gas pressure threshold value. 
     Optionally, the one or more characteristics of the hydraulic system comprise a fluid pressure as detected by a fluid pressure sensor included in the reservoir. 
     Optionally, the processor is arranged to activate the heat providing element to provide heat if the fluid pressure as detected by the fluid pressure sensor included in the reservoir is lower than a fluid pressure threshold value. 
     Optionally, the processor is arranged to control the operation of the heat providing element in accordance with data indicating instructions input by a user. 
     Optionally, the processor is arranged to control the heat providing element based on an ambient characteristic as detected by an ambient characteristic instrument on the aircraft. 
     Optionally, the heat providing element is disabled or enabled based on a correspondence relationship between the value of a characteristic of the hydraulic system and a value of an ambient characteristic. 
     Optionally, the apparatus includes the reservoir. 
     Optionally, the heat providing element is integrated into the reservoir. 
     Optionally, the heat providing element comprises one or more heat generating parts arranged to generate heat to provide heat to the contents of the reservoir. 
     Optionally, the heat providing element is a heat directing element arranged to direct heat to the reservoir from a heat generating component of the hydraulic reservoir. 
     A second aspect of the present invention provides an aircraft comprising an apparatus according to the first aspect. 
     A third aspect of the present invention provides a method of controlling heat provision to a hydraulic system of an aircraft. The method comprises determining a value associated with a physical characteristic with respect to the aircraft; comparing the determined value with temperature control criteria; and activating a heat providing element arranged to provide heat to the contents of a reservoir of the hydraulic system on the basis of the comparison. 
     Optionally, the method comprises, when the heat providing element is activated, circulating hydraulic fluid within a part of the hydraulic system through an orifice using a pump. 
     Optionally, the physical characteristic is a temperature, a gas pressure or a fluid pressure within the reservoir of the hydraulic system. 
     Optionally, the method comprises repeatedly determining the value associated with the physical characteristic with respect to the aircraft; and controlling the operation of the heat providing element according to a feedback control scheme on the basis of the temperature control criteria and the repeatedly determined values. 
     A fourth aspect of the present invention provides a system for controlling one or more characteristics of a hydraulic system of an aircraft. The system comprises a heat providing element arranged to provide heat to the contents of a reservoir of the hydraulic system; one or more sensors arranged to indicate one or more characteristics within the hydraulic system; and a processor. The processor is arranged to: compare the one or more characteristics detected by the one or more sensors to a given operating condition; and operate the heat providing element on the basis of the comparison. 
     A fifth aspect of the present invention provides an apparatus arranged to control parameters of a hydraulic system of an aircraft. The apparatus comprises a temperature control component arranged to control the temperature of a reservoir of the hydraulic system by heating the reservoir. The temperature control component comprises a heater arranged to heat the reservoir; and a controller arranged to implement instructions to control the heater. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a schematic view of an aircraft; 
         FIG. 2  shows a schematic view of a system for use with a hydraulic system of the aircraft of  FIG. 1 ; 
         FIG. 3  shows a schematic view of a hydraulic system of the aircraft of  FIG. 1 ; and 
         FIG. 4  is a flow diagram showing a method of controlling heat provision to the hydraulic system of  FIG. 3 ; 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified schematic view of an aircraft  100 . The aircraft  100  comprises a hydraulic system  102 . The hydraulic system  102  may, for example, comprise linear hydraulic actuators, a hydraulic motor, a hydraulic generator, or other components which function using hydraulic pressure. The hydraulic system  102  of this example is for actuating various components of the aircraft  100 , such as landing gear, landing gear bay doors, cargo doors, etc. In this example, the hydraulic system  102  controls the nose landing gear  104  of the aircraft  100 , as well as the landing gear bay doors (not shown) of the aircraft  100 . 
     The aircraft  100  comprises a hydraulic characteristic control apparatus  106  (hereinafter “control apparatus”) for controlling one or more characteristics of the hydraulic system  102  of the aircraft  100 . For example, the control apparatus  106  may be for modifying the temperature within a reservoir of the hydraulic system  102 , the gas pressure in the reservoir of the hydraulic system  102 , and/or for modifying a fluid pressure in a reservoir of the hydraulic system  102 , etc. In other words, the control apparatus  106  is arranged to control parameters of the hydraulic system  102  of the aircraft  100 . 
     The aircraft  100  also comprises instruments  108 . The instruments  108  may, for example, comprise various ambient characteristic instruments for detecting ambient characteristics in the vicinity of the aircraft  100 . The instruments  108  may also comprise aircraft instruments for detecting characteristics of the aircraft  100 . An example of an ambient characteristic that may be detected is ambient temperature. Other examples include ambient humidity level, altitude, ambient air pressure, etc. The aircraft  100  also includes a computing system  110  which may comprise one or more processors and one or more computer readable storage media. The computing system  110  may control various functions of the aircraft  100 . For example, the computing system  110  may control components of the aircraft  100  on the basis of values of characteristics detected by the instruments  108 . 
     The control apparatus  106  is schematically illustrated in  FIG. 2 . The control apparatus  106  comprises a heat providing element  202  arranged to provide heat to the contents of a reservoir  204  of the hydraulic system  102 . The control apparatus  106  also comprises a processor  206  arranged to control the operation of the heat providing element  202 . The control apparatus  106  therefore comprises a temperature control component arranged to control the temperature of the reservoir  204 , which temperature control component comprises the heater  202  arranged to heat the reservoir  204 , and a controller  206  arranged to implement instructions to control the heater  202 . 
     The reservoir  204  contains a hydraulic fluid  208  and a gas  210 . In the example of  FIG. 2 , the hydraulic fluid  208  and the gas  210  within the reservoir  204  are separated by a piston  211  so that they do not mix. In other examples, the fluid  208  and the gas  210  may be separated by a diaphragm, bellows (e.g. welded metal bellows), or the like. The hydraulic fluid  208  is typically a liquid. It will be understood that hydraulic fluid  208  is circulated through the hydraulic system  102  in order for the hydraulic system  102  to perform its function (for example, actuate, in other words extend or retract, the nose landing gear  104 ). The hydraulic fluid  208  may be any fluid suitable for use in a hydraulic system, for example, the hydraulic fluid  208  may be an oil. Similarly, the gas  210  may be any gas suitable for use in a hydraulic system, for example, Nitrogen gas or Helium gas. The processor  206  of apparatus  106  may be part of the computing system  110  of the aircraft  100 . Alternatively, processor  206  may be provided separately to the computing system  110  of  FIG. 1 . 
     In some examples, the control apparatus  106  may include the reservoir  204 . For example, the control apparatus  106  may be provided along with reservoir  204  for use with a hydraulic system such as hydraulic system  102  of the aircraft  100 . In some examples, the heat providing element  202  may, for example, be integrated into the reservoir  204 . For example, the walls of the reservoir  204  may comprise regions which function as heat generating parts of the heat providing element  202  under the control of the processor  206 . In some examples, the heat providing element  202  may be provided inside the reservoir  204  so as to provide heat to the contents of the reservoir  204  more directly. 
     The control apparatus  106  may form part of a hydraulic characteristic control system  200  (hereinafter “control system”) for controlling one or more characteristics of the hydraulic system  102  of the aircraft  100  to maintain the hydraulic system  102  in accordance with a given operating condition. The given operating condition may comprise, for example, maximum or minimum values, or respective ranges of values of the one or more characteristics of the hydraulic system  102 , in accordance with which the hydraulic system  102  is intended to function correctly e.g. in accordance with which a fault condition of the hydraulic system  102  occurring is unlikely. For example, the given operating condition may comprise a requirement that the temperature within the reservoir  204  is above a temperature threshold, a requirement that the gas pressure of the gas  210  within the reservoir  204  is above a gas pressure threshold and/or a requirement that the fluid pressure of the hydraulic fluid  208  within the reservoir  204  being above a fluid pressure threshold. It will be understood that the temperature, gas pressure and fluid pressure within the reservoir are interdependent. Therefore, the hydraulic characteristic control system  200  may maintain the hydraulic system  102  in accordance with the given operating condition by controlling the temperature within the reservoir  204 , for example. 
     In addition to the hydraulic characteristic control apparatus  106 , the control system  200  may comprise one or more measuring instruments in the form of one or more sensors  212 ,  214 ,  216  arranged to indicate one or more characteristics within the hydraulic system  102 . The processor  206  in the control system  200  may be arranged to compare the one or more characteristics detected by the one or more sensors  212 ,  214 ,  216  to the given operating condition, and operate the heat providing element  202  on the basis of the comparison. Thus, the processor  206  may be arranged to control the heat providing element  202  based on one or more characteristics of the hydraulic system  102  as detected by the sensors  212 ,  214 ,  216 . 
     The one or more sensors  212 ,  214 ,  216  are included in the hydraulic system  102 . In the example of  FIG. 2 , the sensors  212 ,  214 ,  216  are included in the reservoir  204 . In some examples, the one or more sensors  212 ,  214 ,  216  may be positioned away from the reservoir  204  elsewhere in the hydraulic system  102 . The sensor  212  shown in  FIG. 2  may, for example, be a temperature sensor  212  included in the reservoir  204 . In some examples, the one or more characteristics of the hydraulic system  102  (based on which the processor  206  controls the heat providing element  202 ) may comprise the temperature as detected by the temperature sensor  212  included in the reservoir  204 . The temperature as detected by the temperature sensor  212  may be referred to as the reservoir temperature. 
     In the example of  FIG. 2 , the temperature sensor  212  is positioned at a part of the inside of the reservoir  204  submerged in hydraulic fluid  208 . In some examples, the temperature sensor  212  may be positioned at a part of the inside of the reservoir  204  filled with the gas  210  (i.e. not submerged in hydraulic fluid  208 ). The processor  206  may be arranged to activate the heat providing element  202  to provide heat if the temperature as detected by the temperature sensor  212  included in the reservoir  204  is lower than a temperature threshold value. 
     The sensor  214  shown in  FIG. 2  may, for example, be a gas pressure sensor  214  included in the reservoir  204 . In some examples, the one or more characteristics of the hydraulic system  102  (based on which the processor  206  controls the heat providing element  202 ) may comprise a gas pressure as detected by the gas pressure sensor  214  included in the reservoir  204 . The gas pressure as detected by the gas pressure sensor  214  may be referred to as the reservoir gas pressure. In this example, the gas pressure sensor  214  is positioned at a part of the inside of the reservoir  204  filled with the gas  210  (i.e. not submerged in hydraulic fluid  208 ). Alternatively, or in addition, to being arranged to activate the heat providing element  202  to provide heat if the reservoir temperature as detected by the temperature sensor  212  is lower than a temperature threshold value, the processor  206  may be arranged to activate the heat providing element  202  to provide heat if the reservoir gas pressure as detected by the gas pressure sensor  214  is lower than a gas pressure threshold value. 
     The sensor  216  shown in  FIG. 2  may, for example, be a fluid pressure sensor  216  included in the reservoir  204 . In some examples, the one or more characteristics of the hydraulic system  102  (based on which the processor  206  controls the heat providing element  202 ) comprise a fluid pressure as detected by the fluid pressure sensor  216  included in the reservoir  204 . The fluid pressure as detected by the gas pressure sensor  216  may be referred to as the reservoir fluid pressure. In this example, the fluid pressure sensor  216  is positioned at a part of the inside of the reservoir  204  submerged in hydraulic fluid  208 . Alternatively, or in addition, to being arranged to activate the heat providing element  202  responsive to the temperature detected by the temperature sensor  212  and/or the gas pressure detected by the gas pressure sensor  214 , the processor  206  may be arranged to activate the heat providing element  202  to provide heat if the reservoir fluid pressure as detected by the fluid pressure sensor  216  is lower than a fluid pressure threshold value. 
     As mentioned above, the processor  206  is arranged to control the operation of the heat providing element  202 . Also, as described above, the processor  206  may activate the heat providing element  202  based on the reservoir temperature as detected by the temperature sensor  212 , the reservoir gas pressure detected by the gas pressure sensor  214  and/or the reservoir fluid pressure detected by the fluid pressure sensor  216 . Since, as described above, the temperature, gas pressure and fluid pressure within the reservoir  204  are interdependent, the processor may activate the heat providing element  202  responsive to a comparison of any one or more of the detected respective values of these characteristics to their respective thresholds in order to maintain the hydraulic system  102  in accordance with the given operating condition. 
     The heat providing element  202  may comprise one or more heat generating parts arranged to generate heat in order to provide heat to the contents of the reservoir  204 . In the example of  FIG. 2 , the heat providing element  202  comprises a first heat generating part  202   a  and a second heat generating part  202   b  positioned on opposite sides of, and in contact with the reservoir  204  of the hydraulic system  102 . The first and second heat generating parts  202   a ,  202   b  are thus positioned so as to provide heat to the reservoir  204  and therefore to provide heat to the contents of the reservoir  204 . In other examples, the heat providing element may comprise an arrangement different to the one shown in  FIG. 2 . The heat providing element  202  may comprise only one heat generating part or more than two heat generating parts. The heat generating parts may, for example, be positioned differently with respect to the reservoir  204  than as shown in  FIG. 2 . In some examples, the heat providing element  202  may comprise a single heat generating part, for example a heat generating part which surrounds the reservoir  204 . In one example, the single heat generating part is of a hollow cylindrical shape and surrounds the reservoir  204 . In some examples, the heat providing element  202  comprises a single heat generating part which does not substantially completely surround the reservoir  204 . In such examples, the heat generating part may be of physical characteristics (i.e. size, shape, etc) and position with respect to the reservoir  204  such that sufficient heat is provided to the reservoir  204  to control the reservoir temperature. 
     The first and second heat generating parts  202   a ,  202   b  may be placed in contact with the external surface of the reservoir  204  so as to provide good thermal contact between the heat generating parts  202   a ,  202   b  and the reservoir  204 . In other examples, the heat generating parts  202   a ,  202   b  of the heat providing element  202  may be thermally coupled to the surface of the reservoir  204  via a thermally conductive path provided by thermally conductive adhesive, mechanical coupling, the use of a thermal pad, a combination thereof, or the like. In some examples, a lug may be machined on the outside of the reservoir  204  and the heat generating parts of the heat providing element  202  may be fixed to the lug. In further examples, the first and second heat generating parts  202   a ,  202   b  are positioned close to, but separated from, the reservoir  204  such that heat generated by the heat generating parts  202   a ,  202   b  is provided to the reservoir  204 . As described above, the heat providing element  202  may be integrated with the reservoir  204 , or may be provided inside the reservoir  204 . 
     In some examples, the heat generating providing element  202  may be integral to the reservoir  204 . For example, the reservoir  204  may comprise electrically conductive components (e.g. the walls of the reservoirs  204  may comprise conductive material). Current may flow through the electrically conductive components of the reservoir  204  which may generate heat in response. In some examples, the reservoir  204  may be a composite reinforced reservoir, for example, comprising a metallic liner and carbon fibre composite outer shell. In such examples, a heating element (e.g. electric wire) may be woven within the composite material to provide an embedded heat providing element  202 . 
     The first and second heat generating parts  202   a ,  202   b  may, for example, be resistive heaters. For example, the first and second heat generating parts  202   a ,  202   b  may comprise resistive elements which generate heat resistively upon being supplied with electrical power. The first and second heat generating parts  202   a ,  202   b  may, for example, comprise heat generating lamps such as halogen lamps comprising filaments, radiators through which a heated fluid flows in order for the radiators to output heat, inductive heating arrangements comprising inductor coils and susceptor elements, or the like. It should be appreciated that the first heat generating part  202   a  may not be identical to the second heat generating part  202   b . For example, the first heat generating part  202   a  may use a different method of generating heat than the second heat generating part  202   b . More generally various combinations of components and methods for generating heat may be used in the heat providing element  202 . 
     In some examples, the heat providing element  202  does not itself generate heat, but is arranged to direct heat to the reservoir from another heat generating component of the hydraulic system  102 . For example, the heat providing element  202  may be arranged to direct heat to the reservoir  204  from a motor pump of the hydraulic system  102 . In such examples, the processor  206  activating the heat providing element  202  may also activate the component of the hydraulic system  102  from which heat is directed. For example, when the processor  206  activates the heat providing element  202 , e.g. responsive to a temperature, gas pressure and/or fluid pressure threshold, the processor may also activate a pump of the hydraulic system  102  from which the heat providing element  202  directs heat towards the reservoir  204 . 
       FIG. 3  schematically illustrates an example of hydraulic system  102 . Note that to maintain clarity in  FIG. 3 , some of the reference numerals present in  FIG. 2  are omitted, although the hydraulic system  102  of  FIG. 3  may include all components shown in  FIG. 2 . The hydraulic system  102  comprises a pump  302  which pumps hydraulic fluid  208  from the reservoir  204  towards a valve arrangement  304 . In this example, the valve arrangement  304  directs the hydraulic fluid  208  towards a first hydraulic actuator  306  and a second hydraulic actuator  308 . For example, the first hydraulic actuator  306  may be for actuating the nose landing gear  104 , and the second hydraulic actuator may be for actuating the landing gear bay doors of the aircraft  100 . In some examples, the hydraulic system  102  may also comprise an actuator for actuating cargo doors of aircraft  100  (e.g. a third hydraulic actuator). It will be understood that actuators  306 ,  308  may for example comprise pistons to effect movement responsive to hydraulic pressure. The valve arrangement  304  directs the hydraulic fluid  208  towards respective annular piston areas  306   b ,  308   b , as well as full piston areas  306   c ,  308   c  of the actuators  306  and  308 , depending on the desired direction of movement. The hydraulic fluid  208  returns to the reservoir  204  from the area of actuators  306 ,  308  which are not supplied hydraulic fluid  208  by the valve arrangement  304 . For example, if hydraulic fluid is supplied to the annular piston area  306   b  of the first actuator  306 , hydraulic fluid  208  returns to the reservoir from the full piston area  306   c  of the first actuator  306 . The valve arrangement  304  in this example also directs the hydraulic fluid  208  towards an orifice  310 . The valve arrangement  304  comprises valves  306   a ,  308   a  and  310   a  which control the flow of hydraulic fluid  208  to the first actuator  306 , second actuator  308  and the orifice  310 , respectively. 
     In order for the pump  302  to correctly pump the hydraulic fluid  208 , the fluid pressure of hydraulic fluid  208  may be required to be above the fluid pressure threshold for proper pump-priming (i.e. the introduction of hydraulic fluid  208  into pump  302  to prepare it for working). Maintaining the fluid pressure above its respective threshold may also prevent cavitation during pumping (i.e. formation of gas bubbles within the hydraulic fluid  208 ). In examples where the processor  206  controls the heat providing element  202  based on the reservoir temperature and/or the reservoir gas pressure, the temperature threshold and/or the gas pressure threshold may be set so that the reservoir temperature and/or the reservoir gas pressure being above their respective threshold values results in a reservoir fluid pressure which allows the pump  302  to correctly pump the hydraulic fluid  208 . 
     Furthermore, for the pump  302  to work correctly, the fluid viscosity of the hydraulic fluid  208  may be required to be above a given fluid viscosity. Maintaining the reservoir temperature above the temperature threshold may contribute to maintaining the fluid viscosity of the hydraulic fluid  208  above the given fluid viscosity. 
     The given operating condition according to which the hydraulic system  102  is intended to function may depend, for example, on the properties of the hydraulic fluid  208 , the load intended to be moved by the hydraulic actuators  306 ,  308 , etc. Hydraulic reservoirs larger in size may be required for greater reservoir gas pressure. For example, the greatest reservoir gas pressure the hydraulic reservoir  204  is intended to maintain may depend on its size. If the hydraulic system  102  is required to function (i.e. have characteristics in accordance with the given operating condition) in low ambient temperatures, the reservoir  204  may be provided with a gas pressure such that the gas pressure and the fluid pressure within the reservoir  204  are maintained above their respective thresholds at the required low ambient temperatures. However, the control apparatus  106  and the control system  200  advantageously allow hydraulic reservoirs of a given size to be used at lower ambient temperatures compared to prior art systems, while maintaining the reservoir gas pressure above the gas pressure threshold. This is because heat provided by the heat providing element  202  compensates for the lower temperature by maintaining a higher temperature within the reservoir  204 , thereby maintaining the gas and fluid pressures above their respective thresholds. 
     The control apparatus  106  and the control system  200  therefore advantageously allow, for example, a reduction in size of hydraulic reservoirs that may be used at low ambient temperatures. The control apparatus  106  and the control system  200  therefore also advantageously allow hydraulic systems of the aircraft  100  to be placed away from other heat producing components of the aircraft  100 . It will also be understood that hydraulic reservoirs may be provided with lower gas pressures for use at low ambient temperatures when apparatus  106  or system  200  is used. This may advantageously result, for example, in higher pressure differentials in the actuators  306 ,  308 . For example, lower gas pressures permitted by use of the control apparatus  106  or system  200  may allow pressure differentials sufficient for actuators  306 ,  308  to properly function at high ambient temperatures. Use of lower gas pressures may advantageously allow a reduction in the effective surface area of actuator pistons and therefore allow a reduction in the overall actuator size and mass. 
     In the above examples, the processor  206  is arranged to control the heat providing element  202  based on one or more characteristics of the hydraulic system  102  as detected by one or more sensors associated with the hydraulic system  102 . Alternatively, or in addition, the processor  206  may be arranged to control the heat providing element  202  based on an ambient characteristic as detected by an ambient characteristic instrument comprised in the aircraft  100 . For example, the processor  206  may control the heat generating arrangement  206  based on the ambient temperature in the vicinity of the aircraft  100 . The ambient temperature in the vicinity of the aircraft  100  may provide an indication as to the characteristics within the reservoir  204 . For example, a particular ambient temperature may correspond to a particular temperature within the reservoir  204 , a particular gas pressure within the reservoir  204  and a particular fluid pressure within the reservoir  204 . Thus, the processor  206  may control the heat providing element  202  based on an ambient characteristic in order to maintain the hydraulic system  102  in accordance with the given operating condition. 
     In some examples, the processor  206  may disable or enable the heat providing element  202  based on a correspondence relationship between the value of a characteristic of the hydraulic system  102  and a value of an ambient characteristic. For example, in examples in which the processor  206  is arranged to control the heat providing element  202  on the basis of both one or more ambient characteristics, and one or more characteristics of the hydraulic system  102 , the processor  206  may be arranged to activate the heat providing element  202  only if the one or more ambient characteristics and the one or more characteristics of the hydraulic system  102  satisfy the correspondence relationship. In such examples, activation of the heat providing element  202  may be prevented in a situation where one or more sensors (either associated with the hydraulic system  102  or instruments for detecting ambient characteristics) malfunction. For example, the correspondence relationship may be satisfied if the difference between the ambient characteristic in question and the characteristic of the hydraulic system  102  in question is below a difference threshold amount. In a specific example, the correspondence relationship may be satisfied if the difference between the ambient temperature and the reservoir temperature is less than 3° C. For example, if the temperature sensor  212  indicates a temperature within the reservoir  204  of −55° C. whereas the ambient temperature in the vicinity of the aircraft  100  is −10° C., the heat providing element is not activated because such an incoherence between the ambient temperature and the reservoir temperature is unlikely, indicating a possible fault condition with one of the sensors in question. 
     Alternatively or in addition, the processor  206  may be arranged to control the operation of the heat providing element  202  in accordance with data indicating instructions input by a user. For example, ground crew checking the aircraft prior to take-off may input data indicating instructions according to which the processor  206  is to control the heat providing element  202 . For example, the ground crew may measure ambient characteristics and/or characteristics of the hydraulic system  102 , and input data indicating instructions for controlling the heat providing element  202  accordingly. The instructions provided by the user may indicate timings, temperatures, altitudes, etc. based on which the processor  206  is to control the heat providing element  202 . For example, if the ambient temperature is low, the user may input data indicating that the heat providing element  202  is to be activated for a specified period of time. Alternatively or in addition, the user may input data indicating instructions indicating the given operating condition according to which the processor  206  is to control the heat providing element  202 . The user may input data indicating such instructions using data input apparatus comprised in the aircraft  100  or data input apparatus which may be comprised in the control apparatus  106  or control system  200 . 
       FIG. 4  is a flow diagram illustrating a method  400  of controlling heat provision to the hydraulic system  102  of the aircraft  100 . At  402  of method  400 , a value associated with a physical characteristic with respect to the aircraft  100  is determined. For example, the physical characteristic with respect to the aircraft may be a characteristic of the hydraulic system  102  such as temperature within the reservoir  204  of the hydraulic system  102  (as detected by the temperature sensor  212 ), gas pressure within the reservoir  204  of the hydraulic system  102  (as detected by the gas pressure sensor  214 ) or fluid pressure within the reservoir  204  (as detected by the fluid pressure sensor  216 ). Or, the physical characteristic with respect to the aircraft may be an ambient characteristic as detected by an ambient characteristic instrument on the aircraft  100  (for example, an ambient characteristic instruments forming part of the instruments  108  comprised in the aircraft  100 ). The value of the physical characteristic may, for example, be determined by the processor  206  on the basis of values detected by the ambient characteristic instrument or a sensor associated with the hydraulic system  102 . 
     At  404  of the method  400 , the value determined at  402  is compared with temperature control criteria. For example, the temperature control criteria may comprise a threshold value corresponding to the physical characteristic the value of which is determined at  402 . For example, in the case where the reservoir temperature is determined at  402 , the temperature control criteria comprises a reservoir temperature threshold. In other examples, depending on the physical characteristic to be determined at  402 , the temperature control criteria comprises a reservoir gas pressure threshold, a reservoir fluid pressure threshold, or an ambient characteristic threshold (for example, an ambient temperature threshold). 
     At  406  of method  400 , the heat providing element  202  arranged to provide heat to the contents of the reservoir  204  of the hydraulic system  102  is activated on the basis of the comparison at  404  of method  400 . Taking the example of the physical characteristic being the reservoir temperature and the temperature control criteria being a reservoir temperature threshold, at  406 , the processor  206  activates the heat providing element  202  on the basis of the comparison at  404  between the reservoir temperature determined at  402  and the reservoir temperature threshold. For example, the processor  206  activates the heating providing element  202  if the determined reservoir temperature is below the reservoir temperature threshold. 
     The method  400  may also comprise, when the heat providing element  202  is activated, circulating hydraulic fluid  208  within a part of the hydraulic system through the orifice  310  using the pump  302 . Circulating the hydraulic fluid  208  through the orifice  310  may increase the temperature of and reduce the viscosity of the hydraulic fluid  208  which circulates between the orifice  310  and the pump  302 . Circulating the hydraulic fluid  208  through the orifice  310  in combination with activating the heat providing element  202  may contribute to the hydraulic system  102  being maintained in accordance with the given operating condition. 
     The method  400  may also comprise repeatedly determining the value associated with the physical characteristic with respect to the aircraft  100 . The method  400  may also comprise controlling the operation of the heat providing element  202  according to a feedback control scheme on the basis of the temperature control criteria and the repeatedly determined value. For example, the feedback control scheme may be implemented in order to maintain a characteristic of the hydraulic system  102  such that it meets the temperature control criteria. Taking the example in which the physical characteristic is the reservoir gas pressure, the reservoir gas pressure may be repeatedly determined based on the detection by the gas pressure sensor  214 . A feedback control scheme may be implemented on the basis of the temperature control criteria which in this example comprises a reservoir gas pressure threshold. For example, the feedback control scheme may be implemented in order to control the heat providing element  202  to maintain the reservoir gas pressure above the reservoir gas pressure threshold. 
     In examples where the physical characteristic with respect to the aircraft  100  is an ambient characteristic, such as ambient temperature for example, the temperature control criteria may specify an amount of time within a given period of time for the heat providing element  202  to provide heat. In such examples, the feedback control scheme may modify the parameters of the temperature control criteria based on changes in the value of the ambient temperature as the ambient temperature is repeatedly determined. 
     It will be appreciated that various different feedback control schemes may be used as part of method  400 . In some examples, a proportional-integral-derivative feedback control scheme may be used in order to maintain a characteristic of the hydraulic system  102  above its respective threshold value. 
     Although the invention has been described above with reference to one or more preferred examples, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. 
     It should be noted that the term “or” as used herein is to be interpreted to mean “and/or”, unless expressly stated otherwise.