Patent Publication Number: US-2021171205-A1

Title: Environmental control system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This specification is based upon and claims the benefit of priority from UK Patent Application Number GB1917966.2 filed on 9 Dec. 2019, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates to an environmental control system for an aircraft and a controller for the environmental control system. 
     Background of the Disclosure 
     Most aircraft feature an environmental control system (ECS) to supply conditioned air at a suitable temperature and pressure to the cabin. In some cases, the ECS also provides pressurised air to key sub-systems such as wing and engine anti-icing sub-systems. 
     In known systems (e.g. civil aircraft), the source of pressurised air for the ECS is represented by bleeds from the engine core compressors of the gas turbine engines of the aircraft. High pressure and high temperature bleed airflow is extracted from a low pressure port of each engine when the engine is operated at high power settings, such as during cruise. A high pressure port is also provided to supply air during low power settings operations, when the pressure at the low pressure port would not meet the ECS demand. In both cases, a pressure regulating valve is provided to maintain the bleed pressure under acceptable limits. 
     A portion of this bleed is sometimes used for engine or wing anti-icing purposes, or conveyed to other aircraft systems requiring pneumatic actuation. The remaining flow enters an air conditioning pack, where the air is cooled down to meet the cabin conditioning demand. A number of air conditioning packs may be provided that may, for example, each be served by a particular turbine engine of the aircraft. The conditioned airflow is delivered to a mixing manifold where it is mixed with a portion of the cabin air prior to being discharged into the aircraft cabin. 
     The airflow through the air conditioning pack is commonly controlled by a flow control valve assembly (e.g. a butterfly valve). This flow control valve may be controlled so as to provide a target (i.e. desired) flow rate through the air conditioning pack (and into the cabin). 
     The pressure regulating valve (for maintaining bleed pressure under acceptable limits) and the flow control valve both result in energy dissipation from the ECS. In other words, the bleed flow management of the ECS represents an inefficiency that ultimately increases the engine power offtake required to meet ECS demand (so as to in increase aircraft fuel consumption). 
     There is a need for an improved ECS which mitigates at least some of the problems associated with such known systems. 
     SUMMARY OF THE DISCLOSURE 
     In a first aspect there is provided a blower controller for controlling a blower that supplies a pressurised airflow to an air conditioning pack of an aircraft, the blower controller comprising: 
     a pack flow demand adjustment module configured to:
         receive a pack flow demand signal representative of a desired flow rate of an airflow supplied by the air conditioning pack, and a blower condition signal indicative of a condition of an intake airflow received by the blower; and   determine a corrected pack flow demand based on the pack flow demand and the blower condition signal; and       

     a first control signal generator configured to receive the corrected pack flow demand and generate a first control signal to control a first operating parameter of the blower in response to the corrected pack flow demand. 
     The disclosed controller may facilitate the provision of an aircraft environmental control system that does not require a number of pressure regulating and flow control valves present in the known systems discussed above. Such pressure regulating and flow control valves are associated with pressure losses and thus represent inefficiencies in those systems. By providing a controller that facilitates environmental control without these valves, such environmental control may be performed more efficiently. This may lead to reduced power offtake from the engines of the aircraft and thus reduced fuel consumption. 
     Optional features will now be set out. These are applicable singly or in any combination with any aspect. 
     The blower condition signal may be indicative of the pressure of the intake airflow. The blower condition signal may be indicative of the temperature of the intake airflow. The blower condition signal may be indicative of both pressure and temperature of the intake airflow. Thus, for example, the corrected mass flow demand may be calculated according to the following relationship: 
     
       
         
           
             
               ω 
                
               
                   
               
                
               c 
             
             = 
             
               ω 
                
               
                 
                   
                     
                       T 
                       in 
                     
                     
                       T 
                       ref 
                     
                   
                 
                 / 
                 
                   ( 
                   
                     
                       P 
                       in 
                     
                     
                       P 
                       ref 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     where T in  and P in  are the temperature and pressure indicated by the blower condition signal, T ref  and P ref  are reference temperature and pressure (e.g. at sea level), and ω is the pack flow demand. 
     The pack flow demand signal may be representative of a desired mass flow rate. In other embodiments the pack flow demand signal may be representative of a desired volumetric flow rate. 
     The first control signal generator may be configured to generate the first control signal according to a control schedule that determines the first control signal given the corrected pack flow demand. The control schedule may comprise a transfer function that receives the corrected pack flow demand as an input and provides the first control signal as an output. The transfer function may be a linear transfer function. The transfer function may be optimised so as to provide maximum blower efficiency during a particular aircraft flight condition, such as when the aircraft is operating according to a cruise condition. 
     The control schedule may only comprise a single transfer function. Alternatively, the control schedule may comprise a plurality of transfer functions. In such embodiments, the transfer function (for determining the control signal) may be selected by the first control signal generator from the plurality of transfer functions. 
     Each transfer function of the plurality of transfer functions may be representative of an operating state of a system of the aircraft. The first control signal generator may be configured to receive a signal indicative of the operating state of the system and select a transfer function that is representative of the indicated operating state. 
     For example, each transfer function of the plurality of transfer functions may be representative of an operating state of an anti-icing system of the aircraft. In this respect, the first control signal generator may be configured to receive a signal indicative of the operating state of the anti-icing system and select a transfer function of the plurality of transfer functions that is representative of the indicated operating state. 
     The operating state of the anti-icing system may, for example, comprise whether one or more sub-systems of the anti-icing system are in an active state or a deactivated state. Such sub-systems may, for example, comprise a wing anti-icing sub-system or an engine anti-icing sub-system. As will be described further below, in some systems a portion of the air supplied by the blower (controlled by the blower controller) may be diverted to the anti-icing system. Thus, the provision of a plurality of transfer functions (that are selectable by the blower controller) may allow for compensation of this diversion of airflow. 
     In one example, a first transfer function of the plurality of transfer functions of the control schedule may be selected by the first control signal generator when neither the wing anti-icing or engine anti-icing sub-systems are activated (as indicated by the anti-icing sensor). The first transfer function may be referred to as the baseline transfer function. The baseline transfer function may be configured so as to provide maximum blower efficiency during a particular flight condition, such as when the aircraft is operating according to a cruise condition. A second transfer function may be selected when only the engine anti-icing sub system is activated. A third transfer function may be selected when only the wing anti-icing sub-system is activated. Finally, a fourth transfer function may be selected when both the wing anti-icing and engine anti-icing sub-systems are activated. 
     Where each transfer function is a linear transfer function, the transfer functions may differ in gradient. Alternatively or additionally, each transfer function may comprise an offset constant that differs between the transfer functions. As may be appreciated, the offset constant may be reflective of the extra capacity required to supply airflow to the anti-icing system. Thus, the larger the offset constant of a respective transfer function is, the larger the blower speed or variable geometry setting is for a given corrected pack flow demand. 
     Alternatively, or additionally, each transfer function of the plurality of transfer functions may be representative of an operating state of one or more blowers of the aircraft (i.e. further blowers of the aircraft in addition to the abovementioned blower). In this respect, the first control signal generator may be configured to receive a signal indicative of an operating state of a blower of the aircraft and select a transfer function that is representative of the indicated operating state of the blower. 
     The first control signal generator may comprise two control schedules. Both control schedules may comprise a single transfer function. A first control schedule of the two control schedules may be configured to determine a preliminary first control signal given the corrected pack flow demand. A second control schedule may be configured to determine a correction signal given an operating state of a system of the aircraft (e.g. anti-icing system and/or blower/engine system). The second control schedule may thus receive a signal indicative of an operating state of a system of the aircraft. The operating state may be a continuous (rather than discrete) state. For example, the state may be the opening size of a valve of the anti-icing system (i.e. so as to be indicative of the proportion of airflow diverted to the anti-icing system). 
     The first control signal generator may be configured to adjust or correct the preliminary first control signal according to the correction signal so as to generate the first control signal (i.e. for controlling the blower). 
     The controller may comprise a second control signal generator configured to receive the pack flow demand signal and a measured pack flow signal indicative of a measured mass (or volumetric) flow rate of an airflow supplied by the air conditioning pack. The second signal generator may further be configured to compare the pack flow demand signal with the measured pack flow signal. The second control signal generator may be configured to generate a second control signal, in response to the comparison of the pack flow demand and measured pack flow signal, to control a second operating parameter of the blower. In this respect, the second control signal generator may be configured so as to operate as a proportional-integral (PI) or proportional-integral-derivative (PID) controller. 
     The first and second operating parameters may be such that they relate to (i.e. affect) a mass flow rate (or volumetric flow rate) of the blower. The first operating parameter may be a variable geometry position of the blower. The second operating parameter may be a blower speed of the blower (i.e. a rotational speed of an impeller of the blower). 
     Alternatively, the first operating parameter may be a blower speed of the blower. The second operating parameter may be a variable geometry position of the blower. 
     In a second aspect there is provided an aircraft environmental control system comprising:
         a blower configured to supply a pressurised airflow;   an air conditioning pack configured to receive the pressurised airflow from the blower and supply a conditioned airflow to an internal space of the aircraft;   a pack flow demand sub-system configured to determine a pack flow demand, representative of a desired flow rate of the conditioned airflow supplied by the air conditioning pack; and   a blower controller as described above with respect to the first aspect.       

     As is discussed above, the provision of a blower that pressurises airflow for the environmental control system, and an associated controller that controls the blower to meet pack flow demand, may result in a more efficient system. Managing airflow through the pack flow via control of the blower may eliminate the need for a flow control valve at the air conditioning pack or pressure regulating valves, which may in turn avoid the inefficiencies presented by such valves. 
     The blower may be e.g. a centrifugal compressor. The blower may comprise variable geometry and in this respect, may be referred to as a variable geometry compressor. For example, the blower may comprise variable (i.e. movable) outlet guide vanes that, when moved, alter the flow of air discharged from the blower. 
     The blower may be configured for operative connection to a driver for driving (i.e. rotating) the blower. For example, the blower may be driven (i.e. mechanically) by a gas turbine engine of the aircraft (or may be configured to be driven by a gas turbine engine). The blower may be operatively (mechanically) connected to the gas turbine engine (e.g. one or more shafts of the gas turbine engine) by way of a transmission assembly, which may include an ancillary gearbox of the turbine engine. The transmission assembly may be configured to receive a rotational input and provide a rotational output that is different (e.g. in rotational speed) to the rotational input. For example, the transmission assembly may comprise a continuously variable transmission. The continuously variable transmission may allow the blower speed (i.e. speed of rotation of an impeller of the blower) to be independent of the engine shaft speed. 
     The driver may alternatively be an electric motor (i.e. the blower may be driven by an electrical motor). In such embodiments, the blower may be connected to the electric motor directly or via a transmission assembly (e.g. including a continuously variable transmission) as discussed above. 
     The blower may receive an intake airflow from the turbine engine. The turbine engine may comprise a fan and an engine core including one or more compressors, one or more turbines and one or more shafts connecting the compressor(s) and turbine(s). A portion of the air discharged by the turbine engine fan (i.e. downstream of the fan) may enter the engine core, whilst the remaining portion of airflow may flow into a bypass duct of the turbine engine. The intake airflow may be received by the blower from the bypass duct of the turbine engine. For example, the bypass duct may comprise one or more scoops (i.e. outlets) for diverting airflow to the blower. 
     In some embodiments, the system may comprise a pre-cooler in between the blower and the air conditioning pack. The pre-cooler may receive a cooling airflow from the bypass duct of the turbine engine. The cooling airflow may bypass the blower and may exchange heat with the pressurised airflow from the blower in the pre-cooler, so as to cool the pressurised air. For example, the pre-cooler may cool the pressurised air to maintain the temperature of the pressurised air below 200° C. The cooled (and pressurised) airflow may then flow to the air conditioning pack for conditioning. 
     The system may further comprise a blower sensor for measuring a condition of the airflow received by the blower (i.e. the intake airflow). The blower sensor may be a temperature sensor for measuring the temperature of the pressurised airflow supplied by the blower. The blower sensor may alternatively be a pressure sensor for measuring the pressure of the pressurised airflow. The system may comprise both a pressure sensor and a temperature sensor for respectively measuring the pressure and temperature of the pressurised airflow received by the blower. 
     The blower sensor(s) may generate the condition signal indicative of the measured condition of the intake airflow (e.g. indicative of temperature and/or pressure of the intake airflow). The condition signal(s) may be received by the blower controller and, as discussed above with respect to the first aspect, the blower controller may, in response to the condition signal(s), control the blower so as to alter an operating parameter of the blower. 
     The system may further comprise a (e.g. mass) flow rate sensor for measuring the (e.g. mass) flow rate of an airflow through the air conditioning pack. For example, the flow rate sensor may measure the flow rate of the pressurised airflow received at the air conditioning pack, the conditioned air supplied by the air conditioning pack, or an intermediate airflow within the air conditioning pack (i.e. intermediate the received and supplied airflows). 
     The flow rate sensor may generate a pack flow rate signal indicative of the mass (or volumetric) flow rate of the airflow at the air conditioning pack. The pack flow rate signal may be received by the blower controller and, as is discussed above with respect to the first aspect, the blower controller may, in response to the pack flow rate signal, control the blower so as to alter an operating parameter of the blower (e.g. to alter a mass flow rate of the airflow supplied by the blower). 
     The blower controller may control a position of the variable geometry (e.g. outlet vanes) of the blower. For example, the first or second control signal generated by the blower controller may control the position of the variable geometry of the blower (e.g. by way of an actuator). The blower controller may control the position of the variable geometry between an open position and a closed position. In the open position the variable geometry may have a maximum airflow capacity (i.e. permitting the maximum airflow rate for a given blower speed). In the closed position the variable geometry may have an airflow capacity that is less than the airflow capacity when the variable geometry is in the open position (e.g. less than the maximum airflow capacity). In the closed position, the variable geometry may not necessarily be fully closed (i.e. airflow may still be permitted through the variable geometry). 
     The blower controller may control a rotational speed of the blower (i.e. rotational speed of an impeller of the blower/compressor). The first or second control signal may control the blower speed. In this way, the blower controller may control a (mass) flow rate of the air supplied by (i.e. discharged from) the blower. The blower controller may control the speed of the blower via control of the transmission assembly. For example, the blower controller may control the continuously variable transmission of the transmission assembly so as to alter the blower speed. 
     The environmental control system may comprise an anti-icing system. The environmental control system may comprise an anti-icing sensor to detect an operating state of the anti-icing system. The environmental control system may comprise a wing anti-icing sensor and an engine anti-icing sensor to respectively detect an operating state of wing anti-icing and engine anti-icing sub-systems of the anti-icing system. For example, the sensors may detect whether the respective sub-system is in an activated state or a deactivated state. The sensor may alternatively detect a parameter of the anti-icing system or sub-system, such as a valve opening size for a valve controlling airflow to the anti-icing system. 
     The or each anti-icing sensor may generate a signal indicative of the operating state of the anti-icing system and transmit the signal to the blower controller (for use by the controller in a manner described in the first aspect). 
     Such an arrangement may allow the system to accommodate changes in airflow demand from the anti-icing system without the need for additional mass flow sensors for the aircraft anti-icing system. This may reduce the number of failure modes for the system and improve robustness of the system. 
     In some embodiments, the environmental control system may comprise one or more further blowers. Each blower may supply a pressurised airflow to the air conditioning pack, or a further air conditioning pack, for subsequent supply of a conditioned airflow to the internal space of the aircraft. In such embodiments, each of the plurality of transfer functions of the control schedule may additionally or alternatively be associated with an operating condition of one or more of the further blowers. In the event of one or more of the blowers the aircraft become inactive, the environmental control system may be required to compensate for the loss of airflow by increasing the airflow provided by the remaining blower(s). This may occur, for example, as a result of a failure of a gas turbine engine of the aircraft (when power to the blower is supplied by the gas turbine engine). 
     In a third aspect there is provided an aircraft comprising an environmental control system as described above with respect to the second aspect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the figures, in which: 
         FIG. 1  is a schematic of an aircraft environmental control system; 
         FIG. 2  is a schematic of a blower controller according to a first embodiment; 
         FIGS. 3, 4 and 5  are variations of a control schedule for the blower controller of the first embodiment; 
         FIG. 6  is schematic of a blower controller according to a second embodiment; and 
         FIG. 7  is a schematic of a blower controller according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
       FIG. 1  illustrates an aircraft environmental control system  10  comprising a blower  11  for supplying a pressurised airflow  12  and an air conditioning pack  13  that receives the pressurised airflow  12  from the blower  11  and supplies a conditioned airflow  14  to an internal space (cabin)  15  of the aircraft. In the present embodiment, a pre-cooler  65  is disposed between the blower  11  and the air conditioning pack  13  to cool the pressurised airflow  12  prior to entering the air conditioning pack  13 . 
     The blower  11  is in the form of a variable geometry centrifugal compressor that pressurises the airflow and that is driven by a turbine gas engine  17  of the aircraft. The gas turbine engine  17  comprises a fan  18 , a compressor  19  and a turbine,  20 , which are all operatively connected by a shaft  21 . An airflow  22  is supplied to the blower  11  by the fan  18 . For example, the airflow  22  may be supplied from a bypass duct of the turbine engine  17  (e.g. via a scoop in the bypass duct). 
     The blower  11  is operatively connected to the shaft  21  by a transmission assembly  23 , such that rotation of the shaft  21  causes rotation of the impellers of the blower  11 . The transmission assembly  23  comprises a continuously variable transmission (CVT)  24 , which alters the rotational speed of the impeller of the blower  11  (the “blower speed”) relative to the rotational speed of the shaft  21  of the turbine engine  17 . In this way, the blower speed is independent of the shaft  21  rotational speed. 
     The blower  11  is controlled by a blower controller  25 . In particular, the blower controller  25  controls both the blower speed (e.g. via control of the CVT  23 ) and the position of the variable geometry (i.e. position of guide vanes of the variable geometry) of the blower  11  so as to alter the mass flow rate of the pressurised airflow  12  supplied by the blower  11 . In order to provide such control, the blower controller  25  controller generates a variable geometry control signal  26  and a blower speed control signal  27  that are respectively communicated to the blower  11  and the CVT  24 . 
     As will be described in more detail below, the blower speed  27  and variable geometry  26  control signals are both generated in response to a pack flow demand signal  28  received by the blower controller  25  from a pack flow demand sub-system of a zonal controller  29  (which, in other embodiments, may form part of the blower controller  25 ). 
     The zonal controller  29  generates the pack flow demand signal  28  partly based on a plurality of temperature signals received from temperatures sensors  30  measuring the temperature of the air in the cabin  15  and the temperature of the air delivered to the cabin. The zonal controller  29  also receives a temperature selector input  31  and a ventilation flow selector input  32 . The pack flow demand signal  28  is generated based on the ventilation flow selector input  32 . In some cases the pack flow demand signal may be adjusted by the zonal controller  29  based on additional input signals, such as cabin temperature or bleed source. 
     In addition to the pack flow demand signal  28 , the zonal controller  29  generates a trim air valve control signal  33 . This signal  33  controls three trim air valves  34 , which adjust a supply of hot trim air that is combined with the conditioned air  14  from the air conditioning pack  13 . In particular, the conditioned air  14  enters a manifold  35 , where it is split into three separate air streams that are each supplied to different locations (or zones) of the aircraft cabin  15 . Of course, in other embodiments, the aircraft may comprise more zones or less zones (than three zones) and may comprise more than three air streams or less than three air streams. 
     Although not immediately apparent from the figure, this manifold  35  may also receive conditioned airflow from further blowers  16  that are driven by further gas turbine engines of the aircraft. 
     The three trim air valves  34  control three trim air streams that each merge with a corresponding conditioned air stream prior to entering the cabin  15 . In other embodiments, more or less trim air valves may be provided. The trim air streams are hotter than the conditioned air streams, such that by altering the flow rate of a particular trim air stream (i.e. by a corresponding valve  34 ), the system  10  is able to alter the temperature of the air supplied to a particular zone of the cabin  15 . 
     The zonal controller  29  also generates a cooling demand signal  36 , which is transmitted to a pack controller  37  for controlling the air conditioning pack  13 . The pack controller  37  controls the air conditioning pack  13  based on the cooling demand signal  36  and a pack temperature signal received from a temperature sensor  38  and which is indicative of a temperature of the pack  13 . 
     The blower controller  25 , in addition to the pack flow demand signal  28  (from the zonal controller  29 ), receives a pack flow rate signal  40  from a sensor  39  that measures the airflow mass flow rate through the pack  13 . The blower controller  25  additionally receives blower intake condition signals  41  in the form of pressure and temperature signals respectively generated by a pressure sensor  42  and a temperature sensor  43  located at the blower  11  inlet/intake. 
     The blower controller  25  generates the variable geometry control signal  26  and the blower speed control signal  27  based on the condition signals  41 , the pack flow demand signal  28  and the pack flow rate signal  40 , so as to alter the mass flow rate of the pressurised airflow  12  supplied by the blower  11 . As will be described in more detail below, the blower controller  25  may compensate for additional factors. One such factor may be the operating state of an anti-icing system  45  of the aircraft. The environmental control system  10  may divert some of the pressurised air  12  supplied by the blower  11  to the anti-icing system for use by the anti-icing system in de-icing components of the aircraft (e.g. wings and engine). This diversion of the pressurised air  12  means that less air is supplied to the cabin  15  and in some cases this must be compensated for. 
     The operation of the blower controller  25  is apparent from  FIGS. 2, 3 and 4  which schematically depict variations in how the blower controller  25  may be configured. Similar components of these variations have been assigned the same reference numerals. In each of the variations of  FIGS. 2, 3 and 4 , the blower controller  25  receives a blower temperature signal  41   a , a blower pressure signal  41   b , a pack flow demand signal  28  and a pack flow rate signal  40 . Similarly, in each variation, the blower controller  25  generates and transmits a variable geometry control signal  26  and a blower speed control signal  27 . 
     In  FIG. 2 , the blower controller  25  comprises a variable geometry module  46  and a blower speed module  47  that are generally defined by two branches in the schematic. The variable geometry module  46  receives the temperature signal  41   a , pressure signal  41   b  and pack demand signal  28  and processes those signals in a pack flow demand adjustment module  48 . The pack flow demand adjustment module  48  adjusts the pack demand signal  28  based on the temperature  41   a  and pressure  41   b  signals in order to generate a corrected pack demand  49 . The corrected pack demand  49  is received by a first control signal generator  50  that generates the variable geometry control signal  26  based on the corrected pack demand  49 . 
     The blower speed module  47  comprises second control signal generator that includes a pack flow error module  51  that compares the pack flow demand signal  28  with the measured pack flow rate signal  40 . The pack flow error module  51  may be in the form of a PI or PID controller and generates an error signal  52  that is based on the difference between the pack flow demand signal  28  and the pack flow rate signal  40 . This error signal  52  is used by a conversion module  53 , which generates the blower speed control signal  27  (transmitted to the blower  11 ). 
       FIG. 3  illustrates an exemplary control schedule that may be used by the first control signal generator  50  to provide a variable geometry control signal. In this embodiment, the schedule includes a single linear transfer function  54 . The “max” configuration is representative of the variable geometry (i.e. outlet guide vanes) of the compressor  16  being in a position that exhibits the maximum flow capacity (i.e. a fully open position). As is apparent from the figure, the transfer function provides a gradual opening of the variable geometry in response to an increase in the corrected pack flow demand received from the pack demand adjustment module  48 . 
       FIG. 4  depicts a further exemplary control schedule for the first control signal generator  50 , which accommodates the diversion of air to the anti-icing system  45 . This schedule comprises four transfer functions: a first (baseline) transfer function  55  that is selected when no air is diverted to the anti-icing system, a second transfer function  56  that is selected when air is diverted for engine anti-icing only, a third transfer function  57  that is selected when air is diverted for wing anti-icing only, and a fourth transfer function  58  that is selected when air is diverted for both wing and engine anti-icing. 
     As is evident from the figure, each transfer function  55 ,  56 ,  57 ,  58  provides a different relationship between the variable geometry control signal and the received corrected flow demand. In particular, the transfer functions  55 ,  56 ,  57 ,  58  differ in their gradient. All of the transfer functions  55 ,  56 ,  57 ,  58  provide a minimum variable geometry at the same corrected flow demand, but differ in the rate at which the variable geometry is opened with respect to corrected flow demand. As a result, for example, the fourth transfer function  58  (representing both wing and engine anti-icing) reaches the maximum variable geometry at a lower corrected flow demand than the first (baseline) transfer function  55 ). In this way, the system  10  can ensure that air diverted to the anti-icing system  45  is compensated for by an increase in the airflow rate from the blower  11 . 
     Although not shown, it should be appreciated that in order to select the appropriate transfer function  55 ,  56 ,  57 ,  58 , the first control signal generator  50  may receive information regarding the operating state of the anti-icing system  45 . For example, the engine and wing anti-icing sub-systems may each comprise a sensor that detects the state of the sub-system and transmits this to the first control signal generator  50 . Such a sensor may, for example, detect the opening of a valve for controlling air supply to the sub-system. 
     In other embodiments, the control schedule of the first control signal generator  50  may include transfer functions that represent an operating condition one or more blowers of the system. This may be in combination with the anti-icing system state, or as an alternative. In this way, if one blower becomes inactive (e.g. due to an engine failure), the remaining blowers may compensate for the loss of airflow from the inactive blower. 
       FIG. 5  illustrates an alternative schedule to that shown in  FIG. 4  and described above and, accordingly, the transfer functions  55 ,  56 ,  57 ,  58  have been provided with the same reference numerals. Rather than differing in gradient, the transfer functions  55 ,  56 ,  57 ,  58  of  FIG. 5  differ by way of an offset constant. In this way, the rate of opening of the variable geometry in response to corrected flow demand is the same for each of the transfer functions  55 ,  56 ,  57 ,  58 , but the offset constant means that, for example, the fourth transfer function  58  provides a greater variable geometry value than the other transfer functions  55 ,  56 ,  57  for a given corrected flow demand. 
       FIG. 6  illustrates an embodiment of the blower controller  25  that is similar to the embodiment shown in  FIG. 2 , except that the blower speed module  47 ′ of  FIG. 3  takes the form of the variable geometry module  46  of  FIG. 2 , and the variable geometry module  46 ′ of  FIG. 3  takes the form of the blower speed module  47  of  FIG. 2 . In other words, the blower speed module  47 ′ of  FIG. 3  comprises a pack demand adjustment module  48  and the first control signal generator  50 , and the variable geometry module  46 ′ comprises a pack flow error module  51  and conversion module  53  of the second control signal generator (which, in this case, generates the variable geometry control signal  26 ). 
     As may be appreciated the first control signal generator  50 ′ of the embodiment of  FIG. 6  may be similar to those shown in  FIGS. 3, 4 and 5  except that the output provided by the first control signal generator  50 ′ is a blower speed rather than a variable geometry position. 
       FIG. 7  differs from the embodiment of  FIG. 2  in that the first control signal generator comprises first  63  and second  59  control schedules. The first control schedule  63  comprises a single transfer function (such as that shown in  FIG. 3 ). Thus, the first transfer function module  63  does not compensate for e.g. the diversion of airflow to the anti-icing system  45 . The second schedule  59  also comprises a single transfer function that, as will be described below, compensates for external factors (such as the activation of the anti-icing system  45 ). 
     In one example, the second control schedule  59  receives a valve opening signal  60  that is indicative of the state of a valve of the anti-icing system  45  (i.e. indicative of an opening size of the valve). For example, the valve opening signal  60  may be indicative of the opening size of a wing and/or engine anti-icing valve. The second control schedule  59  may be similar to that show in  FIGS. 4 and 5 , except that the input of those transfer functions is the state of the valve and the output is a correction value (transmitted as a correction signal  61 ). 
     The first signal generator  50 ″ of the embodiment of  FIG. 7  further comprises a correction module  62  that determines the variable geometry control signal  26  based on the correction signal  61  and a preliminary control signal  64  received from the first control schedule  63 . In other words, the correction signal “corrects” the preliminary variable geometry controls signal  64  so as to compensate for the provision of airflow to the anti-icing system  45 . 
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.