Abstract:
Airplane ground service equipment includes an air conditioning system for an airplane and also a fluid cooling system for airplane electronics which detachably connects to a first port and a second port on the airplane. The fluid cooling system includes a first connector, a second connector, and a fluid conduit therebetween, the first and second connectors being adapted to be connected to the first and second ports, respectively, such that the connection of the ports to the connectors completes a fluid circuit. A fluid pump directs fluid through the fluid conduit, and a heat exchanger in the fluid conduit removes heat from the fluid circuit and transfers it into the air conditioning system of the ground support equipment. A temperature regulation mechanism stabilizes the temperature of the fluid in the fluid conduit to compensate for changes in the amount of heat that is drawn from the airplane electronics.

Description:
[0001]    This application is a non provisional of provisional application Ser. No. 60/984,002 filed Oct. 31, 2007 (Atty. Docket No. 21585-P1) and provisional application Ser. No. 61/036,727 filed Mar. 14, 2008 (Atty. Docket No. 50-003 ITW 21585-P2). 
       CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0002]    The present patent application is one of a set of commonly owned applications filed on the same day as the present application, sharing some inventors in common, and relating to airplane ground support equipment and carts. The other applications in this set, listed here, are hereby incorporated by reference into the present application: “A Multi-Voltage Power Supply for a Universal Airplane Ground Support Equipment Cart,” James W. Mann, III and David Wayne Leadingham (Ser. No. ______, Atty. Doc. No. 50-002 ITW 21608U); “A Frame and Panel System for Constructing Modules to be Installed on an Airplane Ground Support Equipment Cart,” Jeffrey E. Montminy, Brian A. Teeters, and Kyta Insixiengmay (Ser. No. ______, Atty. Doc. No. 50-004 ITW 21588U); “A System of Fasteners for Attaching Panels onto Modules that are to be Installed on an Airplane Ground Support Equipment Cart,” Jeffrey E. Montminy, Brian A. Teeters, and Kyta Insixiengmay (Ser. No. ______, Atty. Doc. No. 50-005 ITW 21587U); “Airplane Ground Support Equipment Cart Having Extractable Modules and a Generator Module that is Separable from Power and Air Conditioning Modules,” James W. Mann, III and Jeffrey E. Montminy (Ser. No. ______, Atty. Doc. No. 50-006 ITW 21586U); “An Adjustable Air Conditioning Control System for a Universal Airplane Ground Support Equipment Cart,” James W. Mann, III, Jeffrey E. Montminy, Benjamin E. Newell, and Ty A. Newell (Ser. No. ______, Atty. Doc. No. 50-007 ITW 21606U); “A Compact, Modularized Air Conditioning System that can be Mounted Upon an Airplane Ground Support Equipment Cart,” Jeffrey E. Montminy, Kyta Insixiengmay, James W. Mann. III, Benjamin E. Newell, and Ty A. Newell (Ser. No. ______, Atty. Doc. No. 50-008 ITW 21583U); and “Maintenance and Control System for Ground Support Equipment,” James W. Mann, III, Jeffrey E. Montminy, Steven E. Bivens, and David Wayne Leadingham (Ser. No. ______, Atty. Doc. No. 50-009 ITW 21605U). 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates generally to the field of cooling using fluid coolants, and more particularly to airplane ground support equipment that, in addition to providing air and electrical conditioning services to airplanes, is also able to provide airplanes requiring liquid coolants with polyalphaolefin (PAO) or other similar liquid coolants at controlled temperatures and pressures to cool airplane electronics. 
         [0005]    2. Description of the Related Art 
         [0006]    When an airplane is on the ground with its engines shut down, the airplane is typically unable to provide power for its electrical systems and chilled air for its air conditioning systems; and some airplanes are also unable to provide liquid coolant for some critical electronic (or “avionic”) components. It is customary to connect such a grounded airplane to an airplane ground support equipment system. Such a system may have its components mounted upon a mobile equipment cart that is called an airplane ground support equipment cart and that may be parked, placed, or mounted conveniently close to an airplane requiring ground support. Such a cart typically contains an air conditioner that can provide conditioned and cooled air to an airplane plus an electrical power converter that can transform power drawn from the local power grid into power of the proper voltage (AC or DC) and frequency required by the airplane. Such an airplane ground support equipment cart may also contain a diesel engine connected to an electrical generator that enables the cart to provide both air conditioning and also electrical power for an airplane without any connection to the local power grid. And if an airplane requires a source of cooled liquid for its electronics, some carts may also include a source of liquid coolant. 
         [0007]    In the past, particularly with regard to military airplanes, such ground support equipment carts have been custom designed to meet the specialized needs of a single particular type or class of airplane. Hence, a cart designed to support the specific requirements and needs of a first type or class of airplane cannot be used to support the differing specific requirements and needs of other types or classes of airplanes. Different airplanes typically may require different pressures and volumes of cooled air, different amounts of electrical power, different electrical voltage levels, and different electrical frequencies (or direct current). And different airplanes typically may require differing pressures and volumes of cooled liquid for use in cooling onboard electronics. Accordingly, every airport must be supplied with as many different types of ground support equipment carts as there are different types or classes of airplanes that may land and take off at each airport or military base. Problems arise when more airplanes of a particular type arrive at a specific location than there are ground support equipment carts suitably designed to service the needs of that particular type or class of airplane. 
         [0008]    To be more specific, some airplanes require their ground support equipment to provide considerably more airflow at higher pressures than do other airplanes having smaller interiors. Some airplanes require their electrical power to be adjusted to 115 volts of alternating current (A.C.) which alternates, or flows back and forth, 400 times each second (115 volts, 400 Hz A.C.). Other airplanes require 270 volts direct current (270 volts, D.C.) that does not flow back and forth. Yet other airplanes require a source of 28 volts of direct current (28 volts, D.C.). And airplanes also differ in the amount of electrical power that they draw. 
         [0009]    Some airplanes, particularly jet fighters, need an additional source of cooling from their ground support equipment in the form of a liquid coolant that is applied to the so-called avionics systems, including electronics and radar systems. This liquid is typically a polyalphaolefin, or PAO, heat transport fluid or liquid coolant. This fluid is propelled by a pump through one or more heat exchangers within the airplane that cool the liquid using cool air that is present whenever the airplane&#39;s turbo fan propulsion engine is in operation. The cooled liquid is then passed through the avionics. 
         [0010]    When such an airplane&#39;s engine is not in operation, the PAO fluid must be cooled in some other manner to prevent the avionics from overheating. One way to accomplish this is to include in the airplane ground support equipment a PAO pump and a mechanism for cooling the PAO heat transport fluid. A pair of hoses can connect the airplane&#39;s PAO fluid system to the ground support equipment, and a circular flow between the airplane and the ground support equipment is established whereby the PAO fluid flows out of the avionics in the airplane to the ground support equipment where the pump propels the fluid through some form of heat exchange mechanism to cool the fluid, which then flows back into the airplane and into the avionics. Since the temperature and pressure and fluid flow volume requirements for PAO cooling may vary from one type or class of airplane to the next, a PAO cooling system designed to meet the specialized PAO cooling needs of one airplane will not necessarily meet the somewhat different needs of another type or class of airplane. 
         [0011]    As an example of an airplane cart arrangement that provides air and electrical conditioning for an airplane, PCT patent application No. PCT/US2006/043312 (Intl. Pub. No. WO 2007/061622 A1 published on May 31, 2007) discloses an airplane ground support cart that has a modular design of its electrical conditioning components. This cart provides air conditioning and electrical power conversion as well as optional electrical power generation services to airplanes. FIG. 5 reveals that the cart disclosed in this patent application may receive interchangeable, modular power conversion modules. Thus, a module 72, which generates 3-phase 115 volt 400 Hz A.C. power, may be removed and replaced with a module 78, which generates 270 volt D.C. power. FIG. 6 illustrates that this cart may also accept a module 92, which generates 28 volt D.C. electrical power. 
         [0012]    FIG. 2 of the above PCT patent application illustrates a typical arrangement of the mechanical components of a dual air conditioning system within an airplane ground support equipment cart 14. The air conditioner&#39;s mechanical components are spread all across the entire length of the cart 14. Two sets of condenser coils 34 are positioned at one end of the cart 14; and the thickness of the coils 34 and their housing, together with the thickness of the associated cooling fans, occupies roughly one-fifth of the cart&#39;s overall length. A filter and upstream evaporation coil 30 and a downstream evaporation coil 40 and outlet connection 42 (to which can be attached a duct leading to an airplane) are positioned at the other extreme end of the cart 14, occupying somewhat less than one-fifth of the cart&#39;s overall length. A blower fan 32, a discharge plenum 38, and two compressors 36 are shown positioned in the central portions of the cart 14. These mechanical components of the air conditioning system are not confined within a rectangular module within a portion of the volume of the cart 14—these components are spread all across the cart 14 and thus cannot be conveniently removed from the cart for servicing or for use away from the cart 14. Other cart components, such as a diesel engine 54 and generator 56 (shown in FIG. 4 of the PCT application) and an electrical power converter unit 72 (shown in FIG. 5 of the PCT application) are squeezed in among the air conditioning components wherever there is room. This intermixing of non-air-conditioning components with the air-conditioning components greatly complicates servicing of all the components, since they are all crowded into the same cramped space. A service man working on the air conditioner compressors or blowers may find the diesel engine 54 and generator 56 are in the way of these components, for example. 
         [0013]    The air conditioning systems of such a conventional ground support equipment system is also designed to provide a particular volume of cooled air at a particular temperature and pressure to a particular type or class of airplane. If such a system has its cool air ducted into some other type or class of airplane, too much or too little air will flow from the air conditioner system, and this will throw off the balance of the air conditioning system, causing the air to be cooled too little or too much and possibly causing icing of the internal evaporator arrays or damage to the airplane. And the temperature and pressure provided may not be proper for some other type or class of airplane. Likewise, the electrical systems may not be able to supply the needs of differing types or classes of airplanes, and the PAO liquid cooling system may not be properly balanced when used to cool the avionics of differing types or classes of airplanes. 
       SUMMARY OF THE INVENTION 
       [0014]    An embodiment of the invention relates to ground support equipment including an air conditioning system for an airplane and also including a fluid cooling system for airplane electronics which detachably connects to a first port and a second port on the airplane. The fluid cooling system includes a first connector, a second connector, and a fluid conduit there between, the first and second connectors being adapted to be connected to the first and second ports, respectively, such that the connection of the ports to the connectors completes a fluid circuit. A fluid pump directs fluid through the fluid conduit, and a heat exchanger in the fluid conduit removes heat from the fluid circuit and transfers it into the air conditioning system of the ground support equipment. A temperature regulation mechanism stabilizes the temperature of the fluid in the fluid conduit to compensate for changes in the amount of heat that is drawn from the airplane electronics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is an isometric view of an embodiment of a universal airplane ground support equipment cart having a modular design. 
           [0016]      FIG. 2  is an isometric view of the cart shown in  FIG. 1  with the diesel engine and generator module portion that forms the back portion of the cart shown separated from the rest of the cart. 
           [0017]      FIG. 3  is an isometric view of an electrical conversion module of the cart shown in  FIG. 1  to illustrate how it may be slid out of and away from the side of the cart for maintenance purposes or to be replaced with a different module. 
           [0018]      FIG. 4  is a perspective view of a two-stage air conditioning module that is mounted on the front portion of the cart shown in  FIG. 1 , the air conditioning module shown with both of its microchannel condenser coil supporting doors shown swung open to reveal its internal structural details. 
           [0019]      FIG. 5  is a schematic diagram illustrating the flow path of air as it flows through the two-stage air conditioning module shown in  FIG. 4 . 
           [0020]      FIG. 6  is a schematic diagram illustrating the refrigerant circulation paths within the first, or “pre-cool,” air conditioning unit within the two-stage air conditioning module shown in  FIG. 4 , and also showing a heat exchanger which transfers heat from a separate PAO cooling system (not shown) to the refrigerant within this first air conditioning unit. 
           [0021]      FIG. 7  is a schematic diagram illustrating the refrigerant circulation paths within the second, or “post-cool,” air conditioning unit within the two-stage air conditioning module shown in  FIG. 4 . 
           [0022]      FIG. 8  is a schematic diagram of a PAO cooling system within the air conditioning module shown in  FIG. 4  which transfers heat from an airplane to the first, or “pre-cool,” air conditioning unit within the two-stage air conditioning module shown in  FIG. 4 . 
           [0023]      FIG. 9  presents an exploded, perspective view of four plate fin evaporator arrays assembled into a square array, mounted within a rectangular frame, and attached to a funnel-shaped duct that conveys cooled air to the external piping which leads to an airplane. 
           [0024]      FIG. 10  presents a perspective view of the assembly of plate fin evaporator arrays shown in  FIG. 9  mounted between two funnel-shaped ducts that spread the air to be cooled uniformly across the surface of the plate fin evaporator arrays. 
           [0025]      FIG. 11  presents a perspective view of a microchannel condenser unit of the type used in pairs and mounted upon the two doors of the two-stage air conditioning module shown in  FIG. 4 . 
           [0026]      FIG. 12  presents a partly sectional view, taken along the lines A-A in  FIG. 10 , of a microchannel condenser unit illustrating interior details of the air channels. 
           [0027]      FIG. 13  is a block diagram illustrating the signal-carrying bus and the way in which it interconnects the power generating module, two power converter modules, and the two-stage air conditioning module with a control module that includes a display screen with eight pushbuttons and a universal control and diagnostics processor. 
           [0028]      FIG. 14  is a combined flow diagram and state diagram illustrating the normal start-up and run procedures of the overall system and also illustrating the warning, alarm, and shut down states. 
           [0029]      FIG. 15  is a simplified schematic diagram (combining elements taken from  FIGS. 5 ,  6 ,  7 , and  8 ) illustrating the air flow and the refrigerant circulation paths in the pre-cool and post-cool air conditioning units and also in the PAO cooling system, and identifying in particular the eight feedback control loops and controllers that control the operation of these systems and also the temperatures and pressures and set-points that provide input signals to these controllers. 
           [0030]      FIG. 16  is a state diagram illustrating the operation of the compressors within the two air conditioning units. 
           [0031]      FIG. 17  is a flow diagram illustrating how the operation of the blower fan which blows air through the two air conditioner units and into the airplane is automatically controlled through the use of a variable frequency drive for the motor that drives the blower fan. 
           [0032]      FIG. 18  is a schematic diagram showing the connection of the two compressors, the two-speed condenser cooling fan, and the blower fan&#39;s variable frequency drive to a three-phase source of 380 to 500 volt, 50 to 60 Hz electrical power and also showing control signals for the compressors, cooling fan, and blower fan. 
           [0033]      FIG. 19  illustrates and names all of the significant system state signals (temperatures, pressures, etc.) that enter the air conditioning and PAO processor, and it also illustrates all of the significant on/off and 0-to-10 volt output control signals which that processor generates to control all of the air conditioning processes, thereby allowing the air conditioner system to respond flexibly and properly to widely varying load conditions that can be caused by different types and classes of airplanes. 
           [0034]      FIG. 20  presents a block diagram of all the menus and submenus that may be displayed on the face of the control module&#39;s display screen, together with the navigation paths between these menus and submenus. 
           [0035]      FIG. 21  presents a view of the main menu. 
           [0036]      FIG. 22  presents a view of a help menu that appears when the “Help” item is selected on the main menu shown in  FIG. 21 . 
           [0037]      FIG. 23  presents a view of a menu that appears when the airplane “T-50 Golden Eagle” is selected on the main menu shown in  FIG. 21 . 
           [0038]      FIG. 24  presents a view of a help menu that appears when the “Help” item is selected on the “T-50 Golden Eagle” menu shown in  FIG. 23 . 
           [0039]      FIG. 25  presents a view of a maintenance menu that appears when the “Maintenance” item is selected on the main menu shown in  FIG. 21 . 
           [0040]      FIG. 26  presents a view of a scrollable data logging menu and viewing window that appears when the “Data Log Screen” item is selected on the maintenance menu shown in  FIG. 25 . 
           [0041]      FIG. 27  illustrates a view of a pre-cool air conditioning unit&#39;s status values that appears when the “A/C Maintenance” item is selected on the maintenance menu shown in  FIG. 25 . 
           [0042]      FIG. 28  illustrates a view of one of two actuator status and relay status screens that appear when the “Relay Status Screen” item is selected on the maintenance menu shown in FIG.  25 —the values displayed correspond to the more important output control signals generated by the air conditioning and PAO processor shown in  FIG. 19 . 
           [0043]      FIG. 29  presents an exploded isometric view of the control module&#39;s display screen, illustrating that the screen is covered by metal screening that serves as a radio frequency wave blocking screen. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    The detailed description which follows is broken into two sections. Section A presents an introduction to the environment of the present invention, which relates to the design of a PAO liquid cooling system that is incorporated and integrated into the air conditioning system of a modularized universal airplane ground support equipment cart ( FIGS. 1-3 ). Section B presents a detailed description of the PAO liquid cooling system in the context of the complete ground support air conditioning system, including that system&#39;s internal mechanical details ( FIGS. 1-4 ,  9 - 12 , and  29 ), air flow details ( FIG. 5 ), refrigerant and PAO coolant flow path details ( FIGS. 6-8 ), electronic control system details ( FIGS. 13-19 ), and display system and human interaction details ( FIGS. 20-28 ). While the focus of the present invention is the PAO liquid cooling system and the related control systems (disclosed primarily in  FIGS. 6 ,  8 , and  15 ), the PAO system is so closely integrated into the overall air conditioning system that the two systems are described below as a single, integrated system. 
       A. Modular and Universal Airplane Ground Support Equipment Cart 
       [0045]    Airplane ground support equipment carts are wheeled, towable carts or fixed mounted (permanently or temporarily) devices that provide air conditioning, avionics equipment liquid cooling, and electrical power conversion and generation services to airplanes whose engines are shut down. These carts preferably should be conveyed by military and other airplanes to airports and military bases all over the world, so it would be convenient and an advantage to have this equipment be no larger than a standard military equipment conveyance palette. However, many such carts today do not fit one standard palette, and this reduces the numbers of ground support equipment that is available in the field. Traditionally, such ground support equipment carts are custom-designed—they provide such services to only one type or class of airplane. Hence, different carts must be provided for each different type of airplane. Also traditionally, the air conditioning components mounted on such carts are so bulky that they occupy the entire area of the cart, making it necessary to sandwich electrical power conversion and other components wherever there is room and thereby making it extremely awkward to service or replace such cart-mounted components. 
         [0046]    The present invention is embodied in a universal airplane ground support equipment cart—universal in the sense that it is designed to service the varied needs of a variety of types and classes of airplanes, rather than just one type or class. This ground support equipment cart is also modular—its components are rectangular modules that may be easily separated or removed from the cart for service or exchange. The modules may also be used independently of the cart, and modules not needed for a particular type of airplane may be readily removed and used elsewhere, standing by themselves, in a highly flexible manner. Such a cart  10  and several of its modules—an electrical power generation module  14 , an electrical power conversion module  20 , and a dual air conditioning module  400  (which also provides PAO liquid cooling)—are illustrated in simplified form in  FIGS. 1-3 . (Much more detailed drawings of these components are included in this application and also in the related applications cited above). 
         [0047]    In use, the cart  10  is mounted near or drawn up to an airplane (not shown) by a suitable tractor or truck (not shown). An operator connects an air conditioning plenum or air duct  26  from the dual air conditioning module  400  to a cooled air input port (not shown) on the airplane. And if the airplane has avionics or other electronic components that require a supply of liquid coolant, then the operator also connects a pair of PAO liquid coolant conduits  28  from the air conditioning module  400  to a pair of PAO ports on the airplane. The operator then uses a suitable electrical power cable (not shown) to connect an electrical power output port or receptacle (not shown in  FIGS. 1-3 ) on the electrical power conversion module  20  to a matching port or cable on the airplane. To supply the varying needs of different types of airplanes, there may be as many as two electrical power conversion modules  20  the cart  10 , a first module  20  having both a 115 volt, 400 Hz AC power output port and also a separate 270 volt DC power output port, and a second module  1308  ( FIG. 13 ) having a 28 volt DC power output port (one or the other of these modules  20  or  1308  may be removed from the cart  10 ). 
         [0048]    Next, with reference to  FIG. 13 , the operator depresses a “Start” pushbutton  1316  on the front panel of a control module  22  having a display screen  24  that then displays a main menu such as that shown in  FIG. 21 . If the airplane is a T-50 Golden Eagle, the operator depresses one of four pushbuttons  1304  that is adjacent the label “T-50 Golden Eagle” on this menu ( FIG. 21 ), and then the operator depresses one of four pushbuttons  1302  that is adjacent the label “Start” on a “T-50” menu that then appears ( FIG. 23 ). In response, all of the modules automatically reconfigure themselves as needed to service this specific type of airplane with air conditioning of the proper pressure and volume of air, with electrical power of the proper type, voltage, and frequency, and with liquid coolant (if needed). If the operator selects the wrong type of airplane, pressure and air flow measurements can detect this and shut down the system, illuminating a colored status light  1314  to signal an error and displaying an appropriate error message on the control panel  24  to the operator. The system is halted when the operator depresses a “Stop” pushbutton  1318  on the front of the control  22  or a pushbutton  1302  or  1304  that is adjacent the label “Stop” on one of the display screen  24  menus. 
         [0049]    A universal airplane ground support equipment cart is designed to provide flexible support for the needs of many different types and classes of airplanes having widely varying air conditioning and liquid cooling and electrical power support needs. The present invention can provide different pressures and volumes of cooled air and cooled liquid to different airplanes, and it can provide different types and quantities of electrical power to different airplanes. It also provides a simplified, integrated control panel where airplane service personnel can simply select the type of airplane that is to be serviced and have the various appliances on the cart automatically configured to optimize the support for that particular type of airplane. 
         [0050]    A modular airplane ground support equipment cart is one where the different support systems provided by the cart are each confined to rugged, compact, optionally EMI shielded, rectangular modules that may be easily removed, serviced, replaced, and used stand-alone separate from the cart and its other modular components. 
         [0051]    In the cart  10 , for example, a two-stage air conditioning module  400  contains all of the air conditioning components of the cart  10 , including a liquid PAO cooling system. An electrical power converter module  20  contains the power conversion components of the cart  10 , including a 270 volt D.C. supply and a 115 volt 400 Hz A.C. supply; and the module  20  may be replaced or supplemented with another module  1308  ( FIG. 13 ) that includes a 28 volt D.C. supply, providing up to three different types of electrical power conversion in accordance with the specialized needs of different types and classes of airplanes. 
         [0052]    A power supply module  14  contains a diesel engine and a generator for producing 60 cycle, three-phase, 460 volt electrical power when the cart cannot be conveniently hooked up to a 360 to 500 volt, 50 or 60 cycle A.C., three phase supply provided by the local power grid. The power supply module  14  is confined to one end of the cart  10  and may be detached from the cart  10 , as is illustrated in  FIG. 2 . 
         [0053]    Any or all of these modules  14 ,  20 ,  400 , and  1308  may optionally be equipped with an internal transformer (not shown) that transforms the incoming high voltage electrical power down to 120 volts or 240 volts at 50- or 60-Hz and feeds this low voltage to standard, weather protected outlets (not shown) which can be used to provide power to hand tools and to portable lighting equipment and the like, with ground fault protection also provided to these appliances. 
         [0054]    As is illustrated in  FIG. 13 , a control module  22  is mounted on the cart  10  above the power converter module  20 . The control module  22  has on its front panel a pair of start and stop pushbuttons  1316  and  1318 , colored status lights  1314 , and a display screen  24  having sets of four pushbuttons  1302  and  1304  positioned adjacent the display screen  24 &#39;s left and right sides. When turned on, the display screen  24  presents a main menu display, shown in  FIG. 21 , which permits airplane maintenance personnel to select the type of plane that is to be serviced by depressing one of the adjacent pushbuttons  1302  and  1304 . A maintenance menu display, shown in  FIG. 25 , permits service personnel to view and (in some cases) to alter the state of the air conditioning and PAO module  400 , the electrical power converter modules  20  and  1308 , and the power supply module  14 . As is illustrated schematically in  FIG. 13 , all of the modules  14 ,  20 ,  22 ,  400 , and  1308  are automatically networked together by a network  1312  when they are installed upon the cart  10 . In addition, each of the modules  14 ,  20 ,  22 ,  400 , and  1308  is equipped with a network jack (not shown) that can be connected to an external portable computer (not shown) which can then serve as the control module and display for all of the modules, with mouse clicks on the menus shown in  FIGS. 20 to 28  replacing depressions of the pushbuttons  1302  and  1304 . 
         [0055]    The cart  10  is optionally mounted upon two wheel and axle truck assemblies  18  and  19 . In the space on the cart  10  between the power generation module  14  and the two-stage air conditioning module  400 , one or both of the electrical power converter modules  20  and  1308  may be slid into place and attached to the cart  10 , as is illustrated in  FIGS. 2 and 3 . (If both are installed, they may be on opposite sides of the cart, as shown, or they may be installed one above the other.) 
         [0056]    If the power generation module  14  is not required for a particular airplane support task, the module  14  and the wheel and axle truck assembly  19  beneath the module  14  may be completely detached from the rest of the cart  10 , as is illustrated in  FIG. 2 , and removed to be used entirely separately elsewhere, wherever a portable source of 60 Hz, 460 volt, three-phase power is required. As illustrated in  FIGS. 2 and 3 , the electrical power converter modules  20  and  1308  may be slid out on tracks and locked in position to give service personnel convenient access for the servicing of these modules  20  and  1308  and their internal electrical and electronic components. They may also be removed for repair or for use elsewhere as stand-alone power converters, or they may be replaced with different power converter modules that generate different voltages and frequencies as needed for servicing different airplanes. 
       B. Two-Stage Air Conditioning and PAO Liquid Cooling System 
       [0057]    The two-stage air conditioning system and PAO liquid cooling system that is described below has many valuable attributes. Among others: It can achieve a 30-second air conditioner startup, rather than the many minutes that are required to start up conventional airplane ground support equipment air conditioners, due to the close control that is exerted over all aspects of the system and inherently low refrigeration system charge by minimizing internal volume of the refrigeration system (see  FIG. 15  and the accompanying descriptive material presented below). Since the digital control algorithms may be varied dynamically by the processor  1900  to suit unusual conditions, the air conditioner can still operate even if many sensors and controllers are inoperative based upon memory of past operations which can be relied upon to predict conditions in place of actual sensor readings to give control guidance. And as will be explained, an operator indicates on a menu ( FIG. 18 ) which type or class of airplane is to be serviced. If, when the air conditioner is started up initially at a lower blower speed than the final blower speed, the pressure and air flow measurements captured by the temperature, pressure, and power consumption measurement sensors do not correspond to that choice of type or class of airplane, the air conditioner can shut down and give the operator an appropriate warning message that the wrong type of airplane has most likely been selected. Other examples of the system&#39;s attributes are set forth below. An improved user interface is presented in an appendix to this application, where start and stop buttons and colored lamps are added to the display to improve its usability and the menus are adjusted accordingly. 
         [0058]    Referring now to  FIGS. 4 through 12 , the internal mechanical and fluid flow path details of the two-stage air conditioning module  400  are shown. The module  400  contains two air conditioning stages—a pre-cool air conditioner  520  (shown in  FIGS. 5 and 6 ) and a post-cool air conditioner  522  (shown in  FIGS. 5 and 7 ). The flow of air along a path  500  through the two air conditioners  520  and  522  stages are described in  FIG. 5 . The flow of coolant through the two air conditioners  520  and  522  stages are illustrated in  FIG. 6  (pre-cool air conditioner  520 ) and in  FIG. 7  (post-cool air conditioner  522 ). The pre-cool air conditioner  520  has associated with it a PAO liquid cooling system  700 .  FIG. 8  illustrates the flow of avionics liquid coolant through this PAO liquid cooling system  700  and between the system  700  and avionics  825  within an airplane  823 . The mechanical details of each of the air conditioner&#39;s plate fin evaporator arrays are illustrated in  FIGS. 9 and 10 , and the mechanical details of each of the air conditioner&#39;s micro-channel condenser coils are illustrated in  FIGS. 11 and 12 . 
         [0059]      FIG. 4  presents a perspective view of the two-stage air conditioning module  400  as seen from its rear side  402 , with the air duct  26  that conveys air conditioned air to the airplane (not shown) shown extending to the right (in  FIGS. 1 and 2 , the air duct  26  extends to the left). The side  402  is accordingly the side of the module  400  that is not adjacent the electrical power converter modules  20  and  1308  and the control module  22  when these modules  400 ,  20 ,  22 , and  1308  are all mounted on the cart  10 , as shown in  FIG. 1 . Accordingly, the module  400 &#39;s rear side  402  is always accessible for servicing the module  400  and is not blocked by the presence of the other modules. 
         [0060]    A hinged, louvered door  404  is shown swung open from the rear side  402  ( FIG. 4 ) of the module  400 , and this door gives service personnel unfettered access to all the air conditioning and PAO components within the module  400  for service and maintenance procedures, but would not be left open during operation. A second hinged, louvered door  408  is shown swung upwards from the top side of the module  400 . This door  408  gives service personnel access to the PAO system  700  components which are mounted near the top of the module  400 . 
         [0061]    The two louvered doors  404  and  408  each support a pair of thin, microchannel air conditioner condenser coils  406  and  410  the details of which coils are shown in  FIGS. 11 and 12  (discussed below). Each pair of two condenser coils  406  and  410  is associated with a respective one of the two air conditioners  520  and  522  stages mounted within the air conditioning module  400 . A two-speed condenser fan  414  blows air out of a fan portal  418  in one side  416  of the air conditioning module  400 —the side that is not connected by the air duct  26  to the airplane. When both the doors  404  and  408  are closed, the condenser fan  414  sucks air through both of the pairs of microchannel condenser coils  406  and  410 , cooling the refrigerant within the two condenser coils  406  and  410 . The fan  414  blows the air heated by passage through the two condenser coils  406  and  410  out the fan portal  418  on the side  416  of the cart  10  away from where service personnel viewing the display screen  24  or connecting up the air duct  26  or the PAO liquid coolant conduits  28  would normally stand. With reference to  FIGS. 15 and 18 , the fan  414  has low speed  415  and high speed  417  fan control signals which are generated by a controller  1518  which is implemented as an algorithm within the air conditioning and PAO processor  1900 . The controller responds to the ambient temperature and to various temperature and pressure signals shown in  FIGS. 5 ,  6 , and  7  by varying the fan  414  from off to low speed to high speed as needed to aid the processor in maintaining the proper operation of the two air conditioners  520  and  522 . This is another way for the system to adjust rated capacity, a way that is especially useful when the system is running at low capacities—that is, at low ambient conditions. 
         [0062]      FIG. 5  presents a schematic diagram of the air pathway  500  taken by air which is cooled, dehumidified, and compressed as it passes through the two-stage air conditioning module  400 . Outside air shown at  501  is sucked through the pre-cool air conditioner  520  by a blower  508  which then propels the air through the post-cool air conditioner  522  and through the air duct  26  from which it emerges as a stream of cooled, dehumidified, pressurized air that flows directly into the airplane (not shown). 
         [0063]    The pre-cool air conditioner  520  includes as components a first evaporator array  504  ( FIGS. 4 and 5 ) and a pair of the microchannel condenser coils  406  ( FIG. 4 ) plus other components all of which are shown together in  FIG. 6  (described below). The post-cool air conditioner  522  includes as components a second evaporator array  514  ( FIGS. 4 and 5 ) and a second pair of the microchannel condensers  410  ( FIG. 4 ) plus other components all of which are shown in  FIG. 7  (described below). The two air conditioners  520  and  522  are essentially identical except that the pre-cool air conditioner  520  includes a PAO heat exchanger  602  ( FIGS. 6 and 8 ) that absorbs heat from the PAO liquid coolant circuit  700  shown in  FIG. 8 . 
         [0064]    Referring now to  FIGS. 4 and 5 , air  501  that is to be dehumidified and cooled flows along the air pathway  500  first through an air filter  502  and next through the pre-cool air conditioner&#39;s  520  plate fin evaporator array  504 , where the air is partially cooled and dehumidified. The air next flows through a narrowing plenum  505  ( FIG. 4 ) and then onwards to the blower  508 , which propels the air forward at increased pressure. The air next passes through an outlet cone  510  ( FIG. 4 ) designed to convert velocity pressure coming from the blower  508  into static pressure (static regain) before making a turn through an elbow  512  ( FIG. 4 ). The air then flows into an expansion chamber or air funnel  513  ( FIG. 10 ) which contains a baffle plate that spreads out the air so that the air passes uniformly through all parts of the post-cool air conditioner  522 &#39;s plate fin evaporator array  514 . The further cooled and dehumidified air then flows through a narrowing plenum  516  ( FIGS. 4 ,  9 , and  10 ) and through a circular coupling  518  ( FIGS. 4 ,  9 , and  10 ) out the air duct  26  ( FIGS. 1 ,  4 , and  5 ) and onwards to the interior of the airplane (not shown). 
         [0065]    The blower  508  is driven by a variable-speed electric motor  506  the speed of which motor is controlled by the frequency of the motor  506 &#39;s incoming electric power. A voltage-to-frequency converter  525  accepts a serialized digital control signal  1706  which specifies the motor  506 &#39;s frequency and which is supplied by an air conditioner and PAO processor  1900  (a real-time process control computer system—see  FIG. 19 ). The converter  525  responds to that signal  1706  by varying the frequency of the input power to the motor  506  up and down in accord with the frequency called for by the control signal  1706  based upon a control algorithm that monitors the output pressure (measured by the pressure sensor  526 ). The processor  1900  receives a 0-to-10 volt pressure measurement signal from a pressure sensor  526  that measures the pressure within the ring  518  and air duct  26  that supplies cooled air to the airplane (not shown). With reference to  FIGS. 5 and 15 , the processor  1900  compares the pressure read by the pressure sensor  526  to a set-point desired pressure, which may vary from one type and class of plane to the next, and then adjusts the control signal  1706  so as to adjust the blower  508 &#39;s speed to a setting that maintains the pressure within the air duct  26  at or close to the proper pressure that is required to cool the particular type or class of airplane. 
         [0066]    In  FIG. 15 , a controller  1514  is shown symbolically comparing a setpoint pressure Psp to the air duct pressure measured by the pressure sensor  526  and then generating the signal  1706  which controls the blower  508  speed. The controller  1514  is actually implemented digitally within the processor  1900 . The controller  1514  would typically have a proportional component to minimize the pressure error and an integral component to drive that pressure error towards zero over time. The airplane selection process, described below in conjunction with the selection menu shown in  FIG. 21 , can alter the pressure setpoint Psp value as well as other temperature setpoint Tsp values (described below) to customize the air conditioner and PAO controllers shown in  FIG. 15  to the specific needs and requirements of differing types and classes of airplanes. When one of the pushbuttons  1304  adjacent the display screen  24  ( FIG. 13 ) is depressed, for example, to program the modules on the cart  10  to service the T-50 Golden Eagle (see  FIG. 21 ), the optimal temperature Tsp and pressure Psp setpoints for that airplane are selected by the air conditioning and PAO processor  1900  and are placed into a memory of setpoints  1317  ( FIG. 13 ). 
         [0067]      FIG. 17 , which is described below, describes other aspects of the blower  508  control algorithm in somewhat greater detail. 
         [0068]    Differential pressure sensors  528 ,  530 ,  532 , and  534  enable the processor  1900  to monitor the pressure drop across various air conditioning system components. These pressure readings are collected by the processor  1900  and saved in a data log  1319  ( FIG. 13 ) and are used later on for maintenance purposes. For example, an excessive pressure drop across the air filter  502  measured by the differential pressure sensor  528  signals that it soon will be time to clean or replace the air filter  502 . Excessive pressure drop across the evaporator arrays  504  or  514  measured by the differential pressure sensors  530  and  534  can signal icing of an evaporator array that is running too cold or a clogged evaporator array that requires cleaning. The pressure drop across the blower  508 , when compared to the signal  1706  frequency value and also the electrical power applied to the blower  508  (as measured by voltage sensor  1720  and current sensor  1722  both shown in  FIG. 18 ) can indicate the condition of the blower and its motor and whether servicing is needed. This information is saved in the processor  1900 &#39;s data log  1319  ( FIG. 13 ). 
         [0069]    Pressure sensor  536  (see  FIG. 5 ) monitors the outside air pressure, which is recorded by the processor  1900  in the data log  1319 . Pressure sensor  543  ( FIG. 5 ) monitors the output air pressure generated by the blower  508  which is also the air pressure within the air plenums, a pressure that can also be recorded by the processor  1900  in the data log  1319 . RTD (resistor temperature device) temperature sensors  538 ,  540 ,  542 , and  544  monitor the air temperature before and after the air passes through the two evaporator arrays  504  and  514 . These temperature measurements are fed into the processor  1900  which records them in the data log  1319  and can use them for predictive maintenance. As an option, some or all of these temperatures and pressures may be used to adjust the amount of cooling that is generated by each of the two air conditioners, as is illustrated in  FIG. 15 . 
         [0070]      FIGS. 6 and 7  present detailed schematic diagrams of the pre-cool air conditioner  520  and the post-cool air conditioner  522 . In one embodiment of the invention, the refrigerant tubing used in the construction of these air conditioners  520  and  522  is ACR copper tubing, with brazed joints and with many sweated fittings used to achieve a curved path of refrigerant flow. In another embodiment, aluminum tubing is used instead of copper tubing. A tube bender is then used in lieu of many sweated joints, and this reduces the number of parts used on each system, a great cost reducer. A great feature of aluminum is that it makes the system very lightweight and cost less when compared to copper, as aluminum weighs about 70% less than copper and cost approximately one-third as much. In addition, the use of flared fittings would also allow assembly to take place with pre-made lengths and tube configurations where the assembly technician would just need to turn a wrench instead of waiting for a skilled worker certified in copper brazing. This would also make field repairs much quicker than ever before. 
         [0071]      FIG. 6  presents a schematic diagram of the pre-cool air conditioner  520 . With reference to  FIG. 6 , a compressor  601  compresses the refrigerant and sends it along a path  604  to one of the pair of condenser coils  406 , where the refrigerant is cooled by air flowing through the air conditioning module  400  under the impetus of the condenser fan  414 , as was described above, where the refrigerant cools and becomes liquefied. The air conditioning and PAO processor  1900  ( FIG. 19 ) sends out a first on/off pre-cool shutoff signal to a solenoid valve  603  and an on/off pre-cool compressor on signal  1702  which can turn the pre-cool compressor  601  on and off (see  FIGS. 18 and 19 ) and which can shut down the pre-cool air conditioner  520  by shutting off the compressor  601  and isolating the compressor  601  from refrigerant migration by closing the solenoid valve  603 . The shutdown algorithm will then close all the refrigeration valves  620 ,  638 , and  632  ( FIG. 6 ) to further prevent refrigerant migration back to the compressor  601 . 
         [0072]    The cooled and liquefied refrigerant next flows past a charging valve  608 , a filter dryer  606 , and a sight glass  610  over a path  612  to a brazed plate heat exchanger  614  ( FIGS. 4 and 6 ) that is mounted at the very bottom of the air conditioning module  400 , as is shown at  614  in  FIG. 4 . The brazed plate heat exchanger  614  has a multi-purpose in its design: it serves as a liquid refrigerant accumulator that collects any excess liquid refrigerant and any excess oil that may be in the suction line between the compressor  601  and the evaporator array  504  to prevent damage to the compressor  601  (which is designed to pump vapor). The brazed plate heat exchanger  614  also serves as a liquid suction line sub-cooler that sub cools the liquid refrigerant by allowing the expanded gasses flowing along the path  628  and  630  and entering the compressor  601  to absorb heat from the liquid refrigerant in the lines  612  and  618  and in the brazed plate heat exchanger  614 . The liquid line side of the brazed plate heat exchanger  614  acts as a refrigerant receiver, accumulating excess refrigerant charge on the condenser side of the system. The brazed plate heat exchanger  614  increases the capacity and efficiency of the cooling system at high load conditions. Finally, the brazed plate heat exchanger is used to control the suction line superheat, allowing the evaporators to be fully flooded. Flooding the evaporators allows high cooling capacity from the evaporators as well as increasing the evaporator capacity while maintaining higher refrigerant temperature which helps avoid evaporator frosting. 
         [0073]    The path  618  conducts the cooled but still liquefied refrigerant to an electronically controlled expansion valve  620  that is controlled by a 0-to-10 volt signal generated by the processor  1900 . The liquid refrigerant flows through the expansion valve  620  into the low pressure, cool side of the refrigerant circuit, where the liquid begins to vaporize and absorb heat from its surroundings. This boiling liquid passes first through the PAO heat exchanger  602 , where it cools the liquid PAO fluid flowing into a line  622  and out a line  624  which lines lead to the PAO fluid circuit (shown at  700  in  FIG. 8 ). The boiling refrigerant flows onward over the path  626  to the plate fin evaporator array  504  essentially identical to the evaporator array  514  shown in  FIG. 9 , where the refrigerant cools the air that is sucked into the module  400  at  501  ( FIG. 5 ) from the outside air, through the air filter  502  and the plate fin evaporator array  504  and into the blower  508 . The gaseous refrigerant leaves the plate fin evaporator array  504  and flows along the path  628  back through the brazed plate heat exchanger  614  and over the path  630  back to the compressor  601  where it is once again compressed and fed into the pair of condenser coils  406  to be compressed, thus completing the passage of refrigerant through this vapor compression cycle. 
         [0074]    Combined temperature and pressure transducers monitor the condition of the refrigerant throughout this circuit. An RTD (resistor temperature device) temperature and pressure transducer  607  monitors the temperature and pressure of the liquid refrigerant as it leaves the condenser coils  406  and enters the brazed plate heat exchanger  614 . A second RTD temperature and pressure transducer  616  monitors the temperature and pressure of the liquid refrigerant as it leaves the brazed plate heat exchanger  614  over the path  618  and flows through the expansion valve  620 . Another temperature and pressure transducer  634  monitors the temperature and pressure of the gaseous, cooled refrigerant flowing out of the plate fin evaporator array  504 . A pair of temperature and pressure transducers  609  and  611  monitors the temperature and pressure of the gaseous refrigerant entering the compressor  601  and also leaving the compressor  601 . The refrigerant temperature and pressure readings generated by all of these transducers  607 ,  616 ,  634 ,  609 , and  611  and also the pre-cool condenser air output temperature measured by the RTD air temperature transducer  540  are fed into the air conditioning and PAO processor  1900  (see  FIG. 19 ) where these temperatures and pressures may be stored in the data log  1319  ( FIG. 13 ). 
         [0075]    The refrigerant temperatures measured by the RTD temperature transducers  609 ,  616  and  634  and the air temperature measured by the pre-cool air conditioner output RTD temperature transducer  540  are also used for air conditioner control purposes, as is illustrated in  FIG. 15 . 
         [0076]    The pre-cool air conditioner output temperature measured by the RTD temperature transducer  540  is compared to a setpoint temperature, typically 10 degrees Celsius or thereabouts, by means of a controller  1506  which is implemented as a digital process control algorithm within the air conditioner and PAO processor  1900 . As the desired output temperature is adjusted by the user, this setpoint temperature can be altered. This controller  1506  is given both proportional and integral outputs which are summed and used (as a 0-to-10 volt signal) to control an electronic exhaust gas bypass valve  638  (EGBV— FIGS. 6 and 15 ) which, to the degree it is open, permits compressed, hot gas to bypass the condenser coils  406  and the expansion valve  620  and to flow directly from the compressor  601  into the evaporator array  501 , thereby raising the temperature and boiling excess liquid refrigerant within the evaporator array  504 . The processor  1900  continuously adjusts this EGB valve  638  to maintain the air temperature at the pre-cool air conditioner&#39;s plate fin evaporator array  504  outlet at or just above freezing so that the evaporator array  504  is not permitted to ice up. 
         [0077]    The refrigerant temperature (measured by the RTD transducer  616 ) at the outlet of the electronic expansion valve (EEV)  608 , which is the inlet into the PAO heat exchanger  602  and plate fin evaporator array  504 , is fed into another controller  1502  ( FIG. 15 ), which is also implemented as a digital process control algorithm within the air conditioner and PAO processor  1900 . This controller  1502  is also given both proportional and integral outputs which are summed and used (as a 0-to-10 volt signal) to control an electronic evaporator array pressure regulator valve EPR  632  ( FIGS. 6 and 15 ) which valve controls how much cooled, expanded, gaseous refrigerant is permitted to enter the compressor  601 . In this manner, the temperature at the input to the evaporator array  504  and the PAO liquid heat exchanger  602  are controlled and maintained at a setpoint value Tsp, which value is fed into the controller  1502  ( FIG. 15 ). This setpoint is typically kept at 1 degree Celsius. As the desired unit output temperature is adjusted by the user, this setpoint may be altered. The air conditioning and PAO processor  1900  maintains this setpoint value, as well as other similar temperature and pressure setpoint values, in a memory for setpoints  1317  ( FIG. 13 ) where these values may sometimes be altered when different types and classes of airplanes are being serviced. 
         [0078]    The refrigerant temperature (measured by the RTD transducer  616 ) at the outlet of the electronic expansion valve (EEV)  608 , which is the inlet into the PAO heat exchanger  602  and plate fin evaporator array  504 , is compared to the refrigerant temperature (transducer  634 ) at the outlet of the plate fin evaporator array  504  by another controller  1504 , which is also implemented as a digital process control algorithm within the air conditioner and PAO processor  1900 . This controller  1504  may initially be given both proportional and integral outputs which are summed and used (as a 0-to-10 volt signal) to control the electronic expansion valve EEV  608  ( FIGS. 6 and 15 ) which valve controls to what extent the entire evaporator array  504  is thoroughly wetted and participating in the cooling process. Experiments have shown, however, that the controller  1504  may have to be programmed in a nonlinear manner, with the control parameters worked out empirically by experiment and varying from a simple proportional and integral controller to some degree. The EEV  608  is adjusted to maximize the effective cooling area of the evaporator array, as is indicated by a maximum temperature drop across the plate fin evaporator array  504 . The air conditioning and PAO processor  1900  may maintain different control algorithms for the controller  1504  as well as the other controllers  1502  and  1506  in the memory of setpoints  1315  ( FIG. 13 ) so that different control algorithms and strategies may be selected and implemented for different types and classes of airplanes which are being serviced. 
         [0079]      FIG. 7  presents a schematic diagram of the post-cool air conditioner  522 . With reference to  FIG. 7 , a compressor  702  compresses the refrigerant and sends it along a path  704  to one of the pair of condenser coils  410 , where the refrigerant is cooled by air flowing through the air conditioning module  400  under the impetus of the condenser fan  414 , as was described above, where the refrigerant cools and becomes liquefied. The air conditioning and PAO processor  1900  ( FIG. 19 ) sends out a first on/off post-cool shutoff signal to a solenoid valve  703  and an on/off post-cool compressor on signal  1704  which can turn the post-cool compressor  702  on and off (see  FIGS. 18 and 19 ) and which can shut down the post-cool air conditioner  522  by shutting off the compressor  702  and isolating the compressor  702  from refrigerant migration by closing the solenoid valve  703 . The shutdown algorithm will then close all the refrigeration valves  720 ,  738 , and  732  ( FIG. 7 ) to further prevent refrigerant migration back to the compressor  702 . 
         [0080]    The cooled and liquefied refrigerant next flows past a charging valve  708 , a filter dryer  706 , and a sight glass  710  over a path  712  to a brazed plate heat exchanger  714  ( FIGS. 4 and 7 ) that is mounted at the very bottom of the air conditioning module  400 , as is shown at  714  in  FIG. 4 . The brazed plate heat exchanger  714  has a multi-purpose design: it serves as a liquid refrigerant accumulator that collects any excess liquid refrigerant and any excess oil that may be in the suction line between the compressor  702  and the evaporator array  514 , preventing damage to the compressor  702  (which is designed to pump vapor). The brazed plate heat exchanger  714  also serves as a liquid suction line sub-cooler that sub cools the liquid refrigerant by allowing the expanded gasses flowing along the path  728  and  730  and entering the compressor  702  to absorb heat from the liquid refrigerant in the line  712  and  718  and in the brazed plate heat exchanger  714 . The liquid line side of the brazed plate heat exchanger  714  acts as a refrigerant receiver, accumulating excess refrigerant charge on the condenser side of the system. The brazed plate heat exchanger  714  increases the capacity and efficiency of the cooling system at high load conditions. Finally, the brazed plate heat exchanger is used to control the suction line superheat, allowing the evaporators to be fully flooded. Flooding the evaporators allows high cooling capacity from the evaporators as well as increasing the evaporator capacity while maintaining higher refrigerant temperature which helps avoid evaporator frosting. 
         [0081]    The path  718  conducts the cooled but still liquefied refrigerant to an electronically controlled expansion valve  720  that is controlled by a 0-to-10 volt signal generated by the processor  1900 . The liquid refrigerant flows through the expansion valve  720  into the low pressure, cool side of the refrigerant circuit, where the liquid begins to vaporize and absorb heat from its surroundings. This boiling liquid flows to the plate fin evaporator array  514 , shown in  FIGS. 9 and 10 , where the refrigerant cools the air that is blown out through the air duct  26  to the airplane (not shown). The gaseous refrigerant leaves the plate fin evaporator array  514  and flows along the path  728  back through the brazed plate heat exchanger  714  and over the path  730  back to the compressor  702  where it is once again compressed and fed into the pair of condensers  410  to be cooled and liquefied, thus completing the passage all the way through this circular refrigerant circuit. 
         [0082]    Combined temperature and pressure transducers monitor the condition of the refrigerant throughout this circuit. An RTD temperature and pressure transducer  707  monitors the temperature and pressure of the liquid refrigerant as it leaves the pair of condensers  410  and enters the brazed plate heat exchanger  714 . A second RTD temperature and pressure transducer  716  monitors the temperature and pressure of the liquid refrigerant as it leaves the brazed plate heat exchanger  714  over the path  718  and flows through the electronic expansion valve  720 . Another temperature and pressure transducer  734  monitors the temperature and pressure of the gaseous, cooled refrigerant flowing out of the plate fin evaporator array  514 . A pair of temperature and pressure transducers  709  and  711  monitors the temperature and pressure of the gaseous refrigerant entering the compressor  702  and also leaving the compressor  702 . The refrigerant temperature and pressure readings generated by all of these transducers  707 ,  716 ,  734 ,  709 , and  711  and also the post-cool condenser air output temperature measured by the RTD air temperature transducer  544  are fed into the air conditioning and PAO processor  1900  (see  FIG. 19 ) where these temperatures and pressures may be stored in the data log  1319  ( FIG. 13 ). 
         [0083]    The refrigerant temperatures measured by the RTD temperature transducers  709 ,  716  and  734  and the air temperature measured by the pre-cool air conditioner RTD output temperature transducer  544  are also used for air conditioner control purposes, as is illustrated in  FIG. 15 . 
         [0084]    The pre-cool air conditioner output temperature measured by the RTD temperature transducer  544  is compared to a setpoint temperature, typically 10 degrees Celsius or thereabouts, by means of a controller  1512  which is implemented as a digital process control algorithm within the air conditioner and PAO processor  1900 . As the desired output temperature is adjusted by the user, this setpoint temperature can be altered. This controller  1512  is given both proportional and integral outputs which are summed and used (as a 0-to-10 volt signal) to control an electronic exhaust gas bypass valve  738  (EGBV— FIGS. 7 and 15 ) which, to the degree it is open, permits compressed, hot gas to bypass the pair of condenser coils  410  and the expansion valve  720  and to flow directly from the compressor  702  into the evaporator array  514 , thereby raising the temperature and boiling excess refrigerant liquid within the evaporator array  514 . The processor  1900  continuously adjusts this EGB valve  738  to maintain the air temperature at the pre-cool air conditioner&#39;s plate fin evaporator array  514  outlet at or just above freezing so that the evaporator array  514  is not permitted to ice up. 
         [0085]    The refrigerant temperature (measured by RTD transducer  716 ) at the outlet of the electronic expansion valve (EEV)  708 , which is the inlet into the plate fin evaporator array  514 , is fed into another controller  1508  ( FIG. 15 ), which is also implemented as a digital process control algorithm within the air conditioner and PAO processor  1900 . This controller  1508  is also given both proportional and integral outputs which are summed and used (as a 0-to-10 volt signal) to control an electronic evaporator array pressure regulator valve EPR  732  ( FIGS. 7 and 15 ) which valve controls how much cooled, expanded, gaseous refrigerant is permitted to enter the compressor  702 . In this manner, the temperature at the input to the evaporator array  514  is controlled and maintained at a setpoint value Tsp, which value is fed into the controller  1508  ( FIG. 15 ). This setpoint is typically kept at 1 degree Celsius. As the desired unit output temperature is adjusted by the user, the setpoint may be altered. The air conditioning and PAO processor  1900  maintains this setpoint value, as well as other similar temperature and pressure setpoint values, in a memory for setpoints  1317  ( FIG. 13 ) where these values may sometimes be altered when different types and classes of airplanes are being serviced. 
         [0086]    The refrigerant temperature (measured by RTD transducer  716 ) at the outlet of the electronic expansion valve (EEV)  708 , which is the inlet into the plate fin evaporator array  514 , is compared to the refrigerant temperature (transducer  734 ) at the outlet of the plate fin evaporator array  514  by another controller  1510 , which is also implemented as a digital process control algorithm within the air conditioner and PAO processor  1900 . This controller  1510  may initially be given both proportional and integral outputs which are summed and used (as a 0-to-10 volt signal) to control the electronic expansion valve EEV  708  ( FIGS. 7 and 15 ) which valve controls to what extent the entire evaporator array  514  is thoroughly wetted and participating in the cooling process. Experiments have shown, however, that the controller  1510  may have to be programmed in a nonlinear manner, with the control parameters worked out empirically by experiment and varying from a simple proportional and integral controller to some degree. The EEV  708  is adjusted to maximize the effective cooling area of the evaporator array, as is indicated by a maximum temperature drop across the plate fin evaporator array  514 . The air conditioning and PAO processor  1900  may maintain different control algorithms for the controller  1510  as well as the other controllers  1512  and  1508  in the memory of setpoints  1315  ( FIG. 13 ) so that different control algorithms and strategies may be selected and implemented for different types and classes of airplanes which are being serviced. 
         [0087]    The compressors  601  and  702  are part number ZR300-KCE-TWD-250 of Copeland, Sidney, Ohio. The suction line subcoolers or brazed plate heat exchangers  614  and  714  are part number AA6259 of SWEP International, Landskrona, Sweden. Pressure transducers are part number MX5018 provided by Gems Sensors &amp; Controls, Plainville, Conn. The pairs of 60″ by 34″ preassembled microchannel condenser coils  406  and  410  are part number 26944 D13 custom assembled by Trilectron Industries, Palmetto, Fla. 
         [0088]    Clearly, the proper operation of the air conditioner components just described is dependent upon the proper operation of many air and refrigerant temperature and pressure measurements. If any of the measurement instruments fail, the air conditioners  520  and  522 , under the control of the air conditioning processor  1900 , tries to continue operating, with warning messages, substituting for actual temperature and pressure measurements historical temperature and pressure measurements recorded on earlier days when the ambient weather conditions and the type or class of airplane were the same. In this manner, the air conditioning system continues to operate even when some of its sensors and controllers are inoperative. 
         [0089]    A major advantage of the air conditioning system just described is its ability to enter a fully operative state, providing full pressure of air cooled to the proper temperature, within 20 seconds or so of when it is first started up, rather than several minutes later as in conventional air conditioning airplane ground support equipment. 
         [0090]    As explained above, the PAO liquid coolant system  700  derives its cooling from the heat exchanger  602  which is part of the pre-cool air conditioner  520 . The PAO system  700  does not derive its cooling from the post-cool air conditioner  522 . Accordingly, when the PAO system is in operation, it reduces the cooling capacity of the pre-cool air conditioner  520 . The post-cool air conditioner  522  may be adjusted upwards in the amount of cooling which it provides to the air flowing through the air duct  26  to the airplane so that the turning on and off of the PAO system  700  does not necessarily alter the temperature and pressure of the cooled and dehumidified air provided to the airplane by the air duct  26 . 
         [0091]    The PAO cooling system  700  is illustrated schematically in  FIG. 8 . Most of the elements of the PAO cooling system are positioned within the air conditioning module  400  near the top of that module, adjacent to the pair of PAO liquid coolant conduits  28  shown in  FIGS. 1 and 2  that convey the PAO liquid to and from the airplane to shorten the length of the PAO liquid coolant conduits  28  as much as possible. A PAO liquid reservoir  803  is positioned lower down within the module  400  to serve as a reservoir for reserve amounts of the PAO liquid coolant. 
         [0092]    With reference to  FIG. 8 , a PAO fluid pump  805  pumps PAO fluid through a first check valve  807  and through a second check valve  809  to the heat exchanger  602 , which is part of the pre-cool air conditioner  520 , as was explained above. The cooled PAO fluid then flows onwards over a path  811  through filters  813  and over a path  817  to a supply solenoid valve  819  that is turned on and off by the processor  1900  ( FIG. 19 ). When the valve  819  is open, the PAO fluid flows over a path  820  out of the air conditioning module  400  and over a conduit  822  into an airplane  823  where it flows through and cools electronics and avionics components  825 . 
         [0093]    The PAO fluid then flows over a second conduit  824  back from the airplane  823  to the air conditioning module  400  and over a path  826  that leads to the PAO liquid reservoir  803  where it collects, waiting to be drawn back out by the pump  828  and sent back to the heat exchanger  602  again. That completes the PAO liquid coolant circuit. 
         [0094]    The PAO liquid coolant collects in the liquid reservoir  803 . A liquid level sensor  824  signals to the air conditioning and PAO processor  1900  ( FIG. 19 ) when the liquid level is too low. When the PAO system is cabled up to an airplane, there is typically air in the conduits  822  and  824  and possibly in the electronics and avionics as well. When the PAO system is first turned on, the solenoid valve  819  is opened and then the PAO pressure is slowly raised up to the proper operating pressure. Any air present in the system collects above the liquid in the reservoir  803 , and a vacuum pump  833 , actuated by the air conditioning and PAO processor  1900  ( FIG. 19 ), pumps this air out of the liquid reservoir  803 . This prevents overheating of the electronics and avionics  825  caused by air displacing the PAO liquid coolant in the circulating system. 
         [0095]    A 3-way proportional flow regulator valve  828  ( FIGS. 8 and 15 ) controls and continuously adjusts a liquid coolant bypass path  829 - 830  that bypasses the heat exchanger  602  with some of the PAO liquid to reduce the cooling effect. The regulator  828  receives temperature signals directly from a temperature and pressure sensor transducer  832 . Alternatively, the processor  1900  can implement a digital controller which can compare the temperature measured by the transducer  832  to an adjustable setpoint temperature and then adjusts the regulator valve  828  accordingly. 
         [0096]    A bypass path  834  is controlled by an electronically controlled proportional flow restriction valve  821  having a pressure setpoint that can be set and varied by the air conditioning and PAO processor  1900 . As is illustrated in  FIG. 15 , a controller  1516  compares the PAO system output pressure, as measured by the temperature and pressure sensing transducer  832 , to a setpoint pressure Psp (stored in the memory for setpoints  1317  ( FIG. 13 ) and then amplifies the pressure difference using proportional and integral control functions to generate control signals which are summed and then fed as a control signal to the electronically controlled proportional flow restriction valve  821 . The controller  1516  can be implemented as a control algorithm within the air conditioning and PAO processor  1900 . The pressure setpoint Psp may be varied in accordance with the specific needs of different types and classes of airplanes being serviced in response to airplane selection using the main menu shown in  FIG. 21 . It is also possible to have the restriction valve  821  respond directly to pressure indicating signals from the transducer  832  without the use of the processor  1900  and the controller  1516 , and this is the arrangement actually shown in  FIG. 8  (for this reason,  FIG. 19  does not presently show an output signal from the processor  1900  leading to the valve  821 ). 
         [0097]    To protect the PAO system  700  from transients, a bypass valve  835  can be actuated by excessive pressure sensed by the transducer  832  and opened to bypass the heat exchanger  602 , pump  805 , and reservoir  803 . The controller  1516 , implemented within the processor  1900 , opens the bypass valve  835 . 
         [0098]    A PAO hydraulic manifold assembly, part number AGA15700-0-C, which includes the components  819 ,  821 , and  835 , can be obtained from the Rexroth Bosch Group. The PAO pump  833 , Model 4600-20, comes from McNally Industries, Grantsburg, Wis. The PAO heat exchanger  602  is part number AA 6283 of Swep International, Landskrona, Sweden. The PAO pump pressure relief system is part number a971207 zc 04a2 is supplied by Sun Hydraulics, Sarasota, Fla. 
         [0099]    To increase the efficiency and also to decrease the size of the evaporator arrays  504  and  514 , in one embodiment these evaporator arrays are each constructed from four automotive plate fin evaporator arrays  802 ,  804 ,  806 , and  808  ( FIGS. 9 and 10 ) assembled into a roughly square frame  810  and held in place by a cover plate  812 . The assembled frame  810  and plate fin evaporator arrays  802 - 808  shown in  FIG. 8  is used to construct each of the two evaporator arrays  504  and  514 . The evaporator array  514  is shown in  FIGS. 9 and 10  attached to an incoming expansion chamber or air funnel  513  ( FIG. 9 ) which accepts air flowing out of the blower  508  and spreads this air out in a uniform manner over the surface of the four plate fin evaporator arrays  802 - 808  to maximize the cooling efficiency of this unit. Air flows out of the evaporator array  514  into a second funnel  516  which ducts the air to an outgoing cooled air port  518  to which is attached the air duct  26  ( FIGS. 1 ,  2 ,  4 , and  5 ) that conveys the cooled air to the airplane. The evaporator array  504  (not shown in FIGS.  8  and  9 —shown in  FIGS. 4 and 5 ) receives outside air  501  that flows through the air filter  502  directly into the evaporator array  504 . Air flows out of the evaporator array  504  through a funnel  505  ( FIG. 4 ) directly into the blower  508 . The plate and fin design of the evaporator arrays  504  and  514  allows them to be inexpensive, compact, and highly efficient. 
         [0100]    To decrease the size and increase the efficiency of the condenser coils  406  and  410 , each condenser is constructed from a pair of overlaid and interconnected microchannel condenser coils. With reference to  FIG. 4 , these pairs of condenser coils  406  and  410  are long and wide enough to be mounted on the panels or door assemblies  404  and  408 . The door  404  may be swung open to give convenient access to the other mechanical air conditioning components within the module  400 , as is shown. The pairs of condenser coils  406  and  410  are quite thin, so they do not take up much room within the air conditioning module  400 , unlike prior tube and fin arrangements which were much more bulky. 
         [0101]    With reference to  FIGS. 11 and 12 , the microchannel condenser coils (used in pairs to construct the condenser coils  406  and  408 ) are each constructed as a pair of parallel, spaced-apart refrigerant pipes  1002  and  1004  having narrowed or tapered end sections  1006  and  1008  for convenient attachment to copper or flexible tubing. Hollow, rectangular ducts  1010  are mounted between and perpendicular to the pipes  1002  and  1004 , with the ends of the ducts  1010  passing through slots cut partway through the sides of the pipes  1002  and  1004 , as is best shown in  FIG. 12 . The rectangular ducts  1010  are further partitioned internally by partitions  1012  into very small, rectangular channels that provide paths through which the refrigerant may pass between the two pipes  1002  and  1004  flowing through the ducts  1010 . The spaces between the rectangular ducts  1010  are then filled in with folded, thin aluminum fins folded accordion style to maximize heat transfer between the air flowing through the microchannel condenser coil and the refrigerant flowing from the pipe  1002  to the pipe  1004 . These aluminum fins, as well as the arrangement of pairs of condenser coils, force the air to travel a zigzag course, and this further adds to the efficiency of the design. 
         [0102]    Further details concerning the general design of such microchannel condenser coils may be found in U.S. Pat. No. 6,988,538 which issued to Justin P. Merkys, et al. on Jan. 24, 2006. 
         [0103]    Referring now to  FIG. 13 , all of the modules  14 ,  20 ,  22 ,  400 , and  1308  are shown to be networked together by a network  1312 , which in one embodiment is realized using a CAN bus, developed by CIA (CAN In Automation), Erlangen, Germany. Clearly, other bus protocols can also be used, including Ethernet and TCP/IP to network these components together. The CAN bus is one designed particularly for use in a hostile, automotive, outdoors environment. The control module  22  communicates with the can bus network  1312  using a cart network bus driver  1310 , and all the other module-based processors do likewise (not shown in  FIG. 13 ). 
         [0104]    The control module  22  is shown to have a display screen  24  that has an array of four pushbuttons  1302  to its left and a second array of four pushbuttons  1304  to its right, aligned with menu selections on displayed images (see menus and submenus,  FIGS. 21-28 ). The menus are stored within a universal control and diagnostic processor  1306  which manages the display screen  24  and also manages some diagnostics tasks and the like. The processor  1306  inquires over the bus network  1312  as to which modules are present, and it tailors the displayed information accordingly. Menus and diagnostics are not displayed for any module that is not present and operating. 
         [0105]    A hierarchical arrangement of one possible set of menus and other displays is shown in  FIG. 20 . When the system is first turned on, a main screen or menu (shown in detail in  FIG. 21 ) is displayed. This main menu permits the operator of the cart  10  to simply select which of several airplanes the ground support equipment cart is to service. If the operator depresses the pushbutton adjacent the “T-50 Golden Eagle” item, a secondary menu shown in  FIG. 23  is displayed. When the operator depresses the pushbutton adjacent the “Start” item, the air conditioners, one of the power sources, and the PAO liquid cooling system are all started up. The processor  1306  conveys to the processors within other modules, and in particular the air conditioner and PAO processor  1900 , the identity of the plane that is to be serviced (the T-50), and this allows, for example, the air conditioner and PAO processor to adjust the setpoints  1317  that control the operation of the two air conditioners and the PAO system in accordance with the specialized needs of the T-50 class of airplanes.  FIG. 15  illustrates many of the temperature Tsp and pressure Psp setpoints whose settings may be adjusted in this manner to adapt the equipment on the cart  10  to the needs of particular types and classes of airplanes. 
         [0106]    The operator may return to the main menu ( FIG. 21 ) and depress the pushbutton adjacent “Maintenance,” and then a maintenance menu is displayed ( FIG. 25 ). From this maintenance menu, one may navigate to a Data Log display ( FIG. 26 ) where one may scroll through a log of temperatures, pressures, and other data gathered over time. This data log information  1319  ( FIG. 13 ) is also available for further processing by the universal control and diagnostics processor  1306  which can generate reports predicting such things as when certain components will require service or are likely to fail. For example, a gradual increase in the differential pressure across the air filter  502  measured by the differential pressure sensor  528  would enable one to predict when the filter  502  will have to be cleaned or replaced. Other similar maintenance and repair prediction reports can be generated in this manner by the diagnostic processor  1306 . The data log  1319  is maintained by processors (such as the processor  1900 ) within each module, so that this information stays with each module if the modules are moved about and separated. 
         [0107]    Other more focused maintenance reports may be displayed. For example a pre-cool air conditioner status report ( FIG. 27 ) indicates the current status of the air conditioner  520 , indicating such useful things as how much refrigerant is currently bypassing the pair of condenser coils  406  by flowing through the bypass valve  638  to reduce the temperature of the evaporator array  504 , as was explained above. The current settings of the expansion valve  620  and of other valves and the speed of the condenser fan  414  are also indicated, along with the on/off state of the two compressors  601  and  702 . 
         [0108]    Help menus are also provided, as is shown in the illustrative menus shown in  FIGS. 22 and 24 . 
         [0109]      FIG. 29  presents some mechanical details of the display screen  24 . The display screen  24  is a black-and-white, electroluminescent display that is fully operable over extreme ranges of temperature. The display screen  24  is sandwiched together with a metal screen  2902  and with a protective plastic cover plate  2904  all of which are mounted to the side of the control module  22  facing an operator standing before the cart  10 . The screen  2902  provides radio frequency shielding to the display, preventing signals from leaking either into the control module  22  or out of the control module  22 . This rugged, simple arrangement of an all-weather display and eight rugged pushbuttons  1302  and  1304  provides an all-weather display that combines many displays and controls which, in prior designs for ground support equipment carts, were scattered all over the cart, with separate gauges and controls for each appliance, and with no uniformity of control. 
         [0110]    Referring now to  FIG. 14 , the state machine of a master process  1400  for the air conditioner and PAO processor  1900  is shown. When power is applied to the module  400 , the processor  1900  initiates a boot sequence  1402  that prepares the processor  1900  for operation. The boot sequence  1402  enables the processor  1900  to determine whether it is configured as a “stand-alone” or “cart-mounted” module. If the module  400  is cart mounted, it waits (at step  1406 ) for the start command to come in from the CAN data bus after actuation of a START menu command on an airplane-specific menu such as that shown in  FIG. 23 . Otherwise, the processor  1900  seeks discrete signals from its own more primitive user interface (possibly a portable computer plugged into the module  400  using an Ethernet, CAN, or USB portal. 
         [0111]    After the boot sequence  1402 , the processor  1900  enables a data logging sub-machine  1404 . The data logging sub-machine  1404  receives present sensor signals from the module  400  and records them in the data log  1319 . This data log is used by the processors  1900  and  1306  for predictive failure and for enhanced diagnostic functioning, as has been explained. 
         [0112]    After the processor  1900  enables the data logging sub-machine  1404 , it enters an idle state at  1406 . In the idle state  1406 , the processor  1900  waits for an “On” command to arrive, as from the “Start” command on the airplane-specific menu shown in  FIG. 23 . This “On” command may come from the CAR data bus or the user interface of the module  400 . After the processor  1900  receives the “On” command, it exits the idle state  1406  and enters the check power state at  1408 . 
         [0113]    In the check power state  1408 , the processor  1900  performs a self test. Stored default parameters or menu-selected operating parameters are given to the processor  1900  at power up. These operating parameters set the setpoint  1317  temperatures and pressures that the processor  1900  desires to achieve (see the values Tsp and Psp shown in  FIG. 15 ). These operating parameters are adjusted to those appropriate to the output temperatures and pressures (and electrical power) required by any given airplane that may need to be connected to the air conditioning and PAO module  400 . After the check power state is done, the processor  1900  stages the compressors  601  and  702  and the blower  508  on to assure that the air conditioner output temperature (measured by the transducer  544 ) is not permitted to exceed an undesired level. Additionally, the processor  1900  stages all of the remaining big loads on to prevent undue transient loading of the electrical source of power. Thus, the PAO system  700  is gradually brought up to pressure and down in temperature, and the vacuum pump  833  clears the PAO system of air before it comes fully on line. 
         [0114]    While in the check power state  1408 , the processor  1900  also auto-detects the input power type (using transducers  1708  to  1718  shown in  FIG. 18 ) and varies the two air conditioners&#39;  520  and  522  and the PAO system&#39;s  700  settings accordingly, degrading the maximum obtainable performance to reflect less power availability or the need to provide PAO cooling in addition to air cooling. For example, if the processor  1900  detects a lower input voltage than a desired input voltage on the transducers  1710 ,  1712 , and  1716 , the processor  1900  may adjust the setpoints  1317  to provide less cooling to the airplane to compensate for this. This automatic response to changing power conditions allows the user seamless use of the unit regardless of the city or country in which the unit is being operated. 
         [0115]    If the processor  1900  senses no power or abnormal power for ten seconds, it disables any machines presently running, attempts to isolate the power fault, and then enters a system fault triggering alarm state  1422 . Such a system fault is announced with an audible and visible alarm. So long as any power is available to it, the processors  1900  and  1306  continue operate, allowing isolation of the fault and continued use of the remainder of the modules. Capacitors that momentarily store charge provide brief continued running time for the processors  1900  and  1306  following a power failure. In an alternative arrangement, back-up batteries could be provided within each module to provide the module processors with continued power to operate and to perform diagnostics when power is not available for some reason. 
         [0116]    If adequate power is available, the processor  1900  enters the enable sub-state machines state  1410  where it starts up various real-time background processes. From the state  1410 , the processor  1900  proceeds to the run state  1412 . In the run state  1412 , the processor  1900  commences normal operation. Under normal operation, the processor  1900  achieves the desired output parameters (the given setpoint temperatures and pressures) as efficiently as possible by staging the condenser fan  414  to slow and fast settings and by adjusting the air conditioning and PAO parameters to produce the desired output. The selected parameters or setpoints are utilized as is shown in  FIG. 15 , where the controllers  1502 ,  1504 ,  1506 ,  1508 ,  1510 ,  1512 ,  1514 , and  1516  are all implemented as process control digital algorithms executing as control chains instituted within the air conditioning and PAO processor  1900  such that each controller implements a control chain within the processor  1900  that becomes part of one of the feedback control loops shown in  FIG. 15  within the two air conditioners  520  and  522  and the PAO system  700 . Data log processing continues during this normal operation of the processor  1900 . 
         [0117]    Maintenance and diagnostics are also carried out by the two processors  1900  and  1306 . The data log  1319  is collected for use in predictive failure and enhanced diagnostics. In the event of a minor component failure or imminent major component failure, the processor  1306  enters a fail safe state  1418 . If, based on the data collected, there is a danger of continued operation, the processor  1306  announces a fatal system fault and enters the alarm state  1422  and immediately shuts down the unit at  1420 . If the data log  1319  indicates that the unit is operating outside of its normal operating range, the processor  1306  announces a system fault and enters the alarm state  1422  but does not necessarily shut down the entire module  400 . If the data log  1319  indicates that a problem may occur in the near future, the processor  1306  may simply announce a systems warning and enter the fail safe state  1418 . The fail safe state  1418  does not sound an alarm, but it shows an indication on the display  22  as to the nature of the warning. The alarm  1422 , fail safe  1418 , and shutdown  1422  states may be entered from all other states  1416  in the master process  1300  of the processor  1900 . 
         [0118]    The controller continues normal operation in the run state  1412  until it receives an “Off” or “Stop” command, typically from one of the menus shown in  FIGS. 21 ,  23 , and  25 . After receiving the “Off command,” the processor  1900  enters the disable sub-state machines state  1414 . While in this state  1414 , the processor  1900  winds down the operation of all of the system components and stores any data log  1319 . The processor  1900  then returns to the idle state  1406  and awaits another “On” or “Start” command. 
         [0119]    Referring now to  FIG. 16 , a processor  1900  implemented state machine  1501  for one of the compressors  601  or  702  is shown. The compressor state machine  1501  begins in an idle state  1503 . Once the processor  1900  enables the compressor state machine  1501  and there is no current fault, the compressor state machine  1501  enters the wait state  1505 . While in the wait state  1505 , the compressor state machine  1501  runs a short cycle timer to produce a delay. Once the short cycle timer reaches zero, the compressor state machine  1501  moves from the wait state  1505  to the starting state  1507  and starts the compressor  601  or  702 . While in the starting state  1507 , the compressor state machine  1501  pauses for thirty seconds before advancing to the running state  1509 . 
         [0120]    The compressor state machine  1501  remains in the running state  1509 , and the compressor  601  or  702  continues to operate, until the processor  1900  signals for the compressor to be disabled. Once the compressor disable command is received, the compressor state machine  1501  moves from the running state  1509  to the shut down state  1511 . The compressor state machine  1501  may be signaled that the compressor has been disabled during any normal state  1513  in case of a system fault. Upon receipt of such a signal, the compressor state machine  1501  enters the shut down state  1511 . Finally, from the shut down state  1511 , the compressor state machine  1501  reenters the idle state  1503 . 
         [0121]    Referring now to  FIG. 17 , the blower  508  state machine  1600  is shown. The goal of the blower state machine  1600  is to achieve the desired flow rate of air and pressure to meet the operating parameters of any given airplane by controlling the variable speed impeller located within the duct of the two air conditioners. The blower  508  begins in an idle state at  1602 . Once the processor  1900  enables the blower state machine  1600  and provides a pressure operating setpoint, the blower state machine enters a first of two troubleshooting states  1604 . In this state, the impeller is set to low speed to troubleshoot any initial problems, such as a blockage or failure of the blower  508  to operate. The blower state machine  1600  then enters a second troubleshooting state  1606  in which checks of pressure and power to the blower  508  are run to see if an air duct  26  is connected between the cart  10  and an airplane or if, in some other respect, there is bad pressure. If no air duct  26  is connected, or if the air duct  26  is connected to the wrong type or class of airplane, or if sensors on the blower system otherwise sense bad pressure readings for ten seconds, the blower state machine  1600  will enter the alarm state  1622 , giving forth an appropriate warning to the operator. 
         [0122]    If an air duct  26  is connected and there is otherwise good pressure, the blower state machine  1600  will enter the adjust blower motor frequency state  1610 . Here the motor  506  A.C. power frequency is set. The blower state machine  1600  then enters a state  1608  where it checks the pressure change across the blower  508 . If no air duct  26  is connected, or if the air duct  26  is connected to the wrong type or class of airplane, or if sensors on the blower system otherwise sense bad pressure for ten seconds, once again the blower state machine  1600  enters the alarm state  1622 , giving forth an appropriate warning to the operator. If an air duct  26  is connected and if there is good pressure, and if a type or class of airplane has been selected using the menu shown in  FIG. 21 , then the pressure change across the blower  508 , measured by the differential pressure sensor  532  ( FIG. 5 ), and the power consumed by the blower voltage-to-frequency converter  525 , measured by multiplying the voltage  1720  by the current  1722  ( FIG. 18 ), are compared to normal logged values found in the data log  1319  for the type or class of airplane that was selected on the menu shown in  FIG. 21 . If the pressure and power consumed do not correspond to that type of airplane, then the operator is given an alarm  1622  and the idle state  1602  is entered while the problem is checked out. 
         [0123]    The blower state machine  1600  next checks the blower map  1612 . The blower map contains data that helps guide and shape the control algorithm within the processor  1900  that sets the blower motor frequency. This data sets the operational limits of the blower system and also includes information assessing the health of the blower system. 
         [0124]    If the status data of the blower  508  is contained within the blower map, the blower state machine  1600  enters a state  1614  where the blower  508  is permitted to run at a given frequency while the deviation of the cart  10  output pressure (as measured by the pressure transducer  526 ) is checked. If the deviation or error exceeds a threshold value (step  1650 ), then the blower frequency is once again adjusted at step  1615  to minimize the error. 
         [0125]    If, at step  1612 , data for the blower is not found within the blower map, the blower state machine  1600  enters the alarm state  1422  and shuts down the air conditioners. 
         [0126]      FIG. 18  presents a partly block and partly schematic diagram of the signal and electrical power connections to the compressors  601  and  702 , the two speed condenser fan  414 , and the blower  508 , its motor  506 , and its voltage-to-frequency converter  525 . The locations of voltage and current sensors are shown, all of which feed signals into the air conditioning and PAO processor  1900  shown in  FIG. 19 . Signals generated by the processor  1900  (shown in  FIG. 19 ) and fed into the components  601 ,  702 ,  414 , and  525  are also shown in  FIG. 18  to complete the disclosure of all significant signals connecting the processor  1900  to the various air conditioning processes. 
         [0127]      FIG. 19  presents the air conditioning and PAO processor  1900 .  FIG. 19  reveals and lists and categorizes all of the signals that flow from various types of sensors associated with the air conditioning processes and the PAO process into the processor  1900 . It also reveals and lists and categorizes all of the control signals generated by the processor  1900  that flow back to and that control the components of the air conditioning processes and the PAO process. In  FIG. 19 , all the signals are identified by name and by the same reference number that is assigned to the transducer that is the source of an incoming signal or to the device that is the target of an outgoing signal. “PRE-C” is a signal relating to the pre-cool air conditioner  520  shown primarily in  FIG. 6 . “POST-C” is a signal relating to the post-cool air conditioner  522  shown in  FIG. 7 . “PAO” is a signal relating to the PAO liquid coolant processor shown in  FIG. 8 . Many of the signals shown in  FIG. 19  which relate to the actual control of processes are also shown in the process control diagram presented in  FIG. 12 . Other signals originate in or go to  FIG. 18 . The use of these signals has already been explained above. 
         [0128]    While an embodiment of the invention has been disclosed, those skilled in the art will recognize that numerous modifications and changes may be made without departing from the true spirit and scope of the claims as defined by the claims annexed to and forming a part of this specification.