Abstract:
A method of auto-calibrating aircraft cabin pressure sensors in a cabin pressure control system while in service, wherein at a constant sensed cabin pressure during a cruise flight phase at least one pressure sensor output voltage from each of two cabin pressure sensors are recorded. Upon landing and opening of the cabin doors the sensed cabin pressure value is compared to an external field pressure. A new software slope variable and a new software offset variable are computed for the pressure/voltage function using an average computed pressure value in cruise and the recorded field atmospheric pressure sensor voltage value on the ground. Thereafter, a new cabin pressure value is calculated and control software is modified.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to aircraft cabin pressure controls and, more particularly, to a method of auto-calibrating aircraft cabin pressure sensors. 
       BACKGROUND 
       [0002]    For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes. 
         [0003]    As the altitude of an aircraft increases, from its take-off altitude to its “top of climb” or “cruise” altitude, the ambient atmospheric pressure outside of the aircraft decreases. Thus, unless otherwise controlled, air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure at high altitudes. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity. 
         [0004]    Aircraft cabin pressure is often referred to in terms of “cabin altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin altitude to minimize passenger discomfort. 
         [0005]    A typical cabin pressure control system implements pressure controllers, outflow valves and control logic that may, when needed or desired, begin pressurizing the aircraft cabin (or “descending” the aircraft cabin) before take-off, either while taxiing on or to the runway or at the start of the take-off roll down the runway. The pressure control system may implement pressure transducers, also referred to as pressure sensors, to monitor various pressure conditions internal and external the aircraft cabin. During flight the pressure control system enables the aircraft cabin to remain at the appropriate pressure until landing, when de-pressurization takes place. Typically, the pressure sensor components of the cabin pressure system are initially calibrated when placed into service to primary standards set forth by the National Institute of Standards and Technology (NIST). Oftentimes these sensor components are subject to drift due to age, external environmental factors or influence of other system components. To compensate for the drift, the pressure control system hardware or operational software may be fine tuned at various times during the life of the system. When the drift exhibited by the pressure sensors becomes excessive, the sensor components are typically removed from the aircraft and recalibrated, resulting in down time, expense and loss of revenue. 
         [0006]    Hence, there is a need for a method of auto-calibrating a pressure control system, and more particularly the system&#39;s pressure sensors, without the need for removal of the pressure sensor components from the aircraft. More particularly, there is a need for a method that enables the drift experienced by the pressure sensor components to be auto-calibrated while the components are in service. The present invention addresses one or more of these needs. 
       BRIEF SUMMARY 
       [0007]    The present invention provides a method of auto-calibrating a cabin pressure sensor, included as a part of a cabin pressure control system. 
         [0008]    In one embodiment, and by way of example only, the method includes recording a value for each of at least two sensed cabin pressures and at least two pressure sensor output voltages in at least two associated control circuits when an aircraft is in a level flight status at a cruise altitude; when the aircraft is in ground mode, computing an absolute value of a difference between at least one sensed cabin pressure and a field pressure; determining if the computed absolute value is less than a predetermined value; recording the field pressure and at least one value for at least one pressure sensor output voltage when the aircraft is in the ground mode; computing at least one new slope variable for at least one of the associated control circuits; computing at least one new offset variable for at least one of the associated control circuits; and computing at least one new pressure sensor offset and slope calibration for at least one cabin pressure sensor. 
         [0009]    In another exemplary embodiment, the method of auto-calibrating a cabin pressure control system includes recording a value for a sensed cabin pressure (PC 1   cruise  and PC 2   cruise ) and a pressure sensor output voltage (VPC 1   cruise  and VPC 2   cruise ) in an associated control circuit software for each of the active cabin pressure sensor and the standby cabin pressure sensor when an aircraft is at a cruise altitude; when the aircraft is in ground mode, computing an absolute value of a difference between the sensed cabin pressure and a field pressure (PA) for each of the active cabin pressure sensor and the standby cabin pressure sensor; determining if the computed absolute value for each of the active cabin pressure sensor and the standby cabin pressure sensor is less than a value to be determined based on a specific application tolerance stack analysis; recording a value for a pressure sensor output voltage for each of the active cabin pressure sensor (VPC 1 ) and the standby cabin pressure sensor (VPC 2 ) when the aircraft is in the ground mode; computing an average cruise pressure value (PC cruise avg ) for the sensed cabin pressures at the cruise altitude; computing a new slope variable for the associated control circuit software of each of the active cabin pressure sensor (slope 1 ) and the standby cabin pressure sensor (slope 2 ); computing a new offset variable for the associated control circuit software of each of the active cabin pressure sensor (offset 1 ) and the standby cabin pressure sensor (offset 2 ); and computing a new pressure sensor value for each of the active cabin pressure sensor and the standby cabin pressure sensor. 
         [0010]    In yet another exemplary embodiment, the method of auto-calibrating a cabin pressure control system includes recording a value for a sensed cabin pressure (PC 1   cruise  and PC 2   cruise ) and a pressure sensor output voltage (VPC 1   cruise  and VPC 2   cruise ) in an associated control circuit software for each of the first channel cabin pressure sensor and the second channel cabin pressure sensor when an aircraft is at a cruise altitude; when the aircraft is in ground mode, computing an absolute value of a difference between the sensed cabin pressure and a field pressure (PA) for each of the first channel cabin pressure sensor and the second channel cabin pressure sensor; determining if the computed absolute value for each of the first channel cabin pressure sensor and the second channel cabin pressure sensor is less than a value to be determined based on a specific application tolerance stack analysis; recording a value for a pressure sensor output voltage for each of the first channel cabin pressure sensor (VPC 1 ) and the second channel cabin pressure sensor (VPC 2 ) when the aircraft is in the ground mode; computing an average cruise pressure value (PC cruise avg ) for the sensed cabin pressures at the cruise altitude using an equation, 
         [0000]    
       
         
           
             
               PC 
               
                 cruise 
                  
                 
                     
                 
                  
                 avg 
               
             
             = 
             
               
                 
                   PC 
                    
                   
                       
                   
                    
                   
                     1 
                     cruise 
                   
                 
                 + 
                 
                   PC 
                    
                   
                       
                   
                    
                   
                     2 
                     cruise 
                   
                 
               
               2 
             
           
         
       
     
         [0011]    computing a new slope variable for the associated control circuit software of each of the first channel cabin pressure sensor (slope 1 ) and the second channel cabin pressure sensor (slope 2 ) using the equations, 
         [0000]    
       
         
           
             
               slope 
                
               
                   
               
                
               1 
             
             = 
             
               
                 
                   ( 
                   
                     PA 
                     - 
                     
                       PC 
                       
                         cruise 
                          
                         
                             
                         
                          
                         avg 
                       
                     
                   
                   ) 
                 
                 
                   ( 
                   
                     
                       VPC 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       VPC 
                        
                       
                           
                       
                        
                       
                         1 
                         cruise 
                       
                     
                   
                   ) 
                 
               
                
               
                   
               
                
               and 
             
           
         
       
       
         
           
             
               
                 slope 
                  
                 
                     
                 
                  
                 2 
               
               = 
               
                 
                   ( 
                   
                     PA 
                     - 
                     
                       PC 
                       
                         cruise 
                          
                         
                             
                         
                          
                         avg 
                       
                     
                   
                   ) 
                 
                 
                   ( 
                   
                     
                       VPC 
                        
                       
                           
                       
                        
                       2 
                     
                     - 
                     
                       VPC 
                        
                       
                           
                       
                        
                       
                         2 
                         cruise 
                       
                     
                   
                   ) 
                 
               
             
             ; 
           
         
       
     
         [0012]    computing a new offset variable for the associated control circuit software of each of the first channel cabin pressure sensor (offset 1 ) and the second channel cabin pressure sensor (offset 2 ) using the equations, 
         [0000]      offset1 =PA− ( VPC 1*slope1) and offset2 =PA− ( VPC 2*slope2); and 
         [0013]    computing a new pressure sensor value for each of the first channel cabin pressure sensor and the second channel cabin pressure sensor using the equations, 
         [0000]        PC 1=( VPC 1*slope1)+offset1 and  PC 2=( VPC 2*slope2)+offset2. 
         [0014]    Other independent features and advantages of the method of auto-calibrating a cabin pressure sensor included as a part of a cabin pressure control system will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention will hereinafter be described in conjunction with the following drawing figure, wherein: 
           [0016]      FIG. 1  is a functional block diagram of an aircraft cabin pressure control system according to an embodiment; 
           [0017]      FIG. 2  is a flow chart detailing a method of auto-calibrating pressure sensors of an aircraft cabin pressure control system according to an embodiment; and 
           [0018]      FIG. 3  illustrates a graph and supporting data detailing a system that implements an embodiment of the auto-calibration method of the instant invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Before proceeding with the description, it is to be appreciated that the following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
         [0020]    The embodiment disclosed herein is described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present invention. Furthermore, it will be understood by one of skilled in the art that although the specific embodiment illustrated below is directed at a method of auto-calibrating a pressure sensor in a pressure control system in an aircraft, for purposes of explanation, the apparatus may be used in various other embodiments employing calibration of pressure sensors. The following detailed description is, therefore, not to be taken in a limiting sense. 
         [0021]    The following is a novel method of auto-calibrating the cabin pressure sensors in a dual channel cabin pressure control system of an aircraft while the aircraft is in service. The auto-calibration method recalculates the software&#39;s slope and offset variables which are used in the conversion from a pressure sensor output voltage to a calculated cabin pressure for each channel of the system. Turning now to  FIG. 1 , a functional block diagram of an exemplary dual channel aircraft cabin pressure control system  100 , and its interconnections to certain other aircraft systems, is shown. In the depicted embodiment, the system  100  includes redundant first control channel components  102  and second control channel components  104 , also referred to herein as an active first control channel and a standby second control channel, coupled to a single outflow valve  106 . Although not depicted, it will be appreciated that the system  100  may additionally include one or more overpressure relief valves, and one or more negative pressure relief valves. These valves are typically included as part of an aircraft cabin pressure control system but, for clarity and ease of illustration and depiction, are not shown or further described herein. Moreover, before proceeding further with the description of the system  100 , it is noted that the depicted embodiment is merely exemplary and that the system  100  could be implemented with two or more independent outflow valves  106 . 
         [0022]    The active first control channel  102  of the system  100  includes a control unit  108  comprising an instrumentation and control circuit  110  and a valve control circuit  112 . The instrumentation and control circuit  110  is coupled to receive signals from a plurality of sensors and other signal sources. In the depicted embodiment, the illustrated sensor included as a part of the active or first control channel  102  is an active channel or first cabin pressure sensor  114 . The standby or second control channel  104  of the system  100  includes a control unit  116  that, similar to the first control channel control unit  108 , includes an instrumentation and control circuit  118  and a valve control circuit  120 . The instrumentation and control circuit  118  is coupled to receive signals from a standby channel or second cabin pressure sensor  122 . It will be appreciated that the cabin pressure sensors  114  and  122  depicted and described herein are merely exemplary, and that the system  100  could be implemented with additional or different types of sensors. For example, each channel of the system  100  could additionally include additional cabin temperature sensors, one or more atmosphere pressure sensors and/or one or more cabin-to-atmosphere differential pressure sensors. 
         [0023]    The first and second cabin pressure sensors  114  and  122  are disposed and configured to sense absolute cabin pressure, and are preferably implemented as any one of numerous types of analog pressure sensors. Some non-limiting examples of suitable analog pressure sensors include various semiconductor diaphragm pressure sensors, various capacitance pressure sensors, various optical sensors, and various magnetic sensors. In a particular preferred embodiment, however, the first and second cabin pressure sensors  114  and  122  are implemented as piezoelectric strain gauge sensors. 
         [0024]    In an embodiment including one or more cabin temperature sensors, the sensors would be disposed and configured to sense cabin temperature, and preferably implemented as any one of numerous types of analog temperature sensors. Some non-limiting examples of suitable temperature sensors include resistance temperature detectors (RTDs), thermocouples, and various types of optical temperature sensors. When included, the cabin temperature sensors are used to correct the absolute pressure value sensed by the first and second cabin pressure sensors  114  and  122  for environmental temperature changes. 
         [0025]    Each of the instrumentation and control circuits  110  and  118  are configured to communicate with, and receive signals from, an associated aircraft avionics suite  124  and  126 , respectively, via, for example, aircraft interface data buss(es), analog, and/or discrete input/output signals. Based on the signals received from the avionics suites  124  and  126 , as well as signals supplied from the first and second cabin pressure sensors  114  and  122 , the associated instrumentation and control circuit  110  or  118  computes a desired (or commanded) cabin pressure command, an actual (or sensed) cabin pressure, a desired (or commanded) cabin pressure rate-of-change, an actual (or sensed) cabin pressure rate-of-change, temperature corrected cabin pressure, supplies appropriate actuation control signals to the associated valve control circuits  112  or  120 , respectively, and additionally supplies various alarm, indication, warning, and/or control signals. 
         [0026]    Each of the valve control circuits  112  and  120  receives the actuation control signals supplied from their associated instrumentation and control circuit  110  and  118 , respectively. In response to the actuation control signals, which preferably include speed or duty cycle command information and direction information, the valve control circuits  112  and  120  supply valve command signals to the outflow valve  106 , to thereby control the position of the outflow valve  106 , and thereby modulate cabin pressure. 
         [0027]    The outflow valve  106  is preferably mounted on an aircraft bulkhead  130 , and includes a valve body  132 , a valve element  134 , and a valve actuator  136 . The valve body  132  has a flow passage  138  that extends through it, such that when the outflow valve  106  is mounted on the aircraft bulkhead  130 , the flow passage  138  is in fluid communication with the aircraft cabin  140  and the external atmosphere  142 . The valve element  134  is movably mounted on the valve body  132  and extends into the flow passage  138 . The valve element  134  is movable between an open position, in which the aircraft cabin  140  and the external atmosphere  144  are in fluid communication, and a closed position, in which the aircraft cabin  140  is sealed from the external atmosphere. 
         [0028]    The valve actuator  136  is coupled to the valve element  134  and positions the valve element  134  to a commanded position, to thereby control cabin pressure. To do so, the valve actuator  136  is coupled to receive the valve command signals supplied by the valve control circuits  112  and  120 . In response to the supplied valve command signals, the valve actuator  136  moves the valve element  134  to the commanded position. It will be appreciated that the valve actuator  136  may be implemented as any one of numerous types of actuators, but in the depicted embodiment the valve actuator  136  is implemented with a first motor  146  coupled to the first control channel control unit  108 , a second motor  148  coupled to the second control channel control circuit  116 , and a suitable gear train  150 . 
         [0029]    Each of the instrumentation and control circuits  110  and  118  are coupled to receive one or more signals supplied from their respective avionics suite  124  and  126 , and their respective cabin pressure sensor  114  and  122 . The signals from the avionics suites  124  and  126  and the first and second cabin pressure sensors  114  and  122  are representative of the operational mode of the aircraft. In response to the received signals, the respective control units  108  recalculate software variables to auto-calibrate the first and second cabin pressure sensors  114  and  122  according to a method illustrated in  FIG. 2 . 
         [0030]    Turning now to  FIG. 2 , illustrated is a flow chart detailing a method  200  for auto-calibrating the first (active) and second (standby) cabin pressure sensors  114  and  122  of  FIG. 1 . To enable auto-calibration, data is received on a per flight basis, resulting in offset adjustments to the first and second cabin pressure sensors  114  and  122 . More specifically, detailed is the method  200  for recalculating software variables, referred to herein as slope and offset, which are used in the conversion of a pressure sensor output voltage to a calculated cabin pressure for each channel, thereby enabling auto-calibration of the first and second cabin pressure sensors  114  and  122 . 
         [0031]    As previously stated, the first and second cabin pressure sensor  114  and  122  are auto-calibrated on a per flight basis. To begin the process of auto-calibration, the instrumentation and control circuit  110  of the first control channel  102 , hereinafter referred to as the active control channel, and instrumentation and control circuit  118  of the second control channel  104 , hereinafter referred to as the standby control channel, determine if the active control channel  102  and the standby control channel  104  of the system  100  are operational during a sequence of flight as best illustrated at  202 . At a specific point in flight, an active control channel cruise detect flag is set  204 , indicating that the aircraft altitude has not changed within a 1000 feet window for greater than 1 minute. When this prerequisite has been met, the first cabin pressure sensor  114  submits a signal to the instrumentation and control circuit  110  of a sensed pressure value of the active control channel cabin pressure (PC 1   cruise ) and the second cabin pressure sensor  122  submits a signal to the instrumentation and control circuit  118  of a sensed pressure value of the standby control channel cabin pressure (PC 2   cruise ). The PC 1   cruise  and the PC 2   cruise  are simultaneously recorded  206  by control software in the each of the associated instrumentation and control circuits  110  and  118 . In addition, values for an active control channel pressure sensor output voltage (VPC 1   cruise ) and a standby control channel pressure sensor output voltage (VPC 2   cruise ) are recorded  208  by the control software. 
         [0032]    After the aircraft has landed  210  and the cabin doors are opened  212 , also referred to as the aircraft being in ground mode, each instrumentation and control circuit  110  and  118 , and more particularly the associated control software, determines if the absolute value difference between a sensed cabin pressure value at ground and a field pressure (PA) is less than a predetermined value, which is determined based on specific application tolerance stack analysis  214 . The field pressure (PA) may be obtained for each channel from the aircraft avionics suites  124  and  126 , via air data computer information on aircraft interface data buss(es). If the calculated absolute values meet with these criteria, then further calculations resulting in auto-calibration of each of the first and second cabin pressure sensors  114  and  122  are continued. 
         [0033]    Subsequent to the step of calculating the absolute value differences  214 , values for an active control channel pressure sensor output voltage at ground (VPC 1 ) and a standby control channel pressure sensor output voltage at ground (VPC 2 ) are recorded  216  by the control software while the aircraft is in ground mode. Next, the control software determines whether a critical fault  218  exists with regard to the active or first control channel control unit  108  and associated cabin pressure sensor  114  and the standby or second control channel control unit  116  and associated cabin pressure sensor  122 . If no critical faults are detected, a value for the average cabin pressure at cruise altitude (PC cruise avg ) is calculated  220  according to the following formula: 
         [0000]    
       
         
           
             
               PC 
               
                 cruise 
                  
                 
                     
                 
                  
                 avg 
               
             
             = 
             
               
                 
                   
                     PC 
                      
                     
                         
                     
                      
                     
                       1 
                       cruise 
                     
                   
                   + 
                   
                     PC 
                      
                     
                         
                     
                      
                     
                       2 
                       cruise 
                     
                   
                 
                 2 
               
               . 
             
           
         
       
     
         [0034]    If additional accurate cabin pressure sensor readings are available to both the active control channel  102  and the standby control channel  104 , they may be included in calculating the value of PC cruise avg . 
         [0035]    A new slope value used by the control software for the first cabin pressure sensor  114  ( FIG. 1 ) is next calculated  222  using the following equation: 
         [0000]    
       
         
           
             
               slope 
                
               
                   
               
                
               1 
             
             = 
             
               
                 
                   ( 
                   
                     PA 
                     - 
                     
                       PC 
                       
                         cruise 
                          
                         
                             
                         
                          
                         avg 
                       
                     
                   
                   ) 
                 
                 
                   ( 
                   
                     
                       VPC 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       VPC 
                        
                       
                           
                       
                        
                       
                         1 
                         cruise 
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
         [0036]    The new value of slope 1  is limited to be within a slope min  and a slope max , wherein slope min  is the minimum allowable software value of slope and slopemax is the maximum allowable software value of slope, which result in a pressure calculation from sea level to 8,000 ft, a value within acceptable limits, such as Acceptance Test Procedure (ATP) limits as set forth by Honeywell International Inc. 
         [0037]    A new slope value used by the control software for the second cabin pressure sensor  122  ( FIG. 1 ) is next calculated  224  using the following equation: 
         [0000]    
       
         
           
             
               slope 
                
               
                   
               
                
               2 
             
             = 
             
               
                 
                   ( 
                   
                     PA 
                     - 
                     
                       PC 
                       
                         cruise 
                          
                         
                             
                         
                          
                         avg 
                       
                     
                   
                   ) 
                 
                 
                   ( 
                   
                     
                       VPC 
                        
                       
                           
                       
                        
                       2 
                     
                     - 
                     
                       VPC 
                        
                       
                           
                       
                        
                       
                         2 
                         cruise 
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
         [0038]    The new value of slope 2  is limited to be within a slope min  and a slope max  as stated previously with regard to the value of slope  1 . 
         [0039]    To complete the auto-calibration of the first cabin pressure sensor  114  the current calculated value of PC 1  is modified such that it equals the value of PA by recalculating the offset pressure  226 , via the software variable offset 1  using the following equation: 
         [0000]      offset1 =PA− ( VPC 1*slope1). 
         [0040]    To complete the auto-calibration of the second cabin pressure sensor  122  the current calculated value of PC 2  is modified such that it equals the value of PA by recalculating the offset pressure  230 , via the software variable offset 1  using the following equation: 
         [0000]      offset2 =PA− ( VPC 2*slope2). 
         [0041]    The new calculated values  228  and  232  for PC 1  and PC 2  are: 
         [0000]        PC 1=( VPC 1*slope1)+offset1 
         [0000]        PC 2=( VPC 2*slope2)+offset2 
         [0042]    In response to these calculated values for the first and second cabin pressure sensors  114  and  122 , software used by the instrumentation and control circuit  110  for the first cabin pressure sensor  114  and the software used by the instrumentation and control circuit  118  for the second cabin pressure sensor  122  are automatically adjusted for the offset values, or recalibrated, prior to and for use in the next flight sequence. The method  200  is repeated when the aircraft is again in a cruise portion of a flight. Accordingly, the first cabin pressure sensor  114  and the second cabin pressure sensor  122  are continually being auto-calibrated every flight via software adjustments, thereby minimizing any need for removal of the sensors for manual recalibration. 
         [0043]    In support of this method of auto-calibration, reference should now be made to  FIG. 3 , which depicts a graphical representation  300  of supporting data for auto-calibration of a first (active channel) cabin pressure sensor (PC 1 ), similar to the first cabin pressure sensor  114  of  FIG. 1  and a second (standby channel) cabin pressure sensor (PC 2 ), similar to the second cabin pressure sensor  122  of  FIG. 2 . Referring more specifically to the graphical representation  300 , the y-axis shows the difference in pressure readings between the two cabin pressure sensors (PC 1  and PC 2 ) in units of inches of mercury. In light of both pressure sensors (PC 1  and PC 2 ) measuring the same cabin pressure, the difference is due to a calibration error between them. The x-axis corresponds to each step in the method. More specifically, a point  304  represents initial conditions on the ground, a point  306  represents a recording of PC 1  and PC 2  at a cruise altitude, a point  308  represents ground mode and a low cabin to atmospheric differential pressure on ground, a point  310  represents calculations to obtain a new slope variable and offset variable on ground (where the offset is zeroed out and the slope is recalculated), a point  312  represents a recording of PC 1  and PC 2  at a cruise altitude of the next flight. The remaining data points represent a continuation of the method throughout six flight occurrences. As can be seen in the graph  300 , even though a large calibration error between the two cabin pressure sensors (PC 1  and PC 2 ) is initially present, represented at point  304 , after six (6) flights the error is significantly reduced to well within an acceptable range (approx  20  sea level feet). 
         [0044]    Accordingly, disclosed is a method for auto-calibrating aircraft cabin pressure sensors that minimizes the need for removal of the sensors from the aircraft for manual recalibration. The disclosed method enables the drift experienced by the cabin pressure control system, and more specifically the pressure sensors, to be auto-calibrated while the components are in service. 
         [0045]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.