Patent Publication Number: US-2019185185-A1

Title: Quantitative Blockage Analysis Of A Grid In Icing Conditions

Description:
FIELD OF THE INVENTION 
     The invention relates to an analysis system for quantitative blockage analysis of a grid in icing conditions, an aircraft equipped with the analysis system for quantitative blockage analysis, and a method for quantitative blockage analysis of a grid in icing conditions. 
     BACKGROUND OF THE INVENTION 
     For some aircraft operators it is a desire to analyze ice accretion on grids such as an engine&#39;s air intake protection grid. Different technologies are known for monitoring ice accretion on A/C components, however, very little is known about grid icing. 
     BRIEF SUMMARY OF THE INVENTION 
     An aspect of the invention may provide an enhanced possibility for determining an icing state of an aircraft engine&#39;s air intake protection grid. 
     According to an embodiment of the invention, an analysis system for quantitative blockage analysis of a grid in icing conditions comprises at least one optical sensor, a processor unit and a display unit. The at least one optical sensor is configured to continuously capture a series of pictures of a grid and to submit each picture of the series of pictures to the processor unit. The processor unit is configured to determine a color scale value for each of a number of areas in at least a field of each picture, and to count a number of areas in a particular field of a particular picture that show a color scale value greater than an absolute reference value. Further, the processor unit is configured to calculate a time remaining until a critical icing condition is reached based on a model of an icing behavior of the grid and the number of areas that show a color scale value greater than the absolute reference value. The processor is further configured to calculate a warning signal based on the time remaining until the critical icing condition of the grid is reached, and to display the warning signal on the display unit. 
     In an example, the analysis system comprises an optical sensor, which may be a camera, such as an infrared camera or a black and white, i. e. a grey-scale camera. 
     Further, in an example, the analysis system comprises a processor unit, such as central processor unit (CPU). The processor unit is configured to determine a color scale value for each of a number of areas in at least one field of a picture captured by an optical sensor. This means that the processor unit is configured to analyze a particular area, such as a pixel in a field of a picture, such as a field of interest selected by a user. 
     In an example, the processor unit uses an algorithm that determines a brightness of a particular area to determine the color scale value of that area in a picture. Thus, the processor unit may determine a color scale value based on a brightness value of a particular area. The brightness value may be based on information submitted by the optical sensor used to capture the picture. Alternatively or additionally, the processor unit may use an algorithm that determines a color of a particular area to determine the color scale value. Thus, the processor unit may determine a color scale value based on color value of a particular area. The color value may be based on information submitted by the optical sensor. 
     In an example, the display unit may be a screen or lamp, such as an LED. 
     In an example, the processor unit is further configured to count a number of areas in a particular field of a particular picture that show a color scale value greater than an absolute reference value. This means that the processor compares the color space values determined for a particular area with a reference value that is an absolute value and that may be given or that may be dynamically determined by the processor unit using at least one reference picture, for example. 
     In the context of the present invention, an absolute reference value is an absolute value of a reference value that may be pre-given or that may be dynamically calculated based on at least one reference picture, for example. This means, in an example, the absolute reference value may vary over time. 
     Based on the comparison of a color scale value of a particular area and a particular reference value, the processor unit determines if the color scale value of the particular area is greater than, equal to or smaller than the absolute reference value. Since a color scale value that is greater than the absolute reference value means that a respective area is iced, i. e. frozen over, the processor unit is configured to count all iced areas, i.e. all areas that show a color scale value greater than the absolute reference value. 
     In an example, the analysis system is further configured to calculate a warning signal that is indicative of a time remaining until a critical icing condition of the grid is reached. To calculate the warning signal, the processor unit is configured to calculate the time remaining until the critical condition is reached based on a model of an icing behavior of the grid that is based on a fitting parameter determined for the grid and that is used to extrapolate the time remaining until the critical icing condition is reached. 
     By using a model that receives particular areas classified as ice or any value indicative of the particular areas that are classified as ice as an input signal, the model can be used to calculate a time remaining until a critical icing condition, such as a total blocking or a 80% blocking, preferably a 90% blocking, more preferably a 95% blocking or any other blocking condition that is defined as a critical blocking condition is reached. Thus, the critical icing or blocking condition may be a pre-given threshold selected by a pilot, for example. 
     The model may be based on information about an icing behavior of a particular grid. This information may be pre-given or may be calculated based on an icing behavior measured in a time-window after an icing of the grid has been recognized for the first time, for example. Of course, the model may be updated dynamically by resetting the time window for measuring the icing behavior of the grid. 
     Based on the remaining time calculated by the model, a signal can be displayed on a display unit, such as a pilot display in a cockpit of an aircraft that is indicative of the remaining time. This means, that a pilot is informed about the time remaining until a critical blockage condition of a grid that protects an air-intake of an engine of a particular aircraft is reached via the display unit. 
     According to an example, the model is based on a fitting parameter determined for the grid, and the processor is configured to extrapolate the time remaining until the critical icing condition is reached based on the model and the number of areas in the particular field of the particular picture that show a color scale value greater than the absolute reference value. 
     In an example, the processor may be configured to determine an icing signal indicative of the number of areas that show a color scale value greater than the absolute reference value. The icing signal may be a number of areas that show a color scale value greater than the absolute reference value, a percentage of areas that show a color scale value greater than the absolute reference value with respect to all areas of interest or any other mathematical description of the number of areas that show a color scale value greater than the absolute reference value. 
     The icing signal may comprise a graphical element that shows an amount of areas that show a color scale value greater than the absolute reference value with respect to all areas of interest. For this purpose, the icing signal may be based on a color scheme, such as green, yellow and red, wherein “green” stands for a non-critical icing condition, wherein under 21% of all areas show a color scale value greater than the absolute reference value, for example, “yellow” stands for a medium-critical icing condition, wherein over 20% and under 51% of all areas show a color scale value greater than the absolute reference value, for example, and “red” stands for a critical icing condition, wherein over 50% of all areas show a color scale value greater than the absolute reference value, for example. 
     In an example, the icing signal determined by the processor unit is indicative of areas that show a color scale value greater than the absolute reference value, which means that the icing signal is indicative of an amount of ice blocking a passing area for air through a grid. Thus, the icing signal is a quantitative indicator of blockage of a grid in icing conditions. 
     The warning signal may comprise a graphical element indicative of a remaining time until a critical icing condition is reached. 
     According to an example, the processor unit is configured to classify the areas that show a color scale value greater than the absolute reference value as ice and/or to classify the areas that show a color scale value smaller than or equal to the absolute reference value as background. 
     Preferably a grey scale value is chosen as the color scale value. Additionally, a background element may be chosen in a color that has a brightness value that results in a maximum differential brightness value with respect to a brightness value of a color of ice, such as white, for example in a particular picture captured by the optical sensor. 
     A classification of a color scale value of a particular area may be carried out by a classification algorithm such as a support vector machine or an artificial neural network that takes into account the color scale value of the particular area and a particular absolute reference value. This means that the output of the classification is an amount of areas that are blocked, i. e. covered by ice. 
     In an example, for each color scale value of a particular area in a particular field of a particular test picture captured under test conditions, a corresponding absolute reference value may be chosen that is determined for a corresponding area in a corresponding field of a given reference picture. This may allow to increase a classification accuracy of a color scale value of a particular area, particularly for grids that have a varying three-dimensional shape and, therefore, result in varying reference values for particular areas of interest. 
     Preferably, the optical sensor is oriented essentially parallel to an airflow towards the grid. 
     In an example the grid is an air-intake grid of an engine of a helicopter or an aircraft that is driven by a propeller engine or a jet engine. 
     In another advantageous embodiment, the processor unit is configured to calculate the icing signal based on a blockage value calculated as a ratio between a number of areas in a pre-determined blocking field that show a color scale value greater than the absolute reference value and a number of areas in the pre-determined blocking field that show a color scale value smaller than or equal to the absolute reference value. 
     An icing signal that is based on a blockage value may be presented to a pilot of an aircraft using an engine with an air intake that is protected by a grid that is monitored by using the analysis system as described. For this purpose, the icing signal may be displayed on a display unit in a cockpit of the aircraft. 
     In an example, the icing signal is shown by using a graph representing blockage values for at least one area over time. 
     In another advantageous embodiment the processor unit is configured to monitor and to correct light intensity variations in a particular field of areas to be analyzed between particular pictures of a series of pictures captured by the optical sensor. 
     Since light intensity variations in a particular field of interest may decrease an accuracy of a recognition of iced areas, an automatic correction of the light intensity in the field of interest may be used that equals the light intensity for all areas or an average value of all areas in the field of interest. 
     According to another example, the grid is a grid of an air-intake of an engine and the analysis system comprises at least one sensor that is positioned with respect to a flow direction of an airflow through the grid downstream of the grid. The processor unit may be configured to determine at least one differential pressure signal between a pressure of an airflow towards the grid and a pressure determined via the at least one sensor positioned downstream of the grid. The processor unit may further be configured to calculate a time until a critical performance loss of the engine, based on a rate of change of the at least one differential pressure signal. 
     According to another example, the processor unit may further be configured to generate a performance signal being indicative of at least the time until the critical performance loss of the engine and to submit the performance signal to the display unit. The display unit may be configured to output the performance signal submitted by the processor unit. 
     By using a first sensor position upstream of an air-intake grid and a second sensor positioned downstream of the air-intake grid, pressure measurements determined via the second sensor may be used to calculate a differential pressure signal based on a pressure of an airflow measured upstream of the grid, i.e. an airstream towards the grid measured by the first sensor, which may be a pitot probe, for example. 
     Since the airflow downstream of the air-intake grid is closely related to a performance of an engine that is protected by the air-intake grid, a differential pressure signal may be used to calculate a performance loss due to a decreasing airstream through the air-intake grid. Thus, a performance state of an engine protected by the grid may be determined and/or predicted based on the differential pressure signal. 
     According to an example, a rate of a change of the differential pressure signal may be calculated based on a slope of the at least one differential pressure signal. 
     By using the slope of a plurality of differential pressure signals, a future performance, such as a performance loss, may be predicted by extrapolating the slope. 
     The engine may be an engine of a helicopter or a plane. In particular, the engine may be a gas turbine engine. 
     In a further example, the analysis system comprises a first sensor and a second sensor. Thus, the differential pressure signal may be calculated as a differential value between a first pressure determined via the first sensor positioned upstream with respect to the grid and a second pressure determined via the second sensor positioned downstream with respect to the grid. 
     In an example, the first sensor may be a pitot probe. 
     To calculate a differential pressure signal, the first sensor and/or the second sensor may be configured to determine at least one parameter of the following list of parameters: a static pressure of the airstream into the engine, a total pressure of the airstream into the engine, and a temperature of the airstream into the engine. 
     In a further example, the processor unit is configured to calculate coordinates of the areas in the particular field of the particular picture that show a color scale value greater than an absolute reference value and to transmit the coordinates to a de-icing device. 
     A de-icing device, such as an infrared unit or a heating device may be controlled based on coordinates of the areas in the particular field of the particular picture that show a color scale value greater than an absolute reference value. In other words, the de-icing device may be pointed towards areas that are considered as ice. 
     In another example, the at least one pressure sensor comprises a heating. 
     The heating may be configured to warm up the at least one pressure sensor to protect the at least one pressure sensor from icing. The processor unit may be configured to calculate a current performance of the engine. The processor unit may be configured to calculate a performance loss, based on at least one differential pressure signal calculated after the at least one sensor was heated up over a pre-given temperature. The performance loss may be calculated with respect to a predetermined reference value, such as a current engine performance or a known engine performance at a certain temperature. 
     A performance loss may be calculated by using a pre-given function for a particular engine or a percentage scale of an airstream floating into the engine, for example. 
     According to another example, the at least one pressure sensor comprises an actuator that may be configured to rotate the at least one sensor from a first position to a second position in an opposite direction of the first position and vice versa, wherein, in the second position, the at least one sensor faces the grid. The processor unit may be configured to calculate the performance of the engine based on at least one part of the differential pressure signal determined after the at least one sensor was rotated from the first position to the second position. 
     According to another example, the present invention comprises a system for analysis of a power loss caused by blockage of a grid for an air-intake of an engine for an aircraft. The system comprising: at least one sensor, a processor unit and an output unit. The at least one sensor may be positioned with respect to a flow direction of an airflow through the grid downstream of the grid. Further, the processor unit may be configured to determine at least one differential pressure signal between a pressure of an airflow towards the grid and a pressure determined via the at least one sensor positioned downstream of the grid over time. Additionally, or alternatively, the processor unit may be configured to determine a blockage of the grid based on the at least one differential pressure signal. 
     According to an embodiment of the invention, also an aircraft is provided equipped with an analysis system for quantitative blockage analysis of a grid in icing conditions, as described above. 
     The invention further comprises a method for quantitative blockage analysis of a grid in icing conditions, comprising the following steps: 
     a) capturing a series of pictures of a grid by using an optical sensor and submitting each picture of the series of pictures to a processor unit; 
     b) determining a color scale value for each of a number of areas in at least a field of each picture; 
     c) counting a number of areas in a particular field of a particular picture that show a color scale value greater than an absolute reference value; and 
     d) extrapolating a time remaining until a critical icing condition is reached by using a model of an icing behavior of the grid and the number of areas in the particular field of the particular picture that show a color scale value greater than the absolute reference value counted step c); and 
     e) displaying a warning signal on a display unit based on the remaining time calculated by using the model in step d). 
     According to an example, the method comprises calculating the model based on a fitting parameter, wherein the fitting parameter is based on an icing behavior of the grid in an experimental setup or based on at least one picture captured by the optical sensor. 
     According to another example, the method further comprises a displaying step for displaying an icing signal on the display unit, the icing signal being indicative of the number of areas that show a color scale value greater than the absolute reference value. 
     According to an additional example, the method comprises the following steps: 
     f) calculating, by the processor unit, a time until a critical performance loss of the engine, based on a rate of change of the at least one differential pressure signal. 
     According to another example, the method comprises the following steps: 
     g) generating, by the processor unit, a performance signal being indicative of at least the time until the critical performance loss of the engine calculated in step f); 
     h) submitting the performance signal to the display unit; and 
     i) displaying, by the display unit, the performance signal submitted by the processor unit in step h). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics, advantages and potential applications of the present invention result from the following description of the exemplary embodiments illustrated in the figures. In this respect, all described and/or graphically illustrated characteristics also form the object of the invention individually and in arbitrary combination regardless of their composition in the individual claims or their references to other claims. Furthermore, identical or similar objects are identified by the same reference symbols in the figures. 
         FIG. 1  illustrates an exemplary embodiment of an analysis system for quantitative blockage analysis of a grid in icing conditions. 
         FIG. 2  shows an exemplary embodiment of a method for calculating a time remaining until a critical icing condition is reached. 
         FIG. 3  shows an exemplary embodiment of a method for quantitative blockage analysis of a grid in icing conditions. 
         FIG. 4  shows a cross sectional view of an aircraft engine with an exemplary embodiment of an analysis system. 
         FIG. 5  shows an exemplary embodiment of a picture of a grid in icing conditions displayed on a display unit. 
         FIG. 6  shows an exemplary embodiment of an analysis system according to an exemplary embodiment. 
         FIG. 7  shows a diagram for calculation of a differential pressure signal according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In an aspect, the present invention relates to an analysis system for quantitative blockage analysis of a grid in icing conditions by using an optical sensor, such as a grey scale camera and by counting areas in pictures captured by the optical sensor that show a color scale value that is greater than a background represented by a reference value. In other words, the optical sensor is used to detect iced areas based on a difference between color scale values of the iced areas, which are mostly white, and a background color. 
     Based on the detected iced areas, a model of growth of the iced areas may be calculated. The model may be used to calculate a time remaining until reaching a critical condition, such as a total blockage or total icing of a particular grid is reached. 
       FIG. 1  shows an exemplary embodiment of an analysis system  100  for quantitative blockage analysis of a grid  103  in icing conditions. 
     The analysis system  100  comprises an optical sensor  101 , such as an endoscopic camera, a processor unit  105  and a display unit  107 . 
     The optical sensor is oriented parallel to an airflow in order to acquire images in a direction parallel to the airflow towards the grid  103 . 
     The optical sensor may be connected to the processor unit  105  with a communication interface, such as a cable or a wireless connection. 
     The optical sensor  101  submits data of images captured by the optical sensor  101  to the processor unit  105  via the communication interface. 
     The processor unit  105  performs an analysis algorithm that continuously processes the images acquired by the optical sensor  101 . 
     The analysis algorithm determines a color scale value for each area of a number of areas, such as a pixel in at least one field of each picture of a series of pictures. This means, for every area in a field of a particular picture submitted to the processor unit  105 , a color scale value is determined based on the data submitted by the optical sensor  101 . 
     Further, the analysis algorithm counts a number of areas in the at least one field of a particular picture that show a color scale value greater than an absolute reference value. This means that the analysis algorithm compares every color scale value of every area with a respective reference value and, if the color scale value of a particular area is greater than the absolute value of the reference value, the analysis algorithm counts the area as “+1”, for example. Of course, the value of the area that is counted may vary depending on a differential value of the color scale value of the area and the reference value, for example. 
     In an example, all color scale values that are in a range between “0” and a particular reference value, are not considered as ice or as background and all color scale values that are in a range between a particular reference value and “255” are considered as ice. 
     The color scale value determined for every area may be a grey scale value, particularly a grey scale value of a grey scale with a range between “0” and “255”. 
     In another example, the reference value may be calculated from any image of a series of pictures by using a so called “Otsu method” or may be set manually by a user. 
     By counting the number of areas that are considered, i. e. classified as ice, an icing or blocking condition of the grid  103  is determined. This icing condition may be used to calculate a warning signal indicative of a time remaining until total blockage is reached. 
     The warning signal may be calculated using a model that is based on a particular fitting parameter determined for the particular grid  103 . The particular fitting parameter for the particular grid may be determined in an experimental setup or based on pictures or values determined by the optical sensor  101  during a flight, for example. 
     Based on the warning signal displayed on the display unit  107 , a pilot can check the time remaining until a critical icing condition, such as a total blockage is reached. 
       FIG. 2  shows an exemplary embodiment of a method for calculating a warning signal indicative of a time remaining until total critical blockage of a particular grid. 
     Based on information about an icing behaviour, a value of a fitting parameter “A” is calculated in finding step  201  by using equation (1) at a given time “t” and a blockage “y”. As soon as fitting parameter “A” is calculated, it is used to model the icing behaviour of a particular grid to be analysed. 
     For finding the fitting parameter “A” equation (1) may be used based on an icing behavior that was measured in an experimental setup for example. Of course, fitting parameter “A” may also be found based on blockage values “y” determined for numerous timestamps during a flight, for example. This means, that fitting parameter “A” me be found by using equation (1) and at least one blocking value “y” determined at an early stage of an icing process, such as a time window of 200 seconds after an icing of a particular grid has been detected for the first time, for example. Fitting parameter “A” may be calculated by using only one blockage value determined at one timestamp or by using a function of numerous blocking values determined at numerous timestamps. 
         y=−e   −At    (1)
 
     By fitting the factor “A” in a model based on equation (2), a future icing behaviour of the grid is extrapolated, i.e. modelled in modelling step  203 . By finding a final time “t f ” where a critical icing condition, such as a blocking condition of 85% or 100%, for example, is reached, the time remaining until this critical icing condition is reached, can be determined in determination step  205  for a given timestamp “t”. 
     
       
         
           
             
               
                 
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     In equation (2) “y” is the blockage value, “t f ” is the final time, where a critical condition, such as 100% blockage or a blockage value=1 is reached, and “t” is a given timestamp in seconds, for example. 
     As soon as the remaining time has been calculated, the processing unit previously used for the calculating the remaining time or any other processing unit or transmitter may be used to transmit a signal to a display unit that will cause the display unit to display the remaining time in a displaying step  207 . 
       FIG. 3  shows a method  300  for quantitative blockage analysis of a grid in icing conditions. The method may be implemented by an analyzing algorithm as described in connection with  FIG. 1 . 
     Starting at a capturing step  301 , one or more pictures of a grid are captured by using an optical sensor. Further, each picture is submitted to a processor unit. 
     In determination step  303 , color scale values for each of a number of areas in at least one field of each picture are determined. 
     In counting step  305 , a number of areas in a particular field of a particular picture that show a color scale value greater than an absolute reference value is counted based on the color scale values determined in determination step  303 . 
     In displaying step  307 , a warning signal is displayed on a display unit. The warning signal being indicative of a time remaining until a critical icing condition is reached, calculated using on a model of an icing behavior of the grid. The model being calculated based on the number of areas that show a color scale value greater than the absolute reference value counted in counting step  305 . 
       FIG. 4  shows a cross sectional view of an aircraft engine  401  with an air-intake protection grid  403 . The aircraft engine  401  comprises an analysis system  405  with an optical sensor  407  that communicates with a processor unit  409 . The processor unit  409  communicates with a display unit  411 . 
     The processor unit  409  and the display unit  411  may be localized inside an aircraft carrying the aircraft engine  401 . 
     The processor unit  409  is configured to perform the method described in connection with  FIG. 3  based on pictures captured by the optical sensor  407 . This means, that a user, such as a pilot, may supervise an icing condition of the grid  403  by supervising a warning and/or an icing signal displayed on the display unit  411 . Respectively, the user may control the aircraft engine  401  or the aircraft depending on the warning signal and/or the icing signal to change the icing condition of the grid  403  based on the information shown on the display unit  411 . For example, the user may heat up the aircraft engine  401  to avoid further icing of the grid  403 . 
       FIG. 5  shows an exemplary embodiment of an illustration of a grid icing condition displayed on first part of a display unit. 
     In  FIG. 5 , a picture  500  of a grid captured by a camera as described in connection with  FIGS. 1 to 4  is displayed on a display unit. 
     For quantification of the icing of the gird shown in picture  500 , a field of interest  501  is selected in the picture  500 . 
     Afterwards, a color scale value is determined for every pixel in the field of interest  501  and compared with a reference value and all areas that show a color scale value that is greater than an absolute value of the reference value are considered, i. e. classified, as ice. 
     In  FIG. 6 , an air-intake  600  of an engine of an aircraft is shown. The air-intake  600  comprises a grid  601  for protection of an engine sucking air through the air-intake  600 . An airstream flows into the air-intake  600  in a direction indicated by arrow  603 . 
     A first sensor  605  of the aircraft, such as a pitot probe, is arranged upstream of the grid  601  and a second sensor  607  is arranged downstream of the grid  601 . The first sensor  605  measures a pressure of the airstream towards the grid  601  and the second sensor  607  measures a pressure of an airstream that has passed the grid  601 . Since the pressure of the airstream that has passed the grid  601  will be lower than the pressure of the airstream towards the grid  601 , in case the grid  601  shows a blockage  609 , the engine will show a performance loss. 
     The performance of the engine can be determined by calculating a differential pressure signal between a pressure determined by the first sensor  605  and a pressure determined by the second sensor  607 . In other words, if the second sensor  607  measures a pressure that is lower than the pressure measured by the first sensor  605 , the engine will show a performance loss. Of course, the first sensor  605  can be attached anywhere in front, i.e. upstream of the grid  601 , such as in a hull of the aircraft powered by the engine, for example. This means, any pressure sensor of the aircraft may be used as the first sensor. 
     In  FIG. 7  a diagram  700  is shown. On the x-axis  701 , a time scale, for example in [seconds] and on the y-axis  703  a pressure, for example in [mbar] is shown. Line  705  is a downstream pressure measured by a pressure sensor downstream of an air-intake grid for protection of an engine. Line  707  is a total pressure measured by a pressure sensor upstream of the grid. 
     Differential pressure signals  713 ,  715  and  717  are calculated between the downstream pressure  705  and the total pressure  707  measured upstream. In an example, differential pressure signals  713 ,  715  and  717  are each calculated at a certain time, namely when the pressure sensor used for measuring the downstream pressure  705  is turned from a first position to a second position facing the grid. 
     In case the pressure sensor used for measuring the downstream pressure  705  is turned in the second position, the pressure sensor is turned back from the second position to the first position after a given sampling time. The “sampling time” may be chosen to avoid icing of the pressure sensor. Thus, the duration of the sampling time may be chosen dependent from particular test conditions. 
     In the second position, the pressure sensor used for measuring the downstream pressure  705  may measure a total pressure resulting in stronger signals compared to signals determined when the pressure sensor is in the first position for measuring a stationary pressure. 
     In another example, the pressure sensor used for measuring the downstream pressure  705  may be heated instead of moving the pressure sensor from the first position to the second position and vice versa. Thus, a sampling time window for calculation of a differential pressure signal may start as a temperature of the pressure sensor reaches a given threshold, for example. 
     It is preferred to calculate the differential pressure signals  713 ,  715  or  717  only for time windows, in which the pressure sensor faces the grid or a temperature of the pressure sensor has reached the pre-given threshold. 
     For calculation of a particular differential pressure signal, each value determined during a sampling time window, such as a time when the downstream pressure sensor is facing the grid or a temperature of the pressure sensor has reached the pre-given threshold, is compared to a respective value measured from the pressure sensor upstream of the grid. Alternatively, all values determined in a sampling time window may be averaged and compared to averaged values determined by the pressure sensor upstream of the grid in the sampling time window. 
     Any mathematical comparison method, such as a division or a subtraction, for example, may be used to calculate the differential pressure signals  713 ,  715  and  717 . 
     Since in the example shown in  FIG. 7 , the differential pressure signals  713 ,  715  and  717  are decreasing over time, less air is streaming through the grid and a performance of an engine protected by the grid is decreasing. 
     Based on the differential pressure signals  713 ,  715  and  717  a time until a critical power loss occurs can be predicted by extrapolating values of the downstream pressure  705  in the future. For this purpose, a slope of the differential pressure signals  713 ,  715  and  717  may be calculated. 
     Based on the time until the critical power loss, which is determined via the extrapolated values of the downstream pressure  705 , a performance signal may be generated. The performance signal may be indicative of the time remaining until the critical power loss of the engine occurs. The performance signal may be displayed on a display unit. Thus, the performance signal may be shown as a timer on the display unit. 
     In addition, it should be pointed out that “comprising” does not exclude other elements or steps, and “a” or “an” does not exclude a plural number. Furthermore, it should be pointed out that characteristics or steps which have been described with reference to one of the above exemplary embodiments may also be used in combination with other characteristics or steps of other exemplary embodiments described above. Reference characters in the claims are not to be interpreted as limitations. 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.