Patent Publication Number: US-9418564-B2

Title: Method for determining a guidance law for obstacle avoidance by an aircraft, related computer program product, electronic system and aircraft

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to French Patent Application No. 14 00537 filed Mar. 4, 2014, which is herein incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for determining an avoidance guidance law for an aircraft, such as a rotary wing aircraft, to avoid one or more obstacles. The aircraft comprises a collision avoidance system adapted to detect a collision risk with the obstacle(s) and electronic system for determining the avoidance guidance law. 
     BACKGROUND OF THE INVENTION 
     A method known in the art is implemented by the system for determining the avoidance guidance law, and comprises the following steps:
         a) determining one or more set points from among flight path angle and speed set points, at least one set point depending on at least one vertical speed limit value, at least one set point comprising a vertical component in a vertical direction, each vertical speed limit value being provided by the collision avoidance system following the detection of a collision risk with the obstacle(s); and   b) computing the avoidance guidance law as a function of the determined set point(s).       

     The invention also relates to a non-transitory computer-readable medium including a computer program product suitable for carrying out such a determination method. 
     The invention also relates to an electronic system for determining the avoidance guidance law. 
     The invention also relates to an aircraft, such as a rotary wing aircraft, comprising a collision avoidance system adapted to detect a collision risk with one or more obstacles and one such electronic system for determining the avoidance guidance law. 
     A determination method and system of the aforementioned type are known from document EP 1,797,488 B1. 
     The aircraft is a transport airplane, and when the collision avoidance system detects a collision risk, it emits an alarm. The collision avoidance system also uses a vertical speed limit to determine a vertical speed set point. An avoidance guidance law, in particular a load factor set point, is next computed as a function of the determined vertical speed set point. 
     The computed load factor set point depends on a difference between the value of the current vertical speed and the determined vertical speed set point, that difference being multiplied by a variable depending on the current speed of the aircraft. 
     The computed load factor set points are next automatically sent to a flight director that implements a display mode for said computed set points to provide piloting assistance for the crew of the aircraft. 
     However, the avoidance guidance law computed using such a method is not optimal, the power required to keep the flight of the aircraft balanced being adapted to exceed the available power in the event the guidance law is not appropriate to see the avoidance through to completion. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is therefore to propose a method and system making it possible to compute a better avoidance guidance law, by reducing the power required to keep the aircraft&#39;s flight balanced. In general, the new guidance law makes it possible to take performance constraints of the aircraft into account and better manage its energy. 
     To that end, the invention relates to a determination method of the aforementioned type, wherein during step a), at least one determined set point comprises a longitudinal component in a longitudinal direction perpendicular to the vertical direction. 
     With the determination method according to the invention, the use of a speed set point comprising a longitudinal component, such as an airspeed set point comprising both a vertical component and a longitudinal component, makes it possible to compute a better avoidance guidance law, in particular if the aircraft is a rotary wing aircraft. 
     In fact, on a rotary wing aircraft, the total power necessary to fly begins to decrease when the speed increases starting from a zero speed, since the induced power used to suspend the aircraft decreases. When the airspeed continues to increase, the parasitic power resulting from aerodynamic results of the relative wind on the aircraft increases, and the total required power also increases. There is therefore an airspeed where the required power is minimal. This optimal airspeed is also called best climb airspeed. 
     The use of a speed set point comprising a longitudinal component, such as the airspeed set point comprising both a vertical component and a longitudinal component, then makes it possible to determine a guidance law offering better management of the energy and power of the aircraft. The guidance law for example makes it possible to minimize the power necessary to avoid the obstacle(s) by varying said longitudinal component of the speed. The avoidance maneuver is then more lasting and safer with a greater power reserve between the available power and the required power. 
     According to other advantageous aspects of the invention, the determination method comprises one or more of the following features, considered alone or according to any technically possible combination(s):
         during step a), a first set point and a second set point are determined, the first set point being a set point among the vertical speed set point and a flight path angle set point, the second set point being an airspeed set point, the first set point comprising a vertical component, and the second set point comprising a longitudinal component;   at least one set point comprises a target value and a current value, the avoidance guidance law being computed as a function of said current value, and said current value converges towards a target value according to a convergence law;   an authorized range of vertical speed values is determined from the vertical speed limit value(s) provided by the collision avoidance system, and the target value of the vertical speed set point is comprised in said authorized range;   the method further comprises the following step prior to step b):
           +a′) measuring one or more speeds of the aircraft in at least one direction among the vertical and longitudinal directions, and   during step b), the avoidance guidance law is further computed as a function of the measured speed(s);   
           a vertical speed and an airspeed are measured during step a′), and
           when a first variable among the measured vertical speed and a vertical speed set point provided by an automatic pilot device is comprised in the authorized range of vertical speed values, the target value of the vertical speed set point is equal to the first variable and the target value of the airspeed set point is equal to a second variable from among the measured airspeed and an airspeed set point provided by the automatic pilot device,   when the first variable is not comprised in said authorized range, the target value of the vertical speed set point is a value comprised in said authorized range and the target value of the airspeed set point is equal to a best climb airspeed or the second variable;   
           the method further comprises the following step prior to step b):
           +a″) measuring one or more accelerations of the aircraft in a direction among the vertical and longitudinal directions, and   during step b), the avoidance guidance law is further computed as a function of the measured acceleration(s);   
           during step a″), a vertical acceleration and longitudinal acceleration are measured, and the avoidance guidance law is computed, during step b), on the one hand as a function of the set point among the vertical speed set point and the flight path angle and vertical acceleration set point, and on the other hand as a function of the airspeed and longitudinal acceleration set point;   the aircraft is a rotary wing aircraft, and step b) comprises computing at least one command from among an attitude variation and a collective lever variation;   during step b), the attitude variation command is computed using the following equation:
 
 D _THETA_com=− K 1×( IAS _consigne− IAS _mesurée)+ K 2 ×AX _mesurée
       

     where IAS_consigne is the airspeed set point, 
     IAS_mesurée is a measured airspeed, 
     AX_mesurée is a measured longitudinal acceleration, and 
     K1 and K2 are gains depending on at least the altitude and the speed;
         during step b), the collective lever variation command is computed using the following equation:
 
 D _COLL_com= K 3×( VZ _consigne− VZ _mesurée)− K 4 ×AZ _mesurée
       

     where VZ_consigne is the vertical speed set point, 
     VZ_mesurée is a measured vertical speed, 
     AZ_mesurée is a measured vertical acceleration, and 
     K3 and K4 are gains depending on at least the altitude and the speed; and
         the aircraft further comprises an automatic pilot device, and the method further comprises at least one subsequent step after step b) from among:
           +c) displaying the avoidance guidance law computed during step b) on a screen visible by a crew of the aircraft, to provide the crew with assistance in performing an avoidance maneuver; and   +c′) sending the automatic pilot device the avoidance guidance law compute during step b), to perform a maneuver automatically to avoid the obstacle.   
               

     The invention also relates to a non-transitory computer-readable medium including a computer program product comprising software set points which, when implemented by a computer, carry out a method as defined above. 
     The invention also relates to an electronic system for determining an avoidance guidance law for an aircraft, such as a rotary wing aircraft, the aircraft comprising a collision avoidance system adapted to detect a collision risk with the obstacle(s), 
     the system comprising means for determining one or more set points from among flight path angle and speed set points, at least one set point depending on at least one vertical speed limit value, at least one set point comprising a vertical component in a vertical direction, each vertical speed limit value being provided by the collision avoidance system following the detection of a collision risk with the obstacle(s); and means for computing the avoidance guidance law as a function of the determined speed set point(s), 
     wherein at least one determined set point comprises a longitudinal component in a longitudinal direction perpendicular to the vertical direction. 
     The invention also relates to an aircraft, such as a rotary wing aircraft, comprising a collision avoidance system adapted to detect a collision risk with one or more obstacles and an electronic determination system as defined above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These features and advantages of the invention will appear more clearly upon reading the following description, provided solely as a non-limiting example, and done in reference to the appended drawings, in which: 
         FIG. 1  is a diagrammatic view of an aircraft according to the invention, the aircraft comprising a collision avoidance system adapted to detect a collision risk with one or more obstacles, speed and acceleration measurement sensors, flight control members, an automatic pilot device, a data display screen and an electronic system for determining an avoidance guidance law, 
         FIG. 2  is a set of curves showing different powers relative to the aircraft, as well as the total power required by the aircraft to fly, 
         FIG. 3  is a flowchart of a method, according to the invention, for determining an avoidance guidance law, the method being implemented by the determination system of  FIG. 1 , and 
         FIG. 4  is a more detailed flowchart of a step for determining speed set points of the flowchart of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , an aircraft  10 , such as a rotary wing aircraft, comprises a collision avoidance system  12  adapted to detect a collision risk with one or more obstacles, a set of sensors  14  adapted to measure speeds and accelerations of the aircraft  10 , an automatic pilot device  16  of the aircraft and a data display screen  18 . 
     The aircraft  10  also comprises a first control stick  20  and a second control stick  22 , each forming a primary control member that can be manipulated by the crew  24  of the aircraft in order to pilot the aircraft. 
     According to the invention, the aircraft  10  further comprises an electronic system  30  for determining an avoidance guidance law for the aircraft to avoid obstacle(s). 
     The collision avoidance system  12 , also called TCAS (Traffic Collision Avoidance System), is known per se, and is suitable for monitoring the airspace around the aircraft  10 , in particular in order to detect other aircraft equipped with a corresponding active transponder. This detection is independent of the air traffic control done by air traffic controllers. 
     If a collision risk with one or more obstacles is identified, the collision avoidance system  12  is adapted to provide the determination system  30  with one or more vertical speed limit values. The determination system  30  is then adapted to determine an authorized range of vertical speed values from the vertical speed limit value(s) received from the collision avoidance system  12 . 
     When the collision avoidance system  12  only provides a minimum vertical speed value, the authorized range of vertical speed values corresponds to the values above that minimum value. When the collision avoidance system  12  only provides a maximum vertical speed value, the authorized range of vertical speed values corresponds to the values below that maximum value. Lastly, when the collision avoidance system  12  provides both a minimum and maximum vertical speed value, the authorized range of vertical speed values is the set of values comprised between those minimum and maximum values. 
     The set of sensors  14  is suitable for measuring speeds and accelerations of the aircraft  10 , in particular a vertical speed VZ and a vertical acceleration AZ in a vertical direction Z, i.e., a direction normal to the land surface, or a direction passing substantially through the center of the earth. Hereinafter, the measured vertical speed and the measured vertical acceleration are respectively denoted VZ_mesurée and AZ_mesurée. 
     One skilled in the art will of course understand that the invention applies similarly to the case where the flight path angle, also denoted FPA (Flight Path Angle), it is used instead of the vertical speed VZ, knowing that the transition from one variable to the other is done using the following equation: 
     
       
         
           
             
               
                 
                   
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                           V 
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                           ⁢ 
                           Z 
                         
                         
                           V 
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                           X 
                         
                       
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     where VX represents a longitudinal speed in a longitudinal direction X perpendicular to the vertical direction Z. 
     The set of sensors  14  is suitable for measuring an indicated airspeed (IAS), the indicated airspeed measured for the aircraft  10  being denoted IAS_mesurée. The indicated airspeed IAS comprises a vertical component in the vertical direction Z and a longitudinal component in the longitudinal direction X perpendicular to the vertical direction Z. 
     In the following of the description, the airspeed will correspond, by convention, to the indicated airspeed IAS. One skilled in the art will of course understand that the invention applies similarly to the case where the indicated airspeed is the calibrated airspeed (CAS) or the true airspeed (TAS), or the MACH. 
     The set of sensors  14  is also suitable for measuring a longitudinal acceleration AX of the aircraft  10  in a longitudinal direction X, the measured longitudinal acceleration being denoted AX_mesurée. 
     The automatic pilot device  16  is known per se, and when it is activated, makes it possible to act automatically on the trajectory of the aircraft  10 , with no manipulation of one of the primary control members  20 ,  22  by the crew  24  of the aircraft. 
     The display screen  18  is adapted to display data, in particular data from the determination system  30 , for example to provide piloting assistance to the crew  24  of the aircraft. In the example embodiment of  FIG. 1 , the display screen  18  is separate from the determination system  30 . In an alternative that is not shown, the display screen  18  is incorporated into the determination system  30 . 
     The first and second control sticks  20 ,  22  are known per se and form primary control members of the aircraft  10  that are manipulated by the crew  24  to pilot the aircraft. 
     The first control stick  20 , also called collective lever, is suitable for controlling the climb or descent of the rotary wing aircraft  10  in a vertical plane containing the vertical direction Z and longitudinal direction X. 
     The second control stick  22 , also called cyclic control stick or mini-control stick, is suitable for commanding a variation in the attitude of the rotary wing aircraft  10 . 
     The determination system  30  comprises an information processing unit  32  for example formed by a memory  34  and a processor  36  associated with the memory  34 . 
     In the example embodiment of  FIG. 1 , the determination system  30  is separate both from the collision avoidance system  12  and the automatic pilot device  16 . 
     In an alternative that is not shown, the determination system  30  is incorporated into the automatic pilot device  16 . The display screen  18  then for example corresponds to a display screen, not shown, of the automatic pilot device  16 , and the information processing unit  32  corresponds to an information processing unit, not shown, of the automatic pilot device  16 . 
     The memory  34  is adapted to store software  38  for acquiring measured speed values VZ_mesurée, IAS_mesurée and/or acceleration values AZ_mesurée, AX_mesurée from among the values provided by the set of sensors  14 , for the vertical speed limit value(s) from the collision avoidance system  12 , as well as any vertical speed set points VZ_PA and airspeed set points IAS_PA provided by the automatic pilot device  16 . 
     The memory  34  is also adapted to store software  40  for determining one or more speed set points VZ_consigne, IAS_consigne, at least one speed set point VZ_consigne depending on at least one vertical speed limit value, at least one speed set point VZ_consigne, IAS_consigne comprising a vertical component in the vertical direction Z. According to the invention, at least one speed set point IAS_consigne determined by the determination software  40  comprises a longitudinal component in the longitudinal direction X. 
     The memory  34  is also adapted to store software  42  for computing an avoidance guidance law for an obstacle detected by the collision avoidance system  12 , the computation of the guidance law being carried out as a function of the determined speed set point(s) VZ_consigne, IAS_consigne. 
     As an optional addition, the memory  34  is adapted to store display software  44  for displaying data relative to the computed avoidance guidance law on the screen  18 . 
     As an optional addition, the memory  34  is adapted to store software  46  for sending the automatic pilot device  16  data relative to a computed avoidance guidance law, so that the avoidance maneuver is carried out automatically by the automatic pilot device  16 . The data sent to the automatic pilot device  16  for example comprise an attitude variation command D_THETA_com and a collective lever variation command D_COLL_com. 
     The processor  36  is adapted to load and execute each of the software programs  38 ,  40 ,  42 ,  44  and  46 . 
     The acquisition software  38 , the determination software  40  and the computation software  42  respectively form means for acquiring measured speed and/or acceleration values, means for determining one or more speed set points and means for computing the avoidance guidance law. 
     Alternatively, the acquisition means  38 , the determination means  40  and the computation means  42  are made in the form of programmable logic components, or in the form of dedicated integrated circuits. 
     As an optional addition, the display software  44  and the transmission software  46  respectively form means for displaying data on the screen  18  and means for sending data to the automatic pilot device  16 . 
     As an alternative to this addition, the display means  44  and the transmission means  46  are made in the form of programmable logic components, or in the form of dedicated integrated circuits. 
     The acquisition software  38  is, for example, suitable for acquiring both measured vertical speed and airspeed values VZ_mesurée, IAS_mesurée and measured vertical and longitudinal acceleration values AZ_mesurée, AX_mesurée. The acquisition software  38  is also suitable for acquiring the vertical speed limit value(s) from the collision avoidance system  12 , as well as any vertical speed VZ_PA and airspeed IAS_PA set points provided by the automatic pilot device  16 . 
     The determination software  40  is for example suitable for computing a vertical speed set point VZ_consigne and an airspeed set point IAS_consigne. The vertical speed set point VZ_consigne comprises only a vertical component in the vertical direction Z, and the airspeed set point IAS_consigne comprises both a vertical component in the vertical direction Z and a longitudinal component in the longitudinal direction X. 
     In the described example embodiment, each speed set point VZ_consigne, IAS_consigne comprises a target value and a current value, the avoidance guidance law being computed as a function of the current value, and said current value converging toward the target value according to a convergence law. 
     In the following of the description, the target value and the current value of the vertical speed set point VZ_consigne are respectively denoted VZ_cons_cible and VZ_cons_courante. The target value and the current value of the airspeed set point IAS_consigne are respectively denoted IAS_cons_cible and IAS_cons_courante. The target value VZ_cons_cible of the vertical speed set point is comprised in the authorized range of vertical speed values VZ that is determined from the vertical speed limit value(s) provided by the collision avoidance system  12 . 
     In the described example, the vertical speed set point VZ_consigne is then determined as a function of at least one vertical speed limit value provided by the collision avoidance system  12 . 
     When a first variable VZ_référence chosen from among the measured vertical speed VZ_mesurée and any vertical speed set point VZ_PA provided by the automatic pilot device  16  is comprised in the authorized range of vertical speed values, as previously described, the target value VZ_cons_cible of the vertical speed set point is for example equal to the first variable VZ_référence and the target value IAS_cons_cible of the airspeed set point is for example equal to a second variable IAS_référence chosen from among the measured airspeed IAS_mesurée and any airspeed set point IAS_PA provided by the automatic pilot device  16 . 
     Otherwise, when the first variable VZ_référence is not comprised in said authorized range, the target value VZ_cons_cible of the vertical speed set point is a value chosen from said authorized range and the target value IAS_cons_cible of the airspeed is for example equal to a best climb airspeed IAS MM  or the second variable IAS_référence, as will be described in more detail below in reference to  FIG. 4 . 
     When a vertical speed set point VZ_PA is provided by the automatic pilot device  16 , then the first variable VZ_référence is preferably equal to that set point VZ_PA received from the automatic pilot device  16 . If the automatic pilot device  16  has not provided any vertical speed set point to the determination system  30 , the first variable VZ_référence is then equal to the measured vertical speed VZ_mesurée. 
     Similarly, when an airspeed set point IAS_PA is provided by the automatic pilot device  16 , then the second variable IAS_référence is preferably equal to that set point IAS_PA received from the automatic pilot device  16 . If the automatic pilot device  16  has not provided any airspeed set point to the determination system  30 , the second variable IAS_référence is then equal to the measured airspeed IAS_mesurée. 
     One skilled in the art will note that this logic is independent, i.e., separated, between the first variable VZ_référence and the second variable IAS_référence, the automatic pilot device  16  being adapted to provide only a vertical speed set point VZ_PA, or only an airspeed set point IAS_PA, or both a vertical speed set point VZ_PA and an airspeed set point IAS_PA, or no vertical speed set point VZ_PA and no airspeed set point IAS_PA. 
     The best climb airspeed IAS MM , shown in  FIG. 2 , is the airspeed corresponding to a minimum value of the total power required to fly the aircraft  10 , the total required power corresponding to the curve  60  in bold lines in  FIG. 2 . In  FIG. 2 , curve  62  represents the induced power used to sustain the aircraft  10 , curve  64  shows the parasitic power resulting from the aerodynamic effects of the relative wind on the aircraft  10 , and curve  66  shows the profile power resulting from the work of the drag forces on the blades, the total required power being the sum of the induced power, the parasitic power and the profile power. 
     The computation software  42  is suitable for computing the avoidance guidance law for the obstacle(s) as a function of the determined speed set point(s), for example on the one hand as a function of the vertical speed set point VZ_consigne, and in particular the current value VZ_cons_courante of that speed set point, and on the other hand as a function of the airspeed set point IAS_consigne, and in particular the current value IAS_cons_courante of that speed set point. 
     The guidance law calculated by the computation software  42  for example comprises two commands, i.e., a first command depending on the vertical speed set point VZ_consigne and the measured vertical speed VZ_mesurée, and a second command depending on the airspeed set point IAS_consigne and the measured airspeed IAS_mesurée. 
     Additionally, the computed guidance law also depends on the one hand on the vertical acceleration AZ, and on the other hand on the longitudinal acceleration AX. The first command then depends on the vertical speed set point VZ_consigne, in particular the current value VZ_cons_courante of that vertical speed set point, the measured vertical speed VZ_mesurée and the vertical acceleration AZ. Similarly, the second command then depends on the airspeed set point IAS_consigne, in particular the current value IAS_cons_courante of that airspeed set point, the measured airspeed IAS_mesurée and the longitudinal acceleration AX. 
     In the described example embodiment where the aircraft  10  is a rotary wing aircraft, the first command is the collective lever variation command D_COLL_com and the second command is the attitude variation command D_THETA_com. 
     The attitude variation command D_THETA_com for example verifies the following equation:
 
 D _THETA_com=− K 1×( IAS _consigne− IAS _mesurée)+ K 2 ×AX _mesurée  (2)
 
where IAS_consigne is the airspeed set point,
 
IAS_mesurée is a measured airspeed,
 
AX_mesurée is a measured longitudinal acceleration, and
 
K1 and K2 are gains depending on at least the altitude and the speed.
 
     The gain K1 is expressed in degrees per m·s −1 , and is for example comprised between 1 degree per m·s −1  and 6 degrees per m·s −1 , typically equal to 3 degrees per m·s −1 . 
     The gain K2 is expressed in degrees per m·s −2 , and is for example comprised between 0 degrees per m·s −2  and 12 degrees per m·s −2 , typically equal to 6 degrees per m·s −2 . 
     When, in addition, the airspeed set point IAS_consigne comprises the target value IAS_cons_cible and the current value IAS_cons_courante, the attitude variation command D_THETA_com preferably verifies the following equation:
 
 D _THETA_com=− K 1×( IAS _cons_courante− IAS _mesurée)+ K 2 ×AX _mesurée  (3)
 
     The collective lever variation command D_COLL_com for example verifies the following equation:
 
 D _COLL_com= K 3×( VZ _consigne− VZ _mesurée)− K 4 ×AZ _mesurée  (4)
 
where VZ_consigne is the vertical speed set point,
 
VZ_mesurée is a measured vertical speed,
 
AZ_mesurée is a measured vertical acceleration, and
 
K3 and K4 are gains depending on at least the altitude and the speed.
 
     The gain K3 is expressed in % per m·s −1 , and is for example comprised between 1% per m·s −1  and 4% per m·s −1 , typically equal to 2% per m·s −1 . 
     The gain K4 is expressed in % per m·s −2 , and is for example comprised between 0% per m·s −2  and 4% per m·s −2 , typically equal to 1% per m·s −2 . 
     Similarly, when, in addition, the vertical speed set point VZ_consigne comprises the target value VZ_cons_cible and the current value VZ_cons_courante, the collective lever variation command D_COLL_com preferably verifies the following equation:
 
 D _COLL_com= K 3×( VZ _cons_courante− VZ _mesurée)− K 4 ×AZ _mesurée  (5)
 
     The operation of the determination system  30  according to the invention will now be described using  FIGS. 3 and 4  respectively showing a flowchart of the method for determining the avoidance guidance law according to the invention and a detailed flowchart of the step for determining the target values VZ_cons_cible, IAS_cons_cible of the speed set points. 
     During an initial step  100 , vertical and airspeed values VZ_mesurée, IAS_mesurée are measured by the set of sensors  14 , then acquired by the acquisition software  38 . Additionally, vertical and longitudinal acceleration values AZ_mesurée, AX_mesurée are measured by the set of sensors  14 , then acquired by the acquisition software  38 . These different speed and acceleration values are preferably measured at the same moment in time. The acquisition software  38  also acquires the vertical speed limit value(s) from the collision avoidance system  12 , as well as any vertical speed VZ_PA and airspeed IAS_PA set points provided by the automatic pilot device  16 . 
     During step  110 , the determination software  40  next determines the vertical speed set point VZ_consigne and the airspeed set point IAS_consigne, in particular using measured vertical and airspeed values VZ_mesurée, IAS_mesurée previously acquired. This determination step  110  is split into sub-steps shown in  FIG. 4 . 
     In  FIG. 4 , during sub-step  115 , the determination software  40  begins by determining whether the first variable VZ_référence is in the authorized range of vertical speed values. As previously described, the authorized range of vertical speed values is defined from the vertical speed limit value(s) received from the collision avoidance system  12 . 
     If the first variable VZ_référence is in the authorized range of vertical speed values, then during sub-step  120  the determination software  40  determines the target value VZ_cons_cible of the vertical speed set point as being equal to the first variable VZ_référence. 
     Otherwise, during sub-step  125 , the determination software  40  computes the target value VZ_cons_cible of the vertical speed set point by choosing a value within the authorized range of vertical speed values. The selected value is for example the value of said authorized range that is closest to the first variable VZ_référence. Alternatively, the selected value is the value from said authorized range that is closest to the first variable VZ_référence, to or from which a margin is added or subtracted so that the selected target value VZ_cons_cible is separated from said vertical speed limit value(s) by at least that margin. 
     During sub-step  130 , the determination software  40  next determines whether the target value VZ_cons_cible of the vertical speed set point previously computed during sub-step  125  is greater than the first variable VZ_référence. 
     If the computed target value VZ_cons_cible of the vertical speed set point is greater than the first variable VZ_référence, then the determination software  40  next verifies, during sub-step  135 , whether the second variable IAS_référence is lower than the best climb airspeed IAS MM . 
     If the computed target value VZ_cons_cible of the vertical speed set point is, contrariwise, less than the first variable VZ_référence, the determination software  40  verifies, during sub-step  140 , whether the second variable IAS_référence is lower than the best climb airspeed IAS MM . 
     During sub-step  135 , if the second variable IAS_référence is lower than the best climb airspeed IAS MM , then the determination software  40  determines, during sub-step  145 , the target value IAS_cons_cible of the airspeed set point as being equal to the second variable IAS_référence. 
     If, however, during sub-step  135 , the second variable IAS_référence is greater than or equal to the best climb airspeed IAS MM , then the determination software  40  determines, during sub-step  150 , the target value IAS_cons_cible of the airspeed set point as being equal to the best climb airspeed IAS MM . 
     During sub-step  140 , if the second variable IAS_référence is lower than the best climb airspeed IASMM, then the determination software  40  goes on to sub-step  150 , and the target value IAS_cons_cible of the airspeed set point is then equal to the best climb airspeed IAS MM . 
     If, however, during sub-step  140 , the second variable IAS_référence is greater than or equal to the best climb airspeed IASMM, then the determination software  40  goes on to sub-step  145 , and the target value IAS_cons_cible of the airspeed set point is then equal to the second variable IAS_référence. 
     After sub-step  120 , the determination software  40  goes on to sub-step  145 , and the target value IAS_cons_cible of the airspeed set point is then equal to the second variable IAS_référence. 
     In other words, the strategy for determining the target value IAS_cons_cible of the airspeed set point is as follows:
         if the target value VZ_cons_cible of the vertical speed set point is unchanged relative to the first variable VZ_référence (sub-step  120 ), then the target value IAS_cons_cible of the airspeed set point is also unchanged, and equal to the second variable IAS_référence;   if the target value VZ_cons_cible of the vertical speed set point is greater than the first variable VZ_référence (positive response to the test in sub-step  130 ), i.e., if the vertical speed must be increased, and if the second variable IAS_référence is greater than the best climb airspeed IAS MM  (negative response to the test in sub-step  135 ), then the best climb airspeed IAS MM  is used as the target value IAS_cons_cible for the airspeed set point. This makes it possible to use the kinetic energy to facilitate the climb, as shown in  FIG. 2  with the airspeed IAS 1  and arrow F 1 , and decreases the total power required over the longer term, i.e., when the airspeed comes closer to the best climb airspeed.   if the target value VZ_cons_cible of the vertical speed set point is less than or equal to the first variable VZ_référence (negative response to the test in sub-step  130 ), i.e., if the vertical speed must be decreased, and if the second variable IAS_référence is lower than the best climb airspeed IAS MM  (positive response to the test in sub-step  140 ), the best climb airspeed IAS MM  is also used as target value IAS_cons_cible for the airspeed set point. This makes it possible to transfer energy facilitating the descent, as shown in  FIG. 2  with the airspeed IAS 2  and arrow F 2 , and places the aircraft  10  in a good configuration to execute other maneuvers while increasing the available power margin via the decrease of the total required power.   in the other cases (positive response to the test in sub-step  135  or negative response to the test in sub-step  140 ), the target value IAS_cons_cible of the airspeed set point is unchanged, and equal to the second variable IAS_référence.       

     At the end of step  110 , after sub-step  145  or sub-step  150 , the determination software  40  also determines the current values IAS_cons_courante, VZ_cons_courante of the airspeed set point and the vertical speed set point using the target values IAS_cons_cible, VZ_cons_cible previously determined as a function of the convergence law of the current value toward the corresponding target value. 
     The convergence of the current value VZ_cons_courante of the vertical speed set point toward the corresponding target value VZ_cons_cible for example follows an affine law. In other words, the dynamic for the current value VZ_cons_courante to join the target value VZ_cons_cible, i.e., the drift relative to time of the current value VZ_cons_courante, is constant as long as the target value VZ_cons_cible has not been reached. The drift relative to time of the current value VZ_cons_courante is for example equal, in absolute value, to 400 feet/minute per second. The choice of the convergence law then makes it possible to guarantee a vertical acceleration making it possible to ensure a rapid response to avoid the obstacle(s), while retaining a certain power margin. 
     The dynamic for the current value IAS_cons_courante of the airspeed set point to join the target value IAS_cons_cible, i.e., the drift relative to time of the current value IAS_cons_courante, for example depends on the measured airspeed IAS_mesurée and the difference between the target value IAS_cons_cible and the current value IAS_cons_courante of the airspeed set point. The absolute value of the drift relative to time of the current value IAS_cons_courante for example verifies the following equation: 
     
       
         
           
             
               
                 
                   
                      
                     
                       
                         
                           ⅆ 
                           IAS_cons 
                         
                         ⁢ 
                         _courante 
                       
                       
                         ⅆ 
                         t 
                       
                     
                      
                   
                   = 
                   
                     K 
                     × 
                     
                        
                       
                         
                           VZ_cons 
                           ⁢ 
                           _cible 
                         
                         - 
                         
                           VZ_cons 
                           ⁢ 
                           _courante 
                         
                       
                        
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     for values of K*|VZ_cons_cible− VZ _cons_courante| comprised between a minimum longitudinal acceleration AXmin and a maximum longitudinal acceleration AXmax; where K is a gain expressed by knots per second per feet per minute. 
     The sign of the drift relative to time of the current value IAS_cons_courante is that of the difference between the target value and the current value of the airspeed set point, denoted IAS_cons_cible−IAS_cons_courante. The current value IAS_cons_courante of the airspeed set point then converges toward the target value IAS_cons_cible. 
     In other words, the drift relative to time of the current value IAS_cons_courante for example verifies the following equation: 
     
       
         
           
             
               
                 
                   
                     
                       
                         ⅆ 
                         IAS_cons 
                       
                       ⁢ 
                       _courante 
                     
                     
                       ⅆ 
                       t 
                     
                   
                   = 
                   
                     
                       sgn 
                       ⁡ 
                       
                         ( 
                         
                           
                             IAS_cons 
                             ⁢ 
                             _cible 
                           
                           - 
                           
                             IAS_cons 
                             ⁢ 
                             _courante 
                           
                         
                         ) 
                       
                     
                     × 
                     
                       med 
                       ⁡ 
                       
                         ( 
                         
                           
                             K 
                             × 
                             
                                
                               
                                 
                                   VZ_cons 
                                   ⁢ 
                                   _cible 
                                 
                                 - 
                                 
                                   VZ_cons 
                                   ⁢ 
                                   _courante 
                                 
                               
                                
                             
                           
                           , 
                           
                             AX 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             min 
                           
                           , 
                           
                             AX 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             max 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     where sgn is the sign function, which is equal to +1 if the difference (IAS_cons_cible−IAS_cons_courante) is positive, 0 if it is null and −1 if it is negative; 
     med is the median function, which is equal to the median value of the three values (K*|VZ_cons_cible− VZ _cons_courante|), AXmin and AXmax. This makes it possible to limit oneself to values of the longitudinal acceleration AX comprised between the minimum longitudinal acceleration AXmin and the maximum longitudinal acceleration AXmax. 
     As an example, the value of the gain K is equal to 1/500 knots per foot per minute, the minimum and maximum values of the longitudinal acceleration AXmin and AXmax are respectively equal to 1 knot per second and 2 knots per second. 
     With this value example, if the vertical speed deviation is greater than 1000 feet, the current value of the airspeed set point IAS_cons_courante converges at 2 knots per second toward the target value IAS_cons_cible. If the vertical speed deviation is comprised between 500 and 1000 feet, the current value of the airspeed set point IAS_cons_courante converges toward the target value IAS_cons_cible at K times the computed vertical speed deviation. Lastly, if the vertical speed deviation is less than 500 feet, the current value of the airspeed set point IAS_cons_courante converges at 1 knot per second toward the target value IAS_cons_cible. 
     This then makes it possible to accelerate the joining dynamic for the airspeed when the performance requires it, or will require it shortly, or on the contrary offers a slower joining dynamic for the airspeed if the avoidance maneuver does not require an excessively significant dynamic. In the latter case, this then makes it possible to retain a more significant power margin for better safety of the aircraft  10 . 
     Alternatively, the drift relative to time of the convergence law of the current value IAS_cons_courante toward the target value IAS_cons_cible is constant, and for example equal in absolute value to 1 knot per second. 
     During step  160 , shown in  FIG. 3 , the computation software  42  next computes the avoidance guidance law for the obstacle based on the determined speed set point(s). In the described example, the computation software  42  computes the attitude variation command D_THETA_com as a function of the current value IAS_cons_courante of the airspeed set point, the measured airspeed IAS_mesurée and the measured longitudinal acceleration AX_mesurée according to equation (2). The computation software  42  also computes the collective lever variation command D_COLL_com as a function of the current value VZ_cons_courante of the vertical speed set point, the measured vertical speed VZ_mesurée and the measured vertical acceleration AZ_mesurée according to equation (4). 
     After step  160 , the determination system  30  goes on to step  170 , during which its display software  44  manages the display on the screen  18  of data relative to the computed avoidance guidance law. 
     Alternatively or additionally, after step  160 , the determination system  30  goes on to step  180 , during which its transmission software  46  sends the automatic pilot device  16  the data relative to the computed avoidance guidance law, so that the avoidance maneuver is done automatically by the automatic pilot device  16 . The transmission software  46  in particular sends the values of the attitude D_THETA_com and collective lever D_COLL_com variation commands previously computed during step  160 . 
     At the end of step  160 , the determination system  30  returns to step  100  in order to acquire, via its acquisition software  38 , new values of the measured vertical and aircraft velocities VZ_mesurée, IAS_mesurée and measured vertical and longitudinal accelerations AZ_mesurée, AX_mesurée. 
     After having returned to step  100 , the determination step  30  goes on to step  110  in order to determine new speed set points. This new determination is preferably done by varying only the current values VZ_cons_courante and IAS_cons_courante of the vertical speed and airspeed set points toward their respective target values VZ_cons_cible and IAS_cons_cible as a function of the associated convergence laws, and while keeping the target values VZ_cons_cible and IAS_cons_cible determined during the first passage by step  110 . 
     The respective target values VZ_cons_cible and IAS_cons_cible are preferably modified only if there is a modification of the data coming from the collision avoidance system  12  or data coming from the automatic pilot device  16 . In other words, the respective target values VZ_cons_cible and IAS_cons_cible are then modified only if it is necessary to change the avoidance maneuver, for example following a new obstacle, or following an end of obstacle, or following a modification of the trajectory of the obstacle. 
     The determination system  30  and the determination method according to the invention make it possible to compute a better avoidance guidance law by taking into account not only a speed set point comprising a vertical component in the vertical direction Z, as is done in the state of the art while taking into account only the vertical speed set point, but also a speed set point comprising a longitudinal component in the longitudinal direction X perpendicular to the vertical direction Z. The speed set point comprising the longitudinal component is for example the airspeed set point, which comprises both the vertical component and a longitudinal component. 
     The avoidance guidance law thus determined according to the invention then makes it possible to propose better management of the energy of the aircraft  10 , in particular by minimizing the power required to perform the avoidance maneuver. The avoidance maneuver is then more lasting and safer, in particular by bringing the aircraft  10  to a balanced flight point corresponding to the best climb airspeed IAS MM . 
     The energy management also makes it possible to perform faster avoidance maneuvers, in particular in the event of a conversion of the kinetic energy from the forward speed into potential energy to climb more quickly, as shown by arrow F 1  in  FIG. 2 . 
     When each speed set point VZ_consigne, IAS_consigne further has a target value VZ_cons_cible, IAS_cons_cible and a current value VZ_cons_courante, IAS_cons_courante, with a convergence law of the current value toward the corresponding target value, the choice of the convergence laws makes it possible to perform a more natural avoidance maneuver, i.e., closer to the maneuver that would be done by a pilot. 
     According to a second embodiment, the determination system  30  is suitable for determining a flight path angle set point FPA_consigne in place of the vertical speed set point VZ_consigne. In fact, in light of the relationship previously described using equation (1) between the vertical speed VZ and the flight path angle FPA, one skilled in the art will understand that the determination system  30  is also suitable for computing the avoidance guidance law as a function of the determined flight path angle set point FPA_consigne and airspeed set point IAS_consigne. 
     Similarly to what was previously described for the first embodiment, the flight path angle set point FPA_consigne and the airspeed set point IAS_consigne each for example comprise a target value FPA_cons_cible, IAS_cons_cible and a current value FPA_cons_courante, IAS_cons_courante, the avoidance guidance law being computed as a function of the current value, and said current value then converging toward the target value according to a convergence law. 
     The attitude variation command D_THETA_com for example verifies equation (2), or optionally in addition equation (3), previously described for the first reception mode, with unchanged gain values K1, K2. 
     According to this second embodiment, the collective lever variation command D_COLL_com then for example verifies the following equation:
 
 D _COLL_com= K 5×(FPA_consigne−FPA_mesurée)− K 6 ×AZ _mesurée  (8)
 
where FPA_consigne is the flight path angle set point,
 
FPA_mesurée is a measured flight path angle,
 
AZ_mesurée is a measured vertical acceleration, and
 
K5 and K6 are gains depending on at least the altitude and the speed.
 
     The gain K5 is expressed in % per degree, and is for example comprised between 0.2% per degree and 8% per degree, typically equal to 1% per degree. 
     The gain K6 is expressed in % per degree·s −1 , and is for example comprised between 0% per degree·s −1  and 8% per degree·s −1 , typically equal to 0.5% per degree·s −1 . 
     Similarly, when as an optional addition, the flight path angle set point FPA_consigne comprises the target value FPA_cons_cible and the current value FPA_cons_courante, the collective lever variation command D_COLL_com preferably verifies the following equation:
 
 D _COLL_com= K 5×(FPA_cons_courante−FPA_mesurée)− K 6 ×AZ _mesurée  (9)
 
     According to this second embodiment, the flight path angle set point VZ_consigne is then for example determined as a function of at least one vertical speed limit value provided by the collision avoidance system  12 , more specifically as a function of an authorized range of flight path angle values, computed using equation (1) and the authorized range of vertical speed values previously described for the first embodiment. 
     When a third variable FPA_référence chosen from among the measured flight path angle FPA_mesurée and any flight path angle set point FPA_PA provided by the automatic pilot device  16  is comprised in the authorized range of flight path angle values, the target value FPA_cons_cible of the flight path angle set point is for example equal to the third variable FPA_référence, and the target value IAS_cons_cible of the airspeed set point is for example equal to the second variable IAS_référence chosen from among the measured airspeed IAS_mesurée and any airspeed set point IAS_PA provided by the automatic pilot device  16 . 
     Otherwise, when the third variable FPA_référence is not comprised in said authorized range of flight path angle values, the target value FPA_cons_cible of the flight path angle set point is a value chosen from said authorized range of flight path angle values and the target value IAS_cons_cible of the airspeed set point is for example equal to a best climb airspeed IAS MM  or the second variable IAS_référence. 
     The operation of the determination system  30  according to this second embodiment is then similar to that previously described for the first embodiment, each time replacing the vertical speed set point VZ_consigne with the flight path angle set point FPA_consigne, or additionally the current value VZ_cons_courante and the target value VZ_cons_cible of the vertical speed set point by the current value FPA_cons_courante and the target value FPA_cons_cible of the flight path angle set point, respectively, as well as by replacing the first variable VZ_référence with the third variable FPA_référence if applicable, as described above. 
     According to this second embodiment, the determination software  40  thus determines the flight path angle set point FPA_consigne and the airspeed set point IAS_consigne during step  110 , using the measured flight path angle and airspeed values FPA_mesurée, IAS_mesurée acquired previously during step  100 . 
     This determination step  110  is split into sub-steps, similarly to what was previously described for the first embodiment in reference to  FIG. 4 , replacing the first variable VZ_référence with the third variable FPA_référence as well as the current value VZ_cons_courante and the target value VZ_cons_cible of the vertical speed set point by the current value FPA_cons_courante and the target value FPA_cons_cible of the flight path angle set point, respectively. 
     The convergence of the current value FPA_cons_courante of the flight path angle set point toward the corresponding target value FPA_cons_cible for example follows an affine law. The drift relative to time of the current value FPA_cons_courante is for example equal in absolute value to 4 degrees per second. 
     The dynamic for the current value IAS_cons_courante of the airspeed set point to join the target value IAS_cons_cible, i.e., the drift relative to time of the current value IAS_cons_courante, for example depends on the measured airspeed IAS_mesurée and the difference between the target value IAS_cons_cible and the current value IAS_cons_courante of the airspeed set point. 
     According to this second embodiment, the absolute value of the drift relative to time of the current value IAS_cons_courante for example verifies the following equation: 
     
       
         
           
             
               
                 
                   
                      
                     
                       
                         
                           ⅆ 
                           IAS_cons 
                         
                         ⁢ 
                         _courante 
                       
                       
                         ⅆ 
                         t 
                       
                     
                      
                   
                   = 
                   
                     
                       K 
                       ′ 
                     
                     × 
                     
                        
                       
                         
                           FPA_cons 
                           ⁢ 
                           _cible 
                         
                         - 
                         
                           FPA_cons 
                           ⁢ 
                           _courante 
                         
                       
                        
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     for values of the longitudinal acceleration AX comprised between a minimum longitudinal acceleration AXmin and a maximum longitudinal acceleration AXmax; 
     where K′ is a gain expressed in knots per second per degree. 
     The drift relative to time of the current value IAS_cons_courante for example verifies the following equation: 
     
       
         
           
             
                 
             
             ⁢ 
             
               ( 
               11 
               ) 
             
           
         
       
       
         
           
             
               
                 
                   ⅆ 
                   IAS_cons 
                 
                 ⁢ 
                 _courante 
               
               
                 ⅆ 
                 t 
               
             
             = 
             
               
                 sgn 
                 ⁡ 
                 
                   ( 
                   
                     
                       IAS_cons 
                       ⁢ 
                       _cible 
                     
                     - 
                     
                       IAS_cons 
                       ⁢ 
                       _courante 
                     
                   
                   ) 
                 
               
               × 
               
                 
 
               
               ⁢ 
               
                 med 
                 ⁡ 
                 
                   ( 
                   
                     
                       
                         K 
                         ′ 
                       
                       × 
                       
                          
                         
                           
                             FPA_cons 
                             ⁢ 
                             _cible 
                           
                           - 
                           
                             FPA_cons 
                             ⁢ 
                             _courante 
                           
                         
                          
                       
                     
                     , 
                     
                       AX 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       min 
                     
                     , 
                     
                       AX 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       max 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     where sgn is the sign function, which is equal to +1 if the difference (IAS_cons_cible−IAS_cons_courante) is positive, 0 if it is null and −1 if it is negative; 
     med is the median function, which is equal to the median value of the three values (K′*|FPA_cons_cible−FPA_cons_courante|), AXmin and AXmax. This makes it possible to limit oneself to values of the longitudinal acceleration AX comprised between the minimum longitudinal acceleration AXmin and the maximum longitudinal acceleration AXmax. 
     As an example, the value of the gain K′ is equal to 0.25 knots per second per degree, and the minimum and maximum values of the longitudinal acceleration AXmin and AXmax are respectively equals 1 knot per second and 2 knots per second. 
     The advantages of the second embodiment are identical to those of the first embodiment previously described, and are not described again. 
     One skilled in the art will understand that the avoidance guidance law is a guidance law computed in order to avoid the obstacle(s), the guidance law comprising one or more guidance commands for the aircraft  10 . As previously indicated, the computer guidance law for example comprises two commands, i.e., a first command depending on the vertical speed set point VZ_consigne and the measured vertical speed VZ_mesurée, and a second command depending on the airspeed set point IAS_consigne and measured airspeed IAS_mesurée.