Abstract:
A method for aiding the landing of an aircraft on a landing runway comprises the following steps implemented automatically by a processing unit: a) acquiring information relating to the runway; b) acquiring information relating to a current situation of the aircraft; i) determining a landing position of the aircraft on a central longitudinal axis of the runway, as a function of a trajectory determined between a current position of the aircraft and this landing position to comply with at least one constraint relating to a roll angle of the aircraft; j) determining a parameter relating to the landing position; k) determining a deviation between this parameter and a reference value; l) comparing this deviation with a predetermined threshold; and m) as a function of the result of the comparison carried out in step l), commanding, if appropriate, the signaling of an alert via the signaling system of the aircraft.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the French patent application No. 1561785 filed on Dec. 3, 2015, the entire disclosures of which are incorporated herein by way of reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method and to a system for aiding landing of an aircraft on a landing runway, as well as to an aircraft comprising such a system. 
         [0003]    During an approach phase before a landing, an aircraft is generally obliged to follow an approach axis of a landing runway, up to a landing position of the aircraft on the landing runway, for which the wheels of the aircraft touch the landing runway (so-called “touchdown” position). This approach axis can for example correspond to an axis of an instrument-based approach of ILS (“Instrument Landing System”) type Immediately after landing, the aircraft performs a rollout on the landing runway according to a substantially rectilinear trajectory close to a central longitudinal axis of the landing runway. 
         [0004]      FIG. 1  illustrates, viewed from above, an exemplary approach to a landing runway  5  by an aircraft  1 .  FIG. 2  corresponds to a more detailed view around the landing position of the aircraft on the landing runway. This approach is also illustrated, in a side view, by  FIG. 3 . In manual piloting mode, during the approach phase, the pilot must pilot the aircraft  1  along the closest possible trajectory  12  to the approach axis  10  in such a way that, on the one hand, the landing position Td of the aircraft on the landing runway is as close as possible to the central longitudinal axis  4  of the landing runway (so as to avoid putting the aircraft down alongside the landing runway) and that, on the other hand, the speed vector Vtd of the aircraft at this landing position is substantially parallel to the central longitudinal axis of the landing runway (so as to avoid exiting the runway, laterally, during rollout after landing). If a risk of exiting the runway laterally exists, the pilot must perform a go-around and carry out a new approach. During the approach to the landing runway, in particular during the final phase of the approach, the pilot must therefore monitor the trajectory of the aircraft so as to be able to detect such a risk. Document FR2948468 A1 describes a method and a device for aiding the lateral control of an aircraft during the approach phase, making it possible to signal a caution in the cockpit of the aircraft in case a risk of lateral exit from the landing runway is predicted. It would however be beneficial to have alternatives to this method and to this device. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention relates to a method for aiding the landing of an aircraft on a landing runway, the aircraft comprising a set of information sources, a flight management system, a processing unit and a signaling system. The method is noteworthy in that it comprises the following steps implemented in an automatic manner by the processing unit:
       a) acquiring, from the flight management system, information relating to the landing runway;   b) acquiring, from the set of information sources, information relating to a current situation of the aircraft;   i) determining, as a function, on the one hand, of the information relating to the current situation of the aircraft and, on the other hand, of the information relating to the landing runway, a landing position of the aircraft on a central longitudinal axis of the landing runway, this landing position being determined as a function of a trajectory determined between a current position of the aircraft and this landing position so as to comply with at least one constraint relating to a roll angle of the aircraft;   j) determining a parameter relating to the landing position of the aircraft on the central longitudinal axis of the landing runway;   k) determining a deviation between this parameter and a reference value;   l) comparing this deviation with a predetermined threshold; and   m) as a function of the result of the comparison carried out in step l), commanding if appropriate the signaling of an alert by means of the signaling system of the aircraft.       
 
         [0013]    Thus, by virtue of the signaling of an alert in the cockpit of the aircraft, this method makes it possible to forewarn a crew member, in particular a pilot of the aircraft, of a risk of lateral exit from the runway. It thus makes it possible to improve safety since the pilot can then act accordingly to avoid this risk of runway exit, by correcting the trajectory of the aircraft if this is possible without additional risk or by carrying out a go-around so as to attempt a new approach. 
         [0014]    Advantageously, the constraint relating to the roll angle of the aircraft comprises: 
         [0015]    a first condition according to which the roll angle must be less than or equal, in absolute value, to a maximum roll angle value in a flight of the aircraft along the trajectory; and/or 
         [0016]    a second condition according to which the derivative with respect to the time of the roll angle must be less than or equal, in absolute value, to a maximum roll speed value in a flight of the aircraft along the trajectory 
         [0017]    Again advantageously, the parameter relating to the landing position of the aircraft on the central longitudinal axis of the landing runway corresponds to a distance between the current position of the aircraft and the landing position, this distance being considered by projection on a straight line parallel to the central longitudinal axis of the landing runway. Particularly, the reference value corresponds to a current landing distance determined as a function of the information relating to the current situation of the aircraft. In a particular determination mode, the current landing distance is determined by using the following formula: 
         [0000]    
       
      
       Datt=Vx·Ttd  
      
     
         [0018]    in which: 
         [0019]    Datt is the current landing distance; 
         [0020]    Vx is the current ground speed of the aircraft, considered by projection on a straight line parallel to the central longitudinal axis of the landing runway; 
         [0021]    Ttd is a time interval between a current instant and an instant of landing of the aircraft. 
         [0022]    In one embodiment, the method furthermore comprises the following steps:
       f) determining a maximum roll angle, in absolute value, necessary to allow a flight of the aircraft along a trajectory between a current position of the aircraft and a target landing position of the aircraft on the landing runway;   g) comparing this maximum roll angle with a predetermined roll angle limit; and       
 
         [0025]    in step m), the processing unit commands the signaling of the alert if on the one hand the maximum roll angle is greater, in absolute value, than the predetermined roll angle limit and if on the other hand the deviation determined in step k) is greater than the predetermined threshold. 
         [0026]    In a particular embodiment, steps a), b), i), j), k), l), m) and, if appropriate steps f) and g), are implemented in an automatic manner when a current height of the aircraft is less than a predetermined height threshold, the method furthermore comprising the following steps implemented in an automatic and repetitive manner by the processing unit when the current height of the aircraft is greater than this predetermined height threshold:
       a) acquiring, from the flight management system, information relating to the landing runway;   b) acquiring, from the set of information sources, information relating to a current situation of the aircraft;   c) determining, as a function on the one hand of the information relating to the current situation of the aircraft and on the other hand of the information relating to the landing runway, at least one deviation between on the one hand at least one parameter relating to a landing position of the aircraft on the landing runway and on the other hand a reference relating to a central longitudinal axis of the landing runway;   d) comparing this deviation with a predetermined threshold; and   e) as a function of the result of the comparison carried out in step d), commanding if appropriate the signaling of an alert by means of the signaling system of the aircraft.       
 
         [0032]    The invention also relates to a system for aiding landing of an aircraft on a landing runway, the aircraft comprising a set of information sources, a flight management system and a signaling system. The system for aiding landing is noteworthy in that it comprises a processing unit configured to implement the following steps:
       a) acquiring, from the flight management system, information relating to the landing runway;   b) acquiring, from the set of information sources, information relating to a current situation of the aircraft;   i) determining, as a function on the one hand of the information relating to the current situation of the aircraft and on the other hand of the information relating to the landing runway, a landing position of the aircraft on a central longitudinal axis of the landing runway, this landing position being determined as a function of a trajectory determined between a current position of the aircraft and this landing position so as to comply with at least one constraint relating to a roll angle of the aircraft;   j) determining a parameter relating to the landing position of the aircraft on the central longitudinal axis of the landing runway;   k) determining a deviation between this parameter and a reference value;   l) comparing this deviation with a predetermined threshold; and   m) as a function of the result of the comparison carried out in step l), commanding if appropriate the signaling of an alert by means of the signaling system of the aircraft.       
 
         [0040]    Advantageously, the processing unit is configured in such a way that, in step i), the constraint relating to the roll angle of the aircraft comprises: 
         [0041]    a first condition according to which the roll angle must be less than or equal, in absolute value, to a maximum roll angle value in a flight of the aircraft along the trajectory; and/or 
         [0042]    a second condition according to which the derivative with respect to the time of the roll angle must be less than or equal, in absolute value, to a maximum roll speed value in a flight of the aircraft along the trajectory. 
         [0043]    Again advantageously, the processing unit is configured in such a way that, in step j), the parameter relating to the landing position of the aircraft on the central longitudinal axis of the landing runway corresponds to a distance between the current position of the aircraft and the landing position, this distance being considered by projection on a straight line parallel to the central longitudinal axis of the landing runway. Particularly, the processing unit is configured in such a way that, in step k), the reference value corresponds to a current landing distance determined as a function of the information relating to the current situation of the aircraft. In a particular determination mode, the processing unit is configured so as to determine the current landing distance by using the following formula: 
         [0000]    
       
      
       Datt=Vx·Ttd  
      
     
         [0044]    in which: 
         [0045]    Datt is the current landing distance; 
         [0046]    Vx is the current ground speed of the aircraft, considered by projection on a straight line parallel to the central longitudinal axis of the landing runway; 
         [0047]    Ttd is a time interval between a current instant and an instant of landing of the aircraft. 
         [0048]    In one embodiment, the processing unit is configured so as to implement, furthermore, the following steps:
       f) determining a maximum roll angle, in absolute value, necessary to allow a flight of the aircraft along a trajectory between a current position of the aircraft and a target landing position of the aircraft on the landing runway;   g) comparing this maximum roll angle with a predetermined roll angle limit; and       
 
         [0051]    in step m), the processing unit commands the signaling of the alert if on the one hand the maximum roll angle is greater, in absolute value, than the predetermined roll angle limit and if on the other hand the deviation determined in step k) is greater than the predetermined threshold. 
         [0052]    In a particular embodiment, the processing unit is configured so as to:
       implement in an automatic manner steps a), b), i), j), k), l), m) and, if appropriate steps f) and g), when a current height of the aircraft is less than a predetermined height threshold; and   implement the following steps in an automatic and repetitive manner when the current height of the aircraft is greater than this predetermined height threshold:   a) acquiring, from the flight management system, information relating to the landing runway;   b) acquiring, from the set of information sources, information relating to a current situation of the aircraft;   c) determining, as a function on the one hand of the information relating to the current situation of the aircraft and on the other hand of the information relating to the landing runway, at least one deviation between on the one hand at least one parameter relating to a landing position of the aircraft on the landing runway and on the other hand a reference relating to a central longitudinal axis of the landing runway;   d) comparing this deviation with a predetermined threshold; and   e) as a function of the result of the comparison carried out in step d), commanding if appropriate the signaling of an alert by means of the signaling system of the aircraft.       
 
         [0060]    The invention also relates to an aircraft comprising a system for aiding landing such as aforementioned. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0061]    The invention will be better understood on reading the description which follows and on examining the appended figures. 
           [0062]      FIGS. 1 and 2 , already described, illustrate viewed from above an exemplary approach to a landing runway by an aircraft. 
           [0063]      FIG. 3 , already described, corresponds to a side view of the approach represented in  FIGS. 1 and 2 . 
           [0064]      FIG. 4  illustrates a system for aiding landing in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0065]    The aircraft  1  comprises a system for aiding landing  20  such as represented in  FIG. 4 . The system  20  comprises a processing unit  24  (labeled “PROC” in the figure). This processing unit can, in particular, form part of a computer of flight commands of the aircraft, for example a guidance computer of FG (“Flight Guidance”) type, a control computer of FCS (“Flight Control System”) type, etc. The processing unit  24  is connected at input to a set of information sources  22  of the aircraft, comprising, for example, a monitoring unit  22   a , in particular of MMR (“Multi Mode Receiver”) type, and an inertial platform  22   b  of IRS (“Inertial Reference System”) type. The processing unit  24  is connected to the monitoring unit  22   a  by a link  21   a  and to the inertial platform  22   b  by a link  21   b . The processing unit  24  is also connected at an input to a flight management system  32  of the aircraft, for example a computer of FMS (“Flight Management System”) type, by a link  31 . The processing unit  24  is connected at an output, by a link  25 , to a signaling system  30  of the aircraft, for example a system of CDS (“Control and Display System”) type comprising a display unit  28  (labeled DU in the figure for “Display Unit”) and/or a sound signaling device in the cockpit, such as a loudspeaker  29 . 
         [0066]    During operation, in a first embodiment, the processing unit  24  acquires, from the flight management system, information relating to the landing runway. It also acquires, from the set of information sources  22 , information relating to a current situation of the aircraft. This information relating to the current situation of the aircraft corresponds to at least one subset of the following information: the position, the speed, the acceleration and the orientation (angle of roll, angle of attitude, angle of sideslip, etc.) of the aircraft. As a function of the information relating to the current situation of the aircraft and of the information relating to the landing runway, the processing unit  24  determines at least one deviation between on the one hand at least one parameter relating to a landing position of the aircraft on the landing runway and on the other hand a reference relating to a central longitudinal axis of the landing runway. The processing unit  24  compares this deviation with a predetermined threshold and, as a function of the result of the comparison of this deviation with this predetermined threshold, it commands if appropriate the signaling of an alert by means of the signaling system  30  of the aircraft. 
         [0067]    More precisely, in a first variant of the first embodiment, the at least one parameter corresponds to the landing position Td of the aircraft on the landing runway and the at least one deviation corresponds to a deviation Dtd between on the one hand the landing position Td of the aircraft on the landing runway and on the other hand the central longitudinal axis  4  of the landing runway  5 , as represented in  FIG. 2 . The processing unit  24  compares this deviation Dtd with a predetermined distance threshold and it commands the signaling of the alert if this deviation is greater, in absolute value, than the predetermined distance threshold. The predetermined distance threshold can be chosen as a function of the width of the landing runway and of characteristics of the aircraft, in particular its dimensions and the positions of the various landing gear. This predetermined distance threshold can for example be chosen in an interval of 7 meters to 15 meters. The value of the predetermined distance threshold can be refined experimentally so as to obtain robust detection of the cases justifying an alert, without producing too many false alerts. To command the signaling of the alert, the processing unit  24  dispatches a signal, through the link  25 , to the signaling system  30  which then triggers a display of the alert on the display unit  28  and/or a sound signal in the cockpit by means of the loudspeaker  29 . The deviation Dtd can for example be computed using the following equation: 
         [0000]        Dtd=D+V   y   ·Ttd+ 0.5· A   y   ·Ttd   2  
 
         [0068]    in which: 
         [0069]    D is a current deviation, in projection in a horizontal plane, between a current position of the aircraft and a straight line passing through the central longitudinal axis  4  of the landing runway. Several procedures are possible for determining the deviation D:
       according to a first procedure, the current position of the aircraft is measured by at least one of the information sources  22 , for example by a GPS (“Global Positioning System”) receiver of the unit MMR  22   a  or by the inertial platform  22   b . The current position can also correspond to a hybridization of the measurements carried out by the GPS receiver and by the inertial platform. The position of the central longitudinal axis  4  of the landing runway forms part of the information relating to the landing runway acquired from the flight management system  32  by the processing unit  24 . As a function of the current position of the aircraft and of the position of the central longitudinal axis  4  of the landing runway, the processing unit  24  computes the deviation D such as defined above;   according to a second procedure, the set of information sources  22  comprises a receiver of ILS (“Instrument Landing System”) type. Thus, in the case of an instrument-based approach of ILS type, this receiver receives signals corresponding in particular to a beam LOC emitted by an emitter situated on the ground. On the basis of the signals, the receiver of ILS type (or the processing unit  24 ) can estimate, in a known manner, the deviation between the current position of the aircraft and the approach axis  10 . This deviation corresponds to the deviation D;   according to a third procedure, the set of information sources  22  comprises a camera embedded aboard the aircraft and positioned so as to film ahead of the aircraft. This camera therefore makes it possible to film a zone comprising the landing runway. A system for analyzing images is coupled to this camera: it makes it possible to identify the central longitudinal axis of the landing runway by using known techniques, for example a Hough transform. A current height Hgnd of the aircraft relative to a threshold of the landing runway also being known aboard the aircraft (by virtue of the set of information sources  22 ), the processing unit  24  can determine a deviation between the current position of the aircraft and the central longitudinal axis of the landing runway, therefore the deviation D;       
 
         [0073]    These various procedures may optionally be combined to consolidate the estimation of the deviation D; 
         [0074]    Vy is a current lateral speed of the aircraft relative to the ground. It corresponds to the projection of the current speed of the aircraft relative to the ground, on a horizontal axis perpendicular to the central longitudinal axis  4  of the landing runway; 
         [0075]    Ay is a current lateral acceleration of the aircraft relative to the ground. According to a first procedure, it can be obtained by projecting the current acceleration of the aircraft relative to the ground, on the horizontal axis perpendicular to the central longitudinal axis  4  of the landing runway. According to a second procedure, it can be estimated using, for example, the following equation: 
         [0000]        A   y   =g ·tan φ·cos θ
 
         [0076]    in which: 
         [0077]    g is the acceleration due to gravity; 
         [0078]    φ is the current roll angle of the aircraft; and 
         [0079]    θ is the current pitch angle of the aircraft. 
         [0080]    The values of the angles φ and θ are provided, in a usual manner, by the inertial platform  22   b ; and 
         [0081]    Ttd is a time interval between a current instant and an estimated instant of touchdown of the wheels of the aircraft on the landing runway, at the landing position Td. The time interval Ttd can in particular be estimated using the following equation: 
         [0000]    
       
         
           
             Ttd 
             = 
             
               
                 
                   H 
                   gnd 
                 
                 
                   - 
                   
                     VS 
                     gnd 
                   
                 
               
               + 
               
                 T 
                 flare 
               
             
           
         
       
     
         [0082]    in which: 
         [0083]    Hgnd is a current height of the aircraft relative to a threshold of the landing runway, known aboard the aircraft by virtue of the set of information sources  22 ; 
         [0084]    VSgnd is a current vertical speed of the aircraft relative to the ground. It is provided by the set of information sources  22 , in particular by a GPS receiver of the unit MMR  22   a  or by the inertial platform  22   b  (or else by hybridization of values provided by the GPS receiver and by the inertial platform). In the equation, this speed is defined positive upwards. 
         [0085]    Tflare is an optional corrective term making it possible to take account of the additional time taken by the aircraft to carry out a flare maneuver on landing. When this term is used, its value can be fixed (for example 5 seconds) or else result from estimations dependent on the aircraft, its current flight conditions (mass, aerodynamic configuration, etc.). 
         [0086]    In a second variant of the first embodiment, the information relating to the current situation of the aircraft comprises a current roll angle φ of the aircraft. As in the first variant, the at least one parameter corresponds to the landing position Td of the aircraft on the landing runway and the at least one deviation corresponds to a deviation Dtd between on the one hand the landing position Td of the aircraft on the landing runway and on the other hand the central longitudinal axis  4  of the landing runway  5 . The processing unit  24  compares this deviation Dtd with a predetermined distance threshold, as in the first variant. Furthermore, the processing unit  24  compares the current roll angle φ with a predetermined roll angle threshold. This predetermined roll angle threshold can for example be chosen in an interval lying between 3 degrees and 5 degrees. The value of the predetermined roll angle threshold can be refined experimentally so as to obtain robust detection of the cases justifying an alert, without producing too many false alerts. The processing unit  24  commands the signaling of the alert if the following conditions all hold:
       the deviation Dtd is greater, in absolute value, than the predetermined distance threshold;   the current roll angle φ is greater, in absolute value, than the predetermined roll angle threshold; and   the current roll angle tends to deviate the landing position Td of the aircraft on the landing runway from the central longitudinal axis  4  of the landing runway.       
 
         [0090]    In an advantageous manner, the third condition hereinabove corresponds to the following logic equation: 
         [0000]      (( Dtd&gt; 0)AND(φ&lt;0))OR(( Dtd&lt; 0)AND(φ&gt;0))
 
         [0091]    with:
       Dtd&gt;0 when Td is situated to the left of the central longitudinal axis of the landing runway (seen from the aircraft) and Dtd&lt;0 when Td is situated to the right of the axis;   φ&gt;0 when the aircraft banks to the right (that is to say when the right wing of the aircraft is lower than the left wing) and φ&lt;0 when the aircraft banks to the left;   AND corresponds to the logical AND operator; and   OR corresponds to the logical OR operator.       
 
         [0096]    In a third variant of the first embodiment, the at least one parameter corresponds to a speed vector Vtd of the aircraft at its landing position Td on the landing runway and the at least one deviation corresponds to an angular deviation Atd between on the one hand this speed vector Vtd and on the other hand a direction  4   a  of the central longitudinal axis  4  of the landing runway. As represented in  FIG. 2 , the direction  4   a  corresponds to a straight line parallel to the axis  4  and passing through the landing position Td. The processing unit  24  compares this angular deviation Atd with a predetermined angular threshold and it commands the signaling of the alert if this deviation is greater, in absolute value, than this predetermined angular threshold. The value of the predetermined angular threshold can be chosen to be included in an interval between 5 degrees and 15 degrees, for example equal to about 10 degrees. This value can be refined experimentally so as to obtain robust detection of the cases justifying an alert, without producing too many false alerts. In an advantageous manner, the processing unit  24  determines the angular deviation Atd by means of the following equation: 
         [0000]        Atd =Track td −ψ track  
 
         [0097]    in which: 
         [0098]    ψtrack is the direction (expressed in degrees) of the central longitudinal axis of the landing runway relative to North; 
         [0099]    Tracktd is the estimated direction, relative to North, of the speed vector Vtd of the aircraft at the landing position Td. This direction can be estimated by means of the following equation: 
         [0000]    
       
         
           
             
               Track 
               td 
             
             = 
             
               
                 [ 
                 
                   Track 
                   + 
                   
                     
                       
                         
                           Ttd 
                           · 
                           g 
                           · 
                           tan 
                         
                          
                         
                             
                         
                          
                         ϕ 
                       
                       
                         V 
                         gnd 
                       
                     
                     · 
                     
                       180 
                       π 
                     
                   
                 
                 ] 
               
               360 
             
           
         
       
     
         [0100]    in which: 
         [0101]    Track is the direction, relative to North, of the current ground speed vector of the aircraft; 
         [0102]    Ttd is the aforementioned time interval between the current instant and an estimated instant of touchdown of the wheels of the aircraft on the landing runway, at the landing position Td; 
         [0103]    g is the acceleration due to gravity; 
         [0104]    φ is the current roll angle of the aircraft; 
         [0105]    Vgnd is the current ground speed of the aircraft; and 
         [0106]    [ ] 360  symbolizes the operator modulo 360 degrees. 
         [0107]    The third variant of the first embodiment can be combined with one or the other of the first or second variants. According to a first alternative, this combination can be carried out by means of a logical OR of the conditions of activation of the alerts corresponding to the combined variants, thereby allowing greater sensitivity in the detection of cases of alert. According to a second alternative, this combination can be carried out by means of a logical AND of the conditions of activation of the alerts corresponding to the combined variants, thereby allowing greater immunity to false alerts. 
         [0108]    In a second embodiment, the processing unit  24  acquires, from the flight management system  32 , information relating to the landing runway. It also acquires, from the set of information sources  22 , information relating to a current situation of the aircraft. This information relating to the current situation of the aircraft corresponds to at least one subset of the following information: the position, the speed, the acceleration and the orientation (angle of roll, angle of attitude, etc.) of the aircraft. The processing unit  24  determines a maximum roll angle, in absolute value, necessary to allow a flight of the aircraft along a trajectory between a current position of the aircraft and a target landing position of the aircraft on the landing runway. The processing unit  24  compares this maximum roll angle with a predetermined roll angle limit and, as a function of the result of the comparison of the maximum roll angle with the predetermined roll angle limit, it commands if appropriate the signaling of an alert by means of the signaling system  30  of the aircraft. 
         [0109]    In an advantageous manner, the target landing position of the aircraft on the landing runway is situated on the central longitudinal axis  4  of the landing runway. This makes it possible to avoid a lateral deviation of the landing position of the aircraft on the landing runway, with respect to the central longitudinal axis. 
         [0110]    The processing unit  24  determines a trajectory able to be controlled by a pilot of the aircraft so as to rejoin the target landing position from the current position of the aircraft. The maximum roll angle corresponds to the largest roll angle (in absolute value) necessary for a flight of the aircraft  1  along the trajectory. 
         [0111]    The predetermined roll angle limit is chosen so as to correspond to a maximum roll angle that a pilot of the aircraft can control at the current height of the aircraft relative to the ground, without taking any risk of collision of a part of the aircraft (in particular a wing tip) with the terrain. This limit can correspond to a fixed value or else depend on other parameters, such as, for example, the height of the aircraft relative to the terrain or to the threshold of the landing runway, or else the current ground speed of the aircraft. For example, for a height of the aircraft relative to the threshold of the landing runway of less than 100 feet (about 30 meters), the predetermined roll angle limit can be chosen substantially equal to 3 degrees. 
         [0112]    When the result of the comparison of the maximum roll angle with the predetermined roll angle limit is such that the maximum roll angle is greater, in absolute value, than the predetermined roll angle limit, the processing unit  24  dispatches a signal, through the link  25 , to the signaling system  30  which then triggers a display of the alert on the display unit  28  and/or a sound signal in the cockpit by means of the loudspeaker  29 . 
         [0113]    In a still advantageous manner, the processing unit  24  computes the maximum roll angle φTmax by means of the following equation: 
         [0000]    
       
         
           
             
               ϕ 
                
               
                   
               
                
               T 
                
               
                   
               
                
               max 
             
             = 
             
               Arctan 
                
               
                 ( 
                 
                   
                     - 
                     
                       ( 
                       
                         D 
                         + 
                         
                           
                             V 
                             y 
                           
                           · 
                           
                             T 
                             td 
                           
                         
                       
                       ) 
                     
                   
                   
                     
                       0.5 
                       · 
                       
                         T 
                         td 
                         2 
                       
                       · 
                       g 
                       · 
                       cos 
                     
                      
                     
                         
                     
                      
                     θ 
                   
                 
                 ) 
               
             
           
         
       
     
         [0114]    in which the parameters D, Vy, Ttd, g and θ are such as defined with reference to the first embodiment. 
         [0115]    This value of the maximum roll angle corresponds to a case in which the trajectory between the current position of the aircraft and the target landing position is such that it makes it possible to cancel the lateral deviation D after a time Ttd with respect to the current instant, therefore when landing at the target landing position. This trajectory is substantially parabolic between the current position of the aircraft and the target landing position and the roll angle necessary to fly this trajectory is substantially constant and equal to φTmax along the trajectory. 
         [0116]    In a third embodiment, the processing unit  24  acquires, from the flight management system  32 , information relating to the landing runway. It also acquires, from the set of information sources  22 , information relating to a current situation of the aircraft. This information relating to the current situation of the aircraft corresponds to at least one subset of the following information: the position, the speed, the acceleration and the orientation (angle of roll, angle of attitude, etc.) of the aircraft. As a function of the information relating to the current situation of the aircraft and of the information relating to the landing runway, the processing unit  24  determines a landing position of the aircraft on the central longitudinal axis  4  of the landing runway, this landing position being determined as a function of a trajectory determined between a current position of the aircraft and this landing position so as to comply with at least one constraint relating to a roll angle of the aircraft. The processing unit  24  thereafter determines a parameter relating to the landing position of the aircraft on the central longitudinal axis of the landing runway as well as a deviation between this parameter and a reference value. The processing unit  24  compares this deviation with a predetermined threshold, and then, as a function of the result of the comparison carried out between this deviation and this predetermined threshold, it commands, if appropriate, the signaling of an alert by means of the signaling system  30  of the aircraft. 
         [0117]    In an advantageous manner, the constraint relating to the roll angle of the aircraft comprises:
       a first condition according to which the roll angle must be less than or equal (in absolute value) to a maximum value of roll angle during a flight of the aircraft along the trajectory; and/or   a second condition according to which the derivative with respect to time of the roll angle must be less than or equal (in absolute value) to a maximum value of roll rate during a flight of the aircraft along the trajectory.       
 
         [0120]    For the first condition, the maximum value of roll angle is chosen so as to correspond to a roll angle that a pilot of the aircraft can control at the current height of the aircraft relative to the ground, without taking any risk of collision of a part of the aircraft (in particular a wing tip) with the terrain. This maximum value can correspond to a fixed value or else depend on other parameters, such as, for example, the height of the aircraft relative to the terrain or to the threshold of the landing runway, or else the current ground speed of the aircraft. This value can for example be chosen substantially equal to 3 degrees when the height of the aircraft relative to the threshold of the landing runway is less than 100 feet (about 30 meters). For the second condition, the maximum value of roll rate is chosen so as to correspond to a roll rate that a pilot of the aircraft can control at the current height of the aircraft relative to the ground, without taking any risk of collision of a part of the aircraft (in particular a wing tip) with the terrain and/or without fearing to take such a risk. This maximum value can correspond to a fixed value or else depend on other parameters, such as for example the height of the aircraft relative to the terrain or to the threshold of the landing runway, or else the current ground speed of the aircraft. This value can for example be chosen substantially equal to 2 degrees per second when the height of the aircraft relative to the threshold of the landing runway is less than 100 feet (about 30 meters). 
         [0121]    According to one possibility, the parameter relating to the landing position of the aircraft on the central longitudinal axis of the landing runway corresponds to a distance between the current position of the aircraft and the landing position, this distance being considered in projection on a straight line parallel to the central longitudinal axis of the landing runway. In a particular manner, the reference value corresponds to a current landing distance determined as a function of the information relating to the current situation of the aircraft. The term current landing distance designates here a landing distance estimated at the current instant as a function of the information relating to the current situation of the aircraft. This current landing distance is for example determined using the following formula: 
         [0000]    
       
      
       D 
       ldg 
       =V 
       x 
       ·T 
       td  
      
     
         [0122]    in which: 
         [0123]    Dldg is the current landing distance; 
         [0124]    Vx is the current speed of the aircraft relative to the ground, considered in projection on a straight line parallel to the central longitudinal axis of the landing runway; 
         [0125]    Ttd is a time interval between a current instant and an instant of landing of the aircraft. This time interval can be computed as indicated previously with reference to the first embodiment. 
         [0126]    The processing unit  24  determines a deviation between, on the one hand, the distance between the current position of the aircraft and the landing position and, on the other hand, the current landing distance Dldg. The processing unit  24  compares this deviation with a predetermined threshold. If this deviation is greater than the predetermined threshold, the processing unit  24  commands the signaling of an alert by means of the signaling system  30  of the aircraft. The value of the predetermined threshold can be chosen equal to a distance margin of a few tens or hundreds of meters, for example about 200 meters, in such a way that the processing unit  24  commands the signaling of the alert when the distance between the current position of the aircraft and the landing position is greater than the current landing distance Dldg plus the distance margin. 
         [0127]    To determine the distance between the current position of the aircraft and the landing position, the processing unit  24  firstly determines the trajectory between the current position of the aircraft and the landing position. Accordingly, at the current position of the aircraft, the speed vector of the aircraft along the trajectory is considered to be aligned with the current course of the aircraft. The landing position is considered on the central longitudinal axis  4  of the landing runway (without, however, this landing position being frozen longitudinally on this axis), the speed vector of the aircraft then being aligned with the central longitudinal axis. The processing unit  24  determines the trajectory using a known procedure for trajectory generation, while complying with the constraint relating to the roll angle of the aircraft, corresponding for example to a maximum value of roll angle of 3 degrees (in absolute value) and to a maximum roll rate of 2 degrees per second (in absolute value) as mentioned previously. Among the known procedures for trajectory generation, it is for example possible to use Dubins curves (succession of clothoids, of circular arcs, of straight lines, etc.), splines (for example Bezier curves), etc. These procedures can be implemented analytically (by considering a predetermined string of trajectory elements such as circular arcs, straight lines, etc.) or numerically (by searching for curves that optimize the curvature and the derivative of the curvature). Once the trajectory has been determined by the processing unit  24 , the landing position is likewise determined from a longitudinal point of view on the central longitudinal axis  4  of the landing runway. The processing unit  24  then computes the distance between the current position of the aircraft and the landing position, in projection on a straight line parallel to the central longitudinal axis  4  of the landing runway. 
         [0128]    Alternatively, the distance between the current position of the aircraft and the landing position can be computed by using a table comprising as inputs values of lateral deviation of the aircraft with respect to the approach axis, values of angular deviation between the current speed vector of the aircraft and the approach axis and values of height of the aircraft, this table providing as output values of distance between the current position of the aircraft and the landing position on the central longitudinal axis of the landing runway. This table can for example be utilized as is by the processing unit  24  (optionally with the aid of interpolation techniques), or else be utilized in the form of a neural network. 
         [0129]    The third embodiment can be combined with the second embodiment. According to a first alternative, this combination can be carried out by means of a logical OR of the conditions of activation of the alerts corresponding to these two embodiments, thereby allowing greater sensitivity in the detection of cases of alert. According to a second alternative, this combination can be carried out by means of a logical AND of the conditions of activation of the alerts corresponding to these two embodiments, thereby allowing greater immunity to false alerts. 
         [0130]    In a first particular embodiment, the first embodiment (whichever variant is considered) is implemented when the current height of the aircraft relative to a threshold of the landing runway is greater than a first predetermined height threshold A 1 , for example 100 feet (about 30 meters), as represented in  FIG. 3 . The alert, the signaling of which is optionally commanded by the system for aiding landing, then corresponds to an alert of “warning” type making it possible to signal to the pilot of the aircraft that he must correct the current situation of the aircraft in order to avoid exiting the runway laterally. The height of the aircraft relative to the terrain is then considered sufficient for the pilot to be able to attempt a maneuver to correct the trajectory of the aircraft so as to avoid exiting the runway laterally, without taking any risk in respect of the safety of the aircraft. Even if he does not succeed in correcting the trajectory of the aircraft sufficiently, the pilot is thus informed in advance of a risk of exiting the runway laterally, thereby allowing him to prepare himself to effect a go-around in order to attempt a new approach. In an advantageous, although optional, manner, the first embodiment is implemented only from the point in time when the current height of the aircraft relative to the threshold of the landing runway is less than a second predetermined height threshold A 2 , for example 200 feet (about 60 meters), as represented in  FIG. 3 . This makes it possible for the alert of warning type to be signaled only when the aircraft is sufficiently close to the landing runway so as to avoid untimely alerts in the cockpit. 
         [0131]    In a second particular embodiment, that can optionally be combined with the first particular embodiment, the second embodiment and/or the third embodiment (optionally combined with one another as indicated previously) are implemented when the current height of the aircraft relative to the threshold of the landing runway is less than the first predetermined height threshold A 1 , for example 100 feet (about 30 meters). The alert, the signaling of which is optionally commanded by the system for aiding landing, then corresponds to an alert of “caution” type making it possible to signal to the pilot of the aircraft that he can no longer correct the trajectory of the aircraft in order to avoid exiting the runway laterally, without taking some other risk, and that he must then carry out a go-around in order to attempt a new approach. Indeed, the height of the aircraft relative to the terrain is then no longer sufficient to carry out a maneuver to correct the trajectory of the aircraft without the risk either of touching the ground with a wing tip if the trajectory correction requires too high a roll angle, or of landing at a landing position that is too close to the end of the runway if the trajectory correction is carried out with a lower roll angle, or else of bringing about a situation in which the pilot would not be able to control the dynamics of the aircraft to stabilize it while landing at the landing position. 
         [0132]    In another particular embodiment, the first embodiment can also be implemented when the current height of the aircraft relative to the threshold of the landing runway is less than the first predetermined height threshold A 1 . In such a case, the different variants can be used alone or combined with one another as indicated above. The second variant exhibits a greater interest than the other two insofar as it takes account of the current roll angle of the aircraft. These different variants can also be combined with the second embodiment and/or with the third embodiment so as to improve the detection of alerts. When the first embodiment is implemented while the current height of the aircraft relative to the threshold of the landing runway is less than the first predetermined height threshold A 1 , in an advantageous manner the processing unit adapts accordingly the predetermined threshold with which the deviation between, on the one hand, the parameter relating to the landing position of the aircraft on the landing runway and, on the other hand, the reference relating to the central longitudinal axis of the landing runway is compared. More particularly, the processing unit reduces the value of the predetermined threshold when the current height of the aircraft relative to the threshold of the landing runway is less than the first predetermined height threshold A 1 , so as to take account of the proximity of the ground. 
         [0133]    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.