Abstract:
An aircraft terrain avoidance method and device may employ a collision warning section that transmits a caution signal when the aircraft risks colliding with the terrain at the end of a first predetermined period of time. A warning signal is transmitted when the aircraft risks colliding with the terrain at the end of a second predetermined period of time, which is shorter than the first predetermined period of time. A piloting system causes the aircraft to automatically climb with a first gradient corresponding to a predetermined value upon the transmission of the caution signal. An automatic pilot is engaged automatically, if currently disengaged, to force the aircraft to climb with a second gradient, corresponding to the maximum gradient possible in the current flight conditions of the aircraft, upon the transmission of the warning signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a terrain avoidance method and device for an aircraft. 
     BACKGROUND OF THE RELATED ART 
     It is known that many aircraft, in particular civil transport aircraft, are equipped with a collision warning device making it possible to transmit a warning signal in the case of risk of collision of the aircraft with the terrain. With regard to the collision warning device, it can be in particular a device of the TAWS (“Terrain Awareness and Warning System”) type, of the EGPWS (“Enhanced Ground Proximity Warning System”) type or of the GCAS (“Ground Collision Avoidance System”) type. 
     Such a collision warning device is generally capable of generating two warning signals relating to different warning levels, namely a caution and a warning. Generally, a caution is transmitted about 60 seconds before a potential collision of the aircraft with the terrain, when the risk of such a collision is detected. If this collision risk is still beyond doubt about 30 seconds before the predicted impact of the aircraft with the terrain, the said warning device transmits a warning. In the case of a warning, the pilot must of course react immediately. On the other hand, in the case of a caution, the pilot or pilots are supposed to verify the reality of the potential risk of collision with the terrain and to modify his flight path, if this risk is confirmed, in such a way as to avoid such a collision, However, it can happen that the pilots do not react satisfactorily to the warning signals given by a collision warning device, at the risk of causing the destruction of the aircraft at the time of a collision with the terrain. 
     The patent U.S. Pat. No. 4,924,401 proposes a solution aimed at avoiding a collision of the aircraft with the terrain. This solution consists in defining a minimum altitude below which the aircraft must not descend and in piloting the aircraft automatically by means of the automatic pilot, when this minimum altitude is passed through whilst descending, in such a way as to command the aircraft to climb again and thus to prevent any collision with the terrain. 
     However, this known solution is above all adapted to the case where the pilot is unaware even though the aircraft is diving. Because of this, it has the disadvantage of acting very late on the flight path of the aircraft, and the action on this flight path is of course even greater because it is late. Consequently, in the case of a civil transport aircraft, this results in an uncomfortable situation and even a potential danger for the passengers. 
     Furthermore, the risk that this action on the flight path will not make it possible to protect the aircraft from a collision with the terrain is also high because of this late action. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to overcome these disadvantages. It relates to a terrain avoidance method, for an aircraft, that is particularly efficient. 
     For this purpose, the said method according to which there is used a collision warning device capable of transmitting:
     a caution, when the aircraft risks having a collision with the terrain at the end of a first predetermined period of time if it maintains its current flight characteristics (gradient, speed, etc.); and   a warning, when the aircraft risks having a collision with the terrain at the end of a second predetermined period of time if it maintains its current flight characteristics, the said second predetermined period of time being shorter than the said first predetermined period of time,   is noteworthy, according to the invention, in that:   a) if the said collision warning device transmits a caution, the aircraft is automatically subjected to at least one climb command with a first gradient corresponding to a predetermined value; and   b) if the said collision warning device transmits a warning, an automatic pilot of the aircraft, which is engaged automatically if it is disengaged at the time of the transmission of the said warning, automatically and exclusively forces the aircraft to climb with a second gradient corresponding to the maximum gradient possible in the current flight conditions (mass, altitude, etc.) of the aircraft.   

     The method according to the invention has the advantage of acting early on the flight path of the aircraft in such a way as to avoid a collision of the latter with the terrain. In fact, this method makes it possible to place the aircraft on a positive gradient (climb) as soon as a caution is transmitted even if the pilot does not react to this caution. Thus, as soon as the caution is transmitted, action is taken in such a way as to improve the situation of the aircraft with respect to the terrain by increasing its altitude (or by reducing the lowering of its altitude). 
     Furthermore, this action corresponds to a first gradient which is preferably moderate, generally of the order of a few degrees (for example 2°), the effect of which does not therefore reduce the comfort of the passengers. 
     Moreover, if the pilot does not react to a caution and/or if the climb of the aircraft initiated due to the method according to the invention following this caution is not sufficient to avoid any risk of collision with the terrain, the collision warning device transmits a warning (for example about 30 seconds before the predicted impact of the aircraft with the terrain). In a preferred embodiment, the end of a third predetermined period of time (for example 5 seconds) is awaited in order to give the pilot the possibility of carrying out an action in that time intended to avoid a collision with the terrain. If at the end of this third predetermined period of time the pilot has not taken any action aimed at avoiding the collision (or if he has not switched off the warning generated by the said collision warning device in the case in which this warning appears incorrect to him), the automatic pilot automatically initiates a climb of the aircraft at maximum gradient. 
     Furthermore and advantageously, in the case where the pilot carries out an action on a control device of the aircraft, in step a), the said climb command is added to the command which is generated by the said action of the pilot on the said control device. 
     In this way, the aircraft can be maintained in descent (negative gradient) only in the case of a voluntary and maintained action of the pilot on the control device (for example a control column). In the case of caution, the aircraft therefore has a tendency to climb, unless there is a voluntary action by the pilot in order to make it descend. Furthermore, the fact that the aircraft climbs without voluntary action by the pilot has the advantage of contributing to the pilot becoming aware of the risk of collision with the terrain (since the aircraft does not normally climb without a voluntary action by the pilot). 
     Furthermore and advantageously, in the case where the aircraft is climbing, in particular in the absence of any action by the pilot on a control device of the said aircraft, in step a), the said climb command is added to the gradient corresponding to that climb. 
     According to the invention:
     when the automatic pilot is engaged at the time of the transmission of a caution, the said automatic pilot commands (by means of a first integrated function) a gradient corresponding to the said predetermined value;   when the automatic pilot is disengaged at the time of the transmission of a caution, if the aircraft comprises electrical flight controls, the gradient of the aircraft is maintained at the said predetermined value, in the absence of an action by the pilot, by a gradient calculator managing the piloting laws of the said electrical flight controls; and   in the case of an action by the pilot on a control device of the aircraft, the gradient command or instruction corresponding to that action of the pilot is added to the said predetermined value by the calculator (managing the commands generated by the actuation of the control device).   

     It will be noted that there is a continuity of piloting modes according to whether the automatic pilot is engaged or disengaged. This allows a consistency in the behavior of the aircraft no matter what piloting mode is chosen by the pilot. 
     Furthermore and advantageously, in step b), the automatic pilot forces the aircraft into a climb with a thrust generated by engines of the aircraft, corresponding to the maximum thrust possible in the current flight conditions of the aircraft. 
     It will be noted that the method according to the invention allows a progressive reaction according to the level of the alarm: a first moderate reaction in the case of a caution and then a sharp reaction in the case of a warning. This makes it possible to anticipate a climb as early as possible, without notably affecting the comfort of the passengers, and then to initiate a sudden climb when the latter becomes essential in order to save the aircraft from a risk of collision. 
     Furthermore and advantageously, in step b), the automatic pilot forces the aircraft to climb whilst maintaining the angle of incidence of the aircraft below a maximum authorized value, in relation to protection with respect to stalling. 
     The present invention also relates to a terrain avoidance device for an aircraft. 
     According to the invention, the said device of the type comprising a collision warning device capable of transmitting:
     a caution, when the aircraft risks having a collision with the terrain at the end of a first predetermined period of time if it maintains its current flight characteristics; and   a warning, when the aircraft risks having a collision with the terrain at the end of a second predetermined period of time if it maintains its current flight characteristics, the said second predetermined period of time being shorter than the said first predetermined period of time,   is noteworthy in that it furthermore comprises a piloting system of the aircraft, comprising at least:   a means of piloting making it possible to subject the aircraft automatically to at least one climb command with a first gradient corresponding to a predetermined value, when the said collision warning device transmits a caution; and   an automatic pilot which is engaged automatically, if it is disengaged at the time of transmission of a warning by the said collision warning device, and which forces the aircraft, automatically and exclusively, into a climb with a second gradient corresponding to the maximum gradient possible in the current flight conditions of the aircraft, at the time of the transmission of such a warning by the said collision warning device.   

     Advantageously:
     the said means of piloting comprises a first function which is integrated in the automatic pilot and the said first function subjects the aircraft to the said climb command, if the said automatic pilot is engaged at the time of the transmission of a caution; and/or   the said piloting system furthermore comprises at least one control device which is able to be actuated by a pilot of the aircraft, the said piloting means comprises a second function which is integrated in a calculator managing the commands generated by the actuation of the said control device, and the said second function subjects the aircraft to the said climb command if the said automatic pilot is disengaged at the time of the transmission of a caution.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures of the appended drawing will give a good understanding of how the invention may be embodied. In these figures, identical references denote similar elements. 
         FIG. 1  is the block diagram of a terrain avoidance device according to the invention. 
         FIG. 2  is a graphical representation making it possible to describe the essential steps of a method according to the invention. 
         FIGS. 3 and 4  are two diagrammatic views showing two different flight situation of an aircraft, to which the method according to the invention is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The device  1  according to the invention and shown diagrammatically in  FIG. 1  is a terrain (or ground) S avoidance device for an aircraft A, in particular a civil transport aircraft. 
     According to the invention, the said device  1  comprises:
     a collision warning device  2  of the usual type, which is capable of transmitting:
       a caution, when the aircraft A risks having a collision with the terrain S at the end of a first predetermined period of time, for example 60 seconds, starting from the transmission of the caution, if it maintains its current flight characteristics (gradient, speed, etc.); and   a warning, when the aircraft A risks having a collision with the terrain S at the end of a second predetermined period of time, for example 30 seconds starting from the transmission of the warning, if it maintains its current flight characteristics; and   
       a piloting system  3  of the aircraft A, which is connected by a link  2 A to the said collision warning device  2 .   

     With regard to the collision warning device  2 , it can be in particular a device of the TAWS (“Terrain Awareness and Warning System”) type, of the EGPWS (“Enhanced Ground Proximity Warning System”) type or of the GCAS (“Ground Collision Avoidance System”) type. 
     Furthermore, according to the invention, the said piloting system  3  of the aircraft A comprises:
     a means of piloting  4  making it possible to subject the aircraft A automatically to at least one climb command with a first moderate gradient corresponding to a predetermined value, for example 2°, when the said collision warning device transmits a caution; and   an automatic pilot  5  which is engaged automatically, if it is disengaged at the time of transmission of a warning by the said collision warning device  2 , and which forces the aircraft A, automatically and exclusively, into a climb with a second gradient corresponding to the maximum gradient possible in the current flight conditions (mass, altitude, etc.) of the aircraft A, at the time of the transmission of such a warning.   

     As shown in  FIG. 1 , the piloting system  3  furthermore comprises:
     at least one control device  6 , for example a control stick or a mini-control stick, which is able to be actuated by a pilot of the aircraft A, at least for commanding a modification of the gradient of the aircraft A;   a calculator  7  which is connected by a link  8  to the control device  6  and which is intended to manage the commands generated by the actuation of the control device  6 . If the piloting system  3  is of the electrical or optical flight controls type, the said calculator  7  is a normal calculator implementing the piloting laws of the said piloting system  3 ; and   a switching means  9  which is connected by links  10  and  11  respectively to the said calculator  7  and to the said automatic pilot  5  respectively and which, depending on the switching position P 1  or P 2 , transmits, by the intermediary of a link  12 , to normal control surfaces, that are not shown, of the aircraft A, as deflection commands for these control surfaces, either commands coming from the calculator  7 , or commands coming from the automatic pilot  5 .   

     According to the invention, the said piloting means comprise:
     a first function  13  which is integrated in the automatic pilot  5 . The said first function  13  subjects the aircraft A to the said climb command, if the said automatic pilot  5  is engaged at the time of the transmission of a caution; and   a second function  14  which is integrated in the calculator  7  managing the commands generated by the actuation of the said control device  6 . The said function  14  subjects the aircraft A to the said climb command, if the said automatic pilot  5  is disengaged at the time of the transmission of a caution.   

       FIG. 2  is a diagrammatic representation of the method according to the invention implemented by the device  1  shown in  FIG. 1 . 
     As mentioned above, when the collision warning device  2  transmits a caution (step E 1 ), two solutions are possible, depending on whether the automatic pilot  5  is engaged (link L 1 ) or disengaged (link L 2 ) at the time of the transmission of that caution. 
     If the automatic pilot  5  is engaged (step E 2 A), the automatic pilot  5  commands a gradient corresponding to the said predetermined value. On the other hand, if the automatic pilot  5  is not engaged (step E 2 B) and if the aircraft A comprises electrical flight controls, the gradient of the aircraft A is maintained on the said predetermined value, in the absence of an action by the pilot (on the control device  6 ), by the function  14  of the said calculator  7  which, in this case, manages the piloting laws of the said electrical flight controls. In this situation, a command (or a gradient instruction) corresponding to an action of the pilot on the control device  6 , is added if necessary to the said predetermined value, by the said calculator  7 . It will be noted that there is a continuity of piloting modes depending on whether the automatic pilot  5  is engaged or disengaged. In fact, the functions  13  and  14  implement the same type of piloting laws. This allows a consistency of behavior of the aircraft A no matter what piloting mode is chosen by the pilot. 
     In  FIG. 2 , there has been shown:
     a link L 3  illustrating a taking over of the control of the aircraft A by the pilot, that is to say a disengagement of the automatic pilot  5 ; and   a link L 4  illustrating an engagement by the pilot of the automatic pilot  5  (initially disengaged).   

     In  FIG. 3 , there has been shown an example illustrating the said steps E 1 , E 2 A and E 2 B. This example relates to an initial climb of the aircraft A according to a flight path T 0 . The aircraft A is shown at the moment of the transmission of the caution (step E 1 ). In fact it has a risk of having a collision with the terrain S (mountain  15 ) if it continues its flight such as it is, as illustrated by a flight path T 1  (in dashed line) extending the current flight path T 0 . At this moment, the device  1  subjects the aircraft A automatically to an additional climb command having a gradient γ1 which is added to the gradient corresponding to the current flight path T 0  such that the aircraft A no longer follows the flight path T 1  extending the current flight path T 0  but follows a flight path T 2  having additional climb. 
     As illustrated by this example, when the pilot applies an action on the control device  6  of the aircraft A (generating the gradient of the flight path T 0 ), the function  14  of the calculator  7  adds the said climb command (gradient γ1) to the command which is generated by the said action of the pilot on the said control device  6 . 
     The device  1  according to the invention has the advantage of acting early on the flight path of the aircraft A in such a way as to avoid a collision of the latter with the terrain S. In fact, the said device  1  makes it possible to place the aircraft A on a positive gradient (climb) as soon as a caution is transmitted, even if the pilot does not react to this caution. Thus, as soon as the caution is transmitted, action is taken in such a way as to improve the situation of the aircraft A with respect to the terrain S by increasing its altitude. 
     Furthermore, this action corresponds to a first gradient γ1 which is preferably moderate, generally of the order of a few degrees (for example 2°), the effect of which does not therefore reduce the comfort of the passengers of the aircraft A. 
     It will be noted that the aircraft A can be maintained in descent (negative gradient) only in the case of a voluntary and maintained action of the pilot on the control device  6 . The device  1  according to the invention is therefore advantageous in comparison with the prior art since, at the time of the transmission of a caution, the aircraft A has a tendency to climb, unless there is a voluntary action by the pilot in order to make it descend. Furthermore, the fact that the aircraft A climbs without voluntary action by the pilot has the advantage of contributing to the pilot becoming aware of the risk of collision with the terrain S (since the aircraft A does not normally climb without a voluntary action by the pilot). 
     If the pilot does not react to a caution situation and/or if the climb of the aircraft A corresponding to the aforesaid step E 2 A, E 2 B is not sufficient to eliminate the risk of collision with the terrain S (as shown in  FIG. 4 ), the collision warning device  2  transmits a warning (step E 3 ) in the usual manner. 
     In this case, in a preferred embodiment, the end of a third predetermined period of time (for example 5 seconds) is awaited (step E 4 ) in order to give the pilot the possibility of carrying out an appropriate action. The pilot can then carry out an action intended to avoid that collision (link L 5 ), if he considers the warning to be justified. He can also cancel the warning (link L 6 ), if he considers it to be erroneous or unjustified. If at the end of this predetermined period of time the pilot has not taken any action aimed at avoiding this collision and if he has not cancelled this warning, the automatic pilot  5  automatically forces (link L 7 ) the aircraft to climb at maximum gradient (step E 6 ), as previously mentioned, if it is engaged. 
     In the case in which it is not engaged, the automatic pilot  5  is engaged automatically (step E 5 , links L 8 A and L 8 B) before automatically forcing the aircraft A to climb at maximum gradient (step E 6 ). 
     In  FIG. 4 , there has been shown an example illustrating the aforementioned steps of the method according to the invention in the particular case of level flight. Despite a climb with a gradient γ1 on transmission of a caution (step E 1 ), the risk of collision with the terrain S (mountain  15 ) remains such that, in step E 6 , the automatic pilot performs a climb at maximum gradient γ3. 
     It will be noted that the device  1  according to the invention allows a progressive reaction depending on the alarm level: a first moderate reaction in the case of a caution, then a more sudden reaction in the case of a warning. This makes it possible to anticipate a climb of the aircraft A as early as possible, without notably affecting the comfort of the passengers, and then to initiate a sudden climb only when the latter becomes essential in order to save the aircraft A from a collision. 
     In a preferred embodiment, the automatic pilot  5  forces the aircraft A into a climb with a thrust of the engines (not shown) of the aircraft A, corresponding to the maximum thrust possible in the current flight conditions of the aircraft A. 
     Furthermore, according to the invention, using a usual means  16  which is connected by a link  17  to the automatic pilot  5 , a pilot of the aircraft A can disengage (at any time) the automatic pilot  5 , when it is engaged, and pilot the aircraft A manually. 
     Furthermore, according to the invention, the automatic pilot  5  forces the aircraft A to climb, whilst maintaining the angle of incidence of the aircraft A below a maximum authorized value, in relation to protection with respect to stalling. 
     When both of the alarms (caution and warning) become inactive, the aircraft A is returned into an operational speed range (that is to say a speed included between the minimum and maximum values selected by the pilot, given that the implementation of the method according to the invention can exceptionally result in a speed lower than the said minimum value) and into the normal functional modes (that is to say the same modes as those which follow a go-around by means of the automatic pilot  5 ).