Abstract:
A method and device for detecting a risk of collision of an aircraft, having a profile unit having knowledge of the terrain profile, a determination unit for determining effective values of particular flight parameters, a checking unit for verifying whether a flight path determined by the effective values is compatible with the terrain profile, and a transmitting unit for emitting a warning signal in case of incompatibility. The checking unit includes at least one element for calculating a height variation due to an energy transfer and a total slope variation generated by a speed reduction, during an evasive action, an element deter mining an evasive course using the height variation, and an element verifying whether the evasive course determined is compatible with the terrain profile.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a method and a device for detecting a risk of collision of an aircraft, in particular a transport aircraft, with the surrounding terrain. 
     BACKGROUND OF THE INVENTION 
     It is known that the purpose of such a device, for example of the TANS (“Terrain Avoidance and Warning System”) type or of the GPWS (“Ground Proximity Warning System”) type is to detect any risk of collision of the aircraft with the surrounding terrain and to warn the crew when such a risk is detected, such that the latter can then implement a terrain avoidance maneuver. Such a device generally comprises:
         a first means knowing a profile of the terrain located in front of the aircraft;   a second means for determining the effective values of particular flight parameters;   a third means for calculating, from said effective values, an avoidance path that is the best possible representation of the real situation, and for checking if this avoidance path is compatible with said profile of the terrain, at least over a predetermined distance in front of the aircraft; and   a fourth means for transmitting a corresponding warning signal, if said third means detects an incompatibility.       

     In general, said third means uses a model, intended to represent an avoidance maneuver carried out by an aircraft, for calculating the corresponding path. However, the model used which generally takes account of a constant load factor and a fixed path representative of a stabilized state of the aircraft, reproduces fairly well the maneuver implemented by the aircraft. Furthermore, the approximations made make it necessary to take account of large error margins, in order not to overestimate the real performance of the aircraft during an avoidance maneuver. However, the taking into account of the error margins can in particular result in false alarms. The calculation mode and the checking mode used by said third means are not therefore completely reliable. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for detecting a risk of collision of an aircraft with the surrounding terrain which makes it possible to overcome these disadvantages. 
     For this purpose, according to the invention, said method, according to which the following series of successive steps is carried out automatically and repetitively:
     a) a profile of the terrain located in front of the aircraft is taken into account;   b) the effective values of particular flight parameters are determined;   c) from these effective values, an avoidance path comprising at least a pull-out part and a constant slope part is determined and it is projected in front of the aircraft;   d) it is checked if said avoidance path is compatible with said profile of the terrain, at least over a predetermined distance in front of the aircraft; and   e) if an incompatibility is detected in step d), a corresponding warning signal is transmitted,
 
is noteworthy in that:
       in step c):
           there is calculated, using determined effective values of certain of said particular flight parameters, a height variation of the aircraft which is due to a transfer of energy and a total slope variation generated by a speed reduction during an avoidance maneuver; and   a the height variation thus calculated is used for determining an avoidance path which is the best possible representation of reality and which comprises, between the pull-out part and the part at constant slope, an intermediate part which takes account of this height variation; and   
           in step d), the avoidance path thus determined is used to check if that avoidance path is compatible with said profile of the terrain over said predetermined distance in front of the aircraft.   
       

     Thus, because of the invention, there is taken into account a height variation of the aircraft which is due to a transfer of energy and to a thrust variation during the avoidance maneuver, which makes it possible to optimize the model used in step c) and described below. The processings used during this step c) are therefore adapted to be as close as possible to reality. Consequently, the detection of a risk of collision with the terrain takes account of an avoidance maneuver which is very close to the avoidance maneuver actually used if necessary by the aircraft, which, in particular, makes it possible to avoid false alarms and to obtain particularly reliable monitoring. 
     In a first embodiment, said total slope variation generated by a speed reduction corresponds to a thrust variation. 
     Advantageously, in this first embodiment:
         in step b):
           a the effective mass GW of the aircraft is estimated;   the current effective speed VO of the aircraft is measured; and   the current effective slope γO of the aircraft is measured; and   
           in step c), there is calculated said height variation ΔH, using the following equations:
 
Δ H=[K 1·( VO   2   −VF   2 )/2 ·g +( K 2·( VO−VF )+ K 3)/( GW−GWO )]· f ( x )
 
 f ( x )= f ( X−XO;VF;GW;γF−γO )
 
 f ( x )ε[ O; 1]
 
in which:
   K1, K2 and K3 are predetermined parameters depending on the aircraft;   g represents the acceleration of gravity   GWO represents a predetermined constant value of the mass of the aircraft, dependent on said aircraft;   VF represents a constant value of the speed corresponding to the stabilized speed reached at the end of the avoidance maneuver, this value being predetermined and dependent on the aircraft;   γF represents a constant value of the flight slope corresponding to the flight slope with respect to the ground, stabilized at the end of the avoidance maneuver, this value being predetermined and dependent on the aircraft and on status parameters;   X represents the current position of the aircraft on a horizontal axis of a vertical plane of symmetry of the aircraft; and   XO represents the position of the aircraft, on said horizontal axis of said vertical plane, at the start of a height variation phase of said avoidance maneuver.       

     Moreover, in a second embodiment, in step c), said height variation is calculated, step by step, by producing the sum:
         of a first height variation which represents the conversion of kinetic energy into potential energy provoked by the deceleration; and   and of a second height variation which represents the total slope of the step in question.       

     Moreover, advantageously, in order to optimize an initial pull-out phase of the avoidance maneuver:
         in step c):
           there is calculated, using determined effective values of certain of said particular flight parameters, a load factor which is representative of a pull-out phase of the avoidance maneuver; and   the load factor thus calculated is used to determine a pull-out part of the avoidance path, which is the best possible representation of reality; and   
           in step d), the pull-out part thus determined is used to check if the avoidance path is compatible with said profile of the terrain, over said predetermined distance in front of the aircraft.       

     In this case, preferably,
         in step b):
           the effective mass GW of the aircraft is estimated; and   the current effective speed VO of the aircraft is measured; and   
           in step c), said load factor Nz is calculated using the following expression:
 
 Nz=n 0+( n 1 ·GW )+( n 2 ·VO )
   in which n0, n1 and n2 are predetermined parameters.       

     In a particular embodiment, at least certain of said predetermined parameters and if necessary said constant values depend on the effective flight configuration of the aircraft. This makes it possible to improve the representativeness of the modeled avoidance maneuver, in comparison with the maneuver likely to be actually carried out by the aircraft. 
     The present invention also relates to a device for automatically detecting a risk of collision of an aircraft with the surrounding terrain, said device comprising:
         a first means knowing a profile of the terrain located in front of the aircraft;   a second means for determining the effective values of particular flight parameters;   a third means for calculating, from said effective values, an avoidance path, and for checking if said avoidance path is compatible with said profile of the terrain, at least over a predetermined distance in front of the aircraft; and   a fourth means for transmitting a corresponding warning signal, if said third means detects an incompatibility.       

     According to the invention, said device is noteworthy in that said third means comprises at least:
         a first unit for calculating, using determined effective values of some of said particular flight parameters, a height variation of the aircraft which is due to a transfer of energy and to a variation in thrust generated by a speed reduction, during an avoidance maneuver;   a second unit for using the height variation calculated by said first unit, in order to determine an avoidance path that is the best possible representation of reality; and   a third unit for using the avoidance path determined by said second unit in order to check if this avoidance path is compatible with said profile of the terrain, over said predetermined distance in front of the aircraft.       

     In a particular embodiment, said third means furthermore comprises:
         a fourth unit for calculating, using determined effective values of certain of said particular flight parameters, a load factor which is representative of a pull-out phase of the avoidance maneuver;   a fifth unit (corresponding for example to said second unit) for using the load factor, calculated by said fourth unit, in order to determine a pull-out part of the avoidance path, which is the best possible representation of reality; and   a sixth unit (corresponding for example to said third unit) for using this pull-out part determined by said fifth unit, in order to check if said avoidance path is compatible with said profile of the terrain, over said predetermined distance in front of the aircraft.       

    
    
     
       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 indicate similar units. 
         FIG. 1  is the block diagram of a device according to the invention. 
         FIG. 2  is a diagrammatic illustration of a pull-out maneuver taken into account in the present invention. 
         FIG. 3  is a graph making it possible to illustrate essential features of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The purpose of the device  1  according to the invention and shown diagrammatically in  FIG. 1  is to automatically detect any risk of collision of an aircraft A, in particular a military transport aircraft, with the surrounding terrain  2  and to warn the crew of the aircraft A when such a risk is detected, such that the latter can then implement a maneuver TE for avoidance of the terrain  2 , as shown in  FIG. 2 . 
     Such a device  1 , for example of the TAWS (“Terrain Avoidance and Warning System”) type or of the GPWS (“Ground Proximity Warning System”) type, which is installed in the aircraft A, comprises in the usual manner:
         a means  5  which knows the profile of the terrain  6  at least in front of the aircraft A and which comprises for this purpose for example a database containing said terrain profile  6  and/or a means of detection of the terrain such as a radar. Said terrain profile  6  is for example at a predetermined clearance height G above the relief  4 ;   a means  7  described below and for example forming part of a data sources assembly  8 , whose purpose is to determine the effective values of a plurality of particular flight parameters, also described below;   a central unit  9  which is connected by the intermediary of links  11  and  12  respectively to said means  5  and  7 , and whose purpose is to calculate an avoidance path from said effective values of particular flight parameters determined by said means  7 , to project this avoidance path in front of the aircraft A, and to check if said avoidance path thus projected forwards is compatible with said terrain profile  6 , at least over a predetermined distance (for example 10,000 meters) in front of the aircraft A; and   a means  13  which is connected by the intermediary of a link  14  to said central unit  9 , for transmitting a warning signal (sound and/or visual) in the case of detection of a collision risk by said central unit  9 .       

     According to the invention, said central unit  9  comprises:
         a unit  15  for calculating, using effective values (determined by said means  7 ) of certain of said particular flight parameters, as described below, a height variation ΔH of the aircraft A which is due to a transfer of energy and to a variation of total slope generated by a speed reduction, during an avoidance maneuver implemented in order to avoid a terrain  2  in front of the aircraft A; and   a unit  16  which is connected by a link  17  to said unit  15  for using the height variation ΔH calculated by the latter, for the purpose of determining an avoidance path that is the best possible representation of reality; and   a unit  3  which is connected by a link  10  to said unit  16  for using the avoidance path determined by the latter, for the purpose of checking if said avoidance path is compatible with said terrain profile  6 , over said predetermined distance in front of the aircraft A.       

     In order to do this, said unit  3  uses an assistance curve  18  (or avoidance curve) which is calculated by the unit  16 , which is shown in  FIG. 3  and which is considered to reproduce an avoidance maneuver. Said unit  3  makes this assistance curve  18  move rectilinearly in front of the aircraft A and it checks that it does not encounter the terrain profile  6  in front of the current position of the aircraft A, at least over said predetermined distance. Thus, as long as the moved assistance curve  18  thus moved does not encounter the terrain profile  6 , the aircraft A is able to fly over the relief  4  of the terrain  2  which is in front of it. 
     However, when during the movement of the assistance curve  18 , said assistance curve  18  encounters the terrain profile  6 , there is a risk of collision with the latter such that the unit  3  then orders the means  13  to transmit a warning signal, as illustrated by a symbol  19  in  FIG. 2 . At that time, the pilot or an automatic guidance system makes the aircraft A follow an avoidance path TE intended to allow said aircraft A to fly over the relief  4  of the terrain  2  which is in front of it and thus to avoid a collision. 
     Up until the present, a usual assistance curve  18 A (intended to reproduce an avoidance maneuver) comprised, as shown in dotted line in  FIG. 3 :
         a first part  20 A (or pull-out part) representative of a pull-out phase of the avoidance maneuver and intended to allow the aircraft A to regain altitude. This pull-out part  20 A was usually constructed by taking into account a constant load factor of the aircraft A in such a way as to correspond to an arc of circle of constant radius; and   a constant slope part  21 A, which follows this pull-out part  20 A tangentially.       

     Such a usual assistance curve  18 A does not exactly reproduce the avoidance maneuver actually carried out by the aircraft A if necessary, which can in particular give rise to false alarms (relating to a collision risk). 
     The assistance curve  18  according to the invention makes it possible to overcome this disadvantage by reproducing in an optimized manner the avoidance maneuver actually carried out by the aircraft A. For this purpose, according to the invention, said assistance curve  18  comprises, in addition to a special pull-out part  20  described below and a usual constant slope part  21 , an intermediate part  22  taking said height variation ΔH into account. This assistance curve  18  therefore takes account of a dynamic increase in the altitude, starting from the end of said pull-out part  20  and doing this up until the start of said constant slope part  21  (which is therefore vertically shifted upwards by said height variation ΔH, with respect to said usual part  21 A). 
     In order to do this, said means  7  comprises units not shown specifically in order, respectively:
         to estimate the effective mass GW of the aircraft A;   to measure the current effective speed VO of the aircraft A; and   to measure the current effective slope γO (with respect to the ground) of the aircraft A.       

     Moreover, in a first embodiment, for which said total slope variation corresponds to a thrust variation, said unit  15  determines the height variation ΔH, using the preceding effective values and the following expressions:
 
Δ H=[K 1·( VO   2   −VF   2 )/2 ·g +( K 2·( VO−VF )+ K 3)/( GW−GWO )]· f ( x )
 
 f ( x )= f ( X−XO;VF;GW;γF−γO )= X−XO )/[ K 4 ·GW·VF ·(γ F−K 5 ·γO )]
 
 f ( x )ε[ O; 1]
 
in which:
         K1, K2, K3, K4 and K5 are predetermined parameters depending on the aircraft A;   g represents the acceleration of gravity;   GWO represents a predetermined constant value of the mass of the aircraft A, dependent on said aircraft A;   VF represents a constant value of the speed corresponding to the stabilized speed reached at the end of the avoidance maneuver, this value being predetermined and dependent on said aircraft A;   γF represents a constant value of the flight slope corresponding to the flight slope with respect to the ground, stabilized at the end of the avoidance maneuver, this value being predetermined and dependent on the aircraft A and on status parameters;   X represents the current position of the aircraft A on a horizontal axis OX of a vertical plane of symmetry OXZp of the aircraft A; and   XO represents the position of the aircraft A, on said horizontal axis OX of said vertical plane OXZp, at the start of a height variation phase of said avoidance maneuver, as shown in  FIG. 3 .       

     This height variation ΔH is due, as mentioned previously, to an energy transfer (giving rise to a height variation ΔH1) and to a thrust variation (giving rise to a height variation ΔH2):
 
Δ H=ΔH 1 +ΔH 2.
 
     The height variation ΔH1 illustrates a conversion of kinetic energy into potential energy:
 
Δ H 1 =K 1·( VO   2   −VF   2 )/2 ·g.  
 
     Moreover, in this first embodiment, the height variation ΔH2 is due to a thrust variation, which is generated by a speed reduction. 
     Moreover, in a second embodiment, for which the height variation ΔN is due to an energy transfer and to a total slope variation generated by a speed reduction, said height variation ΔH is a function of the geometric slope γ:
 
Δ H≈∫γ·dx  
 
     This geometric slope is however equal to the sum of the total slope γTOT and of an acceleration term:
 
γ=γ TOT −( dVSOL/dt )/ g  
 
     The total slope is equal to the propulsive balance, that is to say to the difference between the thrust and the drag over the mass:
 
γ TOT ≈( P−T )/ m·g  
 
     This total slope γTOT contributes to a height increase ΔH, because it increases as the speed reduces. This increase in total slope, which is inversely proportional to the speed, is due to the increase in thrust inversely proportional to the speed. 
     In fact, this physical phenomenon is particularly accentuated in a turboprop where the preponderant term for the thrust is expressed by:
 
 P ≈Useful power/ V  
 
     The height variation ΔH can therefore be calculated by the unit  15 , in this second embodiment, step by step, as the sum of two contributions:
         ΔH3, which is the conversion of kinetic energy into potential energy caused by the decelerations; and   ΔH4, which represents the total slope of the step in question,
 
that is to say:
 
Δ H=ΔH 3 +ΔH 4
 
with, for a step i
 
Δ H 3=( V   i+1   2   −V   i   2 )/2 ·g  
 
Δ H 4 =tg (γ TOT )·Δ xi  
       

     Moreover, according to the invention, said central unit  9  furthermore comprises:
         a unit  23  for calculating, using the effective values of the mass GW and of the speed VO of the aircraft A (determined by said means  7 ), a load factor Nz which is representative of the real load factor of the aircraft A, during the pull-out phase of an avoidance maneuver really carried out by the aircraft A; and   an additional unit, for example the unit  16  which is connected by a link  24  to said unit  23 , for using said load factor Nz (instead of a predetermined constant load factor, used up until the present) for the purpose of optimizing said pull-out part  20  of the assistance curve  18 . This load factor Nz is taken into account for calculating, in the usual manner, the radius of the arc of circle forming this pull-out part  20 .       

     In a particular embodiment, said unit  23  determines said load factor Nz, using the following expression:
 
 Nz=n 0+( n 1 ·GW )+( n 2 ·VO )
 
in which n0, n1 and n2 are predetermined parameters.
 
     This load factor Nz is closer to reality than is a constant load factor used in the prior art, which makes it possible to increase the conformity of the pull-out part  20  of the assistance curve  18 . 
     The avoidance path TE actually followed by the aircraft A, in the case of detection of a collision risk by the device  1  and of initiation of an avoidance maneuver, is thus reproduced faithfully by the assistance curve  18  determined according to the present invention, as shown in  FIG. 3 . 
     Consequently, the detection of a risk of collision with the terrain  2  used by the device  1  takes account of an avoidance maneuver (in the form of the assistance curve  18 ) which is very close to an avoidance maneuver actually used if necessary by the aircraft A, which in particular makes it possible to avoid false alarms and thus to obtain a particularly reliable monitoring. 
     In a particular embodiment, at least some of said predetermined parameters n0, n1, n2, K1, K2 and K3 or of said constant values GWO, VF and γF (recorded in the central unit  9  or in the assembly  8  of data sources) depend on the effective flight configuration of the aircraft A. In this case, the assembly  8  of data sources comprise means intended for measuring parameters making it possible to determine the current flight configuration of the aircraft A, which depends for example on the flight phase, the aerodynamic configuration (slats, flaps) of the aircraft A, its altitude, etc.