Abstract:
The invention relates to a method for determining the speed of an aircraft that is subject to a time constraint. The invention consists no longer in calculating a single CAS/MACH pair during climb/descent but in adapting the speed in a continuous manner to the bounds of curves of minimum V min  and maximum V max  speed defining a flight envelope of the aircraft. The calculation of these speeds is carried out on the basis of constant maximum and minimum speed setpoints and of a coefficient taking into account a deviation to the time constraint.

Description:
PRIORITY CLAIM 
     This application is a Continuation in Part of Application Publication No. 2008/0300738, filed May 30, 2008 which claims priority to French Patent Application Number 07 03912 filed Jun. 1, 2007. This application also claims priority to French Patent Application Number 08 06232, entitled Method for Determining the Speed of an Aircraft, filed on Nov. 7, 2008. 
     TECHNICAL FIELD 
     The invention relates to the determination of the speed of an aircraft that is subject to a time constraint. 
     BACKGROUND OF THE INVENTION 
     For a few years, thoughts have turned to the increase in traffic and the ensuing loading of air traffic controllers. In order to guarantee the safety and also the economic viability of air transport, it is envisaged, notably in the approach phase, that a time constraint be imposed on a particular waypoint: runway threshold, Initial Approach Fix (IAF), or rallying point for final approach, termed the ATC Merge Point. 
     This allows the air traffic control to guarantee a smoothed flow in the approach, and to manage a stable number of aeroplanes corresponding to the capabilities of the ground facilities and to the limit loading of an air traffic controller. 
     These time constraints can also serve in other operational contexts such as the management of the number of aeroplanes per sector. 
     Aboard the aircraft, the time constraint is in general inserted into a flight management computer termed the FMS (the acronym standing for Flight Management System). A flight management system consists of various functional hardware components which allow the crew to programme a flight using a navigation database. The system calculates a lateral and vertical trajectory making it possible to reach the destination of the flight plan. These calculations are based on the characteristics of the aeroplane and data provided by the crew and the environment of the system. The positioning and guidance functions collaborate to aid the aircraft to remain on this trajectory. 
     The pilot can programme the meeting of a time constraint, termed RTA for Required Time Arrival, at a point of the flight plan on the request of air traffic control for example. In this case the FMS performs an optimization of the trajectory by successive iterations so as to comply with the constraint. 
     To comply with an RTA, the FMS calculates predictions to determine the speed strategy. Once the strategy has been chosen, a re-calculation will take place if the prediction for the time of transit at the constrained point, termed the ETA for Estimated Time of Arrival, departs from a predetermined tolerance. 
     However the speed of an aircraft is confined within a speed envelope defined by two speed profiles: a maximum speed profile and a minimum speed profile. They depend mainly on the weight and the altitude of the aircraft. The maximum speed also depends on the ambient temperature. Other parameters can also come into play depending on the type of aircraft.  FIG. 1  represents a typical evolution of the limit values of a flight envelope  11  for a given altitude and temperature as a function of the aircraft weight. The abscissa axis represents the weights decreasing towards the right, the ordinate axis the speeds. It is noted that the minimum speed V MIN  increases with the weight of the aircraft, while its maximum speed V MAX  decreases onwards of a certain threshold. 
     In the flight management systems according to the known art, the speeds are expressed in a speed unit called CAS, the acronym standing for Calibrated AirSpeed, or in MACH. Nevertheless, the meeting of a time constraint is dependent on the ground speed or GS. The ground speed is the horizontal component of the speed relative to the ground; it is determined by the sum of the air speed and of the wind.  FIG. 2  represents the variation of the air speed as a function of altitude for a given speed expressed in terms of CAS or MACH. It may be noted that for a constant value of CAS, the air speed (and therefore the ground speed) increases with altitude. For a constant value of MACH, the ground speed decreases with altitude. Speed alterations are made rather in terms of CAS at low altitude and in terms of MACH at high altitude. 
     In the flight management systems according to the known art, the speed setpoints are limited to: a CAS/MACH pair for the aircraft climb phase, a few MACH speed values for its cruising phase and a CAS/MACH pair for the descent phase. 
     The setpoint CAS and MACH are dependent on an economic optimization criterion termed CI for Cost Index, weight, altitude, and temperature. 
     The Cost Index is in fact a criterion for optimizing between the time costs CT (“Cost of Time”) and the fuel costs CF (“Cost of Fuel”). The Cost Index is defined by CI=CT/CF. The value of this cost index for an aircraft and a given mission is determined according to criteria specific to each operator, and constrains notably the rules for determining the altitudes and speeds of the flight plan (vertical profile of the flight plan). 
     The maximum speeds (CAS or Mach) may be dependent on the weight and the altitude on certain aircraft, as is the case for  FIG. 1 .  FIG. 2  presents a curve of minimum speed Vmin  201  and a curve of maximum speed Vmax  202  corresponding to a case of initial weight denoted GW 0 , and integrating the lightening of the weight of the aircraft. The minimum speeds (CAS or Mach) take account of the stall speeds with a margin. These minimum speeds are dependent notably on the weight, altitude and temperature. 
     The CAS and MACH setpoints, calculated with the schemes according to the known art, are limited by the envelope. Each of these limits is calculated for a single point of the envelope. For the climb phase or the descent phase, it may happen that the flight envelope at the top or at the bottom is more limiting than the flight envelope during the phase. 
     Several schemes according to the known art make it possible to control the 4D trajectory of the aircraft so as to make it comply with a time constraint. These schemes all perform a convergence in speed, in open loop: the 4D trajectory is reoptimized at regular intervals but is not regulated. These schemes are generally based on a variation of the Cost Index. 
     A flight management system making it possible to comply with a time constraint by varying a cost index “Cost Index” is known through U.S. Pat. No. 5,457,634. Such a system makes it possible notably to calculate an optimal cruising altitude so as to economize on fuel consumption. One of the drawbacks of such a system arises when a time constraint cannot be complied with. The system can then signal that the constraint is deficient though the latter could be complied with by adopting a flight speed closer to the limits of the flight envelope. 
     The invention is aimed at alleviating the problems cited previously by proposing a method for calculating a speed making it possible to comply with a time constraint RTA. The invention consists no longer in calculating a single CAS/MACH pair during climb/descent but in adapting the speed in a continuous manner to the bounds of the curves of minimum Vmin and maximum Vmax speeds when a time constraint may not be achieved by following a single CAS/MACH pair. 
     SUMMARY OF THE INVENTION 
     For this purpose, the subject of the invention is a method for determining the speed of an aircraft that is subject to a time constraint RTA expressed in the form of a fixed date at a determined point, the said aircraft exhibiting a limit speed profile V limit , the said aircraft comprising a flight management system calculating a predicted time of arrival ETA of the aircraft at the said point on following a speed setpoint expressed in the form of a pair of constant speeds CAS,MACH, the flight management system calculating, furthermore, a first arrival time ETA 1  of the aircraft at the said point on following a pair of constant limit speeds CAS limit ,MACH limit , the aircraft following the first speed of the pair CAS limit  when the aircraft flies at an altitude below a predefined altitude Alt Max , the said method being characterized in that it comprises:
         the calculation of a second arrival time ETAenv 2  on flying according to the limit speed profile V limit ,   if the time constraint RTA lies between the first arrival time ETA 1  and the second arrival time ETAenv 2 :   the calculation of a deviation Δ ETA  to the time constraint: Δ ETA =|ETA−RTA|, the said deviation to the time constraint Δ ETA  being equal to the absolute value of the difference between the predicted arrival time ETA of the aircraft at the said point on following a speed setpoint and the time constraint RTA,   the calculation of a first speed profile CAS(alt 1 ) dependent on an altitude on the basis of the limit speed profile V limit , with alt 1  being an altitude between 0 and the predefined altitude Alt Max ,   the calculation of a second speed profile MACH(alt 2 ) dependent on an altitude, on the basis of the limit speed profile V limit , with alt 2  being an altitude between the predefined altitude Alt Max  and a cruising altitude,   the updating of the predicted arrival time ETA by taking into account the calculated first and second speed profiles CAS(alt 1 ) and MACH(alt 2 ),   the calculation of the deviation Δ ETA  to the time constraint: Δ ETA =|ETA−RTA|   if the deviation to the arrival time Δ ETA  is nonzero, the return to the step of calculating the first speed profile CAS(alt 1 ), otherwise the application of the calculated speed profiles to the aircraft.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       According to a characteristic of the invention, the method furthermore comprises a step of calculating a coefficient C as a function of the deviation Δ ETA  to the time constraint and if the deviation Δ ETA  to the time constraint is nonzero, the return to the step of calculating the first speed profile CAS(alt 1 ). 
       According to a variant of the invention, the limit speed profile V limit  is a maximum speed profile V max , the first arrival time ETA 1  being an arrival time ETA Min  of the aircraft following a pair of constant maximum speeds CAS Max ,MACH Max , the second arrival time ETAenv 2  being an arrival time ETAenv Min  following the maximum speed profile V Max . 
       According to another variant of the invention, the limit speed profile V limit  is a minimum speed profile V Min , the first arrival time ETA 1  being an arrival time ETA Max  of the aircraft following a pair of constant minimum speeds CAS Min ,MACH Min , the second arrival time ETAenv 2  being an arrival time ETAenv Max  following the minimum speed profile V Min . 
       The invention also relates to a flight management system of an aircraft comprising a module for constructing a continuous trajectory on the basis of points of a flight plan and a module of predictions for constructing a vertical profile optimized on the trajectory, characterized in that the modules of predictions and for constructing a trajectory comprise means for implementing the method according to the invention. 
       The method according to the invention has the advantage of restoring a time margin by making best use of the capabilities of the aircraft by approaching the limits of the flight envelope. 
       This method operates whatever guidance scheme is chosen: guidance by air speed or TAS for True AirSpeed or guidance by CAS/MACH after conversion. 
       The method performs a guidance by speed by varying the latter in a continuous manner, avoiding jumps in speed setpoint and consequently in engine thrust. 
       Another advantage of the use of the invention is the reduction in the stress to the crew by automatically proposing a solution maximizing the probability of meeting of an RTA and the reduction in the controller&#39;s workload, by decreasing the rate of deficient constraints. 
       The invention will be better understood and other advantages will become apparent on reading the detailed description given by way of nonlimiting example and with the aid of the figures among which: 
         FIG. 1 , already presented, represents an aircraft speed envelope. 
         FIG. 2 , already presented, represents the evolution as a function of altitude of the air speed of an aircraft for a given CAS or MACH speed. 
         FIG. 3  represents curves of minimum and maximum air speeds of an aircraft. 
         FIG. 4  represents the earliest and latest arrival times on using the method according to the invention. 
         FIG. 5  illustrates the main steps of the method according to the invention in the case of a delay in a time constraint. 
         FIG. 6  illustrates the main steps of the method according to the invention in the case of an advance in a time constraint. 
         FIG. 7  illustrates an architecture of a flight management system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The abscissa axis represents true air speeds (or TAS for True AirSpeed). The ordinate axis represents altitudes in feet (or ft). 
     The curves of minimum air speed V min    301  and maximum air speed V max    302  of the figure correspond to a case of initial weight GW 0  and take account of the lightening of the aircraft weight (this is why for example the CAS parts are not ISO CAS). 
     The CAS and MACH setpoints, calculated with the schemes according to the known art, are limited by the envelope. Each of these limits by the envelope is calculated for a single point of the envelope. In the example, the CAS setpoint is limited by the envelope at its value at 22 000 ft to 320 knots (or kts). There exists a first margin  305  between a curve with constant CAS  304  passing through the limit point  303  and the maximum speed V max  curve  302 . This margin  305  is situated at the altitudes below the altitude of limitation by the envelope of 22 000 ft. The first margin  305  represents ranges of speeds, by altitude, flyable by the aircraft and greater than the constant CAS setpoint. 
     In the example, the MACH setpoint is limited by the envelope at a second point  307  corresponding to an altitude of 30 000 ft to a Mach speed of 0.70. There exists a second margin  306  between a curve with constant MACH  308  passing through the second limit point  307  and the maximum speed V max  curve  302 . The second margin  306  is situated at the altitudes below the altitude of limitation by the envelope of 30 000 ft. The second margin  306  represents ranges of speeds, by altitude, flyable by the aircraft and greater than the constant MACH setpoint. 
       FIG. 4  represents a temporal axis  41  along which are represented various predicted times of arrival of an aircraft at a given point. A first point RTA  42  represents a constraint to which the aircraft is subject. A second  43  and a third  44  point ETA min ,ETA max  represent the earliest and latest arrival times on flying respectively according to setpoints of constant maximum and minimum speed. A fourth  45  and a fifth  46  point ETAenv min ,ETAenv max  represent the earliest and latest arrival times on flying respectively according to the maximum and minimum speeds delimiting the aircraft&#39;s flight envelope. The first point  42  being situated outside of the segment formed by the second  43  and the third point  44 , the aircraft is not able to comply with the time constraint by following a speed setpoint calculated with a method according to the known art. But the first point  42  being situated between the fourth  45  and the second point  43 , the time constraint is greater than the earliest arrival time on flying according to the maximum speed authorized by the flight envelope. The constraint can therefore in this case be complied with by adopting an appropriate speed. 
     The invention relates to a method for determining the speed of an aircraft subject to a time constraint RTA. A time constraint can be expressed in the form of a fixed date at a determined point. The aircraft exhibits at least one limit speed profile, in particular: a minimum speed profile V min  and a maximum speed profile V max . The aircraft obeys a specific speed setpoint in a climb phase and a descent phase. The specific speed setpoint is expressed in the form of a pair of constant speeds (CAS,MACH). The aircraft comprises a flight management system making it possible to calculate a predicted arrival time ETA of the aircraft at the said point on following the speed setpoint. 
     In a first variant of implementation of the method according to the invention, the management system also calculates a first arrival time ETA MIN  of the aircraft at the said point on following a pair of constant maximum speeds (CAS MAX ,MACH MAX ). The aircraft flies at the first speed CAS max  of the pair for an altitude lying between 0 and a predefined altitude Alt MAX , termed the crossover altitude, at the second speed MACH MAX  of the pair for an altitude lying between the altitude Alt MAX  and a cruising altitude. 
       FIG. 5  illustrates the main steps of the method according to the invention in the case of a delay in a time constraint. 
     The method according to the invention comprises the following steps:
         the calculation  51  of a second arrival time ETAenv Min  on flying according to the maximum speed profile V max , this is the earliest arrival time on flying at the limits of the flight envelope,   if  54  the time constraint RTA is less than the first arrival time ETA Min  and greater than the second arrival time ETAenv Min :   the calculation  55  of a deviation Δ ETA  to the time constraint: Δ ETA =ETA−RTA   the calculation  56  of a coefficient C as a function of the deviation Δ ETA  to the time constraint; according to a characteristic of the method according to the invention, the coefficient C is calculated according to the following relation:
 
 C =ΔETA/(ETA MIN −ETAenv MIN )
       

     The coefficient C is dependent on the deviation Δ ETA  to the time constraint. It can be calculated according to other relations taking this deviation into account.
         the calculation  57  of a first speed profile CAS(alt 1 ) dependent on an altitude; according to a characteristic of the invention, the said first speed profile satisfies the following equation:
 
CAS(alt 1 )=CAS MAX   +C·[V   max (alt 1 )−CAS max ]
           with alt 1  being an altitude between 0 and the crossover altitude Alt MAX  of the first speed pair (CAS MAX ,MACH MAX ),   
           the calculation of a second speed profile MACH(alt 2 ) dependent on an altitude; according to a characteristic of the invention, the said second speed profile satisfying the following equation:
 
MACH(alt 2 )=MACH MAX   +C·[V   max (alt 2 )−MACH max ]
           with alt 2  being an altitude lying between the crossover altitude Alt MAX  of the first speed pair (CAS MAX ,MACH MAX ) and the cruising altitude of the aircraft;   
           the updating  58  of the predicted arrival time ETA by taking into account the calculated first and second speed profiles CAS(alt 1 ) and MACH(alt 2 ); this calculation can be performed by the flight management system;   the calculation  59  of the deviation Δ ETA  to the time constraint: Δ ETA =ETA−RTA; the predicted time ETA having been updated by taking into account the new speed setpoints, it is possible to calculate a new deviation to the time constraint,   if the deviation Δ ETA  to the time constraint is nonzero  59 , the return to the step  56  of calculating the coefficient C, otherwise the application of the calculated speed profiles to the aircraft.       

     According to a characteristic of the invention, if  52  the time constraint is less than the second arrival time (ETAenv Min ) then  53  the application of the maximum speed profile V max  to the aircraft. In this case, the time constraint cannot be complied with even by flying at the limits of the aircraft&#39;s flight envelope. The effect of this method step is to limit the deviation between the actual arrival time of the aircraft and the time constraint. 
     According to a variant of the invention, the method according to the invention comprises, furthermore, a step of calculating a speed profile, termed the median characteristic speed profile V median , using data of a performance base. The setpoint speed RTA (CAS and MACH) is given by a ratio between this median speed and either Vmax or Vmin. And as in the previous implementation, it is calculated along the mission as a function of the altitude and of the lightening. In this case, in the example of  FIG. 5 , the following are used: 
     When climbing, for the CAS part, a constant CAS between the initial altitude and a transition altitude situated under a first climb altitude alt 1 , and then a convergence towards the envelope in CAS mode. The crossover altitude is then reached in MACH mode, and a constant MACH is thereafter fixed until a second climb altitude alt 2 . 
     When descending, for the CAS part, a constant CAS between the initial altitude and a transition altitude situated under a first descent altitude alt 1 , and then a convergence towards the envelope in CAS mode. The crossover altitude is then reached in MACH mode, and a constant MACH is thereafter fixed until a descent altitude alt 2 . 
     This variant is therefore intermediate between the method and the state of the art, since it proposes a constant CAS/MACH pair over a part of the climb and of the descent, and a variable pair along the envelope. 
     The median speed is calculated as being equal to the mean of the speed profile (in terms of ground speed) arising from the first variant of the method, between the initial altitude and the transition altitude described in the variant (first altitude alt 1  or second altitude alt 2 ). It makes it possible to obtain a constant setpoint speed over an altitude range, and to make the setpoint converge towards the envelope only on the top part of the altitude range. 
     In a second variant of implementation of the method according to the invention, the management system calculates a first arrival time ETA Max  of the aircraft at the said point on following a pair of constant minimum speeds (CAS Min ,MACH Min ). The aircraft flies at the speed CAS Min  for an altitude lying between 0 and a predefined crossover altitude Alt Min  and at the speed MACH Min  for an altitude lying between the second altitude Alt Min  and a cruising altitude. 
       FIG. 6  illustrates the main steps of the method according to the invention in the case of an advance in a time constraint. 
     The method according to the invention comprises the following steps:
         the calculation  61  of a second arrival time ETAenv Max  on flying according to a minimum speed profile V min ,   if  64  the time constraint RTA is greater than the first arrival time ETA Max  and less than the second arrival time ETAenv Max :   the calculation  65  of a deviation Δ ETA  to the time constraint: Δ ETA =RTA−ETA   the calculation  66  of a coefficient C as a function of the deviation Δ ETA  to the time constraint; according to a characteristic of the method according to the invention, the coefficient C is calculated according to the following relation:
 
 C =ΔETA/(ETAenv Max −ETA Max )
   the calculation  67  of a first speed profile CAS(alt 3 ) dependent on an altitude; according to a characteristic of the invention, the said first speed profile satisfying the following equation:
 
CAS(alt 1 )=CAS min   +C·[ CAS min   −V   min (alt 1 )]
           with alt 1  being an altitude between 0 and the predefined altitude (Alt Min ),   
           the calculation of a second speed profile MACH(alt 2 ) dependent on an altitude, the said second speed profile satisfying the following equation:
 
MACH(alt 2 )=MACH min   +C·[ MACH min   −V   min (alt 2 )]
           with alt 2  being an altitude between the predefined altitude Alt Min  and a cruising altitude,   
           the updating  68  of the predicted arrival time ETA by taking into account the calculated first and second speed profiles CAS(alt 1 ) and MACH(alt 2 ),   the calculation  69  of the deviation Δ ETA  to the time constraint: Δ ETA =RTA−ETA   if the deviation to the arrival time Δ ETA  is nonzero  69 , the return to the step  66  of calculating the coefficient C, otherwise the application of the calculated speed profiles to the aircraft.       

     According to a characteristic of the invention, if  62  the time constraint is greater than the second arrival time ETAenv Max  then  63  the application of the minimum speed profile V min  to the aircraft. In this case, the time constraint cannot be complied with even by flying at the limits of the flight envelope of the aircraft. The effect of this method step is to limit the deviation between the actual arrival time of the aircraft and the time constraint. 
     In the case where the time constraint RTA is less than the third arrival time ETA Max  and greater than the first predicted arrival time ETA MIN , the time constraint can be complied with by using schemes according to the known art. 
     According to a variant of the invention, the coefficient C is a piecewise function, dependent on the altitude or on a distance to be traversed until the end of a flight phase where the speed setpoint is applied. 
     According to another variant of the invention, the coefficient C is a linear function dependent on at least one of the following values: the deviation Δ ETA  to the time constraint, the second arrival time ETAenv 2 , the first arrival time ETA 1 , the altitude of the aircraft, a distance to be traversed until the end of a flight phase where the speed setpoint is applied. 
       FIG. 7  illustrates an architecture of a flight management system. The onboard flight management system (FMS) is the computer which determines the geometry of the 4D profile (3D+time-profile of speeds), and dispatches the guidance setpoints to the pilot or to the automatic pilot so as to follow this profile. A flight management system has the following functions described in ARINC standard 702 (Advanced Flight Management Computer System, December 1996). Such a flight management system comprises modules for:
         Navigation LOCNAV,  770 , for performing optimal location of the aircraft as a function of the geolocation means (GPS, GALILEO, VHF radio beacons, inertial platforms);   Flight plan FPLN,  710 , for inputting the geographical elements constituting the skeleton of the route to be followed (departure and arrival procedures, waypoints, airways);   Navigation database NAVDB  730 , for constructing geographical routes and procedures using data included in the bases (points, beacons, interception or altitude legs, etc.);   Performance database, PRF DB  750 , containing the craft&#39;s aerodynamic and engine parameters;   Lateral trajectory TRAJ,  720 : for constructing a continuous trajectory on the basis of the points of the flight plan, complying with the aircraft performance and the confinement constraints;   Predictions PRED,  740 : for constructing a vertical profile optimized on the lateral trajectory;   Guidance, GUID  700 , for guiding in the lateral and vertical planes the aircraft on its 3D trajectory, while optimizing the speed;   Digital datalink DATALINK,  780  for communicating with the control centres and other aircraft.       

     The invention also relates to a flight management system comprising means for implementing the method according to the invention in the trajectory module  720  and predictions module  740 .