Abstract:
A method for automatically managing a lateral trajectory upon triggering an emergency descent includes determining a value of lateral offset and generating an offset setpoint by using the value of lateral offset. The generating an offset setpoint includes calculating a sum of the value of lateral offset and any initial value of lateral offset defined between a central axis of a protected sector that the aircraft travels along and an initial lateral trajectory. The offset setpoint is selected to be the smaller of the sum or a lateral offset maximum, which maintains the aircraft within the protected sector at all times. The aircraft is then operated to move the aircraft to the offset setpoint during the emergency descent, which helps avoid further air traffic that may be located at different altitude levels within the same protected sector, especially along the central axis.

Description:
TECHNICAL FIELD 
     The present invention relates to a method and a device for automatically managing a lateral trajectory of an aircraft, in particular a transport airplane, upon an emergency descent. 
     The solution relates to an automated emergency descent device and, more particularly, to managing the lateral trajectory of the aircraft upon the latter. 
     BACKGROUND 
     As known, civil transport airplanes should be pressurized, as upon a cruise flight, an airplane flies at an altitude being often higher than 30,000 feet (about 9,000 meters), for which the external air is too low in oxygen (and also too cold and too dry) for being compatible with life. Thus, pressurizing systems are provided in airplanes for keeping on board a breathable atmosphere. In particular, the international aeronautic regulation states that any public transport airplane flying at an altitude higher than 20,000 feet (about 6,000 meters ) should be pressurized and that it should establish in the cockpit an equivalent altitude which does not exceed 8,000 feet (about 2,400 meters) upon a normal flight. 
     It may however occur that, as a result of a breakdown or a failure, the pressurization of the airplane could no longer be maintained at an acceptable level. A regulatory procedure then compels the pilot to have the airplane descent, as quickly as possible, at a breathable altitude of 10,000 feet (about 3,000 meters) or at the current security altitude if it is not possible to descent as low as 10,000 feet because of the relief. Such a procedure is referred to as an emergency descent. 
     In such a case, the crew is responsible for different tasks related to initiating the descent, as well as the adjustment of parameters of the descent (speed, target altitude, lateral trajectory, etc.) and this until the airplane flies level at low altitude. 
     When a crew, as a result of the cockpit becoming decompressed or any other event, carries out an emergency descent, they are requested to deviate from the centre of the air traffic way it followed before the event occurred. Such a measure aims at avoiding that, upon the emergency descent, the aircraft comes into conflict with aircrafts flying along the same air traffic way at lower flight levels. Such an operational requirement is explicitly mentioned in document 7030 of the Civil Aviation International Organization, stipulating that the aircraft having to carry out an emergency descent should deviate from its initial itinerary before starting to descent. 
     As most of the aircrafts are not provided with automatic systems for carrying out an emergency descent, the whole tasks to be carried out remain the responsibility of the crew, and amongst them, the requirement of deviating from the central axis of the air traffic way upon the initiation of the maneuver. Such a deviation maneuver generally results for the crew in a reflex action via the heading selector of the autopilot. Such an action results in quickly slaving the autopilot on a new heading setpoint, diverging with respect to the initially followed itinerary. 
     It could happen, however, that in the case of a pressurization loss as a result of which the crew have lost conscience (hypoxia symptoms), the crew is no longer able to apply the above described procedure. 
     In order to overcome such situations, the emergency descent procedure could be automated. 
     In particular, from document FR-2,928,465, a particular method is known for automatically controlling an emergency descent of an aircraft. According to this method, when an emergency descent automatic function is triggered, the following successive operations are carried out: 
     a) a set of vertical setpoints is automatically determined comprising:
         a target altitude representing an altitude to be reached by the aircraft at the end of the emergency descent; and   a target speed representing a speed that the aircraft should respect upon the emergency descent;       

     b) a set of lateral setpoints is automatically determined, representing a lateral maneuver to be carried out upon the emergency descent; and 
     c) the aircraft is automatically guided so that it simultaneously respects said set of vertical setpoints and said set of lateral setpoints until reaching said target altitude that it subsequently maintains, said automatic guidance being able to be interrupted by an action of the pilot of the aircraft. 
     As far as the management of the lateral trajectory within the context of an automated emergency descent is concerned, the following is known:
         from document U.S. Pat. No. 4,314,341, an automated emergency descent to a security altitude. In the case of an emergency descent, this document provides automatically applying a rolling setpoint for a predetermined period of time, followed by folding the wings of the airplane flat. Such a maneuver allows to systematically carry out, in the case of an automated emergency descent, a turn with a defined number of degrees (to the left or to the right) and to deviate from the initial itinerary. If this latter maneuver does allow to deviate from the initially followed air traffic way, it could, for instance, guide the airplane in distress toward an area where no deviation ground is available for allowing a landing or toward an area where the relief is more hilly (higher security altitudes), or even toward air spaces wherein aircrafts are not allowed to fly over or even toward air spaces where the traffic is even denser, which is obviously not wanted when the crew is unconscious (hypoxia); and   from document FR-2,906,921, a method for generating a 3D emergency trajectory for an aircraft, being applicable more specifically to situations requiring an emergency descent to be carried out. The device as described in this document allows to create, in addition to a trajectory in the vertical plane, a lateral trajectory leading to the destination of the flight, taking into account relief and performance constraints. Such a solution, however, requires having available, more specifically, perfectly integrated and reliable data bases of the ground (which is not the case currently). It additionally seems particularly tedious and difficult to be industrially contemplated, with respect to the objective to be achieved, that is allowing an airplane to quickly and perfectly safely reach an altitude, at which the occupants of the airplane are able to autonomously breath and without any additional oxygen supply, and allowing a crew that would initially lost conscience to regain conscience so as to ensure the flight to continue until landing on an airport.       

     Furthermore, the urgent character of situations leading to implementing an emergency descent does not allow the crew to carry out modifications of the active flight itinerary, via the interface of the flight management system, upon the initiation of the emergency descent. Indeed, such modifications would take some time and require a particular attention from the crew. 
     Now, managing the lateral trajectory, along which the emergency descent is carried out, shows to be a particularly important element and should more specifically allow:
         to minimize the risks of collision with aircrafts likely to fly at lower altitudes on the same air traffic way;   to take into account different flight constraints, being considered by the crew until that point (avoidance of areas of turbulences or dangerous meteorological phenomena);   to stay within the protected sector of the followed air traffic way, such sector for which a security altitude is calculated and published on the navigation maps; and   the air control actors to be able to ensure the safety of the airplane in distress and of surrounding aircrafts.       

     Now, as indicated hereinabove, regarding the management of the lateral trajectory upon a non automatic emergency descent, the crew implements simple actions, slaving the autopilot on a selected setpoint, meeting the short term need to deviate from the initial lateral trajectory. Similarly, for aircrafts being already provided with automated systems, managing the lateral trajectory only meets the initial requirement of deviating from the trajectory. 
     Consequently, none of the usual solutions was able to provide and take into account automatically a lateral trajectory able to meet the different operational constraints of an emergency descent maneuver, and this, whatever the initial situation. 
     The present invention aims at solving these drawbacks. It relates to an automatic management method of a lateral trajectory of an aircraft upon an emergency descent, said aircraft having to be laterally guided along an initial lateral trajectory. 
     SUMMARY OF THE INVENTION 
     To this end, according to this invention, said method is remarkable in that, upon triggering the emergency descent, automatically:
         a value of lateral offset is determined being lower than a maximum value and being different from a full value of a few nautical miles (NM); said maximum value being defined so as to ensure that the aircraft stays within the protected sector of the air traffic way, as described hereinunder; and   this value of lateral offset is used for generating an offset setpoint being defined with respect to said initial lateral trajectory and allowing to form an setpoint lateral trajectory that should be laterally followed by the aircraft upon the emergency descent.       

     Thus, the method according to this invention allows the lateral setpoint to be modified automatically upon a failure leading to an initiation of the emergency descent while taking into account the initially followed lateral trajectory. The thus obtained setpoint trajectory has the advantage, as set forth hereinunder, of meeting the operational and regulatory requirements inherent to carrying out an emergency descent, including in the case where the crew lost consciousness as a result of the decompression of the cabin and the cockpit. 
     Said lateral offset value could be determined in different ways within the scope of the present invention. To this end, advantageously:
         said lateral offset value could be a decimal number, having the decimal equal to 5; or   it could also depend on a segment of a flight plan to be followed. Thus, as soon as the flight is prepared, some constraints could be taken into consideration, specific to the contemplated flight for defining the most appropriate strategy, in the case of an emergency descent; or even   it can be determined randomly. This allows to considerably reduce the probability that the selected value is common to several aircrafts flying along the same air traffic way.       

     In a first embodiment, it is considered that the aircraft is laterally guided directly along the initial lateral trajectory (included in the managed mode) upon the initiation of the emergency descent. In this first embodiment, advantageously, said offset setpoint is equal to said value of lateral offset, to which a predetermined offset side is added, preferably the right side. 
     The offset side refers to the right side or the left side, in the direction of which the aircraft is deviated from the value of lateral offset being considered. 
     Moreover, in a second embodiment, it is considered that the aircraft is laterally guided (including in the managed mode) in parallel with the initial lateral trajectory, being laterally offset by an initial value of offset on one side, referred to as the initial side. Such an initial offset could be implemented, for example, in order to avoid an area of meteorological phenomena or dangerous slipstream turbulences, being located along the air traffic way being followed, or even when the crew applies a strategic lateral offset procedure. 
     In this second embodiment, upon initiating or triggering the emergency descent, advantageously:
         the sum of said value of lateral offset and said value of initial offset is calculated; and   as the offset setpoint, the minimum value is selected between said sum and an auxiliary maximum value (allowing to ensure that the aircraft stays within the protected sector of the air traffic way), to said offset setpoint is associated an offset side corresponding to said initial side (so as to avoid the aircraft having to cross the central axis of the air traffic way where the density of the traffic is the highest).       

     Moreover, in a particular embodiment:
         if said initial lateral trajectory is a managed trajectory, the aircraft is guided along the setpoint trajectory, being determined as described hereinabove; and   if said initial lateral trajectory is a selected trajectory, the aircraft is guided along said selected trajectory. This type of navigation is generally used by the crew for a short term management of the flight, and this selected mode of guidance is thus generally temporary. The reasons for which a crew uses a selected mode of guidance instead of a managed mode of guidance could be multiple: instructions from the air control, meteorological avoidance, etc.       

     The present invention therefore allows meeting the operational and regulatory requirements inherent to carrying out an emergency descent, including in the case where the crew lost consciousness as a result of the decompression of the cabin and the cockpit. It more specifically allows:
         to meet the need to deviate from the central axis of the air traffic way being followed and to thus minimize, upon the descent, the probability of a conflict along the same air traffic way;   to continue to take into account different flight constraints, being considered by the crew until that point (avoidance of areas of turbulences or dangerous meteorological phenomena);   the aircraft to take energy upon the interception of the offset trajectory and to thereby improve the descent performances thereof;   the air control actors to be able to ensure the safety of the aircraft in distress and of surrounding aircrafts (predictive trajectory with respect to the initial flight plan known to the control bodies);   to stay within the protected sector of the followed air traffic way, such sector for which a security altitude is calculated and published on the navigation maps; and   the aircraft to continue the flight in parallel to the initial itinerary, along which the crew took care to check, upon the flight preparation, that the deviating grounds able to receive the aircraft could be reached in the case of a depressurization (regulatory operational requirement).       

     The above mentioned method according to this invention, for automatically managing a lateral trajectory of an aircraft upon an emergency descent of an aircraft, is adapted to any type of partially or completely automated emergency descent method. 
     However in a preferred application, this method is used for determining, as a lateral setpoint, an offset setpoint in an automatic controlling process for an emergency descent of an aircraft wherein the following successive operations are carried out: 
     a) a set of vertical setpoints is automatically determined comprising:
         a target altitude setpoint representing an altitude to be reached by the aircraft at the end of the emergency descent; and   a target speed setpoint representing a speed that the aircraft should respect upon the emergency descent;       

     b) a set of lateral setpoints is automatically determined, representing a lateral maneuver to be carried out upon the emergency descent; and 
     c) the aircraft is automatically guided so that it simultaneously respects said set of vertical setpoints and said set of lateral setpoints until reaching said target altitude setpoint. 
     The present invention further relates to a device for automatically managing a lateral trajectory of an aircraft, in particular of a transport airplane, upon an emergency descent. 
     According to this invention, the device is remarkable in that the device includes:
         a lateral offset determination device for automatically determining, upon triggering the emergency descent, a value of lateral offset being lower than a maximum value and being different from a full value of nautical miles; and   an offset setpoint determination device for using automatically this value of lateral offset for generating an offset setpoint being defined with respect to said initial lateral trajectory and allowing to form a setpoint lateral trajectory that should be laterally followed by the aircraft upon the emergency descent.       

     The present invention also relates to a system for automatically controlling an emergency descent of an aircraft, comprising a device of the previous type for automatically managing a lateral trajectory of the aircraft upon such an emergency descent. 
     The present invention further relates to an aircraft, in particular a transport airplane, being provided with a device and/or a system such as mentioned hereinabove. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The FIGS. of the appended drawing will better explain how this invention can be implemented. In these FIGS., like reference numerals relate to like components. 
         FIG. 1  is a block diagram illustrating a device according to this invention. 
         FIGS. 2 and 3  schematically show the flight of an aircraft in a horizontal plane and allow explaining the management of the lateral trajectory upon an emergency descent, in two different situations. 
         FIG. 4  shows the block diagram of a system for automatically controlling an emergency descent of an aircraft, comprising a device according to this invention. 
     
    
    
     DETAILED DESCRIPTION 
     The device  1  according to this invention being schematically shown on  FIG. 1  is intended to automatically manage a lateral trajectory of an aircraft AC, in particular of a transport airplane, upon an emergency descent. A lateral trajectory means the projection on a horizontal plane of the flight trajectory being followed by the aircraft AC. It is considered that the aircraft AC is initially guided, usually, as a function of an initial lateral trajectory TL 0 . 
     According to this invention, the device  1  includes:
         a lateral offset determination device  2  for automatically determining, upon triggering the emergency descent, a value of lateral offset DL being lower than a maximum value DLmax, preferably 5 NM, and being different from a full integer value of nautical miles (1.0, 2.0, 3.0, . . . NM). The maximum value DLmax is defined so as to ensure that the aircraft AC stays in a protected sector of the air traffic way, as set forth hereinunder; and   an offset setpoint determination device  3  being connected via a link  4  to the lateral offset determination device  2  and being formed so as to use, automatically, the value of lateral offset DL so as to generate an offset setpoint CD 1 , CD 2  being defined with respect to the initial lateral trajectory TL 0  and allowing to form a lateral trajectory of setpoint TC 1 , TC 2  that should be laterally followed by the aircraft AC upon the emergency descent.       

     Thus, the device  1  according to this invention allows the lateral setpoint to be modified automatically upon a failure triggering the emergency descent while taking into account the initially followed lateral trajectory TL 0 . The thus obtained setpoint trajectory TC  1 , TC 2  has the advantage, as set forth hereinunder, of meeting the operational and regulatory requirements being inherent to carrying out an emergency descent, including in the case where the crew lost consciousness as a result of a decompression of the cabin and of the cockpit. 
     The lateral offset determination device  2  can determine the value of lateral offset DL in various ways within the scope of the present invention. In particular:
         the lateral offset value DL could be chosen equal to a decimal number, having the tenths position of the decimal equal to 5. As the smallest pitch existing on the flight managing systems is currently 1, such a value, for example, 2.5 NM, allows, on the one hand, to overcome the risks of collision with the other aircrafts flying along the air traffic way and the aircrafts flying offset with respect to the latter, and, on the other hand, to carry out the emergency descent, within the protected sector of this air traffic way; or   the value of lateral offset DL can depend on a segment of the flight plan to be followed. Thus, as soon as the flight is prepared, the crew can take into consideration some constraints being specific to the contemplated flight for defining the most appropriate strategy, in the case of an emergency descent; or even   the value of lateral offset DL can be determined randomly. In this case, preferably, the value of lateral offset DL is defined as a random multiple of the minimum pitch of offset that the aircrafts are able to carry out (considering, for instance, a pitch of 0.1 NM in a near future). This allows to considerably reduce the probability that the selected value is common to several aircrafts flying along the same air traffic way.       

     In a first embodiment shown on  FIG. 2 , the aircraft AC is laterally guided directly along the initial lateral trajectory TL 0  (included in the managed mode) upon triggering the emergency descent in a position P 0 , as a result of a failure being emphasized by a symbol  5 . The lateral trajectory TL of the aircraft AC is initially slaved to the active flight plan of the flight management system (managed lateral trajectory) and no offset is initially inserted. Such a situation could be considered as the nominal case in a cruising phase. 
     This  FIG. 2  further shows the lateral limits  6  and  7  of the protected sector  9  (for which, more specifically, a security altitude is generally calculated and published on navigation maps) of the air traffic way to be followed. The initial lateral trajectory TL 0  is therefore defined according to the central axis  8  of this protected sector  9 . The maximum value DLmax is equal (or optionally lower than) to the distance between the central axis  8  and any one of the lateral limits  6  and  7 . 
     In this first embodiment, the offset setpoint determination device  3  determines an offset setpoint CD 1  (with respect to the central axis  8 ) being equal to the value of lateral offset DL (received from the lateral offset determination device  2 ), with which they associate a predetermined offset side, preferably the right side in the flight direction. Thereby, the lateral trajectory of setpoint TC 1  is obtained, allowing the aircraft AC to avoid another aircraft Al flying in the opposite direction along the central axis  8 . 
     Upon triggering an automated emergency descent function, to be explained hereinunder, the offset setpoint CD 1  to the right is automatically inserted in the active flight plan of the flight management system. The direction of the automatically inserted offset corresponds to the operational practices in service, that requires that a lateral offset occurs to the right by default. 
     As set forth above, the value of lateral offset DL allows, on the one hand, to overcome the risks of collision with the other aircrafts A 1  flying along the air traffic way and with the aircrafts flying offset with respect to the latter, and, on the other hand, to carry out the emergency descent within the protected sector  9  of this air traffic way. 
     Moreover, in a second embodiment, shown on  FIG. 3 , the aircraft AC is laterally guided (including in the managed mode) according to a lateral trajectory TL 1  being parallel to the initial lateral trajectory, defined according to the central axis  10 , being laterally offset by an initial value of lateral offset DL 0  on one side (referred to as the initial side). Such an initial offset could be implemented for avoiding an area  11  of meteorological phenomena. It could also be implemented to avoid an area of dangerous slipstream turbulences, being located along the air traffic way being followed, or even when the crew applies a strategic lateral offset procedure of the SLOP (&lt;&lt;Strategic Lateral Offset Procedure&gt;&gt;) type. 
     In such a case, it is considered that the lateral trajectory of the aircraft AC is slaved to the active flight plan of the flight management system (managed lateral trajectory), but that an offset DL 0  has already been inserted in the latter. 
     In this second embodiment, the offset setpoint determination device  3  includes:
         an offset summing device  10  that, upon triggering the emergency descent, calculates the sum S of the value of lateral offset DL, received from the lateral offset determination device  2 , and of the initial value of offset DL 0 ; and   an offset value comparison and selection device  11  being connected via a link  12  to the offset summing device  10  and selecting, as an offset setpoint CD 2 , the minimum value between the sum S and an auxiliary maximum value (allowing to ensure that the aircraft AC stays in the protected sector  9  of the air traffic way), preferably the value DLmax.       

     With this offset setpoint, the offset value comparison and selection device  11  associate an offset side corresponding to the initial side (so as to avoid the aircraft AC having to cross the central axis  8  of the air traffic way where the density of the traffic is the highest). In the example shown on  FIG. 3 , this side is the left side in the flying direction of the aircraft AC. 
     As an illustration, it is supposed that, in the example of  FIG. 3 , DLmax is equal to 4.5 NM and DL is equal to 2.5 NM. Supposing, in addition, that, in order to avoid the area  11  of dangerous meteorological phenomena, the crew obtained from the local air traffic control body, the authorization to fly in an offset of 3 NM (DL 0 ) to the left, with respect to the central axis  8  of the air traffic way being followed. It is therefore very likely that the other aircrafts A 2  flying along this same way would also have wanted to avoid the disturbed area  11  and they thus also fly in offset. Upon triggering an automatic emergency descent maneuver, the value of the offset is modified for taking into account the offset setpoint CD 2  of 4.5 NM (4.5=Min (3+2.5; 4.5)) to the left. 
     Within the scope of the present invention, if the initial lateral trajectory TL 0  is a managed trajectory, the aircraft AC is guided, upon triggering the emergency descent, along the setpoint trajectory TC 1 , TC 2  being determined as set forth above. 
     Furthermore, in a particular embodiment, if the initial lateral trajectory is a selected trajectory, the aircraft AC is still guided along the selected trajectory, upon a failure occurring, such as a decompression of the cabin for instance. This type of navigation is generally used by the crew for a short term management of the flight, and this selected mode of guidance is thus generally temporary. The reasons for which a crew uses a selected mode of guidance instead of a managed mode of guidance could be multiple: instructions from the air control, meteorological avoidance, for instance. In such a case, no modification of the active flight itinerary of the flight management system is carried out upon triggering a function of emergency descent and the guidance upon the procedure of emergency descent occurs on the current heading (or the current itinerary). 
     The device  1  therefore allows meeting the operational and regulatory requirements being inherent to carrying out an emergency descent, including in the case where the crew lost consciousness as a result of the decompression of the cabin and of the cockpit. It more specifically allows:
         to meet the need to deviate from the central axis of the air traffic way being followed and to thus minimize, upon the descent, the probability of a conflict along the same air traffic way;   to continue to take into account different flight constraints, being considered by the crew until that point (including avoidance of areas  11  of turbulences or dangerous meteorological phenomena);   the aircraft AC to take energy upon the interception of the offset trajectory and to thereby improve the descent performances thereof;   the air control actors to be able to ensure the safety of the airplane in distress and of surrounding aircrafts (predictive trajectory with respect to the initial flight plan known to the control bodies).   to stay within the protected sector  9  of the followed air traffic way, such sector  9  for which a security altitude is calculated and published on the navigation maps; and   the aircraft AC to continue the flight in parallel to the initial itinerary, along which the crew took care to check, upon the flight preparation, that the deviating grounds able to receive the aircraft could be reached in the case of a depressurization (regulatory operational requirement).       

     The device  1  according to this invention further includes an indication device  13  being, for instance, connected to the offset setpoint determination device  3  through a link  14 . Such an indication device  13  allows the pilots to visualize the modifications to the original active flight itinerary and to check the relevance thereof in the case where they remain conscious upon the maneuver. 
     The above mentioned device  1  according to this invention, for automatically managing a lateral trajectory of an aircraft AC upon an emergency descent is adapted to any type of partially or completely automated emergency descent system. 
     However, in a preferred application, such a device  1  is used to form a setpoint lateral trajectory TC 1 , TC 2  that is used by a system  15  for automatically controlling an emergency descent of an aircraft AC. 
     Preferably, such a system  15  for automatically controlling an emergency descent is of the type including, such as shown on  FIG. 4 :
         a triggering device  17  being able to trigger an automatic function of emergency descent;   a controller  18  being connected via a link  19  to the triggering device  17  and being formed so as to implement an automatic function of emergency descent, when it is triggered by the triggering device  17 , automatically carrying out a longitudinal guidance, a lateral guidance and a control of the speed of the aircraft AC; and   a disengaging device  20  being connected via a link  21  to the controller  18  and allowing to control a disengagement of au automatic function of emergency descent being carried out.       

     Such a function of automatic emergency descent thereby allows to bring the aircraft AC back to a breathable altitude (target altitude) and in a stabilized situation, with a view, more specifically, to reanimate (if necessary) the crew and the passengers and to continue the flight. 
     The controller  18  includes:
         a vertical setpoint determination device  22  for automatically determining a set of vertical setpoints, more specifically, comprising:   the target altitude representing the altitude to be reached by the aircraft AC at the end of the emergency descent; and   a target speed representing the speed that the aircraft AC should respect upon the emergency descent;   a lateral setpoint determination device  23  for automatically determining a set of lateral setpoints. Such a set represents a lateral maneuver to be carried out upon the emergency descent; and   an aircraft guidance device  24  for automatically guiding the aircraft, upon triggering an automatic function of emergency descent, so that it simultaneously respects the set of vertical setpoints and the set of lateral setpoints, and this, until reaching the target altitude that it subsequently maintains, as soon as it has reached it.       

     Such a system  15  for automatically controlling an emergency descent could, more specifically, be similar to the system described in document FR-2,928,465 of the Applicant. 
     In such a case, the lateral setpoint determination device  23  includes the device  1  for automatically managing the lateral trajectory of the aircraft AC upon an emergency descent. 
     This system  15  could additionally have more specifically the following characteristics:
         two types of arming could be contemplated: a voluntary arming and an automatic arming.       

     When the crew decides to carry out an emergency descent as a result of a depressurization, a fire alarm or any other reason, they have the possibility to arm the function actuating a dedicated press-button. A logic allows to validate such an arming condition as a function, more specifically, of the current altitude of the aircraft AC. 
     The automatic arming is linked to a depressurization event. It occurs when some criteria involving the air pressure or the variation of the air pressure inside the cabin are met. 
     The arming of the function always precedes triggering thereof;
         the crew keeps at any time the possibility to manually disarm the function, whatever the type of (voluntary or automatic) arming;   two types of triggering are possible as a function of the arming that has preceded.       

     Subsequently to a voluntary arming, the triggering only occurs once the air brakes are completely implemented by the crew. 
     On the other hand, if the arming has been automatic, the triggering also occurs automatically at the end of a count-down initiated upon the arming, if the crew has not reacted by the end thereof. However, if, following a procedure, the crew completely implements the air brakes before the end of the count-down, triggering the function is anticipated with respect to the automatic triggering;
         when the function of automatic emergency descent is triggered, the guidance and the control of the speed of the aircraft occur in the vertical and the lateral planes as follows:   in the vertical plane, the speed to be adopted for carrying out the automatic emergency descent is selected by default by the automatism, so as to minimize the descent time. The crew could freely adjust such a speed upon the maneuver of descent, in order to take into account possible structure damages, and this, without disengaging the function;   the lateral maneuver, carried out simultaneously with the longitudinal maneuver, aims at deviating the aircraft AC from the current itinerary so as to avoid meeting other aircrafts flying on the same itinerary, but at lower altitudes;   getting out of the automatic emergency descent coincides with the capture, then the maintain of the targeted altitude upon the maneuver; and   upon the automated maneuver of emergency descent, the crew can at any time take over on the automatism using usual means: manual action on the joystick, triggering a new mode of guidance of the aircraft AC, disconnection button, adjustment of the speed or of the heading, etc.