Abstract:
According to the invention, a terrain avoidance maneuver is engaged when the alerts of a given pair of alerts are emitted simultaneously, the triggering of one of the alerts depending on the information provided by a first measurement chain and the triggering of the other alert of the pair relying on information provided by a second measurement chain, distinct from and independent of the first chain.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to French Patent Application 0806363, filed Nov. 14, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a terrain avoidance method and system for aircraft, and also to an aircraft provided with such a system. 
     BACKGROUND OF THE INVENTION 
     It is known that numerous aircraft are equipped with a collision alert device making it possible to emit at least one alert signal in the event of a risk of the aircraft colliding with the terrain. Concerning the alert device, it may in particular be a device of TAWS (“Terrain Awareness and Warning System”) type, of EGPWS (“Enhanced Ground Proximity Warning System”) type or of GCAS (“Ground Collision Avoidance System”) type. 
     More precisely, such a TAWS collision alert device is generally capable of emitting the following various types of alerts, the triggerings of which are based on different algorithms and sources of information:
         a first type of alerts, called “reactive” alerts, the triggering of which is based mainly on information provided by a radioaltimeter of the aircraft. There exist various categories of reactive alerts, designated by “Mode  1 ”, “independent Mode  1 ”, “Mode  2 ”, etc., which all detect potential risks of collision of the aircraft with the terrain, but the triggering of which relies on information provided by different measurement chains. Thus, the triggering of the Mode  1  reactive alert depends on the height of the aircraft above the terrain as determined by the radioaltimeter of the aircraft, and also on the vertical speed of the aircraft as provided by the onboard inertial platforms. On the other hand, the triggering of the independent Mode  1  reactive alert depends, on the one hand, on the height of the aircraft above the terrain as determined by onboard GPS (“Global Positioning System”) positioning means and also on a terrain database giving the altitude of the terrain and, on the other hand, on the vertical speed of the aircraft as provided by said GPS means. Moreover, the triggering of the Mode  2  reactive alert is dependent on the height of the aircraft above the terrain as determined by the radioaltimeter and also on the temporal derivative of this height;   a second type of alerts, called “predictive” alerts, the triggering of which relies on information contained in one or more databases (for example of terrain, obstacle, airport runway type, etc.). Predictive alerts such as these are for example called FLTA (“Forward Looking Terrain Alerting”) alerts when the database used is of terrain type, or else PDA (“Premature Descent Alerting”) alerts when the database used is of airport runway type.       

     A terrain avoidance system for a civilian airplane equipped with a collision alert device of TAWS type is known through the document FR-2883403. This system automatically triggers a terrain avoidance maneuver when the collision alert device emits an alert (reactive or predictive) to signal that the airplane risks colliding with the terrain if it maintains its current flight characteristics. Thus, as soon as an alert is emitted, the system acts automatically (without pilot intervention) on the trajectory of the airplane, so as to improve the situation of the latter with respect to the terrain. 
     However, the reactive alerts being emitted after analyzing information provided mainly by the radioaltimeter of the airplane which can only detect the altitude of the terrain under the airplane, it may happen that some of these emitted reactive alerts do not allow genuine anticipation of the risks of the airplane colliding with the relief of the terrain situated ahead of it. 
     Furthermore, the predictive alerts are, for their part, dependent on the reliability and the precision of the databases used to trigger them. Hence, it may transpire that errors are present in these databases, sometimes giving rise to the emission of untimely predictive alerts, whereas in reality there is no substantiated danger of the airplane colliding with the terrain. 
     Moreover, certain terrain geometries are such that they do not bring about the emission of predictive alerts by the TAWS collision alert device. Thus, although the situation of the airplane with respect to the terrain so requires, it may transpire that no terrain avoidance maneuver is triggered if the triggering of such a maneuver relies only on the emission of a predictive alert. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to remedy these drawbacks by refining the aforesaid terrain avoidance system and in particular by reducing the number of untimely triggerings and of non-triggerings of such a maneuver. 
     To this end, according to the invention, the terrain avoidance method for an aircraft equipped with a collision alert device which monitors the flight of said aircraft with respect to the surrounding terrain and which is able to emit at least one alert, when said aircraft risks colliding with the terrain on maintaining its current flight characteristics, according to which method a terrain avoidance maneuver is engaged when engagement conditions are validated, is notable:
         in that said engagement conditions are validated when the alerts of at least one pair of alerts are emitted simultaneously, the triggering of one of the alerts of said pair relying on information provided by a measurement chain and the triggering of the other alert of said pair relying on information provided by another measurement chain; and   in that said measurement chains are distinct and independent.       

     Thus, by using the combination of two alerts (reactive and/or predictive) able to signal one and the same type of risk of the aircraft colliding with the terrain (for example a conflict with the relief, an excessive descent, a premature descent, etc.) but on the basis of information originating from different and independent measurement chains, the number of untimely triggerings of a terrain avoidance maneuver is reduced. 
     Advantageously, when said aircraft is equipped with at least one radioaltimeter, one of the alerts of said pair of alerts can be a Mode  2  reactive alert, the triggering of which depends on the height of said aircraft above the terrain as determined by said radioaltimeter and also on the temporal derivative of this height, and the other alert of said pair of alerts can be an FLTA predictive alert, the triggering of which relies on information contained in at least one topographic database giving the altitude of the terrain. 
     Thus, the combination of a Mode  2  reactive alert and of an FLTA predictive alert makes it possible to detect a potential risk of conflict with the relief of the terrain overflown by the aircraft. 
     As a variant, said aircraft being equipped with at least one radioaltimeter, with inertial platforms and with positioning means of GPS type, one of the alerts of said pair of alerts can advantageously be a Mode  1  reactive alert, the triggering of which depends on the height of said aircraft above the terrain as determined by said radioaltimeter, and also on the vertical speed of said aircraft as provided by said inertial platforms, and the other alert of said pair of alerts can advantageously be an independent Mode  1  alert, the triggering of which depends, on the one hand, on the height of said aircraft above the terrain as determined by said GPS positioning means and also on a topographic database giving the altitude of the terrain and, on the other hand, on the vertical speed of said aircraft as provided by said GPS positioning means. 
     Thus, the combination of a Mode  1  reactive alert and of an independent Mode  1  reactive alert makes it possible to detect an excessive descent of the aircraft liable to lead to a collision with the terrain overflown. 
     Moreover, said engagement conditions can advantageously be validated when at least one of the following alert conditions is satisfied:
         the alerts of a first pair of alerts are emitted simultaneously, the triggering of one of the alerts (for example a Mode  2  reactive alert) of said first pair relying on information provided by a measurement chain and the triggering of the other alert (for example an FLTA predictive alert) of said first pair relying on information provided by another measurement chain;   the alerts of a second pair of alerts are emitted simultaneously, the triggering of one of the alerts (for example a Mode  1  reactive alert) of said second pair relying on information provided by a measurement chain and the triggering of the other alert (for example an independent Mode  1  reactive alert) of said second pair relying on information provided by another measurement chain,
 
said measurement chains for measuring said first pair and said second pair being pairwise distinct and independent.
       

     Thus, the detection of potential risks of collision of the aircraft with the terrain is improved by detecting several different types of risks. The validation of the conditions for engaging said avoidance maneuver can advantageously be accompanied by a broadcasting of a visual and/or audible information cue (for example an alarm) to inform the pilots of the aircraft of a risk of collision with the terrain and warn them of the future triggering of the avoidance maneuver. 
     Moreover, said engagement conditions can be validated when at least one of said alerts is emitted continuously, by said collision alert device, for at least one predetermined duration threshold (for example a few seconds) to ensure that there is a manifest requirement to trigger the terrain avoidance maneuver. 
     Moreover, when said engagement conditions are validated, it is possible to wait a predefined lag before engaging said terrain avoidance maneuver. 
     Thus, in particular when the validation of the engagement conditions is accompanied by the broadcasting of a visual and/or audible information cue, the pilots of the aircraft have the possibility of reacting manually and of extricating the aircraft from a potentially critical situation. 
     In the case where the emission of the aforesaid reactive and predictive alerts is accompanied by the broadcasting of a visual and/or audible warning to warn the pilots of a potential risk of collision, the predefined lag can be reduced with respect to that implemented when there is no warning broadcast, so as to anticipate the terrain avoidance maneuver and thus increase its chances of success. 
     Moreover, the invention also relates to a terrain avoidance system for an aircraft which comprises:
         a collision alert device which monitors the flight of said aircraft with respect to the surrounding terrain and which is able to emit at least one alert, when said aircraft risks colliding with the terrain on maintaining its current flight characteristics; and   an automatic piloting device able to apply a terrain avoidance maneuver to said aircraft automatically when engagement conditions are validated.       

     According to the invention, said terrain avoidance system is noteworthy in that it furthermore comprises a logic device for verifying said engagement conditions, able to verify that the alerts of at least one pair of alerts are emitted simultaneously, the triggering of one of the alerts of said pair relying on information provided by a first measurement chain and the triggering of the other alert of said pair relying on information provided by a second measurement chain, distinct from and independent of said first measurement chain. 
     Preferably, the terrain avoidance system can also comprise comparison means, able to compare the duration of continuous emission of said alerts, emitted by said collision alert device, with a predetermined duration threshold, so that said engagement conditions are validated when at least one of said alerts is emitted continuously for a duration at least equal to said threshold. 
     When said engagement conditions are validated, said automatic piloting device can advantageously wait a predefined lag before engaging said terrain avoidance maneuver. 
     The invention further relates to an aircraft provided with a terrain avoidance system such as mentioned above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures of the appended drawing will elucidate the manner in which the invention may be embodied. In these figures, identical references designate similar elements. 
         FIG. 1  represents, in the form of a schematic diagram, a terrain avoidance system in accordance with the present invention, carried onboard an aircraft. 
         FIG. 2  is a schematic view illustrating a relief avoidance maneuver of the aircraft of  FIG. 1 , in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The terrain avoidance system  1 , in accordance with the present invention and represented schematically in  FIG. 1 , is a terrain avoidance system for an aircraft AC. 
     In this  FIG. 1 , the terrain avoidance system  1  is represented outside the aircraft AC, whereas, in reality, it is mounted onboard the latter. 
     In a standard manner, the system  1  comprises:
         a collision alert device  2 , for example of TAWS type, which monitors the flight of the aircraft AC with respect to the surrounding terrain. This collision alert device  2  is liable to emit reactive and predictive alerts, by way of emission means  3 , when the aircraft AC risks colliding with a relief of said terrain in the event it maintains its current flight characteristics (speed, slope, etc.);   an automatic piloting device  4  of the aircraft AC, connected by way of a link L 1  to the collision alert device  2 . This automatic piloting device  4  is able to determine and to apply a terrain avoidance maneuver to said aircraft AC when engagement conditions (detailed subsequently) are validated. Accordingly, said automatic piloting device  4  can, in a standard manner, determine piloting commands for said aircraft AC (for example a climb command to be applied to the aircraft AC to bring about a gain of height with respect to the relief of the terrain, or else a climb command for the aircraft AC as well as a heading command to be applied to said aircraft AC). Said automatic piloting device  4  can form part of a standard automatic pilot of the aircraft AC;   control means  5  which are connected to the automatic piloting device  4 , by way of the link L 2 . As a variant, they can be integrated, at least in part, into the automatic piloting device  4 . The control means  5  are able to receive the piloting commands originating from said automatic piloting device  4 . They comprise for example calculation means  6  for determining airfoil deflection commands, on the basis of the piloting commands received, and means of actuation  7  of at least one airfoil G of said aircraft AC which receive these airfoil deflection commands, by way of the link L 3 , and move said airfoil G accordingly by means of the link L 4 ; and   indication means  8 , connected to said automatic piloting device  4 , by way of the link L 5 . These indication means  8  can make it possible to forewarn the pilot or pilots of the aircraft AC when an avoidance maneuver is applied to it. Such an indication can consist of a visual information cue displayed on a viewing screen (for example mounted in the flight deck of the aircraft AC) and/or of an audible alarm. Furthermore, they can also allow the broadcasting of a visual and/or audible warning upon the emission of reactive and/or predictive alerts by the collision alert device  2 .       

     According to the invention, the collision alert device  2  furthermore comprises a verification logic device  9  for verifying said engagement conditions, which is connected to said means for emitting alerts  3 . As a variant, the verification logic device  9  can be integrated into the automatic piloting device  4 . 
     In one embodiment of the invention, this verification logic device  9  comprises:
         a first AND logic gate  10  with two inputs and one output. One of the two inputs of the first AND gate  10  is intended to receive an alert signal S 1  representative of a Mode  2  reactive alert, by way of the link L 6 . The other input of the first AND gate  10  is on the other hand intended to receive an alert signal S 2  representative of an FLTA predictive alert, by way of the link L 7 . Thus, the first AND logic gate  10  is able to emit as output, by way of the link L 8 , a first signal S 3  representative of a risk of collision (and more precisely of a conflict with the relief of the terrain overflown by the aircraft AC) when it receives simultaneously, on each of its inputs, the alert signals S 1  and S 2 ;   a second AND logic gate  11  with two inputs and one output. One of the two inputs of the second AND gate  11  is intended to receive an alert signal S 4  representative of a Mode  1  reactive alert, by way of the link L 9 . The other input of the second AND gate  11  is on the other hand intended to receive an alert signal S 5  representative of an independent Mode  1  reactive alert, by way of the link L 10 . Thus, the second AND logic gate  11  is able to emit as output, by way of the link L 11 , a second signal S 6  representative of a risk of collision (and more precisely of an excessive descent of the aircraft AC) when it receives simultaneously, on each of its inputs, the alert signals S 4  and S 5 ; and   an OR logic gate  12  with two inputs and one output. One of the two inputs of the OR logic gate  12  is connected to the output of the first AND logic gate  10 , by way of the link L 8 , and the other is connected to the output of the second AND logic gate  11 , by way of the link L 11 . The OR logic gate  12  is able to deliver as output, by way of the link L 12 , a confirmation signal S 7  representative of a risk of collision of the aircraft AC with the terrain when it receives at least one of the signals S 3  or S 6 .       

     Of course, in a variant of the embodiment of the invention, different combinations of the aforesaid alerts or combinations of some of the aforesaid alerts with other alerts (for example a PDA predictive alert) can be envisaged so as to detect different types of risks of collision. 
     The verification logic device  9  also comprises comparison means  13  for comparing the duration of emission of each of the alerts emitted by the means for emitting alerts  3  with a predetermined duration threshold (for example a few seconds). Thus, the comparison means  13  are able to deliver as output, by way of the link L 13 , a signal S 8  representative of the fact that at least one of the emitted alerts is emitted continuously for at least said duration threshold. 
     It comprises moreover a third logic gate  14  of AND type, with two inputs and one output. It is able to receive as input the signals S 7  and S 8 , by way respectively of the links L 12  and L 13 , and to deliver as output, by way of the link L 1 , a signal S 9  representative of the requirement to engage a terrain avoidance maneuver. 
     According to the embodiment, the conditions for engaging a terrain avoidance maneuver are satisfied when:
         at least one of said following alert conditions is satisfied:
           the Mode  2  reactive alert and the FLTA predictive alert are emitted simultaneously by the emission means  3  of the collision alert device  2 ;   the Mode  1  reactive alert and the independent Mode  1  reactive alert are emitted simultaneously by the emission means  3  of the collision alert device  2 ; and   
           at least one of said alerts (Mode  1 , independent Mode  1 , Mode  2  or FLTA) is emitted continuously for at least the predetermined duration threshold.       

     The validation of the conditions for engaging said avoidance maneuver can be accompanied by the broadcasting of a visual and/or audible information cue (for example an alarm) to inform the pilots of the aircraft of a risk of collision with the terrain and warn them of the future triggering of the avoidance maneuver. 
     When the engagement conditions are validated, the terrain avoidance maneuver, determined by the automatic piloting device  4  of the system  1 , can be applied to the aircraft AC with a predefined lag (for example a few seconds) after the validation of the engagement conditions, so as to allow the pilots to react manually. 
     Moreover, in the case where the emission of the aforesaid reactive and predictive alerts is accompanied by the broadcasting of a visual and/or audible warning to inform the pilots of a potential risk of collision, the predefined lag can be reduced with respect to that implemented when there is no warning broadcast, so as to anticipate the terrain avoidance maneuver and thus increase its chances of success. 
     Represented in  FIG. 2  is an example illustrating a terrain S relief avoidance maneuver by the aircraft AC, in which it is assumed that the avoidance maneuver is triggered automatically as soon as the engagement conditions are satisfied (that is to say the predefined lag is zero). The aircraft AC is represented at the moment of the emission, by the collision alert device  2 , of the signal S 7  ( FIG. 1 ) representative of the requirement to engage an avoidance maneuver. The aircraft AC does indeed risk colliding with the terrain S (mountain  15 ) if it continues its flight unaltered, as illustrated by the trajectory T 1  (broken line) extending the current trajectory T 0 . At this moment, the terrain avoidance system  1  subjects the aircraft AC automatically to an additional climb command exhibiting a slope γ which is added to the slope corresponding to the current trajectory T 0 , so that the aircraft AC then no longer follows the trajectory T 1 , but an additional climb trajectory T 1 ′.