Device for aiding the piloting of an aircraft, especially a rotary-wing aircraft and in particular a helicopter

The present invention relates to a device for aiding the piloting of an aircraft, especially a rotary-wing aircraft, in particular a helicopter. According to the invention, said device (1) comprises first means (2) for determining, on the one hand, the slope and the heading of a preset path of the aircraft and, on the other hand, the slope and the heading of a flight path which the aircraft can take in order to join up with the preset path, and second means (3) for presenting, on a display screen (5), simultaneously a first characteristic sign (S1), whose position on the display screen (5) is representative of the slope and of the heading of the preset path and a second characteristic sign (S2), whose position on the display screen (5) is representative of the slope and of the heading of the flight path.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a device for aiding the piloting of an
 aircraft, especially a rotary-wing aircraft and in particular a
 helicopter.
 More precisely, it relates to a device making it possible to present
 information cues to a pilot of the aircraft, which are capable of
 providing the latter with indications regarding the exact position of said
 aircraft with respect to the surrounding space, especially in poor
 visibility, or even in the complete absence of visibility. Such cues are
 generally very useful, or even indispensable, when approaching a landing
 strip.
 2. Description of Related Art
 On aeroplanes, especially civil transport aeroplanes, a device for
 presenting cues of this type is known, and is associated with a guidance
 beam or axis (for example of the "ILS" type) transmitted from the ground
 in proximity to a landing strip and which is intended to guide the
 aeroplane during the approach to the strip, with a view to landing.
 To this end, the known device provides the pilot(s) with two cues presented
 on a display screen, namely:
 a first cue ("LOC") comprising an arrow which the pilot must orient in
 terms of heading on the guidance axis and a deviation bar, whose lateral
 displacement on the screen with respect to the arrow is representative of
 the offset between the guidance beam and the position of the aircraft; and
 a second cue ("GLIDE") which indicates the relative vertical position
 between said guidance axis and the position of the aircraft.
 These cues relate to values which are proportional to the angle between
 said guidance axis and the line of aim of the point at which the aircraft
 expects to touch down.
 Such a known device for presenting cues is well suited to an approach made
 in a straight manner, with a fixed and relatively gentle slope, for
 example 3.degree., as is generally the case for an aeroplane.
 However, when the approach has to be made according to a preset path which
 exhibits several successive segments exhibiting different headings and
 slopes, as well as relatively large values of slope of up to 15.degree.,
 as may be the case for a helicopter, the aforesaid cues are not
 appropriate and are moreover inadequate.
 This is because a cue in angular form obliges the pilot to pass through a
 particular point in space. Now, such a constraint is not useful in respect
 of the various points of transition of a path having a plurality of
 segments, with the exception of the last segment terminating in the
 landing strip.
 Moreover:
 an approach at gentle slope, around 3.degree., is not well suited to the
 deployment capabilities of helicopters; and
 helicopters must be able to land on unprepared zones which are not
 furnished with guidance beams.
 Consequently, the known and aforesaid device is not satisfactory for
 guidance along complex preset paths.
 The document FR-2 666 428 describes a process for displaying pilot-aid
 symbols on a screen aboard an aircraft. This known document teaches in
 particular the displaying, according to a three-dimensional
 representation, of the approach segments situated on the path which the
 aeroplane must travel. To this end, the current segment of travel, as well
 as the next segment, are displayed respectively according to two
 parallelepipeds, in perspective.
 Furthermore, the document FR-2 752 051 discloses a device for assisting the
 guidance of a vehicle over a path. This known device comprises means for
 determining the slope and the heading of a flight path which the vehicle
 can take in order to join up with a preset path and means for displaying a
 guidance window, into which must be brought and wherein must be maintained
 a symbol representing the speed vector of the aeroplane, during the
 guidance of the aeroplane. This guidance window is centered on a preset
 point which represents the direction of guidance to be followed by the
 aircraft, in terms of heading and slope. The coordinates of this preset
 point are defined by the point of intersection between a sphere (of fixed
 radius) centered on the position of the aeroplane and the preset path.
 However, the pilot has no information regarding the discrepancies of slope
 and of heading between the preset and flight paths, and this may be very
 detrimental in certain flight conditions.
 SUMMARY OF THE INVENTION
 The object of the present invention is to remedy these drawbacks. It
 relates to a pilot-aid device capable of presenting to a pilot of an
 aircraft, and especially of a helicopter, cues enabling him to carry out,
 in a simple and efficient manner, piloting along a complex preset path,
 exhibiting for example a plurality of segments of different slopes and
 headings.
 To this end, according to the invention, the device for aiding the piloting
 of an aircraft, especially a rotary-wing aircraft, in particular a
 helicopter, comprises:
 first means for determining the slope and the heading of a flight path
 which said aircraft can take in order to join up with a preset path; and
 second means for presenting, on a display screen, a second characteristic
 sign, whose position on said display screen is representative both of the
 slope and of the heading of said flight path,
 is noteworthy in that said first means moreover determine the slope and the
 heading of the preset path of the aircraft, in that said second means
 moreover present, simultaneously with said second characteristic sign, a
 first characteristic sign, whose position on said display screen is
 representative both of the slope and of the heading of said preset path,
 and in that said slope and said heading of the flight path are such that
 the distance between said first and second characteristic signs on said
 display screen is dependent:
 according to a first direction representing the heading, on the distance in
 a horizontal plane between the position of the aircraft and said preset
 path; and
 according to a second direction representing the slope, on the distance in
 a vertical plane between the position of the aircraft and said preset
 path.
 Thus, by virtue of the invention, the pilot(s) is (are) presented not with
 cues in angular form, but with cues representative of distance offsets,
 specified below, thereby making it possible to remedy the aforesaid
 drawbacks.
 Moreover, by virtue of the invention, cues are also provided which relate
 to the slope, both of the guidance path and of the preset path, this not
 being the case for the aforesaid device disclosed by the document FR-2 752
 051. These cues provided simultaneously are necessary in particular when
 various segments of the preset path exhibit arbitrary slopes which may
 have high values. This is because, when the aircraft is for example
 situated below the preset path, it is necessary to know the slope of the
 latter so as to bring about a greater descent slope of the aircraft, so
 that it can join up with this preset path. Moreover, by virtue of the
 invention, the discrepancies in heading and in slope between said preset
 and guidance paths are deduced easily and rapidly by a pilot from the
 distances between said first and second characteristic signs, along said
 first and second directions respectively.
 Furthermore, the device in accordance with the invention is not bound by a
 guidance axis and can be used anywhere.
 Furthermore, preferably:
 the discrepancy in slope p and the discrepancy in heading r between said
 preset and flight paths satisfy the following relations which depend on
 the distances dv and dh in meters m respectively in the vertical plane and
 the horizontal plane between the position of the aircraft (H) and said
 preset path (T):
 if dv&gt;25 m, p is proportional to d
 if dv&lt;25 m, p is proportional to dv/5
 if dh&gt;25 m, r is proportional to dh
 if dh&lt;25 m, r is proportional to dh/5; and
 when the preset path comprises a plurality of successive segments, the
 current segment, used to determine the slope and the heading of said
 preset path, is advantageously the one which is closest to the position of
 the aircraft.
 It will be noted that within the context of the present invention, said
 display screen can be either a usual so-called "head down" screen provided
 on the aircraft's instrument panel, or a usual so-called "head up" screen,
 that is to say a transparent screen through which the pilot can see the
 exterior. This "head up" screen can be fixed with respect to the cabin or
 fixed with respect to the pilot's head.
 In the case of a "head up" screen, the device in accordance with the
 invention makes it possible to render compatible a so-called "natural"
 piloting carried out by pilots and a so-called "instrument-based" piloting
 carried out by automatic piloting means, in the form of a single assisted
 natural piloting mode. The assistance so provided is incorporated into the
 natural piloting so as to enhance it without disturbing it.
 Furthermore, advantageously, said second means moreover present, on said
 display screen:
 a third characteristic sign, for example in the form of straight lines,
 linking together said first and second characteristic signs, thus making
 it possible to depict to the pilot the limits of the aerial course which
 he must follow in order to join up with said preset path and therefore
 eases the understanding and evaluation of the various cues presented;
 and/or
 a fourth characteristic sign indicating the direction in which the aircraft
 is flying, thereby easing and simplifying the piloting since it is then
 sufficient for the pilot to direct the aircraft so that said fourth
 characteristic sign is and remains superimposed on said second
 characteristic sign indicating the advised flight path; and/or
 a fifth characteristic sign indicating the direction of a point in space
 which will be reached by said aircraft within a specified duration, thus
 making it possible to join up with the preset path within said specified
 duration if the pilot carries out a piloting making it possible to
 superimpose this fifth characteristic sign on said second characteristic
 sign.
 Furthermore, in order to yet further ease the understanding of the cues
 presented, advantageously, said second characteristic sign exhibits the
 shape of an ellipse, of variable geometric configuration, that is to say
 of variable shape and/or variable size. The shape of the ellipse can thus
 in particular vary, corresponding to the perspective view, from the
 aircraft, of a circle centered on the preset path. As regards the size of
 the ellipse, it can for example vary as a function of the actual speed of
 the aircraft, thus making it possible to provide the pilot with a speed
 cue in addition.
 Another speed cue can be obtained by presenting a sixth characteristic
 sign, for example using dashes, which is associated with said second
 characteristic sign and is spaced therefrom by a distance dependent on the
 difference between the actual speed of the aircraft and a preset speed.
 This provides a simple and effective aid to piloting since it is then
 sufficient to superimpose said sixth and second characteristic signs, so
 as to bring the actual speed of the aircraft to said preset speed and thus
 obtain suitable piloting.
 Furthermore, advantageously, said second means can present, moreover, on
 said display screen;
 at least one auxiliary sign which is intended to deputize for a
 characteristic sign which has exited the field of view represented on said
 display screen and which is situated, according to a field exit direction,
 at a predetermined distance from the actual position of said exited
 characteristic sign; and/or
 a first additional sign warning of a modification, within a predetermined
 duration, of the preset path, for example a change of heading and/or of
 slope. This first additional sign disappears when said first
 characteristic sign is superimposed thereon, that is to say when the path
 modification has been made; and/or
 a second additional sign indicating the position of the ground relative to
 a zone in space connected with said aircraft. This second additional sign
 is preferable associated with the fourth characteristic sign indicating
 the direction in which the aircraft is flying.
 Moreover, in a preferred embodiment, this second additional sign is an arc
 of a circle, whose size varies as a function of position, and which is
 centered on said fourth characteristic sign. The appropriate piloting then
 assumes that said second additional sign becomes and remains a
 semi-circle.

DETAILED DESCRIPTION OF THE INVENTION
 The device 1 in accordance with the invention and represented
 diagrammatically in FIG. 1C is intended to provide an aid for the piloting
 of an aircraft, especially a rotary-wing aircraft, in this instance a
 helicopter H, by presenting cues which are useful in said piloting.
 According to the invention, said device 1 which is on board said helicopter
 H comprises to this end:
 means 2, comprising for example sensors and computers, for determining in a
 usual manner, on the one hand, the slope and the heading of a preset path
 T of the helicopter H and, on the other hand, the slope and the heading of
 a flight path which said helicopter H can take in order to join up with
 said preset path T; and
 means 3 linked by a link 4 to said means 2 and presenting, on a display
 screen 5, simultaneously a characteristic sign S1, for example a rhomb,
 whose position on said display screen 5 is representative both of the
 slope and of the heading of said preset path T and a characteristic sign
 S2, for example a circle, whose position on said display screen 5 is
 representative both of the slope and of the heading of said flight path.
 In order to clearly highlight the relative position between the helicopter
 H and the preset path T, and to clearly explain the arrangement of the
 various signs on the screen 5 of FIG. 1C, we have represented:
 in FIG. 1A, the projection onto a horizontal plane of the path T, of the
 position of the helicopter H and of the speed vector V of the latter, said
 speed vector V and said path T forming an angle a; and
 in FIG. 1B, the projection onto a vertical plane OXZ of said elements T, H
 and V.
 The preset path T comprises a plurality of successive segments L1 to L4 of
 different slopes and headings. Segment L3 which is the closest, a
 horizontal distance dh and a vertical distance dv from the current
 position of the helicopter H, is regarded as the current segment, with
 respect to which the processings and presentations of cues specified below
 are carried out.
 As may be seen in FIG. 1C, said display screen 5 also indicates:
 a horizon line 6;
 a slope (or pitching) scale 7;
 a heading (or azimuth) scale 8; and
 a sign 9 illustrating the orientation of the speed vector V of the
 helicopter H and therefore indicating the direction in which the latter is
 flying.
 In the vertical plane (FIG. 1B), the segment L3 exhibits a slope P, for
 example -8.degree.. The sign S1 is therefore placed on the scale 7 at said
 slope P.
 Moreover, in the horizontal plane (FIG. 1A), the segment L3 is oriented in
 terms of heading at a.degree., for example 5.degree., with respect to the
 heading of the helicopter H. The sign S1 is therefore placed on the scale
 8 at a distance D representative of the value a.degree., as represented in
 FIG. 1C.
 Said sign S1 therefore indicates directly the slope and the heading of the
 preset path T.
 This sign S1 may possibly be accompanied by an indication regarding the
 origin of the preset path T, for example FMS as represented in FIG. 1C, to
 indicate that the path is calculated by a navigation computer on board the
 helicopter H, or ATC, to indicate that it is calculated by aerial control
 means provided on the ground and that it is transmitted by radio link to
 the helicopter H or to the means 2.
 Additionally, the sign S2 provides an indication enabling the pilot to join
 up with the preset path T according to a predefined strategy. This sign S2
 is centered on the screen 5 at a slope and at a heading (or at a slope
 discrepancy and at a heading discrepancy with respect to the sign S1)
 which are computed by the means 2 as being optimal given the distance
 (vertical distance dv and horizontal distance dh) which separates the
 helicopter H from the current segment L3 of the preset path T and given
 the manner in which it is desired that the helicopter H join up with said
 preset path T (soft approach or more direct approach).
 By way of example, in the vertical plane OXZ, the helicopter H which is
 situated above the segment L3 will have to take a slope (for example -15)
 greater than the slope P (for example -8) of this segment L3, in order to
 be able to join up with the latter.
 A similar situation occurs in the horizontal plane in respect of the
 heading values.
 According to the invention, on the screen 5:
 the discrepancy in slope p computed by the means 2, between the signs S1
 and S2, is dependent on the vertical distance dv; and
 the discrepancy in heading r between these signs S1 and S2 is dependent on
 the horizontal distance dh.
 Within the context of the present invention, said means 2 may use various
 laws, especially as a function of the chosen approach strategy, to compute
 the values p and r from the values dv and dh.
 However, in a preferred embodiment, these values are computed from the
 following relations:
 if dv&gt;25 m, p is proportional to d
 if dv&lt;25 m, p is proportional to dv/5
 if dh&gt;25 m, r is proportional to dh
 if dh&lt;25 m, r is proportional to dh/5.
 Thus, by virtue of the invention, the piloting of the helicopter H consists
 simply in making the sign 9 indicating the direction in which the
 helicopter H is flying coincide with the sign S2, thereby enabling said
 helicopter H to join up with and follow the preset path T. Naturally, when
 it joins up with this path, the signs S1 and S2 are centered on the same
 point of the screen 5.
 According to the invention, the screen 5 moreover comprises a sign S3
 formed of two lines linking the center of the sign S1 to the ends of the
 sign S2 (that is to say to tangents to the circle forming said sign S2).
 This sign S3 gives a simple representation in perspective which suggests
 the course to be followed in order to join up with the preset path T.
 FIGS. 2A and 3A, 2B and 3B, 2C and 3C represent figures which are similar
 to FIGS. 1A, 1B and 1C respectively, but which illustrate situations and
 positions where there is a difference between the helicopter H and the
 preset path T.
 In FIGS. 2A, 2B and 2C, the situation is close to that illustrated in FIG.
 1C, the helicopter H is situated above the segment L3, but it exhibits a
 slope which is slightly greater than that of this segment.
 FIGS. 3A, 3B and 3C illustrate a situation in which the helicopter H is
 situated inside a "preset tunnel" 10 representing a tubular shape centered
 on the current segment L3. Moreover the slope of the helicopter H is very
 close to that of this segment L3.
 Consequently, the sign S1 is situated inside the sign S2, the signs S1, S2
 and 9 being almost superimposed.
 Within the context of the present invention, said signs S1 and S2 may
 exhibit different shapes.
 By way of example, said sign S1 may in particular be:
 a rhomb, as represented in FIGS. 1C and 3C;
 a point which is not represented, thereby making it possible to prune the
 presentation on the screen 5; or
 a symbol illustrating a landing strip, as represented in FIG. 2C. This
 symbol can, for example, be presented solely when guidance is carried out
 with respect to the last segment L4 of the preset path T, which culminates
 at the landing point, so as to alert the pilot of the imminence of
 landing.
 According to the invention, the means 2 can moreover determine a prediction
 speed, which can be equal to the sum of the current speed and of a value
 proportional to the derivative of the speed.
 To illustrate this prediction speed, the screen 5 comprises a sign S4
 represented in FIG. 8 and associated for example with the sign 9. This
 sign S4, for example an arrow, indicates the point in space which will be
 reached by the helicopter H within a specified duration, if it maintains
 its current accelerations constant.
 Said sign S4 therefore enables the helicopter H to join up with the path T
 within said specified duration if the guidance superimposes the tip of the
 arrow S4 on the center of the circle S2.
 This sign S4, which is merged with the sign 9 when the helicopter is flying
 in a straight line, can take a plurality of shapes, especially that of an
 arrow.
 By way of example, it can take the shape:
 of a two-dimensional symbol which is fixed and is situated at a distance in
 front of the helicopter H, corresponding to a specified duration of
 flight;
 of a symbolic object representing a three-dimensional object in space,
 which is deformable as a function of the angle of sight and which is
 situated at a distance in front of the helicopter, corresponding to a
 specified duration of flight; or
 of a symbolic object representing a three-dimensional object in space,
 which is deformable as a function of the angle of sight, but which is also
 spread over two distances corresponding to two specified durations of
 flight.
 In the case where the preset path exhibits a curve, a corrected prediction
 speed is formed so as to maintain the helicopter H on said preset path, by
 carrying out the aforesaid piloting mode.
 To this end, the prediction speed is corrected by a value which is
 dependent on the centrifugal force due to the turn imposed.
 With a prediction speed thus corrected, as soon as the helicopter H enters
 the turn, the sign S4 is shifted outside the turn by the defined
 correction and the pilot must bring said sign S4 back onto the sign S2 in
 order to carry out the piloting in accordance with the invention. On
 exiting the turn, the correction is deleted.
 This correction therefore gives a piloting indication enabling the pilot to
 start turning in time, without waiting to be off the preset path to
 receive the order to return thereto.
 Additionally, the sign S2 can be an ellipse of variable configuration, that
 is to say of variable shape and/or variable dimension.
 In a first embodiment, the shape of the ellipse varies as a function of the
 angle of rotation of the circle formed by the preset tunnel 10 represented
 in FIG. 3A, with respect to the axis of aim of the center of this circle
 from the helicopter H.
 Additionally, during piloting, when the speed of the helicopter H is
 relatively low, a side wind may subject the latter to a sides lip which
 may in particular cause the signs 9 and S4 to exit the field of view
 represented on the display screen 5. The pilot then no longer sees the
 sign 9 or S4 which he must bring to the center of the circle S2 for the
 piloting in accordance with the invention.
 To remedy this drawback, in the example represented in FIG. 4 where the
 signs 9 and S2 exit sideways with respect to the plane of the screen 5,
 that is to say in the direction of heading, two auxiliary signs 9ad and
 S2ad are provided, situated on the screen 5, at a predetermined distance
 Ra, from said signs 9 and S2 respectively, along said direction of
 heading, but at the same slope as said signs 9 and S2.
 The piloting then consists in bringing said auxiliary sign 9ad into
 superposition with said auxiliary sign S2ad.
 It will moreover be noted that:
 the distance Ra is, for example, equal to the length of the radius of the
 circle forming the screen 5;
 when the guidance is carried out, not by means of the sign 9, but by means
 of the sign S4, it is possible to provide a similar additional sign
 associated with said sign S4; and
 when exit takes place vertically, additional signs are provided, offset in
 the slope direction, at the same heading as the signs S2, 9 and/or S4
 which have exited.
 Furthermore, as may be seen in FIG. 5, there may be provision according to
 the invention for an extra additional sign S1ad intended to warn the pilot
 of a next change of the current segment of the preset path T, for example
 the next passage from the segment L3 to the segment L4.
 This sign S1ad is set in place at a specified duration before the scheduled
 change.
 As the point of change is approached, that is to say the present position
 on the current segment (L3 for example) approaches the next segment (L4
 for example), the sign S1 approaches said additional sign S1ad.
 When the change is made, the two signs S1 and S1ad are superimposed and
 said sign S1ad disappears.
 Furthermore, an extra additional sign S3ad illustrating the course to be
 followed can be associated with said additional sign S1ad.
 By way of example, the various auxiliary or additional signs 9ad, S1ad,
 S2ad and S3ad can exhibit the same shape as the signs 9, S1, S2 and S3
 with which they are associated, but with a representation as dashes.
 Additionally, to aid the pilot in controlling and maintaining the
 helicopter at an appropriate speed, a sign S5 is provided which is
 centered around the sign 9 indicating the direction of the speed vector V
 of the helicopter H, as represented in FIG. 7B.
 Its shape is designed to inform the pilot of the relative position of the
 ground 11 with respect to a region in space connected with the helicopter
 H, by displaying directly the intersection with the ground of a conical
 volume 12 centered on the speed vector V. This solution assumes that the
 means 2 know the distance from the ground in this direction, either with
 the aid of a radar or laser telemeter, or with the help of a radio probe
 and/or a digitized terrain database.
 Said sign S5 is composed of a circle of fixed diameter centered on the sign
 9 and of a chord 14 of this circle, whose height is dependent on the
 intersection between the cone 12 and the ground 11.
 The cone 12 exhibits a length kV, k being a value which is for example
 proportional to the current slope of the helicopter H and V being the
 speed of the helicopter H.
 Naturally, as long as said cone 12 does not touch the ground 11 (FIG. 6A),
 the sign S5 does not appear (FIG. 6B). In FIG. 6B, the position which said
 sign S5 would take if it were present has been indicated by dashes.
 On the other hand, when the cone 12 touches the ground 11, as illustrated
 by a part 13 drawn thicker in FIG. 7A, said sign S5 appears on the screen
 5 (FIG. 7B). The height of the chord 14 in the circle forming the sign S5
 therefore depends on the size of said part 13.
 This sign S5 enables in particular the pilot to descend with a given slope
 at any speed and to retain this speed until the cone 12 touches the ground
 11, this being depicted by the chord 14 or intersection bar which will
 climb in the circle. He will then have to slow down until this chord 14 is
 level with the sign 9, hence until it becomes a diameter of the circle
 illustrated by dashes in FIG. 6B, and will have to Be control his speed in
 order to retain the chord 14 at this level.
 It will be noted that when the length of the cone 12 depends on the slope,
 the aforesaid symbology has the advantage of aiding the pilot by imposing
 a maximum speed on him, as a function of the slope which he actually
 takes, even if this slope is not that of the preset path T. This amounts
 to advising him of a descent slope, and for any slope taken, of a maximum
 speed not to be exceeded, even if the slope followed is not the advised
 slope. The device 1 therefore imposes a speed on him only when proximity
 to the ground 11 so demands, that is to say when it is vital to slow down
 so as not to hit the ground 11.
 This symbology (sign S5) therefore represents important advantages, not
 only to ease piloting, but also for safety.
 Additionally, if the sign S2 is an ellipse of size and position varying
 with speed, provision may be made, as represented in FIG. 8, for a second
 ellipse (sign S6) drawn dashed, which is also centered on the course to be
 followed, but at a distance from the ellipse S2 which is dependent on the
 difference between the actual speed of the helicopter H and a preset speed
 determined by the means 2.
 Piloting then consists in superimposing the ellipse S6 on the ellipse S2
 and therefore in bringing the actual speed of the helicopter H to said
 preset speed.
 It will be noted that in the example represented in FIG. 8, as the ellipse
 S6 is of larger size than the ellipse S2, the actual speed of the
 helicopter H is greater than its preset speed.
 It will furthermore be noted that the means 2 can be associated with a
 usual so-called "head down" screen or with a usual so-called "head up"
 screen.
 The "head down" display has the advantage of adapting easily to numerous
 helicopters or aeroplanes already equipped with electronic screens, of
 "EFIS" type. These screens present secure piloting cues. However, their
 use near to the ground constrains the pilot to abandon sight of the
 exterior so as to look at his dashboard, this generally being deemed by
 him as incurring a risk. The cues presented must therefore be accurate,
 clear and easily interpretable so as to compensate for the loss of
 exterior sight, this naturally being the case with the device 1 in
 accordance with the invention.
 On the other hand, the advantage of a "head up" screen is that it renders
 compatible a "natural" piloting and an "instrument-based" piloting, by
 associating these two modes with a single assisted natural mode of
 piloting. The assistance proposed is incorporated into the natural
 piloting so as to enhance it, while disturbing it as little as possible.
 The "fly by instruments"/"fly by sight" transition is thus eased.
 It will be noted that the use of a head up visor ("HUD":"Head Up Display")
 in a civil helicopter is an alternative solution to automatic piloting
 during the approach. In principle, the pilot replaces the servocontrols,
 the HUD/pilot pair fulfilling the function of the automatic piloting.
 During the approach to the ground, the "head up" guidance exhibits two
 fundamental benefits for the pilot:
 to be able to acquire a cue regarding the helicopter without switching his
 gaze to the interior of the cabin; and
 to obtain a flight cue superimposed on the real world and in correspondence
 therewith, thereby exempting said pilot of the need to make the
 intellectual correlation between the symbolized cue and the exterior
 visual markers.
 The "head up" display makes it possible moreover, if necessary, to improve
 the perception of the exterior world, through the presentation of an
 artificial visual image, thereby enabling the pilot to regain the concepts
 of relief and space in a natural way.