Method and device for assistance with the piloting of an aircraft

The present invention relates to a method of assisting the piloting of an aircraft at low altitude. According to the invention, a hypothetical curve associated with an optimal theoretical trajectory for clearing an obstacle is calculated and, in the field in front of the aircraft: all the obstacles situated in a search area are determined; the tops of the obstacles determined are compared with the hypothetical curve; an obstacle is defined as a dangerous obstacle if the top of the obstacle is situated highest with respect to the hypothetical curve; and the coordinates of the top of the dangerous obstacle are communicated.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and a device for assistance with 
the piloting of an aircraft, making it possible, in particular, to 
determine the obstacles which are dangerous for said aircraft in its 
flight space. 
In flight at low altitude, the pilot of an aircraft, in particular of a 
helicopter, sees in front of him, at low incidence angles, a very large 
number of points on the ground or of obstacles which he generally locates 
well in direction but not always in distance. 
The intellectual processing of this set of poorly located points often 
represents a heavy and difficult workload, all the more so since 
visibility may be poor and the obstacles too thin to be distinguished from 
afar. 
These drawbacks are particularly perceptible when the pilot is using night 
vision devices or cameras which give an image without color or relief in a 
reduced field. 
SUMMARY OF THE INVENTION 
The object of the present invention is to remedy these drawbacks. It 
relates to a method of assistance with the piloting of an aircraft, in 
particular for flights at low altitude, making it possible to detect the 
obstacles which are dangerous for said aircraft. 
To this end, according to the invention, the method of assisting the 
piloting of an aircraft at low altitude, making it possible to detect the 
obstacles in relief which are dangerous for said aircraft in the field in 
front of the latter, is noteworthy in that: 
on the basis of the maneuvering capabilities of the aircraft, a 
hypothetical curve is calculated, related to said aircraft and associated 
with an optimal theoretical trajectory for clearing an obstacle in a 
vertical plane; and 
in the field in front of the aircraft, the following steps are performed: 
a) all the obstacles situated in a search area are determined; 
b) the tops of said obstacles determined are compared with said 
hypothetical curve; 
c) that obstacle is defined as dangerous whose top is situated highest with 
respect to said hypothetical curve; and 
d) the coordinates of said top of this dangerous obstacle are communicated. 
Thus, by virtue of the invention, it is possible, among all the potential 
obstacles, visible or not to the pilot, to identify the obstacle 
presenting an immediate danger and, if it is on the trajectory of the 
aircraft, necessarily giving rise to an immediate avoidance maneuver on 
the part of the pilot. 
Moreover, this reduction in the number of obstacles to be taken into 
account considerably eases the task of observation by the pilot. 
Advantageously, the field in front of the aircraft is subdivided into 
angular sectors and, in each of said angular sectors, said steps a, b, c 
and d are performed. Thus, the pilot has available an aid not only to 
vertical avoidance, in a defined direction, but also an aid for lateral 
avoidance, on the basis of the information obtained from each of said 
angular sectors. 
Advantageously, furthermore, in each of said angular sectors, the 
coordinates of a reference point situated at a predefined distance above 
the top of the dangerous obstacle are calculated, said predefined distance 
corresponding to the minimum desired height for overflying an obstacle. 
Thus, by presenting said reference point to the pilot, he is shown the 
point in space through which he has to pass in order to perform the most 
advantageous overflight of the dangerous obstacle, that is to say as low 
as possible and presenting no risk of collision. Obviously, said minimum 
overflight height is adapted to the characteristics of the aircraft and to 
the type of mission to be carried out. By way of example, a height of 20 
meters may be envisaged for a helicopter. 
In order to present the preceding results as effectively as possible to the 
pilot of the aircraft, characteristic signs are superimposed on the image 
of the external landscape, which are arranged at the points in space 
corresponding to said determined coordinates. 
Thus, when these coordinates are those of the reference points, the pilot 
directly sees the point in space through which he has to pass in order to 
perform an optimal overflight. When these coordinates are those of the 
tops of the dangerous obstacles, this superimposition makes it possible to 
reveal these dangerous obstacles which are not always clearly visible 
directly, and the ease of recognition of which often varies with the 
metereological conditions. This superimposition consequently offers the 
pilot an effective aid to observation, by facilitating recognition of the 
dangerous obstacles, above all in conditions of poor visibility. 
Advantageously, a piloting curve is determined, corresponding to a smoothed 
curve linking all said reference points and said piloting curve, as well 
as a characteristic sign indicating the position of the speed vector of 
the aircraft collimated at infinity are superimposed on the image of the 
external landscape. 
This piloting curve thus divides the image of the external landscape into 
two parts, a lower part which lies between the ground and said piloting 
curve and which the aircraft must avoid at all costs, by reason of the 
presence of said dangerous obstacles, and an upper part which is situated 
above said piloting curve and which includes no dangerous obstacle. It 
will be noted, moreover, that, in order to perform a flight both as low as 
possible and without danger for the aircraft, it is sufficient for the 
pilot to steer the aircraft towards a point of said piloting curve since, 
by definition, this piloting curve links together the reference points 
which are defined, as described previously, by taking account of the 
minimum height for overflying an obstacle. 
Consequently, in order to perform an optimal overflight, it is sufficient 
for the pilot to steer the aircraft so as to superimpose on said piloting 
curve said sign which is characteristic of the speed vector, which, 
obviously, indicates the direction of flight of the aircraft. 
By virtue of the invention, it is thus possible to perform directional 
guidance by superimposition of the speed vector and of the piloting curve. 
It will be noted, moreover, that said piloting curve will implicitly 
furnish a distance indication, since the angular offset between the top of 
an obstacle and the piloting curve is proportional to the distance between 
this obstacle and the aircraft. 
In order not to overload the image of the external landscape, sent to the 
pilot, advantageously only the central part of said piloting curve 
including the points in space which are accessible to the aircraft with a 
predefined limit roll are superimposed on said image of the landscape. 
This then makes it possible, moreover, to indicate clearly to the pilot 
the part of space which he is capable of reaching from his current 
position. Moreover, the continuous presence of all of this accessible part 
of the piloting curve allows the pilot to keep a check on the consistency 
of said piloting curve with the r al obstacles seen. It will be noted that 
the value of said limit roll depends, in particular, on the type of 
aircraft in question. For a helicopter, for example, a value of 45.degree. 
may be used. 
It will be noted that the piloting, by super-imposition of the speed vector 
and of the piloting curve, does not always make it possible to pass 
exactly horizontal, at the minimum overflight height, above the obstacle 
in question. In order to remedy this drawback, while allowing the aircraft 
more rapidly to reach the height sought in order to be placed in 
horizontal position before the overflight, it can be guided with the aid 
of a piloting curve offset towards the aircraft, determined in a way 
identical to said piloting curve, by using, in the calculation, modified 
coordinates of the dangerous obstacles, obtained in each case by the 
reduction according to the same ratio, for example one quarter, of the 
horizontal component of said coordinates. 
Advantageously, moreover, an auxiliary piloting curve is calculated, 
determined in a way identical to said piloting curve, by taking account of 
the obstacles situated at a longer distance from the aircraft than said 
dangerous obstacles, and said auxiliary piloting curve is superimposed on 
the image of the external landscape, for example by way of a trace in 
broken lines, so as to differentiate it from said piloting curve. This 
auxiliary piloting curve makes it possible to reveal the obstacles which 
are situated further than the dangerous obstacles of the piloting curve 
and which are to be taken into account after said dangerous obstacles have 
been cleared. This information may, in particular, assist the pilot in the 
choice of a possible lateral diversion. 
It is also possible to use said auxiliary piloting curve differently, 
without superimposing it on the image. It is possible, for example, to use 
the information supplied by said auxiliary piloting curve to differentiate 
certain parts of the piloting curve, by indicating to the pilot the 
directions to be avoided, that is to say those where the auxiliary 
piloting curve is high, and the directions to seek, that is to say those 
where the auxiliary piloting curve is low. This differentiation may be 
obtained by different thicknesses of the trace of the piloting curve, by 
the use of dotted lines or of colors. 
Advantageously, an optimal avoidance point is determined, corresponding to 
the point of the piloting curve towards which the aircraft has to steer in 
order to perform the optimal avoidance trajectory above the obstacles. 
According to the invention, in order to determine said optimal avoidance 
point: 
for each reference point of the piloting curve, a compensated relative 
distance D satisfying the relationship D=h+kv is calculated, h being the 
horizontal distance and v the vertical distance between said reference 
point and the position of the speed vector, and k being a predefined 
coefficient; 
said calculated compensated relative distances are compared with each 
other; and 
the optimal avoidance point is determined as being the reference point the 
compensated relative distance of which is smallest. 
It may sometimes be sufficient, for determining the optimal avoidance 
point, to use only the reference points of three angular sectors, namely 
the central angular sector in front of the aircraft, in which the speed 
vector lies, as well as the adjacent angular sectors situated respectively 
to right and to left of this central angular sector. 
Said optimal avoidance point may be used in two different ways. On the one 
hand, a characteristic sign arranged at said optimal avoidance point may 
be superimposed on the image off the external landscape. Thus, in order to 
perform an optimal avoidance, it is sufficient to make the sign 
characteristic of the speed vector correspond with said sign 
characteristic of the optimal avoidance point. Moreover, it is possible to 
communicate the coordinates of said optimal avoidance point to the 
automatic pilot of the aircraft, in order to perform automatic avoidance. 
Moreover, a characteristic sign arranged at the point of said piloting 
curve through which said aircraft will pass while keeping its current roll 
is superimposed on the image of the external landscape. 
Advantageously, said hypothetical curve consists of a straight-line part, 
of length P, parallel to the speed vector of the aircraft and of a 
circular arc, of radius R, depending on the maneuvering capabilities of 
the aircraft, for pitch-up and pitch-down, in the vertical plane. 
Preferably, the length P of said straight-line part of said hypothetical 
curve is proportional to the speed of the aircraft, which makes it 
possible to obtain a duration (warning duration) which is necessary for 
the aircraft to cover the distance P, which is fixed whatever the speed of 
the aircraft. This fixed warning duration, for example 3 seconds or 5 
seconds for a helicopter, corresponds to the maximum duration left 
available to the pilot to commence an avoidance maneuver, after having 
been made aware of the order to avoid a dangerous obstacle. 
Moreover, the radius R of said circular arc satisfies the relationship 
R=Rc+Rp, with Rc the pitch-up limit radius, dependent on the pitch-up 
limit load factor, and Rp the pitch-down limit radius, dependent on the 
pitch-down limit load factor. 
It will be noted that the pitch-down limit load factor is generally 
determined by taking account of considerations relating to the comfort of 
the pilot, since the capabilities of the aircraft in terms of load factor 
downwards are not greatly reduced as a function of mass, of temperature or 
of the atmospheric pressure. A value generally acknowledged by pilots is 
0.6 g, g being the acceleration due to gravity. 
In contrast, the pitch-up limit load factor capable of being taken by the 
aircraft depends very substantially on the previously quoted parameters, 
namely the mass, the temperature and the pressure, as well as on the speed 
and the attitudes of the aircraft. 
Moreover, as the piloting curve is derived by the calculation of several 
points, it is possible either to take the same value for the calculation 
at each point of this piloting curve, or to take a different value having 
regard to the fact that the aircraft has to roll in order to reach the 
points of the piloting curve which are not exactly in front of the latter 
and that, therefore, this rolling reduces the limit load factor which it 
is capable of taking. 
In the context of the present invention, it is thus possible to determine, 
in various different possible ways, said pitch-up limit load factor used 
for the calculation of the pitch-up limit radius Rc. For example, a fixed 
value may be taken which the aircraft is capable of taking throughout the 
flight envelope, or predefined values may be used varying as a function of 
the speed of the aircraft. According to another possibility, said pitch-up 
limit load factor may be determined directly from the temperature, the 
pressure, the mass and the speed of the aircraft. 
Advantageously, in each angular sector, said search area is limited to a 
distance L from the aircraft, satisfying the relationship L=P+R, which 
avoids taking account of obstacles which cannot be defined as dangerous 
obstacles, since they are out of range of said hypothetical curve, of 
length L, related to the aircraft. 
In order to implement the method according to the present invention, a 
device is provided including: 
a rangefinder sensor intended to measure the direction and the distance of 
all the obstacles situated in said search areas; 
a computer linked to said rangefinder sensor calculating said hypothetical 
curve on the basis of recorded data, defining the dangerous obstacles, and 
capable of calculating the coordinates of the reference points and said 
piloting curve; and 
a visual display device linked to said computer, capable of presenting to 
the pilot of the aircraft, in a fashion superimposed on the external 
landscape or on the image of the latter, the characteristic signs of said 
coordinates, said piloting curve, as well as the characteristic sign 
indicating the position of the speed vector. 
Advantageously, said device includes an audible alarm, which is associated 
with a time base, which is linked to said visual display device and which 
is triggered when said speed vector is below said piloting curve for a 
predefined duration, for example half of the warning duration. 
Moreover, said visual display device filters the variations in the piloting 
curve and presents to the pilot, progressively, the alterations in said 
piloting curve, which makes it possible to avoid abrupt jumps by the 
piloting curve which are likely to discomfort or tire the pilot needlessly 
.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The present invention is intended to assist the pilot of an aircraft, in 
particular of a helicopter, to maintain the lowest possible trajectory in 
a dangerous environment, by indicating to said pilot the obstacles which 
are dangerous for said aircraft in its flight space. 
For determination of these dangerous obstacles, a hypothetical curve F is 
used, related to the aircraft A, the mode of derivation of which is 
explained below, with reference to FIGS. 1A, 1B and 1C. 
The calculation of the hypothetical curve F is performed in a reference 
system (Ox, Oy), the origin O representing the aircraft A, likened to a 
point, the axis Ox being parallel to the speed vector .sub.v of the 
aircraft A, in a horizontal direction in the example represented, and the 
axis Oy being perpendicular to the speed vector .sub.v , that is to say 
vertical, and directed upwards in this example. 
In order to clear an obstacle 1, represented diagrammatically, and to move 
to the horizontal, in line with its top 2, that is to say at the level of 
a point 3 situated slightly above said top 2, the aircraft A covers a 
trajectory T1 including, on the one hand, a circular arc 5 with radius of 
curvature Rc from the initial position at O up to a point 4, and, on the 
other hand, a circular arc 6 with radius of curvature Rp (not represented) 
from the point 4 up to the overflight point 3. 
The radii of curvature Rc and Rp, corresponding respectively to the 
pitch-up and pitch-down limit radii, depend on the maneuvering 
capabilities of the aircraft A. They are calculated respectively from 
pitch-up and pitch-down limit load factors, as described previously. 
The top 2, as well as the point 3, is on a circular arc 7, tangent to the 
speed vector .sub.v at the point O and having a radius R=Rc+Rp. 
The trajectory T1, as it is represented in FIG. 1A, is the trajectory for 
clearing the obstacle 1 without a margin, since the overflight point 3 is 
slightly above the top 2 of the obstacle 1. In order to clear this 
obstacle 1 with a vertical margin H, corresponding, for example, to the 
minimum height prescribed for overflying an obstacle, the circular arc 7 
is offset by a distance H downwards, with respect to the aircraft, that is 
to say along the Oy axis, in the direction of the negative coordinates, as 
represented in FIG. 1B. 
Moreover, in order for the pilot of the aircraft to have a certain reaction 
time DR (or warning time) available before commencing the maneuver for 
clearing the obstacle 1, the circular arc 7 is offset by a distance P 
forwards, that is to say along the Ox axis, as represented in FIG. 1C. It 
will be noted that, in order to have a fixed warning duration DR, for 
example 3 seconds, whatever the speed V of the aircraft, said distance P 
is defined to be proportional to this speed V, on the basis of the 
relationship P=DR.V. 
Under these conditions, the aircraft A performs a clearance trajectory T2 
corresponding to the optimal trajectory for clearing an obstacle, having 
regard to the maneuvering capabilities of the aircraft. The slopes of this 
trajectory T2 are softer than those of the trajectory T1 which has been 
represented in broken lines in FIG. 1C and which is a limit trajectory. 
Thus the hypothetical curve F sought is obtained. The latter consists of a 
straight-line part 8 of length P and of the circular arc 7. 
The steps previously described in order to explain the calculation of the 
hypothetical curve F make it possible clearly to reveal the advantage of 
this curve and to show its links with the theoretical trajectory T2 for 
optimal clearance. 
It will be noted that the trajectory for clearance of an obstacle 1 is 
optimal, that is to say identical to said trajectory T2, if the clearance 
is triggered at the very moment when the obstacle 1 to be cleared touches 
said hypothetical curve F. 
In the reference system (Ox, Oy), the circle (not represented) which 
includes the circular arc 7 has the equation: 
EQU (x-P).sup.2 +[y-(R-H)].sup.2 =R.sup.2, 
which has the root y=R-H-.sqroot.(R.sup.2 -(x-P).sup.2). 
Consequently, in said reference system (Ox, Oy), the equation of the 
hypothetical curve F is: 
##EQU1## 
Said hypothetical curve F is thus limited to a distance L=P+R from the 
point O. 
This hypothetical curve F is used to determine the obstacles which are 
dangerous for the aircraft, as will be seen below. 
To this end, only a field 10 in front of the aircraft is of interest, as 
represented in FIG. 2, including at least the part of the space which the 
aircraft is capable of reaching from its current position, having regard 
to the characteristics of its dynamic performance. In accordance with the 
invention, said frontal field 10 is divided into angular sectors 11 to 14, 
with the same angular aperture .alpha.. 
In each of said angular sectors 11 to 14: 
all the obstacles 15 to 20 situated in a search area, explained below, are 
determined, by means of a rangefinder detector, for example; 
the tops 2 of said determined obstacles 15 to 20 are compared with said 
hypothetical curve F related to the aircraft and not represented in FIG. 
2; 
that obstacle is defined as dangerous obstacle 16, 18 and 20 whose top 2 is 
situated highest with respect to said hypothetical curve F; and 
the position of said top 2 of this dangerous obstacle 16, 18 and 20 is 
recorded. 
Obviously, when a sector includes no apparent obstacle, as for example 
sector 12, a point 21 on the ground (see FIG. 3) is defined as dangerous 
obstacle. 
In general, a rangefinder detector, such as for example a radar, is used, 
associated with a laser beam which regularly and systematically scans the 
space lying in front of said detector. Said detector, in each direction, 
specifically records the range of the ground which sends back an echo. All 
the points detected, such as for example trees, the ground or cables, of 
which the direction and the range with respect to the aircraft is 
therefore known, are considered as obstacles capable of being used in the 
implementation of the invention. 
According to the invention, the altitude (coordinate along y, according to 
FIG. 1) of each obstacle is calculated, in a reference system related to 
the aircraft, for example. Next, the altitude difference between each 
obstacle y(O) and the corresponding point y(F) of the hypothetical curve F 
is calculated. The obstacle defined as dangerous is that whose difference 
y(O)-y(F) is the largest in algebraic value, whatever the nature of this 
obstacle (trees, ground, cable, etc.). 
Furthermore, since the hypothetical curve F is, by definition, limited to 
the distance L=P+R, said search areas associated with the various angular 
sectors 11 to 14 are likewise limited to a distance L from the aircraft, 
since the obstacles which are situated at a greater distance are then out 
of range of the hypothetical curve F and can therefore not be defined as 
dangerous obstacles. 
The foregoing results are then presented to the pilot of the aircraft, by 
superimposition on an image 22 of the external landscape, for example 
directly by means of a clear visor or of a helmet visor or by presentation 
on a screen (head-down or head midway) in superimposition with a camera 
image (visible or infrared spectrum). 
Various modes of presentation are possible. 
According to a first possibility, characteristic signs, in the form of 
reticles 23, arranged at the tops 2 of the dangerous obstacles 16, 18 and 
20, are superimposed on the image 22 of the external landscape, as 
represented in FIG. 3. 
This solution exhibits a double advantage. On the one hand, it makes it 
possible, among all the existing obstacles 15 to 20, to designate those 
which are defined as dangerous and on which the pilot should concentrate 
all his attention. On the other hand, it contributes effective assistance 
to observation, which is particularly useful in poor visibility, by 
indicating to the pilot the presence of obstacles which are possibly not 
recognized. 
For the sake of clarification, a specific reticle 24 may be used when the 
dangerous obstacle relates to a point on the ground, such as, for example, 
the point 21. 
According to another possibility, characteristic signs are superimposed on 
the image of the external landscape in the form of reticles 25 arranged at 
points in space 26 to 29 (reference points) lying at a determined height 
above the tops 2 of the dangerous obstacles. By using, as height, the 
minimum prescribed height H for overflying an obstacle, the pilot is 
clearly shown the point in space through which he has to pass in order to 
perform an optimal overflight, that is to say an overflight which is as 
low as possible and without danger of collision. 
In order to facilitate such a maneuver, a reticle 30 is additionally 
superimposed on the image 22 of the external landscape, this reticle 
indicating the position of the speed vector of the aircraft collimated at 
infinity, that is to say the direction of flight of the aircraft A. Thus, 
in order to perform an optimal clearance of a dangerous obstacle 16, 18 or 
20, it is sufficient for the pilot to steer the aircraft A in such a way 
as to superimpose the reticle 30 on the reticle 25 of the reference point 
26, 28 or 29 associated with this dangerous obstacle 16, 18 or 20. 
According to a third information presentation possibility, a piloting curve 
31 corresponding to a smooth curve linking all the reference points 26 to 
29, is superimposed on the image 22 of the external landscape, as 
represented in FIG. 4. This piloting curve 31 divides the image 22 of the 
external landscape into two parts, a lower part 32 which the aircraft must 
avoid, by reason of the presence of dangerous obstacles 16, 18 and 20, and 
an upper part 33 including no dangerous obstacles. Thus, in order to 
perform the lowest possible flight without danger of collision, it is 
sufficient to pilot the aircraft A so as to superimpose the reticle 30 of 
the speed vector on said piloting curve 31. 
In order not to overload the image 22 sent to the pilot, only a central 
part 34 of said piloting curve 31 is represented, as represented in FIG. 
5, by deleting the lateral parts 35 and 36 including points in space which 
are not accessible to the aircraft with a predefined limit roll. In this 
FIG. 5, traces 37 and 38 have been represented in broken lines, which 
correspond to the hypothetical trajectories at maximum roll, respectively 
to the right and to the left, capable of being performed by an aircraft 
located at a point 39. 
It is obvious that the various points of said central part 34 of the 
piloting curve 31 are more or less easily accessible to the aircraft. In 
order to facilitate the choice by the pilot, an optimal avoidance point is 
determined, corresponding to the point of the central part 34 towards 
which the aircraft A has to steer in order to perform optimal avoidance 
trajectory above the obstacles. 
To this end, the following steps are performed: 
for each reference point 27 to 29 of the central part 34 of the piloting 
curve 31, a compensated relative distance D is calculated, satisfying the 
relationship D=h+kv, being the horizontal distance and v the vertical 
distance between said reference point 27 to 29 and the position 30 of the 
speed vector, as represented for the reference point 28 in FIG. 4, and 
being a predefined coefficient; 
said calculated compensated relative distances are compared with each 
other; and 
the reference point whose compensated relative distance is smallest is 
determined as optimal avoidance point. 
The coefficient is chosen to be positive and greater than 1, so as to favor 
lateral avoidance of an obstacle with respect to vertical clearance. 
In order to assist the pilot to achieve a good trajectory, for example to 
pass between two high obstacles, a characteristic sign, in the form of a 
reticle 48, is superimposed on the point 49 of the piloting curve 31, 
through which the aircraft will pass if it maintains its current roll, 
said current roll being indicated by the position of the reticle 30 of the 
speed vector, as represented in FIG. 5. 
Thus, for example, in order to pass between two high obstacles, it is 
sufficient for the pilot to steer the aircraft in such a way as to 
superimpose the reticle 48 on the point in space and on said piloting 
curve, through which he wants to pass, and for him to maintain this 
position. 
In FIG. 6, a device 40 in accordance with the invention for implementing 
the method described above has been represented. This device 40 includes: 
a rangefinder sensor 41 intended to measure the direction and the distance 
of all the obstacles 15 to 20 situated in the various search areas; 
a computer 42 linked by means of a link 43 to said rangefinder sensor 41, 
calculating said hypothetical curve F on the basis of recorded data, 
defining the dangerous obstacles 16, 18 and 20, and capable of calculating 
the coordinates of the tops 2 of these dangerous obstacles, the 
coordinates of the reference points 26 to 29 and said piloting curve 31; 
and 
a visual display device 44 linked to said computer 42, by means of a link 
45, and capable of presenting to the pilot of the aircraft, in a manner 
superimposed posed on the external landscape, said characteristic signs 23 
and 25, said piloting curve 31, as well as the reticle 30 indicating the 
position of the speed vector. 
Moreover, an audio alarm 46 associated with a time base, which is not 
represented, is linked to said visual display device 44, via a link 47. 
This audio alarm 46 is triggered when the reticle 30 of the speed vector 
is below the piloting curve 31 for a predefined duration, thus giving 
warning of the urgency of an avoidance maneuver.