- a database system containing terrain data and obstacle data,

- an input device onboard the aircraft for selecting a target position,

- a display device onboard the aircraft for displaying information to a pilot of the aircraft,

- a processor onboard the aircraft,

the processor being connected to the input device and to the display device, and the processor being configured to determine an instrument approach procedure for the aircraft on the basis of the selected target position and the terrain data and obstacle data from the database.

The aircraft may be a rotary-wing aircraft, for example a helicopter, or a fixed-wing aircraft, such as an airplane.

EP 1 198720 discloses a system that utilizes a global positioning system for creating an approach to a position on the ground from a location above the ground. When the pilot wants to land the aircraft, the pilot uses an input device to enter coordinates or to select a desired point on the ground displayed on a digital moving map as well as other information such as desired landing direction. Thereafter, a processor onboard the aircraft creates a precision approach for the aircraft to that position on the ground from the in-flight position of the aircraft. The approach includes direction, elevation, and distance to the position on the ground. The approach may also include altitude penalties for obstacles and elevation changes in the terrain. The system includes a real-time mapping device, such as a Doppler radar or a diode laser, that identifies obstacles in the approach. The identified obstacles are then compared to obstacle data in a database to verify the validity of the obstacle data in the database. This verification allows the system to modify the approach if necessary based upon the identified obstacles.

This known system can only be used when the aircraft is already in the vicinity of the landing position on the ground. In addition, this system is not able to assist the pilot in case of a missed approach, for example, when the pilot loses visual contact with the runway at a late stage in the approach, e.g. when an unexpected patch of ground fog obstructs the pilot's visibility.

It is an object of the invention to provide an improved system for determining an approach procedure for an aircraft. This object is achieved according to the invention by a system for determining an instrument approach procedure for an aircraft, the system comprising:

- a database system containing terrain data and obstacle data,

- an input device onboard the aircraft for selecting a target position,

- a display device onboard the aircraft for displaying information to a pilot of the aircraft,

- a processor onboard the aircraft,

the processor being connected to the input device and to the display device, and the processor being configured to determine an instrument approach procedure for the aircraft on the basis of the selected target position and the terrain data and obstacle data from the database,

wherein

the processor is configured to determine, on the basis of the selected target position, a final approach path segment and a missed approach path segment of the instrument approach procedure, the final approach path segment being defined by a flight path that begins at a final approach waypoint at a final approach waypoint altitude and ends at a missed approach point, at a lower altitude than the final approach waypoint altitude, for deciding whether the aircraft may continue to a landing position on the ground or a missed approach procedure is started, and the missed approach path segment being defined by a flight path that begins at the missed approach point and ends at a higher altitude than the altitude of the missed approach point, which missed approach path segment is followed by the aircraft when it is decided at the missed approach point that the missed approach procedure is started, wherein, for example, the missed approach path segment is a substantially straight flight path, and

the system comprises a memory containing obstacle clearance surface data, on the basis of which, for each of the final approach path segment and the missed approach path segment, at least one obstacle clearance surface is defined that extends at a pre-determined orientation and at a pre-determined distance below said path segment, and

the processor is configured to determine an obstacle clearance altitude (OCA) for the instrument approach procedure as the lowest altitude at which any obstacle identified by the terrain data and obstacle data in the database system does not penetrate any of the obstacle clearance surfaces in the final approach path segment and the missed approach path segment, and

the system is configured to display the obstacle clearance altitude (OCA) on the display device.

According to the invention, the system determines the instrument approach procedure onboard the aircraft. The final approach path segment and the missed approach path segment are determined by calculating the coordinates of the final approach waypoint and the coordinates of the missed approach point. Incidentally, the final approach waypoint is a waypoint or "fix", whereas the missed approach point may be a waypoint or "fix" or may not be a waypoint or "fix" depending on the type of approach procedure. However, the missed approach point is a point that is calculated by the system according to the invention in any type of approach procedure. In the process, the processor may also determine the course or direction of the missed approach path segment. After the final approach path segment and the missed approach path segment have been determined, the obstacle clearance altitude (OCA) is calculated by analysing, for each of the final approach path segment and the missed approach path segment, if any obstacle identified by the terrain data and obstacle data in the database system penetrates the obstacle clearance surface below said approach path segment. The dimensions, orientation and shape of the obstacle clearance surfaces may vary along the final approach path segment and the missed approach path segment. For each of the final approach path segment and the missed approach path segment, the dimensions, orientation and shape of the obstacle clearance surfaces, including the minimum obstacle clearance (MOC) distance, i.e. the vertical distance between the obstacle clearance surface and the final approach path segment or the missed approach path segment situated above it, are stored in the memory. The parameters defining the obstacle clearance surfaces depend on the type of instrument approach procedure. If any obstacle penetrates the obstacle clearance surface at any point along the final approach path segment or the missed approach path segment, the obstacle clearance altitude (OCA) is increased by the processor in such a manner that the obstacle clearance surface is situated just above the obstacle or obstacles. Thus, the obstacle clearance altitude (OCA) is the lowest altitude at which the obstacle clearance surface is not penetrated by any obstacle anywhere along the final approach path segment and the missed approach path segment. The calculated obstacle clearance altitude (OCA) is displayed on the display device to the pilot. Of course, other information from the calculated instrument approach procedure may also be displayed on the display device to the pilot, for example the coordinates of the final approach waypoint (final approach fix) and, optionally, coordinates of other waypoints or fixes.

The system according to the invention does not require any ground-based infrastructure. Also, the system according to the invention can be used to determine the instrument approach procedure at any moment in time, for example, the pilot can enter the desired target position when the aircraft is in-flight at a considerable distance therefrom. It is even possible for the desired target position to be entered already before take-off.

Furthermore, the instrument approach procedure determined by the system according to the invention provides for a missed approach path segment. The obstacle clearance altitude (OCA) is determined by taking into consideration not only the final approach path segment but also the missed approach path segment, which increases safety.

The instrument approach procedure determined by the system according to the invention can be a standard procedure laid down by definitions and descriptions of the International Civil Aviation Organization ("ICAO"), for example a procedure referred to as "Point-in-Space" procedure ("PinS") or a procedure referred to as "Localizer-Precision with Vertical guidance" procedure ("LPV"), sometimes also referred to as "Approach Procedure with Vertical guidance" procedure ("APV").

When the system according to the invention is configured to determine a PinS procedure, the target position is a position above the ground, and the missed approach point is a waypoint or "fix" that is defined by the target system, i.e. the same waypoint as the PinS. Preferably, the system is configured to display the missed approach point on the display device. In this latter case, the processor is not only configured to calculate the coordinates of the missed approach point in the procedure, but the missed approach point is also presented to the pilot as a waypoint or "fix" on the display device. After the aircraft has reached the missed approach point, the pilot either continues to proceed visually to a suitable landing position on the ground or the pilot starts the missed approach procedure. In the PinS procedure, the missed approach path segment may be defined by a substantially straight line. The PinS procedure is only applicable to rotorcraft, such as helicopters.

According to the invention, with the PinS procedure, the obstacle clearance surface of the final approach segment may comprise a primary area and two secondary areas, wherein the primary area of the final approach segment is oriented substantially horizontally and extends at a minimum obstacle clearance (MOC) distance below the final approach segment, and wherein each secondary area of the final approach segment slopes upwards from a longitudinal edge of the primary area of the final approach segment on either side of said primary area. Likewise, it is possible that the obstacle clearance surface of the missed approach segment comprises a primary area and two secondary areas, wherein the primary area of the missed approach segment is oriented substantially horizontally and extends at a minimum obstacle clearance (MOC) distance below the missed approach segment, and wherein each secondary area of the missed approach segment slopes upwards from a longitudinal edge of the primary area of the missed approach segment on either side of said primary area. Thus, with the system according to the invention, an instrument approach procedure in accordance with PinS can be calculated onboard the aircraft, and the obstacle clearance altitude (OCA) can be determined by taking into consideration both the final approach path segment and the missed approach path segment of said PinS procedure.

When the system according to the invention is configured to determine an LPV procedure, the target position is the landing position on the ground, and the missed approach point is situated on a substantially straight line from the final approach waypoint to the landing position on the ground. Preferably, the system is configured to display the selected target position, i.e. the landing position on the ground on the display device. In this case, the missed approach point is not a waypoint or "fix" that is presented on the display device to the pilot. The missed approach point is defined by a pre-determined decision altitude. At the decision altitude, the pilot decides either to continue and land on the selected landing position on the ground or to start the missed approach procedure. The LPV procedure can be applied both to rotary-wing and fixed-wing aircraft.

With an LPV procedure, the obstacle clearance surfaces may be defined in a runway coordinate system. The landing position on the ground is situated on a runway having a centre line, and the runway coordinate system has an origin situated at a runway threshold of the runway, and mutually perpendicular axes X, Y and Z extending from the origin, wherein the Z-axis extends vertically, wherein the X-axis extends parallel to the centre line of the runway, towards the final approach waypoint, and wherein the Y-axis extends transversely to the centre line of the runway according to the right-hand rule. In such a runway coordinate system, the final approach path segment extends in the X-Z plane.

According to the invention, with the LPV procedure, the obstacle clearance surface of the final approach path segment may comprise at least one central surface, which is referred to as W-surface, which has a centre line that extends in the X-Z plane, and which W-surface extends at right angles to the X-Z plane, and which W-surface is inclined with respect to the X-Y plane. As a result, the W-surface is also inclined with respect to the Y-Z plane. The W- surface is symmetrical with respect to the X-Z plane. In most cases, the W-surface is substantially rectangular. The W-surface slopes downwards as seen in the direction of the runway.

It is also possible according to the invention that the obstacle clearance surface of the final approach path segment comprises two central surfaces that each have a centre line that extends in the X-Z plane, which are referred to as W-surface and W-surface, respectively, wherein the W-surface is a continuation of the w-surface as seen in the direction of the runway, and which W- and W-surfaces each extend at right angles to the X-Z plane, and which surfaces are inclined with respect to the X-Y plane at different angles (and thus the W- and W-surfaces are also inclined with respect to the Y-Z plane at different angles). The W- surface and the W-surface are aligned with each other and each slope downward as seen in the direction of the runway. For example, the W-surface that is situated closer to the runway extends at an angle with respect to the X-Y plane that is greater than the W-surface that is situated further away from the runway.

In an embodiment of the invention, the W-surface comprises two opposed longitudinal edges on either side of said W-surface, wherein the obstacle clearance surface of the final approach segment comprises two side surfaces, which are each referred to as X-surface, and which X-surfaces each slope upward from the longitudinal edges of the W-surface at an angle with respect to the X-Y plane, to the X-Z plane, and also to the Y-Z plane. The X- surfaces each slope downward as seen in the direction of the runway. At the same time, each X-surface slopes upward from the respective longitudinal edge of the W-surface. When the obstacle clearance surface of the final approach path segment also includes a W- surface, each of the X-surfaces also slopes upward from the longitudinal edges of the W- surface. As seen in a cross-section parallel to the Y-Z plane, the W-surface and the adjacent X-surfaces define the shape of a trough. The same applies to the W'-surface and the adjacent X-surfaces.

According to the invention, with the LPV procedure, the obstacle clearance surface of the missed approach path segment may also comprise at least one central surface, which is referred to as Z-surface, which has a centre line that extends in the X-Z plane, and which Z- surface extends at right angles to the X-Z plane, and which Z-surface is inclined with respect to the X-Y plane (and thus also to the Y-Z plane). The Z-surface of the missed approach segment is symmetrical with respect to the X-Z plane, and preferably, the Z-surface has the shape of a isosceles trapezium or a plurality of isosceles trapeziums that are aligned with each other. The Z-surface slopes upward as seen in the direction away from the runway.

Furthermore, the Z-surface comprises two opposed longitudinal edges on either side of said Z-surface, said longitudinal edges being non-parallel when the Z-surface has the shape of a isosceles trapezium, wherein the obstacle clearance surface of the missed approach segment comprises two side surfaces, which are each referred to as Y-surface, and which Y-surfaces each slope upward from the longitudinal edges of the Z-surface at an angle with respect to the X-Y plane, to the X-Z plane, and also to the Y-Z plane. The Y- surfaces each slope upward as seen in the direction away from the runway. At the same time, each Y-surface slopes upward from the respective longitudinal edge of the Z-surface. As seen in a cross-section parallel to the Y-Z plane, the Z-surface and the adjacent Y- surfaces also define the shape of a trough. The Y-surfaces may also extend along the final approach segment adjacent to the longitudinal edges of the X-surfaces.

In a preferred embodiment of the system according to the invention, the processor is configured to determine an initial approach path segment and/or an intermediate path segment of the instrument approach procedure, the initial approach path segment being defined by a flight path that begins at an initial approach waypoint and ends at an intermediate waypoint, and the intermediate path segment being defined by a flight path that begins at the intermediate waypoint and ends at the final approach waypoint, wherein the initial approach waypoint and the intermediate waypoint are preferably at the same altitude as the final approach waypoint altitude, and wherein, on the basis of the obstacle clearance surface data, for each of the initial approach path segment and/or the intermediate path segment, at least one obstacle clearance surface is defined that extends at a pre-determined orientation and at a pre-determined distance below said path segment, and the processor is configured to determine the obstacle clearance altitude (OCA) for the instrument approach procedure as the lowest altitude at which any obstacle identified by the terrain data and obstacle data in the database system does not penetrate any of the obstacle clearance surfaces in the final approach path segment and the missed approach path segment and the initial approach path segment and/or the intermediate path segment.

In this case, the system according to the invention not only determines the final approach path segment and the missed approach path segment, but also the intermediate path segment, and preferably also the initial approach path segment. In most instrument approach procedures, the procedure includes at least one waypoint or "fix" ahead of the final approach waypoint. Usually, the instrument approach procedure includes two additional waypoints or "fixes" ahead of the final approach waypoint, i.e. the intermediate waypoint (intermediate fix) and the initial approach waypoint (initial approach fix). However, some instrument approach procedures do not determine the initial approach waypoint, i.e. they only calculate one waypoint or "fix" ahead of the final approach waypoint. Furthermore, even if both the intermediate fix and the initial approach fix are determined, the pilot may decide to disregard the initial approach fix and fly directly to the intermediate fix.

The intermediate path segment is determined by calculating the coordinates of the intermediate fix. The initial approach path segment is determined by calculating the coordinates of the initial approach fix. On the basis of the obstacle clearance data stored in the memory of the system, an obstacle clearance surface is defined for each of the intermediate path segment and/or the initial approach path segment. Thus, the processor calculates the obstacle clearance altitude (OCA) for the instrument approach procedure by taking into consideration obstacles in the intermediate path segment and/or initial approach segment as well, which is advantageous for safety. In this case, the system may be configured to display on the display device not only the obstacle clearance altitude (OCA), but also, for example the final approach fix, the initial approach fix and/or the intermediate fix coordinates.

The system according to the invention takes into consideration any obstacles along the initial approach path segment and/or along the intermediate path segment by assessing obstacle clearance surfaces associated with said path segments. Preferably, the obstacle clearance surface of the initial approach path segment comprises a primary area and two secondary areas, wherein the primary area of the initial approach path segment is oriented substantially horizontally and extends at a minimum obstacle clearance (MOC) distance below the initial approach path segment, and wherein each secondary area of the initial approach path segment slopes upwards from a longitudinal edge of the primary area of the initial approach path segment on either side of said primary area. Likewise, it is preferred that the obstacle clearance surface of the intermediate path segment comprises a primary area and two secondary areas, wherein the primary area of the intermediate path segment is oriented substantially horizontally and extends at a minimum obstacle clearance (MOC) distance below the intermediate path segment, and wherein each secondary area of the intermediate path segment slopes upwards from a longitudinal edge of the primary area of the intermediate path segment on either side of said primary area. Obstacle clearance surfaces of this type result in high levels of safety.

Such obstacle clearance surfaces may be configured to comply with ICAO

regulations. For example, the minimum obstacle clearance (MOC) distance is 246 ft in the final approach path segment (with the PinS procedure), 500 ft in the intermediate path segment and 1000 ft in the initial approach path segment. The width of the primary area may be, for example, between 2-4 nm. The width of the primary area may vary along the path segments. The two secondary areas are situated on either side of the primary area.

In an embodiment according to the invention, the database system contains instrument approach procedure data of a plurality of different instrument approach procedures, such as the PinS procedure and/or the LPV procedure, wherein the input device is configured to select an instrument approach procedure from said plurality of instrument approach procedures, i.e. to allow a pilot to select a desired procedure from the plurality of different instrument approach procedures, and wherein the system is configured to determine the selected instrument approach procedure. In this case, the pilot may select the type of instrument approach procedure, for example Pins or LPV. On the basis of the selected instrument approach procedure, the system according to the invention calculates the coordinates of the final approach waypoint and the missed approach point as well as the obstacle clearance altitude (OCA) and displays the relevant parameters to the pilot. For example, with the PinS procedure, the coordinates of the final approach waypoint, the coordinates of the missed approach point (i.e. the PinS waypoint) and the obstacle clearance altitude (OCA), and optionally other data, may be depicted on the display device.

In an embodiment according to the invention, the input device is configured to select an approach glide slope and/or an approach course or direction, at least for the final approach path segment, and/or an initial altitude, wherein the processor is configured to calculate the final approach waypoint and the missed approach point on the basis of the selected approach glide slope and/or the selected approach course and/or the selected initial altitude. For example, the system according to the invention asks the pilot not only to enter the target position, but also the desired approach glide slope, the desired approach course and the desired initial altitude. Usually, the initial altitude defines the final approach waypoint altitude. On the basis of the selected target position, the selected approach glide slope, the selected approach course and the selected initial altitude , the system according to the invention calculates the coordinates of the final approach waypoint and the coordinates of the missed approach point. Thus, the final approach path segment can be determined using the input from the pilot. The missed approach path segment starts from the missed approach point and is also determined by the system, for example by autonomously selecting a course or direction for the missed approach path and specifying an altitude to which the aircraft has to climb. Instead, the system may select a missed approach holding waypoint to which the aircraft should be directed during the missed approach procedure. It should be noted that the system may also autonomously select a desired approach glide slope and/or a desired approach course and/or a desired initial altitude, wherein the autonomously selected values are used to calculate the coordinates of the final approach waypoint and the coordinates of the missed approach point.

In a preferred embodiment according to the invention, the input device is configured to select an optimization function for determining, after an obstacle clearance altitude (OCA) of a first instrument approach procedure has been determined by the processor, a second, alternative instrument approach procedure, wherein the processor is configured to modify the selected approach glide slope and/or the selected initial altitude and/or the selected approach course, after the optimization function has been selected, and to determine a modified final approach path segment and a modified missed approach path segment, and, optionally, a modified initial approach path segment and/or a modified intermediate path segment, on the basis of the selected target position and the modified approach glide slope and/or the modified initial altitude and/or the modified approach course, and wherein, on the basis of the obstacle clearance surface data, for each of said modified path segments, at least one obstacle clearance surface is defined that extends at a pre-determined orientation and at a pre-determined distance below said modified path segments, and the processor is configured to determine a modified obstacle clearance altitude (modified OCA) for the second, alternative instrument approach procedure as the lowest altitude at which any obstacle identified by the terrain data and obstacle data in the database system does not penetrate any of the obstacle clearance surfaces in said modified path segments, and wherein the processor is configured to compare the modified obstacle clearance altitude (modified OCA) with the obstacle clearance altitude (OCA) of the first instrument approach procedure. The modified obstacle clearance altitude (modified OCA) may be displayed on the display device.

Thus, the system determines the obstacle clearance altitude (OCA) of a first instrument approach procedure, which is displayed on the display device. The obstacle clearance altitude (OCA) of the first instrument approach procedure calculated by the system may be considered too high, for example a cloud base is situated below the obstacle clearance altitude (OCA). When the pilot selects the optimization function, the system analyses if the calculated obstacle clearance altitude (OCA) can be decreased by changing the glide scope for the final approach path segment and/or the initial altitude and/or the approach course that have been selected by the pilot. The modified glide slope and/or the modified initial altitude lie within a predetermined operational range. As a result, an obstacle that penetrated one of the obstacle clearance surfaces associated with the calculated first instrument approach procedure may no longer penetrate. Thus, when the optimization function is selected, an attempt is made by the system to generate a second, alternative instrument approach procedure to the selected target position having a lower obstacle clearance altitude (OCA). If the system arrives at a second, alternative instrument approach procedure having a lower obstacle clearance altitude (modified OCA), the latter is displayed on the display device, preferably together with the coordinates of the modified final approach waypoint, and preferably also together with the coordinates of the modified intermediate waypoint and/or the modified initial approach waypoint. If the modified obstacle clearance altitude (modified OCA) of the second, alternative instrument approach procedure is still not satisfactory to the pilot, he may then decide to select another target position or another approach procedure.

In an embodiment according to the invention, the processor is configured to determine the obstacle clearance altitude (OCA) by:

- defining an estimated obstacle clearance altitude that is equal to the minimum obstacle clearance (MOC) distance at the missed approach point,

- determining if any obstacle identified by the terrain data and obstacle data in the database system penetrates any of the obstacle clearance surfaces, and,

if it is determined that no obstacles penetrate the obstacle clearance surfaces, determine that the estimated obstacle clearance altitude is the minimum obstacle clearance altitude (OCA), and

if it is determined that one or more obstacles penetrate the obstacle clearance surfaces, increase the estimated obstacle clearance altitude so that said obstacles no longer penetrate any of the obstacle clearance surfaces, and determine that the increased estimated obstacle clearance altitude is the obstacle clearance altitude (OCA).

Thus, the system is configured to determine a first estimate of the obstacle clearance altitude by ignoring any obstacles. Then, said first estimate is checked against the obstacles identified by the terrain data and obstacle data in the database system. If one or more obstacles penetrate the obstacle clearance surfaces, said first estimate is increased so that said obstacles no longer penetrate the obstacle clearance surfaces. This results in the obstacle clearance altitude (OCA) that is displayed on the display device.

In this case, it is possible that the processor is configured to determine, if it is determined that one or more obstacles penetrate the obstacle clearance surface in the missed approach segment, a height by which said obstacles project into the obstacle clearance surface of the missed approach segment, and wherein the processor is configured to increase the estimated obstacle clearance altitude by said height so that said obstacles no longer penetrate the obstacle clearance surface in the missed approach segment. Thus, obstacles in the missed approach path segment are avoided by increasing the obstacle clearance altitude by the height by which said obstacles penetrate into the obstacle clearance surface. Thus, said obstacle clearance altitude (OCA) defines a safe altitude for the missed approach path segment.

The invention also relates to a method for determining an instrument approach procedure for an aircraft, wherein use is made of a system comprising:

- a database system containing terrain data and obstacle data,

- an input device onboard the aircraft for selecting a target position,

- a display device onboard the aircraft for displaying information to a pilot of the aircraft,

- a processor onboard the aircraft,

the processor being connected to the input device and to the display device, and the method comprises:

- determining by the processor an instrument approach procedure for the aircraft on the basis of the selected target position and the terrain data and obstacle data from the database system,

wherein

the processor determines, on the basis of the selected target position, a final approach path segment and a missed approach path segment of the instrument approach procedure, the final approach path segment being defined by a flight path that begins at a final approach waypoint at a final approach waypoint altitude and ends at a missed approach point, at a lower altitude than the final approach waypoint altitude, for deciding whether the aircraft may continue to a landing position on the ground or a missed approach procedure is started, and the missed approach path segment being defined by a flight path that begins at the missed approach point and ends at a higher altitude than the altitude of the missed approach point, which missed approach path segment is followed by the aircraft when it is decided at the missed approach point that the missed approach procedure is started, and the system comprises a memory containing obstacle clearance surface data, on the basis of which, for each of the final approach path segment and the missed approach path segment, at least one obstacle clearance surface is defined that extends at a pre-determined orientation and at a pre-determined distance below said path segment, and

the processor determines an obstacle clearance altitude (OCA) for the instrument approach procedure as the lowest altitude at which any obstacle identified by the terrain data and obstacle data in the database system does not penetrate any of the obstacle clearance surfaces in the final approach path segment and the missed approach path segment, and the obstacle clearance altitude (OCA) is displayed on the display device.

The method according to the invention has the same advantages as the system according to the invention as described above. In addition, one or more of the features of the system according to the invention as described above, and also one or more of the features of the system claims, individually or in any combination of features, may be applied to the method according to the invention.

The invention will now be explained in more detail, only by way of example, with reference to the figures.

Figure 1 is a schematic illustration of a first instrument approach procedure for an aircraft that is determined by the system according to the invention.

Figure 2 is a schematic illustration of a second instrument approach procedure for an aircraft that is calculated by the system according to the invention.

Figure 3 is a block diagram of a system for determining an instrument approach procedure for an aircraft according to an exemplary embodiment of the invention, wherein the system is integrated in the flight management system (FMS) of an aircraft.

Figure 4 is a cross-section according to IV-IV in figure 1.

Figure 5 is a schematic top view of obstacle clearance surfaces in the final approach segment and the missed approach segment in the instrument approach procedure shown in figure 2.

Figure 6 is a schematic side view of the obstacle clearance surfaces shown in figure

5.

Figure 7 is a schematic top view of a third instrument approach procedure for an aircraft that is calculated by the system according to the invention.

Figure 8 is a cross-section according to VIII-VIII in figure 7.

The system for determining an instrument approach procedure is installed onboard an aircraft. The aircraft can be a rotary-wing aircraft, for example a helicopter, or a fixed-wing aircraft. The system may constitute a module of a flight management system (FMS) onboard the aircraft or the system may be a dedicated module onboard the aircraft.

The instrument approach procedure shown in figure 1 is a so-called "Point-in-Space"

("PinS") procedure. The PinS procedure applies only to rotorcraft. The PinS procedure comprises an initial approach waypoint or initial approach fix (IAF) 8, an intermediate waypoint or intermediate fix (IF) 9, a final approach waypoint or final approach fix (FAF) 10 and a missed approach point 11. In the PinS procedure, the missed approach point 11 is also a waypoint or "fix", which is referred to as the "Point-in-Space".

The PinS procedure comprises an initial approach path segment 3 between the IAF 8 and the IF 9, an intermediate path segment 4 between the IF 9 and the FAF 10, and a final approach path segment 5 between the FAF 10 and the missed approach point or Point-in- Space 11. The initial approach path segment 3, the intermediate path segment 4 and the final approach path segment 5 each have a length that is defined by the length of said flight path when projected onto the surface of the earth, as shown by X _F AS, XIS and X| _A s in figure 1. The approach glide slope for the final approach path segment is indicated by Θ.

Thus, the PinS procedure brings the aircraft to the Point-in-Space 11 with the pilot flying by reference to instruments. Normally, the pilot proceeds visually from the Point-in- Space 11 to find a suitable landing spot 12, as schematically indicated by visual flight path 14. However, if the pilot, for some reason, has not gained visual ground contact by the time the Point-in-Space 11 is reached, or the pilot considers that the weather is not good enough for the visual path segment, for example an unexpected patch of ground fog may obstruct the pilot's visibility, the pilot will start a missed approach procedure. During the missed approach procedure, the aircraft flies from the missed approach point, i.e. the Point-in-Space 11 , to a higher altitude according to a straight flight path. The straight flight path that is followed by the aircraft in the missed approach procedure defines a missed approach segment 6.

Alternatively, the missed approach segment 6 may comprise a missed approach holding waypoint (not shown), to which the aircraft is directed in the missed approach procedure.

The instrument landing procedure shown in figure 2 is a so-called "Localizer-Precision with Vertical guidance" ("LPV") procedure, sometimes also named "Approach Procedure with Vertical guidance" ("APV") procedure. The LPV procedure can be used by both rotary-wing and fixed-wing aircraft. The LPV procedure falls in the class of "precision approach" (it is sometimes referred to as "precise approach" rather than "precision approach"). Like the PinS procedure, the LPV procedure comprises an initial approach waypoint or initial approach fix (IAF) 8, an intermediate waypoint or intermediate fix (IF) 9, a final approach waypoint or final approach fix (FAF) 10 and a missed approach point 11. In the LPV procedure, the missed approach point 11 is not a waypoint that is displayed to the pilot. The missed approach point 11 is a calculated point in the procedure which lies at a decision altitude (DA) that is predetermined in the procedure.

The LPV procedure also comprises an initial approach path segment 3 between the IAF 8 and the IF 9, an intermediate path segment 4 between the IF 9 and the FAF 10, and a final approach path segment 5 between the FAF 10 and the missed approach point 11. The initial approach path segment 3, the intermediate path segment 4 and the final approach path segment 5 each have a length that is defined by the length of said flight path when projected onto the surface of the earth, as shown by X _F AS, XIS and X| _A s in figure 2. The approach glide slope for the final approach path segment is indicated by Θ.

With the LPV procedure, the pilot decides at the decision altitude either to continue and land on the landing position on the ground 12 ahead or to start the missed approach procedure. During the missed approach procedure, the aircraft flies from the missed approach point, i.e. from the decision altitude (DA) to a higher altitude according to a straight flight path. The straight flight path that is followed by the aircraft in the missed approach procedure defines a missed approach segment 6. Alternatively, the missed approach segment 6 may comprise a missed approach holding waypoint (not shown), to which the aircraft is directed in the missed approach procedure.

Although the PinS procedure and the LPV procedure are shown in figures 1 and 2 with straight path segments 3, 4, 5, said path segments 3, 4, 5 may be curved or at an angle with respect to each other, in particular the initial approach path segment 3, and also the intermediate path segment 4. This is also shown in the instrument approach procedure according to figure 7.

Figure 3 schematically shows a system 1 for determining the instrument approach procedure shown in figure 1 or 2 onboard the aircraft. The system 1 shown in figure 3 is integrated in the flight management system (FMS) 20 of the aircraft. In operation, the pilot may call a list of procedures in the FMS 20, for example arrival procedures and approach procedures. With the system 1 according to the invention integrated into the FMS 20, the pilot can select that, for example, the PinS or LPV procedure illustrated in figures 1 and 2 or another type of procedure is to be calculated by the system 1.

The system 1 comprises an input device 23 onboard the aircraft for selecting a target position. With the PinS procedure, the target position is the Point-in-Space 11 , i.e. the Point- in-Space 11 is directly entered by the pilot into the input device 23. For the LPV procedure, the target position is a landing position on the ground. The pilot may also select a desired approach glide slope, a desired approach course or approach direction for the final approach path segment 5 and a desired initial altitude by means of the input device 23. Incidentally, the pilot may also be asked by the system to enter additional data into the input device 23 of the system. If the landing position on the ground is in an urban area with many obstacles around, the pilot might consider finding a clear landing area. When arriving at the Point-in-Space 11 in the PinS procedure, the flight will proceed visually until a suitable landing spot is found. With the LPV procedure, the aircraft will land straight ahead onto a runway or onto a Final Approach and Take-Off (FATO) area.

The system 1 comprises a processor which calculates the lengths of the path segments X _F AS, XIS and X| _A s and the coordinates of the IAF 8, the IF 9, the FAF 10 and the missed approach point 11 on the basis of the selected target position, the selected approach glide slope, the selected approach course and the selected initial altitude. The processor also determines the course or direction of the missed approach path segment 6. Thus, the initial approach path segment 3, the intermediate path segment 4 and the final approach path segment 5 can be determined using the input from the pilot. Alternatively, the system 1 may autonomously select an approach glide slope and/or an approach course and/or an initial altitude, wherein the autonomously selected values are used to calculate the coordinates of the IAF 8, the IF 9, the FAF 10 and the missed approach point 1 1.

The system 1 comprises a database system 22 which is connected to the FMS 20. The database system 22 may comprise one or more databases. The database system 22 contains terrain data and obstacle data. Furthermore, the system comprises a memory (not shown) which contains obstacle clearance surface data, on the basis of which, for each of the initial approach path segment 3, the intermediate path segment 4, the final approach path segment 5 and the missed approach path segment 6, an obstacle clearance surface 30 is defined that extends at a pre-determined orientation and at a pre-determined distance below said path segment 3, 4, 5, 6.

As shown in figures 1 and 4, which illustrate the PinS procedure, the obstacle clearance surface 30 for the initial approach path segment 3 comprises a primary area 31 and two secondary areas 32. The primary area 31 is oriented substantially horizontally and extends at a minimum obstacle clearance distance (MOCI _AS ) below the initial approach path segment 3, as shown in figure 8. The minimum obstacle clearance (MOC) distance is given for any type of procedure. For example, with the PinS procedure, the minimum obstacle clearance (MOCI _AS ) distance is 1000 ft in the initial approach path segment 3. The width w of the primary area in the initial approach path segment 3 may be, for example, between 2-4 nm. The secondary areas 32 are situated on either side of the primary area 31. The secondary areas 32 slope upwards from the longitudinal edges of the primary area 31. The obstacle clearance surface 30 for the intermediate path segment 4 also comprises a primary area 31 and two secondary areas 32, similar to the initial approach path segment 3. The primary area 31 lies at a minimum obstacle clearance (MOC| _S ) distance below the intermediate path segment 4 (500 ft with the PinS procedure). The obstacle clearance surface 30 for the final approach path segment 5 also has a primary area 31 and adjacent secondary areas 32. The primary area 31 of the final approach path segment 3 is situated at a minimum obstacle clearance (MOC _FA ) below the altitude of the Point-in-Space 11. With this PinS procedure, MOCF _A is 246 ft (see figures 7 and 8).

With the LPV procedure, the obstacle clearance surface 30 for the initial approach path segment 3 and the intermediate path segment 4 is similar to the PinS procedure, i.e. the obstacle clearance surface 30 comprises a primary area 31 and two adjacent secondary areas 32. Of course, the minimum obstacle clearance (MOC) distance may be different for the LPV procedure. The obstacle clearance surfaces 30 for the final approach segment 5 and the missed approach segment 6 are shown in figures 5 and 6.

With the LPV procedure illustrated in figures 2, 5 and 6, the obstacle clearance surfaces 30 may be defined in a runway coordinate system. The landing position on the ground is situated on a runway having a centre line. The runway coordinate system has an origin situated at a runway threshold of the runway, and mutually perpendicular axes X, Y and Z extending from the origin. The Z-axis extends vertically, the X-axis extends parallel to the centre line of the runway, towards the final approach waypoint 10, and the Y-axis extends transversely to the centre line of the runway according to the right-hand rule. In such a runway coordinate system, the final approach path segment 5 extends in the X-Z plane.

With the LPV procedure shown in figures 5 and 6, the obstacle clearance surface 30 of the final approach path segment 5 comprises a central surface, which is referred to as W- surface, which has a centre line that extends in the X-Z plane. The W-surface extends at right angles to the X-Z plane, and is inclined with respect to the X-Y plane and with respect to the Y-Z plane. The W-surface is symmetrical with respect to the X-Z plane. In this case, the W-surface is substantially rectangular. The W-surface slopes downwards as seen in the direction of the runway. In this example, the W-surface extends partially in the intermediate path segment 4.

As shown in figures 5 and 6, the obstacle clearance surface 30 of the final approach path segment 5 also comprises a further central surface having a centre line that extends in the X-Z plane, which are referred to as W'-surface. The W'-surface is a continuation of the W-surface as seen in the direction of the runway, i.e. the W'-surface is situated closer to the runway. The W-surface is also substantially rectangular, and also extends at right angles to the X-Z plane, and is inclined with respect to the X-Y plane and to the Y-Z plane, but at a different angle than the W-surface. Thus, the W-surface and the W-surface are aligned with each other and each slope downward as seen in the direction of the runway, whereas they are connected to each other by a discontinuity. As shown in figure 6, the W'-surface extends at an angle with respect to the X-Y plane that is greater than the W-surface that is situated further away from the runway.

The W-surface and W'-surface comprise two opposed longitudinal edges 34 on either side. The obstacle clearance surface 30 of the final approach segment 5 comprises two side surfaces, which are each referred to as X-surface, which X-surfaces each slope upward from the longitudinal edges 34 of the W-surface and W'-surface at an angle with respect to the X- Y plane, to the X-Z plane, and also to the Y-Z plane. Thus, on the one hand, the X-surfaces each slope downward as seen in the direction of the runway, whereas on the other hand, the X-surfaces each slopes upward from the respective longitudinal edge 34 of the W-surface. As seen in a cross-section parallel to the Y-Z plane, the W-surface and the adjacent portion of the X-surfaces define the shape of a trough. The same applies to the W'-surface and the adjacent portion of the X-surfaces. Figures 5 and 6 also illustrate that the X-surfaces may extend partially in the intermediate path segment 4.

With the LPV procedure, the obstacle clearance surface 30 of the missed approach path segment 6 also comprise at least one central surface, which is referred to as Z-surface, which has a centre line that extends in the X-Z plane. The Z-surface extends at right angles to the X-Z plane, and is inclined with respect to the X-Y plane and also to the Y-Z plane. The Z-surface of the missed approach segment is symmetrical with respect to the X-Z plane. As shown in figure 5, the Z-surface has the shape of a number of isosceles trapeziums that are aligned with each other. The Z-surface slopes upward as seen in the direction away from the runway (see figure 6).

Furthermore, the Z-surface comprises two opposed longitudinal edges 35 on either side of said Z-surface. The longitudinal edges 35 are non-parallel as the Z-surface has the shape of a number of isosceles trapeziums. The obstacle clearance surface 30 of the missed approach segment 6 comprises two side surfaces, which are each referred to as Y-surface. The Y-surfaces each slope upward from the longitudinal edges 35 of the Z-surface at an angle with respect to the X-Y plane, to the X-Z plane, and also to the Y-Z plane. In addition, the Y-surfaces each slope upward as seen in the direction away from the runway (see figure 6). As seen in a cross-section parallel to the Y-Z plane, the Z-surface and the adjacent portions of the Y-surfaces also define the shape of a trough. As shown in figures 5 and 6, the Y-surfaces also extend partially along the final approach segment 5 adjacent to the longitudinal edges of the X-surfaces.

As explained above, the dimensions, shape and orientation of the obstacle clearance surfaces 30 depend on the type of procedure and also on the path segment. However, they are given and pre-determined in the initial approach path segment 3, the intermediate path segment 4, the final approach path segment 5 and the missed approach path segment 6 for each specific type of procedure. Therefore, obstacle clearance surface data relating to the dimensions, shape and orientation of the obstacle clearance surfaces 30 are stored in a memory of the system for each of said path segments 3, 4, 5, 6 and for each type of procedure. Once the initial approach path segment 3, the intermediate path segment 4, the final approach path segment 5 and the missed approach path segment 6 are determined by the system 1 , the obstacle clearance surface 30 follows directly from the obstacle clearance surface data stored in the memory.

After calculating the coordinates of the IAF 8, the IF 9, the FAF 10 and the missed approach point 11 , i.e. after determining the initial approach path segment 3, the

intermediate path segment 4, the final approach path segment 5 and the missed approach path segment 6, the processor determines an obstacle clearance altitude (OCA) for the instrument approach procedure. The obstacle clearance altitude (OCA) is calculated by checking if any obstacles identified by the terrain data and obstacle data in the database system 22 penetrate any of the obstacle clearance surfaces 30. The obstacle clearance altitude (OCA) for the instrument approach procedure is the lowest altitude at which any obstacle identified by the terrain data and obstacle data in the database system 22 does not penetrate the obstacle clearance surfaces 30 in any of said path segments 3, 4, 5, 6, i.e. at the obstacle clearance altitude (OCA) the aircraft is safeguarded from collision with any obstacles.

Next, the calculated obstacle clearance distance (OCA) and the coordinates of the

IAF 8, the IF 9, the FAF 10 are displayed on a display device 24 of the system 1 , which may be the display of the FMS 20. With the PinS procedure shown in figure 1 , the coordinates of the missed approach decision point 1 1 , i.e. the Point-in-Space, are also displayed on the display device 24. Wth the LPV procedure shown in figure 2, instead of the missed approach point 1 1 , the coordinates of the landing position 12 on the ground are displayed on the display device 24.

The pilot is then asked to accept or optimize the instrument approach procedure calculated by the system 1. The pilot can select the optimization function when he considers that the calculated obstacle clearance altitude (OCA) is still too high, for example because of a cloud base below said altitude.

When the pilot selects the optimization function, the system 1 analyses if the calculated obstacle clearance altitude (OCA) that has been displayed on the display device 24 can be decreased by changing the parameters selected by the pilot, e.g. the glide scope Θ for the final approach path segment 5, the initial altitude and/or the approach course selected by the pilot. As a result, an obstacle that penetrated the obstacle clearance surface 30 associated with the calculated instrument approach procedure may no longer penetrate. Thus, when the optimization function is selected, an attempt is made by the system 1 to generate a modified instrument approach procedure with a lower obstacle clearance altitude (modified OCA). If the system 1 arrives at such a modified instrument approach procedure, said lower obstacle clearance altitude (modified OCA) is displayed on the display device 24 together with the associated coordinates of the IAF 8, the IF 9, the FAF 10 and the Point-in- Space 11/landing position 12 on the ground. Next, the pilot is again asked to accept. If the modified obstacle clearance altitude (OCA) is still not acceptable, the pilot may decide to select another target position 12 or another approach procedure.

After the pilot has accepted the instrument approach procedure calculated by the system 1 (optimized or not), the coordinates of the IAF 8, the I F 9, the FAF 10 and the Point- in-Space 1 1/landing position 12 on the ground can be loaded into the approach section of the current flight plan. If there is no current flight plan, it may become a dedicated "approach flight plan". When approaching the IAF 8, the pilot activates the calculated instrument approach procedure. The system 1 then computes flight path deviations from the desired approach flight path by means of the flight guidance module 25. The computed deviations are shown on the pilot's flight display. The pilot may also select an automatic approach by switching on the autopilot mode. In this mode, the autopilot accepts flight guidance data from the flight guidance module 25.

Thus, the system according to the invention can provide an instrument approach procedure even if there is no current flight plan or no approach procedures exist for the destination. Therefore, the system according to the invention can be used at airports and landing sites that are not equipped with any ground-based infrastructure.

The system according to the invention is not limited to determining the instrument approach procedures shown in figures 1 and 2, i.e. the system according to the invention may also calculate other instrument approach procedures, for example an instrument approach procedure referred to as "SBAS Offshore Approach Procedure" ("SOAB"), wherein SBAS is an abbreviation of "Satellite Based Augmentation System".