WO2015101978A1

WO2015101978A1 - Autonomous emergency descending and landing of aircrafts

Info

Publication number: WO2015101978A1
Application number: PCT/IL2014/051126
Authority: WO
Inventors: Ohad ROZENBERG
Original assignee: Israel Aerospace Industries Ltd.
Priority date: 2013-12-31
Filing date: 2014-12-25
Publication date: 2015-07-09
Also published as: IL230249A0; IL230249A

Abstract

The presently disclosed subject matter includes, inter alia, a system, a method and a computer program product of autonomous descent of an aircraft (e.g. UAV), with a non-operating engine, from a current position to a desired position in the sky. The disclosed subject matter can be used for example for enabling a UAV with a non-operating engine to glide from a point of engine failure detection through an exit point of a descending spiral to a landing window. Once the landing window has been reached, the aircraft can execute an autonomous landing procedure and land at a selected landing site notwithstanding engine failure, and also a possible uplink communication failure.

Description

AUTONOMOUS EMERGENCY DESCENDING AND LANDING OF AIRCRAFTS

FIELD OF THE PRESENTLY DISCLOSED SUBJECT MATTER

The presently disclosed subject matter relates to the field of autonomously controlled aircrafts

BACKGROUND

In forced landing situations, an aircraft is forced to land as a result of a crucial technical problem such as engine failure. In such cases, it is imperative to enable safe landing of the aircraft in order to avoid endangerment of passengers, civilians on the ground, as well as to avoid damage to the surroundings of the landing area. Furthermore, due to the general high price tag of aircrafts, it is also desirable to avoid any damage which may be caused to the aircraft as a result of uncontrolled descent of the aircraft to the ground.

Forced landing may be encountered in both piloted aircraft as well as unmanned aerial vehicles (UAVs). While using Unmanned Aerial vehicles in military applications has been widespread for many years, in recent years the use of UAVs for civilian applications is continuously growing. Civilian applications include for example, highway traffic monitoring, search & rescue, and border patrol. Nevertheless, the integration of UAVs in routine civilian use is impeded due to insufficient level of safety, which is unacceptable by authorities in populated areas. This requirement is particularly important in cases where a UAV is required to make a forced landing.

Published documents considered to be relevant as background to the presently disclosed subject matter are listed below. Acknowledgement of the documents herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.

A paper entitled "Simulation of a Fixed-wing UAV Forced Landing with Dynamic Path Planning" by: Pillar Eng, Dr. Luis Mejias, Prof. Rodney Walker; Australian Research Centre for Aerospace Automation (ARCAA), School of Engineering Systems, Queensland University of Technology, describes the current status of a program to develop automated forced landing techniques for a fixed-wing Unmanned Aerial Vehicle (UAV). The paper outlines two dynamic path planning algorithms that were developed based on processes used by human pilots in forced landings. To evaluate the performances of these algorithms, a simulation environment was created using a non-linear 6 degree-of-freedom aircraft model. The simulation also modeled prevailing wind conditions which are a major factor in the forced landing planning process. A paper entitled "Towards Flight Trials for an Autonomous UAV Emergency

Landing using Machine Vision" by: D.L. Fitzgerald, Australian Research Centre for Aerospace Automation (ARCAA) Autonomous System Laboratory, CSIRO, presents the evolution and status of a number of research programs focused on developing an automated fixed wing UAV landing system. Results obtained in each of the three main areas of research, such as vision-based site identification, path and trajectory planning and multi-criteria decision making, are presented.

US Patent Application, Publication Number US 6531/0264312 disclosed a routing tool. The routing tool is configured to determine a landing site for an aircraft by receiving flight data. The routing tool identifies at least one landing site proximate to a flight path and generates a spanning tree between the landing site and the flight path. According to some embodiments, the landing sites are determined in real-time during flight. Additionally, the landing sites may be determined at the aircraft or at a remote system or device in communication with the aircraft. In some embodiments, the routing tool generates one or more spanning trees before flight. The spanning trees may be based upon a flight plan, and may be stored in a data storage device.

SUMMARY

According to an aspect of the presently disclosed subject matter there is provided a method of autonomous gliding of an aircraft with a non-operating engine from a current position in the sky to a desired position, the method comprising: with the help of at least one processing unit, performing at least the following operations: obtaining information with respect to a landing approach path; the landing approach path comprising at least a descending spiral section followed by a U-turn approach section; the U-turn approach section terminating with a landing window located at a predefined altitude; obtaining information with respect to an exit altitude threshold; generating instructions to exit the descending spiral and proceed to the U-turn approach section, if aircraft real-time altitude is equal to or lower than the exit altitude threshold; wherein exiting from a descending spiral is performed with flaps in retracted position; while gliding along the U-turn approach section, repeatedly determining flaps-extension-altitude; the flaps-extension-altitude being an altitude at which the flaps are to be extended in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range; and generating instructions to extend flaps if real-time altitude of the aircraft is equal to or lower than flaps-extension-altitude.

In addition to the above features, the method according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (i) to (x) below, in any desired combination or permutation: i) . The method further comprising: while gliding from flaps extension point towards the landing window, repeatedly determining flaps-retraction-altitude; the flaps-retraction-altitude being an altitude at which the flaps are to be retracted in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range; and retracting flaps if real-time altitude of the aircraft is equal to or lower than flaps- retraction-altitude. ii) . The method further comprises calculating the exit altitude threshold comprising: obtaining information indicative of aircraft altitude loss in each descending circle (ACAL) in the descending spiral; obtaining information indicative of a desired exit altitude (DEA); setting the exit altitude threshold to a value which equals to (DEA + ^ ¹); wherein the desired exit altitude is determined to enable the aircraft to exit the descending spiral, glide along the U-turn approach section and arrive at the landing window in an acceptable altitude range, assuming flaps are in retracted position for half the distance of the U-turn approach section and are in extended position for half of the distance of the U-turn approach section. iii) . The method further comprises generating the landing approach path comprising: obtaining information indicative of aircraft situation data including at least: current position of aircraft and glide ratio of aircraft; obtaining information indicative of position of landing site; determining a descending spiral located above landing site area; determining a U-turn approach section comprising a downwind leg for allowing the aircraft, an upwind leg and a U-turn leg connecting the downwind and upwind legs, the downwind leg terminating at the landing window; the U-turn approach section is determined according to current wind conditions to enable the aircraft to glide along the downwind leg substantially in the wind direction and enable the aircraft to glide along the upwind leg substantially opposite to the wind direction. iv) . wherein determination of flaps-extension-altitude comprises: determining estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be extended when aircraft is at a selected altitude; increasing the value of the selected altitude if estimated future altitude is greater than altitude of the landing window; decreasing the value of the selected altitude if estimated future altitude is lower than altitude of the landing window; repeating determination of estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be extended at the selected altitude until the value of the selected altitude has converged to a value within the acceptable altitude range of the landing window; setting the flaps-extension-altitude as the selected altitude. v). wherein determination of flaps-retraction-altitude comprises: determining an estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be retracted when aircraft is at a second selected altitude; decreasing the value of the second selected altitude if estimated future altitude is greater than altitude of the landing window; increasing the value of the second selected altitude if estimated future altitude is lower than altitude of the landing window; repeating determination of estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be retracted at the second selected altitude until the value of the second selected altitude converged to a value within the acceptable altitude range of the landing window; setting the flaps-retraction- altitude as the second selected altitude. vi) . The method further comprises : determining a respective predicted glide ratio for each segment along the U- turn approach section; a segment being a leg or a part thereof; wherein the determination of each respective predicted glide ratio takes into consideration wind conditions and position of flaps in the respective segment; based on at least the current position of the aircraft and the glide ratio in each segment, calculating altitude loss in the segment; and combining the altitude loss of all segments and determining an estimated altitude loss along the U-turn approach section. vii) . The method further comprises: determining the estimated future aircraft altitude based on a difference between a current altitude of the aircraft and the estimated altitude loss. viii). wherein the non-operating engine is a result of engine failure; the method further comprising initiating an autonomous emergency descending procedure responsive to identification of engine failure. ix) . Wherein the aircraft being capable of descending from the landing window and landing on a selected landing site; the method further comprises, upon arrival to landing window, initiating an autonomous landing procedure and autonomously landing at the landing site. x) . wherein the aircraft is an unmanned aerial vehicle.

According to another aspect of the presently disclosed subject matter there is provided an emergency control unit onboard an aircraft configured to enable autonomous gliding of an aircraft with a non-operating engine from a current position in the sky to a desired position; the emergency control unit being operatively connectable to a flight control unit onboard the aircraft and comprises at least one processing unit; the processing unit comprising computer memory operatively connected to at least one processor and configured to: obtain information with respect to a landing approach path; the landing approach path comprising at least a descending spiral section followed by a U-turn approach section; the U-turn approach section terminating with a landing window located at a predefined altitude; obtain information with respect to an exit altitude threshold; generating instructions to the flight control unit to exit the descending spiral and proceed to U-turn approach section, if aircraft real-time altitude is equal to or lower than the exit altitude threshold; wherein exiting from a descending spiral is performed with flaps in retracted position; while the aircraft is gliding along the U- turn approach section, repeatedly determine flaps-extension-altitude; the flaps- extension-altitude being an altitude at which the flaps are to be extended in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range; and generate instructions to flight control unit to extend flaps, if real-time altitude of the aircraft is equal to or lower than flaps-extension-altitude.

In addition to the above features, the control unit according to this aspect of the presently disclosed subject matter can optionally comprise one or more of features (xi) to (xiv) listed below, in any desired combination or permutation:

xi). Wherein the flight control unit comprises or is operatively connected to a flaps control unit configured to control flaps position in response to a command received from the flight control unit. xii). Wherein the emergency control unit comprises at least a first processing unit and a second processing unit, each processing unit configured to operate individually; the first processing unit is configured to repeatedly determine the flaps-extension-altitude while the aircraft is gliding along the U-turn approach section; and feed the determined flaps-extension-altitude to the second processing unit; the second processing unit is configured to repeatedly determine whether real-time altitude of the aircraft is equal to or lower than flaps-extension-altitude received from the first processing unit; wherein an operation performed by the second processing unit can be repeated in a higher frequency than an operation performed by the first processing unit. xiii) . Wherein the emergency control unit, comprising a landing site determination module, is configured for determining a suitable landing site. xiv) . Wherein the emergency control unit, comprising an autonomous landing control module, is configured to initiate an autonomous landing procedure, upon arrival to landing window, and generate instructions for landing the aircraft at the landing site. xv) . Wherein the emergency control unit is installed and operating on board an Unmanned Aerial Vehicle.

According to another aspect of the presently disclosed subject matter there is provided an aircraft comprising an emergency control unit as disclosed herein.

According to yet another aspect of the presently disclosed subject matter there is provided a non-transitory program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method of autonomous gliding of an aircraft with a non-operating engine from a current position in the sky to a desired position, the method comprising: obtaining information with respect to a landing approach path; the landing approach path comprising at least a descending spiral section followed by a U-turn approach section; the U-turn approach section terminating with a landing window located at a predefined altitude; obtaining information with respect to an exit altitude threshold; generating instructions to exit the descending spiral and proceed to the U-turn approach section, if aircraft real-time altitude is equal to or lower than the exit altitude threshold; wherein exiting from descending spiral is performed with flaps in retracted position; while gliding along the U-turn approach section, repeatedly determining flaps-extension-altitude; the flaps-extension-altitude being an altitude at which the flaps are to be extended in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range; and generating instructions to extend flaps if real-time altitude of the aircraft is equal to or lower than flaps-extension-altitude. The control unit and the computer storage device, disclosed in accordance with the presently disclosed subject matter can optionally comprise one or more of features (i) to (x) listed above, mutatis mutandis, in any desired combination or permutation. BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the presently disclosed subject matter and to see how it may be carried out in practice, the subject matter will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

Fig. 1 is a flowchart showing an example of a sequence of operations which are performed in a forced landing scenario, in accordance with the presently disclosed subject matter;

Fig. 2 is a schematic illustration of a landing approach path for guiding an aircraft in a forced landing scenario to a selected landing site, in accordance with the presently disclosed subject matter; Fig. 3 is a flowchart showing an example of a sequence of operations performed in accordance with the presently disclosed subject matter;

Fig. 4 is a flowchart showing an example of a sequence of operations performed for determining flaps-extension-altitude, in accordance with the presently disclosed subject matter; and Fig. 5 is a flowchart showing an example of a sequence of operations performed for determining flaps-retraction-altitude, in accordance with the presently disclosed subject matter; and Fig. 6 is a functional block diagram schematically illustrating an example of an aircraft comprising an onboard emergency control unit, in accordance with the presently disclosed subject matter.

DETAILED DESCRIPTION In the drawings and descriptions set forth, identical reference numerals indicate those components that are common to different embodiments or configurations. Elements in the drawings are not necessarily drawn to scale.

Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "obtaining ", "generating", "determining", "calculating", or the like, include action and/or processes of a computer that manipulate and/or transform data into other data, said data represented as physical quantities, e.g. such as electronic quantities, and/or said data representing the physical objects. The terms "computer", "computerized device", "processing unit" or variation thereof should be expansively construed to cover any kind of electronic device with data processing capabilities, including, by way of non-limiting example, a personal computer, a server, a computing system, a communication device, a processor (e.g. digital signal processor (DSP), a microcontroller, a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), any other electronic computing device, and or any combination thereof.

As used herein, the phrase "for example," "such as", "for instance" and variants thereof describe non-limiting embodiments of the presently disclosed subject matter. Reference in the specification to "one case", "some cases", "other cases" or variants thereof means that a particular feature, structure or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the presently disclosed subject matter. Thus the appearance of the phrase "one case", "some cases", "other cases" or variants thereof does not necessarily refer to the same embodiment(s).

It is appreciated that certain features of the presently disclosed subject matter, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the presently disclosed subject matter, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. In embodiments of the presently disclosed subject matter, fewer, more and/or different stages than those shown in Figs. 1, 3, 4 and 5 may be executed. In embodiments of the presently disclosed subject matter one or more stages illustrated in Figs. 1, 3, 4 and 5 may be executed in a different order and/or one or more groups of stages may be executed simultaneously. Fig. 6 illustrates a general schematic of the system architecture in accordance with an embodiment of the presently disclosed subject matter. The modules in Fig. 6 may be centralized in one location or dispersed over more than one location. In other embodiments of the presently disclosed subject matter, the system may comprise fewer, more, and/or different modules than those shown in Fig. 6. In the following description the term "repeatedly" should be broadly construed to include any one or more of: "continuously", "periodic repetition" and " non-periodic repetition", wherein periodic repetition is characterized by intervals of constant length between repetitions, and non-periodic repetition is characterized by intervals of varying length between repetitions. In the following description the term "aircraft situation data" should be broadly construed to include any information pertaining to conditions of the aircraft and environmental conditions around the aircraft. Aircraft situation data include both raw data (e.g. wind velocity) and data which is calculated based on the raw data (e.g. glide ratio, glide distance). Aircraft situation data can include for example any one or more of the following: position of the aircraft (including 3D (x,y,z) location in space and orientation(pitch, yaw, turn)), direction, groundspeed, airspeed, residual fuel level, wind speed, wind velocity, altitude, aircraft's weight, glide ratio, glide distance, glide speed, aircraft's drag coefficient, aircraft's lift coefficient, aircraft's operational condition (e.g. whether the engine is operative or not, whether uplink communication is operative or not), etc. Bearing the above in mind, attention is now drawn to Fig. 1 showing a flowchart of a sequence of operations which are performed in a forced landing scenario, in accordance with the presently disclosed subject matter. The operations described with reference to Fig. 1 can be executed, for example, by an emergency control unit installed onboard an aircraft. An onboard emergency control unit 610 is described in detail with reference to Fig 6 below. The aircraft may be a piloted aircraft or a UAV, either one may be equipped with emergency autonomous descending and landing capability.

An onboard emergency control unit is a device comprising one or more processing units each comprising in turn one or more computer processors operatively connected to a computer memory (including non-transitory computer memory). The processing unit is configured to provide instructions to the aircraft's flight control unit (e.g. 650) to enable safe descent and landing in a forced landing scenario. Fig. 1 is described in conjunction with Fig. 2 showing a schematic illustration of a landing approach path for guiding an aircraft to a selected landing site, in accordance with the presently disclosed subject matter.

Upon detection of a need for forced landing a (e.g. in case of engine failure) an autonomous descending procedure is initiated (block 101). An emergency control unit onboard the aircraft obtains information indicative of a preferred landing site suited for landing the aircraft (block 103). The landing site can be selected using various principles and/or methods for selecting a landing site in emergency landing scenarios which are known per se. For example, a landing site can be selected from a database storing information with respect to different alternative landing sites located in the vicinity of the position of the aircraft. Alternatively, according to another example, an appropriate landing site can be selected by an onboard subsystem configured to identify (e.g. by visual analysis of the ground below the aircraft) and classify forced landing sites located on the ground in the vicinity of the aircraft's position. The landing site can be selected based on various parameters including for example, the estimated glide distance of the aircraft, the aircraft's fuel level, the wind conditions (selecting a landing site with a runway which is approachable at a desired angle with respect to wind direction, i.e. an upwind leg substantially in the wind direction and a downwind leg substantially opposite to the wind direction), the altitude of the aircraft at the time of engine failure detection, etc. A preferred landing site can be automatically selected by the onboard emergency control unit. Optionally, if the aircraft is a piloted aircraft, the landing site can be selected by the pilot. If uplink communication between a control station and the aircraft is operative, information indicative of a selected landing site can be provided from the control station.

The aircraft further obtains information indicative of a landing approach path (block 105). The landing approach path is used for guiding the aircraft from its current position to a landing window located at the end of the landing approach path. The aircraft can be configured to autonomously generate a landing approach path (e.g. with the help of an onboard emergency control unit). Alternatively, information with respect to the landing approach path can be autonomously obtained from an external source. For example, if uplink communication with the aircraft is available, the information can be obtained from a remote control station. Optionally, the operations described above with reference to block 103 and

105 can be repeatedly performed as a background procedure during normal flight, before a forced landing scenario is encountered. The relevant emergency information including available landing sites and landing approach paths leading to respective landing sites, is constantly updated based on real-time aircraft situation data. If a need for a forced landing arises, the relevant emergency information can be made immediately available and thus valuable time is saved. Additionally, if uplink communication with the aircraft is available, information with respect to the available landing sites can be transmitted to a remote control station to allow an operator to monitor the procedure. As illustrated in Fig. 2 the landing approach path is constructed as a series of waypoints (WP) guiding the UAV from its current position to landing window 10. The position of the landing window is determined at a certain height and position with respect to the selected landing site. Given the position of the landing window and the aircraft's situation data, the aircraft can autonomously and safely land on the preferred landing site 5. Waypoints in Fig. 2 are illustrated by way of example only. More waypoints, less waypoints and/or different waypoints can be used instead of those illustrated.

The landing approach path can be divided into a number of sections. The first section is an initial descent section (1) directing the UAV from its initial position towards a descending spiral section (2) which is connected in turn to U-turn approach section (3) terminating at landing window 10.

According to one example the position of the descending spiral is determined to be located above the runway of a preferred landing site 5 (as indicated by the spiral projection in Fig. 2). Thus, once the aircraft obtains information indicative of the position of the landing site, the position of a suitable descending spiral can be determined accordingly.

The aircraft glides along the initial descent section, joins the descending spiral section and loses altitude by descending along one or more descending circles (block 107). Once the aircraft descends to a desired altitude, the aircraft exits the descending spiral section (block 109). As explained below in more detail, the aircraft strives to exit the descending spiral at an altitude as close as possible to a desired exit altitude. The aircraft proceeds to the U-turn approach section (3) of the landing approach path (block 111). The U-turn approach section comprises a downwind leg 3a, an upwind leg 3c and a U-turn approach section 3b connecting the downwind and upwind legs. The U-turn approach section is determined according to current wind conditions to enable the UAV to glide along the downwind leg substantially in the wind direction and glide along the upwind leg substantially opposite to the wind direction. The upwind leg ends at a respective landing window 10. Once the aircraft arrives to the landing window (block 113) an autonomous landing procedure can be initiated for autonomously landing the aircraft on the preferred landing site 5 (block 115). For example, an autonomous landing procedure can be implemented in a UAV, enabling the UAV to land, even if uplink communication with the control stations is unavailable.

The landing approach path may comprise more than one alternative U-turn approach section 3. Each U-turn approach section begins at a respective exit point at the descending spiral section 2 and ends at a respective landing window. Fig. 2 illustrates an example of two alternative U-turn approach sections providing landing approach to landing site 5. The U-turn approach section which is more suitable for landing the aircraft is selected to guide the aircraft to a respective landing window 10.

The specific attributes of the U-turn approach section (including for example the direction and length of each leg) can be determined based on various parameters. For example, in addition to the position of the landing window and the aircraft's situation data mentioned above, U-turn approach section can be determined based on obstacles, such as trees or buildings, located in the vicinity of the selected landing site. In order to enable safe autonomous landing of the aircraft, it is important that the aircraft approaches the landing window within an acceptable altitude range, which defines the altitude of the landing window. To this end, an exit point from the descending spiral is determined at a desired exit altitude which enables the aircraft to safely glide through the U-turn approach section and reach the landing window in a desired altitude range.

When the aircraft exits the descending spiral it proceeds to glide towards the direction of the U-turn approach section. Thus, once the aircraft reaches the desired exit altitude, the aircraft may be required to glide an additional distance within the descending spiral and lose more altitude before exiting the descending spiral and proceeding to the U-turn approach section. In such cases, the actual exit point of the aircraft from the descending spiral would be at an altitude which is lower than the desired exit altitude.

For example, assuming the desired exit altitude is reached immediately after the aircraft glides pass the exit point directing the aircraft towards the U-turn approach section, the aircraft continues to glide a distance of approximately an additional full circle within the descending spiral before it can reach the exit point again and advance to the U-turn approach section.

Thus, while the desired exit altitude is determined based, inter alia, on the altitude of the landing window, the altitude of the actual exit point from the descending spiral may be different than the desired exit altitude.

As a consequence of the difference between the desired exit altitude and actual exit altitude, the aircraft may reach the landing window at the wrong altitude, a situation which may lead to an inaccurate and possibly hazardous landing. This scenario is obviously complicated when the aircraft is a UAV with inoperable uplink communication and no rectifying instructions can be transmitted to the UAV from a remote control station. Furthermore, autonomous landing scenarios of this sort require overcoming the challenge of constantly changing aircraft situation data parameters (e.g. changes in wind velocity).

Fig. 3 is a flowchart showing an example of a sequence of operations performed in accordance with the presently disclosed subject matter. In general, operations described with reference to Fig. 3 can be executed with the help of an onboard emergency control unit 610 described below with reference to Fig. 6. Fig. 3 provides a more detailed description of the descending procedure which is described above with reference to Fig. 1.

Block 301 in Fig 3 corresponds to block 120 in Fig. 1 and refers to the same operation described above with reference to blocks 101 to 105. According to the presently disclosed subject matter, a U-turn approach section is determined for guiding the aircraft while gliding from the descending spiral to the landing window. The position of the landing window at the end of the U-turn approach section is determined, inter alia, based on the position of the selected landing site and a required altitude of the landing window allowing the aircraft to autonomously land at the selected landing site.

Information indicative of a desired exit altitude for exiting from the descending spiral is determined in the aircraft (block 303). Information with respect to the desired exit altitude can be determined once the position of the descending spiral and landing window are known.

Obtaining the information with respect to the desired exit altitude, before entry into the descending spiral or immediately after entry, would assist the aircraft to exit the descending spiral at correct exit point and avoid unnecessary delay. An onboard emergency control unit 610 can be configured to determine the desired exit altitude (e.g. with the help of exit control module 625).

In order to determine a desired exit altitude (DEA- desired exit altitude) aircraft situation data is obtained. The relevant situation data includes, for example, information with respect to the position of the aircraft and position of the landing window, as well as information with respect to estimated and/or measured environmental conditions (e.g. wind velocity). The obtained information is used for calculating the aircraft's glide ratio and a respective desired exit altitude, which would allow the aircraft to safely glide through the U-turn approach section and reach the landing window within an acceptable altitude range. In general, glide ratio can be calculated by dividing ground speed by sink rate. The aircraft's glide ratio is determined while assuming the aircraft's flaps are in retracted position for half the distance of the U-turn approach section and are in extended position for the other half. Notably, the wind velocity is likely to vary in the different legs of the U-turn approach section. Accordingly, the glide ratio of the aircraft along the U-turn approach section is determined while considering changes in the wind conditions along the U-turn approach section and its impact on the glide ratio. A respective glide ratio is determined for gliding along each leg or different parts thereof, taking into consideration the specific wind conditions in each leg or part thereof. For example, a glide ratio can be repeatedly determined during gliding along the U-turn approach section, thus dividing the section into segments, each having a respective glide ratio value.

Information with respect to the aircraft altitude loss in each circle in the descending spiral (ACAL) is obtained as well (block 305).

Optionally, if uplink communication is available, part or all of the information obtained at the aircraft can be provided by an external source such as a control station. Otherwise, the entire information obtained by the aircraft can be determined by onboard emergency control unit 610. Information with respect to aircraft altitude loss in each circle of the descending spiral (ACAL) can be determined, inter alia, based on the aircraft turning radius and the current glide ratio. During the descent along the descending spiral, the turning radius of the aircraft in each descending circle is maintained constant.

Having determined the DEA and the altitude loss in each circle, while the aircraft is gliding down along the descending spiral, the aircraft's altitude is monitored (block 307) and it is determined whether the real-time altitude ( TA) is equal to or lower than an exit altitude threshold (block 309).

The exit altitude threshold equals to the desired exit altitude + half the altitude loss in one circle in the descending spiral, mathematically represented by the expression:

RTA < ψΕΑ + ^ - Once the aircraft descends to an altitude which is equal to or lower than the exit altitude threshold, the aircraft strives to exit from the descending spiral and proceeds to the U-turn approach section.

The exit altitude threshold confines the possible discrepancy between the desired exit altitude (DEA) and the actual exit altitude to a value which equals:

At one extreme, if an altitude which is equal to: (DEA + ^^) is reached immediately after the aircraft passes the exit point connecting the descending spiral to the U-turn approach section, the aircraft must glide for another full descending circle before exiting from the descending spiral. In such cases, the aircraft will leave the descending spiral at an altitude which is lower by than the DEA.

At the other extreme, if an altitude which is equal to: (DEA + ^ ¹), is reached right before the aircraft passes the exit point connecting the descending spiral to the U-turn approach section, the aircraft will immediately exit the descending spiral. In such cases the aircraft will exit the descending spiral at an altitude which is higher by than the DEA.

The aircraft exits from the descending spiral with the flaps in retracted position (exerting less drag on the aircraft than in extended position) and follows the U-turn approach section towards the landing window (block 311). While gliding along the U-turn approach section, a gilding simulation process is executed. During the gliding simulation process, flaps-extension-altitude is determined (block 313). Flaps-extension-altitude is the altitude at which the flaps should be extended in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range. Fig. 4 is a flowchart showing an example of a sequence of operations performed for determining flaps-extension-altitude, in accordance with the presently disclosed subject matter. These operations can be accomplished for example by onboard emergency control unit 610 (e.g. with the help of gliding simulation module 629).

The gliding simulation process exemplified with reference to Fig. 4 is performed as an iterative process where in each iteration the future altitude of the aircraft at the end of the U-turn approach section is determined while assuming the flaps are extended at a different altitude.

At block 401 an estimated future aircraft altitude at the end of the U-turn approach section is determined, assuming the flaps will be extended when the aircraft is at a selected altitude. The initially selected altitude can be any altitude between the altitude of the exit point from the descending spiral and the altitude of the landing window, located at the end of the U-turn approach section.

It has been explained that desired exit altitude is determined while assuming the flaps are extended for half the length of the U-turn approach section and retracted for the other half. Thus, the gliding simulation process can start for example, with a selected altitude which equals to the altitude of a point located in the middle between the altitude of the exit point from the descending spiral and the altitude of the landing window.

An estimated altitude loss at the end of the U-turn approach section can be determined based on various parameters including for example: the selected altitude, data characterizing the U-turn approach section (e.g. the location of the landing window and the length of each leg) as well as real-time aircraft situation data including current position of the aircraft and the glide ratio.

The aircraft's glide ratio is determined while assuming the aircraft's flaps are in retracted position up until the selected altitude is reached and are in extended position for the rest of the distance of the U-turn approach section up until the landing window.

As explained above, a respective glide ratio is determined for gliding along each leg or different parts thereof, while taking into consideration the specific wind conditions and position of the flaps in each leg or part thereof. Thus, a predicted glide ratio may be determined for different segments (a segment being a leg or a part of a leg) along the U-turn approach section. An estimated altitude loss in each segment can be determined based on the position of the aircraft and glide ratio along the segment. An overall estimated altitude loss along the U-turn approach section can then be calculated by combining the altitude loss calculated for each segment.

Knowing the current altitude of the aircraft and the estimated altitude loss of the aircraft by the time it reaches the end of the U-turn approach section, the estimated future altitude of the aircraft at the end of the U-turn approach section can be determined based on the difference between the two.

Once the value of an estimated future altitude of the aircraft is obtained, it is determined whether this value is different than the altitude of the landing window (block 403). Notably, an acceptable altitude for reaching the landing window may be defined by an altitude range and not a discrete value. In such cases it is determined whether the value of the estimated future altitude is within the defined range.

If the value of the estimated future altitude of the aircraft is within the prescribed range of the landing window, the flaps-extension-altitude is determined as the selected altitude (block 413). Otherwise, it is further determined whether the estimated future altitude is greater than the altitude of the landing window (block 405). If so, the value of the selected altitude is increased (block 407) and if not so, the value of the selected altitude is decreased (block 409).

Flaps in extended position speed up the aircraft's descent and increase the glide ratio. Thus, if the selected altitude is decreased, the flaps will remain retracted for a longer time and as a result the aircraft will descend slower. If, on the other hand, the selected altitude is increased, the aircraft will descend faster.

Determination of the estimated future aircraft altitude at the end of the U- turn approach section is repeated, assuming the flaps will be extended when the aircraft is at the now updated, selected altitude (block 411). The exemplified gliding simulation process operates as a converging algorithm. The operations described above with reference to blocks 403-411 are repeated until the value of the selected altitude is within the acceptable altitude range of the landing window. Once the value of the selected altitude falls within the acceptable altitude range, the flaps-extension-altitude is set as the selected altitude (block 413).

As long as the aircraft is gliding at a higher altitude than the flaps-extension- altitude and the flaps remain retracted, the flaps-extension-altitude is repeatedly calculated (as explained above with reference to Fig. 4). Due to changes in the aircraft situation data, the value of the flaps-extension-altitude may change (for example, in case wind velocity has changed) this may affect the glide ratio, which would in turn change the value of the flaps-extension-altitude. If an updated flaps- extension-altitude is determined which is different than the previously determined flaps-extension-altitude, the updated flaps-extension-altitude is used instead of the previous one.

Reverting to Fig. 3, once the flaps-extension-altitude is determined, it is repeatedly determined, whether the real-time altitude of the aircraft, is equal to or lower than the flaps-extension-altitude (block 315). If it is indeed determined that the real-time altitude of the aircraft is equal to or lower than the flaps-extension- altitude, the flaps are extended (block 317).

Optionally, after the flaps are extended, an estimated future aircraft altitude at the end of the U-turn approach section is determined assuming the flaps will be retracted when the aircraft is at another selected altitude at some location between the point where the flaps were extended and the landing window (block 319). Retracting the flaps can help to more accurately bring the aircraft to the landing window at the desired altitude.

Fig. 5 is a flowchart showing an example of a sequence of operations performed for determining flaps-retraction-altitude, in accordance with the presently disclosed subject matter. The operations described in fig. 5 can be accomplished for example by onboard emergency control unit 610 (e.g. with the help of gliding simulation module 629). The operations in fig. 5 follow the same principles as those described above with reference to fig. 4 and therefore the description of these operations is not repeated in full detail.

At block 501 an estimated future aircraft altitude at the end of the U-turn approach section is determined assuming the flaps will be retracted when the aircraft is at a selected altitude. The initially selected altitude can be any altitude between the altitude of the aircraft at the location along the u-turn approach section, where the flaps were extended (flaps extension point) and the altitude of the landing window. For example, the initial selected altitude can be the altitude of a point located in the middle, between these two points.

The future estimated altitude of the aircraft is determined based on the same parameters and principles as described above with reference to fig. 4. This time, however, the flaps are initially in extended position.

The glide ratio along the remaining part of the U-turn approach section is calculated, in a similar manner to calculation described with reference to Fig. 4. Based on the glide ratio, altitude loss of the aircraft during gliding along the U-turn approach section can be determined. Knowing the current altitude of the aircraft and the estimated altitude loss at the end of the U-turn approach section, the estimated future altitude of the aircraft at the landing window can be determined. Once the value of an estimated future altitude of the aircraft is obtained, it is determined whether this value is different than the altitude of the landing window (block 503).

If the value of the estimated future altitude of the aircraft is within the prescribed range of the landing window, the flaps-retraction-altitude is determined as the selected altitude (block 513). Otherwise, it is further determined whether the estimated future altitude is greater than the altitude of the landing window (block 505). If so, the value of the selected altitude is decreased (block 507) and if not so, the value of the selected altitude is increased (block 509). Determination of the estimated future aircraft altitude at the end of the U- turn approach section is repeated assuming the flaps will be retracted when the aircraft is at the now updated, selected altitude (block 511).

The operations described above with reference to blocks 503-511 are repeated until the value of the selected altitude has converged to a value within the acceptable altitude range of the landing window. Once the value of the selected altitude falls within the acceptable altitude range, the flaps-retraction-altitude is set as the selected altitude (block 513).

As long as the aircraft is gliding at a higher altitude than the flaps-retraction- altitude and the flaps remain extended, the flaps-retraction-altitude is repeatedly calculated (as explained with reference to Fig. 4). Due to changes in the aircraft situation data the value of the flaps-retraction-altitude may change for example as a result of a change in wind velocity.

Reverting to Fig. 3, once the flaps-extension-altitude is determined, it is repeatedly determined, whether the real-time altitude of the aircraft is equal to or lower than the flaps-retraction-altitude (block 321) and if so, the flaps are retracted (block 323).

At block 325 it is determined whether the aircraft has reached the landing window. Arrival to the landing window can be determined with the help of navigation assisting devices such as a GPS receiver and altimeter operating on the UAV. Once the landing window is reached, an autonomous landing procedure can be initiated (block 327). As mentioned above, an autonomous landing procedure is a sequence of operations which enables the aircraft to safely land at the selected landing site. Fig. 6 is a functional block diagram schematically illustrating an aircraft emergency control unit, in accordance with the presently disclosed subject matter. While fig. 6 refers to a UAV, this is done by way of example only, and the same elements and principles described with reference to fig. 6 are similarly applicable in a piloted aircraft. Onboard emergency control unit 610 is suitably mounted on UAV 600 and is operatively connected to various devices and subsystems onboard the aircraft. Emergency control unit 610 is configured to control operations of the UAV in the event of engine failure, to enable the UAV to safely glide and reach a landing window and land at a selected landing site. Emergency control unit 610 can be further configured to control various other operations in addition to engine failure situations (e.g. take-off, flight, maneuvering, landing, shutting off), but this is not necessarily so.

Emergency control unit 610 may be fully automated, but in some implementations it may also react to commands issued by another system or an operator. For example, responsive to a command, emergency control unit 610 may terminate its autonomous control of UAV 600, and be controlled by another system or by a human operator.

Onboard emergency control unit 610 comprises at least one processing unit 620 connected to a number of input interfaces. Input interface include for example: airspeed input interface 601a configured to obtain data indicative of airspeed of the UAV from one or more air speed detectors 601 (e.g. implemented as Pitot tubes); navigation system input interface 603a configured to obtain data indicative of UAV position and heading from navigation assisting device such as GPS receiver 603 and I NS (not shown); altitude input interface 605a configured to obtain data indicative of current altitude of the UAV from altimeter 605. Altimeter 605 can be implemented, for example, as a pressure altimeter, a sonic altimeter, a radar altimeter, a GPS based altimeter, and so forth. It is noted that control unit 610 can be further configured to obtain additional information (including situation data of all kinds) indicative of flight and state of UAV 600. It is noted that the list above is given by way of non-limiting example only and onboard emergency control unit 610 can be operatively connected to additional types of input interfaces and/or various devices to those specified above.

Control unit 610 comprises one or more processing units. Each processing unit comprises or is operatively connected to one or more computer processors and computer memory (volatile and non-volatile). Processing unit 620 can comprise the following modules:

• Landing site determination module 621 configured to obtain information with respect to a selected landing site as described above with reference to block 103 in Fig. 1 and block 120 in Fig. 3. According to one example, landing site determination module 621 can be configured to obtain information from data- repository 622 with respect to one or more candidate landing sites located on the ground in the vicinity of the UAVs real-time position, and select from among the candidate landing sites a landing site most suited for landing the UAV. Selection of the landing site can be based, inter alia, on information received from the various onboard devices and input interfaces. For example, current residual fuel level (from fuel level indicator 607), residual power (from power source 609), current position (from GPS 603), altitude (from altimeter 605) and wind velocity (from airspeed detector 601). Based on this information, additional relevant data such as gliding distance can be determined.

• Emergency landing approach path generator 623 configured to generate an emergency landing approach path once a landing site is selected, as described above with reference to block 105 in Fig. 1. Landing approach path generator 623 is configured to generate a landing approach path guiding the UAV from its current position to a landing window located above the landing site. Determination of a landing approach path can be based, inter alia, on information received from the various onboard devices and input interfaces. For example, landing approach path generator 623 can be configured to obtain real-time glide ratio. Landing site determination module 631 and emergency landing approach path generator 623 can be configured to continuously operate during normal operation of the UAV and provide real-time information with respect to available landing sites and respective landing approach paths leading to these landing sites. Thus, real-time information is readily available in case of engine failure. • Exit control module 625 is configured to control the exit of the aircraft from the descending spiral as described above with reference to fig. 3, blocks 303 to 311. Exit control module 625 can be configured to determine a desired exit altitude from the descending spiral as described above with reference to block 303. The exit altitude is determined to enable the UAV to glide and reach the landing window within an acceptable altitude range. Exit control unit 625 is further configured to determine the altitude loss in each descending circle in the descending spiral as described above with reference to block 305 in Fig. 3. This can be accomplished for example with the aid of information obtained from navigation assisting devices such as GPS 603 and altimeter 605. Exit control module 625 can be also configured to obtain the real-time altitude and guide the UAV to exit the descending spiral if the real-time altitude is equal to or lower than the desired exit altitude as described above with reference to blocks 307 to 311 in fig. 3. Exit control module 625 can be configured to provide instructions to navigation module 635, for example for guiding the aircraft to exit the descending spiral. Navigation module 635 is configured in general to control UAV flights control unit 650 for guiding the UAV along the landing approach path.

• Control unit 610 can comprise a glide ratio determination module 627, configured to repeatedly calculate the glide ratio for the benefit of other modules that make use of this information. Glide ratio can be calculated based on at least groundspeed data (e.g. from GPS 603) and altitude data (e.g. from altimeter 605). Glide ratio is utilized for example, by emergency landing approach path 623, exit control module 625 and gliding simulation module 629.

• Gliding simulation module 629 configured to determine flaps- extension-altitude as described above with reference to blocks 313 in fig. 3 and in fig. 4. Gliding simulation module 629 can be further configured to determine flaps- retraction-altitude as described above with reference to blocks 319 in fig. 3 and in fig. 5. Information indicative of the determined flaps-extension-altitude and flaps- retraction-altitude is transmitted to flaps position module 631. • Flaps positioning module 631 can be configured to generate instructions for switching the flaps to a desired position. This can be done for example, based on the output of gliding simulation module 629, as described above with reference to blocks 315, 317, 321 and 323 in Fig. 3. The instructions are transmitted to flaps control unit 651 configured, responsive to the received instruction, to change the position of flaps 663. Flaps control unit 651 can be comprised within or otherwise operatively connected to UAV flight control unit 650.

Gliding simulation module 629 and flaps position module 631 can be configured in two separate processing units, each operating individually. Gliding simulation module 629 is configured to determine and repeatedly update flaps- extension-altitude (and flaps-retraction-altitude), based on real-time aircraft situation data. Once determined, the flaps-extension-altitude (or flaps-retraction- altitude) is fed to flaps position module 631 which is configured in turn to repeatedly compare the latest received flaps-extension-altitude (or flaps-retraction-altitude) to current altitude of the UAV and instructs to change the position of the flaps accordingly.

The operations performed by gliding simulation module 629 are more processing intensive than those performed by flaps position module 631. Accordingly, each processing unit can perform its assigned operations at a different speed. For example, gliding simulation module 629 can be configured to determine an updated flaps-extension-altitude (or flaps-retraction-altitude) at an average frequency of 10 times each second. Flaps position module 631 can be configured to perform the comparison between the flaps-extension-altitude and the current altitude at an average frequency of 100 times per second. Each time a new flaps- extension-altitude or flaps-retraction-altitude value is calculated, the value is fed to flaps position module 631 which uses the most updated value in its calculations. The separation into two individual processing units enables to improve the overall processing efficiency of the two processing units.

• Autonomous landing control module 633 is configured to control the UAV during the autonomous landing procedure which is executed upon arrival to the landing window. Autonomous landing control module is configured to generate instructions to various UAV systems such as, flaps, rudder, wheels etc.

• Navigation module 635 is configured to control the UVA in order to maintain its course along a desired progress path. The progress path can be a predetermined path to a certain destination (stored for example in data-repository 622) or can be a progress path provided in real-time from a control station. The progress path can also be an emergency landing approach path generated responsive to engine failure as disclosed herein. Navigation module 635 is configured to generate instructions to UAV flight control unit 650, comprising flaps control unit 653, configured to control flaps 663 position. UAV flight control unit 650 can additionally comprise one or more other control units 653 configured to control various other devices such as: rudder 661, wheels 665, and others 667 (e.g. ailerons, elevators, etc).

It is to be understood that the system according to the presently disclosed subject matter may be a suitably programmed computer. Likewise, the presently disclosed subject matter contemplates a non-transitory computer program being readable by a computer for executing the method of the presently disclosed subject matter. The presently disclosed subject matter further contemplates a machine- readable memory (transitory and non-transitory) tangibly embodying a program of instructions executable by the machine for executing the method of the presently disclosed subject matter.

It is also to be understood that the presently disclosed subject matter is not limited in its application to the details set forth in the description contained herein or illustrated in the drawings. The presently disclosed subject matter is capable of other embodiments and of being practiced and carried out in various ways. Hence, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for designing other structures, methods, and systems for carrying out the several purposes of the present presently disclosed subject matter.

Claims

CLAIMS:

1. A method of autonomous gliding of an aircraft with a non-operating engine from a current position in the sky to a desired position, the method comprising: with the help of at least one processing unit, performing at least the following operations: obtaining information with respect to a landing approach path; the landing approach path comprising at least a descending spiral section followed by a U-turn approach section; the U-turn approach section terminating with a landing window located at a predefined altitude; determining information with respect to an exit altitude threshold; generating instructions to exit the descending spiral and proceed to the U- turn approach section, if aircraft real-time altitude is equal to or lower than the exit altitude threshold; wherein exiting from descending spiral is performed with flaps in retracted position; while gliding along the U-turn approach section, repeatedly determining flaps-extension-altitude; the flaps-extension-altitude being an altitude at which the flaps are to be extended in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range; and generating instructions to extend flaps if real-time altitude of the aircraft is equal to or lower than flaps-extension-altitude.

2. The method according to claim 1 further comprising: while gliding from flaps extension point towards the landing window, repeatedly determining flaps-retraction-altitude; the flaps-retraction-altitude being an altitude at which the flaps are to be retracted in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range; and retracting flaps if real-time altitude of the aircraft is equal to or lower than flaps-retraction-altitude.

3. The method according to any one of the preceding claims further comprising calculating the exit altitude threshold comprising: determining aircraft altitude loss in each descending circle (ACAL) in the descending spiral; determining information indicative of a desired exit altitude (DEA); setting the exit altitude threshold to a value which equals to (DEA + ^^); wherein the desired exit altitude is determined to enable the aircraft to exit the descending spiral, glide along the U-turn approach section and arrive at the landing window in an acceptable altitude range, assuming flaps are in retracted position for half the distance of the U-turn approach section and are in extended position for half of the distance of the U-turn approach section.

4. The method according to any one of the preceding claims further comprising, generating the landing approach path comprising: obtaining information indicative of aircraft situation data including at least: current position of aircraft and glide ratio of aircraft; obtaining information indicative of position of a landing site; determining a descending spiral located above the landing site area; determining a U-turn approach section comprising a downwind leg, an upwind leg and a U-turn leg connecting the downwind and upwind legs, the downwind leg terminating at the landing window; the U-turn approach section is determined according to current wind conditions to enable the aircraft to glide along the downwind leg substantially in the wind direction and enable the aircraft to glide along the upwind leg substantially opposite to the wind direction.

5. The method according to any one of the preceding claims wherein determination of flaps-extension-altitude comprises: determining estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be extended when aircraft is at a selected altitude; increasing the value of the selected altitude if estimated future altitude is greater than altitude of the landing window; decreasing the value of the selected altitude if estimated future altitude is lower than altitude of the landing window; repeating determination of estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be extended at the selected altitude until the value of the selected altitude has converged to a value within the acceptable altitude range of the landing window; setting the flaps-extension-altitude as the selected altitude.

6. The method according to any one of claims 2 to 5 wherein determination of flaps-retraction-altitude comprises: determining an estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be retracted when aircraft is at a second selected altitude; decreasing the value of the second selected altitude if estimated future altitude is greater than altitude of the landing window; increasing the value of the second selected altitude if estimated future altitude is lower than altitude of the landing window; repeating determination of estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be retracted at the second selected altitude until the value of the second selected altitude has converged to a value within the acceptable altitude range of the landing window; setting the flaps-retraction-altitude as the second selected altitude.

7. The method according to any one of claims 5 to 6 further comprising : determining a respective predicted glide ratio for each segment along the U- turn approach section; a segment being a leg or a part thereof; wherein the determination of each respective predicted glide ratio takes into consideration wind conditions and position of flaps in the respective segment; based on at least the current position of the aircraft and the glide ratio in each segment, calculating altitude loss in the segment; and combining the altitude loss of all segments and determining an estimated altitude loss along the U-turn approach section.

8. The method according to claim 7 further comprising: determining the estimated future aircraft altitude based on a difference between a current altitude of the aircraft and the estimated altitude loss.

9. The method according to any one of the preceding claims wherein the non-operating engine is a result of engine failure; the method further comprising; initiating an autonomous emergency descending procedure responsive to identification of engine failure.

10. The method according to any one of the preceding claims wherein the aircraft is capable of descending from the landing window and landing on a selected landing site; the method further comprising: upon arrival to the landing window, initiating an autonomous landing procedure and autonomously landing at the landing site.

11. The method according to any one of the preceding claims wherein the aircraft is an unmanned aerial vehicle.

12. An emergency control unit onboard an aircraft configured to enable autonomous gliding of an aircraft with a non-operating engine from a current position in the sky to a desired position; the emergency control unit being operatively connectable to a flight control unit onboard the aircraft and comprises at least one processing unit; the processing unit comprising computer memory operatively connected to at least one processor and configured to: obtain information with respect to a landing approach path; the landing approach path comprising at least a descending spiral section followed by a U-turn approach section; the U-turn approach section terminating with a landing window located at a predefined altitude; determining information with respect to an exit altitude threshold; generating instructions to the flight control unit to exit the descending spiral and proceed to U-turn approach section, if aircraft real-time altitude is equal to or lower than the exit altitude threshold; wherein exiting from a descending spiral is performed with flaps in retracted position; while the aircraft is gliding along the U-turn approach section, repeatedly determine flaps-extension-altitude; the flaps-extension-altitude being an altitude in which the flaps are to be extended in order to enable the aircraft to glide and reach the landing window at an acceptable altitude range; and generate instructions to flight control unit to extend flaps, if real-time altitude of the aircraft is equal to or lower than flaps-extension-altitude.

13. The emergency control unit according to claim 12 wherein the flight control unit comprises or is operatively connected to a flaps control unit configured to control flaps position in response to a command received from the flight control unit.

14. The emergency control unit according to any one of claims 12 to 13, further configured to: repeatedly determine flaps-retraction-altitude, while gliding from flaps extension point towards the landing window; the flaps-retraction-altitude being an altitude at which the flaps are to be retracted in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range; and generate instructions to the flight control unit, instructing to retract flaps if real-time altitude of the aircraft is equal to or lower than the flaps-retraction- altitude.

15. The emergency control unit according to any one of claims 12 to 14, further configured to: calculate the exit altitude threshold, the calculation comprising: determine aircraft altitude loss in each descending circle (ACAL) in the descending spiral; determine information indicative of a desired exit altitude (DEA); setting the exit altitude threshold to a value which equals to (DEA + ^^); wherein the desired exit altitude is determined to enable the aircraft to exit the descending spiral, glide along the U-turn approach section and arrive at the landing window in an acceptable altitude range, assuming flaps are in retracted position for half the distance of the U-turn approach section and are in extended position for half of the distance of the U-turn approach section.

16. The emergency control unit according to any one of claims 12 to 15, further configured to generate the landing approach path comprising: obtaining information indicative of aircraft situation data including at least: current position of aircraft and glide ratio of aircraft; obtaining information indicative of position of landing site; determining a descending spiral located above landing site area; determining a U-turn approach section comprising a downwind leg, an upwind leg and a u-turn leg connecting the downwind and upwind legs, the downwind leg terminating at the landing window; the U-turn approach section is determined, according to current wind conditions, to enable the aircraft to glide along the downwind leg substantially in the wind direction and enable the aircraft to glide along the upwind leg substantially opposite to the wind direction.

17. The emergency control unit according to any one of claims 12 to 16 wherein the processing unit is configured to perform the following operations for determination of the flaps-extension-altitude: determining estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be extended when aircraft is at a selected altitude; increasing the value of the selected altitude if estimated future altitude is greater than altitude of the landing window; decreasing the value of the selected altitude if estimated future altitude is lower than altitude of the landing window; repeating determination of estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be extended at the selected altitude until the value of the selected altitude converged to a value within the acceptable altitude range of the landing window; and setting the flaps-extension-altitude as the selected altitude.

18. The emergency control unit according to any one of claims 12 to 17 wherein the processing unit is configured to perform the following operations for determination of the flaps-retraction-altitude: determine estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be retracted when aircraft is at a selected altitude; decrease the value of the selected altitude if estimated future altitude is greater than altitude of the landing window; increase the value of the selected altitude if estimated future altitude is lower than altitude of the landing window; repeat determination of estimated future aircraft altitude at the end of the U-turn approach section if flaps are to be retracted at the selected altitude until the value of the selected altitude has converged to a value within the acceptable altitude range of the landing window; and set the flaps-retraction-altitude as the selected altitude.

19. The emergency control unit according to any one of claims 13 to 18 wherein the processing unit is further configured to: determine a respective predicted glide ratio for each segment along the U- turn approach section; a segment being a leg or a part thereof; wherein the determination of each respective predicted glide ratio takes into consideration wind conditions and position of flaps in the respective segment; based on at least the current position of the aircraft and the glide ratio in each segment, calculate altitude loss in the segment; and combine the altitude loss of all segments and determine an estimated altitude loss along the U-turn approach section.

20. The emergency control unit according to any one of claims 12 to 19 comprising at least a first processing unit and a second processing unit, each processing unit configured to operate individually; the first processing unit is configured to repeatedly determine the flaps- extension-altitude and/or the flaps-retraction-altitude while the aircraft is gliding along the U-turn approach section; and feed the determined flaps-extension-altitude and/or the flaps-retraction-altitude to the second processing unit; the second processing unit is configured to repeatedly determine whether real-time altitude of the aircraft is equal to or lower than flaps-extension-altitude and/or the flaps-retraction-altitude received from the first processing unit and generate instructions to change flaps position accordingly; wherein an operation performed by the second processing unit can be repeated at a higher frequency than an operation performed by the first processing unit.

21. The emergency control unit according to any one of claims 12 to 20 comprising a landing site determination module configured to determine a suitable landing site.

22. The emergency control unit according to any one of claims 12 to 20 comprising an autonomous landing control module; wherein the aircraft is capable of descending from the landing window and landing on a selected landing site; the autonomous landing control module configured to initiate an autonomous landing procedure, upon arrival to landing window, and generate instructions for landing the aircraft at the landing site.

23. The emergency control unit according to any one of claims 12 to 22 installed and operating on board an Unmanned Aerial Vehicle.

24. An aircraft comprising an emergency control unit according to any one of claims 12 to 23.

25. A non-transitory program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method of autonomous gliding of an aircraft with a non-operating engine from a current position in the sky to a desired position, the method comprising: obtaining information with respect to a landing approach path; the landing approach path comprising at least a descending spiral section followed by a U-turn approach section; the U-turn approach section terminating with a landing window located at a predefined altitude; obtaining information with respect to an exit altitude threshold; generating instructions to exit the descending spiral and proceed to the U- turn approach section, if aircraft real-time altitude is equal to or lower than the exit altitude threshold; wherein exiting from a descending spiral is performed with flaps in retracted position; while gliding along the U-turn approach section, repeatedly determining flaps-extension-altitude; the flaps-extension-altitude being an altitude at which the flaps are to be extended in order to enable the aircraft to glide and reach the landing window in an acceptable altitude range; and generating instructions to extend flaps if real-time altitude of the aircraft is equal to or lower than flaps-extension-altitude.