Patent Description:
A system on an aircraft may be configured to issue an affirmative aircraft landing signal in response to a landing event occurring. For example, a landing event may begin when an aircraft is close to the ground and the pilot has visual contact with the runway, followed possibly by initial ground contact with one or more of the aircraft's tyres, then by sustained ground contact and, finally, when weight is increased on the wheels.

It is important to determine accurately when certain stages of an aircraft landing event have occurred. This is partly so that aircraft retardation functions can be deployed at an appropriate moment. Retardation functions may include one or more of thrust reversers, ground spoilers, wheel braking and/or lift dumpers, for instance, and may generally be referred to herein as 'braking'. A delay in accurately determining when the certain stages of an aircraft landing event has or have occurred may cause a delay in initiating retardation functions, which may in turn influence the stopping capability and/or distance of the aircraft. These factors may influence which airports certain aircraft can access, and/or place weight restrictions on certain aircraft that need to access certain airports, where, for example, the airports have a restricted runway length.

Some known aircraft landing systems are adapted to detect relative movement of certain landing gear elements to establish when ground contact has occurred. Such systems may be referred to as weight-on-wheel (WoW) detection systems using WoW detectors. As aircraft sizes are generally increasing, so too are the pre-loads used in aircraft suspension systems. As such, the time between an aircraft contacting the ground and an aircraft system detecting relative movement within a suspension system may increase. This may result in delayed aircraft braking, and the need for longer runways or the exclusion of some aircraft from operating in some airports. <CIT> discloses an aircraft landing gear warning system that incorporates means of determining the aircraft's proximity to the ground and the status of the landing gear. <CIT> discloses an apparatus and a method for determining and indicating a distance between a landing small airplane and a landing surface. <CIT> discloses a computer that accepts inputs from a surface monitor to determine whether the terrain below the aircraft is water or land.

A first aspect of the present invention provides a system to detect aircraft ground proximity, the system comprising: a transmitter for transmitting a radio frequency signal along an extended landing gear of the aircraft so that the landing gear is configured to act as a transmitting antenna of the radio frequency signal; a sensor configured to detect a parameter of the transmitted radio frequency signal, and a controller configured, on the basis of an output of the sensor, to issue a landing signal when the detected parameter or a change, detected by the controller, in the detected parameter meets a predetermined criterion, wherein the predetermined criterion is indicative of a certain aircraft ground proximity.

Optionally, the detected parameter is affected by contact between the extended landing gear and the ground, and the parameter meets the predetermined criterion upon contact between the extended landing gear and the ground, whereby the issued landing signal indicates zero distance between the aircraft and the ground.

Optionally, the transmitter is connected to the extended landing gear at a distance from a point of the aircraft from which the extended landing gear extends that is an odd-multiple of a quarter wavelength of the radio frequency signal.

Optionally, the extended landing gear is configured to act as a receiving antenna of the radio frequency signal transmitted by the transmitter and is configured, in use, to receive the radio frequency signal transmitted along the landing gear, and the sensor is configured to detect a parameter of the received radio frequency signal.

Optionally, the extended landing gear is a first extended landing gear, and wherein a second extended landing gear of the aircraft is configured to act as a receiving antenna of the radio frequency signal, wherein the sensor is associated with the second extended landing gear and is configured to detect a parameter of the radio frequency signal at the second extended landing gear, wherein the transmitter and the sensor are electrically connected only when the first extended landing gear and the second extended landing gear are both in contact with the ground, and wherein the controller is configured to issue the landing signal when it is determined, based on the output of the sensor, that both of the first and the second landing gears are in contact with the ground.

Optionally, the detected parameter comprises capacitance between the first extended landing gear and the second extended landing gear.

Optionally, the detected parameter comprises an electrical current between the transmitter and the sensor.

Also disclosed, but not claimed, is a system to detect aircraft ground proximity, the system comprising a transmitter configured to transmit a radio frequency signal as an electric current along an extended landing gear of an aircraft and through a coil positioned at a distal end of the extended landing gear, a sensor configured to detect a parameter related to the coil, the detected parameter indicative of a distance between the coil and the ground, and a controller configured, on the basis of an output of the sensor, to issue a landing signal when the detected parameter indicates that the distance between the coil and the ground meets a threshold distance.

The transmitter of this unclaimed system may be configured to generate an oscillating magnetic current in the coil when the radio frequency signal is transmitted through the coil, and the detected parameter comprises a rate of oscillation of the magnetic current.

The transmitter of this system may be configured to generate a magnetic field around the coil, and the parameter is detectably changed when an electrically-conductive penetrates the magnetic field. Optionally, the parameter comprises a parameter of one or more eddy currents generated in the electrically-conductive object that penetrates the magnetic field.

The detected parameter of this system may comprise an inductance of the coil.

The coil may be positioned on an underside of the landing gear, wherein the underside of the landing gear faces the ground when the landing gear is extended.

Also disclosed is an unclaimed system to detect aircraft landing, the system comprising a controller of an aircraft, the controller configured to determine whether a parameter of a radio frequency signal transmitted through part of the aircraft indicates that the proximity of the aircraft to the ground meets a threshold, and issue a landing signal when the threshold is met.

For this system, the threshold may be met only when the distance between the aircraft and the ground is zero.

A second aspect of the present invention provides an aircraft comprising a landing gear, at least one electrically-conductive tyre mounted on the landing gear; and a system according to the first aspect of the present invention, wherein the landing gear of the aircraft is the extended landing gear of the system.

A third aspect of the present invention provides a method to detect aircraft ground proximity, the method comprising: transmitting a radio frequency signal along an extended landing gear of the aircraft so that the landing gear is configured to act as a transmitting antenna of the radio frequency signal, detecting a parameter of the transmitted radio frequency signal, and issuing a landing signal when the detected parameter or a change in the detected parameter meets a predetermined criterion, wherein the predetermined criterion is indicative of aircraft ground proximity.

Optionally, the method comprises transmitting the radio frequency signal from a point on the extended landing gear that is an odd-multiple of a quarter wavelength of the radio frequency signal from a body of the aircraft.

Also disclosed is an unclaimed method to detect aircraft ground proximity, the method comprising transmitting a radio frequency signal as an electric current along an extended landing gear of the aircraft and through a coil positioned at a distal end of the extended landing gear; detecting a parameter related to the coil, the detected parameter indicative of a distance between the coil and the ground; and issuing a landing signal when the detected parameter indicates that the distance between the coil and the ground meets a threshold distance. Optionally, the method comprises generating an oscillating magnetic current in the coil when the radio frequency signal is transmitted through the coil, wherein the detected parameter comprises a rate of oscillation of the magnetic current.

The following disclosure relates to apparatus and processes for detecting aircraft ground proximity during aircraft landing. For aircraft landing, the landing gears of the aircraft are in an extended position. In some aircraft, particularly light aircraft, one or more of the landing gears are fixed in an extended position throughout flight. In other aircraft, the landing gears are stowed in a retracted position during flight, and are moved to an extended position for landing, as is well-known in the art. The landing gears contact the ground upon landing and support the weight of the aircraft.

Unless otherwise stated, aircraft ground proximity is taken as the distance between the ground and the lowest part of a tyre mounted on a distal end of an extended landing gear. Systems according to the invention may be configured to also detect zero distance between the aircraft and the ground, for example when the tyre contacts the ground.

Embodiments of the present invention provide a system to detect aircraft landing. The system comprises a controller, such as the controller <NUM> shown in <FIG>. The controller is configured to, in use, determine whether a parameter of a radio frequency signal transmitted through part of an aircraft indicates that a proximity of the aircraft to the ground meets a threshold, and issue a landing signal when the threshold is met. The threshold may be a predetermined distance between the aircraft and the ground. The threshold may be a distance of zero between the aircraft and the ground.

The landing signal issued by the controller may be sent to other aircraft systems, which may automatically activate aircraft retardation functions to reduce the velocity of the aircraft in response to the landing signal. The landing signal may be sent to the cockpit of the aircraft to indicate to the aircraft crew the proximity of the aircraft to the ground.

<FIG> illustrates a simplified diagram of a system <NUM> according to an embodiment of the invention. The system <NUM> is configured to detect aircraft ground proximity. In some embodiments, the system <NUM> is configured to detect that a distance between an aircraft <NUM> that comprises the system <NUM> and the ground is below certain threshold. In some embodiments, the system is configured to detect that the distance between the aircraft <NUM> and the ground is zero. The system <NUM> comprises a transmitter <NUM> for transmitting a radio frequency (RF) signal along an extended landing gear <NUM> of the aircraft <NUM>. The radio frequency signal is an electric current that alternates as a radio frequency. The landing gear <NUM> is configured to act as a transmitting antenna of the RF signal. The system <NUM> comprises a sensor <NUM> configured to detect a parameter of the radio frequency signal transmitted by the transmitter <NUM>. The parameter may be affected by the RF signal transmitted by the transmitter <NUM>. The system comprises a controller <NUM> configured to detect a change in the parameter detected on the basis of an output of the sensor <NUM>, and to issue a landing signal when the change in the detected parameter meets a predetermined criterion. The predetermined criterion is indicative of a certain aircraft ground proximity.

The aircraft landing gear <NUM> shown in <FIG> comprises a strut <NUM>, a bogie <NUM> at a distal end of the landing gear <NUM> and one or more tyres <NUM> mounted to the bogie <NUM>. In some embodiments, the landing gear <NUM> may not comprise a bogie <NUM>, and the tyres <NUM> may be instead mounted to another structure at the distal end of the landing gear <NUM>. The landing gear <NUM> extends from the aircraft body <NUM> from a point A. The tyres <NUM> are electrically-conductive, as is known in the art. In other embodiments, the aircraft landing gear may comprise skids instead of tyres <NUM>. It is to be understood that reference to tyres when describing any embodiments of the present invention may equally apply to skids, wherein the "ground" is water. The landing gear <NUM> may be a nose landing gear or a main landing gear of the aircraft <NUM>.

In some embodiments, the transmitter <NUM> is positioned on the landing gear <NUM>, and may be at or near point <NUM>. In other embodiments, as shown in <FIG>, the transmitter <NUM> is physically located in the aircraft body <NUM> and is electrically connected to point <NUM>, for example by an electrically-conductive wire or other electrically-conductive path. In some embodiments, the sensor <NUM> is positioned on the landing gear <NUM>, and may be at or near point <NUM>. In other embodiments, as shown in <FIG>, the sensor <NUM> is physically located in the aircraft body <NUM> and is electrically connected to point <NUM>, for example by an electrically-conductive wire or other electrically-conductive path.

In the embodiment shown in <FIG>, the parameter detected by the sensor <NUM> is affected by contact between the extended landing gear <NUM> and the ground (not shown). The parameter meets the predetermined criterion upon contact between the extended landing gear and the ground whereby the issued landing signal indicates zero distance between the aircraft and the ground. In some embodiments, the parameter meets the predetermined criterion when at least one of the tyres <NUM> is in contact with the ground.

In the embodiment shown in <FIG>, the landing gear <NUM> is configured to act as a transmitting antenna of the RF signal. That is, the landing gear <NUM> comprises an electrically conductive path between the point <NUM> at which the transmitter <NUM> is connected to the extended landing gear <NUM> and the tyres <NUM>. The point <NUM> at which the transmitter <NUM> is connected to the extended landing gear <NUM> is at a distance X from the point A, from which the extended landing gear <NUM> extends from the aircraft body <NUM>. In some embodiments, the distance X is substantially equal to any odd-multiple of a quarter wavelength of the RF signal, for example <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM> etc., which can be more generally defined as X = (2y-<NUM>) λ/<NUM> wavelength, where y is an integer and λ is the wavelength of the RF signal.

The point <NUM> being positioned at such a distance (X = (2y-<NUM>) λ/<NUM>) from the aircraft body <NUM> is significant because, in use, the RF signal transmitted by the transmitter <NUM> will be reflected off the aircraft body <NUM>, rather than being conducted by the aircraft body <NUM>. This can be advantageous. For example, the RF signal transmitted by the transmitter <NUM> may not disrupt other systems on the aircraft <NUM>. For example, the RF signal may be transmitted only from the distal end of the landing gear <NUM>, remote from point A.

Dependent on the type of aircraft <NUM> and the position of the landing gear <NUM> on the aircraft <NUM>, the aircraft body <NUM> may, for example, be part of the wing of the aircraft <NUM> or the fuselage of the aircraft <NUM>.

In some embodiments, the detected parameter comprises a frequency of the RF signal. The frequency may be affected by contact between at least one of the tyres <NUM> of the landing gear <NUM> at the ground because such contact changes the effective length of the antenna due to the electrically-conductive properties of the at least one tyre <NUM> and the ground. In some embodiments, the parameter detected by the sensor <NUM> may comprise the power of the RF signal. Because the tyres <NUM> are electrically-conductive, they conduct the RF signal, which is then conducted by the ground upon contact between at least one of the tyres <NUM> and the ground, thus changing the detected power of the RF signal.

In the embodiment shown in <FIG>, the landing gear <NUM> is configured to act as a receiving antenna of the RF signal transmitted by the transmitter <NUM> and is configured, in use, to receive the radio frequency transmitted along the landing gear <NUM>. The landing gear <NUM> may be configured to receive a reflection of the RF signal transmitted along the landing gear <NUM>. In some embodiments, the sensor <NUM> is configured to detect a parameter of the reflected the RF signal received at the landing gear <NUM>.

<FIG> illustrates a simplified diagram of a system <NUM> according to an embodiment of the invention. The system <NUM> is configured to detect that a distance between the aircraft <NUM> and the ground is zero. The system <NUM> comprises a transmitter <NUM>, a receiver <NUM> and a controller <NUM> which are substantially the same as the transmitter <NUM>, receiver <NUM> and controller <NUM> described with reference to <FIG>, and will therefore not be described again in detail here.

The aircraft <NUM> shown in <FIG> comprises a first landing gear <NUM> and a second landing gear <NUM>. The first landing gear <NUM> and the second landing gear <NUM> are on opposite sides of the aircraft <NUM>. The first landing gear <NUM> is substantially the same as the landing gear <NUM> described above with reference to <FIG>. The second landing gear is substantially the same as the first landing gear, and like features have been given corresponding reference numbers, but increased by <NUM>, and corresponding letters. The system <NUM> comprises a transmitter <NUM>, a sensor <NUM> and a controller <NUM>. The second landing gear <NUM> is configured to act as a receiving antenna of the RF signal transmitted by the transmitter <NUM> along the first landing gear <NUM>. The transmitter <NUM> and the sensor <NUM> are electrically connected only when the first extended landing gear <NUM> and the second extended landing gear <NUM> are both in contact with the ground. The controller <NUM> is configured to issue the landing signal when it is determined, based on the output of the sensor <NUM>, that both of the first and the second landing gears <NUM>, <NUM> are in contact with the ground.

The transmitter <NUM> shown in <FIG> is for transmitting an RF signal along the first landing gear <NUM>. In some embodiments, the transmitter <NUM> is positioned on the first landing gear <NUM>, and may be at point <NUM>. In other embodiments, as shown in <FIG>, the transmitter <NUM> is physically located in the aircraft body <NUM> and is electrically connected to point <NUM>, for example by an electrically-conductive wire or other electrically-conductive path. The sensor <NUM> is configured to detect a parameter of the RF signal at a point <NUM> on the second landing gear <NUM>. In some embodiments, the sensor <NUM> is positioned on the second landing gear <NUM>, and may be at point <NUM>. In other embodiments, as shown in <FIG>, the sensor <NUM> is physically located in the aircraft body <NUM> and is electrically connected to point <NUM>, for example by an electrically-conductive wire or other electrically-conductive path.

In some embodiments, the points <NUM>,<NUM> are located on their respective landing gear <NUM>, <NUM> at a distance X from the respective point A, from which the respective landing gear <NUM> extends from the aircraft body <NUM>. In some embodiments, the distance X is substantially equal to any odd-multiple of a quarter wavelength of the RF signal, for example <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM> etc., which can be defined as X = (2y-<NUM>) λ/<NUM>, where y is an integer and λ is the wavelength of the RF signal. In the embodiment shown in <FIG>, the points <NUM>, <NUM> are an equal respective distance X from the respective point A. In other embodiments, the points <NUM>, <NUM> may be at differing distances from the respective point A compared to one another. The position of the points <NUM>, <NUM> shown in <FIG> helps to prevent conduction of the RF signal transmitted by the transmitter <NUM> through the aircraft body <NUM>. If the RF signal were conducted through the aircraft body <NUM>, a parameter of the RF signal conducted through the aircraft body <NUM> would be detected by the sensor <NUM> and may not be affected by aircraft ground proximity.

The system <NUM> shown in <FIG> is configured so that the landing signal is issued by the controller <NUM> only when an electrical connection is made between the transmitter <NUM> and the sensor <NUM>. The electrical connection is made when at least one of the tyres <NUM> of the first landing gear <NUM> and at least one of the tyres <NUM> of the second landing gear <NUM> make contact with the ground, so that the RF signal is conducted from the point <NUM> on the first landing gear <NUM>, through the at least one tyre <NUM> of the first landing gear <NUM>, through the ground, through the at least one tyre <NUM> of the second landing gear <NUM>, and to the point <NUM>, where a parameter of the RF signal is detected by the sensor <NUM>.

In some embodiments, the sensor <NUM> is configured to measure capacitance. When the aircraft <NUM> is in flight, the sensed capacitance will have a different value compared to when a tyre <NUM>, <NUM> on each of the landing gears <NUM>, <NUM> is in contact with the ground. Any suitable sensor could be used, as known in the art. The predetermined criterion may be a threshold capacitance and/or a threshold change in capacitance, so that a measured capacitance meeting the predetermined criterion is indicative of aircraft ground contact. In some embodiments, the sensor <NUM> may be used in the same way, but may be configured instead to detect electrical current between the transmitter <NUM> and the sensor <NUM>.

<FIG> illustrates a simplified diagram of an alternative example system <NUM> that is not claimed. The system <NUM> is configured to detect aircraft ground proximity. In some examples, the system <NUM> is configured to detect that a distance between an aircraft <NUM> that comprises the system <NUM> and the ground is below certain threshold. In some examples, the system <NUM> is configured to detect that a distance between the aircraft <NUM> and the ground is zero. The system <NUM> comprises a transmitter <NUM> configured to transmit a RF signal along a landing gear <NUM> and through a coil <NUM> positioned at a distal end of the landing gear <NUM>, remote from an aircraft body <NUM>. The system <NUM> comprises a sensor <NUM> configured to detect a parameter related to the coil <NUM>. The parameter is indicative of a distance between the coil <NUM> and the ground. A controller <NUM> comprised in the system <NUM> is configured to detect a change in the parameter detected by the sensor <NUM> and to issue a landing signal when it is determined that the parameter indicates that the distance between the coil <NUM> and the ground meets a threshold distance.

In some examples, the transmitter <NUM> and/or the sensor <NUM> are positioned on the landing gear <NUM>. In other embodiments, as shown in <FIG>, the transmitter <NUM> and/or the sensor <NUM> are physically located in the aircraft body <NUM> and are electrically connected to the landing gear <NUM>, for example by an electrically-conductive wire or other electrically-conductive path.

In the example shown in <FIG>, the coil <NUM> is positioned on an underside of the landing gear <NUM>. The underside of the landing gear <NUM> faces the ground when the landing gear <NUM> is extended. In the example shown in <FIG>, the underside of the landing gear <NUM> is an underside of the bogie <NUM>. In other examples, the coil <NUM> may be positioned elsewhere on the landing gear <NUM>.

Transmitting an RF signal from the transmitter <NUM> to the coil <NUM> generates a magnetic field around the coil <NUM>. In some examples, as the aircraft <NUM> approaches the ground, the generated magnetic field penetrates the electrically-conductive ground, which detectably changes the parameter related to the coil <NUM> that is detected by the sensor <NUM>. In some examples, the parameter comprises an inductance of the coil <NUM>.

In some examples, the magnetic field induces eddy currents in conductive materials that penetrate the magnetic field. For example, eddy currents may be induced in a portion of the landing gear <NUM> adjacent to the coil <NUM>. The eddy currents in the landing gear <NUM> may be affected by contact between at least one of the tyres <NUM> and the ground. By way of further example, eddy currents may be induced in the ground as the aircraft <NUM> approaches the ground. The eddy currents in the ground may be affected by the proximity of the aircraft <NUM> to the ground. In such examples, the parameter may comprise a parameter of the eddy currents in the landing gear <NUM> and/or the ground.

In some examples, the transmitter <NUM> causes the generation of an oscillating magnetic current in the coil <NUM> when the RF signal is transmitted through the coil <NUM>. The detected parameter may comprise a rate of oscillation of the magnetic current. In other examples, the parameter may comprise current in the coil <NUM>.

In some examples, the threshold distance may be set so that the landing signal is issued after ground contact and before the full weight of the aircraft <NUM> is taken by the wheels. When the aircraft is stationary on the ground, the tyres deflect under the weight of the aircraft, meaning that the coil <NUM> is closer to the ground than at initial contact between the aircraft and the ground. At the moment when the aircraft tyres <NUM> make contact with the ground, a majority of the aircraft weight may still be lifted by the wings. The tyres <NUM> therefore do not necessarily fully deflect immediately upon initial ground contact. In some embodiments, the threshold distance may be set so that the landing signal is issued before the aircraft <NUM> is in contact with the ground, but when it is detected that ground contact is imminent. For example, the threshold distance may be set as up to <NUM>, <NUM> or <NUM> greater than a vertical distance between the coil <NUM> and the bottom of the tyres <NUM>. Such examples can help to account for a delay between issue of the landing signal and the activation of aircraft retardation functions.

<FIG> is a schematic diagram of an aircraft <NUM> according to an embodiment of the invention. The aircraft <NUM> comprises wings <NUM>, a fuselage <NUM>, two main landing gears <NUM> and a nose landing gear <NUM>. Upon each landing gear <NUM>, <NUM> is mounted at least one electrically-conductive tyre <NUM>. In the embodiment shown in <FIG>, the main landing gears <NUM> extend from the aircraft wings <NUM> and the nose landing gear <NUM> extends from the fuselage <NUM>.

The aircraft <NUM> comprises one or more systems according to embodiments of the invention, such as the systems <NUM>, <NUM>, described with reference to <FIG>. In embodiments in which the landing gear of the system according to an embodiment of the invention is a main landing gear <NUM>, the aircraft body <NUM> may be a respective wing <NUM> of the aircraft <NUM>. In other embodiments, the main landing gear <NUM> may extend from the aircraft fuselage <NUM>. In embodiments in which the landing gear of the system according to an embodiment of the invention is the nose landing gear <NUM>, the aircraft body <NUM> is the aircraft fuselage <NUM>.

In some embodiments, the aircraft <NUM> comprises a first system <NUM> and a second system <NUM>, one or each of which embodying the invention, and other aircraft systems (not shown) are configured to activate aircraft retardation functions only when both the first and second systems <NUM>, <NUM> have issued a landing signal. In some embodiments, the first and second systems <NUM>, <NUM> have a common controller configured to issue a landing signal only when it is determined that the predetermined criterion for each system <NUM>, <NUM> has been met. This can help to prevent premature activation of aircraft retardation functions, which may occur due to an incorrect reading by one of the systems <NUM>, <NUM>. In some embodiments, the aircraft comprises two systems <NUM> embodying the invention, each of the systems <NUM> being associated with a respective one of the main landing gears <NUM>. In some such embodiments, the transmitters <NUM> of each system <NUM> may be configured to transmit RF signals of different frequencies to one another and/or to transmit RF signals that are phase shifted relative to one another, to help prevent interference between the two systems <NUM>. In some embodiments, the two systems <NUM> may have a common controller <NUM> configured to issue a landing signal only when it is determined that the predetermined criterion for each system <NUM> has been met.

<FIG> is a flow diagram showing a method <NUM> according to an embodiment of the present invention. The method <NUM> comprises: transmitting <NUM> an RF signal along an extended aircraft landing gear so that the landing gear is configured to act as a transmitting antenna of the radio frequency signal, detecting <NUM> a parameter of the transmitted RF signal; and issuing <NUM> a landing signal when a change in the detected parameter meets a predetermined criterion. The predetermined criterion is indicative of aircraft ground proximity. In some embodiments, the predetermined criterion may be indicative of zero distance between the aircraft and the ground.

In some embodiments, the transmitting <NUM> and the detecting <NUM> occur only when the aircraft landing gear is in an extended position. In some embodiments, the transmitting <NUM> and the detecting <NUM> are initiated when the altitude of the aircraft falls below an altitude threshold, or when the speed of the aircraft falls below a speed threshold.

The method <NUM> may be performed by a system according to an embodiment of the invention, for example a system <NUM>, <NUM>, described with reference to any one of <FIG>. The transmitting <NUM> may be performed by a transmitter <NUM>, <NUM>, which may be positioned on the extended aircraft landing gear or may be positioned within a body of the aircraft, such as the wing or fuselage of the aircraft. The detecting <NUM> may be performed by a sensor <NUM>, <NUM>. The issuing <NUM> may be performed by a controller <NUM>, <NUM>,.

In some embodiments, the method <NUM> comprises transmitting <NUM> the RF signal from a point on the extended aircraft landing gear that is an odd-multiple of a quarter wavelength of the RF signal from point from which the landing gear extends from a body of the aircraft, for example <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM> etc., which can be defined as (2y-<NUM>) λ/<NUM>, where y is an integer and λ is the wavelength of the RF signal.

Some embodiments of the invention comprise an aircraft, such as the aircraft <NUM> shown in <FIG>, configured to perform the method <NUM>. In some embodiments, the aircraft comprises wings, a fuselage, two main landing gears and a nose landing gear. Upon at least one of the landing gears is mounted at least one electrically-conductive tyre. In some embodiments, the aircraft comprises a transmitter to transmit an RF signal along a landing gear of the aircraft, a receiver to detect a parameter related to the transmitted RF signal, and a controller to issue the landing signal when a change in the detected parameter meets a predetermined criterion.

It is to be noted that the term "or" as used herein is to be interpreted to mean "and/or", unless expressly stated otherwise.

Claim 1:
A system to detect aircraft ground proximity, the system comprising:
a transmitter (<NUM>) for transmitting a radio frequency signal along an extended landing gear (<NUM>) of the aircraft,
a sensor (<NUM>) configured to detect a parameter of the transmitted radio frequency signal, and
a controller (<NUM>) configured, on the basis of an output of the sensor (<NUM>), to issue a landing signal when the detected parameter or a change, detected by the controller (<NUM>), in the detected parameter meets a predetermined criterion, wherein the predetermined criterion is indicative of a certain aircraft ground proximity,
characterized in that
the landing gear (<NUM>) is configured to act as a transmitting antenna of the radio frequency signal.