Patent Description:
Modem aircraft typically have landing gear comprising a plurality of undercarriages which support the aircraft when it is on the ground. Some of the undercarriages have braking wheels which are operable to provide a braking force to decelerate the aircraft when a braking torque is applied by a set of brakes. One or more of the undercarriages is steerable and may be rotated to steer the aircraft.

During some ground manoeuvres, the aircraft may experience unintended changes in heading, for example due to external factors such as cross-winds, gusts and variations in a runway surface, as well as aircraft factors such as uneven tyre pressure, asymmetric braking, asymmetric engine thrust or component failures. To compensate for these asymmetries, some aircraft include heading control systems which can control the nosewheel angle without input from the flight crew. Examples are described in <CIT> and <CIT>.

Aircraft with shorter wheelbases tend to be more sensitive to steering inputs and so the range of angles within which a heading control system can control the nose wheel without risk of severe lateral movement is reduced. Accordingly, the degree to which the heading control system is able to control the nosewheel angle during ground manoeuvres tends to be lower.

Furthermore, to reduce levels of noise and pollution at airports, and to reduce fuel consumption, it is common practice to perform certain ground manoeuvres, such as taxiing, with one or more engines not producing significant thrust. For example, in certain dual engine aircraft, it is common prior to take-off to taxi to the runway using only one engine, and/or to shut down one of the engines shortly after landing. This results in an asymmetry in the thrust to produce a yawing moment, which is exacerbated in shorter wheelbase aircraft, which commonly have only two engines: one engine mounted under each wing.

The present invention mitigates the above-mentioned problems and accordingly may provide an improved system and method for controlling the heading of an aircraft.

<FIG> is a plan view of an aircraft <NUM> having a fuselage <NUM> and wings including a left wing <NUM> and a right wing 104R extending outwardly from the fuselage <NUM>. The aircraft <NUM> is aligned with a set of axes including a longitudinal axis denoted by an arrow labelled x, which is parallel to the direction of the aircraft <NUM> in straight and level flight and a lateral axis denoted by an arrow labelled y in a direction perpendicular to the x axis. The aircraft <NUM> has a centre of gravity <NUM>.

During ground operations, the aircraft <NUM> might, in some circumstances be propelled forward by its engines, which in the example shown in <FIG>, include a left engine <NUM> and a right engine 108R. In other examples the aircraft might include more than one left and right engines. The left engine <NUM> might be mounted on or under the left wing <NUM> and the right engine <NUM> might be mounted on or under the left wing 104R.

When both engines <NUM>, 108R are providing the same amount of thrust, indicated by thrust vectors <NUM>, 109R shown as arrows in <FIG>, the engines do not provide a yawing moment about the centre of gravity <NUM>.

<FIG> is a side view of an aircraft such as the aircraft <NUM> shown in <FIG>. The aircraft <NUM> has landing gear which supports the aircraft when it is on the ground and controls the movement of the aircraft <NUM> during ground manoeuvres such as landing, taxiing and take off. The landing gear comprises a main landing gear (referred to hereinafter as the MLG <NUM>) and a nose landing gear (referred to hereinafter as the NLG <NUM>). During ground manoeuvres, at speeds at which a vertical stabilizer <NUM> of the aircraft does not produce enough of an aerodynamic effect to steer the aircraft <NUM>, the NLG <NUM> is steerable to steer the aircraft <NUM>. To steer the aircraft <NUM>, the NLG <NUM> is moved by an angle with respect to the x axis to steer the aircraft <NUM>. Such movements of the NLG <NUM> can be manually steered under control of the flight crew using a tiller, steering wheel, or other steering mechanism provided in the cockpit of the aircraft <NUM>.

In the absence of other yawing moments, when the both the left and right engines <NUM>, 108R are providing equal thrust, the thrust provided by the engines <NUM>, 108R is applied equally about the centre of gravity <NUM> and the aircraft <NUM> will advance along the x axis. During ground manoeuvres, the aircraft <NUM> will experience forces that, without correction, may cause the heading of the aircraft <NUM> to deviate from the x axis. For example, influences such as crosswinds, cambers on the taxiways and runway, asymmetry in tyre pressures or tyre radii of different tyres of the MLG <NUM>, or braking effects in different wheels of the MLG <NUM>, for example, may result in the heading of the aircraft <NUM> deviating even while the thrust provided by each of the engines <NUM>, 108R is equal, and without any input from the flight crew.

To reduce the workload of the flight crew in correcting these deviations, some aircraft are equipped with a control system, referred to herein as a heading control system, that can adjust the angle of the NLG <NUM> without manual input from the flight crew. The degree to which such heading control systems can adjust the angle of the NLG <NUM> (referred to hereinafter as an "authority") is limited to ensure that the heading control system cannot make unsafe adjustments to the NLG <NUM>, and cannot mask potentially unsafe states to the flight crew; for example, the limits may be determined to prevent the aircraft overcompensating from deviations from an intended heading (for example, veering to one side or the other) that may, in some circumstances, cause the aircraft <NUM> to manoeuvre away from paved areas of an airfield, and/or provide an uncomfortable feeling to passengers in the cabin of the aircraft <NUM>. The authority may define a range of angles within which the NLG <NUM> can be controlled to provide a range of steering angles 114a as shown in <FIG>.

<FIG> illustrates a heading control system <NUM> according to an embodiment. The control system <NUM> is a computerised device implemented by a controller <NUM>, which may be a processor executing software instructions stored in a memory <NUM> on the basis of inputs received via an interface <NUM>. Although, in the example shown in <FIG>, the heading control system <NUM> is implemented in software executed by hardware (controller <NUM>) in some examples, the heading control system <NUM> may be implemented entirely in hardware.

The controller <NUM> may be arranged to provide proportional-integral-derivative (PID) control, in which gain settings for proportional, integral and derivative terms may be summed together to provide an overall gain value that is then applied to a feedback loop controlling an angle of the NLG <NUM>. In other examples, other forms of controller may be used. For example, the controller <NUM> may provide only proportional and integral control (a so-called PI controller), only proportional and derivative control (a so-called PD controller) or only proportional control.

The controller <NUM> may be arranged to operate in a number of modes with each mode having an associated set of gain terms corresponding to the proportional, integral and derivative terms applied to the controller <NUM>.

For example, the controller <NUM> may operate in a first mode when the aircraft is operating with both engines <NUM>, 108R providing equal thrust vectors <NUM>, 109R, as shown in <FIG>.

The interface <NUM> is arranged to receive incoming data referred to hereinafter as a bias signal <NUM> and to transmit outgoing data referred to hereinafter as a control signal <NUM>.

The bias signal <NUM> may include data that the controller <NUM> may apply or use to determine gain values to generate the control signals <NUM> for controlling the NLG <NUM>.

For example, in the first mode, gain terms applied by the controller <NUM> may be selected or "tuned" such that the controller <NUM> provides a control signal <NUM> indicating a deviation from a determined (e.g. straight-line, along the x-axis) heading, with equal priority given to corrections to the left (port) and right (starboard). That is, the authority of the controller <NUM> (i.e. the range of angles within the NLG <NUM> is permitted to be controlled) is centred on the longitudinal axis of the aircraft <NUM> (i.e. the x-axis) and is substantially equal from port to starboard (left to right). The controller <NUM> of the heading control system <NUM> may have authority to provide automated control of the position of the NLG <NUM>. That is the heading control system <NUM> is an automated steering system of the aircraft that, within certain limits that constrain the authority of the heading control system <NUM>, can provided automated control of the NLG <NUM> to steer the aircraft. The limits within which the heading control system <NUM> can control or vary the NLG <NUM> may be a predetermined angular range, which may be stored, for example, in the memory <NUM>. For example, the controller <NUM> may have authority to control the position of the NLG <NUM> within a range of, for example, ± <NUM> degrees of the position set by the pilot (e.g. using the tiller provided in the cockpit). In some examples, the controller <NUM> may have authority to control the position of the NLG <NUM> within a range of ± <NUM> degrees. In some examples, the controller <NUM> may have authority to control the position of the NLG <NUM> within a range of ± <NUM> degrees. In other examples, such as may be implemented in smaller aircraft, the controller <NUM> may have authority to control the position of the NLG <NUM> within a larger range of angles (e.g. ± <NUM> degrees).

In the first mode, the gain terms applied by the controller <NUM> are arranged to provide a substantially equal response whether the controller <NUM> is sending a control signal <NUM> to actuate the NLG <NUM> to turn towards the left (port) or right (starboard). In some examples, the gain terms are determined or selected to provide a relatively slow response with minimal overshoot. That is, the gain terms are selected or determined to be relatively heavily "damped".

The controller <NUM> may, responsive to receipt of the received bias signal <NUM>, operate in a second mode in which the gain terms applied by the controller <NUM> are unequal so that the aircraft <NUM> is more sensitive in respect of control signals <NUM> applied to one side of the aircraft <NUM> or the other.

For example, the bias signal <NUM> may indicate to the controller <NUM> that aircraft is biased to one side or another, and the controller <NUM> may take corrective action in response. The bias signal <NUM> may be determined based on any information that the controller <NUM> can use to determine an asymmetry in the forces applied to the aircraft <NUM>. For example, when the aircraft <NUM> is being operated in a so-called "one-engine-on" mode in which a dual engine aircraft's thrust is provided by only one of the aircraft's engines, the controller <NUM> may determine, select or apply a different set of gain terms based on the bias signal <NUM>. In some examples, the bias signal <NUM> may be based on Boolean signals (i.e. a <NUM> or a <NUM>) indicating whether the respective engines are on or off. In one example, the bias signal <NUM> is based on the statuses of engine oil pressure switches of the respective port and starboard engines <NUM>, 108R of the aircraft <NUM>. For example, if one engine pressure switch (for example that of the port engine <NUM>) indicates that the left engine is on and the other engine pressure switch indicates that the other engine (for example, the starboard engine 108R) is off, then the bias signal <NUM> may indicate an offset between the port and starboard (left and right).

In other examples, the bias signal <NUM> may be based on relative settings applied to the port and starboard engines <NUM>, 108R of the aircraft <NUM>. For example, the bias signal <NUM> may be based on one or more of: throttle settings of the respective engines <NUM>, 108R, temperature measurements from at least one portion of the respective engines <NUM>, 108R, pressure measurements from at least one portion of the respective engines <NUM>, 108R (such as an oil pressure), or a fan speed of the respective engines <NUM>, 108R.

In some examples, the controller <NUM> may determine an offset angle based on the relative settings or values resulting from those settings. For example, the offset angle may be determined based on a difference between the relative settings or a ratio of the relative settings.

The bias signal <NUM> may additionally or alternatively be generated based on inputs not relating to inputs from the flight crew but instead from external influences such as crosswinds such as that shown by the arrow <NUM> in <FIG>.

The bias signal <NUM> in effect defines an angle by which the authority is offset from the longitudinal (i.e. x-axis) of the aircraft. In one example, the bias signal <NUM> is based on (e.g. indicates) readings from one or more sensors of the aircraft. For example, the bias signal <NUM> may indicate a reading, such as a temperature or pressure reading, of each of the port engine <NUM> and the starboard engine 108R. The controller <NUM> may then determine the offset angle based on a relationship between the two readings. For example, data stored in the memory <NUM> may indicate a relationship between the offset angle and a difference between, or a ratio of, the two readings. The stored data may be determined based on, for example, the statuses of engine oil pressure switches of the respective port and starboard engines <NUM>, 108R of the aircraft <NUM>. In other examples, the stored data may be based on one or more of: throttle settings of the respective engines <NUM>, 108R, temperature measurements from at least one portion of the respective engines <NUM>, 108R, pressure measurements from at least one portion of the respective engines <NUM>, 108R (such as an oil pressure), or a fan speed of the respective engines <NUM>, 108R.

In any event the controller <NUM> is arranged to receive the bias signal <NUM> and, on the basis of the bias signal <NUM>, define an offset angle that defines a difference between an angular range centred on the longitudinal axis of the aircraft and an angular range centred on an angle offset from the longitudinal axis of the aircraft.

In the second mode, the gain terms applied by the controller <NUM> are arranged to provide an unequal response when the controller <NUM> sends a control signal <NUM> to actuate the NLG <NUM> to turn towards the left (port) or right (starboard). For example, the gain terms applied, if is a bias towards the right (for example if the left engine <NUM> is providing thrust without thrust being provided by the right engine 108R, as shown in <FIG>, the gain settings applied by the controller <NUM> may accordingly be unequal. For example, a control signal <NUM> representing control instructions to control the NLG <NUM> to the right of the x-axis may be heavily damped such that the response to such instructions is relatively slow response with minimal overshoot. Whereas, a control signal <NUM> representing control instructions to control the NLG <NUM> to the left of the x-axis may be less heavily damped so that the response is quicker but there is a greater degree of overshoot. In some examples the gain settings for the less heavily damped side may be selected to provide critical damping to balance overshoot and response time.

<FIG> is a plan view of an aircraft <NUM> such as the aircraft of <FIG> and <FIG>, in a situation which may cause the controller <NUM> to operate in the second mode. In the example shown in <FIG>, only a left single thrust vector <NUM> is applied and/or a crosswind vector <NUM> is applied to the aircraft to produce a yawing motion about the centre of gravity <NUM> of the aircraft <NUM>.

As a result, the aircraft <NUM> experiences a yawing force about the centre of gravity <NUM> resulting in generation of a bias signal <NUM> indicating, for the example shown in <FIG>, a bias towards the right (starboard) of the aircraft <NUM> about the centre of gravity <NUM>.

In the second mode, as shown in <FIG>, the controller <NUM> may generate or apply gain terms different to those generated or applied in the first mode. In particular the gain terms generated or applied by the controller <NUM> in the second mode may result in the authority being shifted or expanded in one direction to define a range of angles within which the NLG <NUM> can be controlled to provide a different range of steering angles 114b as shown in <FIG>. The range of angles 114b is different to the range of angles 114a shown in <FIG> to the extent that it is unequal in that, in the example shown in <FIG>, gain terms applied to corrections (i.e. steering corrections) that cause the aircraft <NUM> to steer towards the left (port) are arranged to be less heavily damped (for example critically damped) than gain terms applied to corrections that cause the aircraft <NUM> to steer towards the right (starboard). For example, such gain terms provide for an increased degree of control of the NLG <NUM> in one direction (i.e. port rather than starboard) while the controller <NUM> operates within the safe limits of adjustment of the NLG <NUM>.

For example, in the example shown in <FIG>, in which the left engine <NUM> is providing thrust <NUM>, an increase of the yaw about the centre of gravity in a clockwise direction (as viewed from above) will result in a bias signal <NUM> commensurate with (e.g. proportional to) the yawing force, which will form the basis of the control signal <NUM> determined by the or applied by the controller <NUM>.

<FIG> is a flow diagram illustrating a method <NUM> of operating a heading control system of an aircraft, such as the heading control system <NUM> described above with reference to <FIG>.

At block <NUM>, a bias signal <NUM> indicating a bias towards a port or a starboard side of an aircraft is received.

At block <NUM>, based on the received bias signal <NUM>, an offset angle is generated. The offset signal defines an offset from a longitudinal axis of the aircraft (i.e. the x-axis referred to in <FIG>).

At block <NUM>, the nose wheel of the aircraft is controlled within the defined angular range.

In some examples, the control system <NUM> may determine, from the offset signal, whether the aircraft is configured to travel in a substantially forward direction on the ground and the heading control system may be disabled if it is determined that the aircraft is not configured to be travelling in a substantially forward direction on the ground. For example, the determination may be made on the basis of one or more of: GPS data indicating a taxi-able position, a position of the aircraft rudder controls, a measured degree of acceleration, a measured degree of compression of one or more landing gear. For example, during turns the heading control system may be disabled because the heading to which the aircraft is changing and therefore the heading control system has no fixed target heading to aim for. In some examples, a dynamic heading that changes during a turn may be specified; in such examples, the heading control system may not be disabled.

In some examples, the method comprises determining whether, after landing, the aircraft has travelled in a substantially forward direction for greater than a predetermined time, and enabling the heading control system if the aircraft has travelled in a substantially forward direction for greater than the predetermined time. During a landing phase, the heading control system may be disabled because at high speeds small changes to the steering angle can cause large lateral movement of the aircraft, which may be dangerous.

Claim 1:
A heading control system (<NUM>) for an aircraft (<NUM>), the heading control system being arranged to maintain a heading of an aircraft by controlling a nose wheel angle of the aircraft, the heading control system comprising:
an interface (<NUM>) arranged to receive a bias signal (<NUM>) indicating a bias towards the port or the starboard of the aircraft; and
one or more processors arranged to:
determine, based on the bias signal, an offset angle defining an offset from a longitudinal axis of the aircraft; and
characterised in that the one or more processors are further arranged to perform a control process to control the nose wheel angle within an angular range centred on the offset angle.