GB2563852A

GB2563852A - Aircraft braking

Info

Publication number: GB2563852A
Application number: GB1710218.7A
Authority: GB
Inventors: Howell George; Crane Nicholas
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2017-06-27
Filing date: 2017-06-27
Publication date: 2019-01-02
Also published as: GB201710218D0

Abstract

Disclosed is a braking system 200 for an aircraft. The aircraft comprises a landing gear arrangement comprising a pair of wheels 110L, 110R sharing a common axis of rotation, each wheel having a brake. The braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit 202 and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit 204. In both modes the braking system is arranged to reduce hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation. The backup hydraulic circuit may include an accumulator arranged to receive pressurized hydraulic fluid the primary hydraulic circuit. Also disclosed is an aircraft with the abovementioned system, a method of operating a braking system, and a computer program which executes the method.

Description

AIRCRAFT BRAKING

TECHNICAL FIELD

[0001] The present invention relates to a braking system for an aircraft.

BACKGROUND

[0002] Aircraft systems often use hydraulic power to transmit force to a mechanical component to move that component, for example. For example, many aircraft have one or more braking systems that can be used to retard the aircraft by appropriate transmission of hydraulic fluid to hydraulic actuators in wheel brakes.

[0003] Many such hydraulic aircraft systems are powered from a central hydraulic power source. That source may, for example, comprise an electric pump powered by the aircraft engines to generate hydraulic pressure.

[0004] Hydraulic braking systems perform a critical function), and so a backup braking mechanism, powered by an alternate source of hydraulic power, may be provided to provide hydraulic pressure to enable the aircraft to brake if a main source of hydraulic pressure fails.

[0005] The present application discloses an improvement to such a backup braking system.

SUMMARY

[0006] A first aspect of the present invention provides a braking system for an aircraft, wherein the aircraft comprises a landing gear arrangement comprising a pair of wheels sharing a common axis of rotation, the pair of wheels comprising a first wheel and a second wheel, each having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit, wherein: in the first mode, the braking system is arranged to reduce hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair in when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation; and in the second mode, the braking system is arranged to reduce hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation.

[0007] Optionally, the expected speed is determined on the basis of a model of the speed of rotation of the respective first and second wheels during braking.

[0008] Optionally, the expected speed is determined based on relative movement between the aircraft and the ground.

[0009] Optionally the relative movement between the aircraft and the ground is determined on the basis of GPS data indicating the position of the aircraft.

[0010] Optionally, the expected speed is determined based on a rate of rotation of the second wheel.

[0011] Optionally, the braking system comprises: a first set of servo valves operating in the first mode, the first set of servo valves providing hydraulic power from a primary hydraulic power source; and a second set of servo valves, different from the first set of servo valves and operating in the second mode, the second set of servo valves providing hydraulic power from a backup hydraulic power source.

[0012] Optionally, in the first mode, the primary hydraulic circuit is a hydraulic circuit powered by the aircraft.

[0013] Optionally, the backup hydraulic circuit comprises an accumulator arranged to receive a supply of pressurised hydraulic fluid from the normal supply.

[0014] Optionally, the braking system comprises a first type of manifold and a second type of manifold, different to the first type of manifold, wherein: the first type of manifold comprises one servo valve fluidically coupled to the primary hydraulic circuit and one servo valve fluidically coupled to the backup hydraulic circuit; and the second type of manifold comprises two servo valves fluidically coupled to the primary hydraulic circuity and two servo valves fluidically coupled to the backup hydraulic circuit.

[0015] Optionally, the braking system comprises a plurality of manifolds, each manifold comprising one servo valve fluidically coupled to the primary hydraulic circuit and one servo valve fluidically coupled to the backup hydraulic circuit.

[0016] Optionally, in each of the first and second modes, the first wheel and the second wheel are both supplied from the same hydraulic circuit.

[0017] A second aspect of the present invention provides an aircraft comprising a braking system according to the first aspect of the present invention.

[0018] A third aspect of the present invention provides a method of operating a braking system for an aircraft, the aircraft comprising a landing gear arrangement comprising a pair of wheels sharing a common axis of rotation, the pair of wheels comprising a first wheel and a second wheel, each having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit, the method comprising: in the first mode, reducing hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation; and in the second mode, reducing hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation.

[0019] A fourth aspect of the present invention provides a computer program which, when executed by a processor in a braking system for an aircraft system, the aircraft comprising a landing gear arrangement comprising a pair of wheels sharing a common axis of rotation, the pair of wheels comprising a first wheel and a second wheel, each having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit, causes the processor to: in the first mode, reduce hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation; and in the second mode, reducing hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation.

[0020] A fifth aspect of the present invention provides a braking system for an aircraft, wherein the aircraft comprises a landing gear arrangement comprising a pair of wheels sharing a common axis of rotation, the pair of wheels comprising a first wheel and a second wheel each having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit, wherein in each of the first mode and the second mode, in response to detecting a skid condition of the first wheel, but not the second wheel, the braking system is arranged to reduce hydraulic pressure to the brake of the first wheel while maintaining hydraulic pressure to the brake of the second wheel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [0022] Figure lisa schematic diagram of a front elevation of a known landing gear leg assembly according to an embodiment of the invention; [0023] Figure 2 is a schematic diagram of a braking system of an aircraft according to an embodiment of the invention; [0024] Figure 3 is a flow diagram illustrating an example of a method of operating a braking system according to an embodiment of the invention; [0025] Figure 4 is a schematic diagram of a braking system of an aircraft according to an embodiment of the invention; [0026] Figure 5 is a schematic diagram of a braking system of an aircraft according to an embodiment of the invention; and [0027] Figure 6 is a schematic diagram showing a front view of an example of an aircraft of an embodiment of the invention.

DETAILED DESCRIPTION

[0028] Figure 1 illustrates major elements of an exemplary landing gear leg assembly referred to hereinafter as a “leg”) 100 for an aircraft. The leg comprises a shock strut 102 (for example, an oleo-pneumatic shock strut) having a main fitting 104 attached to the aircraft (for example to a wing or to the aircraft fuselage) and a slider 106 (or post), which can slide into the main fitting 104 against the pressure of a fluid or fluids to form a shock absorbing element connected, at its lower end, to a bogie beam assembly 108. The bogie beam assembly 108 supports at least one pair 110 of wheels, including a left wheel 110L at the left side of the leg 100 and a right wheel 11 OR at the right side of the leg 100. In some examples, the left and right wheels 110L, 110R may be connected by a common axle. In other examples, each wheel 110L, 110R may have its own dedicated axle, that substantially share a common axis of rotation, such that the left and right wheels 110L, 110R can rotate substantially independently of one another.

[0029] Each wheel 110L, 110R comprises a wheel brake 112L, 112R for providing a wheel braking function (under the control of the pilot). Braking by wheel braking is provided by the wheel brakes 112L, 112R is typically initiated when sustained ground contact has occurred.

[0030] In order to maintain control of the aircraft under braking, the wheel brakes 112L, 112R of the aircraft are controlled to prevent “skidding” (i.e. slipping or sliding of the wheels 110L, 110R over the ground).

[0031] In order to determine whether the wheels are skidding, the aircraft may comprise a so-called “antiskid” system for determining whether the wheels 110L 110R are rotating at an expected speed. For example, a wheel 110L, 110R may comprise a sensor, such as a tachometer, which provides an indication of a rate of rotation of the wheel 110L, 110R, which may be translated into an indicated ground speed of the aircraft. The ground speed indicated by the rotation of the wheel 110L, 11 OR may be compared with the expected speed to determine whether the wheel 110L, 11 OR is skidding. For example, if the ground speed indicated by the rotation of the wheel 110L, 11 OR is less than the expected speed (or below the expected ground speed by a predetermined threshold amount), then the antiskid system may determine that the wheel 110L, 11 OR is skidding.

[0032] The expected speed may be determined in a number of ways.

[0033] In some embodiments, the antiskid system may comprise a model including data representing and expected speed (or rate of wheel rotation) as a function of time after braking commences. In such embodiments, the antiskid system may determine that the wheel 110L, 11 OR is skidding if the speed indicated by the rotation of the wheel 110L, 11 OR is less than a speed predicted by the model (or is below the speed predicted by the model by more than a threshold amount).

[0034] In other embodiments, the expected speed may be determined from direct measurements of the ground speed of the aircraft. For example, the antiskid system may receive data indicative of the speed or time varying location of the aircraft based on, for example, GPS data. In such embodiments, the antiskid system may compare the speed indicated by the rotation of the wheel 110L, 110R with the measured ground speed, and determine that the wheel 110L, 11 OR is skidding if the speed indicated by the rotation of the wheel 110L, 11 OR is less than the measured ground speed (or is below the measured ground speed by more than a threshold amount).

[0035] In other embodiments, the expected speed may be determined based on a rate fo rotation of one of the wheels 110L, 110R. For example, the speed (or rate of rotation) of one wheel (for example the left wheel 110L of a pair 110 of wheels) may be compared with the speed (or rate of rotation) of another wheel (for example the right wheel 110R of a pair 110 of wheels), and the antiskid system may determine that one of the wheels 110L, 11 OR is skidding if the speed indicated by the rotation of that wheel 110L, 11 OR is less than a speed of the other wheel 110R, 110L (or is below the speed of the other wheel 110R, 110L by more than a threshold amount).

[0036] As explained above, critical hydraulic systems, such as wheel braking systems, include a backup mechanism, powered by an alternate source of hydraulic power. In current implementations, the antiskid system operates in different modes of operation, depending on whether they are powered by a primary hydraulic power supply, or a backup hydraulic power supply. In particular, the response of the antiskid system is different if the braking system is being powered by a primary source of hydraulic power (referred to herein as “normal mode”) than its response if the braking system is being powered by a backup source of hydraulic power (referred to herein as “alternate mode”).

[0037] In normal mode, when a wheel 110L, 11 OR is determined to be skidding, the brake 112L, 112R of that wheel 110L, 110R is released, and then reapplied, whereas the brake of the other wheel 110R, 110L of the pair 110 is not released (if it is not determined to be skidding). However, in alternate mode, when a wheel 110L, 11 OR is determined to be skidding, the brake 112L, 112R of that wheel 110L, 11 OR and the brakes of the other wheel 110R, 110L of the pair 110 are both released, and then reapplied.

[0038] The reason for this is to reduce the number of components required in the backup braking system. In particular, a hydraulic circuit for supplying hydraulic fluid to the brakes 112L, 112R in normal mode comprises one servo valve (referred to as a “Normal Servo Valve”) per wheel brake 112L, 112R, whereas the hydraulic circuit for supplying hydraulic fluid to the brakes 112L, 112R in alternate mode comprises one servo valve (referred to as an “Alternate Servo Valve”) per pair 112 of wheel brakes 112L, 112R.

[0039] However, this results in a reduced braking efficiency when the antiskid system is operating in alternate mode compared to when the antiskid system is operating in normal mode.

[0040] Figure 2 is a simplified schematic diagram of a braking system 200 for providing hydraulic power to a braking system in a landing gear leg assembly, such as the leg 100 described above with reference to Figure 1, according to the invention.

[0041] For clarity, the landing gear assembly depicted in Figure 2 is a simplified landing gear assembly comprising one pair 110 of wheels 110F, 110R per leg. However, in other examples, such as those described below with reference to Figures 4 and 5, each leg 100 may support more than one pair 110 of wheels. For example, each leg may support two or three (or more) pairs 110 of wheels 110L, 110R.

[0042] In particular, in the example shown in Figure 2, the left leg comprises two wheels labelled 1 and 2, and the right leg comprises two wheels labelled 3 and 4.

[0043] The braking system 200 is, in normal operation, powered by a “Normal” hydraulic power supply (referred to hereinafter as a “Normal Supply”) 202. The breaking system 200 also includes a backup supply of hydraulic power (referred to hereinafter as an “Alternate Supply”) 204, for providing hydraulic pressure to the braking system in the event of a failure of the normal supply 202.

[0044] The normal supply 202 is fluidically connected, via hydraulic lines (such as hydraulic pipes), to a selector valve (referred to as a “Normal Selector Valve”) 206. The normal selector valve 206 is fluidically connected, via hydraulic lines, to servo valves (referred to as “Normal Servo Valves”) 208. Each of the normal servo valves 208 is fluidically connected, via hydraulic lines, to a wheel brake 112L, 112R of a respective wheel 110L, 11 OR.

[0045] Similarly, the alternate supply 204 is fluidically connected, via hydraulic lines (such as hydraulic pipes), to a selector valve (referred to as a “Alternate Selector Valve”) 210. The alternate selector valve 210 is fluidically connected, via hydraulic lines, to servo valves (referred to as “Alternate Servo Valves”) 212. Each of the alternate servo valves 208 is fluidically connected, via hydraulic lines, to a wheel brake 112L, 112R of a respective wheel 110L, 11 OR. Therefore, in contrast to current systems, in the braking system shown in Figure 2, both the normal and alternate (i.e. primary and backup) systems comprise one servo valve per wheel brake 112L, 112R. That is, there is one alternate servo valve 212 per wheel brake 112L, 112R (rather than one alternate servo valve per pair of wheel brakes 112L, 112R) [0046] The braking system 200 comprises a controller 214 arranged to detect a failure condition of the normal supply 202. For example, the controller 214 may comprise a pressure sensor for sensing a pressure provided by the normal supply 202 and a processor to receive a pressure signal (indicated by the dashed arrow 214a) and evaluate the pressure signal. The processor may be programmed to send a control signal (indicated by the dashed arrow 214b) to activate the alternate supply 204 in the event that a sensed pressure falls below a threshold pressure, for example. The controller 214 may be implemented in hardware and/or software.

[0047] The alternate supply 204 is for providing hydraulic pressure to enable braking of the aircraft in the event of a failure of the normal supply 202. For example, in relation to the landing gear arrangement 100 depicted in Figure 1, the alternate supply 204 may be arranged to provide hydraulic pressure to enable operation of the wheel brakes 112L, 112R while the aircraft is on the ground.

[0048] The normal supply 202 may be, for example, a central hydraulic power source for the aircraft, which may comprise an electric pump powered by the aircraft engines to generate hydraulic pressure.

[0049] The alternate supply 204 may, for example, comprise an accumulator, which is a pressure storage reservoir in which hydraulic fluid is held under a hydraulic pressure generated by an external source. In particular, the accumulator may be selected to provide a sufficient volume of pressurised hydraulic fluid to operate a brake 112L, 112R (or brakes) in the event of a failure of the normal supply 202.

[0050] In some embodiments, in normal use, the normal supply 202 may maintain a predetermined hydraulic pressure for a predetermined volume of hydraulic fluid in the accumulator forming the alternate supply 204.

[0051] Similar to current implementation (as described above), in normal mode, when a wheel 110L, 110R is determined to be skidding, the brake 112L, 112R of that wheel 110L, 110R is released, and then reapplied, whereas the brake 112R, 112L of the other wheel 110R, 110L of the pair 110 is not released (if it is not determined to be skidding). However, in contrast to the current implementation, in alternate mode, when a wheel 110L, 11OR is determined to be skidding, the brake 112L, 112R of that wheel 110L, 110R is released, and then reapplied, whereas the brake 112R, 112L of the other wheel 110R, 110L of the pair 110 is not released (if it is not determined to be skidding). As a result, when operating in alternate mode (i.e. powered by the alternate supply 204), braking efficiency is maintained (or at least improved with respect to the current implementation).

[0052] In order to provide substantially independent braking of the wheels 110L, 11 OR of a pair 110 of wheels in alternate mode, the braking system shown in Figure 2 comprises a hydraulic circuit for supplying hydraulic fluid to the brakes 112F, 112R in normal mode that comprises one servo valve (referred to as a “Normal Servo Valve”) per wheel brake 112F, 112R, and a hydraulic circuit for supplying hydraulic fluid to the brakes 112F, 112R in alternate mode that also comprises one servo valve (referred to as an “Alternate Servo Valve”) per wheel brake 112F, 112R.

[0053] Figure 3 is a flow diagram illustrating a method 300 of operating an aircraft braking system, such as the braking system 200 described above with reference to Figure 2; that is, a braking system for an aircraft comprising a pair of wheels substantially sharing a common axis of rotation, each wheel having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit.

[0054] At block 302, the braking system is operating in the first mode. In the first mode, hydraulic pressure to the brake of the first wheel of the pair is reduced when there is a difference between a speed determined by a rate of rotation of the first wheel and an expected speed, whereas hydraulic pressure to the brake of the second wheel of the pair is maintained when there is a correspondence between a speed determined by a rate of rotation of the second wheel and the expected speed.

[0055] For example, as described above with reference to Figure 2, the pressure supplied by the normal supply 202 may be detected by a pressure sensor and the detected pressure may be evaluated by the controller 214.

[0056] At block 304, the braking system is operating in the second mode. In the second mode, hydraulic pressure to the brake of the first wheel of the pair is reduced when there is a difference between a speed determined by a rate of rotation of the first wheel and an expected speed, whereas hydraulic pressure to the brake of the second wheel of the pair is maintained when there isa correspondence between a speed determined by a rate of rotation of the second wheel and the expected speed.

[0057] For example, as described above with reference to Figure 2, the controller 214 may send a control signal 214b to operate the alternate selector valve 210 to provide a transmission path for hydraulic fluid between the alternate supply 204 and the alternate servo valves 208, which is response to determining that a respective wheel 110L, 110R is skidding are arranged to release hydraulic pressure to the brake 112L, 112R of that respective wheel 110L, 11 OR.

[0058] Although in the braking system 200 described above with reference to Figure 2 a single normal supply is described, it will be understood that some embodiments of the invention, may include multiple normal (i.e. primary or main) sources of hydraulic pressure for which a respective alternate supply (i.e. backup source of hydraulic pressure) provides backup hydraulic power. In particular, the wheel brakes of different pairs of wheels may be supplied by different hydraulic systems. In some examples, the wheel brakes of a single pair of wheels may each be supplied by a different hydraulic systems.

[0059] Figure 4 is a simplified schematic diagram of a braking system 400 for providing hydraulic power to a landing gear assembly comprising more than one pair 110 of wheels 110L, 110R. In the particular example shown in Figure 4, each leg 100 comprises three pairs of wheels 110; that is six pairs of wheel (three on the left leg 100 and three on the right leg 100, numbered 1 to 12.

[0060] Similar to the braking system 200 described above with reference to Figure 2, the braking system 400 shown in Figure 4 comprises a normal (i.e. primary) supply 202 and an alternate (i.e. backup) supply 204. However, in the example shown in Figure 4, the brakes 112L, 112R are supplied by two, separate and independent hydraulic systems, of which only one is shown, for clarity. In some examples, the two systems may be named “green” and “yellow” to distinguish between the two systems.

[0061] In the example shown in Figure 4, the servo valves (both normal and alternate servo valves) are housed in servo valve manifolds 402. In particular, the example shown in Figure 4 comprises two types of manifold; single manifolds 402a and double manifolds 402b.

[0062] Single manifolds 402a supply hydraulic power, in both normal and alternate modes, to one wheel 110F, 110R of a pair 110 (in the example shown in Figure 4, the single manifolds 402a in the “green” system shown supply hydraulic power to the left wheel 110F of the centre pair of wheels 110, whereas the single manifolds 402a in the “yellow” system not shown supply hydraulic power to the right wheel 11 OR of the centre pair of wheels 110).

[0063] Double manifolds supply hydraulic power, in both normal and alternate modes, to both wheels 11 OF, 11 OR of a pair 110 (in the example shown in Figure 4, the double manifolds 402a in the “green” system shown supply hydraulic power to the front pair 110 of wheels 110F, 110R, whereas the double manifolds 402a in the “yellow” system not shown supply hydraulic power to the front pair 110 of wheels 110F, 110R.

[0064] Each of the single manifolds 402a comprises a normal servo valve 404 and an alternate servo valve 406. Each of the double manifolds 402b comprises a pair of normal servo valves 404 and a pair of alternate servo valves 406. Therefore, in common with the braking system 200 described above with reference to Figure 2, in the braking system 400 shown in Figure 4, in both normal and alternate modes, when a wheel 11 OF, 11 OR is determined to be skidding, the brake 112F, 112R of that wheel 110F, 110R is released, and then reapplied, whereas the brake 112R, 112F of the other wheel 110R, 11 OF of the pair 110 is not released (if it is not determined to be skidding). As a result, when operating in alternate mode (i.e. powered by the alternate supply 204), braking efficiency is maintained (or at least improved with respect to the current implementation).

[0065] Figure 5 is a simplified schematic diagram of a further braking system 500 for providing hydraulic power to a landing gear assembly comprising more than one pair 110 of wheels 110F, 110R. In common with the example shown in Figure 4, each leg 100 comprises three pairs of wheels 110; that is six pairs of wheel (three on the left leg 100 and three on the right leg 100, numbered 1 to 12.

[0066] Similar to the braking system 200 described above with reference to Figure 2, the braking system 400 shown in Figure 5 comprises a normal (i.e. primary) supply 202 and an alternate (i.e. backup) supply 204. Further, similar to the example shown in Figure 4, the brakes 112F, 112R are supplied by two, separate and independent hydraulic systems, of which only one is shown, for clarity.

[0067] In the example shown in Figure 5, the servo valves (both normal and alternate servo valves) are each housed in the same kind of servo valve manifolds 502, corresponding to the single manifolds 402a described above with reference to Figure 4.

[0068] Each of the manifolds 502 comprises a normal servo valve 404 and an alternate servo valve 406. Therefore, also in common with the braking system 200 described above with reference to Figure 2, in the braking system 500 shown in Figure 5, in both normal and alternate modes, when a wheel 110F, 110R is determined to be skidding, the brake 112F, 112R of that wheel 110F, 110R is released, and then reapplied, whereas the brake 112R, 112F of the other wheel 110R, 110F of the pair 110 is not released (if it is not determined to be skidding). As a result, when operating in alternate mode (i.e. powered by the alternate supply 204), braking efficiency is maintained (or at least improved with respect to the current implementation).

[0069] The braking system 400 described with reference to Figure 4 is advantageous over the braking system 500 described with reference in that it requires a less complex system of hydraulic lines (i.e. pipes). However, the braking system 500 described with reference to Figure 5 is advantageous over the braking system 400 described with reference in that it can be implemented by duplicating existing single manifolds for each wheel 110L, 11 OR.

[0070] In the event of a failure of the normal supply 202, hydraulic fluid may no longer be supplied to the alternate supply 204, or the normal supply 202 may not be able to maintain a specified pressure of hydraulic fluid in the alternate supply 204 (since, for example, hydraulic fluid in an accumulator may leak). Therefore, the alternate supply 204 is selected to provide a sufficient volume of hydraulic fluid, pressurised to a sufficient pressure, to provide hydraulic power to each of the brakes 110L, 11 OR in a given hydraulic system. This enables the braking system to provide backup hydraulic power to the brakes 110L, 110R independently in in alternate mode, which in turns improves the braking efficiency of the braking system when operating in alternate mode.

[0071] In some embodiments, braking system 200 described above with reference to Figure 2 may be installed in a vehicle. Referring to Figure 6, there is shown a schematic front view of an example of a vehicle according to an embodiment of the invention. In the example of Figure 6, the vehicle is an aircraft 600. The aircraft 600 may comprise one or more legs 100 comprising one or braking systems 602, such as the braking system 200 described above with reference to Figure 2. In other embodiments, the vehicle may be other than an aircraft, such as a road vehicle, a rail vehicle, a watercraft or a spacecraft.

[0072] The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

[0073] It is to be noted that the term “or” as used herein is to be interpreted to mean “and/or”, unless expressly stated otherwise.

Claims

CLAIMS:

1. A braking system for an aircraft, wherein the aircraft comprises a landing gear arrangement comprising a pair of wheels sharing a common axis of rotation, the pair of wheels comprising a first wheel and a second wheel each having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit, wherein: in the first mode, the braking system is arranged to reduce hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation; and in the second mode, the braking system is arranged to reduce hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation.

2. A braking system according to claim 1, wherein the expected speed is determined on the basis of a model of the speed of rotation of the respective first and second wheels during braking.

3. A braking system according to claim 1, wherein the expected speed is determined based on relative movement between the aircraft and the ground.

4. A braking system according to claim 3, wherein the relative movement between the aircraft and the ground is determined on the basis of GPS data indicating the position of the aircraft.

5. A braking system according to claim 1, wherein the expected speed is determined based on a rate of rotation of the second wheel.

6. A braking system according to any preceding claim, comprising: a first set of servo valves operating in the first mode, the first set of servo valves providing hydraulic power from a primary hydraulic power source; and a second set of servo valves, different from the first set of servo valves and operating in the second mode, the second set of servo valves providing hydraulic power from a backup hydraulic power source.

7. A braking system according to any preceding claim, wherein, in the first mode, the primary hydraulic circuit is a hydraulic circuit powered by the aircraft.

8. A braking system according to any preceding claim, wherein the backup hydraulic circuit comprises an accumulator arranged to receive a supply of pressurised hydraulic fluid from the primary hydraulic circuit.

9. A braking system according to any preceding claim, comprising a first type of manifold and a second type of manifold, different to the first type of manifold, wherein: the first type of manifold comprises one servo valve fluidically coupled to the primary hydraulic circuit and one servo valve fluidically coupled to the backup hydraulic circuit; and the second type of manifold comprises two servo valves fluidically coupled to the primary hydraulic circuit and two servo valves fluidically coupled to the backup hydraulic circuit.

10. A braking system according to any one of claims 1 to 8, comprising a plurality of manifolds, each manifold comprising one servo valve fluidically coupled to the primary hydraulic circuit and one servo valve fluidically coupled to the backup hydraulic circuit.

11. A braking system according to any preceding claim, wherein, in each of the first and second modes, the first wheel and the second wheel are both supplied from the same hydraulic circuit.

12. An aircraft comprising a braking system according to any preceding claim.

13. A method of operating a braking system for an aircraft, wherein the aircraft comprises a landing gear arrangement comprising a pair of wheels sharing a common axis of rotation, the pair of wheels comprising a first wheel and a second wheel, each having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit, the method comprising: in the first mode, reducing hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation; and in the second mode, reducing hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation.

14. A computer program which, when executed by a processor in a braking system for an aircraft, wherein the aircraft comprises a landing gear arrangement comprising a pair of wheels sharing a common axis of rotation, the pair of wheels comprising a first wheel and a second wheel, each having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit, causes the processor to: in the first mode, reduce hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair when there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation; and in the second mode, reduce hydraulic pressure to the brake of the first wheel of the pair when there is a difference between a detected rate of rotation of the first wheel and an expected rate of rotation, while maintaining hydraulic pressure to the brake of the second wheel of the pair in response to detecting that there is a correspondence between a detected rate of rotation of the second wheel and the expected rate of rotation.

15. A braking system for an aircraft, wherein the aircraft comprises a landing gear arrangement comprising a pair of wheels sharing a common axis of rotation, the pair of wheels comprising a first wheel and a second wheel each having a brake, wherein the braking system is operable in a first mode in which the braking system is supplied with hydraulic pressure from a primary hydraulic circuit and a second mode in which the braking system is supplied with hydraulic pressure from a backup hydraulic circuit, wherein in each of the first mode and the second mode, in response to detecting a skid condition of the first wheel, but not the second wheel, the braking system is arranged to reduce hydraulic pressure to the brake of the first wheel while maintaining hydraulic pressure to the brake of the second wheel.