Improved carbon brake control for aircraft

0001This invention relates to a method for controlling carbon aircraft brakes as described in the preamble of claim 1, and to a system for controlling such brakes as described in the preamble of claim 7. Such a method and system are known from the prior art document EP-A-0 329 373.

0002Modern aircraft which are designed to carry large passenger or cargo payloads are often provided with carbon brakes on each of the wing or body mounted wheels. The nose wheel is typically not braked. Carbon brakes are preferred because of their light weight and performance characteristics and generally comprise a piston housing and parts, a torque plate and a carbon heat sink stack. This stack contains all the friction surfaces which, when compressed, cause the wheel to decrease its speed. The stack comprises a pressure plate, rotor disks, stator disks and backing plate. Carbon composite rotors are connected to the wheel through the rotor drive keys and turn with the wheel. Carbon composite stators, pressure plates and backing plate are connected to the torque tube and do not turn. Braking friction is caused when the rotors are compressed against the stators.

0003While carbon brakes are preferred for weight and performance reasons over steel brakes, the cost of replacing the stack divided by the number of landing cycles between replacements is much higher than for steel brakes.

0004Aircraft brake control systems are designed with separate pedal controls for the left and right brakes. When one of the brake pedals is depressed, all the brakes on that side of the aircraft are commanded to apply simultaneously and equally. By applying all brakes equally, the heat energy absorbed by each individual brake is minimized. For steel brakes, brake life is largely determined by the total amount of energy absorbed by each brake and is largely unaffected by the number of brake applications that accumulate that energy. Hence, brake control systems that apply all brakes simultaneously and equally provide economic operation of steel brakes and minimize exposure to overheating of any individual brake. However, direct application of this method to carbon brakes does not extend and may significantly shorten their lives, as carbon brake wear has been found to correlate significantly with the number of brake applications and to not be significantly affected by the energy absorbed during each.

0005By far the largest number of brake applications occur during ordinary taxiing, so in order to extend the life of carbon brakes and substantially reduce their operating cost, the above-identified prior art document EP-A-0 329 373 discloses a method and a system for controlling the carbon brakes of an aircraft in such a way that only some of the brakes are applied in response to brake applications under ordinary taxiing conditions. An alternaring wheel braking pattern is established to minimize brake wear at each braked wheel and yet to promote even distribution of absorbed energy among all the brakes. This, in turn, prevents overheating of any individual brake. The extended brake-wear system is activated only when aircraft ground speed and brake intensity are typical of taxi operations. Aircraft speed and brake pedal depression are sensed so that brakes at all wheels will be operative in critical braking situations such as landing, parking, or emergency stopping.

0006In order to prevent the brake disablement system from being activated during the landing run of the aircraft, when it is decelerated to a speed below the treshold speed where the system would normally be activated, the cited prior art document proposes a "weight switch" which is triggered at touchdown, and which starts a time delay during which the system is not functional.

0007The present invention seeks to provide an improved method and system for controlling carbon aircraft brakes in order to extend their service life. According to the invention, this is accomplished by the features of the characterizing portion of claims 1 and 7, respectively.

0008The invention will be better understood in terms of the Figures and detailed description which follow.

0009Figure 1 is a simplified schematic of a subsystem for aircraft brakes which alternately disables one of two brakes in order to limit the number of brake applications and extend carbon brake life.

0010Figure 2 is a schematic view of a sixteen wheel and brake landing gear configuration for a wide bodied aircraft showing a brake disable circuit which would be activated under low braking pressure and aircraft speed conditions representative of taxi braking to disable half the brakes and thereby extend brake life.

0011For carbon brakes, the landings to wear-out ratio is strongly dependent on the number of brake applications rather than the energy absorbed by a brake during each application. For commercial passenger aircraft, the brakes may be applied an average of twenty times per landing cycle. The brakes are generally applied during landing absorbing several million Newtonmetres (foot-pounds) for heavy wide-bodied aircraft and once to stop the wheels from spinning before they are retracted after take-off. Both of these are "high speed" brake applications, and are typically at moderate hydraulic pressures less than about 10.3 MPa (1500 psi) hydraulic pressure. The balance of the brake applications are "taxi snubs" for steering or low speed braking. They create hydraulic brake fluid pressures generally less than about 10.3 MPa (1500 psi) and absorb about 0.68 M Newtonmetres (0.5 MFP) average per snub for wide-bodied aircraft. These taxi snubs account for a significant amount of brake energy temperature buildup, and for carbon brakes, most of the wear since carbon brake wear is dependent on the number of brake applications. Occasionally, "emergency" brake applications may be made at higher pressures (up to 20.7 MPa (3000 psi) hydraulic fluid pressure), but such emergency braking is an insignifcant wear factor.

0012Conventional brake wear control systems provide for applying all brakes equally, gently, and simultaneously during normal taxi braking. In accordance with a preferred embodiment of this invention, the life of carbon brakes is extended by minimizing the number of brake applications while distributing the heat energy absorbed substantially equally among all the brakes. This is accomplished by alternately applying only a selected number of brakes rather than all the brakes during each normal taxi braking operation.

0013A simplified example of a preferred embodiment of this invention is shown schematically in Figure 1. A left wheel 2 and right wheel 4 are on the same side of an airplane and are actuated by the one of the two brake pedals in the cockpit. Wheel 2 has a carbon brake 6 and right wheel 4 a carbon brake 8. In this embodiment, the antiskid control system 10 is integral to the brake disable system. Left and right wheel speed sensors 12 and 14, electronically measure wheel speeds and input the signals generated to the antiskid control circuit 10. Signals from antiskid control circuit 10 are outputted through diodes 16 and 18 to left and right hydraulic antiskid valves 20 and 22. The signals from wheel speed sensors 12 and 14 are integrated by antiskid control circuit 10 and outputted to brake disable control circuit 24.

0014Brake metering valve 26 which is responsive to a call for braking from the cockpit is located in brake hydraulic line 28. The static line pressure is low, pressure during taxi snubs is higher, and pressure during parking and emergency braking is relatively higher still. This pressure is measured at metered brake pressure sensor 30. The signal from sensor 30 is inputted to brake disable control circuit 24.

0015The system works in accordance with the invention as follows. The speeds of wheels 2 and 4 are sensed through sensors 12 and 14 and processed in antiskid control circuit 10 to determine aircraft speed. That aircraft speed signal is inputted to brake disable circuit 24. The desired intensity of braking action is sensed by the metered brake pressure sensor 30 and is also inputted to brake disable circuit 24. Inside brake disable control circuit 24, the metered pressure signal is compared against a first predetermined value, 690 kPa (100 psi) for example, to detect when a brake application has been commanded. At the moment at which a brake application is detected, a comparison is made between the aircraft speed signal and a predetermined value for aircraft speed in brake release logic circuit 32. If the speed is higher than the predetermined value, 64 kph (40mph), for example, then brake disablement is not disabled. Subsequently, comparison is continuously made inside brake release logic 32 between the metered pressure signal value and a second predetermined value. If the pressure is greater than the second predetermined value, greater than 10.3 MPa (1500 psi), for example, then the brake disable control circuit 24 does not disable any brakes. That is, if heavy braking intensity is called for, all the brakes are applied. If and only if aircraft speed at the time of brake application and metered brake pressure are lower than their predetermined maximum values will brake release logic circuit 32 be activated.

0016As indicated by bipolar knife switch 36, only one of the two antiskid valves 20 and 22 will be commanded to release its respective brake through left diode 38 or right diode 40 when brake release logic 32 triggers. Brake select logic circuit 42 remembers which brake was last disabled and switches switch 36 when a new brake application has been detected by brake disable circuit 24.

0017Brake disable logic 24 responds to both the metered pressure signal and the aircraft speed signal at the time of brake application. Thereafter, logic circuit 24 responds only to the metered pressure signal from sensor 30 until the metered brake pressure returns to the no-braking system pressure. This ensures that following a high speed brake application, such as a landing, the brake release command will not be produced, and half the brakes will not be released, as the aircraft decelerates through the brake disable speed threshold. The disable signal would then only be produced at low speed after the brakes were released, then reapplied.

0018If an emergency stop, i.e., high metered pressure is sensed by brake disable circuit 24, then brake release logic 32 removes the brake release command so that both brakes 6 and 8 are applied, thus insuring full aircraft braking capability when it is needed. Similarly, if a higher speed stop, such as a landing stop or rejected take off, is sensed by brake disable circuit 24 from the aircraft speed signal, then the brake release logic 32 removes the brake release command so that both brakes 6 and 8 may share the braking energy, preventing overheating of an individual brake or brakes.

0019While the desired braking intensity has been described in terms of metered braking pressure, other input to the brake disable circuit providing like information would be equally useful. For example, the acceleration and throw of the brake pedal in the cockpit could be monitored or the rate of brake temperature increase. Similarly, input other than aircraft speed such as wheel speed or aircraft ground speed measured independently of the wheel speed could be inputted to the brake disable circuit. Such alternatives will be apparent to those skilled in the art.

0020The invention has been described specifically in Figure 1 in terms of a brake pair on one side of an aircraft. However, systems in accordance with this invention for aircraft with other numbers and arrangements of carbon braked wheels could be readily adapted by persons skilled in the art. For example, Figure 2 shows the wheel configuration for a wide-bodied Boeing 747-400™ series aircraft equipped with a carbon brake on each main gear wheel. The nose wheel which is not braked is not shown.

0021Referring to Figure 2, there are four four-wheel trucks located under the left wing 44, left body 46, right body 48 and right wing 50 of an aircraft. Using truck 44 as an example, wheels 52 and 54 on one side, and 56 and 58 on the other side of a four-wheel axle frame 60 each provide input to a brake disable circuit 62 like that described in Figure 1. A metered brake pressure signal would also be provided to each like brake disable circuit. Thus, when both the aircraft speed at time of brake application and metered brake pressure are below target values, half of the sixteen brakes would be disabled. For example, brakes on wheels 52 and 54 on the left side of the truck 60 would be alternately disabled during successive brake applications as would the brakes on wheels 56 and 58.

0022Since Carbon brake wear is a function of the number of applications, and since the vast majority of brake applications occur during taxiing, the life of carbon brakes is significantly improved by practicing this invention. For example, if half the brakes are applied during each taxi brake application, brake wear life could nearly double. The life of carbon brakes might be proportionately extended even further by disabling even more than half the brakes during each braking cycle. System logic insures maximum braking capability during emergency braking, i.e., high pressure, conditions. Overheating of individual brakes is prevented because system logic alternates between brakes to share the braking energy among all the brakes.

0023Other system refinements such as redundant metered pressure sensors could be added to improve failure mode performance. Also, means could be provided to smooth brake pedal control responsiveness in the cockpit between partial brake and full brake transitions. That is, the back pressure on the brake pedal could be adjusted so that equal pedal depression results in equal braking responsiveness irrespective of how many brakes are being disabled at a given time. Brake temperature could also be considered in the brake disabling algorithm to prevent disablement if some brakes are too hot from previous brake applications.

0024While the invention has been described in terms of specific embodiments thereof, other forms may be readily adapted by one skilled in the art. Accordingly, the scope of the invention is to be limited only in accordance with the following claims.