Fluid-pressure and analogous brake systems – Speed-controlled – With failure responsive means
Reexamination Certificate
1999-07-22
2003-02-04
Graham, Matthew C. (Department: 3683)
Fluid-pressure and analogous brake systems
Speed-controlled
With failure responsive means
C303S155000, C303SDIG009
Reexamination Certificate
active
06513885
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to aircraft landing gear braking systems, and more particularly concerns an improved system for protection against inadvertent braking, and multiply redundant separated brake control channels.
2. Description of Related Art
Automatic braking systems have been commonly provided on commercial aircraft to aid the deceleration of the aircraft upon landing. As the size and complexity of aircraft have increased, the automatic braking systems have also become more complex and computerized. Modem anti-skid systems incorporated into aircraft braking systems commonly optimize braking efficiency by adapting to runaway conditions and other factors which affect braking in order to optimize deceleration typically corresponding to the level of brake pressure selected by the pilot.
A catastrophic failure mode can occur in a conventional brake-by-wire control system that results in uncommanded brake application on one or more wheels during takeoff of the aircraft. Since uncommanded braking during takeoff can have serious consequences, and at the very least can result in unnecessary and accelerated wear to the braking system, it is desirable to configure the braking system to reduce the possibility of these undesirable results. The overriding primary consideration is, of course, safety, although considerations of reliability are also significant.
High performance digital brake-by-wire control systems have been developed and installed on several aircraft including light commercial jet transports and modern business jets that use brake pressure feedback and enhanced built-in-test capability. Brake torque control is also used to further enhance brake control. Such digital brake control systems have achieved excellent braking performance over all runway conditions, and in RTO (Refused Take Off) and landing configurations. With sophisticated brake control algorithms, optimum braking performance is assured regardless of conditions, and the same software configuration can be used with assured range of brake and hydraulic configurations.
Two specific catastrophic failure modes that need to be addresses by an aircraft braking control system architecture are: a) the inadvertent application of any brake during the takeoff roll, and b) the complete loss of braking. The problem of inadvertent application of any brake during the takeoff roll sets the following design requirements: 1) no single failure shall result in the application of any brake during take off; and 2) the probability of any combination of failures leading to any brake being applied during take off shall be extremely improbable (less than 1×10
−9
). The second catastrophic hazard, the loss of all braking, sets the following design requirements: 1) no single failure shall lead to loss of all braking; and 2) the probability of any combination of failures leading to loss of all braking shall be extremely improbable (less than 1×10
−9
). These high performance requirements preclude the exclusive use of software. In addition, another commonly known braking control architecture has the disadvantage that the active brake control hydraulic fluid channels are connected to a single coil within the brake control valve, which provides a single point of failure that can result in catastrophic failure in the event of failure at that point.
Redundancy is typically achieved by use of a master or monitor channel that is used to monitor the operational status of hydraulic fluid braking control channels, and the monitor channel can command a first control channel to turn off and a second control channel to commence control, for example. Another method of redundancy management uses two control channels with one active control channel, and a second, inactive control channel in standby mode. When the active channel shuts down, the standby channel takes over control. However, both the master-slave and the active-standby systems can permit a single failure within the master or the active channel to cause a major breakdown in the redundancy management system.
In addition, loss of braking can also occur as a result of the antiskid function, requiring accounting for the probability that normal, alternate, emergency, and ultimate brake systems will be depressurised by a single failure of the anti-skid system. Loss of braking can occur owing to incorrect antiskid activity as a result of control system failure or loss of aircraft power. Another significant failure is the loss of gear retraction braking, which could allow a wheel with a loose tire tread to enter the wheel well while spinning. The hardwired interlock used to prevent application of brakes during take-off, typically conflicts with the requirement to stop the wheels during climb when the thrust levers are advanced.
Furthermore, the need to preclude asymmetric braking as a result of the loss of braking, or extra braking on one main landing gear set the following design requirements: 1) combinations of failures leading to the loss of all braking on either main landing gear shall be improbable (1×10
6
); and 2) combinations of failures leading to extra braking on either main landing gear shall be improbable (1×10
−6
). Touchdown and aquaplaning protection is provided by comparing wheel speeds with the groundspeed signal from the Air Data Inertial Reference Units (ADIRU). Typically any main gear aft wheel that is at a velocity 50 knots or more below the ADIRU groundspeed value is given a brake release signal. Undesired asymmetrical release of brakes can result from a false ADIRU signal, or from unwanted pressure being applied to any brake.
A need therefore continues to exist for an improved aircraft landing gear braking control system. The present invention addresses these and other needs.
SUMMARY OF THE INVENTION
Briefly, and in general terms, the present invention provides for a braking control system for dual redundant control of hydraulically operated wheel brakes of aircraft landing gear providing protection against inadvertent braking, and separation of braking control through primary and secondary braking control channels using an interface with dual coil brake control valves. The braking control system is safe, reliable, maintainable, lightweight, and affordable, and provides for a redundant brake-by-wire control architecture using a primary dual redundant brake-by-wire braking system, and a secondary dual redundant analog brake-by-wire system. Positive hydraulic system selection between the normal primary and alternate secondary hydraulic braking systems is performed using solenoid operated shutoff valves (SOSV). The primary braking system control of a center landing gear, if one is present, is split between right and left pedals, with the front axle of center gear landing controlled by left pedals, and the aft axle of center landing gear controlled by right pedals, and locked wheel protection is performed on a tandem basis rather than on an axle basis to prevent fault propagation. Alternate braking is performed on a paired wheel basis through the alternate hydraulic system. The primary braking system includes pressure and antiskid control performed using dual coil servo valves with pressure feedback, autobrake control employing primary brake system servo valves, and an equal load distribution provided by pressure feedback control. Emergency braking is also provided, allowing braking when all electrical power generation and all hydraulic power generation is lost. Parking brake and ultimate braking modes are also provided, using hydraulic power stored in accumulators.
The present invention accordingly provides for a braking control system for dual redundant control of hydraulically operated wheel braking for an aircraft having landing gear that can move between a retracted position and an actuated position, the landing gear having a plurality of wheels and a corresponding plurality of wheel brakes for said plurality of wheels, and a plurality of brake pedals for controlling oper
Mackness Robert F.
Salamat Bijan
Burch Melody M.
Fulwider Patton Lee & Utecht LLP
Graham Matthew C.
Hydro-Aire, Inc.
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