This invention relates to a method and system for increasing accuracy of clamping force of electric brakes of aircraft, and more particularly relates to a method and system for increasing accuracy of clamping force of electric aircraft carbon brakes providing greater accuracy for low brake clamping force commands by dedicating a portion of a plurality of electric brake actuators of each brake to low brake clamping force commands, without otherwise affecting normal braking. The present invention also relates to a system and method for aircraft brake metering to alleviate structural loading, and more particularly relates to a system and method for metering aircraft brakes to alleviate structural loading of an aircraft by delaying a full onset of braking for a preset period of time, such as on any brake-by-wire aircraft where a brake metering function can be modified.
Commercial aircraft commonly have landing gear with electrically actuated brakes for wheels mounted to the wing and body of the aircraft. The electrically actuated brakes are typically carbon brakes including a torque plate and a carbon heat sink stack containing the friction surfaces that are clamped together by four electric brake actuators with a clamping brake force to cause a wheel to decrease its speed of rotation. In such a conventional airplane carbon brake system, when braking is commanded, either by a pilot's actuation of a brake pedal or automatic braking, it causes the friction surfaces of the carbon brakes to make contact, creating brake torque to slow down the rotational speed of the wheel, and through contact with the ground, the taxi speed of airplane.
As is described in U.S. Pat. No. 7,441,844, it is possible to reduce brake wear of electrically operated aircraft carbon brakes, once braking has been commenced, by maintaining a minimum light residual clamping brake force when braking is no longer commanded, such as when a pilot stops pressing on a brake pedal, or otherwise during a commanded release of braking during automatic braking. During taxiing of commercial aircraft, particularly at low speeds, steering of the aircraft is typically controlled by braking, and an unequal distribution of brake energy due to inaccurate metering of brake clamping force can in some instances interfere with the directional stability of aircraft, particularly when a minimum light residual clamping brake force is maintained during taxiing when braking is no longer commanded. Unequal distribution of brake energy due to inaccurate metering of brake clamping force can also result in damage to wheels and brakes from exposure to excessively high temperatures. It has been found that it is not possible with currently available electrical braking systems to achieve a brake clamping force accuracy required by current industry standards for the Boeing 787, and to prevent unequal distribution of brake energy.
Repeated rapid brake applications at low speed can also create fatigue loads on aircraft structural components. This is most apparent when the aircraft is taxiing at low speed and full rapid braking is commanded by a pilot by rapidly depressing the brake pedals. On some aircraft, such a condition will cause the nose gear of the aircraft (which does not typically have brakes) to bounce off the runway, because sudden braking loads are presented as a torque about the fuselage and reacted as a down force on the nose gear of the aircraft. Over the life of an aircraft, the design of the aircraft must take into account these structural loads so that the aircraft structure will not fail prematurely.
Previous known methods for alleviating airplane structural load during rapid low speed braking typically have involved the delayed application of some, but not all of the available brakes to reduce the aircraft structural loads and fatigue. Such selective delayed application of less than all available aircraft brakes, such as by applying only some of the available brakes while the remainder of the available brakes are not applied during an initial brake application during taxi conditions, can reduce aircraft structural loads.
One such conventional electric brake system for an aircraft is known that employs a brake control process to reduce high dynamic structural loading of the aircraft landing gear and lurching of the aircraft that can be caused by braking maneuvers. The system obtains current aircraft speed to determine that the aircraft is in taxi mode, current brake pedal deflection position, and deflection rate to determine whether to delay the onset of a desired braking condition.
It would be desirable to provide a system and method for alleviating aircraft structural loads during braking that does not require a logic condition of first determining that the aircraft is in taxi mode, and that does not require releasing and then reapplying some, but not all of the brakes during low speed braking. It would be desirable to provide a system and method for metering aircraft brakes to alleviate structural loading of an aircraft that does not rely upon the delayed application of brakes, but instead delays a full onset of braking for a preset period of time, to reduce aircraft structural loading, such as on any brake-by-wire aircraft where a brake metering function can be modified. The present invention meets these and other needs.