Basicamente funciona evitando que se bloqueen las ruedas cuando se aplican los frenos. De esta manera se optimiza la performance de frenado, especialmente en condiciones de pista mojada o contaminada.
Para mas detalles te copio el sistema del MD. No varÃa mucho con respecto a otros equipos, ya que el pricipio de funcionamiento es el mismo.
ANTISKID SYSTEM - DESCRIPTION AND OPERATION
1. General
A. The antiskid system is an automatic, electrically controlled means of preventing main gear wheels
from skidding during brake application. Each wheel is controlled independently. The system consists
of a solid state circuitry control box, four speed sensing transducers, four dual servo valves, four
failure annunciator displays, a system arming switch and a test switch. (Figure 1)
B. Skids are detected by comparing present wheel speed (as sensed by a transducer mounted in the
axle) to a reference velocity. This reference velocity is based on past wheel speed and deceleration
and is determined by the control circuitry. When wheel speed drops below this reference level an
error signal is generated. When the error signal reaches a predetermined threshold, a control signal
is sent to the servo valves which reduces the brake pressure to that wheel. As the wheel begins to
spin up following the skid, pressure is gradually reapplied. Both the error threshold and
reapplication note are chosen to maximize friction between tire and runway and thereby minimizing
stopping distance. Note that the antiskid system operates only when the pressure applied by the pilot
is sufficient to cause a skid condition. If the antiskid system is inoperative (not armed), the pilot’s
metered pressure will be applied directly to the brakes regardless of skidding.
2. Antiskid System Control Box
A. The control box contains a solid-state printed circuit card for each wheel, two self test cards, and a
wheels not rolling card. The main wheel cards contain the brains of the antiskid system. They accept
a sine wave input, with frequency proportional to velocity, from the transducers. This sine wave is
frequency doubled and converted to a DC voltage level, and compared with a reference velocity. An
error signal is generated which is input to the control circuits, pressure bias modulation (PBM), and
transit control. PBM integrates the error signal to provide smooth control under normal conditions.
Transit control provides rapid brake release when a deep skid occurs. The outputs of these control
circuits are combined and sent to valve drivers which provide the control signals to the two servo
valves which control pressure to the brake. The main wheel cards also provide the wheels with
locked wheel and touchdown protection. Touchdown protection releases the inboard wheels in flight
to prevent an immediate skid on touch down. Locked wheel pairing provides a pressure release
signal from a paired wheel control card in the event of a lockup. The wheel pairings are as follows:
left inboard with right outboard; right inboard with left outboard.
B. The two self-test cards provide a means for checking the components of the antiskid system. If any
component is inoperative, one of the failure annunciator displays will remain on after self-test. The
test card does not single out the component, but does indicate the portion of the system involved.
C. The wheels not rolling card senses that one or more main gear wheels is in a skid or lockup
condition. For the light to come on the aircraft must be on the ground, with the ground speed above
40 knots, and one or more wheels must be spinning at less than eighty percent of ground speed. The
light will not come on while the aircraft is in flight or when the ground speed is below 40 knots.
D. The control box, located in the aft electronics accessory compartment, is secured to a rack by two
lock clamps. The control box has two electrical receptacles: one to accommodate the test equipment,
and one to receive wiring from, and to, the other antiskid components and power source. The front
panel contains a meter and necessary switches to trouble shoot the system and isolate a fault down
to the LRU level.
MD-80
AIRCRAFT MAINTENANCE MANUAL
EFFECTIVITY
