From the CJ3 Manual:
WHEEL BRAKES
The Model 525B uses hydraulically powered main landing gear brakes. Crew inputs to the brake metering valve are mechanically transmitted via a series of cables from the toe brakes on the rudder pedals. The brake metering valve regulates hydraulic pressure to the brakes based on pilot or copilot input. An electronic anti-skid system monitors the main gear wheel speeds and
reduces brake pressure as necessary to optimize stopping distance and prevent wheel lock-up.
A parking brake valve is used to trap pressurized fluid in the brake lines and is controlled by a control knob in the cockpit. The pneumatic brake system is a back-up system used to supply pressure to the brake assemblies in the event of hydraulic brake system failure. It supplies pressurized nitrogen from a bottle in the nose directly to shuttle valves on the brakes and is controlled by a lever in the cockpit.
Brake Metering Valve & Cable System Operation
The brakes are operated by a separate, closed center hydraulic system with an independent reservoir, pump/electric motor, and accumulator. A pressure switch located near the fluid end of the accumulator senses brake system pressure and commands the pump on and off accordingly.
There is no cockpit switch for the brake pump. The pump is powered on any time the gear handle is in the down position and the accumulator pressure is below 1175 ± 75 psig.
When the accumulator pressure reaches 1500 ± 50 psig, the power is removed for the pump. A separate low pressure switch built into the pump monitors the system for low pressure. If the system pressure drops below (900 ± 50 psig) and the gear handle is down, the low pressure switch
causes the (PWR BRK LOW PRESS) annunciator to illuminate. During normal operation, the accumulator provides pressurized fluid to the brake metering valve which regulates pressure (0 to 1000 +50/-20 psi) to the brake assemblies in proportion to the brake pedal deflection of the pilot’s
or co-pilot’s brake pedals. Braking can be controlled independently from either cockpit position.
The cable system that transmits pedal deflection to the brake metering valve is designed such that pedal inputs by the pilot do not cause the copilots pedals to move, and vice versa.
If the pilots and copilots pedals are depressed simultaneously, the brake system accepts the highest input. Since the brake system is a cable controlled ‘power brake system’, the braking feel force at the pedals is created by the springs in the mixer, the springs within the brake metering
valve and a proportional hydraulic feedback force generated by the brake metering valve.
Anti-skid System Operation
The cockpit controls for the anti-skid system consist of a single ANTISKID switch which is a ON/OFF lever lock switch located just right of the gear handle. The primary function of the antiskid system is to provide maximum braking efficiency under all runway conditions. In addition, the anti-skid system provides touchdown protection, which prevents braking until adequate wheel spin-up has occurred and locked wheel crossover protection that prevents adverse differential braking.
Anti-skid Protection
Anti-skid protection is provided to allow maximum braking efficiency, which in turn minimizes landing distances. If the pilot applies enough brake pedal force to cause slippage between the tires and the runway, the wheel speed transducer data received by the control box will indicate a sudden deceleration for the slipping wheel. The control box will determine the severity of the impending skid and send the appropriate current signal to the anti-skid servo valve to reduce brake pressure accordingly. Dual servo valves reduce pressure for either brake independently.
Therefore, a single wheel skid will result in the reduction of brake pressure at the skidding wheel only. Anti-skid protection will be available unless the touchdown protection mode is active.
Touchdown Protection
Touchdown protection is provided to prevent the application of brake pressure prior to wheel spin up. During a landing, the wheels must be allowed to spin up to provide the anti-skid system a ‘reference’ velocity to which individual wheel speeds can be compared. Touchdown protection is
active only when an AIR signal is sensed by both main gear squat switches. In touchdown protection mode, the control box commands the anti-skid servo valves to dump all brake pressure. The full dump command will remain active for 3 seconds after WOW or until wheel spin up has occurred. Under normal circumstances, the wheels will spin up almost immediately after
touchdown. Therefore, the system incorporates a spin up override feature. When the velocity of a wheel exceeds 59+2 kts, touchdown protection is overridden and brake pressure application is allowed to that wheel. Each wheel is independent in regard to spin up override, specifically;
touchdown protection mode is overridden for each wheel independently only when the speed of a given wheel is in excess of 59+2 kts. The wheel spin up override will remain active until the wheel velocity falls below 15+2 kts.
Locked Wheel Crossover Protection
Locked wheel crossover protection prevents inadvertent turning of the aircraft due to differential braking caused by adverse runway conditions. The velocities of the two wheels are compared to each other to determine if one wheel is locked. If the velocity of one wheel falls to less than 30%
of the velocity of the other wheel, the control box will send a full dump command to the anti-skid servo valve controlling the slower wheel. The full dump will remain in effect until the velocity of the slow wheel increases above the 30% threshold. The locked wheel crossover feature is
inactive at wheel speeds below 25 kts to allow for low speed taxiing maneuvers.
Anti-skid System Self Monitoring
The anti-skid system performs continuous integrity checks on the wheel speed transducer circuits, the anti-skid servo valve circuit and the regulated power to the control box. If a fault is detected during the continuous monitoring the ANTISKID INOP annunciator is illuminated and a
signal is sent to the anti-skid fault display unit. The fault display unit is located in the RH nose compartment on the forward side of the forward pressure bulkhead. The fault display unit consists of 5 rotary flags to aid in the troubleshooting of an ANTISKID INOP indication. There is one rotary
flag for each of the following conditions: LH transducer fault, RH transducer fault, servo valve fault, control box fault and a squat switch disagree. Both main gear squat switch signals are monitored and compared. If the signals disagree for more than 12.8+1 seconds, the squat switch disagree flag is tripped. However, this fault does not cause the ANTISKID INOP annunciator to illuminate.
There is also one condition, low supply voltage, which causes the ANTISKID INOP annunciator to illuminate but does not result in a tripped flag on the fault display unit. The continuous monitor function evaluates the voltage supplied to the control box. Anytime the input voltage is less than
7.0+1 volts, the ANTISKID INOP annunciator is illuminated but the control box flag will not be tripped. This feature is intended to alert the crew that the anti-skid system is either switched off or is unavailable due to insufficient power. In addition to continuous monitoring, the anti-skid control
box performs a dynamic self test which is initiated by any of the following events:
- initial power-up of the anti-skid system
- transition of the gear handle to the down position
- selection of (ANTISKID) on the rotary test switch
During a dynamic self test, a signal is sent to illuminate the ANTISKID INOP annunciator. Upon successful completion of the test, the ANTISKID INOP annunciator will extinguish. If a fault is detected during the self test, the annunciator will remain illuminated. A dynamic self test performed in the air takes approximately 3 seconds, while a dynamic self test performed on the
ground takes approximately 6 seconds. The dynamic self test routine is inhibited if wheel speed is
greater than 15+5 kts.