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response to cockpit brake pedal commands. The left and right pilot and copilot
brake pedals are mechanically linked to the left and right brake metering valves in
the nose wheel well. The metering valves open in response to brake pedal
deflection, porting increased pressure with greater brake pedal application. From
the brake metering valves, pressurized hydraulic fluid is routed to the augmenter
valve in the main wheel well. The augmenter valve increases hydraulic fluid flow
rates and intensifies hydraulic pressure to compensate for any system response
loss caused by the distance between the brake metering valves in the nose wheel
well and the main landing gear. From the augmenter valve, hydraulic pressure
then passes through the anti-skid control valve modules, hydraulic line fuses,
OPERATING MANUAL
2A-32-00 PRODUCTION AIRCRAFT SYSTEMS
Page 32
January 31/02
Revision 6
pressure transducers and brake shuttle valves and each brake piston assembly.
Hydraulic fuses detect a broken or damaged hydraulic line indicated by increased
fluid flow and close to prevent system fluid loss. The anti-skid control valve
modules regulate hydraulic pressure to the brake assemblies to maintain wheel
rotation during braking. Pressure transducers electronically transmit readings of
hydraulic pressure applied to the brakes to the Data Acquisition Units (DAUs). The
DAUs formulate this information for display on the cockpit Engine Instrument/
Crew Alerting System (EICAS). See Figure 14 for a schematic of the HMAB
system.
The left and right brake metering valves are plumbed to both the Combined and
Auxiliary hydraulic systems. If Combined system pressure drops below 1500 psi,
internal stops in the metering valves shift, allowing Auxiliary system pressure to
operate the brakes with a slight increase in pedal effort required. The Auxiliary
hydraulic pump will pressurize automatically (if armed) when the loss of
Combined system pressure requires increased brake pedal travel and
subsequent engagement of the pedal limit switches. The limit switches in
combination with the Combined system low pressure switch complete the circuit
to power the Auxiliary pump. When the Auxiliary system is in use, the augmenter
valve is bypassed, and pressure is routed through alternate anti-skid modules to
the emergency brake shuttles and hydraulic fuses, and then to the brakes.
If Combined or Auxiliary hydraulic pressure are not available, a parking /
emergency brake system with stored accumulator pressure may be used to stop
the aircraft. The brake accumulator is in the nose wheel well, and has a nitrogen
gas pre-charge of 1200 psi. The accumulator is shown in Figure 10. When normal
hydraulic pressure is initially applied to the aircraft, the accumulator is fully
pressurized with hydraulic fluid at 3000 psi. Accumulator pressure may be read at
the accumulator in the nose wheel well, or on the indicator on the copilot lower
instrument panel. The accumulator, when fully charged contains sufficient
pressurized hydraulic fluid for approximately 5 to 6 brake applications. The PARK
/ EMER BRAKE handle on the cockpit center console is connect by cable to the
modulating valve on the right side of the nose wheel well. Brake modulation can
be accomplished by pulling out the handle between ¼ and ½ inch. Handle travel
beyond approximately ½ inch results in the application of full accumulator
pressure. No anti-skid is available with emergency braking. To apply accumulator
pressure for parking the aircraft, the PARK / EMER BRAKE handle is pulled up
and then rotated clockwise. To release the PARK / EMER BRAKE, pull the handle
up and rotate counterclockwise.
An anti-rotation valve is incorporated in the braking system to stop wheel spin
upon landing gear retraction. When the landing gear is down and locked, the antirotation
valve is closed, but as the gear retracts, pressure from the main landing
gear timer valve is routed to the anti-rotation valve where it is reduced to 400±50
psi, and applied to the wheel brakes through the parking / emergency brake lines.
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-32-00
Page 33
January 31/02
Revision 6
2A-32-42: Hydro-Mechanical Analog Braking (HMAB) Anti-skid
Protection
1. General Description
NOTE:
The following system description applies to aircraft
S/N 1214 and subsequent, S/N 1000 - 1213 with ASC
307 installed, and S/N 1183.
Hydro-mechanical Analog Braking (HMAB) anti-skid protection includes a primary
system and an alternate system. The primary system provides anti-skid protection
during normal operations using the Combined hydraulic system. The alternate
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